Ultrafast X-ray absorption study of longitudinal-transverse phonon coupling in electrolyte aqueous solution

DEFF Research Database (Denmark)

Jiao, Yishuo; Adams, Bernhard W.; Dohn, Asmus Ougaard

2017-01-01

Ultrafast X-ray absorption spectroscopy is applied to study the conversion of longitudinal to transverse phonons in aqueous solution. Permanganate solutes serve as X-ray probe molecules that permit the measurement of the conversion of 13.5 GHz, longitudinal phonons to 27 GHz, transverse phonons...

Specularity of longitudinal acoustic phonons at rough surfaces

Science.gov (United States)

Gelda, Dhruv; Ghossoub, Marc G.; Valavala, Krishna; Ma, Jun; Rajagopal, Manjunath C.; Sinha, Sanjiv

2018-01-01

The specularity of phonons at crystal surfaces is of direct importance to thermal transport in nanostructures and to dissipation in nanomechanical resonators. Wave scattering theory provides a framework for estimating wavelength-dependent specularity, but experimental validation remains elusive. Widely available thermal conductivity data presents poor validation since the involvement of the infinitude of phonon wavelengths in thermal transport presents an underconstrained test for specularity theory. Here, we report phonon specularity by measuring the lifetimes of individual coherent longitudinal acoustic phonon modes excited in ultrathin (36-205 nm) suspended silicon membranes at room temperature over the frequency range ˜20 -118 GHz. Phonon surface scattering dominates intrinsic Akhiezer damping at frequencies ≳60 GHz, enabling measurements of phonon boundary scattering time over wavelengths ˜72 -140 nm . We obtain detailed statistics of the surface roughness at the top and bottom surfaces of membranes using HRTEM imaging. We find that the specularity of the excited modes are in good agreement with solutions of wave scattering only when the TEM statistics are corrected for projection errors. The often-cited Ziman formula for phonon specularity also appears in good agreement with the data, contradicting previous results. This work helps to advance the fundamental understanding of phonon scattering at the surfaces of nanostructures.

Enhancing of optic phonon contribution in hydrodynamic phonon transport

Science.gov (United States)

de Tomas, C.; Cantarero, A.; Lopeandia, A. F.; Alvarez, F. X.

2015-10-01

In the framework of the kinetic-collective model of phonon heat transport, we analyze how each range of the phonon frequency spectrum contributes to the total thermal conductivity both in the macro and the nanoscale. For this purpose, we use two case study samples: naturally occurring bulk silicon and a 115 nm of diameter silicon nanowire. We show that the contribution of high-energy phonons (optic branches) is non-negligible only when N-collisions are strongly present. This contribution increases when the effective size of the sample decreases, and it is found to be up to a 10% at room temperature for the 115 nm nanowire, corroborating preliminar ab-initio predictions.

Coherent phonon optics in a chip with an electrically controlled active device.

Science.gov (United States)

Poyser, Caroline L; Akimov, Andrey V; Campion, Richard P; Kent, Anthony J

2015-02-05

Phonon optics concerns operations with high-frequency acoustic waves in solid media in a similar way to how traditional optics operates with the light beams (i.e. photons). Phonon optics experiments with coherent terahertz and sub-terahertz phonons promise a revolution in various technical applications related to high-frequency acoustics, imaging, and heat transport. Previously, phonon optics used passive methods for manipulations with propagating phonon beams that did not enable their external control. Here we fabricate a phononic chip, which includes a generator of coherent monochromatic phonons with frequency 378 GHz, a sensitive coherent phonon detector, and an active layer: a doped semiconductor superlattice, with electrical contacts, inserted into the phonon propagation path. In the experiments, we demonstrate the modulation of the coherent phonon flux by an external electrical bias applied to the active layer. Phonon optics using external control broadens the spectrum of prospective applications of phononics on the nanometer scale.

Quasiparticle properties of a coupled quantum-wire electron-phonon system

DEFF Research Database (Denmark)

Hwang, E. H.; Hu, Ben Yu-Kuang; Sarma, S. Das

1996-01-01

We study leading-order many-body effects of longitudinal-optical phonons on electronic properties of one-dimensional quantum-wire systems. We calculate the quasiparticle properties of a weakly polar one-dimensional electron gas in the presence of both electron-phonon and electron-electron interac......We study leading-order many-body effects of longitudinal-optical phonons on electronic properties of one-dimensional quantum-wire systems. We calculate the quasiparticle properties of a weakly polar one-dimensional electron gas in the presence of both electron-phonon and electron......-electron interactions, The leading-order dynamical screening approximation (GW approximation) is used to obtain the electron self-energy, the quasiparticle spectral function, and the quasiparticle damping rate in our calculation by treating electrons and phonons on an equal footing. Our theory includes effects (within...... theoretical results for quasiparticle properties....

Optical phonons in cubic AlxGa1-xN approached by the modified random element isodisplacement model

International Nuclear Information System (INIS)

Liu, M.S.; Bursill, L.A.; Prawer, S.

1998-01-01

The behaviour of longitudinal and transverse optical phonons in cubic Al x Ga l-x N are derived theoretically as a function of the concentration x (0≤x≤1). The calculation is based on a Modified Random Element Isodisplacement model which considers the interactions from the nearest neighbor and second neighbor atoms. We find one-mode behavior in Al x Ga l-x N where the phonon frequency in general varies continuously and approximately linearly with x. (author)

Analysis of Longitudinal Waves in Rod-Type Piezoelectric Phononic Crystals

Directory of Open Access Journals (Sweden)

Longfei Li

2016-04-01

Full Text Available Phononic crystals can be used to control elastic waves due to their frequency bands. This paper analyzes the passive and active control as well as the dispersion properties of longitudinal waves in rod-type piezoelectric phononic crystals over large frequency ranges. Based on the Love rod theory for modeling the longitudinal wave motions in the constituent rods and the method of reverberation-ray matrix (MRRM for deriving the member transfer matrices of the constituent rods, a modified transfer matrix method (MTMM is proposed for the analysis of dispersion curves by combining with the Floquet–Bloch principle and for the calculation of transmission spectra. Numerical examples are provided to validate the proposed MTMM for analyzing the band structures in both low and high frequency ranges. The passive control of longitudinal-wave band structures is studied by discussing the influences of the electrode’s thickness, the Poisson’s effect and the elastic rod inserts in the unit cell. The influences of electrical boundaries (including electric-open, applied electric capacity, electric-short and applied feedback control conditions on the band structures are investigated to illustrate the active control scheme. From the calculated comprehensive frequency spectra over a large frequency range, the dispersion properties of the characteristic longitudinal waves in rod-type piezoelectric phononic crystals are summarized.

Temperature dependence of phonons in pyrolitic graphite

International Nuclear Information System (INIS)

Brockhouse, B.N.; Shirane, G.

1977-01-01

Dispersion curves for longitudinal and transverse phonons propagating along and near the c-axis in pyrolitic graphite at temperatures between 4 0 K and 1500 0 C have been measured by neutron spectroscopy. The observed frequencies decrease markedly with increasing temperature (except for the transverse optical ''rippling'' modes in the hexagonal planes). The neutron groups show interesting asymmetrical broadening ascribed to interference between one phonon and many phonon processes

Electron-optical phonon coupling in superconductors

International Nuclear Information System (INIS)

Rietschel, H.

1975-01-01

The role of the optical phonons in superconductivity is investigated in the case of compounds with different atomic masses Msub(k). It is shown that the electron mass enhancement factor lambda is independent of Msub(k) if the force constant matrix is mass independent. However, when using lambda to calculate Tsub(c), it must be decomposed into its acoustical and optical contributions, which depend separately on Msub(k). Interference scattering from a light and a heavy mass is studied and its contributions to lambda within the free electron approximation. Numerical results are presented for a rocksalt structure crystal with nearest and next nearest neighbour coupling. These results indicate that the optical phonon contributions to lambda may substantially increase Tsub(c). (orig.) [de

Optical-phonon-induced frictional drag in coupled two-dimensional electron gases

DEFF Research Database (Denmark)

Hu, Ben Yu-Kuang

1998-01-01

The role of optical phonons in frictional drag between two adjacent but electrically isolated two-dimensional electron gases is investigated. Since the optical phonons in III-V materials have a considerably larger coupling to electrons than acoustic phonons (which are the dominant drag mechanism ...

Optical pumping of hot phonons in GaAs

International Nuclear Information System (INIS)

Collins, C.L.; Yu, P.Y.

1982-01-01

Optical pumping of hot LO phonons in GaAs has been studied as a function of the excitation photon frequency. The experimental results are in good agreement with a model calculation which includes both inter- and intra-valley electron-phonon scatterings. The GAMMA-L and GAMMA-X intervalley electron-phonon interactions in GaAs have been estimated

Interface phonon effect on optical spectra of quantum nanostructures

International Nuclear Information System (INIS)

Maslov, Alexander Yu.; Proshina, Olga V.; Rusina, Anastasia N.

2009-01-01

This paper deals with theory of large radius polaron effect in quantum wells, wires and dots. The interaction of charge particles and excitons with both bulk and interface optical phonons is taken into consideration. The analytical expression for polaron binding energy is obtained for different types of nanostructures. It is shown that the contribution of interface phonons to the polaron binding energy may exceed the bulk phonon part. The manifestation of polaron effects in optical spectra of quantum nanostructures is discussed.

Unified treatment of coupled optical and acoustic phonons in piezoelectric cubic materials

DEFF Research Database (Denmark)

Willatzen, Morten; Wang, Zhong Lin

2015-01-01

A unified treatment of coupled optical and acoustic phonons in piezoelectric cubic materials is presented whereby the lattice displacement vector and the internal ionic displacement vector are found simultaneously. It is shown that phonon couplings exist in pairs only; either between the electric...... piezoelectricity in a cubic structured material slab. First, it is shown that isolated optical phonon modes generally cannot exist in piezoelectric cubic slabs. Second, we prove that confined acousto-optical phonon modes only exist for a discrete set of in-plane wave numbers in piezoelectric cubic slabs. Third...... potential and the lattice displacement coordinate perpendicular to the phonon wave vector or between the two other lattice displacement components. The former leads to coupled acousto-optical phonons by virtue of the piezoelectric effect. We then establish three new conjectures that entirely stem from...

Subterahertz Longitudinal Phonon Modes Propagating in a Lipid Bilayer Immersed in an Aqueous Medium

Science.gov (United States)

Zakhvataev, V. E.

2018-04-01

The properties of subterahertz longitudinal acoustic phonon modes in the hydrophobic region of a lipid bilayer immersed in a compressible viscous aqueous medium are investigated theoretically. An approximate expression is obtained for the Mandelstam-Brillouin components of the dynamic structure factor of a bilayer. The analysis is based on a generalized hydrodynamic model of the "two-dimensional lipid bilayer + three-dimensional fluid medium" system, as well as on known sharp estimates for the frequencies and lifetimes of long-wavelength longitudinal acoustic phonons in a free hydrated lipid bilayer and in water, obtained from inelastic X-ray scattering experiments and molecular dynamics simulations. It is shown that, for characteristic values of the parameters of the membrane system, subterahertz longitudinal phonon-like excitations in the hydrophobic part of the bilayer are underdamped. In this case, the contribution of the viscous flow of the aqueous medium to the damping of a longitudinal membrane mode is small compared with the contribution of the lipid bilayer. Quantitative estimates of the damping ratio agree well with the experimental results for the vibration mode of the enzyme lysozyme in aqueous solution [1]. It is also shown that a coupling between longitudinal phonon modes of the bilayer and relaxation processes in its fluid environment gives rise to an additional peak in the scattering spectrum, which corresponds to a non-propagating mode.

Phonon excitations in multicomponent amorphous solids

International Nuclear Information System (INIS)

Vakarchuk, I.A.; Migal', V.M.; Tkachuk, V.M.

1988-01-01

The method of two-time temperature-dependent Green's functions is used to investigate phonon excitations in multicomponent amorphous solids. The equation obtained for the energy spectrum of the phonon excitations takes into account the damping associated with scattering of phonons by structure fluctuations. The quasicrystal approximation is considered, and as an example explicit expressions are obtained for the case of a two-component amorphous solid for the frequencies of the acoustical and optical modes and for the longitudinal and transverse velocities of sound. The damping is investigated

Phonon spectra in SiO2 glasses

International Nuclear Information System (INIS)

Perez R, J.F.; Jimenez S, S.; Gonzalez H, J.; Vorobiev, Y.V.; Hernandez L, M.A.; Parga T, J.R.

1999-01-01

Phonon spectra in SiO 2 sol-gel made glasses annealed under different conditions are investigated using infrared absorption and Raman scattering. These data are compared with those obtained in commercial optical-quality quartz. All the materials exhibit the same phonon bands, the exact position and the intensity depend on the measuring technique and on the sample preparation method. The phonon spectra in this material are interpreted on the basis of a simple quasi-linear description of elastic waves in an O-Si-O chain. It is shown that the main features observed in the range 400-1400 cm -1 can be predicted using a quasi-linear chain model in which the band at 1070 cm -1 is assigned to the longitudinal optical waves in the O-Si-O chain with the smallest possible wavelength at the Brillouin zone boundary, the band located around 450 cm -1 is assigned to the transversal optical waves and the band at 800 cm -1 to the longitudinal acoustical waves with the same wavelength. The degree of structural disorder can be also deduced within the framework of the proposed model. (Author)

Towards phonon photonics: scattering-type near-field optical microscopy reveals phonon-enhanced near-field interaction

International Nuclear Information System (INIS)

Hillenbrand, Rainer

2004-01-01

Diffraction limits the spatial resolution in classical microscopy or the dimensions of optical circuits to about half the illumination wavelength. Scanning near-field microscopy can overcome this limitation by exploiting the evanescent near fields existing close to any illuminated object. We use a scattering-type near-field optical microscope (s-SNOM) that uses the illuminated metal tip of an atomic force microscope (AFM) to act as scattering near-field probe. The presented images are direct evidence that the s-SNOM enables optical imaging at a spatial resolution on a 10 nm scale, independent of the wavelength used (λ=633 nm and 10 μm). Operating the microscope at specific mid-infrared frequencies we found a tip-induced phonon-polariton resonance on flat polar crystals such as SiC and Si 3 N 4 . Being a spectral fingerprint of any polar material such phonon-enhanced near-field interaction has enormous applicability in nondestructive, material-specific infrared microscopy at nanoscale resolution. The potential of s-SNOM to study eigenfields of surface polaritons in nanostructures opens the door to the development of phonon photonics--a proposed infrared nanotechnology that uses localized or propagating surface phonon polaritons for probing, manipulating and guiding infrared light in nanoscale devices, analogous to plasmon photonics

Infrared-active optical phonons in LiFePO4 single crystals

Science.gov (United States)

Stanislavchuk, T. N.; Middlemiss, D. S.; Syzdek, J. S.; Janssen, Y.; Basistyy, R.; Sirenko, A. A.; Khalifah, P. G.; Grey, C. P.; Kostecki, R.

2017-07-01

Infrared-active optical phonons were studied in olivine LiFePO4 oriented single crystals by means of both rotating analyzer and rotating compensator spectroscopic ellipsometry in the spectral range between 50 and 1400 cm-1. The eigenfrequencies, oscillator strengths, and broadenings of the phonon modes were determined from fits of the anisotropic harmonic oscillator model to the data. Optical phonons in a heterosite FePO4 crystal were measured from the delithiated ab-surface of the LiFePO4 crystal and compared with the phonon modes of the latter. Good agreement was found between experimental data and the results of solid-state hybrid density functional theory calculations for the phonon modes in both LiFePO4 and FePO4.

Modelling exciton–phonon interactions in optically driven quantum dots

DEFF Research Database (Denmark)

Nazir, Ahsan; McCutcheon, Dara

2016-01-01

We provide a self-contained review of master equation approaches to modelling phonon effects in optically driven self-assembled quantum dots. Coupling of the (quasi) two-level excitonic system to phonons leads to dissipation and dephasing, the rates of which depend on the excitation conditions...

Quantum Theory of Conditional Phonon States in a Dual-Pumped Raman Optical Frequency Comb

Science.gov (United States)

Mondloch, Erin

In this work, we theoretically and numerically investigate nonclassical phonon states created in the collective vibration of a Raman medium by the generation of a dual-pumped Raman optical frequency comb in an optical cavity. This frequency comb is generated by cascaded Raman scattering driven by two phase-locked pump lasers that are separated in frequency by three times the Raman phonon frequency. We characterize the variety of conditioned phonon states that are created when the number of photons in all optical frequency modes except the pump modes are measured. Almost all of these conditioned phonon states are extremely well approximated as three-phonon-squeezed states or Schrodinger-cat states, depending on the outcomes of the photon number measurements. We show how the combinations of first-, second-, and third-order Raman scattering that correspond to each set of measured photon numbers determine the fidelity of the conditioned phonon state with model three-phonon-squeezed states and Schrodinger-cat states. All of the conditioned phonon states demonstrate preferential growth of the phonon mode along three directions in phase space. That is, there are three preferred phase values that the phonon state takes on as a result of Raman scattering. We show that the combination of Raman processes that produces a given set of measured photon numbers always produces phonons in multiples of three. In the quantum number-state representation, these multiples of three are responsible for the threefold phase-space symmetry seen in the conditioned phonon states. With a semiclassical model, we show how this three-phase preference can also be understood in light of phase correlations that are known to spontaneously arise in single-pumped Raman frequency combs. Additionally, our semiclassical model predicts that the optical modes also grow preferentially along three phases, suggesting that the dual-pumped Raman optical frequency comb is partially phase-stabilized.

Electronic response and longitudinal phonons of a charge-density-wave distorted linear chain

International Nuclear Information System (INIS)

Giuliani, G.

1978-01-01

The longitudinal-phonon spectrum of an incommensurate charge-density-wave distorted linear chain at T = 0 K are calculated. This is done by direct numerical evaluation of the full static-electronic-response matrix. The electronic band structure assumed for this purpose is that of a mean-field theory 1-D Peierls insulator. The present results show how, within this simplified, but self-consistent picture, the phase and amplitude modes connect to, and interact with, the ordinary longitudinal-phonon branch. Effects due to our inclusion of (0,2ksub(F)) scattering along with the usual (-2ksub(F), 2ksub(F)) are also pointed out. An alternative approximate expression for the 1-D electronic-response matrix is also given. (author)

Light-induced nonthermal population of optical phonons in nanocrystals

Science.gov (United States)

Falcão, Bruno P.; Leitão, Joaquim P.; Correia, Maria R.; Soares, Maria R.; Wiggers, Hartmut; Cantarero, Andrés; Pereira, Rui N.

2017-03-01

Raman spectroscopy is widely used to study bulk and nanomaterials, where information is frequently obtained from spectral line positions and intensities. In this study, we monitored the Raman spectrum of ensembles of semiconductor nanocrystals (NCs) as a function of optical excitation intensity (optical excitation experiments). We observe that in NCs the red-shift of the Raman peak position with increasing light power density is much steeper than that recorded for the corresponding bulk material. The increase in optical excitation intensity results also in an increasingly higher temperature of the NCs as obtained with Raman thermometry through the commonly used Stokes/anti-Stokes intensity ratio. More significantly, the obtained dependence of the Raman peak position on temperature in optical excitation experiments is markedly different from that observed when the same NCs are excited only thermally (thermal excitation experiments). This difference is not observed for the control bulk material. The inefficient diffusion of photogenerated charges in nanoparticulate systems, due to their inherently low electrical conductivity, results in a higher steady-state density of photoexcited charges and, consequently, also in a stronger excitation of optical phonons that cannot decay quickly enough into acoustic phonons. This results in a nonthermal population of optical phonons and thus the Raman spectrum deviates from that expected for the temperature of the system. Our study has major consequences to the general application of Raman spectroscopy to nanomaterials.

Strong Carrier–Phonon Coupling in Lead Halide Perovskite Nanocrystals

Science.gov (United States)

2017-01-01

We highlight the importance of carrier–phonon coupling in inorganic lead halide perovskite nanocrystals. The low-temperature photoluminescence (PL) spectrum of CsPbBr3 has been investigated under a nonresonant and a nonstandard, quasi-resonant excitation scheme, and phonon replicas of the main PL band have been identified as due to the Fröhlich interaction. The energy of longitudinal optical (LO) phonons has been determined from the separation of the zero phonon band and phonon replicas. We reason that the observed LO phonon coupling can only be related to an orthorhombically distorted crystal structure of the perovskite nanocrystals. Additionally, the strength of carrier–phonon coupling has been characterized using the ratio between the intensities of the first phonon replica and the zero-phonon band. PL emission from localized versus delocalized carriers has been identified as the source of the observed discrepancies between the LO phonon energy and phonon coupling strength under quasi-resonant and nonresonant excitation conditions, respectively. PMID:29019652

LO-phonon and plasmon coupling in neutron-transmutation-doped GaAs

International Nuclear Information System (INIS)

Kuriyama, K.; Sakai, K.; Okada, M.

1996-01-01

Coupling between the longitudinal-optic (LO) phonon mode and the longitudinal plasma mode in neutron-transmutation-doped (NTD) semi-insulating GaAs was studied using Raman-scattering spectroscopy and a Fourier-transform infrared spectrometer. When the electron concentration due to the activation of NTD impurities (Ge Ga and Se As ) approaches ∼8x10 16 cm -3 , the LO-phonon endash plasmon coupling is observed. This behavior is consistent with the free-electron absorption due to the activation of NTD impurities in samples annealed above 600 degree C. copyright 1996 The American Physical Society

Electrical modulation and switching of transverse acoustic phonons

Science.gov (United States)

Jeong, H.; Jho, Y. D.; Rhim, S. H.; Yee, K. J.; Yoon, S. Y.; Shim, J. P.; Lee, D. S.; Ju, J. W.; Baek, J. H.; Stanton, C. J.

2016-07-01

We report on the electrical manipulation of coherent acoustic phonon waves in GaN-based nanoscale piezoelectric heterostructures which are strained both from the pseudomorphic growth at the interfaces as well as through external electric fields. In such structures, transverse symmetry within the c plane hinders both the generation and detection of the transverse acoustic (TA) modes, and usually only longitudinal acoustic phonons are generated by ultrafast displacive screening of potential gradients. We show that even for c -GaN, the combined application of lateral and vertical electric fields can not only switch on the normally forbidden TA mode, but they can also modulate the amplitudes and frequencies of both modes. By comparing the transient differential reflectivity spectra in structures with and without an asymmetric potential distribution, the role of the electrical controllability of phonons was demonstrated as changes to the propagation velocities, the optical birefringence, the electrically polarized TA waves, and the geometrically varying optical sensitivities of phonons.

One phonon resonant Raman scattering in free-standing quantum wires

International Nuclear Information System (INIS)

Zhao, Xiang-Fu; Liu, Cui-Hong

2007-01-01

The scattering intensity (SI) of a free-standing cylindrical semiconductor quantum wire for an electron resonant Raman scattering (ERRS) process associated with bulk longitudinal optical (LO) phonon modes and surface optical (SO) phonon modes is calculated separately for T=0 K. The Frohlich interaction is considered to illustrate the theory for GaAs and CdS systems. Electron states are confined within a free-standing quantum wire (FSW). Single parabolic conduction and valence bands are assumed. The selection rules are studied. Numerical results and a discussion are also presented for various radii of the cylindrical

Acousto-optical phonon excitation in cubic piezoelectric slabs and crystal growth orientation effects

DEFF Research Database (Denmark)

Willatzen, Morten; Duggen, Lars

2017-01-01

In this paper we investigate theoretically the influence of piezoelectric coupling on phonon dispersion relations. Specifically we solve dispersion relations for a fully coupled zinc-blende freestanding quantum well for different orientations of the crystal unit cell. It is shown that the phonon...... mode density in GaAs can change by a factor of approximately 2–3 at qx a = 1 for different crystal-growth directions relative to the slab thickness direction. In particular, it is found that optical and acoustic phonon modes are always piezoelectrically coupled, independent of the crystal...... that the piezoelectric effect leads to a drastically enhanced coupling of acoustic and optical phonon modes and increase in the local phonon density of states near the plasma frequency where the permittivity approaches zero....

Phonons, defects and optical damage in crystalline acetanilide

Science.gov (United States)

Kosic, Thomas J.; Hill, Jeffrey R.; Dlott, Dana D.

1986-04-01

Intense picosecond pulses cause accumulated optical damage in acetanilide crystals at low temperature. Catastrophic damage to the irradiated volume occurs after an incubation period where defects accumulate. The optical damage is monitored with subanosecond time resolution. The generation of defects is studied with damage-detected picosecond spectroscopy. The accumulation of defects is studied by time-resolved coherent Raman scattering, which is used to measure optical phonon scattering from the accumulating defects.

The anharmonic phonon decay rate in group-III nitrides

International Nuclear Information System (INIS)

Srivastava, G P

2009-01-01

Measured lifetimes of hot phonons in group-III nitrides have been explained theoretically by considering three-phonon anharmonic interaction processes. The basic ingredients of the theory include full phonon dispersion relations obtained from the application of an adiabatic bond charge model and crystal anharmonic potential within the isotropic elastic continuum model. The role of various decay routes, such as Klemens, Ridley, Vallee-Bogani and Barman-Srivastava channels, in determining the lifetimes of the Raman active zone-centre longitudinal optical (LO) modes in BN (zincblende structure) and A 1 (LO) modes in AlN, GaN and InN (wurtzite structure) has been quantified.

Bulk-like-phonon polaritons in one-dimensional photonic superlattices

Science.gov (United States)

Gómez-Urrea, H. A.; Duque, C. A.; Mora-Ramos, M. E.

2017-05-01

We investigate the properties of a one-dimensional photonic superlattice made of alternating layers of air and wurtzite aluminum nitride. The Maxwell equations are solved for any admissible values of the angle of incidence by means of the transfer matrix formalism. The band structure of the frequency spectrum is obtained, as well as the density of states and transmittance associated to both the TM and TE modes. The dispersion relations indicate that for oblique incidence and TM modes there is a component of the electric field oriented along the growth direction of the structure that couples with the longitudinal optical phonon oscillations of the aluminum nitride thus leading to the appearance of longitudinal phonon polaritons in the system.

The hydrogen-bond network of water supports propagating optical phonon-like modes.

Science.gov (United States)

Elton, Daniel C; Fernández-Serra, Marivi

2016-01-04

The local structure of liquid water as a function of temperature is a source of intense research. This structure is intimately linked to the dynamics of water molecules, which can be measured using Raman and infrared spectroscopies. The assignment of spectral peaks depends on whether they are collective modes or single-molecule motions. Vibrational modes in liquids are usually considered to be associated to the motions of single molecules or small clusters. Using molecular dynamics simulations, here we find dispersive optical phonon-like modes in the librational and OH-stretching bands. We argue that on subpicosecond time scales these modes propagate through water's hydrogen-bond network over distances of up to 2 nm. In the long wavelength limit these optical modes exhibit longitudinal-transverse splitting, indicating the presence of coherent long-range dipole-dipole interactions, as in ice. Our results indicate the dynamics of liquid water have more similarities to ice than previously thought.

Froehlih coupling with LO-phonons in quantum dots. Huang-Rhys phenomena

International Nuclear Information System (INIS)

Banyai, L.

1991-01-01

The quantum coupling between photoexcited carriers and longitudinal optical (LO) phonons in zero-dimensional structures (quantum dots) is considered. A classical model of the electron-phonon interaction is presented. The polarization field is then quantized and the Huang-Rhys phenomenon is observed. Effects induced by the quantization of the electron system are also considered. Finally, the modifications of the theory due to specific aspects of quantum dots are discussed. (Author)

Electron-longitudinal-acoustic-phonon scattering in double-quantum-dot based quantum gates

International Nuclear Information System (INIS)

Zhao Peiji; Woolard, Dwight L.

2008-01-01

We propose a nanostructure design which can significantly suppress longitudinal-acoustic-phonon-electron scattering in double-quantum-dot based quantum gates for quantum computing. The calculated relaxation rates vs. bias voltage exhibit a double-peak feature with a minimum approaching 10 5 s -1 . In this matter, the energy conservation law prohibits scattering contributions from phonons with large momenta; furthermore, increasing the barrier height between the double quantum dots reduces coupling strength between the dots. Hence, the joint action of the energy conservation law and the decoupling greatly reduces the scattering rates. The degrading effects of temperatures can be reduced simply by increasing the height of the barrier between the dots

Interband optical absorption in the Wannier-Stark ladder under the electron-LO-phonon resonance condition

International Nuclear Information System (INIS)

Govorov, A.O.

1993-08-01

Interband optical absorption in the Wannier-Stark ladder in the presence of the electron-LO-phonon resonance is investigated theoretically. The electron-LO-phonon resonance occurs when the energy spacing between adjacent Stark-ladder levels coincides with the LO-phonon energy. We propose a model describing the polaron effect in a superlattice. Calculations show that the absorption line shape is strongly modified due to the polaron effect under the electron-LO-phonon resonance condition. We consider optical phenomena in a normal magnetic field that leads to enhancement of polaron effects. (author). 17 refs, 5 figs

Electron mobility limited by optical phonons in wurtzite InGaN/GaN core-shell nanowires

Science.gov (United States)

Liu, W. H.; Qu, Y.; Ban, S. L.

2017-09-01

Based on the force-balance and energy-balance equations, the optical phonon-limited electron mobility in InxGa1-xN/GaN core-shell nanowires (CSNWs) is discussed. It is found that the electrons tend to distribute in the core of the CSNWs due to the strong quantum confinement. Thus, the scattering from first kind of the quasi-confined optical (CO) phonons is more important than that from the interface (IF) and propagating (PR) optical phonons. Ternary mixed crystal and size effects on the electron mobility are also investigated. The results show that the PR phonons exist while the IF phonons disappear when the indium composition x < 0.047, and vice versa. Accordingly, the total electron mobility μ first increases and then decreases with indium composition x, and reaches a peak value of approximately 3700 cm2/(V.s) when x = 0.047. The results also show that the mobility μ increases as increasing the core radius of CSNWs due to the weakened interaction between the electrons and CO phonons. The total electron mobility limited by the optical phonons exhibits an obvious enhancement as decreasing temperature or increasing line electron density. Our theoretical results are expected to be helpful to develop electronic devices based on CSNWs.

High-speed asynchronous optical sampling for high-sensitivity detection of coherent phonons

International Nuclear Information System (INIS)

Dekorsy, T; Taubert, R; Hudert, F; Schrenk, G; Bartels, A; Cerna, R; Kotaidis, V; Plech, A; Koehler, K; Schmitz, J; Wagner, J

2007-01-01

A new optical pump-probe technique is implemented for the investigation of coherent acoustic phonon dynamics in the GHz to THz frequency range which is based on two asynchronously linked femtosecond lasers. Asynchronous optical sampling (ASOPS) provides the performance of on all-optical oscilloscope and allows us to record optically induced lattice dynamics over nanosecond times with femtosecond resolution at scan rates of 10 kHz without any moving part in the set-up. Within 1 minute of data acquisition time signal-to-noise ratios better than 10 7 are achieved. We present examples of the high-sensitivity detection of coherent phonons in superlattices and of the coherent acoustic vibration of metallic nanoparticles

Temperature dependence of Raman scattering by optical phonons in ZnTe

International Nuclear Information System (INIS)

Simmonds, P.E.; Martin, A.D.

1987-01-01

Measurements of the temperature dependence of Raman scattering by optical phonons between temperatures 5 K and 293 K in the II-VI semiconductor ZnTe are reported. Typical Raman spectra for ZnTe at different temperatures are shown and values of the measured LO and TO phonon Raman shifts and linewidths are given for T = 5, 77, and 293 K. The measured linewidth of the LO Raman line as a function of temperature is plotted and compared with model predictions based on various three- and four-phonon processes

Normal processes of phonon-phonon scattering and thermal conductivity of germanium crystals with isotopic disorder

CERN Document Server

Kuleev, I G

2001-01-01

The effect of normal processes of the phonon-phonon scattering on the thermal conductivity of the germanium crystals with various isotopic disorder degrees is considered. The phonon pulse redistribution in the normal scattering processes both inside each oscillatory branch (the Simons mechanism) and between various phonon oscillatory branches (the Herring mechanism) is accounted for. The contributions of the longitudinal and cross-sectional phonons drift motion into the thermal conductivity are analyzed. It is shown that the pulse redistribution in the Herring relaxation mechanism leads to essential suppression of the longitudinal phonons drift motion in the isotopically pure germanium crystals. The calculations results of thermal conductivity for the Herring relaxation mechanism agree well with experimental data on the germanium crystals with various isotopic disorder degrees

Renormalisation of Nonequilibrium Phonons Under Strong Perturbative Influences.

Science.gov (United States)

Mehta, Sushrut Madhukar

Effects of strong perturbative influences, namely the presence of a narrow distribution of acoustic phonons, and the presence of an electron plasma, on the dynamics of nonequilibrium, near zone center, longitudinal optical phonons in GaP have been investigated in two separate experiments. The study of the effects of the interaction between the LO phonons and a heavily populated, narrow distribution of acoustic phonons lead to the observation of a new optically driven nonequilibrium phonon state. Time Resolved Coherent Antistokes Raman Scattering (TR-CARS), with picosecond resolution, was used to investigate the new mode. In order to achieve high occupation numbers in the acoustic branch, the picosecond laser pulses used were amplified up to 1.0 GW/cm^2 peak power per laser beam. An important characteristic property of the new state which differentiates it from the well known LO phonon state is the fact that rather than having the single decay rate observed under thermal equilibrium, the new state has two decay rates. Moreover, these two decay rates depend strongly on the distribution of the acoustic phonon occupation number. The coupling of the LO phonons with an electron plasma, on the other hand, was investigated by measurements of the shape of the Raman scattered line associated with the phonon-plasmon coupled mode. The plasma was generated by thermal excitation of carriers in doped samples. It was possible to study a large variety of plasma excitations by controlling the concentration of the dopant and the ambient temperature. A complete, self consistant model based on standard dielectric response theory is presented, and applied to the measurements of the phonon-plasmon coupled mode. It is possible to recover, via this model, the effective coupled mode damping rate, the plasma damping rate, and the plasma frequency as functions of ambient temperature, or the carrier concentration.

Thermal conductivity of electron-doped CaMnO3 perovskites: Local lattice distortions and optical phonon thermal excitation

International Nuclear Information System (INIS)

Wang Yang; Sui Yu; Wang Xianjie; Su Wenhui; Liu Xiaoyang; Fan, Hong Jin

2010-01-01

The thermal transport properties of a series of electron-doped CaMnO 3 perovskites have been investigated. Throughout the temperature range 5-300 K, phonon thermal conductivity is dominant, and both electron and spin wave contributions are negligible. The short phonon mean free paths in this system result in the relatively low thermal conductivities. The strong phonon scatterings stem from the A-site mismatch and bond-length fluctuations induced by local distortions of MnO 6 octahedra. The thermal conductivity in the magnetically ordered state is enhanced as a result of the decrease in spin-phonon scattering. The results also indicate that above the magnetic ordering temperature, observable thermal excitation of optical phonons occurs. The contribution of optical phonons to thermal conductivity becomes non-negligible and is proposed to play an important role in the glass-like thermal transport behavior (i.e. positive temperature dependence of the thermal conductivity) in the paramagnetic state. These features can be understood in terms of an expression of thermal conductivity that includes both acoustic and optical phonon terms.

Emergence of an Out-of-Plane Optical Phonon (ZO) Kohn Anomaly in Quasifreestanding Epitaxial Graphene.

Science.gov (United States)

Politano, Antonio; de Juan, Fernando; Chiarello, Gennaro; Fertig, Herbert A

2015-08-14

In neutral graphene, two prominent cusps known as Kohn anomalies are found in the phonon dispersion of the highest optical phonon at q=Γ (LO branch) and q=K (TO branch), reflecting a significant electron-phonon coupling (EPC) to undoped Dirac electrons. In this work, high-resolution electron energy loss spectroscopy is used to measure the phonon dispersion around the Γ point in quasifreestanding graphene epitaxially grown on Pt(111). The Kohn anomaly for the LO phonon is observed at finite momentum q~2k_{F} from Γ, with a shape in excellent agreement with the theory and consistent with known values of the EPC and the Fermi level. More strikingly, we also observe a Kohn anomaly at the same momentum for the out-of-plane optical phonon (ZO) branch. This observation is the first direct evidence of the coupling of the ZO mode with Dirac electrons, which is forbidden for freestanding graphene but becomes allowed in the presence of a substrate. Moreover, we estimate the EPC to be even greater than that of the LO mode, making graphene on Pt(111) an optimal system to explore the effects of this new coupling in the electronic properties.

Experimental evidence of zone-center optical phonon softening by accumulating holes in thin Ge

Directory of Open Access Journals (Sweden)

Shoichi Kabuyanagi

2016-01-01

Full Text Available We discuss the impact of free carriers on the zone-center optical phonon frequency in germanium (Ge. By taking advantage of the Ge-on-insulator structure, we measured the Raman spectroscopy by applying back-gate bias. Phonon softening by accumulating holes in Ge film was clearly observed. This fact strongly suggests that the phonon softening in heavily-doped Ge is mainly attributed to the free carrier effect rather than the dopant atom counterpart. Furthermore, we propose that the free carrier effect on phonon softening is simply understandable from the viewpoint of covalent bonding modification by free carriers.

Observation of coherent optical phonons excited by femtosecond laser radiation in Sb films by ultrafast electron diffraction method

Energy Technology Data Exchange (ETDEWEB)

Mironov, B. N.; Kompanets, V. O.; Aseev, S. A., E-mail: isanfemto@yandex.ru [Russian Academy of Sciences, Institute of Spectroscopy (Russian Federation); Ischenko, A. A. [Moscow Technological University, Institute of High Chemical Technologies (Russian Federation); Kochikov, I. V. [Moscow State University (Russian Federation); Misochko, O. V. [Russian Academy of Sciences, Institute of Solid State Physics (Russian Federation); Chekalin, S. V.; Ryabov, E. A. [Russian Academy of Sciences, Institute of Spectroscopy (Russian Federation)

2017-03-15

The generation of coherent optical phonons in a polycrystalline antimony film sample has been investigated using femtosecond electron diffraction method. Phonon vibrations have been induced in the Sb sample by the main harmonic of a femtosecond Ti:Sa laser (λ = 800 nm) and probed by a pulsed ultrashort photoelectron beam synchronized with the pump laser. The diffraction patterns recorded at different times relative to the pump laser pulse display oscillations of electron diffraction intensity corresponding to the frequencies of vibrations of optical phonons: totally symmetric (A{sub 1g}) and twofold degenerate (E{sub g}) phonon modes. The frequencies that correspond to combinations of these phonon modes in the Sb sample have also been experimentally observed.

Electron-confined LO-phonon scattering in GaAs-Al0.45Ga0.55As ...

Indian Academy of Sciences (India)

resonance [17] – at high field where a longitudinal-optical (LO) phonon mediates a transition ..... This competition is controlled through the factor. Γ = γLLw ... [3] L Eaves, F W Sheard and G A T Toombs, Band structure engineering in semicon-.

Tunable infrared reflectance by phonon modulation

Science.gov (United States)

Ihlefeld, Jon F.; Sinclair, Michael B.; Beechem, III, Thomas E.

2018-03-06

The present invention pertains to the use of mobile coherent interfaces in a ferroelectric material to interact with optical phonons and, ultimately, to affect the material's optical properties. In altering the optical phonon properties, the optical properties of the ferroelectric material in the spectral range near-to the phonon mode frequency can dramatically change. This can result in a facile means to change to the optical response of the ferroelectric material in the infrared.

Tunable infrared reflectance by phonon modulation

Energy Technology Data Exchange (ETDEWEB)

Ihlefeld, Jon F.; Sinclair, Michael B.; Beechem, III, Thomas E.

2018-03-06

The present invention pertains to the use of mobile coherent interfaces in a ferroelectric material to interact with optical phonons and, ultimately, to affect the material's optical properties. In altering the optical phonon properties, the optical properties of the ferroelectric material in the spectral range near-to the phonon mode frequency can dramatically change. This can result in a facile means to change to the optical response of the ferroelectric material in the infrared.

The effects of optical phonon on the binding energy of bound polaron in a wurtzite ZnO/MgxZn1−xO quantum well

International Nuclear Information System (INIS)

Zhao, Feng-Qi; Guo, Zi-Zheng; Zhu, Jun

2014-01-01

An improved Lee-Low-Pines intermediate coupling method is used to study the energies and binding energies of bound polarons in a wurtzite ZnO/Mg x Zn 1−x O quantum well. The contributions from different branches of long-wave optical phonons, i.e., confined optical phonons, interface optical phonons, and half-space optical phonons are considered. In addition to electron-phonon interaction, the impurity-phonon interaction, and the anisotropy of material parameters, such as phonon frequency, electron effective mass, and dielectric constant, are also included in our computation. Ground-state energies, binding energies and detailed phonon contributions from various phonons as functions of well width, impurity position and composition are presented. Our result suggests that total phonon contribution to ground state and binding energies in the studied wurtzite ZnO/Mg 0.3 Zn 0.7 O quantum wells varies between 28–23 meV and 62–45 meV, respectively, which are much larger than the corresponding values (about 3.2–1.8 meV and 1.6–0.3 meV) in GaAs/Al 0.3 Ga 0.7 As quantum wells. For a narrower quantum well, the phonon contribution mainly comes from interface and half-space phonons, for a wider quantum well, most of phonon contribution originates from confined phonons. The contribution from all the phonon modes to binding energies increases slowly either when impurity moves far away from the well center in the z direction or with the increase in magnesium composition (x). It is found that different phonons have different influences on the binding energies of bound polarons. Furthermore, the phonon contributions to binding energies as functions of well width, impurity position, and composition are very different from one another. In general, the electron-optical phonon interaction and the impurity center-optical phonon interaction play an important role in electronic states of ZnO-based quantum wells and cannot be neglected.

Simulation of Terahertz Frequency Sources. Polar-Optical Phonon Enhancement of Harmonic Generation in Schottky Diodes

National Research Council Canada - National Science Library

Gelmont, Boris

2002-01-01

... polar optical vibration frequency When a high frequency input signal is applied to a frequency multiplier device polar-optical phonons can enhance the non-linearities inherent in this device, enabling...

Observation of antiphase coherent phonons in the warped Dirac cone of Bi2Te3

Science.gov (United States)

Golias, E.; Sánchez-Barriga, J.

2016-10-01

In this Rapid Communication we investigate the coupling between excited electrons and phonons in the highly anisotropic electronic structure of the prototypical topological insulator Bi2Te3 . Using time- and angle-resolved photoemission spectroscopy we are able to identify the emergence and ultrafast temporal evolution of the longitudinal-optical A1 g coherent-phonon mode in Bi2Te3 . We observe an antiphase behavior in the onset of the coherent-phonon oscillations between the Γ K ¯ and the Γ M ¯ high-symmetry directions that is consistent with warping. The qualitative agreement between our density-functional theory calculations and the experimental results reveals the critical role of the anisotropic coupling between Dirac fermions and phonon modes in the topological insulator Bi2Te3 .

Phonon Raman spectra of colloidal CdTe nanocrystals: effect of size, non-stoichiometry and ligand exchange

Directory of Open Access Journals (Sweden)

Lokteva Irina

2011-01-01

Full Text Available Abstract Resonant Raman study reveals the noticeable effect of the ligand exchange on the nanocrystal (NC surface onto the phonon spectra of colloidal CdTe NC of different size and composition. The oleic acid ligand exchange for pyridine ones was found to change noticeably the position and width of the longitudinal optical (LO phonon mode, as well as its intensity ratio to overtones. The broad shoulder above the LO peak frequency was enhanced and sharpened after pyridine treatment, as well as with decreasing NC size. The low-frequency mode around 100 cm-1 which is commonly related with the disorder-activated acoustical phonons appears in smaller NCs but is not enhanced after pyridine treatment. Surprisingly, the feature at low-frequency shoulder of the LO peak, commonly assigned to the surface optical phonon mode, was not sensitive to ligand exchange and concomitant close packing of the NCs. An increased structural disorder on the NC surface, strain and modified electron-phonon coupling is discussed as the possible reason of the observed changes in the phonon spectrum of ligand-exchanged CdTe NCs. PACS: 63.20.-e, 78.30.-j, 78.67.-n, 78.67.Bf

Anomalous dispersion of optical phonons in La2-xSrxCuO4 at low temperatures

International Nuclear Information System (INIS)

Bishoyi, K.C.; Rout, G.C.; Behera, S.N.

2001-01-01

Inelastic neutron scattering measurements of cuprate system show that a discontinuity in dispersion develops in the middle of the highest energy of optical phonon at low temperatures. We present here a microscopic theory to explain the phonon anomaly in doped cuprate system in normal state. Anti-ferromagnetism due to copper moments is introduced in the electronic Hamiltonian. Phonon coupling to the hybridisation between conduction electrons of the system and the doped f-electrons is incorporated. The phonon self energy due to electron-phonon interaction, which involves the electronic density response function, is evaluated explicitly by Zubarev's Green's function technique in finite temperature and small wave vector limit. The temperature dependence of phonon frequency and the anomalous phonon dispersion are calculated numerically and studied by varying the position of the f-level (ε f ), the effective electron-phonon coupling strength (g), staggered field (h), and the hybridisation parameter (V). (author)

Ultrafast electron-optical phonon scattering and quasiparticle lifetime in CVD-grown graphene.

Science.gov (United States)

Shang, Jingzhi; Yu, Ting; Lin, Jianyi; Gurzadyan, Gagik G

2011-04-26

Ultrafast quasiparticle dynamics in graphene grown by chemical vapor deposition (CVD) has been studied by UV pump/white-light probe spectroscopy. Transient differential transmission spectra of monolayer graphene are observed in the visible probe range (400-650 nm). Kinetics of the quasiparticle (i.e., low-energy single-particle excitation with renormalized energy due to electron-electron Coulomb, electron-optical phonon (e-op), and optical phonon-acoustic phonon (op-ap) interactions) was monitored with 50 fs resolution. Extending the probe range to near-infrared, we find the evolution of quasiparticle relaxation channels from monoexponential e-op scattering to double exponential decay due to e-op and op-ap scattering. Moreover, quasiparticle lifetimes of mono- and randomly stacked graphene films are obtained for the probe photon energies continuously from 1.9 to 2.3 eV. Dependence of quasiparticle decay rate on the probe energy is linear for 10-layer stacked graphene films. This is due to the dominant e-op intervalley scattering and the linear density of states in the probed electronic band. A dimensionless coupling constant W is derived, which characterizes the scattering strength of quasiparticles by lattice points in graphene.

Study of optical phonon modes of CdS nanoparticles using Raman

Indian Academy of Sciences (India)

In this paper we report the study of optical phonon modes of nanoparticles of CdS using Raman spectroscopy. Nanoparticle sample for the present study was synthesized through chemical precipitation technique. The CdS nanoparticles were then subjected to heat treatment at low temperature (150°C) for extended time ...

Engineering dissipation with phononic spectral hole burning

Science.gov (United States)

Behunin, R. O.; Kharel, P.; Renninger, W. H.; Rakich, P. T.

2017-03-01

Optomechanics, nano-electromechanics, and integrated photonics have brought about a renaissance in phononic device physics and technology. Central to this advance are devices and materials supporting ultra-long-lived photonic and phononic excitations that enable novel regimes of classical and quantum dynamics based on tailorable photon-phonon coupling. Silica-based devices have been at the forefront of such innovations for their ability to support optical excitations persisting for nearly 1 billion cycles, and for their low optical nonlinearity. While acoustic phonon modes can persist for a similar number of cycles in crystalline solids at cryogenic temperatures, it has not been possible to achieve such performance in silica, as silica becomes acoustically opaque at low temperatures. We demonstrate that these intrinsic forms of phonon dissipation are greatly reduced (by >90%) by nonlinear saturation using continuous drive fields of disparate frequencies. The result is a form of steady-state phononic spectral hole burning that produces a wideband transparency window with optically generated phonon fields of modest (nW) powers. We developed a simple model that explains both dissipative and dispersive changes produced by phononic saturation. Our studies, conducted in a microscale device, represent an important step towards engineerable phonon dynamics on demand and the use of glasses as low-loss phononic media.

Phonon impact on optical control schemes of quantum dots: Role of quantum dot geometry and symmetry

Science.gov (United States)

Lüker, S.; Kuhn, T.; Reiter, D. E.

2017-12-01

Phonons strongly influence the optical control of semiconductor quantum dots. When modeling the electron-phonon interaction in several theoretical approaches, the quantum dot geometry is approximated by a spherical structure, though typical self-assembled quantum dots are strongly lens-shaped. By explicitly comparing simulations of a spherical and a lens-shaped dot using a well-established correlation expansion approach, we show that, indeed, lens-shaped dots can be exactly mapped to a spherical geometry when studying the phonon influence on the electronic system. We also give a recipe to reproduce spectral densities from more involved dots by rather simple spherical models. On the other hand, breaking the spherical symmetry has a pronounced impact on the spatiotemporal properties of the phonon dynamics. As an example we show that for a lens-shaped quantum dot, the phonon emission is strongly concentrated along the direction of the smallest axis of the dot, which is important for the use of phonons for the communication between different dots.

Dynamical Fano-Like Interference between Rabi Oscillations and Coherent Phonons in a Semiconductor Microcavity System.

Science.gov (United States)

Yoshino, S; Oohata, G; Mizoguchi, K

2015-10-09

We report on dynamical interference between short-lived Rabi oscillations and long-lived coherent phonons in CuCl semiconductor microcavities resulting from the coupling between the two oscillations. The Fourier-transformed spectra of the time-domain signals obtained from semiconductor microcavities by using a pump-probe technique show that the intensity of the coherent longitudinal optical phonon of CuCl is enhanced by increasing that of the Rabi oscillation, which indicates that the coherent phonon is driven by the Rabi oscillation through the Fröhlich interaction. Moreover, as the Rabi oscillation frequency decreases upon crossing the phonon frequency, the spectral profile of the coherent phonon changes from a peak to a dip with an asymmetric structure. The continuous wavelet transformation reveals that these peak and dip structures originate from constructive and destructive interference between Rabi oscillations and coherent phonons, respectively. We demonstrate that the asymmetric spectral structures in relation to the frequency detuning are well reproduced by using a classical coupled oscillator model on the basis of dynamical Fano-like interference.

Optical phonon scattering on electronic mobility in Al2O3/AlGaN/AlN/GaN heterostructures

Science.gov (United States)

Zhou, X. J.; Qu, Y.; Ban, S. L.; Wang, Z. P.

2017-12-01

Considering the built-in electric fields and the two-mode property of transverse optical phonons in AlGaN material, the electronic eigen-energies and wave functions are obtained by solving Schrödinger equation with the finite difference method. The dispersion relations and potentials of the optical phonons are given by the transfer matrix method. The mobility of the two dimensional electron gas influenced by the optical phonons in Al2O3/AlGaN/AlN/GaN heterostructures is investigated based on the theory of Lei-Ting force balance equation. It is found that the scattering from the half-space phonons is the main factor affecting the electronic mobility, and the influence of the other phonons can be ignored. The results show that the mobility decreases with increasing the thicknesses of Al2O3 and AlN layers, but there is no definite relationship between the mobility and the thickness of AlGaN barrier. The mobility is obviously reduced by increasing Al component in AlGaN crystal to show that the effect of ternary mixed crystals is important. It is also found that the mobility increases first and then decreases as the increment of the fixed charges, but decreases always with increasing temperature. The heterostructures constructed here can be good candidates as metal-oxide-semiconductor high-electron-mobility-transistors since they have higher electronic mobility due to the influence from interface phonons weakened by the AlN interlayer.

Ultrafast atomic-scale visualization of acoustic phonons generated by optically excited quantum dots

Directory of Open Access Journals (Sweden)

Giovanni M. Vanacore

2017-07-01

Full Text Available Understanding the dynamics of atomic vibrations confined in quasi-zero dimensional systems is crucial from both a fundamental point-of-view and a technological perspective. Using ultrafast electron diffraction, we monitored the lattice dynamics of GaAs quantum dots—grown by Droplet Epitaxy on AlGaAs—with sub-picosecond and sub-picometer resolutions. An ultrafast laser pulse nearly resonantly excites a confined exciton, which efficiently couples to high-energy acoustic phonons through the deformation potential mechanism. The transient behavior of the measured diffraction pattern reveals the nonequilibrium phonon dynamics both within the dots and in the region surrounding them. The experimental results are interpreted within the theoretical framework of a non-Markovian decoherence, according to which the optical excitation creates a localized polaron within the dot and a travelling phonon wavepacket that leaves the dot at the speed of sound. These findings indicate that integration of a phononic emitter in opto-electronic devices based on quantum dots for controlled communication processes can be fundamentally feasible.

Phonon dynamics and Urbach energy studies of MgZnO alloys

Energy Technology Data Exchange (ETDEWEB)

Huso, Jesse, E-mail: jhuso@vandals.uidaho.edu; Che, Hui; Thapa, Dinesh; Canul, Amrah; Bergman, Leah [Department of Physics, University of Idaho, Moscow, Idaho 83844-0903 (United States); McCluskey, M. D. [Department of Physics and Astronomy, Washington State University, Pullman, Washington 99164-2814 (United States)

2015-03-28

The Mg{sub x}Zn{sub 1−x}O alloy system is emerging as an environmentally friendly choice in ultraviolet lighting and sensor technologies. Knowledge of defects which impact their optical and material properties is a key issue for utilization of these alloys in various technologies. The impact of phase segregation, structural imperfections, and alloy inhomogeneities on the phonon dynamics and electronic states of Mg{sub x}Zn{sub 1−x}O thin films were studied via selective resonant Raman scattering (SRRS) and Urbach analyses, respectively. A series of samples with Mg composition from 0–68% were grown using a sputtering technique, and the optical gaps were found to span a wide UV range of 3.2–5.8 eV. The extent of the inherent phase segregation was determined via SRRS using two UV-laser lines to achieve resonance with the differing optical gaps of the embedded cubic and wurtzite structural domains. The occurrence of Raman scattering from cubic structures is discussed in terms of relaxation of the selection rules due to symmetry breaking by atomic substitutions. The Raman linewidth and Urbach energy behavior indicate the phase segregation region occurs in the range of 47–66% Mg. Below the phase segregation, the longitudinal optical phonons are found to follow the model of one-mode behavior. The phonon decay model of Balkanski et al. indicates that the major contributor to Raman linewidth arises from the temperature-independent term attributed to structural defects and alloy inhomogeneity, while the contribution from anharmonic decay is relatively small. Moreover, a good correlation between Urbach energy and Raman linewidth was found, implying that the underlying crystal dynamics affecting the phonons also affect the electronic states. Furthermore, for alloys with low Mg composition structural defects are dominant in determining the alloy properties, while at higher compositions alloy inhomogeneity cannot be neglected.

Attenuation process of the longitudinal phonon mode in a TeO2 crystal in the 20-GHz range

Science.gov (United States)

Ohno, S.; Sonehara, T.; Tatsu, E.; Koreeda, A.; Saikan, S.

2017-06-01

We experimentally investigated the hypersonic attenuation process of a longitudinal mode (L-mode) sound wave in TeO2 from room temperature to a lower temperature using Brillouin scattering and impulsive stimulated thermal scattering (ISTS) measurements. For precise measurement of the Brillouin linewidth at low temperatures, whereby the mean free path of the phonon becomes longer than the sample length, it is indispensable that the phonon should propagate along the phonon-resonance direction. To figure out the suitable direction, we defined two indices characterizing a degree of phonon divergence and a purity of propagation direction. The best direction that we found from these indices is [110] direction in TeO2, and it was used to discuss the temperature and frequency dependences of Brillouin spectra. We extracted the temperature dependence of the attenuation rate of T4 from the modulated Brillouin spectra due to the phonon resonance below Debye temperature. The frequency dependence ω1 of the hypersonic attenuation was also estimated from the polarization dependence of the Brillouin linewidth. Theoretically, it predicted that the L-mode phonon attenuation at low temperatures in TeO2 is a result of Herring's process, which shows the attenuation behavior of ω2T3 . The ω1T4 dependence is not allowed in Herring's process but is allowed by the L +L →L process, which has been considered to be forbidden so far. We evaluated the thermal phonon lifetime using ISTS and established that it was finite even at 20 K, thereby allowing the L +L →L process. Therefore, we conclude that the L +L →L process dominates the attenuation of an L-mode phonon in TeO2 in the low-temperature region.

Lattice instability and soft phonons in single-crystal La/sub 2-//sub x/Sr/sub x/CuO4

International Nuclear Information System (INIS)

Boeni, P.; Axe, J.D.; Shirane, G.

1988-01-01

The dispersion of the low-lying phonon branches of several doped and undoped single crystals of La/sub 2-//sub x/Sr/sub x/CuO 4 have been investigated by using inelastic-neutron-scattering techniques. The zone-center modes are in good agreement with Raman measurements. The reported peaks in the phonon density of states show up at energies that correspond to extrema in the dispersion curves of the transverse and longitudinal acoustic branches near the zone boundary. The tetragonal-to-orthorhombic phase transition is caused by a softening of transverse-optic-phonon mode at the X point. The rotational nature of the soft mode leads to moderate weak electron-phonon coupling and the mode is unlikely to enhance significantly conventional phonon mediated superconductivity. We did not observe any evidence for the predicted breathing-mode instability near the zone boundary

Impact of optical phonon scattering on inversion channel mobility in 4H-SiC trenched MOSFETs

Science.gov (United States)

Kutsuki, Katsuhiro; Kawaji, Sachiko; Watanabe, Yukihiko; Onishi, Toru; Fujiwara, Hirokazu; Yamamoto, Kensaku; Yamamoto, Toshimasa

2017-04-01

Temperature characteristics of the channel mobility were investigated for 4H-SiC trenched MOSFETs in the range from 30 to 200 °C. The conventional model of channel mobility limited by carrier scattering is based on Si-MOSFETs and shows a greatly different channel mobility from the experimental value, especially at high temperatures. On the other hand, our improved mobility model taking into account optical phonon scattering yielded results in excellent agreement with experimental results. Moreover, the major factors limiting the channel mobility were found to be Coulomb scattering in a low effective field (<0.7 MV/cm) and optical phonon scattering in a high effective field.

Hot-phonon generation in THz quantum cascade lasers

Science.gov (United States)

Spagnolo, V.; Vitiello, M. S.; Scamarcio, G.; Williams, B. S.; Kumar, S.; Hu, Q.; Reno, J. L.

2007-12-01

Observation of non-equilibrium optical phonons population associated with electron transport in THz quantum cascade lasers is reported. The phonon occupation number was measured by using a combination of micro-probe photoluminescence and Stokes/Anti-Stokes Raman spectroscopy. Energy balance analysis allows us to estimate the phonon relaxation rate, that superlinearly increases with the electrical power in the range 1.5 W - 1.95 W, above laser threshold. This observation suggests the occurrence of stimulated emission of optical phonons.

Photon-phonon-enhanced infrared rectification in a two-dimensional nanoantenna-coupled tunnel diode

International Nuclear Information System (INIS)

Kadlec, Emil A.; Jarecki, Robert L.; Starbuck, Andrew; Peters, David W.; Davids, Paul S.

2016-01-01

The interplay of strong infrared photon-phonon coupling with electromagnetic confinement in nanoscale devices is demonstrated to have a large impact on ultrafast photon-assisted tunneling in metal-oxide-semiconductor (MOS) structures. Infrared active optical phonon modes in polar oxides lead to strong dispersion and enhanced electric fields at material interfaces. We find that the infrared dispersion of SiO_2 near a longitudinal optical phonon mode can effectively impedance match a photonic surface mode into a nanoscale tunnel gap that results in large transverse-field confinement. An integrated 2D nanoantenna structure on a distributed large-area MOS tunnel-diode rectifier is designed and built to resonantly excite infrared surface modes and is shown to efficiently channel infrared radiation into nanometer-scale gaps in these MOS devices. This enhanced-gap transverse-electric field is converted to a rectified tunneling displacement current resulting in a dc photocurrent. We examine the angular and polarization-dependent spectral photocurrent response of these 2D nanoantenna-coupled tunnel diodes in the photon-enhanced tunneling spectral region. Lastly, our 2D nanoantenna-coupled infrared tunnel-diode rectifier promises to impact large-area thermal energy harvesting and infrared direct detectors.

Towards a Quantum Interface between Diamond Spin Qubits and Phonons in an Optical Trap

Science.gov (United States)

Ji, Peng; Momeen, M. Ummal; Hsu, Jen-Feng; D'Urso, Brian; Dutt, Gurudev

2014-05-01

We introduce a method to optically levitate a pre-selected nanodiamond crystal in air or vacuum. The nanodiamond containing nitrogen-vacancy (NV) centers is suspended on a monolayer of graphene transferred onto a patterned substrate. Laser light is focused onto the sample, using a home-built confocal microscope with a high numerical aperture (NA = 0.9) objective, simultaneously burning the graphene and creating a 3D optical trap that captures the falling nano-diamond at the beam waist. The trapped diamond is an ultra-high-Q mechanical oscillator, allowing us to engineer strong linear and quadratic coupling between the spin of the NV center and the phonon mode. The system could result in an ideal quantum interface between a spin qubit and vibrational phonon mode, potentially enabling applications in quantum information processing and sensing the development of quantum information storage and processing.

Polar optical phonons in a semiconductor quantum-well: The complete matching problem

International Nuclear Information System (INIS)

Nieto, J.M.; Comas, F.

2007-01-01

Confined polar optical phonons in a semiconductor quantum-well (QW) are studied by applying a phenomenological theory which was proposed a few years ago and is based on a continuum approach. This theory considers the coupled character of the electromechanical vibrations and takes due account of both the electric and mechanical boundary conditions. In the present work, we have applied the so-called complete matching problem in contrast with all previous published works on the subject, where more restrictive approximate boundary conditions has been applied. We also consider the effects of strains at the interfaces on the phonon spectra. Comparisons with previous works are made, while we focused on the study of a ZnTe/CdTe/ZnTe QW

Terahertz instability of surface optical-phonon polaritons that interact with surface plasmon polaritons in the presence of electron drift

International Nuclear Information System (INIS)

Sydoruk, O.; Solymar, L.; Shamonina, E.; Kalinin, V.

2010-01-01

Traveling-wave interaction between optical phonons and electrons drifting in diatomic semiconductors has potential for amplification and generation of terahertz radiation. Existing models of this interaction were developed for infinite materials. As a more practically relevant configuration, we studied theoretically a finite semiconductor slab surrounded by a dielectric. This paper analyzes the optical-phonon instability in the slab including the Lorentz force and compares it to the instability in an infinite material. As the analysis shows, the slab instability occurs because of the interaction of surface optical-phonon polaritons with surface plasmon polaritons in the presence of electron drift. The properties of the instability depend on the slab thickness when the thickness is comparable to the wavelength. For large slab thicknesses, however, the dispersion relation of the slab is similar to that of an infinite material, although the coupling is weaker. The results could be used for the design of practical terahertz traveling-wave oscillators and amplifiers.

Phonon dispersion evolution in uniaxially strained aluminum crystal

Science.gov (United States)

Parthasarathy, Ranganathan; Misra, Anil; Aryal, Sitaram; Ouyang, Lizhi

2018-04-01

The influence of loading upon the phonon dispersion of crystalline materials could be highly nonlinear with certain particular trends that depend upon the loading path. In this paper, we have calculated the influence of [100] uniaxial strain on the phonon dispersion and group velocities in fcc aluminum using second moments of position obtained from molecular dynamics (MD) simulation at 300 K. In contrast to nonlinear monotonic variation of both longitudinal and transverse phonon frequencies along the Δ , Λ and Σ lines of the first Brillouin zone under tension, transverse phonon branches along the Λ line show inflection at specific wavevectors when the compressive strain exceeds 5%. Further, the longitudinal group velocities along the high-symmetry Δ line vary non-monotonically with strain, reaching a minimum at 5% compressive strain. Throughout the strain range studied, the equilibrium positions of atoms displace in an affine manner preserving certain static structural symmetry. We attribute the anomalies in the phonon dispersion to the non-affine evolution of second moments of atomic position, and the associated plateauing of force constants under the applied strain path.

Linear and non-linear infrared response of one-dimensional vibrational Holstein polarons in the anti-adiabatic limit: Optical and acoustical phonon models

Science.gov (United States)

Falvo, Cyril

2018-02-01

The theory of linear and non-linear infrared response of vibrational Holstein polarons in one-dimensional lattices is presented in order to identify the spectral signatures of self-trapping phenomena. Using a canonical transformation, the optical response is computed from the small polaron point of view which is valid in the anti-adiabatic limit. Two types of phonon baths are considered: optical phonons and acoustical phonons, and simple expressions are derived for the infrared response. It is shown that for the case of optical phonons, the linear response can directly probe the polaron density of states. The model is used to interpret the experimental spectrum of crystalline acetanilide in the C=O range. For the case of acoustical phonons, it is shown that two bound states can be observed in the two-dimensional infrared spectrum at low temperature. At high temperature, analysis of the time-dependence of the two-dimensional infrared spectrum indicates that bath mediated correlations slow down spectral diffusion. The model is used to interpret the experimental linear-spectroscopy of model α-helix and β-sheet polypeptides. This work shows that the Davydov Hamiltonian cannot explain the observations in the NH stretching range.

Confined and interface phonons in combined cylindrical nanoheterosystem

Directory of Open Access Journals (Sweden)

O.M.Makhanets

2006-01-01

Full Text Available The spectra of all types of phonons existing in a complicated combined nanoheterosystem consisting of three cylindrical quantum dots embedded into the cylindrical quantum wire placed into vacuum are studied within the dielectric continuum model. It is shown that there are confined optical (LO and interface phonons of two types: top surface optical (TSO and side surface optical (SSO modes of vibration in such a nanosystem. The dependences of phonon energies on the quasiwave numbers and geometrical parameters of quantum dots are investigated and analysed.

Electrons and Phonons in Semiconductor Multilayers

Science.gov (United States)

Ridley, B. K.

1996-11-01

This book provides a detailed description of the quantum confinement of electrons and phonons in semiconductor wells, superlattices and quantum wires, and shows how this affects their mutual interactions. It discusses the transition from microscopic to continuum models, emphasizing the use of quasi-continuum theory to describe the confinement of optical phonons and electrons. The hybridization of optical phonons and their interactions with electrons are treated, as are other electron scattering mechanisms. The book concludes with an account of the electron distribution function in three-, two- and one-dimensional systems, in the presence of electrical or optical excitation. This text will be of great use to graduate students and researchers investigating low-dimensional semiconductor structures, as well as to those developing new devices based on these systems.

Electron phonon couplings in 2D perovskite probed by ultrafast photoinduced absorption spectroscopy

Science.gov (United States)

Huynh, Uyen; Ni, Limeng; Rao, Akshay

We use the time-resolved photoinduced absorption (PIA) spectroscopy with 20fs time resolution to investigate the electron phonon coupling in the self-assembled hybrid organic layered perovskite, the hexyl ammonium lead iodide compound (C6H13NH3)2 (PbI4) . The coupling results in the broadening and asymmetry of its temperature-dependence photoluminescence spectra. The exact time scale of this coupling, however, wasn't reported experimentally. Here we show that using an ultrashort excitation pulse allows us to resolve from PIA kinetics the oscillation of coherent longitudinal optical phonons that relaxes and self-traps electrons to lower energy states within 200 fs. The 200fs relaxation time is equivalent to a coupling strength of 40meV. Two coupled phonon modes are also identified as about 100 cm-1 and 300 cm-1 from the FFT spectrum of the PIA kinetics. The lower energy mode is consistent with previous reports and Raman spectrum but the higher energy one hasn't been observed before.

Ultrafast optical generation of squeezed magnon states and long lifetime coherent LO phonons

Science.gov (United States)

Zhao, Jimin

2005-12-01

Ultrafast optical pulses have been used to generate, probe, and control low-energy elementary excitations in crystals. In particular, we report the first experimental demonstration of the generation of quantum squeezed states of magnons (collective spin-wave excitations) in a magnetic material, and new progress in experimental investigation of anharmonic interactions in a semiconductor. The mechanism for the magnon squeezing is two-magnon impulsive stimulated Raman scattering (ISRS). Femtosecond laser pulses have been used to coherently correlate degenerate counter-propagating magnons in the antiferromagnetic insulator MnF2. In the squeezed state, fluctuations of the magnetization of a crystallographic unit cell vary periodically in time and are reduced below that of the ground-state quantum noise. Similar experiments were also performed in another antiferromagnetic insulator, FeF2, for which the squeezing effect is one order of magnitude larger. We have also investigated the anharmonic interaction of the low-frequency E2 phonon in ZnO through ISRS. Temperature dependence of the linewidth and frequency indicates that the two-phonon up-conversion process is the dominant decay channel and isotopic disorder may be the main limit on the lifetime at low temperature. We have observed the longest lifetime of an optical phonon mode in a solid (211 ps at 5 K). And we have found that pump-probe experiments, compared with spontaneous Raman spectroscopy, have extremely high accuracy in determining the frequency of a low-lying excitation.

CO{sub 2} INFRARED PHONON MODES IN INTERSTELLAR ICE MIXTURES

Energy Technology Data Exchange (ETDEWEB)

Cooke, Ilsa R. [Department of Chemistry, University of Virginia, McCormick Road, Charlottesville, VA 22904 (United States); Fayolle, Edith C.; Öberg, Karin I., E-mail: irc5zb@virginia.edu [Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138 (United States)

2016-11-20

CO{sub 2} ice is an important reservoir of carbon and oxygen in star- and planet-forming regions. Together with water and CO, CO{sub 2} sets the physical and chemical characteristics of interstellar icy grain mantles, including desorption and diffusion energies for other ice constituents. A detailed understanding of CO{sub 2} ice spectroscopy is a prerequisite to characterize CO{sub 2} interactions with other volatiles both in interstellar ices and in laboratory experiments of interstellar ice analogs. We report laboratory spectra of the CO{sub 2} longitudinal optical (LO) phonon mode in pure CO{sub 2} ice and in CO{sub 2} ice mixtures with H{sub 2}O, CO, and O{sub 2} components. We show that the LO phonon mode position is sensitive to the mixing ratio of various ice components of astronomical interest. In the era of the James Webb Space Telescope , this characteristic could be used to constrain interstellar ice compositions and morphologies. More immediately, LO phonon mode spectroscopy provides a sensitive probe of ice mixing in the laboratory and should thus enable diffusion measurements with higher precision than has been previously possible.

Strong anharmonicity in the phonon spectra of PbTe and SnTe from first principles

Science.gov (United States)

Ribeiro, Guilherme A. S.; Paulatto, Lorenzo; Bianco, Raffaello; Errea, Ion; Mauri, Francesco; Calandra, Matteo

2018-01-01

At room temperature, PbTe and SnTe are efficient thermoelectrics with a cubic structure. At low temperature, SnTe undergoes a ferroelectric transition with a critical temperature strongly dependent on the hole concentration, while PbTe is an incipient ferroelectric. By using the stochastic self-consistent harmonic approximation, we investigate the anharmonic phonon spectra and the occurrence of a ferroelectric transition in both systems. We find that vibrational spectra strongly depend on the approximation used for the exchange-correlation kernel in density-functional theory. If gradient corrections and the theoretical volume are employed, then the calculation of the phonon frequencies as obtained from the diagonalization of the free-energy Hessian leads to phonon spectra in good agreement with experimental data for both systems. In PbTe we evaluate the linear thermal expansion coefficient γ =2.3 ×10-5K-1 , finding it to be in good agreement with experimental value of γ =2.04 ×10-5K-1 . Furthermore, we study the phonon spectrum and we do reproduce the transverse optical mode phonon satellite detected in inelastic neutron scattering and the crossing between the transverse optical and the longitudinal acoustic modes along the Γ X direction. The phonon satellite becomes broader at high temperatures but its energy is essentially temperature independent, in agreement with experiments. We decompose the self-consistent harmonic free energy in second-, third-, and fourth-order anharmonic terms. We find that the third- and fourth-order terms are small. However, treating the third-order term perturbatively on top of the second-order self-consistent harmonic free energy overestimates the energy of the satellite associated with the transverse optical mode. On the contrary, a perturbative treatment on top of the harmonic Hamiltonian breaks down and leads to imaginary phonon frequencies already at 300 K. In the case of SnTe, we describe the occurrence of a ferroelectric

Polar Mixing Optical Phonon Spectra in Wurtzite GaN Cylindrical Quantum Dots: Quantum Size and Dielectric Effects

International Nuclear Information System (INIS)

Zhang Li; Liao Jianshang

2010-01-01

The interface-optical-propagating (IO-PR) mixing phonon modes of a quasi-zero-dimensional (QoD) wurtzite cylindrical quantum dot (QD) structure are derived and studied by employing the macroscopic dielectric continuum model. The analytical phonon states of IO-PR mixing modes are given. It is found that there are two types of IO-PR mixing phonon modes, i.e. ρ-IO/z-PR mixing modes and the z-IO/ρ-PR mixing modes existing in QoD wurtzite QDs. And each IO-PR mixing modes also have symmetrical and antisymmetrical forms. Via a standard procedure of field quantization, the Froehlich Hamiltonians of electron-(IO-PR) mixing phonons interaction are obtained. Numerical calculations on a wurtzite GaN cylindrical QD are performed. The results reveal that both the radial-direction size and the axial-direction size as well as the dielectric matrix have great influence on the dispersive frequencies of the IO-PR mixing phonon modes. The limiting features of dispersive curves of these phonon modes are discussed in depth. The phonon modes 'reducing' behavior of wurtzite quantum confined systems has been observed obviously in the structures. Moreover, the degenerating behaviors of the IO-PR mixing phonon modes in wurtzite QoD QDs to the IO modes and PR modes in wurtzite Q2D QW and Q1D QWR systems are analyzed deeply from both of the viewpoints of physics and mathematics. (interdisciplinary physics and related areas of science and technology)

Phonon dynamics of graphene on metals

Science.gov (United States)

Taleb, Amjad Al; Farías, Daniel

2016-03-01

The study of surface phonon dispersion curves is motivated by the quest for a detailed understanding of the forces between the atoms at the surface and in the bulk. In the case of graphene, additional motivation comes from the fact that thermal conductivity is dominated by contributions from acoustic phonons, while optical phonon properties are essential to understand Raman spectra. In this article, we review recent progress made in the experimental determination of phonon dispersion curves of graphene grown on several single-crystal metal surfaces. The two main experimental techniques usually employed are high-resolution electron energy loss spectroscopy (HREELS) and inelastic helium atom scattering (HAS). The different dispersion branches provide a detailed insight into the graphene-substrate interaction. Softening of optical modes and signatures of the substrate‧s Rayleigh wave are observed for strong graphene-substrate interactions, while acoustic phonon modes resemble those of free-standing graphene for weakly interacting systems. The latter allows determining the bending rigidity and the graphene-substrate coupling strength. A comparison between theory and experiment is discussed for several illustrative examples. Perspectives for future experiments are discussed.

Acoustic phonons in the hexagonal perovskite CsNiCl3 around the Gamma-point

DEFF Research Database (Denmark)

Visser, D.; Monteith, A.R.; Rønnow, H.M.

2000-01-01

The acoustic phonon dispersion curves of the hexagonal perovskite CsNiCl3 were measured at room temperature in the vicinity of the Gamma-point along the [0 0 1] and [1 1 0] directions. The derived velocity of sound values for the longitudinal and transverse acoustic phonons are compared with the ......The acoustic phonon dispersion curves of the hexagonal perovskite CsNiCl3 were measured at room temperature in the vicinity of the Gamma-point along the [0 0 1] and [1 1 0] directions. The derived velocity of sound values for the longitudinal and transverse acoustic phonons are compared...

Spin waves in terbium. II. Magnon-phonon interaction

International Nuclear Information System (INIS)

Jensen, J.; Houmann, J.G.

1975-01-01

The selection rules for the linear couplings between magnons and phonons propagating in the c direction of a simple basal-plane hcp ferromagnet are determined by general symmetry considerations. The acoustic-optical magnon-phonon interactions observed in the heavy-rare-earth metals have been explained by Liu as originating from the mixing of the spin states of the conduction electrons due to the spin-orbit coupling. We find that this coupling mechanism introduces interactions which violate the selection rules for a simple ferromagnet. The interactions between the magnons and phonons propagating in the c direction of Tb have been studied experimentally by means of inelastic neutron scatttering. The magnons are coupled to both the acoustic- and optical-transverse phonons. By studying the behavior of the acoustic-optical coupling, we conclude that it is a spin-mixed-induced coupling as proposed by Liu. The coupled magnon--transverse-phonon system for the c direction of Tb is analyzed in detail, and the strengths of the couplings are deduced as a function of wave vector by combining the experimental studies with the theory

On-chip photonic-phononic emitter-receiver apparatus

Science.gov (United States)

Cox, Jonathan Albert; Jarecki, Jr., Robert L.; Rakich, Peter Thomas; Wang, Zheng; Shin, Heedeuk; Siddiqui, Aleem; Starbuck, Andrew Lea

2017-07-04

A radio-frequency photonic devices employs photon-phonon coupling for information transfer. The device includes a membrane in which a two-dimensionally periodic phononic crystal (PnC) structure is patterned. The device also includes at least a first optical waveguide embedded in the membrane. At least a first line-defect region interrupts the PnC structure. The first optical waveguide is embedded within the line-defect region.

Mapping momentum-dependent electron-phonon coupling and nonequilibrium phonon dynamics with ultrafast electron diffuse scattering

Science.gov (United States)

Stern, Mark J.; René de Cotret, Laurent P.; Otto, Martin R.; Chatelain, Robert P.; Boisvert, Jean-Philippe; Sutton, Mark; Siwick, Bradley J.

2018-04-01

Despite their fundamental role in determining material properties, detailed momentum-dependent information on the strength of electron-phonon and phonon-phonon coupling (EPC and PPC, respectively) across the entire Brillouin zone has remained elusive. Here we demonstrate that ultrafast electron diffuse scattering (UEDS) directly provides such information. By exploiting symmetry-based selection rules and time resolution, scattering from different phonon branches can be distinguished even without energy resolution. Using graphite as a model system, we show that UEDS patterns map the relative EPC and PPC strength through their profound sensitivity to photoinduced changes in phonon populations. We measure strong EPC to the K -point TO phonon of A1' symmetry (K -A1' ) and along the entire TO branch between Γ -K , not only to the Γ -E2 g phonon. We also determine that the subsequent phonon relaxation of these strongly coupled optical phonons involve three stages: decay via several identifiable channels to TA and LA phonons (1 -2 ps), intraband thermalization of the non-equilibrium TA/LA phonon populations (30 -40 ps) and interband relaxation of the TA/LA modes (115 ps). Combining UEDS with ultrafast angle-resolved photoelectron spectroscopy will yield a complete picture of the dynamics within and between electron and phonon subsystems, helping to unravel complex phases in which the intertwined nature of these systems has a strong influence on emergent properties.

Electron-phonon coupling from finite differences

Science.gov (United States)

Monserrat, Bartomeu

2018-02-01

The interaction between electrons and phonons underlies multiple phenomena in physics, chemistry, and materials science. Examples include superconductivity, electronic transport, and the temperature dependence of optical spectra. A first-principles description of electron-phonon coupling enables the study of the above phenomena with accuracy and material specificity, which can be used to understand experiments and to predict novel effects and functionality. In this topical review, we describe the first-principles calculation of electron-phonon coupling from finite differences. The finite differences approach provides several advantages compared to alternative methods, in particular (i) any underlying electronic structure method can be used, and (ii) terms beyond the lowest order in the electron-phonon interaction can be readily incorporated. But these advantages are associated with a large computational cost that has until recently prevented the widespread adoption of this method. We describe some recent advances, including nondiagonal supercells and thermal lines, that resolve these difficulties, and make the calculation of electron-phonon coupling from finite differences a powerful tool. We review multiple applications of the calculation of electron-phonon coupling from finite differences, including the temperature dependence of optical spectra, superconductivity, charge transport, and the role of defects in semiconductors. These examples illustrate the advantages of finite differences, with cases where semilocal density functional theory is not appropriate for the calculation of electron-phonon coupling and many-body methods such as the GW approximation are required, as well as examples in which higher-order terms in the electron-phonon interaction are essential for an accurate description of the relevant phenomena. We expect that the finite difference approach will play a central role in future studies of the electron-phonon interaction.

Proposal for an optomechanical traveling wave phonon-photon translator

Energy Technology Data Exchange (ETDEWEB)

Safavi-Naeini, Amir H; Painter, Oskar, E-mail: safavi@caltech.edu, E-mail: opainter@caltech.edu [Thomas J Watson, Sr., Laboratory of Applied Physics, California Institute of Technology, Pasadena, CA 91125 (United States)

2011-01-15

In this paper, we describe a general optomechanical system for converting photons to phonons in an efficient and reversible manner. We analyze classically and quantum mechanically the conversion process and proceed to a more concrete description of a phonon-photon translator (PPT) formed from coupled photonic and phononic crystal planar circuits. The application of the PPT to RF-microwave photonics and circuit QED, including proposals utilizing this system for optical wavelength conversion, long-lived quantum memory and state transfer from optical to superconducting qubits, is considered.

Demonstration of suppressed phonon tunneling losses in phononic bandgap shielded membrane resonators for high-Q optomechanics.

Science.gov (United States)

Tsaturyan, Yeghishe; Barg, Andreas; Simonsen, Anders; Villanueva, Luis Guillermo; Schmid, Silvan; Schliesser, Albert; Polzik, Eugene S

2014-03-24

Dielectric membranes with exceptional mechanical and optical properties present one of the most promising platforms in quantum opto-mechanics. The performance of stressed silicon nitride nanomembranes as mechanical resonators notoriously depends on how their frame is clamped to the sample mount, which in practice usually necessitates delicate, and difficult-to-reproduce mounting solutions. Here, we demonstrate that a phononic bandgap shield integrated in the membrane's silicon frame eliminates this dependence, by suppressing dissipation through phonon tunneling. We dry-etch the membrane's frame so that it assumes the form of a cm-sized bridge featuring a 1-dimensional periodic pattern, whose phononic density of states is tailored to exhibit one, or several, full band gaps around the membrane's high-Q modes in the MHz-range. We quantify the effectiveness of this phononic bandgap shield by optical interferometry measuring both the suppressed transmission of vibrations, as well as the influence of frame clamping conditions on the membrane modes. We find suppressions up to 40 dB and, for three different realized phononic structures, consistently observe significant suppression of the dependence of the membrane's modes on sample clamping-if the mode's frequency lies in the bandgap. As a result, we achieve membrane mode quality factors of 5 × 10(6) with samples that are tightly bolted to the 8 K-cold finger of a cryostat. Q × f -products of 6 × 10(12) Hz at 300 K and 14 × 10(12) Hz at 8 K are observed, satisfying one of the main requirements for optical cooling of mechanical vibrations to their quantum ground-state.

First-principles study on the electronic structure, phonons and optical properties of LaB_6 under high-pressure

International Nuclear Information System (INIS)

Chao, Luomeng; Bao, Lihong; Wei, Wei; O, Tegus; Zhang, Zhidong

2016-01-01

The electronic structure, phonons and optical properties of LaB_6 compound under different pressure have been studied by first-principles calculation. The electronic structure calculation shows that the d band along the M-Γ direction of the Brillouin zone moves up with increasing pressure and the band minimum is above the Fermi level at 45 GPa. The pressure-induced charge transfer from La to B atoms is reflected in the upshift of d band along the M-Γ direction with pressure. The calculated phonon dispersion curve at zero pressure is in good agreement with the experimental results. However, the phonon dispersion under high pressure does not show any information about the phase transition at 10 GPa, which was reported previously. The acoustic and optical phonon modes harden all the way with increasing pressure. In addition, the dielectric function is in accordance with the Drude model in the pressure range of 0 GPa–35 GPa and follows the Lorentz model at 45 GPa. The LaB_6 compound exhibits better visible light transmittance performance with the increasing pressure in the range of 0 GPa–35 GPa and visible light transmittance peak would be shifted towards ultraviolet region. - Highlights: • Physical properties of LaB_6 under high pressure have been theoretically studied. • Predict an electronic topological transition occurs at 45 GPa for LaB_6. • Predict a pressure-induced charge transfer from La to B atoms. • The phonon modes at Γ point show an increasing trend with increasing pressure. • The LaB_6 exhibits better heat-shielding performance with the increasing pressure.

Confined longitudinal acoustic phonon modes in free-standing Si membranes coherently excited by femtosecond laser pulses

OpenAIRE

Hudert, Florian; Bruchhausen, Axel; Issenmann, Daniel; Schecker, Olivier; Waitz, Reimar; Erbe, Artur; Scheer, Elke; Dekorsy, Thomas; Mlayah, Adnen; Huntzinger, Jean-Roch

2009-01-01

In this Rapid Communication we report the first time-resolved measurements of confined acoustic phonon modes in free-standing Si membranes excited by fs laser pulses. Pump-probe experiments using asynchronous optical sampling reveal the impulsive excitation of discrete acoustic modes up to the 19th harmonic order for membranes of two different thicknesses. The modulation of the membrane thickness is measured with fm resolution. The experimental results are compared with a theoretical model in...

Temperature dependence of Brillouin light scattering spectra of acoustic phonons in silicon

International Nuclear Information System (INIS)

Olsson, Kevin S.; Klimovich, Nikita; An, Kyongmo; Sullivan, Sean; Weathers, Annie; Shi, Li; Li, Xiaoqin

2015-01-01

Electrons, optical phonons, and acoustic phonons are often driven out of local equilibrium in electronic devices or during laser-material interaction processes. The need for a better understanding of such non-equilibrium transport processes has motivated the development of Raman spectroscopy as a local temperature sensor of optical phonons and intermediate frequency acoustic phonons, whereas Brillouin light scattering (BLS) has recently been explored as a temperature sensor of low-frequency acoustic phonons. Here, we report the measured BLS spectra of silicon at different temperatures. The origins of the observed temperature dependence of the BLS peak position, linewidth, and intensity are examined in order to evaluate their potential use as temperature sensors for acoustic phonons

Understanding photon sideband statistics and correlation for determining phonon coherence

Science.gov (United States)

Ding, Ding; Yin, Xiaobo; Li, Baowen

2018-01-01

Generating and detecting coherent high-frequency heat-carrying phonons have been topics of great interest in recent years. Although there have been successful attempts in generating and observing coherent phonons, rigorous techniques to characterize and detect phonon coherence in a crystalline material have been lagging compared to what has been achieved for photons. One main challenge is a lack of detailed understanding of how detection signals for phonons can be related to coherence. The quantum theory of photoelectric detection has greatly advanced the ability to characterize photon coherence in the past century, and a similar theory for phonon detection is necessary. Here, we reexamine the optical sideband fluorescence technique that has been used to detect high-frequency phonons in materials with optically active defects. We propose a quantum theory of phonon detection using the sideband technique and found that there are distinct differences in sideband counting statistics between thermal and coherent phonons. We further propose a second-order correlation function unique to sideband signals that allows for a rigorous distinction between thermal and coherent phonons. Our theory is relevant to a correlation measurement with nontrivial response functions at the quantum level and can potentially bridge the gap of experimentally determining phonon coherence to be on par with that of photons.

Four-phonon processes in the thermal conductivity of GaSb

International Nuclear Information System (INIS)

Aliev, M.I.; Arasly, D.G.; Guseinov, R.E.

1978-01-01

Phonon thermal conductivity of GaSb in the 300-700 K temperature range is studied by the light pulsed heating which is aimed at estimation of contributions of different polarized branches of acoustic oscillations into lattice thermal conductivity. The role of optico-acoustic interactions and multiphonon processes in phonon-phonon scattering at high temperatures is discussed. It is shown that the X thermal conductivity caused by the current carriers is negligibly small, and the Xsub(ph) phonon conductivity changes depending on temperature according to the Xsub(ph) approximately Tsup(-1.4) law. While calculating Xsub(ph) according to the Holland model taking into account phonon scattering on point defects the phonon thermal conductivity is given as a sum of contributions from longitudinal and transverse low-frequency Xsub(th1) and high-frequency Xsub(th2) acoustic phonons. It is established that at T>500 K Xsub(ph) is caused only by high-frequency transverse phonons and to explain the observed Xsub(ph) dependence on temperature it is necessary to introduce four-phonon process along with the three-phonon processes into intraphonon scattering

Hot LO-phonon limited electron transport in ZnO/MgZnO channels

Science.gov (United States)

Šermukšnis, E.; Liberis, J.; Matulionis, A.; Avrutin, V.; Toporkov, M.; Özgür, Ü.; Morkoç, H.

2018-05-01

High-field electron transport in two-dimensional channels at ZnO/MgZnO heterointerfaces has been investigated experimentally. Pulsed current-voltage (I-V) and microwave noise measurements used voltage pulse widths down to 30 ns and electric fields up to 100 kV/cm. The samples investigated featured electron densities in the range of 4.2-6.5 × 1012 cm-2, and room temperature mobilities of 142-185 cm2/V s. The pulsed nature of the applied field ensured negligible, if any, change in the electron density, thereby allowing velocity extraction from current with confidence. The highest extracted electron drift velocity of ˜0.5 × 107 cm/s is somewhat smaller than that estimated for bulk ZnO; this difference is explained in the framework of longitudinal optical phonon accumulation (hot-phonon effect). The microwave noise data allowed us to rule out the effect of excess acoustic phonon temperature caused by Joule heating. Real-space transfer of hot electrons into the wider bandgap MgZnO layer was observed to be a limiting factor in samples with a high Mg content (48%), due to phase segregation and the associated local lowering of the potential barrier.

Phonons in a one-dimensional Yukawa chain: Dusty plasma experiment and model

International Nuclear Information System (INIS)

Liu Bin; Goree, J.

2005-01-01

Phonons in a one-dimensional chain of charged microspheres suspended in a plasma were studied in an experiment. The phonons correspond to random particle motion in the chain; no external manipulation was applied to excite the phonons. Two modes were observed, longitudinal and transverse. The velocity fluctuations in the experiment are analyzed using current autocorrelation functions and a phonon spectrum. The phonon energy was found to be unequally partitioned among phonon modes in the dusty plasma experiment. The experimental phonon spectrum was characterized by a dispersion relation that was found to differ from the dispersion relation for externally excited phonons. This difference is attributed to the presence of frictional damping due to gas, which affects the propagation of externally excited phonons differently from phonons that correspond to random particle motion. A model is developed and fit to the experiment to explain the features of the autocorrelation function, phonon spectrum, and the dispersion relation

Resonant surface-enhanced Raman scattering by optical phonons in a monolayer of CdSe nanocrystals on Au nanocluster arrays

Energy Technology Data Exchange (ETDEWEB)

Milekhin, Alexander G., E-mail: milekhin@isp.nsc.ru [A.V. Rzhanov Institute of Semiconductor Physics, pr. Lavrentjeva, 13, 630090 Novosibirsk (Russian Federation); Novosibirsk State University, Pirogov str. 2, 630090 Novosibirsk (Russian Federation); Sveshnikova, Larisa L.; Duda, Tatyana A. [A.V. Rzhanov Institute of Semiconductor Physics, pr. Lavrentjeva, 13, 630090 Novosibirsk (Russian Federation); Rodyakina, Ekaterina E. [A.V. Rzhanov Institute of Semiconductor Physics, pr. Lavrentjeva, 13, 630090 Novosibirsk (Russian Federation); Novosibirsk State University, Pirogov str. 2, 630090 Novosibirsk (Russian Federation); Dzhagan, Volodymyr M. [Semiconductor Physics, Technische Universität Chemnitz, D-09107 Chemnitz (Germany); Sheremet, Evgeniya [Solid Surfaces Analysis, Technische Universität Chemnitz, D-09107 Chemnitz (Germany); Gordan, Ovidiu D. [Semiconductor Physics, Technische Universität Chemnitz, D-09107 Chemnitz (Germany); Himcinschi, Cameliu [Institut für Theoretische Physik, TU Bergakademie Freiberg, 09596 Freiberg (Germany); Latyshev, Alexander V. [A.V. Rzhanov Institute of Semiconductor Physics, pr. Lavrentjeva, 13, 630090 Novosibirsk (Russian Federation); Novosibirsk State University, Pirogov str. 2, 630090 Novosibirsk (Russian Federation); Zahn, Dietrich R.T. [Semiconductor Physics, Technische Universität Chemnitz, D-09107 Chemnitz (Germany)

2016-05-01

Highlights: • Regular Au nanocluster and dimer arrays as well as single Au dimers are fabricated. • Resonant SERS by monolayers of CdSe nanocrystals deposited on the Au nanostructures is observed. • LO energy change for CdSe NCs on different single Au dimers indicates SERS by single or a few NCs. - Abstract: Here we present the results on an investigation of resonant Stokes and anti- Stokes surface-enhanced Raman scattering (SERS) by optical phonons in colloidal CdSe nanocrystals (NCs) homogeneously deposited on arrays of Au nanoclusters using the Langmuir–Blodgett technology. The thickness of deposited NCs, determined by transmission and scanning electron microscopy, amounts to approximately 1 monolayer. Special attention is paid to the determination of the localized surface plasmon resonance (LSPR) energy in the arrays of Au nanoclusters as a function of the nanocluster size by means of micro-ellipsometry. SERS by optical phonons in CdSe NCs shows a significant enhancement factor with a maximal value of 2 × 10{sup 3} which depends resonantly on the Au nanocluster size and thus on the LSPR energy. The deposition of CdSe NCs on the arrays of Au nanocluster dimers enabled us to study the polarization dependence of SERS. It was found that a maximal SERS signal is observed for the light polarization along the dimer axis. Finally, SERS by optical phonons was observed for CdSe NCs deposited on the structures with a single Au dimer. A difference of the LO phonon energy is observed for CdSe NCs on different single dimers. This effect is explained as the confinement-induced shift which depends on the CdSe nanocrystal size and indicates quasi-single NC Raman spectra being obtained.

Freeform Phononic Waveguides

Directory of Open Access Journals (Sweden)

Georgios Gkantzounis

2017-11-01

Full Text Available We employ a recently introduced class of artificial structurally-disordered phononic structures that exhibit large and robust elastic frequency band gaps for efficient phonon guiding. Phononic crystals are periodic structures that prohibit the propagation of elastic waves through destructive interference and exhibit large band gaps and ballistic propagation of elastic waves in the permitted frequency ranges. In contrast, random-structured materials do not exhibit band gaps and favour localization or diffusive propagation. Here, we use structures with correlated disorder constructed from the so-called stealthy hyperuniform disordered point patterns, which can smoothly vary from completely random to periodic (full order by adjusting a single parameter. Such amorphous-like structures exhibit large band gaps (comparable to the periodic ones, both ballistic-like and diffusive propagation of elastic waves, and a large number of localized modes near the band edges. The presence of large elastic band gaps allows the creation of waveguides in hyperuniform materials, and we analyse various waveguide architectures displaying nearly 100% transmission in the GHz regime. Such phononic-circuit architectures are expected to have a direct impact on integrated micro-electro-mechanical filters and modulators for wireless communications and acousto-optical sensing applications.

Intrinsic to extrinsic phonon lifetime transition in a GaAs-AlAs superlattice.

Science.gov (United States)

Hofmann, F; Garg, J; Maznev, A A; Jandl, A; Bulsara, M; Fitzgerald, E A; Chen, G; Nelson, K A

2013-07-24

We have measured the lifetimes of two zone-center longitudinal acoustic phonon modes, at 320 and 640 GHz, in a 14 nm GaAs/2 nm AlAs superlattice structure. By comparing measurements at 296 and 79 K we separate the intrinsic contribution to phonon lifetime determined by phonon-phonon scattering from the extrinsic contribution due to defects and interface roughness. At 296 K, the 320 GHz phonon lifetime has approximately equal contributions from intrinsic and extrinsic scattering, whilst at 640 GHz it is dominated by extrinsic effects. These measurements are compared with intrinsic and extrinsic scattering rates in the superlattice obtained from first-principles lattice dynamics calculations. The calculated room-temperature intrinsic lifetime of longitudinal phonons at 320 GHz is in agreement with the experimentally measured value of 0.9 ns. The model correctly predicts the transition from predominantly intrinsic to predominantly extrinsic scattering; however the predicted transition occurs at higher frequencies. Our analysis indicates that the 'interfacial atomic disorder' model is not entirely adequate and that the observed frequency dependence of the extrinsic scattering rate is likely to be determined by a finite correlation length of interface roughness.

Multiple interruption of optically generated acoustic phonons in ruby

International Nuclear Information System (INIS)

Dijkhuis, J.I.

1979-01-01

This thesis clarifies the rate-determining processes which tend to equilibrate the bottlenecked 29 cm -1 phonons with the temperature bath in stationary experiments. In addition, the direct relaxation between the Zeeman components of E is measured, revealing at high pumping, both continuous and time-resolved, a strong phonon bottleneck. (Auth.)

Quantum Phonon Optics: Squeezing Quantum Noise in the Atomic Displacements.

Science.gov (United States)

Hu, X.; Nori, F.

1996-03-01

We have investigated(X. Hu and F. Nori, Physical Review B, in press; preprints.) coherent and squeezed quantum states of phonons. Squeezed states are interesting because they allow the possibility of modulating the quantum fluctuations of atomic displacements below the zero-point quantum noise level of phonon vacuum states. We have studiedfootnotemark[1] the possibility of squeezing quantum noise in the atomic displacement using a polariton-based approach and also a method based on the three-phonon anharmonic interaction. Our focus here is on the first approach. We have diagonalized the polariton Hamiltonian and calculated the corresponding expectation values and fluctuations of both the atomic displacement and the lattice amplitude operators (the later is the phonon analog of the electric field operator for photons). Our results shows that squeezing of quantum fluctuations in the atomic displacements can be achieved with appropriate initial states of both photon and phonon fields. The degree of squeezing is directly related to the crystal susceptibility, which is indicative of the interaction strength between the incident light and the crystal.

Reststrahlen Band Optics for the Advancement of Far-Infrared Optical Architecture

Science.gov (United States)

Streyer, William Henderson

. Computational models of the emission indicated the samples had significantly higher power efficiency than a blackbody at the same temperature in the same wavelength band. Chapter 5 presents selective thermal emission in the far-infrared from samples of patterned gallium phosphide. The selective absorption of the samples occurs in the material's Reststrahlen band and can be attributed to surface phonon polariton modes. The surfaces of the samples were grated via wet etching to provide the additional momentum necessary for free space photons to couple into and out of the surface phonon polariton modes. Upon heating the samples, selective thermal emission of the surface phonon polariton modes was observed. Chapter 6 investigates a potential means of linking lattice vibrations to free space photons. Lightly doped films of gallium arsenide were grown by molecular beam epitaxy and wet etched with 1D gratings. The light doping served to modify the material's intrinsic permittivity and extend the region of its Reststrahlen band. Though the extension of the region with negative real permittivity was small, it extended beyond the longitudinal optical phonon energy of the material, which stands as the high energy boundary of the unmodified material's Reststrahlen band. Hybrid surface polariton modes were observed at energies near the longitudinal optical phonon energy where they are not supported on the surface of the intrinsic material -- offering a potential bridge between bulk optical phonon populations and free space photons. Chapter 7 presents preliminary results exploring the prospect of exploiting an absorption resonance known as the Berreman mode as a mechanism to link optical phonons to free space photons. The Berreman mode is a strong absorption resonance that occurs near the longitudinal optical phonon energy at moderate angles of incidence in polar semiconductors. Preliminary results demonstrate selective thermal emission consistent with the expected spectral position of the

Lifetime of the phonons in the PLT ceramic

Energy Technology Data Exchange (ETDEWEB)

Barba-Ortega, J., E-mail: jjbarba@unal.edu.co; Joya, M. R., E-mail: mrinconj@unal.edu.co [Departamento de Física, Universidad Nacional de Colombia, carrera 30 # 45-03, Bogotá 1149 (Colombia); Londoño, F. A., E-mail: flondono@fisica.udea.edu.co [Instituto de Física, Universidad de Antioquia, Calle 67 #53-108 Of.6-105, Medellin (Colombia)

2014-11-05

The lifetimes at higher temperatures on lanthanum-modified lead titanate (PLT) are mainly due to the anharmonic decay of optical phonons into low-energy phonons. The temperature-independent contributions from inherent crystal defects and from boundary scattering become comparable to the phonon scattering contribution at lower temperatures. The thermal interaction is large at higher temperatures which decreases the phonon mean free path, and so the decay lifetime decreases as the temperature of the system is increased. This leads to the increased line width at higher temperatures. We made an estimate of the lifetimes for different concentrations and temperatures in PLT.

Inverse Edelstein effect induced by magnon-phonon coupling

Science.gov (United States)

Xu, Mingran; Puebla, Jorge; Auvray, Florent; Rana, Bivas; Kondou, Kouta; Otani, Yoshichika

2018-05-01

We demonstrate a spin to charge current conversion via magnon-phonon coupling and an inverse Edelstein effect on the hybrid device Ni/Cu (Ag )/Bi 2O3 . The generation of spin current (Js≈108A/m2 ) due to magnon-phonon coupling reveals the viability of acoustic spin pumping as a mechanism for the development of spintronic devices. A full in-plane magnetic field angle dependence of the power absorption and a combination of longitudinal and transverse voltage detection reveals the symmetric and asymmetric components of the inverse Edelstein effect voltage induced by Rayleigh-type surface acoustic waves. While the symmetric components are well studied, asymmetric components still need to be explored. We assign the asymmetric contributions to the interference between longitudinal and shear waves and an anisotropic charge distribution in our hybrid device.

Electron nonelastic scattering by confined and interface polar optical phonons in a modulation-doped AlGaAs/GaAs/AlGaAs quantum well

CERN Document Server

Pozela, K

2001-01-01

The calculations of electron scattering rates by polar optical (PO) phonons in an AlGaAs/GaAs/AlGaAs quantum well (QW) with a different width and doping level are performed. The electron-PO-phonon scattering mechanisms which are responsible for the alternate dependence of electron mobility on a QW width, as well as for the decrease of conductivity in the QW with increasing electron concentration are determined. It is shown that the degeneration of electron gas decreases the electron scattering rate by PO-phonon emission and increases the scattering rate by phonon absorption. The competition between the decrease of the intrasubband scattering and the increase of the intersubband scattering by PO-phonon absorption is responsible for the alternate changes of the mobility with a QW width

Optical and acoustic sensing using Fano-like resonances in dual phononic and photonic crystal plate

Energy Technology Data Exchange (ETDEWEB)

Amoudache, Samira [Institut d' Electronique, de Microélectronique et de Nanotechnologie, Université de Lille 1, 59655 Villeneuve d' Ascq (France); Laboratoire de Physique et Chimie Quantique, Université Mouloud Mammeri, B.P. 17 RP, 15000 Tizi-Ouzou (Algeria); Moiseyenko, Rayisa [Department of Physics, Technical University of Denmark, DTU Physics, Building 309, DK-2800 Kongens Lyngby (Denmark); Pennec, Yan, E-mail: yan.pennec@univ-lille1.fr; Rouhani, Bahram Djafari [Institut d' Electronique, de Microélectronique et de Nanotechnologie, Université de Lille 1, 59655 Villeneuve d' Ascq (France); Khater, Antoine [Institut des Molécules et Matériaux du Mans (IMMM), UMR CNRS 6283, l' UNAM, Université du Maine, 72085 Le Mans (France); Lucklum, Ralf [Institute of Micro and Sensor Systems (IMOS), Otto-von-Guericke-University, P.O. Box 4120, D-39016 Magdeburg (Germany); Tigrine, Rachid [Laboratoire de Physique et Chimie Quantique, Université Mouloud Mammeri, B.P. 17 RP, 15000 Tizi-Ouzou (Algeria)

2016-03-21

We perform a theoretical study based on the transmissions of optical and acoustic waves normally impinging to a periodic perforated silicon plate when the embedded medium is a liquid and show the existence of Fano-like resonances in both cases. The signature of the resonances appears as well-defined asymmetric peaks in the phononic and photonic transmission spectra. We show that the origin of the Fano-like resonances is different with respect to the nature of the wave. In photonic, the origin comes from guided modes in the photonic plate while in phononic we show that it comes from the excitation of standing waves confined inside the cavity coming from the deformation of the water/silicon edges of the cylindrical inclusion. We finally use these features for sensing and show ultra-sensitivity to the light and sound velocities for different concentrations of analytes.

Optical properties of Eu{sup 3+}-doped antimony-oxide-based low phonon disordered matrices

Energy Technology Data Exchange (ETDEWEB)

Som, Tirtha; Karmakar, Basudeb, E-mail: basudebk@cgcri.res.i [Glass Technology Laboratory, Glass Division, Central Glass and Ceramic Research Institute (Council of Scientific and Industrial Research), 196 Raja S C Mullick Road, Kolkata 700032 (India)

2010-01-27

A new series of monolithic Eu{sub 2}O{sub 3}-doped high antimony oxide (40-80 mol%) content disordered matrices (glasses) of low phonon energy (about 600 cm{sup -1}) in the K{sub 2}O-B{sub 2}O{sub 3}-Sb{sub 2}O{sub 3} (KBS) system was prepared by the melt-quench technique. Infrared reflection spectroscopy was used to establish the low phonon energy of the glasses. Amorphicity and devitrification of the glasses were confirmed by x-ray diffraction analysis. UV-vis absorption spectra of Eu{sup 3+} have been measured and the band positions have been justified with quantitative calculation of the nephelauxetic parameter and covalent bonding characteristics of the host. These Eu{sub 2}O{sub 3}-doped glasses upon excitation at 393 nm radiation exhibit six emission bands in the range 500-750 nm due to their low phonon energy. Of these, the magnetic dipole {sup 5}D{sub 0} -> {sup 7}F{sub 1} transition shows small Stark splitting while the electric dipole {sup 5}D{sub 0}->{sup 7}F{sub 2} transition undergoes remarkable Stark splitting into two components. They have been explained by the crystal field effect. The Judd-Ofelt parameters, {Omega}{sub t{sub =2,4,6}}, were also evaluated and the change of {Omega}{sub t} with the glass composition was correlated with the asymmetric effect at Eu{sup 3+} ion sites and the fundamental properties like covalent character and optical basicity. We are the first to report the spectroscopic properties of the Eu{sup 3+} ion in KBS low phonon antimony glasses.

The lattice dynamical studies of rare earth compounds: electron-phonon interactions

International Nuclear Information System (INIS)

Jha, Prafulla K.; Sanyal, Sankar P.; Singh, R.K.

2002-01-01

During the last two decades chalcogenides and pnictides of rare earth (RE) atoms have drawn considerable attention of the solid state physicists because of their peculiar electronic, magnetic, optical and phonon properties. Some of these compounds e.g. sulphides and selenides of cerium (Ce), samarium (Sm), yttrium (Y), ytterbium (Yb), europium (Eu) and thulium (Tm) and their alloys show nonintegral valence (between 2 and 3), arising due to f-d electron hybridization at ambient temperature and pressure. The rare earth mixed valence compounds (MVC) reviewed in this article crystallize in simple cubic structure. Most of these compounds show the existence of strong electron-phonon coupling at half way to the zone boundary. This fact manifests itself through softening of the longitudinal acoustic mode, negative value of elastic constant C 12 etc. The purpose of this contribution is to review some of the recent activities in the fields of lattice dynamics and allied properties of rare earth compounds. The present article is primarily devoted to review the effect of electron-phonon interactions on the dynamical properties of rare earth compounds by using the lattice dynamical model theories based on charged density deformations and long-range many body forces. While the long range charge transfer effect arises due to f-d hybridization of nearly degenerate 4f-5d bands of rare earth ions, the density deformation comes into the picture of breathing motion of electron shells. These effects of charge transfer and charge density deformation when considered in the lattice dynamical models namely the three body force rigid ion model (TRM) and breathing shell model (BSM) are quite successful in explaining the phonon anomalies in these compounds and undoubtedly unraveled many important physical process governing the phonon anomalies in rare earth compounds

Correlation between phonon anomaly along [211] and the Fermi surface nesting features with associated electron-phonon interactions in Ni2FeGa: A first principles study

International Nuclear Information System (INIS)

Chabungbam, Satyananda; Sahariah, Munima B.

2015-01-01

First principles calculation reaffirms the presence of phonon anomaly along [211] direction in Ni 2 FeGa shape memory alloy supporting the experimental findings of J. Q. Li et al. Fermi surface scans have been performed in both austenite and martensite phase to see the possible Fermi nesting features in this alloy. The magnitude of observed Fermi surface nesting vectors in (211) plane exactly match the phonon anomaly wavevectors along [211] direction. Electron-phonon calculation in the austenite phase shows that there is significant electron-phonon coupling in this alloy which might arise out of the lattice coupling between lower acoustic modes and higher optical modes combined with the observed strong Fermi nesting features in the system. - Highlights: • Transverse acoustic (TA 2 ) modes show anomaly along [211] direction in Ni 2 FeGa. • The phonon anomaly wavevector has been correlated with the Fermi nesting vectors. • Electron-phonon coupling calculation shows significant coupling in this system. • Max. el-ph coupling occurs in transition frequencies from acoustic to optical modes

Quasiparticle recombination and 2 Δ-phonon-trapping in superconducting tunneling junctions

International Nuclear Information System (INIS)

Eisenmenger, W.; Lassmann, K.; Trumpp, H.J.; Krauss, R.

1976-01-01

The experimental recombination lifetime Tsub(eff) of quasiparticles in superconducting films in general exceeds tge intrinsic recombination lifetime tau sub(R) by phonon trapping. On the basis of geometric acoustic propagation and reabsorption of phonons emitted in quasiparticle recombination, tau sub(eff) is calculated as a function of film thickness d taking into account longitudinal and transverse phonon reabsorption, bulk loss process and acoustical phonon transmission into the substrate. With increasing thickness d three characteristic ranges are found: range 1 with film thickness d small compared to the phonon reabsorption mean free path Λsub(w) range 2 with d larger than Λsub(w) and dominating boundary losses, and range 3, also with d larger than Λsub(w) but with dominating bulk losses. (orig./HPOE) [de

Intrinsic to extrinsic phonon lifetime transition in a GaAs–AlAs superlattice

International Nuclear Information System (INIS)

Hofmann, F; Garg, J; Chen, G; Maznev, A A; Nelson, K A; Jandl, A; Bulsara, M; Fitzgerald, E A

2013-01-01

We have measured the lifetimes of two zone-center longitudinal acoustic phonon modes, at 320 and 640 GHz, in a 14 nm GaAs/2 nm AlAs superlattice structure. By comparing measurements at 296 and 79 K we separate the intrinsic contribution to phonon lifetime determined by phonon–phonon scattering from the extrinsic contribution due to defects and interface roughness. At 296 K, the 320 GHz phonon lifetime has approximately equal contributions from intrinsic and extrinsic scattering, whilst at 640 GHz it is dominated by extrinsic effects. These measurements are compared with intrinsic and extrinsic scattering rates in the superlattice obtained from first-principles lattice dynamics calculations. The calculated room-temperature intrinsic lifetime of longitudinal phonons at 320 GHz is in agreement with the experimentally measured value of 0.9 ns. The model correctly predicts the transition from predominantly intrinsic to predominantly extrinsic scattering; however the predicted transition occurs at higher frequencies. Our analysis indicates that the ‘interfacial atomic disorder’ model is not entirely adequate and that the observed frequency dependence of the extrinsic scattering rate is likely to be determined by a finite correlation length of interface roughness. (paper)

Optical and acoustic sensing using Fano-like resonances in dual phononic and photonic crystal plate

DEFF Research Database (Denmark)

Amoudache, Samira; Moiseyenko, Rayisa; Pennec, Yan

2016-01-01

We perform a theoretical study based on the transmissions of optical and acoustic waves normally impinging to a periodic perforated silicon plate when the embedded medium is a liquid and show the existence of Fano-like resonances in both cases. The signature of the resonances appears as well-defi...... of standing waves confined inside the cavity coming from the deformation of the water/silicon edges of the cylindrical inclusion. We finally use these features for sensing and show ultra-sensitivity to the light and sound velocities for different concentrations of analytes.......-defined asymmetric peaks in the phononic and photonic transmission spectra. We show that the origin of the Fano-like resonances is different with respect to the nature of the wave. In photonic, the origin comes from guided modes in the photonic plate while in phononic we show that it comes from the excitation...

Phonons and their dispersion in model ferroelastics Hg2Hal2

Science.gov (United States)

Roginskii, E. M.; Kvasov, A. A.; Markov, Yu. F.; Smirnov, M. B.

2012-05-01

Dispersion relations of the acoustic and optical phonon frequencies have been calculated and plotted, and the density of states of the phonon spectrum of Hg2Cl2 and Hg2Br2 crystals has been derived. The effect of hydrostatic pressure on the frequencies of acoustic and optical phonons and their dispersion has been theoretically analyzed. It has been found that an increase in the pressure leads to a strong softening of the slowest acoustic TA branch (the soft mode) at the X point of the Brillouin zone boundary, which is consistent with the phenomenological Landau theory and correlates with experiment.

Current & Heat Transport in Graphene Nanoribbons: Role of Non-Equilibrium Phonons

Science.gov (United States)

Pennington, Gary; Finkenstadt, Daniel

2010-03-01

The conducting channel of a graphitic nanoscale device is expected to experience a larger degree of thermal isolation when compared to traditional inversion channels of electronic devices. This leads to enhanced non-equilibrium phonon populations which are likely to adversely affect the mobility of graphene-based nanoribbons due to enhanced phonon scattering. Recent reports indicating the importance of carrier scattering with substrate surface polar optical phonons in carbon nanotubes^1 and graphene^2,3 show that this mechanism may allow enhanced heat removal from the nanoribbon channel. To investigate the effects of hot phonon populations on current and heat conduction, we solve the graphene nanoribbon multiband Boltzmann transport equation. Monte Carlo transport techniques are used since phonon populations may be tracked and updated temporally.^4 The electronic structure is solved using the NRL Tight-Binding method,^5 where carriers are scattered by confined acoustic, optical, edge and substrate polar optical phonons. [1] S. V. Rotkin et al., Nano Lett. 9, 1850 (2009). [2] J. H. Chen, C. Jang, S. Xiao, M. Ishigami and M. S. Fuhrer, Nature Nanotech. 3, 206 (2008). [3] V. Perebeinos and P. Avouris, arXiv:0910.4665v1 [cond-mat.mes-hall] (2009). [4] P. Lugli et al., Appl. Phys. Lett. 50, 1251 (1987). [5] D. Finkenstadt, G. Pennington & M.J. Mehl, Phys. Rev. B 76, 121405(R) (2007).

Low-loss, infrared and terahertz nanophotonics using surface phonon polaritons

Directory of Open Access Journals (Sweden)

Caldwell Joshua D.

2015-04-01

Full Text Available The excitation of surface-phonon-polariton (SPhP modes in polar dielectric crystals and the associated new developments in the field of SPhPs are reviewed. The emphasis of this work is on providing an understanding of the general phenomenon, including the origin of the Reststrahlen band, the role that optical phonons in polar dielectric lattices play in supporting sub-diffraction-limited modes and how the relatively long optical phonon lifetimes can lead to the low optical losses observed within these materials. Based on this overview, the achievements attained to date and the potential technological advantages of these materials are discussed for localized modes in nanostructures, propagating modes on surfaces and in waveguides and novel metamaterial designs, with the goal of realizing low-loss nanophotonics and metamaterials in the mid-infrared to terahertz spectral ranges.

Rode's iterative calculation of surface optical phonon scattering limited electron mobility in N-polar GaN devices

International Nuclear Information System (INIS)

Ghosh, Krishnendu; Singisetti, Uttam

2015-01-01

N-polar GaN channel mobility is important for high frequency device applications. Here, we report theoretical calculations on the surface optical (SO) phonon scattering rate of two-dimensional electron gas (2DEG) in N-polar GaN quantum well channels with high-k dielectrics. Rode's iterative calculation is used to predict the scattering rate and mobility. Coupling of the GaN plasmon modes with the SO modes is taken into account and dynamic screening is employed under linear polarization response. The effect of SO phonons on 2DEG mobility was found to be small at >5 nm channel thickness. However, the SO mobility in 3 nm N-polar GaN channels with HfO 2 and ZrO 2 high-k dielectrics is low and limits the total mobility. The SO scattering for SiN dielectric on GaN was found to be negligible due to its high SO phonon energy. Using Al 2 O 3 , the SO phonon scattering does not affect mobility significantly only except the case when the channel is too thin with a low 2DEG density

Pump pulse duration dependence of coherent phonon amplitudes in antimony

Energy Technology Data Exchange (ETDEWEB)

Misochko, O. V., E-mail: misochko@issp.ac.ru [Russian Academy of Sciences, Institute of Solid State Physics (Russian Federation)

2016-08-15

Coherent optical phonons of A{sub 1k} and E{sub k} symmetry in antimony have been studied using the femtosecond pump–probe technique. By varying the pump-pulse duration and keeping the probe duration constant, it was shown that the amplitude of coherent phonons of both symmetries exponentially decreases with increasing pulse width. It was found that the amplitude decay rate for the fully symmetric phonons with larger frequency is greater than that of the doubly degenerate phonons, whereas the frequency and lifetime for coherent phonons of both symmetries do not depend on the pump-pulse duration. Based on this data, the possibility of separation between dynamic and kinematic contributions to the generation mechanism of coherent phonons is discussed.

Active and passive vibration isolation in piezoelectric phononic rods with external voltage excitation

Directory of Open Access Journals (Sweden)

Qicheng Zhang

2017-05-01

Full Text Available Active piezoelectric materials are applied to one-dimensional phononic crystals, for the control of longitudinal vibration propagation both in active and passive modes. Based on the electromechanical coupling between the acoustical vibration and electric field, the electromechanical equivalent method is taken to theoretically predict the transmission spectrum of the longitudinal vibration. It is shown that the phononic rod can suppress the vibration efficiently at the frequencies of interest, by actively optimizing the motions of piezoelectric elements. In an illustrated phononic rod of 11.2cm long, active tunable isolations of more than 20dB at low frequencies (500Hz-14kHz are generated by controlling the excitation voltages of piezoelectric elements. Meanwhile, passive fixed isolation at high frequencies (14k-63kHz are presented by its periodicity characteristics. Finite element simulations and vibration experiments on the rod demonstrate the effectiveness of the approach in terms of its vibration isolation capabilities and tunable characteristics. This phononic rod can be manufactured easily and provides numerous potential applications in designing isolation mounts and platforms.

Screening-induced surface polar optical phonon scattering in dual-gated graphene field effect transistors

Energy Technology Data Exchange (ETDEWEB)

Hu, Bo, E-mail: hubo2011@semi.ac.cn

2015-03-15

The effect of surface polar optical phonons (SOs) from the dielectric layers on electron mobility in dual-gated graphene field effect transistors (GFETs) is studied theoretically. By taking into account SO scattering of electron as a main scattering mechanism, the electron mobility is calculated by the iterative solution of Boltzmann transport equation. In treating scattering with the SO modes, the dynamic dielectric screening is included and compared to the static dielectric screening and the dielectric screening in the static limit. It is found that the dynamic dielectric screening effect plays an important role in the range of low net carrier density. More importantly, in-plane acoustic phonon scattering and charged impurity scattering are also included in the total mobility for SiO{sub 2}-supported GFETs with various high-κ top-gate dielectric layers considered. The calculated total mobility results suggest both Al{sub 2}O{sub 3} and AlN are the promising candidate dielectric layers for the enhancement in room temperature mobility of graphene in the future.

Electronic and optical properties of ZrB12 and YB6. Discussion on electron-phonon coupling

International Nuclear Information System (INIS)

Teyssier, J.; Kuzmenko, A.; Marel, D. van der; Lortz, R.; Junod, A.; Filippov, V.; Shitsevalova, N.

2006-01-01

We report the optical properties of high-quality single crystals of low temperature superconductors zirconiumdodecaboride ZrB 12 (T c =5.95 K) and yttrium hexaboride YB 6 (T c =7.15 K) in the range 6 meV-4.6 eV at room temperature. The experimental optical conductivity was extracted from the analysis of the reflectivity in the infrared range and ellipsometry measurement of the dielectric function in the visible range. The electronic band structure of these compounds was calculated by the self-consistent full-potential LMTO method and used to compute the interband part of the optical conductivity and the plasma frequency Ω p . A good agreement was observed between the interband part of the experimental optical conductivities and the band structure calculations. Different methods combining optical spectroscopy, resistivity, specific heat measurements and results of band structure calculations are used to determine the electron-phonon coupling constant. (copyright 2006 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)

Phonon-impurity relaxation and acoustic wave absorption in yttrium-aluminium garnet crystals with impurities

International Nuclear Information System (INIS)

Ivanov, S.N.; Kotelyanskij, I.M.; Medved', V.V.

1983-01-01

The experimental results of investigations of the influence of substitution impurities in the yttrium-aluminium garnet lattice on absorption of high-frequency acoustic waves are presented. It is shown that the phonon-impurity relaxation processses affect at most the wave absorption and have resonance character when the acoustic wave interacts with the thermal phonon group in the vicinity of the perturbed part of the phonon spectrum caused by the impurity. The differences of time values between inelastic and elastic thermal phonons relaxations determined from the data on longitudinal and shear waves in pure and impurity garnet crystals are discussed

Unraveling the acoustic electron-phonon interaction in graphene

DEFF Research Database (Denmark)

Kaasbjerg, Kristen; Thygesen, Kristian S.; Jacobsen, Karsten W.

2012-01-01

Using a first-principles approach we calculate the electron-phonon couplings in graphene for the transverse and longitudinal acoustic phonons. Analytic forms of the coupling matrix elements valid in the long-wavelength limit are found to give an almost quantitative description of the first...... that the intrinsic effective acoustic deformation potential of graphene is Ξeff=6.8 eV and that the temperature dependence of the mobility μ~T-α in the Bloch-Gru¨neisen regime increases beyond an α=4 dependence even in the absence of screening when the true coupling matrix elements are considered. The α>4...

Magnon and phonon thermometry with inelastic light scattering

Science.gov (United States)

Olsson, Kevin S.; An, Kyongmo; Li, Xiaoqin

2018-04-01

Spin caloritronics investigates the interplay between the transport of spin and heat. In the spin Seebeck effect, a thermal gradient across a magnetic material generates a spin current. A temperature difference between the energy carriers of the spin and lattice subsystems, namely the magnons and phonons, is necessary for such thermal nonequilibrium generation of spin current. Inelastic light scattering is a powerful method that can resolve the individual temperatures of magnons and phonons. In this review, we discuss the thermometry capabilities of inelastic light scattering for measuring optical and acoustic phonons, as well as magnons. A scattering spectrum offers three temperature sensitive parameters: frequency shift, linewidth, and integrated intensity. We discuss the temperatures measured via each of these parameters for both phonon and magnons. Finally, we discuss inelastic light scattering experiments that have examined the magnon and phonon temperatures in thermal nonequilibrium which are particularly relevant to spin caloritronic phenomena.

Hydrostatic stress dependence of the exciton-phonon coupled states in cylindrical quantum dots

International Nuclear Information System (INIS)

El Moussaouy, A.; Bria, D.; Nougaoui, A.

2005-01-01

We investigate theoretically the effects of compressive stress on the binding energy of an exciton in a cylindrical quantum dot (QD) using a variational procedure within the effective mass approximation. The stress was applied in the z direction and the interaction between the charge carriers (electron and hole) and confined longitudinal optical (LO) phonon modes was taken into account. Specific applications of these results are given for GaAs QDs embedded in a Ga 1-x Al x As semiconductor. The result shows that the binding energy and the polaronic correction increases linearly with increasing stress. Moreover, we obtain the binding energy and the polaronic contribution in the limit in which the QD turns into a quantum well

Creation of high-energy phonons by four-phonon processes in anisotropic phonon system of He II

International Nuclear Information System (INIS)

Adamenko, I.N.; Nemchenko, K.E.; Slipko, V.A.; Kitsenko, Yu.A.; Wyatt, A.F.G.

2007-01-01

The problem of the creation of high-energy phonons (h-phonons) by a pulse of low-energy phonons (I-phonons) moving from a heater to a detector in superfluid helium, is solved. The rate of h-phonon creation is obtained and it is shown that created h-phonons occupy a much smaller solid angle in momentum space, than the I-phonons. Analytical expression for the creation rate of h-phonon, along the symmetry axis of a pulse, are derived. It allows us to get useful approximate analytical expressions for creation rate of h-phonons. The time dependences of the parameters which describe the I-phonon pulse are obtained. This shows that half of the initial energy of I-phonon pulse can be transferred into h-phonons. The results of the calculations are compared with experimental data and we show that this theory explains a number of experimental results. The value of the momentum, which separates the I- and h-phonon subsystems, is found

Pseudospins and Topological Effects of Phonons in a Kekulé Lattice

Science.gov (United States)

Liu, Yizhou; Lian, Chao-Sheng; Li, Yang; Xu, Yong; Duan, Wenhui

2017-12-01

The search for exotic topological effects of phonons has attracted enormous interest for both fundamental science and practical applications. By studying phonons in a Kekulé lattice, we find a new type of pseudospin characterized by quantized Berry phases and pseudoangular momenta, which introduces various novel topological effects, including topologically protected pseudospin-polarized interface states and a phonon pseudospin Hall effect. We further demonstrate a pseudospin-contrasting optical selection rule and a pseudospin Zeeman effect, giving a complete generation-manipulation-detection paradigm of the phonon pseudospin. The pseudospin and topology-related physics revealed for phonons is general and applicable for electrons, photons, and other particles.

Tunable phonon-induced transparency in bilayer graphene nanoribbons.

Science.gov (United States)

Yan, Hugen; Low, Tony; Guinea, Francisco; Xia, Fengnian; Avouris, Phaedon

2014-08-13

In the phenomenon of plasmon-induced transparency, which is a classical analogue of electromagnetically induced transparency (EIT) in atomic gases, the coherent interference between two plasmon modes results in an optical transparency window in a broad absorption spectrum. With the requirement of contrasting lifetimes, typically one of the plasmon modes involved is a dark mode that has limited coupling to the electromagnetic radiation and possesses relatively longer lifetime. Plasmon-induced transparency not only leads to light transmission at otherwise opaque frequency regions but also results in the slowing of light group velocity and enhanced optical nonlinearity. In this article, we report an analogous behavior, denoted as phonon-induced transparency (PIT), in AB-stacked bilayer graphene nanoribbons. Here, light absorption due to the plasmon excitation is suppressed in a narrow window due to the coupling with the infrared active Γ-point optical phonon, whose function here is similar to that of the dark plasmon mode in the plasmon-induced transparency. We further show that PIT in bilayer graphene is actively tunable by electrostatic gating and estimate a maximum slow light factor of around 500 at the phonon frequency of 1580 cm(-1), based on the measured spectra. Our demonstration opens an avenue for the exploration of few-photon nonlinear optics and slow light in this novel two-dimensional material.

Probing the interatomic potential of solids with strong-field nonlinear phononics

Science.gov (United States)

von Hoegen, A.; Mankowsky, R.; Fechner, M.; Först, M.; Cavalleri, A.

2018-03-01

Nonlinear optical techniques at visible frequencies have long been applied to condensed matter spectroscopy. However, because many important excitations of solids are found at low energies, much can be gained from the extension of nonlinear optics to mid-infrared and terahertz frequencies. For example, the nonlinear excitation of lattice vibrations has enabled the dynamic control of material functions. So far it has only been possible to exploit second-order phonon nonlinearities at terahertz field strengths near one million volts per centimetre. Here we achieve an order-of-magnitude increase in field strength and explore higher-order phonon nonlinearities. We excite up to five harmonics of the A1 (transverse optical) phonon mode in the ferroelectric material lithium niobate. By using ultrashort mid-infrared laser pulses to drive the atoms far from their equilibrium positions, and measuring the large-amplitude atomic trajectories, we can sample the interatomic potential of lithium niobate, providing a benchmark for ab initio calculations for the material. Tomography of the energy surface by high-order nonlinear phononics could benefit many aspects of materials research, including the study of classical and quantum phase transitions.

Scattering of phonons by dislocations

International Nuclear Information System (INIS)

Anderson, A.C.

1979-01-01

By 1950, an explicit effort had been launched to use lattice thermal conductivity measurements in the investigation of defect structures in solids. This technique has been highly successful, especially when combined with the measurements of other properties such as optical absorption. One exception has been the study of dislocations. Although dislocations have a profound effect on the phonon thermal conductivity, the mechanisms of the phonon-dislocation interaction are poorly understood. The most basic questions are still debated in the literature. It therefore is pointless to attempt a quantitative comparison between an extensive accumulation of experimental data on the one hand, and the numerous theoretical models on the other. Instead, this chapter will attempt to glean a few qualitative conclusions from the existing experimental data. These results will then be compared with two general models which incorporate, in a qualitative manner, most of the proposed theories of the phonon-dislocation interaction. Until very recently, measurement of thermal conductivity was the only means available to probe the interaction between phonons and defects at phonon frequencies above the standard ultrasonic range of approx. = 10 9 Hz. The introductory paragraphs provide a brief review of the thermal-conductivity technique and the problems which are encountered in practice. There is also a brief presentation of the theoretical models and the complications that may occur in more realistic situations

Dominant phonon polarization conversion across dimensionally mismatched interfaces: Carbon-nanotube-graphene junction

Science.gov (United States)

Shi, Jingjing; Lee, Jonghoon; Dong, Yalin; Roy, Ajit; Fisher, Timothy S.; Ruan, Xiulin

2018-04-01

Dimensionally mismatched interfaces are emerging for thermal management applications, but thermal transport physics remains poorly understood. Here we consider the carbon-nanotube-graphene junction, which is a dimensionally mismatched interface between one- and two-dimensional materials and is the building block for carbon-nanotube (CNT)-graphene three-dimensional networks. We predict the transmission function of individual phonon modes using the wave packet method; surprisingly, most incident phonon modes show predominantly polarization conversion behavior. For instance, longitudinal acoustic (LA) polarizations incident from CNTs transmit mainly into flexural transverse (ZA) polarizations in graphene. The frequency stays the same as the incident mode, indicating elastic transmission. Polarization conversion is more significant as the phonon wavelength increases. We attribute such unique phonon polarization conversion behavior to the dimensional mismatch across the interface, and it opens significantly new phonon transport channels as compared to existing theories where polarization conversion is neglected.

Optical and acoustic phonon modes in strained InGaAs/GaAs rolled up tubes

Science.gov (United States)

Angelova, T.; Shtinkov, N.; Ivanov, Ts.; Donchev, V.; Cantarero, A.; Deneke, Ch.; Schmidt, O. G.; Cros, A.

2012-05-01

Rolled-up semiconductor tubes of various diameters made of alternating In0.215Ga0.785As/GaAs layers have been investigated by means of Raman scattering. The optical and acoustic phonon modes of individual tubes have been studied and compared with the characteristics of the surrounding material. After tube formation, the frequency of the phonon modes shifts with respect to the as-grown material and disorder activated modes are observed. The frequency shifts are related to the residual strain in the tubes through the deformation potential approximation. Good agreement with atomistic valence force field simulations and x-ray micro-diffraction measurements is found. By comparison with x-ray data, a Raman strain constant K = 0.65 is proposed for In0.215Ga0.785As. In the low frequency range, acoustic mode doublets are observed on the tubes that are absent in the surrounding material. They show clear evidence of the formation of periodic superlattices after the rolling-up process, and give insight into the quality of their interfaces.

The Influence of the Optical Phonons on the Non-equilibrium Spin Current in the Presence of Spin-Orbit Couplings

Science.gov (United States)

Hasanirokh, K.; Phirouznia, A.; Majidi, R.

2016-02-01

The influence of the electron coupling with non-polarized optical phonons on magnetoelectric effects of a two-dimensional electron gas system has been investigated in the presence of the Rashba and Dresselhaus spin-orbit couplings. Numerical calculations have been performed in the non-equilibrium regime. In the previous studies in this field, it has been shown that the Rashba and Dresselhaus couplings cannot generate non-equilibrium spin current and the spin current vanishes identically in the absence of other relaxation mechanisms such as lattice vibrations. However, in the current study, based on a semiclassical approach, it was demonstrated that in the presence of electron-phonon coupling, the spin current and other magnetoelectric quantities have been modulated by the strength of the spin-orbit interactions.

Phonon tunneling through a double barrier system

International Nuclear Information System (INIS)

Villegas, Diosdado; León-Pérez, Fernando de; Pérez-Álvarez, R.; Arriaga, J.

2015-01-01

The tunneling of optical and acoustic phonons at normal incidence on a double-barrier is studied in this paper. Transmission coefficients and resonance conditions are derived theoretically under the assumption that the long-wavelength approximation is valid. It is shown that the behavior of the transmission coefficients for the symmetric double barrier has a Lorentzian form close to resonant frequencies and that Breit–Wigner's formula have a general validity in one-dimensional phonon tunneling. Authors also study the so-called generalized Hartman effect in the tunneling of long-wavelength phonons and show that this effect is a numerical artifact resulting from taking the opaque limit before exploring the variation with a finite barrier width. This study could be useful for the design of acoustic devices

Phonon tunneling through a double barrier system

Energy Technology Data Exchange (ETDEWEB)

Villegas, Diosdado [Departamento de Física, Universidad Central “Marta Abreu” de Las Villas, CP 54830, Santa Clara, Villa Clara (Cuba); Instituto de Física, Universidad Autónoma de Puebla, 18 Sur y San Claudio, Edif. 110A, Ciudad Universitaria, 72570 Puebla (Mexico); León-Pérez, Fernando de [Centro Universitario de la Defensa de Zaragoza, Ctra. de Huesca s/n, E-50090 Zaragoza (Spain); Pérez-Álvarez, R. [Universidad Autónoma del Estado de Morelos, Av. Universidad 1001, CP 62209 Cuernavaca (Mexico); Arriaga, J., E-mail: arriaga@ifuap.buap.mx [Instituto de Física, Universidad Autónoma de Puebla, 18 Sur y San Claudio, Edif. 110A, Ciudad Universitaria, 72570 Puebla (Mexico)

2015-04-15

The tunneling of optical and acoustic phonons at normal incidence on a double-barrier is studied in this paper. Transmission coefficients and resonance conditions are derived theoretically under the assumption that the long-wavelength approximation is valid. It is shown that the behavior of the transmission coefficients for the symmetric double barrier has a Lorentzian form close to resonant frequencies and that Breit–Wigner's formula have a general validity in one-dimensional phonon tunneling. Authors also study the so-called generalized Hartman effect in the tunneling of long-wavelength phonons and show that this effect is a numerical artifact resulting from taking the opaque limit before exploring the variation with a finite barrier width. This study could be useful for the design of acoustic devices.

Manipulation of Phonons with Phononic Crystals

Energy Technology Data Exchange (ETDEWEB)

Leseman, Zayd Chad [Univ. of New Mexico, Albuquerque, NM (United States)

2015-07-09

There were three research goals associated with this project. First, was to experimentally demonstrate phonon spectrum control at THz frequencies using Phononic Crystals (PnCs), i.e. demonstrate coherent phonon scattering with PnCs. Second, was to experimentally demonstrate analog PnC circuitry components at GHz frequencies. The final research goal was to gain a fundamental understanding of phonon interaction using computational methods. As a result of this work, 7 journal papers have been published, 1 patent awarded, 14 conference presentations given, 4 conference publications, and 2 poster presentations given.

Elastic wave surfaces and phonon focussing for the A-15 compounds

International Nuclear Information System (INIS)

Viswanathan, K.S.

1981-01-01

It is shown that the section of the energy surface corresponding to the longitudinal mode by the principal xy-plane for the A-15 compounds will degenerate into four points at the corners of a square at very low temperatures in the cubic phase. When the quasi-shear mode propagating along the (110) direction becomes soft, simultaneously the longitudinal mode will exhibit unusually high phonon focussing. (author)

Flexural phonon limited phonon drag thermopower in bilayer graphene

Science.gov (United States)

Ansari, Mohd Meenhaz; Ashraf, SSZ

2018-05-01

We investigate the phonon drag thermopower from flexural phonons as a function of electron temperature and carrier concentration in the Bloch-Gruneisen regime in non-strained bilayer graphene using Boltzmann transport equation approach. The flexural phonons are expected to be the major source of intrinsic scattering mechanism in unstrained bilayer graphene due to their large density. The flexural phonon modes dispersion relation is quadratic so these low energy flexural phonons abound at room temperature and as a result deform the bilayer graphene sheet in the out of plane direction and affects the transport properties. We also produce analytical result for phonon-drag thermopower from flexural phonons and find that phonon-drag thermopower depicts T2 dependence on temperature and n-1 on carrier concentration.

Measuring the effective phonon density of states of a quantum dot in cavity quantum electrodynamics

DEFF Research Database (Denmark)

Madsen, Kristian Høeg; Nielsen, Per Kær; Kreiner-Møller, Asger

2013-01-01

We employ detuning-dependent decay-rate measurements of a quantum dot in a photonic-crystal cavity to study the influence of phonon dephasing in a solid-state quantum-electrodynamics experiment. The experimental data agree with a microscopic non-Markovian model accounting for dephasing from...... longitudinal acoustic phonons, and the analysis explains the difference between nonresonant cavity feeding in different nanocavities. From the comparison between experiment and theory we extract the effective phonon density of states experienced by the quantum dot in the nanocavity. This quantity determines...

Strongly correlated quasi-one-dimensional bands: Ground states, optical absorption, and phonons

International Nuclear Information System (INIS)

Campbell, D.K.; Gammel, J.T.; Loh, E.Y. Jr.

1989-01-01

Using the Lanczos method for exact diagonalization on systems up to 14 sites, combined with a novel ''phase randomization'' technique for extracting more information from these small systems, we investigate several aspects of the one-dimensional Peierls-Hubbard Hamiltonian, in the context of trans-polyacetylene: the dependence of the ground state dimerization on the strength of the electron-electron interactions, including the effects of ''off-diagonal'' Coulomb terms generally ignored in the Hubbard model; the phonon vibrational frequencies and dispersion relations, and the optical absorption properties, including the spectrum of absorptions as a function of photon energy. These three different observables provide considerable insight into the effects of electron-electron interactions on the properties of real materials and thus into the nature of strongly correlated electron systems. 29 refs., 11 figs

Phonon anomalies in optical spectra of LiNbO3 single crystals

Directory of Open Access Journals (Sweden)

ANDREJA VALCIC

2004-06-01

Full Text Available LiNbO3 single crystals were grown by the Czochralski technique in an air atmosphere. The critical crystal diameter Dc = 1.5 cm and the critical rate of rotation wc = 35 rpm were calculated by equations from the hydrodynamics of the melt. The domain inversion was carried out at 1430 K using a 3.75 V/cm electric field for 10 min. The obtained crystals were cut, polished and etched to determine the presence of dislocations and single domain structures. The optical properties were studied by infrared and Raman spectroscopy as a function of temperature. With decreasing temperature, an atypical behaviour of the phonon modes could be seen in the ferroelectrics LiNbO3. The obtained results are discussed and compared with published data.

Effect of Holstein phonons on the electronic properties of graphene

International Nuclear Information System (INIS)

Stauber, T; Peres, N M R

2008-01-01

We obtain the self-energy of the electronic propagator due to the presence of Holstein polarons within the first Born approximation. This leads to a renormalization of the Fermi velocity of 1%. We further compute the optical conductivity of the system at the Dirac point and at finite doping within the Kubo formula. We argue that the effects due to Holstein phonons are negligible and that the Boltzmann approach, which does not include inter-band transitions and can thus not treat optical phonons due to their high energy of ℎω 0 ∼ 0.1-0.2 eV, remains valid

FT-IR reflection spectra of single crystals: resolving phonons of different symmetry without using polarised radiation

Directory of Open Access Journals (Sweden)

METODIJA NAJDOSKI

2000-07-01

Full Text Available Fourier-transform infrared (FT-IR reflection spectra, asquired at nearnormal incidence, were recorded from single crystals belonging to six crystal systems: CsCr(SO42.12H2O (alum, cubic, K2CuCl2·2H2O (Mitscherlichite, tetragonal, CaCO3 (calcite, hexagonal, KHSO4 (mercallite, orthorhombic, CaSO4·2H2O (gypsum, monoclinic and CuSO4·5H2O (chalcantite, triclinic. The acquired IR reflection spectra were further transformed into absorption spectra, employing the Kramers-Kronig transformation. Except for the cubic alums, the spectra strongly depend on the crystal face from which they were recorded; this is a consequence of anisotropy. Phonons of a given symmetry (E-species, in tetragonal/hexagonal and B-species, in monoclinic crystals may be resolved without using a polariser. The spectrum may be simplified in the case of an orthorhombic crystal, as well. The longitudinal-optical (LO and transversal-optical (TO mode frequencies were calculated in the case of optically isotropic and the simplified spectra of optically uniaxial crystals.

Optical properties of a single-colour centre in diamond with a green zero-phonon line

International Nuclear Information System (INIS)

Smith, Jason M; Grazioso, Fabio; Patton, Brian R; Dolan, Philip R; Markham, Matthew L; Twitchen, Daniel J

2011-01-01

We report the photoluminescence characteristics of a colour centre in diamond grown by plasma-assisted chemical vapour deposition. The colour centre emits with a sharp zero-phonon line at 2.330 eV (λ=532 nm) and a lifetime of 3.3 ns, thus offering potential for a high-speed single-photon source with green emission. It displays a vibronic emission spectrum with a Huang-Rhys parameter of 2.48 at 77 K. Hanbury-Brown and Twiss measurements reveal that the electronic level structure of the defect includes a metastable state that can be populated from the optically excited state.

Modulation of thermal conductivity in kinked silicon nanowires: phonon interchanging and pinching effects.

Science.gov (United States)

Jiang, Jin-Wu; Yang, Nuo; Wang, Bing-Shen; Rabczuk, Timon

2013-04-10

We perform molecular dynamics simulations to investigate the reduction of the thermal conductivity by kinks in silicon nanowires. The reduction percentage can be as high as 70% at room temperature. The temperature dependence of the reduction is also calculated. By calculating phonon polarization vectors, two mechanisms are found to be responsible for the reduced thermal conductivity: (1) the interchanging effect between the longitudinal and transverse phonon modes and (2) the pinching effect, that is, a new type of localization, for the twisting and transverse phonon modes in the kinked silicon nanowires. Our work demonstrates that the phonon interchanging and pinching effects, induced by kinking, are brand-new and effective ways in modulating heat transfer in nanowires, which enables the kinked silicon nanowires to be a promising candidate for thermoelectric materials.

Low-Field Mobility and Galvanomagnetic Properties of Holes in Germanium with Phonon Scattering

DEFF Research Database (Denmark)

Lawætz, Peter

1968-01-01

acoustic scattering, no overall consistency is found between available galvanomagnetic data and deformation potentials derived directly from experiments on strained Ge. The discrepancies may be ascribed to ionized-impurity scattering, but at higher temperatures where optical phonon scattering is operative......A theoretical calculation of the low-field galvanomagnetic properties of holes in Ge has been carried out incorporating all relevant details of the band structure. The scattering is limited to acoustic and optical phonons and is described by the deformation potentials a, b, d, and d0. For pure......, the deviations are still appreciable. We are led to conclude that the deformation-potential theory of phonon scattering needs reconsideration, and a nontrivial correction is pointed out....

Enhancement of phononic band gaps in ternary/binary structure

International Nuclear Information System (INIS)

Aly, Arafa H.; Mehaney, Ahmed

2012-01-01

Based on the transfer matrix method (TMM) and Bloch theory, the interaction of elastic waves (normal incidence) with 1D phononic crystal had been studied. The transfer matrix method was obtained for both longitudinal and transverse waves by applying the continuity conditions between the consecutive unit cells. Dispersion relations are calculated and plotted for both binary and ternary structures. Also we have investigated the corresponding effects on the band gaps values for the two types of phononic crystals. Furthermore, it can be observed that the complete band gaps are located in the common frequency stop-band regions. Numerical simulations are performed to investigate the effect of different thickness ratios inside each unit cell on the band gap values, as well as unit cells thickness on the central band gap frequency. These phononic band gap materials can be used as a filter for elastic waves at different frequencies values.

Band structures and localization properties of aperiodic layered phononic crystals

Energy Technology Data Exchange (ETDEWEB)

Yan Zhizhong, E-mail: zzyan@bit.edu.cn [Department of Applied Mathematics, Beijing Institute of Technology, Beijing 100081 (China); Zhang Chuanzeng [Department of Civil Engineering, University of Siegen, D-57078 Siegen (Germany)

2012-03-15

The band structures and localization properties of in-plane elastic waves with coupling of longitudinal and transverse modes oblique propagating in aperiodic phononic crystals based on Thue-Morse and Rudin-Shapiro sequences are studied. Using transfer matrix method, the concept of the localization factor is introduced and the correctness is testified through the Rytov dispersion relation. For comparison, the perfect periodic structure and the quasi-periodic Fibonacci system are also considered. In addition, the influences of the random disorder, local resonance, translational and/or mirror symmetries on the band structures of the aperiodic phononic crystals are analyzed in this paper.

Four-phonon scattering significantly reduces intrinsic thermal conductivity of solids

Science.gov (United States)

Feng, Tianli; Lindsay, Lucas; Ruan, Xiulin

2017-10-01

For decades, the three-phonon scattering process has been considered to govern thermal transport in solids, while the role of higher-order four-phonon scattering has been persistently unclear and so ignored. However, recent quantitative calculations of three-phonon scattering have often shown a significant overestimation of thermal conductivity as compared to experimental values. In this Rapid Communication we show that four-phonon scattering is generally important in solids and can remedy such discrepancies. For silicon and diamond, the predicted thermal conductivity is reduced by 30% at 1000 K after including four-phonon scattering, bringing predictions in excellent agreement with measurements. For the projected ultrahigh-thermal conductivity material, zinc-blende BAs, a competitor of diamond as a heat sink material, four-phonon scattering is found to be strikingly strong as three-phonon processes have an extremely limited phase space for scattering. The four-phonon scattering reduces the predicted thermal conductivity from 2200 to 1400 W/m K at room temperature. The reduction at 1000 K is 60%. We also find that optical phonon scattering rates are largely affected, being important in applications such as phonon bottlenecks in equilibrating electronic excitations. Recognizing that four-phonon scattering is expensive to calculate, in the end we provide some guidelines on how to quickly assess the significance of four-phonon scattering, based on energy surface anharmonicity and the scattering phase space. Our work clears the decades-long fundamental question of the significance of higher-order scattering, and points out ways to improve thermoelectrics, thermal barrier coatings, nuclear materials, and radiative heat transfer.

Use of thulium-sensitized rare earth-doped low phonon energy crystalline hosts for IR sources.

Science.gov (United States)

Ganem, Joseph; Bowman, Steven R

2013-11-01

Crystalline hosts with low phonon energies enable novel energy transfer processes when doped with rare earth ions. Two applications of energy transfer for rare earth ions in thulium-sensitized low phonon energy crystals that result in infrared luminescence are discussed. One application is an endothermic, phonon-assisted cross-relaxation process in thulium-doped yttrium chloride that converts lattice phonons to infrared emission, which raises the possibility of a fundamentally new method for achieving solid-state optical cooling. The other application is an optically pumped mid-IR phosphor using thulium-praseodymium-doped potassium lead chloride that converts 805-nm diode light to broadband emission from 4,000 to 5,500 nm. These two applications in chloride crystals are discussed in terms of critical radii calculated from Forster-Dexter energy transfer theory. It is found that the critical radii for electric dipole-dipole interactions in low phonon energy chloride crystals are comparable to those in conventional oxide and fluoride crystals. It is the reduction in multi-phonon relaxation rates in chloride crystals that enable these additional energy transfer processes and infrared luminescence.

Quantum decoherence of phonons in Bose-Einstein condensates

Science.gov (United States)

Howl, Richard; Sabín, Carlos; Hackermüller, Lucia; Fuentes, Ivette

2018-01-01

We apply modern techniques from quantum optics and quantum information science to Bose-Einstein condensates (BECs) in order to study, for the first time, the quantum decoherence of phonons of isolated BECs. In the last few years, major advances in the manipulation and control of phonons have highlighted their potential as carriers of quantum information in quantum technologies, particularly in quantum processing and quantum communication. Although most of these studies have focused on trapped ion and crystalline systems, another promising system that has remained relatively unexplored is BECs. The potential benefits in using this system have been emphasized recently with proposals of relativistic quantum devices that exploit quantum states of phonons in BECs to achieve, in principle, superior performance over standard non-relativistic devices. Quantum decoherence is often the limiting factor in the practical realization of quantum technologies, but here we show that quantum decoherence of phonons is not expected to heavily constrain the performance of these proposed relativistic quantum devices.

3D continuum phonon model for group-IV 2D materials

DEFF Research Database (Denmark)

Willatzen, Morten; Lew Yan Voon, Lok C.; Gandi, Appala Naidu

2017-01-01

. In this paper, we use the model to not only compare the phonon spectra among the group-IV materials but also to study whether these phonons differ from those of a compound material such as molybdenum disulfide. The origin of quadratic modes is clarified. Mode coupling for both graphene and silicene is obtained......, contrary to previous works. Our model allows us to predict the existence of confined optical phonon modes for the group-IV materials but not for molybdenum disulfide. A comparison of the long-wavelength modes to density-functional results is included....

Phonon density of states and anharmonicity of UO2

Science.gov (United States)

Pang, Judy W. L.; Chernatynskiy, Aleksandr; Larson, Bennett C.; Buyers, William J. L.; Abernathy, Douglas L.; McClellan, Kenneth J.; Phillpot, Simon R.

2014-03-01

Phonon density of states (PDOS) measurements have been performed on polycrystalline UO2 at 295 and 1200 K using time-of-flight inelastic neutron scattering to investigate the impact of anharmonicity on the vibrational spectra and to benchmark ab initio PDOS simulations performed on this strongly correlated Mott insulator. Time-of-flight PDOS measurements include anharmonic linewidth broadening, inherently, and the factor of ˜7 enhancement of the oxygen spectrum relative to the uranium component by the increased neutron sensitivity to the oxygen-dominated optical phonon modes. The first-principles simulations of quasiharmonic PDOS spectra were neutron weighted and anharmonicity was introduced in an approximate way by convolution with wave-vector-weighted averages over our previously measured phonon linewidths for UO2, which are provided in numerical form. Comparisons between the PDOS measurements and the simulations show reasonable agreement overall, but they also reveal important areas of disagreement for both high and low temperatures. The discrepancies stem largely from a ˜10 meV compression in the overall bandwidth (energy range) of the oxygen-dominated optical phonons in the simulations. A similar linewidth-convoluted comparison performed with the PDOS spectrum of Dolling et al. obtained by shell-model fitting to their historical phonon dispersion measurements shows excellent agreement with the time-of-flight PDOS measurements reported here. In contrast, we show by comparisons of spectra in linewidth-convoluted form that recent first-principles simulations for UO2 fail to account for the PDOS spectrum determined from the measurements of Dolling et al. These results demonstrate PDOS measurements to be stringent tests for ab inito simulations of phonon physics in UO2 and they indicate further the need for advances in theory to address the lattice dynamics of UO2.

Enhancement of coherent acoustic phonons in InGaN multiple quantum wells

Science.gov (United States)

Hafiz, Shopan D.; Zhang, Fan; Monavarian, Morteza; Avrutin, Vitaliy; Morkoç, Hadis; Özgür, Ümit

2015-03-01

Enhancement of coherent zone folded longitudinal acoustic phonon (ZFLAP) oscillations at terahertz frequencies was demonstrated in InGaN multiple quantum wells (MQWs) by using wavelength degenerate time resolved differential transmission spectroscopy. Screening of the piezoelectric field in InGaN MQWs by photogenerated carriers upon femtosecond pulse excitation gave rise to terahertz ZFLAPs, which were monitored at the Brillouin zone center in the transmission geometry. MQWs composed of 10 pairs InxGa1-xN wells and In0.03Ga0.97N barriers provided coherent phonon frequencies of 0.69-0.80 THz depending on the period of MQWs. Dependences of ZFLAP amplitude on excitation density and wavelength were also investigated. Possibility of achieving phonon cavity, incorporating a MQW placed between two AlN/GaN phonon mirrors designed to exhibit large acoustic gaps at the zone center, was also explored.

Phonon superradiance and phonon laser effect in nanomagnets.

Science.gov (United States)

Chudnovsky, E M; Garanin, D A

2004-12-17

We show that the theory of spin-phonon processes in paramagnetic solids must take into account the coherent generation of phonons by the magnetic centers. This effect should drastically enhance spin-phonon rates in nanoscale paramagnets and in crystals of molecular nanomagnets.

The Importance of Phonons with Negative Phase Quotient in Disordered Solids.

Science.gov (United States)

Seyf, Hamid Reza; Lv, Wei; Rohskopf, Andrew; Henry, Asegun

2018-02-08

Current understanding of phonons is based on the phonon gas model (PGM), which is best rationalized for crystalline materials. However, most of the phonons/modes in disordered materials have a different character and thus may contribute to heat conduction in a fundamentally different way than is described by PGM. For the modes in crystals, which have sinusoidal character, one can separate the modes into two primary categories, namely acoustic and optical modes. However, for the modes in disordered materials, such designations may no longer rigorously apply. Nonetheless, the phase quotient (PQ) is a quantity that can be used to evaluate whether a mode more so shares a distinguishing property of acoustic vibrations manifested as a positive PQ, or a distinguishing property of an optical vibrations manifested as negative PQ. In thinking about this characteristic, there is essentially no intuition regarding the role of positive vs. negative PQ vibrational modes in disordered solids. Given this gap in understanding, herein we studied the respective contributions to thermal conductivity for several disordered solids as a function of PQ. The analysis sheds light on the importance of optical like/negative PQ modes in structurally/compositionally disordered solids, whereas in crystalline materials, the contributions of optical modes are usually small.

Optical coherence tomography and subclinical optical neuritis in longitudinally extensive transverse myelitis

Directory of Open Access Journals (Sweden)

Prakash Kumar Sinha

2017-01-01

Full Text Available Objective: The aim is to compare the retinal nerve fiber layer (RNFL thickness of longitudinally extensive transverse myelitis (LETM eyes without previous optic neuritis with that of healthy control subjects. Methods: Over 20 LETM eyes and 20 normal control eyes were included in the study and subjected to optical coherence tomography to evaluate and compare the RNFL thickness. Result: Significant RNFL thinning was observed at 8 o'clock position in LETM eyes as compared to the control eyes (P = 0.038. No significant differences were seen in other RNFL measurements. Conclusion: Even in the absence of previous optic neuritis LETM can lead to subclinical axonal damage leading to focal RNFL thinning.

Prediction of phonon-mediated superconductivity in hole-doped black phosphorus.

Science.gov (United States)

Feng, Yanqing; Sun, Hongyi; Sun, Junhui; Lu, Zhibin; You, Yong

2018-01-10

We study the conventional electron-phonon mediated superconducting properties of hole-doped black phosphorus by density functional calculations and get quite a large electron-phonon coupling (EPC) constant λ ~ 1.0 with transition temperature T C ~ 10 K, which is comparable to MgB 2 when holes are doped into the degenerate and nearly flat energy bands around the Fermi level. We predict that the softening of low-frequency [Formula: see text] optical mode and its phonon displacement, which breaks the lattice nonsymmorphic symmetry of gliding plane and lifts the band double degeneracy, lead to a large EPC. These factors are favorable for BCS superconductivity.

Prediction of phonon-mediated superconductivity in hole-doped black phosphorus

Science.gov (United States)

Feng, Yanqing; Sun, Hongyi; Sun, Junhui; Lu, Zhibin; You, Yong

2018-01-01

We study the conventional electron-phonon mediated superconducting properties of hole-doped black phosphorus by density functional calculations and get quite a large electron-phonon coupling (EPC) constant λ ~ 1.0 with transition temperature T C ~ 10 K, which is comparable to MgB2 when holes are doped into the degenerate and nearly flat energy bands around the Fermi level. We predict that the softening of low-frequency B3g1 optical mode and its phonon displacement, which breaks the lattice nonsymmorphic symmetry of gliding plane and lifts the band double degeneracy, lead to a large EPC. These factors are favorable for BCS superconductivity.

Phonon structures of GaN-based random semiconductor alloys

Science.gov (United States)

Zhou, Mei; Chen, Xiaobin; Li, Gang; Zheng, Fawei; Zhang, Ping

2017-12-01

Accurate modeling of thermal properties is strikingly important for developing next-generation electronics with high performance. Many thermal properties are closely related to phonon dispersions, such as sound velocity. However, random substituted semiconductor alloys AxB1-x usually lack translational symmetry, and simulation with periodic boundary conditions often requires large supercells, which makes phonon dispersion highly folded and hardly comparable with experimental results. Here, we adopt a large supercell with randomly distributed A and B atoms to investigate substitution effect on the phonon dispersions of semiconductor alloys systematically by using phonon unfolding method [F. Zheng, P. Zhang, Comput. Mater. Sci. 125, 218 (2016)]. The results reveal the extent to which phonon band characteristics in (In,Ga)N and Ga(N,P) are preserved or lost at different compositions and q points. Generally, most characteristics of phonon dispersions can be preserved with indium substitution of gallium in GaN, while substitution of nitrogen with phosphorus strongly perturbs the phonon dispersion of GaN, showing a rapid disintegration of the Bloch characteristics of optical modes and introducing localized impurity modes. In addition, the sound velocities of both (In,Ga)N and Ga(N,P) display a nearly linear behavior as a function of substitution compositions. Supplementary material in the form of one pdf file available from the Journal web page at http://https://doi.org/10.1140/epjb/e2017-80481-0.

Electronic and optical properties of ZrB{sub 12} and YB{sub 6}. Discussion on electron-phonon coupling

Energy Technology Data Exchange (ETDEWEB)

Teyssier, J.; Kuzmenko, A.; Marel, D. van der; Lortz, R.; Junod, A. [Departement de Physique de la Matiere Condensee, Universite de Geneve, Quai Ernest-Ansermet 24, 1211 Geneve 4 (Switzerland); Filippov, V.; Shitsevalova, N. [Institute for Problems of Materials Science NANU, Kiev (Ukraine)

2006-09-15

We report the optical properties of high-quality single crystals of low temperature superconductors zirconiumdodecaboride ZrB{sub 12} (T{sub c}=5.95 K) and yttrium hexaboride YB{sub 6} (T{sub c}=7.15 K) in the range 6 meV-4.6 eV at room temperature. The experimental optical conductivity was extracted from the analysis of the reflectivity in the infrared range and ellipsometry measurement of the dielectric function in the visible range. The electronic band structure of these compounds was calculated by the self-consistent full-potential LMTO method and used to compute the interband part of the optical conductivity and the plasma frequency {omega}{sub p}. A good agreement was observed between the interband part of the experimental optical conductivities and the band structure calculations. Different methods combining optical spectroscopy, resistivity, specific heat measurements and results of band structure calculations are used to determine the electron-phonon coupling constant. (copyright 2006 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)

Visualization of Longitudinal and Transverse Components of Strongly Focused Optical Field by means of Photo-Reactive Azopolymers

Directory of Open Access Journals (Sweden)

Kharitonov A.V.

2015-01-01

Full Text Available Most important problems in modern photonics are fabrication, visualization and characterization of nanomaterials at optical frequencies. A number of optical techniques uses tightly focused laser beams to access longitudinal electromagnetic fields, which are directed towards the wave vector. In this Letter, the distribution of transverse and longitudinal optical fields in tightly focused laser beams, polarized in a new fashion, is investigated. Polarization dependent fingerprints of transverse and longitudinal optical fields are experimentally captured by means of photoinduced surface deformations in azobenzene polymer thin films.

Strain effects on the optical conductivity of gapped graphene in the presence of Holstein phonons beyond the Dirac cone approximation

Energy Technology Data Exchange (ETDEWEB)

Yarmohammadi, Mohsen, E-mail: m.yarmohammadi69@gmail.com [Young Researchers and Elite Club, Kermanshah Branch, Islamic Azad University, Kermanshah (Iran, Islamic Republic of)

2016-08-15

In this paper we study the optical conductivity and density of states (DOS) of doped gapped graphene beyond the Dirac cone approximation in the presence of electron-phonon (e-ph) interaction under strain, i.e., within the framework of a full π-band Holstein model, by using the Kubo linear response formalism that is established upon the retarded self-energy. A new peak in the optical conductivity for a large enough e-ph interaction strength is found which is associated to transitions between the midgap states and the Van Hove singularities of the main π-band. Optical conductivity decreases with strain and at large strains, the system has a zero optical conductivity at low energies due to optically inter-band excitations through the limit of zero doping. As a result, the Drude weight changes with e-ph interaction, temperature and strain. Consequently, DOS and optical conductivity remains stable with temperature at low e-ph coupling strengths.

Phonon thermal transport through tilt grain boundaries in strontium titanate

Energy Technology Data Exchange (ETDEWEB)

Zheng, Zexi; Chen, Xiang; Yang, Shengfeng; Xiong, Liming; Chen, Youping [Department of Mechanical and Aerospace Engineering, University of Florida, Gainesville, Florida 32611 (United States); Deng, Bowen; Chernatynskiy, Aleksandr [Department of Materials Science and Engineering, University of Florida, Gainesville, Florida 32611 (United States)

2014-08-21

In this work, we perform nonequilibrium molecular dynamics simulations to study phonon scattering at two tilt grain boundaries (GBs) in SrTiO{sub 3}. Mode-wise energy transmission coefficients are obtained based on phonon wave-packet dynamics simulations. The Kapitza conductance is then quantified using a lattice dynamics approach. The obtained results of the Kapitza conductance of both GBs compare well with those obtained by the direct method, except for the temperature dependence. Contrary to common belief, the results of this work show that the optical modes in SrTiO{sub 3} contribute significantly to phonon thermal transport, accounting for over 50% of the Kapitza conductance. To understand the effect of the GB structural disorder on phonon transport, we compare the local phonon density of states of the atoms in the GB region with that in the single crystalline grain region. Our results show that the excess vibrational modes introduced by the structural disorder do not have a significant effect on phonon scattering at the GBs, but the absence of certain modes in the GB region appears to be responsible for phonon reflections at GBs. This work has also demonstrated phonon mode conversion and simultaneous generation of new modes. Some of the new modes have the same frequency as the initial wave packet, while some have the same wave vector but lower frequencies.

Phonon thermal transport through tilt grain boundaries in strontium titanate

International Nuclear Information System (INIS)

Zheng, Zexi; Chen, Xiang; Yang, Shengfeng; Xiong, Liming; Chen, Youping; Deng, Bowen; Chernatynskiy, Aleksandr

2014-01-01

In this work, we perform nonequilibrium molecular dynamics simulations to study phonon scattering at two tilt grain boundaries (GBs) in SrTiO 3 . Mode-wise energy transmission coefficients are obtained based on phonon wave-packet dynamics simulations. The Kapitza conductance is then quantified using a lattice dynamics approach. The obtained results of the Kapitza conductance of both GBs compare well with those obtained by the direct method, except for the temperature dependence. Contrary to common belief, the results of this work show that the optical modes in SrTiO 3 contribute significantly to phonon thermal transport, accounting for over 50% of the Kapitza conductance. To understand the effect of the GB structural disorder on phonon transport, we compare the local phonon density of states of the atoms in the GB region with that in the single crystalline grain region. Our results show that the excess vibrational modes introduced by the structural disorder do not have a significant effect on phonon scattering at the GBs, but the absence of certain modes in the GB region appears to be responsible for phonon reflections at GBs. This work has also demonstrated phonon mode conversion and simultaneous generation of new modes. Some of the new modes have the same frequency as the initial wave packet, while some have the same wave vector but lower frequencies

Magnon Broadening Effect by Magnon-Phonon Interaction in Colossal Magnetoresistance Manganites

OpenAIRE

Furukawa, Nobuo

1999-01-01

In order to study the magnetic excitation behaviors in colossal magnetoresistance manganites, a magnon-phonon interacting system is investigated. Sudden broadening of magnon linewidth is obtained when a magnon branch crosses over an optical phonon branch. Onset of the broadening is approximately determined by the magnon density of states. Anomalous magnon damping at the brillouine zone boundary observed in low Curie temperature manganites is explained.

Preface: Phonons 2007

Science.gov (United States)

Perrin, Bernard

2007-06-01

logo.jpg" ALT="Conference logo"/> The conference PHONONS 2007 was held 15-20 July 2007 in the Conservatoire National des Arts et Métiers (CNAM) Paris, France. CNAM is a college of higher technology for training students in the application of science to industry, founded by Henri Grégoire in 1794. This was the 12th International Conference on Phonon Scattering in Condensed Matter. This international conference series, held every 3 years, started in France at Sainte-Maxime in 1972. It was then followed by meetings at Nottingham (1975), Providence (1979), Stuttgart (1983), Urbana-Champaign (1986), Heidelberg (1989), Ithaca (1992), Sapporo (1995), Lancaster (1998), Dartmouth (2001) and St Petersburg (2004). PHONONS 2007 was attended by 346 delegates from 37 different countries as follows: France 120, Japan 45, Germany 25, USA 25, Russia 21, Italy 13, Poland 9, UK 9, Canada 7, The Netherlands 7, Finland 6, Spain 6, Taiwan 6, Greece 4, India 4, Israel 4, Ukraine 4, Serbia 3, South Africa 3, Argentina 2, Belgium 2, China 2, Iran 2, Korea 2, Romania 2, Switzerland 2, and one each from Belarus, Bosnia-Herzegovina, Brazil, Bulgaria, Egypt, Estonia, Mexico, Moldova, Morocco, Saudi Arabia, Turkey. There were 5 plenary lectures, 14 invited talks and 84 oral contributions; 225 posters were presented during three poster sessions. The first plenary lecture was given by H J Maris who presented fascinating movies featuring the motion of a single electron in liquid helium. Robert Blick gave us a review on the new possibilities afforded by nanotechnology to design nano-electomechanical systems (NEMS) and the way to use them to study elementary and fundamental processes. The growing interest for phonon transport studies in nanostructured materials was demonstrated by Arun Majumdar. Andrey Akimov described how ultrafast acoustic solitons can monitor the optical properties of quantum wells. Finally, Maurice Chapellier told us how phonons can help tracking dark matter. These 328

Path-integral Monte Carlo study of phonons in the bcc phase of Helium-3

OpenAIRE

Sorkin, V.; Polturak, E.; Adler, Joan

2006-01-01

Using Path Integral Monte Carlo and the Maximum Entropy method, we calculate the dynamic structure factor of solid He-3 in the bcc phase at a finite temperature of T = 1.6 K and a molar volume of 21.5 cm^3. From the single phonon dynamic structure factor, we obtain both the longitudinal and transverse phonon branches along the main crystalline directions, [001], [011] and [111]. Our results are compared with other theoretical predictions and available experimental data.

Evidence for phonon-mediated coupling in superconducting Ba0.6K0.4BiO3

International Nuclear Information System (INIS)

Hinks, D.G.; Dabrowski, B.; Richards, D.R.; Jorgensen, J.D.; Pei, S.; Zasadzinski, J.F.

1989-01-01

Superconducting Ba 0.6 K 0.4 BiO 3 , with a T c of 30 K, shows a large 18 O isotope effect which indicates that phonons are involved in the pairing mechanism. Infrared reflectivity measurements indicate a value for the superconducting gap consistent with moderate coupling (2Δ/k T c = 3.5 ± 0.5). A mediating energy for pairing of about 40 meV would be required to obtain a T c of 30 K. Strong coupling of electrons by optical phonons (which are present in this material with energies up to 80 meV) could account for the observed transition temperature. Recent tunneling spectroscopy shows the presence of strongly coupled optical phonons in the 40 to 70 meV region, indicating that superconductivity in this material may be phonon mediated

Theoretical approach to the phonon modes and specific heat of germanium nanowires

Energy Technology Data Exchange (ETDEWEB)

Trejo, A.; López-Palacios, L.; Vázquez-Medina, R.; Cruz-Irisson, M., E-mail: irisson@ipn.mx

2014-11-15

The phonon modes and specific heat of Ge nanowires were computed using a first principles density functional theory scheme with a generalized gradient approximation and finite-displacement supercell algorithms. The nanowires were modeled in three different directions: [001], [111], and [110], using the supercell technique. All surface dangling bonds were saturated with Hydrogen atoms. The results show that the specific heat of the GeNWs at room temperature increases as the nanowire diameter decreases, regardless the orientation due to the phonon confinement and surface passivation. Also the phonon confinement effects could be observed since the highest optical phonon modes in the Ge vibration interval shifted to a lower frequency compared to their bulk counterparts.

Microscopic theory of multiple-phonon-mediated dephasing and relaxation of quantum dots near a photonic band gap

Science.gov (United States)

Roy, Chiranjeeb; John, Sajeev

2010-02-01

We derive a quantum theory of the role of acoustic and optical phonons in modifying the optical absorption line shape, polarization dynamics, and population dynamics of a two-level atom (quantum dot) in the “colored” electromagnetic vacuum of a photonic band-gap (PBG) material. This is based on a microscopic Hamiltonian describing both radiative and vibrational processes quantum mechanically. We elucidate the extent to which phonon-assisted decay limits the lifetime of a single photon-atom bound state and derive the modified spontaneous emission dynamics due to coupling to various phonon baths. We demonstrate that coherent interaction with undamped phonons can lead to an enhanced lifetime of a photon-atom bound state in a PBG. This results in reduction of the steady-state atomic polarization but an increase in the fractionalized upper state population in the photon-atom bound state. We demonstrate, on the other hand, that the lifetime of the photon-atom bound state in a PBG is limited by the lifetime of phonons due to lattice anharmonicities (breakup of phonons into lower energy phonons) and purely nonradiative decay. We also derive the modified polarization decay and dephasing rates in the presence of such damping. This leads to a microscopic, quantum theory of the optical absorption line shapes. Our model and formalism provide a starting point for describing dephasing and relaxation in the presence of external coherent fields and multiple quantum dot interactions in electromagnetic reservoirs with radiative memory effects.

Microscopic theory of multiple-phonon-mediated dephasing and relaxation of quantum dots near a photonic band gap

International Nuclear Information System (INIS)

Roy, Chiranjeeb; John, Sajeev

2010-01-01

We derive a quantum theory of the role of acoustic and optical phonons in modifying the optical absorption line shape, polarization dynamics, and population dynamics of a two-level atom (quantum dot) in the ''colored'' electromagnetic vacuum of a photonic band-gap (PBG) material. This is based on a microscopic Hamiltonian describing both radiative and vibrational processes quantum mechanically. We elucidate the extent to which phonon-assisted decay limits the lifetime of a single photon-atom bound state and derive the modified spontaneous emission dynamics due to coupling to various phonon baths. We demonstrate that coherent interaction with undamped phonons can lead to an enhanced lifetime of a photon-atom bound state in a PBG. This results in reduction of the steady-state atomic polarization but an increase in the fractionalized upper state population in the photon-atom bound state. We demonstrate, on the other hand, that the lifetime of the photon-atom bound state in a PBG is limited by the lifetime of phonons due to lattice anharmonicities (breakup of phonons into lower energy phonons) and purely nonradiative decay. We also derive the modified polarization decay and dephasing rates in the presence of such damping. This leads to a microscopic, quantum theory of the optical absorption line shapes. Our model and formalism provide a starting point for describing dephasing and relaxation in the presence of external coherent fields and multiple quantum dot interactions in electromagnetic reservoirs with radiative memory effects.

First-principles prediction of phononic thermal conductivity of silicene: A comparison with graphene

International Nuclear Information System (INIS)

Gu, Xiaokun; Yang, Ronggui

2015-01-01

There has been great interest in two-dimensional materials, beyond graphene, for both fundamental sciences and technological applications. Silicene, a silicon counterpart of graphene, has been shown to possess some better electronic properties than graphene. However, its thermal transport properties have not been fully studied. In this paper, we apply the first-principles-based phonon Boltzmann transport equation to investigate the thermal conductivity of silicene as well as the phonon scattering mechanisms. Although both graphene and silicene are two-dimensional crystals with similar crystal structure, we find that phonon transport in silicene is quite different from that in graphene. The thermal conductivity of silicene shows a logarithmic increase with respect to the sample size due to the small scattering rates of acoustic in-plane phonon modes, while that of graphene is finite. Detailed analysis of phonon scattering channels shows that the linear dispersion of the acoustic out-of-plane (ZA) phonon modes, which is induced by the buckled structure, makes the long-wavelength longitudinal acoustic phonon modes in silicene not as efficiently scattered as that in graphene. Compared with graphene, where most of the heat is carried by the acoustic out-of-plane (ZA) phonon modes, the ZA phonon modes in silicene only have ∼10% contribution to the total thermal conductivity, which can also be attributed to the buckled structure. This systematic comparison of phonon transport and thermal conductivity of silicene and graphene using the first-principle-based calculations shed some light on other two-dimensional materials, such as two-dimensional transition metal dichalcogenides

The determination of the acoustical phonon dispersion branches of CePd3 by inelastic neutron diffraction

International Nuclear Information System (INIS)

Severing, A.

1985-07-01

The result of this thesis is, that the phonon dispersion of CePd 3 , measured by inelastic neutron scattering, does not show any phonon softening effects due to valence fluctuations as it is observed in the phonon dispersion spectra of TmSe, SmS under pressure and SMsub(.75)Ysub(.25)S in dependence of the temperature. Even at low temperature no softening effects could be detected in comparison to the room temperature data. However we see indications for the existence of intermediate valence induced electron-phonon couplings in the linewidth of the longitudinal acoustic phonon in -direction with reduced wavevector xi=0.2. This phonon seems to be broadened at 135 K. If this broadening is a real intermediate valence effect, this effects manifest themselves much weaker in CePd 3 than in the substances with NaCl-structure. The question whether such couplings realy exist can only be answered by further measurements. (orig.)

Phonon induced optical gain in a current carrying two-level quantum dot

Energy Technology Data Exchange (ETDEWEB)

Eskandari-asl, Amir, E-mail: amir.eskandari.asl@gmail.com [Department of Physics, Shahid Beheshti University, G.C. Evin, Tehran 1983963113 (Iran, Islamic Republic of); School of Nano Science, Institute for Research in Fundamental Sciences (IPM), P.O. Box: 19395-5531, Tehran, Iran (Iran, Islamic Republic of)

2017-05-15

In this work we consider a current carrying two level quantum dot (QD) that is coupled to a single mode phonon bath. Using self-consistent Hartree-Fock approximation, we obtain the I-V curve of QD. By considering the linear response of our system to an incoming classical light, we see that depending on the parametric regime, the system could have weak or strong light absorption or may even show lasing. This lasing occurs at high enough bias voltages and is explained by a population inversion considering side bands, while the total electron population in the higher level is less than the lower one. The frequency at which we have the most significant lasing depends on the level spacing and phonon frequency and not on the electron-phonon coupling strength.

Phonon Routing in Integrated Optomechanical Cavity-waveguide Systems

Science.gov (United States)

2015-08-20

cavity (bottom beam of Fig. 1b), allowing for evanescent cou- pling of laser light into and out of the cavity. A single optical fiber taper is used to...couple light into the on- chip coupling waveguide, and a photonic crystal mirror is etched in to the end of the optical coupling waveguide so that light...coupled into the nanobeam cavity can be recollected by the optical fiber taper as per Ref. [36]. Figure 1c shows the band structure of the phonon

Detecting the phonon spin in magnon-phonon conversion experiments

Science.gov (United States)

Holanda, J.; Maior, D. S.; Azevedo, A.; Rezende, S. M.

2018-05-01

Recent advances in the emerging field of magnon spintronics have stimulated renewed interest in phenomena involving the interaction between spin waves, the collective excitations of spins in magnetic materials that quantize as magnons, and the elastic waves that arise from excitations in the crystal lattice, which quantize as phonons. In magnetic insulators, owing to the magnetostrictive properties of materials, spin waves can become strongly coupled to elastic waves, forming magnetoelastic waves—a hybridized magnon-phonon excitation. While several aspects of this interaction have been subject to recent scrutiny, it remains unclear whether or not phonons can carry spin. Here we report experiments on a film of the ferrimagnetic insulator yttrium iron garnet under a non-uniform magnetic field demonstrating the conversion of coherent magnons generated by a microwave field into phonons that have spin. While it is well established that photons in circularly polarized light carry a spin, the spin of phonons has had little attention in the literature. By means of wavevector-resolved Brillouin light-scattering measurements, we show that the magnon-phonon conversion occurs with constant energy and varying linear momentum, and that the light scattered by the phonons is circularly polarized, thus demonstrating that the phonons have spin.

The Electron-Phonon Interaction in Strongly Correlated Systems

International Nuclear Information System (INIS)

Castellani, C.; Grilli, M.

1995-01-01

We analyze the effect of strong electron-electron repulsion on the electron-phonon interaction from a Fermi-liquid point of view and show that the electron-electron interaction is responsible for vertex corrections, which generically lead to a strong suppression of the electron-phonon coupling in the v F q/ω >>1 region, while such effect is not present when v F q/ω F is the Fermi velocity and q and ω are the transferred momentum and frequency respectively. In particular the e-ph scattering is suppressed in transport properties which are dominated by low-energy-high-momentum processes. On the other hand, analyzing the stability criterion for the compressibility, which involves the effective interactions in the dynamical limit, we show that a sizable electron-phonon interaction can push the system towards a phase-separation instability. Finally a detailed analysis of these ideas is carried out using a slave-boson approach for the infinite-U three-band Hubbard model in the presence of a coupling between the local hole density and a dispersionless optical phonon. (author)

Terahertz lasers and amplifiers based on resonant optical phonon scattering to achieve population inversion

Science.gov (United States)

Williams, Benjamin S. (Inventor); Hu, Qing (Inventor)

2009-01-01

The present invention provides quantum cascade lasers and amplifier that operate in a frequency range of about 1 Terahertz to about 10 Terahertz. In one aspect, a quantum cascade laser of the invention includes a semiconductor heterostructure that provides a plurality of lasing modules connected in series. Each lasing module includes a plurality of quantum well structure that collectively generate at least an upper lasing state, a lower lasing state, and a relaxation state such that the upper and the lower lasing states are separated by an energy corresponding to an optical frequency in a range of about 1 to about 10 Terahertz. The lower lasing state is selectively depopulated via resonant LO-phonon scattering of electrons into the relaxation state.

Absence of phase-dependent noise in time-domain reflectivity studies of impulsively excited phonons

KAUST Repository

Hussain, A.

2010-06-17

There have been several reports of phase-dependent noise in time-domain reflectivity studies of optical phonons excited by femtosecond laser pulses in semiconductors, semimetals, and superconductors. It was suggested that such behavior is associated with the creation of squeezed phonon states although there is no theoretical model that directly supports such a proposal. We have experimentally re-examined the studies of phonons in bismuth and gallium arsenide, and find no evidence of any phase-dependent noise signature associated with the phonons. We place an upper limit on any such noise at least 40–50 dB lower than previously reported.

Absence of phase-dependent noise in time-domain reflectivity studies of impulsively excited phonons

KAUST Repository

Hussain, A.; Andrews, S. R.

2010-01-01

There have been several reports of phase-dependent noise in time-domain reflectivity studies of optical phonons excited by femtosecond laser pulses in semiconductors, semimetals, and superconductors. It was suggested that such behavior is associated with the creation of squeezed phonon states although there is no theoretical model that directly supports such a proposal. We have experimentally re-examined the studies of phonons in bismuth and gallium arsenide, and find no evidence of any phase-dependent noise signature associated with the phonons. We place an upper limit on any such noise at least 40–50 dB lower than previously reported.

3D continuum phonon model for group-IV 2D materials

KAUST Repository

Willatzen, Morten

2017-06-30

A general three-dimensional continuum model of phonons in two-dimensional materials is developed. Our first-principles derivation includes full consideration of the lattice anisotropy and flexural modes perpendicular to the layers and can thus be applied to any two-dimensional material. In this paper, we use the model to not only compare the phonon spectra among the group-IV materials but also to study whether these phonons differ from those of a compound material such as molybdenum disulfide. The origin of quadratic modes is clarified. Mode coupling for both graphene and silicene is obtained, contrary to previous works. Our model allows us to predict the existence of confined optical phonon modes for the group-IV materials but not for molybdenum disulfide. A comparison of the long-wavelength modes to density-functional results is included.

Phonon-assisted damping of plasmons in three- and two-dimensional metals

Science.gov (United States)

Caruso, Fabio; Novko, Dino; Draxl, Claudia

2018-05-01

We investigate the effects of crystal lattice vibrations on the dispersion of plasmons. The loss function of the homogeneous electron gas (HEG) in two and three dimensions is evaluated numerically in the presence of electronic coupling to an optical phonon mode. Our calculations are based on many-body perturbation theory for the dielectric function as formulated by the Hedin-Baym equations in the Fan-Migdal approximation. The coupling to phonons broadens the spectral signatures of plasmons in the electron-energy loss spectrum (EELS) and it induces the decay of plasmons on timescales shorter than 1 ps. Our results further reveal the formation of a kink in the plasmon dispersion of the two-dimensional HEG, which marks the onset of plasmon-phonon scattering. Overall, these features constitute a fingerprint of plasmon-phonon coupling in EELS of simple metals. It is shown that these effects may be accounted for by resorting to a simplified treatment of the electron-phonon interaction which is amenable to first-principles calculations.

Phonon-assisted optical bands of nanosized powdery SrAl{sub 2}O{sub 4}:Eu{sup 2+} crystals: Evidence of a multimode Pekarian

Energy Technology Data Exchange (ETDEWEB)

Nazarov, M. [School of Materials and Mineral Resources Engineering, Universiti Sains Malaysia, Engineering Campus, 14300 Nibong Tebal, Pulau Pinang (Malaysia); Institute of Applied Physics, Academiei Street 5, Chisinau MD-2028 (Moldova, Republic of); Brik, M.G. [Institute of Physics, University of Tartu, Riia 142, Tartu 51014 (Estonia); Spassky, D. [Institute of Physics, University of Tartu, Riia 142, Tartu 51014 (Estonia); Skobeltsyn Institute of Nuclear Physics, M.V. Lomonosov Moscow State University, 119991 Moscow (Russian Federation); Tsukerblat, B., E-mail: tsuker@bgu.ac.il [Department of Chemistry, Ben-Gurion University of the Negev, Beer-Sheva 84105 (Israel); Palii, A. [Institute of Applied Physics, Academiei Street 5, Chisinau MD-2028 (Moldova, Republic of); Nazida, A. Nor [School of Materials and Mineral Resources Engineering, Universiti Sains Malaysia, Engineering Campus, 14300 Nibong Tebal, Pulau Pinang (Malaysia); Faculty of Art and Design, Universiti Teknologi MARA (Perak), Seri Iskandar 32610, Bandar Baru Seri Iskandar, Perak (Malaysia); Ahmad-Fauzi, M.N. [School of Materials and Mineral Resources Engineering, Universiti Sains Malaysia, Engineering Campus, 14300 Nibong Tebal, Pulau Pinang (Malaysia)

2013-12-09

A stoichiometric powder composed of nanosized grains of SrAl{sub 2}O{sub 4}:Eu{sup 2+} was synthesized by combustion method at 500 °C with the subsequent calcination at 1000 °C. The zero-phonon line position, parameter of the Stokes shift, heat release factor and effective phonon energy were studied experimentally and analyzed in the framework of the multimode Pekar–Huang–Rhys model. Experimental data show that the optical 4f–5d transitions in Eu{sup 2+} ion exhibit a broad asymmetric electron–vibrational bands with a pronounced structure near the maxima. The form-function of the absorption and luminescence bands are theoretically analyzed in the framework of the model of the linear electron–vibrational interaction assuming strong coupling with the local vibration (estimated Pekar–Huang–Rhys parameter a=2S=10 and frequency ℏω=509 cm{sup −1}) and relatively weak interaction with the crystal phonons. The last results in an effective temperature dependent broadening of the discrete lines corresponding to the local vibrations and to a specific shape of the whole phonon assisted band (multimode Pekarian). Providing specific interrelation between the key parameters the calculated absorption and luminescence bands exhibit peculiar temperature dependent structured peaks in a qualitative agreement with the experimental data.

Quantum theory of phonon-mediated decoherence and relaxation of two-level systems in a structured electromagnetic reservoir

Science.gov (United States)

Roy, Chiranjeeb

In this thesis we study the role of nonradiative degrees of freedom on quantum optical properties of mesoscopic quantum dots placed in the structured electromagnetic reservoir of a photonic crystal. We derive a quantum theory of the role of acoustic and optical phonons in modifying the optical absorption lineshape, polarization dynamics, and population dynamics of a two-level atom (quantum dot) in the "colored" electromagnetic vacuum of a photonic band gap (PBG) material. This is based on a microscopic Hamiltonian describing both radiative and vibrational processes quantum mechanically. Phonon sidebands in an ordinary electromagnetic reservoir are recaptured in a simple model of optical phonons using a mean-field factorization of the atomic and lattice displacement operators. Our formalism is then used to treat the non-Markovian dynamics of the same system within the structured electromagnetic density of states of a photonic crystal. We elucidate the extent to which phonon-assisted decay limits the lifetime of a single photon-atom bound state and derive the modified spontaneous emission dynamics due to coupling to various phonon baths. We demonstrate that coherent interaction with undamped phonons can lead to enhanced lifetime of a photon-atom bound state in a PBG by (i) dephasing and reducing the transition electric dipole moment of the atom and (ii) reducing the quantum mechanical overlap of the state vectors of the excited and ground state (polaronic shift). This results in reduction of the steady-state atomic polarization but an increase in the fractionalized upper state population in the photon-atom bound state. We demonstrate, on the other hand, that the lifetime of the photon-atom bound state in a PBG is limited by the lifetime of phonons due to lattice anharmonicities (break-up of phonons into lower energy phonons) and purely nonradiative decay. We demonstrate how these additional damping effects limit the extent of the polaronic (Franck-Condon) shift of

Anharmonic longitudinal motion of bases and dynamics of nonlinear excitation in DNA.

Science.gov (United States)

Di Garbo, Angelo

2016-01-01

The dynamics of the transcription bubble in DNA is studied by using a nonlinear model in which torsional and longitudinal conformations of the biomolecule are coupled. In the absence of forcing and dissipation the torsional dynamics is described by a perturbed kink of the Sine-Gordon DNA model, while the longitudinal conformational energy propagate as phonons. It was found that for random initial conditions of the longitudinal conformational field the presence of the kink promotes the creation of phonons propagating along the chain axis. Moreover, the presence of forcing, describing the active role of RNA polymerase, determines in agreement to the experimental data a modulation of the velocity of the transcription bubble. Lastly, it was shown that the presence of dissipation impacts the dynamic of the phonon by reducing the amplitude of the corresponding conformational field. On the contrary, dissipation and forcing modulate the velocity of the transcription bubble alone.

Raman analysis of phonon modes in a short period AlN/GaN superlattice

Science.gov (United States)

Sarkar, Ketaki; Datta, Debopam; Gosztola, David J.; Shi, Fengyuan; Nicholls, Alan; Stroscio, Michael A.; Dutta, Mitra

2018-03-01

AlN/GaN-based optoelectronic devices have been the subject of intense research underlying the commercialization of efficient devices. Areas of considerable interest are the study of their lattice dynamics, phonon transport, and electron-phonon interactions specific to the interface of these heterostructures which results in additional optical phonon modes known as interface phonon modes. In this study, the framework of the dielectric continuum model (DCM) has been used to compare and analyze the optical phonon modes obtained from experimental Raman scattering measurements on AlN/GaN short-period superlattices. We have observed the localized E2(high), A1(LO) and the E1(TO) modes in superlattice measurements at frequencies shifted from their bulk values. To the best of our knowledge, the nanostructures used in these studies are among the smallest yielding useful Raman signatures for the interface modes. In addition, we have also identified an additional spread of interface phonon modes in the TO range resulting from the superlattice periodicity. The Raman signature contribution from the underlying AlxGa1-xN ternary has also been observed and analyzed. A temperature calibration was done based on Stokes/anti-Stokes ratio of A1(LO) using Raman spectroscopy in a broad operating temperature range. Good agreement between the experimental results and theoretically calculated calibration plot predicted using Bose-Einstein statistics was obtained.

Exciton-phonon coupling in a CsPbBr3 single nanocrystal

Science.gov (United States)

Ramade, Julien; Andriambariarijaona, Léon Marcel; Steinmetz, Violette; Goubet, Nicolas; Legrand, Laurent; Barisien, Thierry; Bernardot, Frédérick; Testelin, Christophe; Lhuillier, Emmanuel; Bramati, Alberto; Chamarro, Maria

2018-02-01

We have performed micro-photoluminescence measurements on a single CsPbBr3 nanocrystal (NC) with a size comparable to the Bohr diameter (7 nm). When the NC has an orthorhombic crystal symmetry, we observe an exciton fine structure composed of three peaks linearly polarized. We took advantage of the polarization properties of micro-photoluminescence to monitor in situ both the energy and linewidth of individual peaks when increasing temperature. We reveal that two regimes exist, at low and high temperature, which are dominated by acoustic or longitudinal optical phonon (Fröhlich term) couplings, respectively. The acoustic contribution does not change when the energy of the excitonic transition varies in the range of 2.46-2.62 eV, i.e., with NC sizes corresponding to this range. We find that line broadening is mainly ruled by the Fröhlich term, which is consistent with the polar nature of CsPbBr3.

Correlation between surface phonon mode and luminescence in nanocrystalline CdS thin films: An effect of ion beam irradiation

International Nuclear Information System (INIS)

Kumar, Pragati; Agarwal, Avinash; Saxena, Nupur; Singh, Fouran; Gupta, Vinay

2014-01-01

The influence of swift heavy ion irradiation (SHII) on surface phonon mode (SPM) and green emission in nanocrystalline CdS thin films grown by chemical bath deposition is studied. The SHII of nanocrystalline CdS thin films is carried out using 70 MeV Ni ions. The micro Raman analysis shows that asymmetry and broadening in fundamental longitudinal optical (LO) phonon mode increases systematically with increasing ion fluence. To analyze the role of phonon confinement, spatial correlation model (SCM) is fitted to the experimental data. The observed deviation of SCM to the experimental data is further investigated by fitting the micro Raman spectra using two Lorentzian line shapes. It is found that two Lorentzian functions (LFs) provide better fitting than SCM fitting and facilitate to identify the contribution of SPM in the observed distortion of LO mode. The behavior of SPM as a function of ion fluence is studied to correlate the observed asymmetry (Γ a /Γ b ) and full width at half maximum of LO phonon mode and to understand the SHII induced enhancement of SPM. The ion beam induced interstitial and surface state defects in thin films, as observed by photoluminescence (PL) spectroscopy studies, may be the underlying reason for enhancement in SPM. PL studies also show enhancement in green luminescence with increase in ion fluence. PL analysis reveals that the variation in population density of surface state defects after SHII is similar to that of SPM. The correlation between SPM and luminescence and their dependence on ion irradiation fluence is explained with the help of thermal spike model.

Acoustic phonon dynamics in thin-films of the topological insulator Bi2Se3

International Nuclear Information System (INIS)

Glinka, Yuri D.; Babakiray, Sercan; Johnson, Trent A.; Holcomb, Mikel B.; Lederman, David

2015-01-01

Transient reflectivity traces measured for nanometer-sized films (6–40 nm) of the topological insulator Bi 2 Se 3 revealed GHz-range oscillations driven within the relaxation of hot carriers photoexcited with ultrashort (∼100 fs) laser pulses of 1.51 eV photon energy. These oscillations have been suggested to result from acoustic phonon dynamics, including coherent longitudinal acoustic phonons in the form of standing acoustic waves. An increase of oscillation frequency from ∼35 to ∼70 GHz with decreasing film thickness from 40 to 15 nm was attributed to the interplay between two different regimes employing traveling-acoustic-waves for films thicker than 40 nm and the film bulk acoustic wave resonator (FBAWR) modes for films thinner than 40 nm. The amplitude of oscillations decays rapidly for films below 15 nm thick when the indirect intersurface coupling in Bi 2 Se 3 films switches the FBAWR regime to that of the Lamb wave excitation. The frequency range of coherent longitudinal acoustic phonons is in good agreement with elastic properties of Bi 2 Se 3

Unraveling the interlayer-related phonon self-energy renormalization in bilayer graphene.

Science.gov (United States)

Araujo, Paulo T; Mafra, Daniela L; Sato, Kentaro; Saito, Riichiro; Kong, Jing; Dresselhaus, Mildred S

2012-01-01

In this letter, we present a step towards understanding the bilayer graphene (2LG) interlayer (IL)-related phonon combination modes and overtones as well as their phonon self-energy renormalizations by using both gate-modulated and laser-energy dependent inelastic scattering spectroscopy. We show that although the IL interactions are weak, their respective phonon renormalization response is significant. Particularly special, the IL interactions are mediated by Van der Waals forces and are fundamental for understanding low-energy phenomena such as transport and infrared optics. Our approach opens up a new route to understanding fundamental properties of IL interactions which can be extended to any graphene-like material, such as MoS₂, WSe₂, oxides and hydroxides. Furthermore, we report a previously elusive crossing between IL-related phonon combination modes in 2LG, which might have important technological applications.

Molecular-dynamics theory of the temperature-dependent surface phonons of W(001)

International Nuclear Information System (INIS)

Wang, C.Z.; Fasolino, A.; Tosatti, E.

1987-04-01

We study the temperature-dependent zone-boundary surface phonons across the c(2x2)→1x1 reconstruction phase transition of the clean W(001) surface. Velocity-velocity correlations and hence the phonon spectral densities are calculated by molecular dynamics for the surface atoms of a finite thickness (001) slab, with interatomic potentials established in a previous study of the surface statics. Our calculated k = (1/2,1/2)(2π/a) surface phonon are dominated by three main low-frequency modes. Of these, the longitudinal and the shear horizontal are reconstruction-related and display critical broadening and softening at the phase transition, while the third, the shear vertical, is basically unaffected. The reconstruction phase mode, shear horizontal, appears to be responsible for the phase fluctuations which destroy long-range order at the transition. (author). 30 refs, 12 figs

Effect of magnon-phonon interaction on transverse acoustic phonon excitation at finite temperature

International Nuclear Information System (INIS)

Cheng Taimin; Li Lin; Xianyu Ze

2007-01-01

A magnon-phonon interaction model is developed on the basis of two-dimensional square Heisenberg ferromagnetic system. By using Matsubara Green function theory transverse acoustic phonon excitation is studied and transverse acoustic phonon excitation dispersion curves is calculated on the main symmetric point and line in the first Brillouin zone. On line Σ it is found that there is hardening for transverse acoustic phonon on small wave vector zone (nearby point Γ), there is softening for transverse acoustic phonon on the softening zone and there is hardening for transverse acoustic phonon near point M. On line Δ it is found there is no softening and hardening for transverse acoustic phonon. On line Z it is found that there is softening for transverse acoustic phonon on small wave vector zone (nearby point X) and there is hardening for transverse acoustic phonon nearby point M. The influences of various parameters on transverse acoustic phonon excitation are also explored and it is found that the coupling of the magnon-phonon and the spin wave stiffness constant play an important role for the softening of transverse acoustic phonon

Nonlocal electron-phonon coupling in the pentacene crystal: Beyond the Γ-point approximation

KAUST Repository

Yi, Yuanping

2012-01-01

There is currently increasing interest in understanding the impact of the nonlocal (Peierls-type) electron-phonon mechanism on charge transport in organic molecular semiconductors. Most estimates of the non-local coupling constants reported in the literature are based on the Γ-point phonon modes. Here, the influence of phonon modes spanning the entire Brillouin zone (phonon dispersion) on the nonlocal electron-phonon couplings is investigated for the pentacene crystal. The phonon modes are obtained by using a supercell approach. The results underline that the overall nonlocal couplings are substantially underestimated by calculations taking sole account of the phonons at the Γ point of the unit cell. The variance of the transfer integrals based on Γ-point normal-mode calculations at room temperature is underestimated in some cases by 40% for herringbone-type dimers and by over 80% for cofacial dimers. Our calculations show that the overall coupling is somewhat larger for holes than for electrons. The results also suggest that the interactions of charge carriers (both electrons and holes) with acoustic and optical phonons are comparable. Therefore, an adequate description of the charge-transport properties in pentacene and similar systems requires that these two electron-phonon coupling mechanisms be treated on the same footing. © 2012 American Institute of Physics.

Long-Wavelength Phonon Scattering in Nonpolar Semiconductors

DEFF Research Database (Denmark)

Lawætz, Peter

1969-01-01

The long-wavelength acoustic- and optical-phonon scattering of carriers in nonpolar semiconductors is considered from a general point of view. The deformation-potential approximation is defined and it is shown that long-range electrostatic forces give a nontrivial correction to the scattering...... of the very-short-range nature of interactions in a covalent semiconductor....

Microscopic theory of the phonon frequencies in BCC barium

International Nuclear Information System (INIS)

Oli, B.A.

1988-09-01

The phonon dispersion frequencies are calculated from first principles for bbc barium using a resonance pseudopotential model which incorporates the effect of s-d hybridization. It was also possible using this scheme to account for the anomalous feature of the Ba dispersion curve observed experimentally in the (ξ,0,0) direction where the frequencies of the transverse branch are higher than the frequencies of the longitudinal branch. The frequencies obtained were also used to calculate the phonon density of states by the linear-analytic tetrahedra method of zone integration. The results of these calculations are qualitatively in good agreement with experimental data, and provide further support to the interpretation of the anomalous behaviour in the (ξ,0,0) direction as arising from s-d hybridization. (author). 27 refs, 4 figs, 3 tabs

Phonon dispersion relations for caesium thiocyanate

International Nuclear Information System (INIS)

Irving, M.A.; Smith, T.F.; Elcombe, M.M.

1984-01-01

Room temperature phonon dispersion relations for frequencies below 2 THz have been measured, along the three orthorhombic axes and selected diagonal directions by neutron inelastic scattering, for caesium thiocyanate. These curves, which represent 13 acoustic modes and 11 optic modes of vibration, do not agree with the dispersion behaviour calculated from the rigid-ion model developed by Ti and Ra to describe their Raman scattering observations

Optical gain coefficients of silicon: a theoretical study

Science.gov (United States)

Tsai, Chin-Yi

2018-05-01

A theoretical model is presented and an explicit formula is derived for calculating the optical gain coefficients of indirect band-gap semiconductors. This model is based on the second-order time-dependent perturbation theory of quantum mechanics by incorporating all the eight processes of photon/phonon emission and absorption between the band edges of the conduction and valence bands. Numerical calculation results are given for Si. The calculated absorption coefficients agree well with the existing fitting formula of experiment data with two modes of phonons: optical phonons with energy of 57.73 meV and acoustic phonons with energy of 18.27 meV near (but not exactly at) the zone edge of the X-point in the dispersion relation of phonons. These closely match with existing data of 57.5 meV transverse optical (TO) phonons at the X4-point and 18.6 meV transverse acoustic (TA) phonons at the X3-point of the zone edge. The calculated results show that the material optical gain of Si will overcome free-carrier absorption if the energy separation of quasi-Fermi levels between electrons and holes exceeds 1.15 eV.

Phonon manipulation with phononic crystals.

Energy Technology Data Exchange (ETDEWEB)

Kim Bongsang; Hopkins, Patrick Edward; Leseman, Zayd C.; Goettler, Drew F.; Su, Mehmet F. (University of New Mexico, Albuquerque, NM); El-Kady, Ihab Fathy; Reinke, Charles M.; Olsson, Roy H., III

2012-01-01

In this work, we demonstrated engineered modification of propagation of thermal phonons, i.e. at THz frequencies, using phononic crystals. This work combined theoretical work at Sandia National Laboratories, the University of New Mexico, the University of Colorado Boulder, and Carnegie Mellon University; the MESA fabrication facilities at Sandia; and the microfabrication facilities at UNM to produce world-leading control of phonon propagation in silicon at frequencies up to 3 THz. These efforts culminated in a dramatic reduction in the thermal conductivity of silicon using phononic crystals by a factor of almost 30 as compared with the bulk value, and about 6 as compared with an unpatterned slab of the same thickness. This work represents a revolutionary advance in the engineering of thermoelectric materials for optimal, high-ZT performance. We have demonstrated the significant reduction of the thermal conductivity of silicon using phononic crystal structuring using MEMS-compatible fabrication techniques and in a planar platform that is amenable to integration with typical microelectronic systems. The measured reduction in thermal conductivity as compared to bulk silicon was about a factor of 20 in the cross-plane direction [26], and a factor of 6 in the in-plane direction. Since the electrical conductivity was only reduced by a corresponding factor of about 3 due to the removal of conductive material (i.e., porosity), and the Seebeck coefficient should remain constant as an intrinsic material property, this corresponds to an effective enhancement in ZT by a factor of 2. Given the number of papers in literature devoted to only a small, incremental change in ZT, the ability to boost the ZT of a material by a factor of 2 simply by reducing thermal conductivity is groundbreaking. The results in this work were obtained using silicon, a material that has benefitted from enormous interest in the microelectronics industry and that has a fairly large thermoelectric power

Light scattering by surface phonons in crystals

International Nuclear Information System (INIS)

Albuquerque, D.L.

1980-01-01

Theory of inelastic light scattering by surface acoustic phonons homogeneous crystals is presented. The Green functions are determined by the use of a classical linear response method and used to evaluate the Brillouin cross section. The acoustic modes are found from solutions to the acoustical-wave equation and boundary conditions appropriated. Two light-scattering mechanisms, amely the surface corrugation and bulk elasto-optic effect are analyzed by deriving optical fields which satisfy both the acousto-optically driven wave equation and the electromagnetic boundary conditions. No restrictions are imposed concerning the angle of incidence of the light. Some representative computed Brillouin ineshapes are also presented and their features discussed. (author) [pt

Phonon effects on the radiative recombination of excitons in double quantum dots

Science.gov (United States)

Karwat, Paweł; Sitek, Anna; Machnikowski, Paweł

2011-11-01

We study theoretically the radiative recombination of excitons in double quantum dots in the presence of carrier-phonon coupling. We show that the phonon-induced pure dephasing effects and transitions between the exciton states strongly modify the spontaneous emission process and make it sensitive to temperature, which may lead to nonmonotonic temperature dependence of the time-resolved luminescence. We show also that, under specific resonance conditions, the biexcitonic interband polarization can be coherently transferred to the excitonic one, leading to an extended lifetime of the total coherent polarization, which is reflected in the nonlinear optical spectrum of the system. We study the stability of this effect against phonon-induced decoherence.

Surface phonons

CERN Document Server

Wette, Frederik

1991-01-01

In recent years substantial progress has been made in the detection of surface phonons owing to considerable improvements in inelastic rare gas scattering tech­ niques and electron energy loss spectroscopy. With these methods it has become possible to measure surface vibrations in a wide energy range for all wave vectors in the two-dimensional Brillouin zone and thus to deduce the complete surface phonon dispersion curves. Inelastic atomic beam scattering and electron energy loss spectroscopy have started to play a role in the study of surface phonons similar to the one played by inelastic neutron scattering in the investigation of bulk phonons in the last thirty years. Detailed comparison between experimen­ tal results and theoretical studies of inelastic surface scattering and of surface phonons has now become feasible. It is therefore possible to test and to improve the details of interaction models which have been worked out theoretically in the last few decades. At this point we felt that a concise, co...

Active tuning of surface phonon polariton resonances via carrier photoinjection

Science.gov (United States)

Dunkelberger, Adam D.; Ellis, Chase T.; Ratchford, Daniel C.; Giles, Alexander J.; Kim, Mijin; Kim, Chul Soo; Spann, Bryan T.; Vurgaftman, Igor; Tischler, Joseph G.; Long, James P.; Glembocki, Orest J.; Owrutsky, Jeffrey C.; Caldwell, Joshua D.

2018-01-01

Surface phonon polaritons (SPhPs) are attractive alternatives to infrared plasmonics for subdiffractional confinement of infrared light. Localized SPhP resonances in semiconductor nanoresonators are narrow, but that linewidth and the limited extent of the Reststrahlen band limit spectral coverage. To address this limitation, we report active tuning of SPhP resonances in InP and 4H-SiC by photoinjecting free carriers into nanoresonators, taking advantage of the coupling between the carrier plasma and optic phonons to blueshift SPhP resonances. We demonstrate state-of-the-art tuning figures of merit upon continuous-wave excitation (in InP) or pulsed excitation (in 4H-SiC). Lifetime effects cause the tuning to saturate in InP, and carrier redistribution leads to rapid (electronic and phononic excitations.

Observation of chiral phonons

KAUST Repository

Zhu, Hanyu; Yi, Jun; Li, Ming-yang; Xiao, Jun; Zhang, Lifa; Yang, Chih-Wen; Kaindl, Robert A.; Li, Lain-Jong; Wang, Yuan; Zhang, Xiang

2018-01-01

Chirality reveals symmetry breaking of the fundamental interaction of elementary particles. In condensed matter, for example, the chirality of electrons governs many unconventional transport phenomena such as the quantum Hall effect. Here we show that phonons can exhibit intrinsic chirality in monolayer tungsten diselenide. The broken inversion symmetry of the lattice lifts the degeneracy of clockwise and counterclockwise phonon modes at the corners of the Brillouin zone. We identified the phonons by the intervalley transfer of holes through hole-phonon interactions during the indirect infrared absorption, and we confirmed their chirality by the infrared circular dichroism arising from pseudoangular momentum conservation. The chiral phonons are important for electron-phonon coupling in solids, phonon-driven topological states, and energy-efficient information processing.

Observation of chiral phonons

KAUST Repository

Zhu, Hanyu

2018-02-01

Chirality reveals symmetry breaking of the fundamental interaction of elementary particles. In condensed matter, for example, the chirality of electrons governs many unconventional transport phenomena such as the quantum Hall effect. Here we show that phonons can exhibit intrinsic chirality in monolayer tungsten diselenide. The broken inversion symmetry of the lattice lifts the degeneracy of clockwise and counterclockwise phonon modes at the corners of the Brillouin zone. We identified the phonons by the intervalley transfer of holes through hole-phonon interactions during the indirect infrared absorption, and we confirmed their chirality by the infrared circular dichroism arising from pseudoangular momentum conservation. The chiral phonons are important for electron-phonon coupling in solids, phonon-driven topological states, and energy-efficient information processing.

Optimization of phononic filters via genetic algorithms

Energy Technology Data Exchange (ETDEWEB)

Hussein, M I [University of Colorado, Department of Aerospace Engineering Sciences, Boulder, Colorado 80309-0429 (United States); El-Beltagy, M A [Cairo University, Faculty of Computers and Information, 5 Dr. Ahmed Zewail Street, 12613 Giza (Egypt)

2007-12-15

A phononic crystal is commonly characterized by its dispersive frequency spectrum. With appropriate spatial distribution of the constituent material phases, spectral stop bands could be generated. Moreover, it is possible to control the number, the width, and the location of these bands within a frequency range of interest. This study aims at exploring the relationship between unit cell configuration and frequency spectrum characteristics. Focusing on 1D layered phononic crystals, and longitudinal wave propagation in the direction normal to the layering, the unit cell features of interest are the number of layers and the material phase and relative thickness of each layer. An evolutionary search for binary- and ternary-phase cell designs exhibiting a series of stop bands at predetermined frequencies is conducted. A specially formulated representation and set of genetic operators that break the symmetries in the problem are developed for this purpose. An array of optimal designs for a range of ratios in Young's modulus and density are obtained and the corresponding objective values (the degrees to which the resulting bands match the predetermined targets) are examined as a function of these ratios. It is shown that a rather complex filtering objective could be met with a high degree of success. Structures composed of the designed phononic crystals are excellent candidates for use in a wide range of applications including sound and vibration filtering.

Optimization of phononic filters via genetic algorithms

International Nuclear Information System (INIS)

Hussein, M I; El-Beltagy, M A

2007-01-01

A phononic crystal is commonly characterized by its dispersive frequency spectrum. With appropriate spatial distribution of the constituent material phases, spectral stop bands could be generated. Moreover, it is possible to control the number, the width, and the location of these bands within a frequency range of interest. This study aims at exploring the relationship between unit cell configuration and frequency spectrum characteristics. Focusing on 1D layered phononic crystals, and longitudinal wave propagation in the direction normal to the layering, the unit cell features of interest are the number of layers and the material phase and relative thickness of each layer. An evolutionary search for binary- and ternary-phase cell designs exhibiting a series of stop bands at predetermined frequencies is conducted. A specially formulated representation and set of genetic operators that break the symmetries in the problem are developed for this purpose. An array of optimal designs for a range of ratios in Young's modulus and density are obtained and the corresponding objective values (the degrees to which the resulting bands match the predetermined targets) are examined as a function of these ratios. It is shown that a rather complex filtering objective could be met with a high degree of success. Structures composed of the designed phononic crystals are excellent candidates for use in a wide range of applications including sound and vibration filtering

Hypersonic phononic crystals.

Science.gov (United States)

Gorishnyy, T; Ullal, C K; Maldovan, M; Fytas, G; Thomas, E L

2005-03-25

In this Letter we propose the use of hypersonic phononic crystals to control the emission and propagation of high frequency phonons. We report the fabrication of high quality, single crystalline hypersonic crystals using interference lithography and show that direct measurement of their phononic band structure is possible with Brillouin light scattering. Numerical calculations are employed to explain the nature of the observed propagation modes. This work lays the foundation for experimental studies of hypersonic crystals and, more generally, phonon-dependent processes in nanostructures.

Influence of quasibound states on the carrier capture in quantum dots

DEFF Research Database (Denmark)

Magnúsdóttir, Ingibjörg; Uskov, A.V.; Ferreira, R.

2002-01-01

The interaction of carriers in quantum-dot quasibound states with longitudinal optical phonons is investigated. For a level separation between the quasibound state and a discrete quantum-dot state in the vicinity of the phonon energy, a strong electron-phonon coupling occurs. A mixed electron...

Tri-component phononic crystals for underwater anechoic coatings

International Nuclear Information System (INIS)

Zhao, Honggang; Liu, Yaozong; Wen, Jihong; Yu, Dianlong; Wen, Xisen

2007-01-01

Localized resonance in phononic crystal, composed of three-dimensional arrays of composite units, has been discovered recently. The composite unit is a high-density sphere coated by soft silicon rubber. In this Letter, the absorptive properties induced by the localized resonance are systemically investigated. The mode conversions during the Mie scattering of a single coated lead sphere in unbounded epoxy are analyzed by referring the elements of the scattering matrix. Then the anechoic properties of a slab containing a plane of such composite scatterers are investigated with the multiple-scattering method by accounting the effects of the multiple scattering and the viscous dissipation. The results show that the longitudinal to transverse mode conversion nearby the locally resonant region is an effective way to enhance the anechoic performance of the finite slab of phononic crystal. Then, the influences of the viscoelasticity of the silicon rubber and the coating thickness on the acoustic properties of the finite slab are investigated for anechoic optimization. Finally, we synthetically consider the destructive scattering in the finite slab of phononic crystal and the backing, and design an anechoic slab composed of bi-layer coated spheres. The results show that the most of the incident energy is absorbed at the desired frequency band

Correlation between surface phonon mode and luminescence in nanocrystalline CdS thin films: An effect of ion beam irradiation

Energy Technology Data Exchange (ETDEWEB)

Kumar, Pragati, E-mail: pkumar.phy@gmail.com; Agarwal, Avinash [Department of Physics, Bareilly College, Bareilly 243 005, Uttar Pradesh (India); Saxena, Nupur; Singh, Fouran [Inter University Accelerator Centre, Aruna Asaf Ali Marg, New Delhi 110 067 (India); Gupta, Vinay [Department of Physics and Astrophysics, University of Delhi, Delhi 110 007 (India)

2014-07-28

The influence of swift heavy ion irradiation (SHII) on surface phonon mode (SPM) and green emission in nanocrystalline CdS thin films grown by chemical bath deposition is studied. The SHII of nanocrystalline CdS thin films is carried out using 70 MeV Ni ions. The micro Raman analysis shows that asymmetry and broadening in fundamental longitudinal optical (LO) phonon mode increases systematically with increasing ion fluence. To analyze the role of phonon confinement, spatial correlation model (SCM) is fitted to the experimental data. The observed deviation of SCM to the experimental data is further investigated by fitting the micro Raman spectra using two Lorentzian line shapes. It is found that two Lorentzian functions (LFs) provide better fitting than SCM fitting and facilitate to identify the contribution of SPM in the observed distortion of LO mode. The behavior of SPM as a function of ion fluence is studied to correlate the observed asymmetry (Γ{sub a}/Γ{sub b}) and full width at half maximum of LO phonon mode and to understand the SHII induced enhancement of SPM. The ion beam induced interstitial and surface state defects in thin films, as observed by photoluminescence (PL) spectroscopy studies, may be the underlying reason for enhancement in SPM. PL studies also show enhancement in green luminescence with increase in ion fluence. PL analysis reveals that the variation in population density of surface state defects after SHII is similar to that of SPM. The correlation between SPM and luminescence and their dependence on ion irradiation fluence is explained with the help of thermal spike model.

Longitudinal optical monitoring of blood flow in breast tumors during neoadjuvant chemotherapy

Science.gov (United States)

Cochran, J. M.; Chung, S. H.; Leproux, A.; Baker, W. B.; Busch, D. R.; DeMichele, A. M.; Tchou, J.; Tromberg, B. J.; Yodh, A. G.

2017-06-01

We measure tissue blood flow markers in breast tumors during neoadjuvant chemotherapy and investigate their correlation to pathologic complete response in a pilot longitudinal patient study (n  =  4). Tumor blood flow is quantified optically by diffuse correlation spectroscopy (DCS), and tissue optical properties, blood oxygen saturation, and total hemoglobin concentration are derived from concurrent diffuse optical spectroscopic imaging (DOSI). The study represents the first longitudinal DCS measurement of neoadjuvant chemotherapy in humans over the entire course of treatment; it therefore offers a first correlation between DCS flow indices and pathologic complete response. The use of absolute optical properties measured by DOSI facilitates significant improvement of DCS blood flow calculation, which typically assumes optical properties based on literature values. Additionally, the combination of the DCS blood flow index and the tissue oxygen saturation from DOSI permits investigation of tissue oxygen metabolism. Pilot results from four patients suggest that lower blood flow in the lesion-bearing breast is correlated with pathologic complete response. Both absolute lesion blood flow and lesion flow relative to the contralateral breast exhibit potential for characterization of pathological response. This initial demonstration of the combined optical approach for chemotherapy monitoring provides incentive for more comprehensive studies in the future and can help power those investigations.

Control of coherent information via on-chip photonic-phononic emitter-receivers.

Science.gov (United States)

Shin, Heedeuk; Cox, Jonathan A; Jarecki, Robert; Starbuck, Andrew; Wang, Zheng; Rakich, Peter T

2015-03-05

Rapid progress in integrated photonics has fostered numerous chip-scale sensing, computing and signal processing technologies. However, many crucial filtering and signal delay operations are difficult to perform with all-optical devices. Unlike photons propagating at luminal speeds, GHz-acoustic phonons moving at slower velocities allow information to be stored, filtered and delayed over comparatively smaller length-scales with remarkable fidelity. Hence, controllable and efficient coupling between coherent photons and phonons enables new signal processing technologies that greatly enhance the performance and potential impact of integrated photonics. Here we demonstrate a mechanism for coherent information processing based on travelling-wave photon-phonon transduction, which achieves a phonon emit-and-receive process between distinct nanophotonic waveguides. Using this device, physics--which supports GHz frequencies--we create wavelength-insensitive radiofrequency photonic filters with frequency selectivity, narrow-linewidth and high power-handling in silicon. More generally, this emit-receive concept is the impetus for enabling new signal processing schemes.

Control of coherent information via on-chip photonic–phononic emitter–receivers

Science.gov (United States)

Shin, Heedeuk; Cox, Jonathan A.; Jarecki, Robert; Starbuck, Andrew; Wang, Zheng; Rakich, Peter T.

2015-01-01

Rapid progress in integrated photonics has fostered numerous chip-scale sensing, computing and signal processing technologies. However, many crucial filtering and signal delay operations are difficult to perform with all-optical devices. Unlike photons propagating at luminal speeds, GHz-acoustic phonons moving at slower velocities allow information to be stored, filtered and delayed over comparatively smaller length-scales with remarkable fidelity. Hence, controllable and efficient coupling between coherent photons and phonons enables new signal processing technologies that greatly enhance the performance and potential impact of integrated photonics. Here we demonstrate a mechanism for coherent information processing based on travelling-wave photon–phonon transduction, which achieves a phonon emit-and-receive process between distinct nanophotonic waveguides. Using this device, physics—which supports GHz frequencies—we create wavelength-insensitive radiofrequency photonic filters with frequency selectivity, narrow-linewidth and high power-handling in silicon. More generally, this emit-receive concept is the impetus for enabling new signal processing schemes. PMID:25740405

Light scattering by surface phonons in crystals

International Nuclear Information System (INIS)

Albuquerque, E.L. de

1981-01-01

A theory of inelastic light scattering by surface acoustic phonons in homogeneous crystals is presented. The Green functions are determined by the use of a classical linear response method and used to evaluate the Brillouin cross section. The acoustic modes are found from solutions to the acoustical-wave equation and boundary conditions appropriated. Two light-scattering mechanisms, namely the surface corrugation and bulk elasto-optic effect are analyzed by deriving optical fields which satisfy both the acousto-optically driven wave equation and the electromagnetic boundary conditions. No restrictions are imposed concerning the angle of incidence of the light. Some representative computed Brillouin lineshapes are also presented and their features discussed. (Author) [pt

Effect of growth temperature on the morphology and phonon properties of InAs nanowires on Si substrates

Directory of Open Access Journals (Sweden)

Lei Wen

2011-01-01

Full Text Available Abstract Catalyst-free, vertical array of InAs nanowires (NWs are grown on Si (111 substrate using MOCVD technique. The as-grown InAs NWs show a zinc-blende crystal structure along a < 111 > direction. It is found that both the density and length of InAs NWs decrease with increasing growth temperatures, while the diameter increases with increasing growth temperature, suggesting that the catalyst-free growth of InAs NWs is governed by the nucleation kinetics. The longitudinal optical and transverse optical (TO mode of InAs NWs present a phonon frequency slightly lower than those of InAs bulk materials, which are speculated to be caused by the defects in the NWs. A surface optical mode is also observed for the InAs NWs, which shifts to lower wave-numbers when the diameter of NWs is decreased, in agreement with the theory prediction. The carrier concentration is extracted to be 2.25 × 1017 cm-3 from the Raman line shape analysis. A splitting of TO modes is also observed. PACS: 62.23.Hj; 81.07.Gf; 63.22.Gh; 61.46.Km

Phonon engineering for nanostructures.

Energy Technology Data Exchange (ETDEWEB)

Aubry, Sylvie (Stanford University); Friedmann, Thomas Aquinas; Sullivan, John Patrick; Peebles, Diane Elaine; Hurley, David H. (Idaho National Laboratory); Shinde, Subhash L.; Piekos, Edward Stanley; Emerson, John Allen

2010-01-01

Understanding the physics of phonon transport at small length scales is increasingly important for basic research in nanoelectronics, optoelectronics, nanomechanics, and thermoelectrics. We conducted several studies to develop an understanding of phonon behavior in very small structures. This report describes the modeling, experimental, and fabrication activities used to explore phonon transport across and along material interfaces and through nanopatterned structures. Toward the understanding of phonon transport across interfaces, we computed the Kapitza conductance for {Sigma}29(001) and {Sigma}3(111) interfaces in silicon, fabricated the interfaces in single-crystal silicon substrates, and used picosecond laser pulses to image the thermal waves crossing the interfaces. Toward the understanding of phonon transport along interfaces, we designed and fabricated a unique differential test structure that can measure the proportion of specular to diffuse thermal phonon scattering from silicon surfaces. Phonon-scale simulation of the test ligaments, as well as continuum scale modeling of the complete experiment, confirmed its sensitivity to surface scattering. To further our understanding of phonon transport through nanostructures, we fabricated microscale-patterned structures in diamond thin films.

First Principles Modeling of Phonon Heat Conduction in Nanoscale Crystalline Structures

International Nuclear Information System (INIS)

Mazumder, Sandip; Li, Ju

2010-01-01

of optical phonons, and (2) by developing a suite of numerical algorithms for solution of the BTE for phonons. The suite of numerical algorithms includes Monte Carlo techniques and deterministic techniques based on the Discrete Ordinates Method and the Ballistic-Diffusive approximation of the BTE. These methods were applied to calculation of thermal conductivity of silicon thin films, and to simulate heat conduction in multi-dimensional structures. In addition, thermal transport in silicon nanowires was investigated using two different first principles methods. One was to apply the Green-Kubo formulation to an equilibrium system. The other was to use Non-Equilibrium Molecular Dynamics (NEMD). Results of MD simulations showed that the nanowire cross-sectional shape and size significantly affects the thermal conductivity, as has been found experimentally. In summary, the project clarified the role of various phonon modes - in particular, optical phonon - in non-equilibrium transport in silicon. It laid the foundation for the solution of the BTE in complex three-dimensional structures using deterministic techniques, paving the way for the development of robust numerical tools that could be coupled to existing device simulation tools to enable coupled electro-thermal modeling of practical electronic/optoelectronic devices. Finally, it shed light on why the thermal conductivity of silicon nanowires is so sensitive to its cross-sectional shape.

Interface-guided mode of Lamb waves in a two-dimensional phononic crystal plate

International Nuclear Information System (INIS)

Huang Ping-Ping; Yao Yuan-Wei; Zhang Xin; Li Jing; Hu Ai-Zhen; Wu Fu-Gen

2015-01-01

We investigate the interface-guided mode of Lamb waves in a phononic crystal heterostructures plate, which is composed of two different semi-infinite phononic crystal (PC) plates. The interface-guided modes of the Lamb wave can be obtained by the lateral lattice slipping or by the interface longitudinal gliding. Significantly, it is observed that the condition to generate the interface-guided modes of the Lamb wave is more demanding than that of the studied fluid–fluid system. The interface-guided modes are strongly affected not only by the relative movement of the two semi-infinite PCs but also by the thickness of the PC plate. (paper)

Negative refraction imaging of solid acoustic waves by two-dimensional three-component phononic crystal

International Nuclear Information System (INIS)

Li Jing; Liu Zhengyou; Qiu Chunyin

2008-01-01

By using of the multiple scattering methods, we study the negative refraction imaging effect of solid acoustic waves by two-dimensional three-component phononic crystals composed of coated solid inclusions placed in solid matrix. We show that localized resonance mechanism brings on a group of flat single-mode bands in low-frequency region, which provides two equivalent frequency surfaces (EFS) close to circular. The two constant frequency surfaces correspond to two Bloch modes, a right-handed and a left-handed, whose leading mode are respectively transverse (T) and longitudinal (L) modes. The negative refraction behaviors of the two kinds of modes have been demonstrated by simulation of a Gaussian beam through a finite system. High-quality far-field imaging by a planar lens for transverse or longitudinal waves has been realized separately. This three-component phononic crystal may thus serve as a mode selector in negative refraction imaging of solid acoustic waves

Decoherence in semiconductor cavity QED systems due to phonon couplings

DEFF Research Database (Denmark)

Nielsen, Per Kær; Mørk, Jesper

2014-01-01

We investigate the effect of electron-phonon interactions on the coherence properties of single photons emitted from a semiconductor cavity QED (quantum electrodynamics) system, i.e., a quantum dot embedded in an optical cavity. The degree of indistinguishability, governing the quantum mechanical...

Density of phonon states in the light-harvesting complex II of green plants

CERN Document Server

Pieper, J K; Irrgang, K D; Renger, G

2002-01-01

In photosynthetic antenna complexes, the coupling of electronic transitions to low-frequency vibrations of the protein matrix (phonons) plays an essential role in light absorption and ultra-fast excitation energy transfer (EET). The model calculations presented here indicate that inelastic neutron scattering experiments provide invaluable information on the phonon density of states for light-harvesting complex II, which may permit a consistent interpretation of contradictory results from high-resolution optical spectroscopy. (orig.)

Soft phonon modes driven huge difference on lattice thermal conductivity between topological semimetal WC and WN

Science.gov (United States)

Guo, San-Dong; Chen, Peng

2018-04-01

Topological semimetals are currently attracting increasing interest due to their potential applications in topological qubits and low-power electronics, which are closely related to their thermal transport properties. Recently, the triply degenerate nodal points near the Fermi level of WC are observed by using angle-resolved photoemission spectroscopy. In this work, by solving the Boltzmann transport equation based on first-principles calculations, we systematically investigate the phonon transport properties of topological semimetals WC and WN. The predicted room-temperature lattice thermal conductivities of WC (WN) along the a and c directions are 1140.64 (7.47) W m-1 K-1 and 1214.69 (5.39) W m-1 K-1. Considering the similar crystal structure of WC and WN, it is quite interesting to find that the thermal conductivity of WC is more than two orders of magnitude higher than that of WN. It is found that, different from WN, the large acoustic-optical (a-o) gap prohibits the acoustic+acoustic → optical (aao) scattering, which gives rise to very long phonon lifetimes, leading to ultrahigh lattice thermal conductivity in WC. For WN, the lack of an a-o gap is due to soft phonon modes in optical branches, which can provide more scattering channels for aao scattering, producing very short phonon lifetimes. Further deep insight can be attained from their different electronic structures. Distinctly different from that in WC, the density of states of WN at the Fermi level becomes very sharp, which leads to destabilization of WN, producing soft phonon modes. It is found that the small shear modulus G and C44 limit the stability of WN, compared with WC. Our studies provide valuable information for phonon transports in WC and WN, and motivate further experimental studies to study their lattice thermal conductivities.

Parity-Time Synthetic Phononic Media

DEFF Research Database (Denmark)

Christensen, Johan; Willatzen, Morten; Velasco, V. R.

2016-01-01

media, have been devised in many optical systems with the ground breaking potential to create nonreciprocal structures and one-way cloaks of invisibility. Here we demonstrate a feasible approach for the case of sound where the most important ingredients within synthetic materials, loss and gain......, are achieved through electrically biased piezoelectric semiconductors. We study first how wave attenuation and amplification can be tuned, and when combined, can give rise to a phononic PT synthetic media with unidirectional suppressed reflectance, a feature directly applicable to evading sonar detection....

Optical and structural characterization of GaSb and Te-doped GaSb single crystals

International Nuclear Information System (INIS)

Tirado-Mejia, L.; Villada, J.A.; Rios, M. de los; Penafiel, J.A.; Fonthal, G.; Espinosa-Arbelaez, D.G.; Ariza-Calderon, H.; Rodriguez-Garcia, M.E.

2008-01-01

Optical and structural properties of GaSb and Te-doped GaSb single crystals are reported herein. Utilizing the photoreflectance technique, the band gap energy for doped samples was obtained at 0.814 eV. Photoluminescence (PL) spectra showed a peak at 0.748 eV that according to this research, belongs to electronic states of pure GaSb and not to the longitudinal optical (LO) phonon replica as has been reported by other authors. Analysis of the full width at half maximum (FWHM) values of X-ray diffraction, as well as micro-Raman peaks showed that the inclusion of Te decreases the crystalline quality

Electron mobility variance in the presence of an electric field: Electron-phonon field-induced tunnel scattering

International Nuclear Information System (INIS)

Melkonyan, S.V.

2012-01-01

The problem of electron mobility variance is discussed. It is established that in equilibrium semiconductors the mobility variance is infinite. It is revealed that the cause of the mobility variance infinity is the threshold of phonon emission. The electron-phonon interaction theory in the presence of an electric field is developed. A new mechanism of electron scattering, called electron-phonon field-induced tunnel (FIT) scattering, is observed. The effect of the electron-phonon FIT scattering is explained in terms of penetration of the electron wave function into the semiconductor band gap in the presence of an electric field. New and more general expressions for the electron-non-polar optical phonon scattering probability and relaxation time are obtained. The results show that FIT transitions have principle meaning for the mobility fluctuation theory: mobility variance becomes finite.

The Effect of Phonon Relaxation Process on Absorption Spectra ...

African Journals Online (AJOL)

In this work we study the effect of phonon relaxation process on the absorption spectra using the Green's function technique. The Green's function technique which is widely used in many particle problems is used to solve the Kubo formula which describes the optical absorption process. Finally the configurational diagram is ...

Longitudinal analysis of progression in glaucoma using spectral-domain optical coherence tomography.

Science.gov (United States)

Wessel, Julia M; Horn, Folkert K; Tornow, Ralf P; Schmid, Matthias; Mardin, Christian Y; Kruse, Friedrich E; Juenemann, Anselm G; Laemmer, Robert

2013-05-01

To compare the longitudinal loss of RNFL thickness measurements by SD-OCT in healthy individuals and glaucoma patients with or without progression concerning optic disc morphology. A total of 62 eyes, comprising 38 glaucomatous eyes with open angle glaucoma and 24 healthy controls, were included in the study (Erlangen Glaucoma Registry, NTC00494923). All patients were investigated annually over a period of 3 years by Spectralis SD-OCT measuring peripapillary RNFL thickness. By masked comparative analysis of photographs, the eyes were classified into nonprogressive and progressive glaucoma cases. Longitudinal loss of RNFL thickness was compared with morphological changes of optic disc morphology. Mixed model analysis of annual OCT scans revealed an estimated annual decrease of the RNFL thickness by 2.12 μm in glaucoma eyes with progression, whereas glaucoma eyes without progression in optic disc morphology lost 1.18 μm per year in RNFL thickness (P = 0.002). The rate of change in healthy eyes was 0.60 μm and thereby also significantly lower than in glaucoma eyes with progression (P < 0.001). The intrasession variability of three successive measurements without head repositioning was 1.5 ± 0.7 μm. The loss of mean RNFL thickness exceeded the intrasession variability in 60% of nonprogressive eyes, and in 85% of progressive eyes after 3 years. LONGITUDINAL MEASUREMENTS OF RNFL THICKNESS USING SD-OCT SHOW A MORE PRONOUNCED REDUCTION OF RNFL THICKNESS IN PATIENTS WITH PROGRESSION COMPARED WITH PATIENTS WITHOUT PROGRESSION IN GLAUCOMATOUS OPTIC DISC CHANGES. (www.clinicaltrials.gov number, NTC00494923.).

Phonon and free-charge carrier properties in group-III nitride heterostructures investigated by spectroscopic ellipsometry and optical Hall effect

Science.gov (United States)

Schoeche, Stefan

The material class of group-III nitrides gained tremendous technological importance for optoelectronic and high-power/high-frequency amplification devices. Tunability of the direct band gap from 0.65 eV (InN) to 6.2 eV (AlN) by alloying, high breakthrough voltages and intrinsic mobilities, as well as the formation of highly mobile 2d electron gases (2DEG) at heterointerfaces make these compounds ideal for many applications. GaN and Ga-rich alloys are well studied and current research is mainly device-oriented. For example, choice and quality of the gate dielectric significantly influence device performance in high-electron mobility transistors (HEMT) which utilize highly mobile 2DEGs at heterointerfaces. Experimental access to the 2DEG channel properties without influence from parasitic currents or contact properties are desirable. In- and Al-rich ternary alloys are less explored than Ga-rich compounds. For InN and In-rich alloys, while many material parameters such as stiffness constants or effective mass values are largely unknown, reliable p-type doping is a major challenge, also because p-type conducting channels are buried within highly conductive n-type material formed at the surface and interfaces preventing electrical characterization. For AlN and high-Al content alloys, doping mechanisms are not understood and reliable fabrication of material with high free-charge carrier (FCC) concentrations was achieved just recently. Difficulties to form ohmic contacts impair electrical measurements and optical characterization is impeded by lack of high-energy excitation sources. In this work, spectroscopic ellipsometry over the wide spectral range from the THz to VUV in combination with optical Hall effect (generalized ellipsometry with applied magnetic field) from THz to MIR are applied in order to investigate the phonon modes and FCC properties in group-III nitride heterostructures. Adequate model descriptions and analysis strategies are introduced which allow

Longitudinally extensive optic neuritis in neuromyelitis optica spectrum disorder.

Science.gov (United States)

Pula, John H; Kattah, Jorge C; Keung, Bonnie; Wang, Huaping; Daily, Jennifer

2014-10-15

Neuomyelitis optica, sarcoid, and multiple sclerosis can all cause optic neuritis. Further means of distinguishing the causes of optic neuritis among these etiologies would be valuable for the clinician. This is a retrospective, cohort study from a single university based hospital and neuro-ophthalmology clinic. Blinded interpretation of orbit MRIs was performed on patients with acute optic neuritis from multiple sclerosis (n=25), sarcoid (n=5) and neuromyelitis optica spectrum disorder (n=6). A length of >40 mm anterior visual pathway enhancement distinguished neuromyelitis optica spectrum disorder from multiple sclerosis (p=0.0376). No statistically significant differences were found for presence of pain or papillitis, however there was a trend for bilateral involvement and chiasmal involvement in neuromyelitis optica spectrum disorder compared to multiple sclerosis. In acute optic neuritis, enhancing anterior visual pathway lesion length >40 mm helps differentiate neuromyelitis optica spectrum disorder from multiple sclerosis. This degree of involvement can be considered longitudinally extensive optic neuritis. Further characterization is necessary as this degree of enhancement occurs in other clinical syndromes besides neuromyelitis optica. Copyright © 2014 Elsevier B.V. All rights reserved.

Supra-ballistic phonons

International Nuclear Information System (INIS)

Russell, F.M.

1989-05-01

Energetic particles moving with a solid, either from nuclear reactions or externally injected, deposit energy by inelastic scattering processes which eventually appears as thermal energy. If the transfer of energy occurs in a crystalline solid then it is possible to couple some of the energy directly to the nuclei forming the lattice by generating phonons. In this paper the transfer of energy from a compound excited nucleus to the lattice is examined by introducing a virtual particle Π. It is shown that by including a Π in the nuclear reaction a substantial amount of energy can be coupled directly to the lattice. In the lattice this particle behaves as a spatially localized phonon of high energy, the so-called supra-ballistic phonon. By multiple inelastic scattering the supra-ballistic phonon eventually thermalizes. Because both the virtual particle Π and the equivalent supra-ballistic phonon have no charge or spin and can only exist within a lattice it is difficult to detect other than by its decay into thermal phonons. The possibility of a Π removing excess energy from a compound nucleus formed by the cold fusion of deuterium is examined. (Author)

From Planck's quanta to phonon in solids

International Nuclear Information System (INIS)

Martinez- Duart, J. M; Melo, O. de

2008-01-01

Planck's 1900 published results on the black body radiation had the first application in the quantification of radiation. This quantum hypothesis explained several noteworthy light- matter interaction effects in 1905. These were the electron emission, Stokes law and gas ionization. As soon as two years later, A. Einstein derived an expression for the specific heat of solids, applying the quantum hypothesis to the mechanical oscillation of the atoms. In the present work, the main ideas which led to the concept of phonon are discussed. From an historical point of view, the developments due to Einstein, Born, Debye, among others are analyzed and most important properties of the phonons are presented. Finally, the importance of this entity in the theory of solids is explained, in particular regarding the thermal and optical properties as well as the electrical conductivity

Acousto-optical interaction of surface acoustic and optical waves in a two-dimensional phoxonic crystal hetero-structure cavity.

Science.gov (United States)

Ma, Tian-Xue; Zou, Kui; Wang, Yue-Sheng; Zhang, Chuanzeng; Su, Xiao-Xing

2014-11-17

Phoxonic crystal is a promising material for manipulating sound and light simultaneously. In this paper, we theoretically demonstrate the propagation of acoustic and optical waves along the truncated surface of a two-dimensional square-latticed phoxonic crystal. Further, a phoxonic crystal hetero-structure cavity is proposed, which can simultaneously confine surface acoustic and optical waves. The interface motion and photoelastic effects are taken into account in the acousto-optical coupling. The results show obvious shifts in eigenfrequencies of the photonic cavity modes induced by different phononic cavity modes. The symmetry of the phononic cavity modes plays a more important role in the single-phonon exchange process than in the case of the multi-phonon exchange. Under the same deformation, the frequency shift of the photonic transverse electric mode is larger than that of the transverse magnetic mode.

Optimizing phonon space in the phonon-coupling model

Science.gov (United States)

Tselyaev, V.; Lyutorovich, N.; Speth, J.; Reinhard, P.-G.

2017-08-01

We present a new scheme to select the most relevant phonons in the phonon-coupling model, named here the time-blocking approximation (TBA). The new criterion, based on the phonon-nucleon coupling strengths rather than on B (E L ) values, is more selective and thus produces much smaller phonon spaces in the TBA. This is beneficial in two respects: first, it curbs the computational cost, and second, it reduces the danger of double counting in the expansion basis of the TBA. We use here the TBA in a form where the coupling strength is regularized to keep the given Hartree-Fock ground state stable. The scheme is implemented in a random-phase approximation and TBA code based on the Skyrme energy functional. We first explore carefully the cutoff dependence with the new criterion and can work out a natural (optimal) cutoff parameter. Then we use the freshly developed and tested scheme for a survey of giant resonances and low-lying collective states in six doubly magic nuclei looking also at the dependence of the results when varying the Skyrme parametrization.

Depth dependent modification of optical constants arising from H+ implantation in n-type 4H-SiC measured using coherent acoustic phonons

Directory of Open Access Journals (Sweden)

Andrey Baydin

2016-06-01

Full Text Available Silicon carbide (SiC is a promising material for new generation electronics including high power/high temperature devices and advanced optical applications such as room temperature spintronics and quantum computing. Both types of applications require the control of defects particularly those created by ion bombardment. In this work, modification of optical constants of 4H-SiC due to hydrogen implantation at 180 keV and at fluences ranging from 1014 to 1016 cm−2 is reported. The depth dependence of the modified optical constants was extracted from coherent acoustic phonon spectra. Implanted spectra show a strong dependence of the 4H-SiC complex refractive index depth profile on H+ fluence. These studies provide basic insight into the dependence of optical properties of 4H silicon carbide on defect densities created by ion implantation, which is of relevance to the fabrication of SiC-based photonic and optoelectronic devices.

Phononic crystals fundamentals and applications

CERN Document Server

Adibi, Ali

2016-01-01

This book provides an in-depth analysis as well as an overview of phononic crystals. This book discusses numerous techniques for the analysis of phononic crystals and covers, among other material, sonic and ultrasonic structures, hypersonic planar structures and their characterization, and novel applications of phononic crystals. This is an ideal book for those working with micro and nanotechnology, MEMS (microelectromechanical systems), and acoustic devices. This book also: Presents an introduction to the fundamentals and properties of phononic crystals Covers simulation techniques for the analysis of phononic crystals Discusses sonic and ultrasonic, hypersonic and planar, and three-dimensional phononic crystal structures Illustrates how phononic crystal structures are being deployed in communication systems and sensing systems.

Hyperbolic phonon polaritons in hexagonal boron nitride (Conference Presentation)

Science.gov (United States)

Dai, Siyuan; Ma, Qiong; Fei, Zhe; Liu, Mengkun; Goldflam, Michael D.; Andersen, Trond; Garnett, William; Regan, Will; Wagner, Martin; McLeod, Alexander S.; Rodin, Alexandr; Zhu, Shou-En; Watanabe, Kenji; Taniguchi, T.; Dominguez, Gerado; Thiemens, Mark; Castro Neto, Antonio H.; Janssen, Guido C. A. M.; Zettl, Alex; Keilmann, Fritz; Jarillo-Herrero, Pablo; Fogler, Michael M.; Basov, Dmitri N.

2016-09-01

Uniaxial materials whose axial and tangential permittivities have opposite signs are referred to as indefinite or hyperbolic media. While hyperbolic responses are normally achieved with metamaterials, hexagonal boron nitride (hBN) naturally possesses this property due to the anisotropic phonons in the mid-infrared. Using scattering-type scanning near-field optical microscopy, we studied polaritonic phenomena in hBN. We performed infrared nano-imaging of highly confined and low-loss hyperbolic phonon polaritons in hBN. The polariton wavelength was shown to be governed by the hBN thickness according to a linear law persisting down to few atomic layers [1]. Additionally, we carried out the modification of hyperbolic response in meta-structures comprised of a mononlayer graphene deposited on hBN [2]. Electrostatic gating of the top graphene layer allows for the modification of wavelength and intensity of hyperbolic phonon polaritons in bulk hBN. The physics of the modification originates from the plasmon-phonon coupling in the hyperbolic medium. Furthermore, we demonstrated the "hyperlens" for subdiffractional focusing and imaging using a slab of hBN [3]. References [1] S. Dai et al., Science, 343, 1125 (2014). [2] S. Dai et al., Nature Nanotechnology, 10, 682 (2015). [3] S. Dai et al., Nature Communications, 6, 6963 (2015).

Photon-phonon laser on crystalline silicon: a feasibility study

International Nuclear Information System (INIS)

Zadernovsky, A A

2015-01-01

We discuss a feasibility of photon-phonon laser action in bulk silicon with electron population inversion. It is well known, that only direct gap semiconductors are used as an active medium in optical lasers. In indirect gap semiconductors, such as crystalline silicon, the near-to-gap radiative electron transitions must be assisted by emission or absorption of phonons to conserve the momentum. The rate of such two-quantum transitions is much less than in direct gap semiconductors, where the similar radiative transitions are single-quantum. As a result, the quantum efficiency of luminescence in silicon is too small to get it as a laser material. Numerous proposals to overcome this problem are aimed at increasing the rate of radiative recombination. We suggest enhancing the quantum efficiency of luminescence in silicon by stimulating the photon part of the two-quantum transitions by light from an appropriate external laser source. This allows us to obtain initially an external-source-assisted lasing in silicon and then a true photon-phonon lasing without any external source of radiation. Performed analysis revealed a number of requirements to the silicon laser medium (temperature, purity and perfection of crystals) and to the intensity of stimulating radiation. We discuss different mechanisms that may hinder the implementation of photon-phonon lasing in silicon

Research on FBG-based longitudinal-acousto-optic modulator with Fourier mode coupling method.

Science.gov (United States)

Li, Zhuoxuan; Pei, Li; Liu, Chao; Ning, Tigang; Yu, Shaowei

2012-10-20

Fourier mode coupling model was first applied to achieve the spectra property of a fiber Bragg grating (FBG)-based longitudinal-acousto-optic modulator. Compared with traditional analysis algorithms, such as the transfer matrix method, the Fourier mode coupling model could improve the computing efficiency up to 100 times with a guarantee of accuracy. In this paper, based on the theoretical analysis of this model, the spectra characteristics of the modulator in different frequencies and acoustically induced strains were numerically simulated. In the experiment, a uniform FBG was modulated by acoustic wave (AW) at 12 different frequencies. In particular, the modulator responses at 563 and 885.5 KHz with three different lead zirconate titanate (PZT) loads applied were plotted for illustration, and the linear fitting of experimental data demonstrated a good match with the simulation result. The acoustic excitation of the longitudinal wave is obtained using a conic silica horn attached to the surface of a shear-mode PZT plate paralleled to the fiber axis. This way of generating longitudinal AW with a transversal PZT may shed light on the optimal structural design for the FBG-based longitudinal-acousto-optic modulator.

Surface phonons on Bi2Sr2CaCu2O8+δ

Science.gov (United States)

Phelps, R. B.; Akavoor, P.; Kesmodel, L. L.; Demuth, J. E.; Mitzi, D. B.

1993-11-01

We report measurements of surface optical phonons on Bi2Sr2CaCu2O8+δ with high-resolution electron-energy-loss spectroscopy (HREELS). In addition to peaks near 50 and 80 meV (403 and 645 cm-1), which have been previously observed, our loss spectra exhibit a peak at 26 meV (210 cm-1). Loss spectra were measured at temperatures from 45 to 146 K, and the temperature dependence of the peaks was found to be weak. The 50 and 80 meV peaks shift to lower frequency by ~1.5 meV over this temperature range. All three peaks are attributed to surface optical phonons. The identification of particular bulk modes corresponding to the surface modes observed with HREELS is discussed.

Acoustic phonons mediated non-equilibrium spin current in the presence of Rashba and Dresselhaus spin–orbit couplings

International Nuclear Information System (INIS)

Hasanirokh, K.; Phirouznia, A.

2013-01-01

Influence of electrons interaction with longitudinal acoustic phonons on magnetoelectric and spin-related transport effects are investigated. The considered system is a two-dimensional electron gas system with both Rashba and Dresselhaus spin–orbit couplings. The works which have previously been performed in this field, have revealed that the Rashba and Dresselhaus couplings cannot be responsible for spin current in the non-equilibrium regime. In the current Letter, a semiclassical method was employed using the Boltzmann approach and it was shown that the spin current of the system, in general, does not go all the way to zero when the electron–phonon coupling is taken into account. It was also shown that spin accumulation of the system could be influenced by electron–phonon coupling.

Modeling of phonon- and Coulomb-mediated capture processes in quantum dots

DEFF Research Database (Denmark)

Magnúsdóttir, Ingibjörg

2003-01-01

This thesis describes modeling of carrier relaxation processes in self-assembled quantum-dot-structures, with particular emphasis on carrier capture processes in quantum dots. Relaxation by emission of lontitudinal optical (LO) phonons is very efficient in bulk semiconductors and nanostructures...... of higher dimensionality. Here, we investigate carrier capture processes into quantum dots, mediated by emission of one and two LO phonons. In these investigations is is assumed that the dot is empty initially. In the Case of single-phonon capture we also investigate the influence of the presence...... of a charge in the quantum-dot state to which the capture takes place. In general, capture rates are of the same order as capture rates into an empty dot state, but in some cases the dot-size interval for which the capture process is energetically allowed, is considerably reduced.The above calculations...

Investigation of the dispersion of phonon modes in CdI2 single crystals by a method of inelastic scattering of thermal neutrons

International Nuclear Information System (INIS)

Piroga, S.A.

2001-01-01

Experimental observation using a method of inelastic scattering of thermal neutrons the longitudinal phonons in the G-Z, G-X and G-L directions in CdI 2 singe crystal has been obtained. The phonon subsystem observed in the case of CdI 2 single crystals is two dimensional. This is because of the fact that interlayer interactions are weak in compare to intra layer interactions

Theory of the Influence of Phonon-Phonon and Electron-Phonon Interactions on the Scattering of Neutrons by Crystals

International Nuclear Information System (INIS)

Kokkedee, J.J.J.

1963-01-01

As predicted by harmonic theory the coherent inelastic spectrums of neutrons, scattered by a single, non-conducting crystal, for a particular angle of scattering consists of a number of delta-function peaks superposed on a continuous background. The peaks correspond to one-phonon processes in which one phonon is absorbed or emitted by the neutron; the background arises from multi-phonon processes. When anharmonic forces (phonon-phonon interactions) are present, the delta-function peaks are broadened into finite peaks, while their central frequencies are shifted with respect to the harmonic values. In the case of a metal there is in addition to phonon-phonon interactions an interaction between phonons and conduction electrons, which also gives a contribution to the displacement and broadening oftheone-phononpeaks. Continuing earlier work of Van Hove (sho considered the relatively simple case of a non-conductin crystal in its ground state (T = 0 o K) ), we have studied the shifts and widths of the scattering peaks as a 'result of the above-mentioned interactions by means of many particle perturbation theory, making extensive use of diagram techniques. Prerequisite to the entire discussion is the assumption that, independent of the strength of the interactions, the width of each peak is small compared to the value of the frequency at its centre; only then the peaks can be considered as being well defined with respect to the background to higher order in the interactions. This condition is expected to be fulfilled for temperatures which are not too high and values of the phonon wave vector which are not too large. Our procedure yields closed formulae for the partial scattering function describing the peaks, which can be evaluated to arbitrarily high accuracy. In particular an expansion for calculating the line shift and line width in powers of u/d and in terms of simple connected diagrams is obtained (u is an average atomic or ionic displacement, d is the smallest

Cavity-type hypersonic phononic crystals

International Nuclear Information System (INIS)

Sato, A; Fytas, G; Pennec, Y; Djafari-Rouhani, B; Yanagishita, T; Masuda, H; Knoll, W

2012-01-01

We report on the engineering of the phonon dispersion diagram in monodomain anodic porous alumina (APA) films through the porosity and physical state of the material residing in the nanopores. Lattice symmetry and inclusion materials are theoretically identified to be the main factors which control the hypersonic acoustic wave propagation. This involves the interaction between the longitudinal and the transverse modes in the effective medium and a flat band characteristic of the material residing in the cavities. Air and filled nanopores, therefore, display markedly different dispersion relations and the inclusion materials lead to a locally resonant structural behavior uniquely determining their properties under confinement. APA films emerge as a new platform to investigate the rich acoustic phenomena of structured composite matter. (paper)

Analytical theory and method for longitudinal magneto-optical Kerr effect of optically anisotropic magnetic film

Energy Technology Data Exchange (ETDEWEB)

Wang, Xiao, E-mail: sps_wangx@ujn.edu.cn [School of Physics and Technology, University of Jinan, Jinan 250022 (China); School of Material Science and Engineering, University of Jinan, Jinan 250022 (China); Lian, Jie [School of Information Science and Engineering, Shandong University, Jinan 250100 (China); Li, Ping; Xu, XiJin [School of Physics and Technology, University of Jinan, Jinan 250022 (China); Li, MengMeng [School of Information Science and Engineering, Shandong University, Jinan 250100 (China)

2017-01-15

The Fresnel equations are solved to analyze the reflection and propagation properties of the ordinary and extraordinary light of the optically anisotropic magnetic film. Using the boundary and propagation matrix, the longitudinal magneto-optical Kerr rotation expression is derived. After that, simulations are performed on optically anisotropic and isotropic Co/SiO{sub 2} film. Results show that for Co material in the thin-film limit, the anisotropic Co can provide larger max rotations than the isotropic Co in the visible region. This is because that the refractive index discrepancy of optically anisotropic Co film reduces the Fresnel reflective coefficient r{sub pp,} which improves the Kerr rotation. This makes the optically anisotropic Co film more effective in magneto optical sensor design and device fabrication. - Highlights: • In this work, using the boundary matrix and media propagation matrix developed by Zak and S.D.Bader,we get the analytical solution of the magneto-optical Kerr rotation of the optical anisotropic magnetic film. • Results show that for film in the thin-film limit, the anisotropic Co can provide larger maximum rotations than the isotropic Co. • The improvement of Kerr rotation can be attributed to the refractive index discrepancy of optically anisotropic Co film which reduce the Fresnel reflective coefficient rpp.

Electric field-induced valley degeneracy lifting in uniaxial strained graphene: evidence from magnetophonon resonance

OpenAIRE

Assili, Mohamed; Haddad, Sonia; Kang, Woun

2015-01-01

A double peak structure in the magneto-phonon resonance (MPR) spectrum of uniaxial strained graphene, under crossed electric and magnetic fields, is predicted. We focus on the $\\Gamma$ point optical phonon modes coupled to the inter-Landau level transitions $0 \\leftrightarrows \\pm 1$ where MPR is expected to be more pronounced at high magnetic field. We derive the frequency shifts and the broadenings of the longitudinal (LO) and transverse (TO) optical phonon modes taking into account the eff...

Phonon operators for deformed nuclei

International Nuclear Information System (INIS)

Solov'ev, V.G.

1982-01-01

The mathematical formalism with the phonon operators independent of the signature of the angular momentum projection turns out to be inadequate for describing excited states of deformed nuclei. New phonon operators are introduced which depend on the signature of the angular momentum projection on the symmetry axis of a deformed nucleus. It is shown that the calculations with the new phonons take correctly into account the Pauli principle in two-phonon components of wave functions. The results obtained differ from those given by the phonons independent of the signature of the angular momentum projection. The new phonons must be used in deformed nuclei at taking systematically the Pauli principle into account and in calculations involving wave functions of excited states having components with more than one-phonon operator

Blue and red shifted temperature dependence of implicit phonon shifts in graphene

Science.gov (United States)

Mann, Sarita; Jindal, V. K.

2017-07-01

We have calculated the implicit shift for various modes of frequency in a pure graphene sheet. Thermal expansion and Grüneisen parameter which are required for implicit shift calculation have already been studied and reported. For this calculation, phonon frequencies are obtained using force constants derived from dynamical matrix calculated using VASP code where the density functional perturbation theory (DFPT) is used in interface with phonopy software. The implicit phonon shift shows an unusual behavior as compared to the bulk materials. The frequency shift is large negative (red shift) for ZA and ZO modes and the value of negative shift increases with increase in temperature. On the other hand, blue shift arises for all other longitudinal and transverse modes with a similar trend of increase with increase in temperature. The q dependence of phonon shifts has also been studied. Such simultaneous red and blue shifts in transverse or out plane modes and surface modes, respectively leads to speculation of surface softening in out of plane direction in preference to surface melting.

Controlling competing electronic orders via non-equilibrium acoustic phonons

Science.gov (United States)

Schuett, Michael; Orth, Peter; Levchenko, Alex; Fernandes, Rafael

The interplay between multiple electronic orders is a hallmark of strongly correlated systems displaying unconventional superconductivity. While doping, pressure, and magnetic field are the standard knobs employed to assess these different phases, ultrafast pump-and-probe techniques opened a new window to probe these systems. Recent examples include the ultrafast excitation of coherent optical phonons coupling to electronic states in cuprates and iron pnictides. In this work, we demonstrate theoretically that non-equilibrium acoustic phonons provide a promising framework to manipulate competing electronic phases and favor unconventional superconductivity over other states. In particular, we show that electrons coupled to out-of-equilibrium anisotropic acoustic phonons enter a steady state in which the effective electronic temperature varies around the Fermi surface. Such a momentum-dependent temperature can then be used to selectively heat electronic states that contribute primarily to density-wave instabilities, reducing their competition with superconductivity. We illustrate this phenomenon by computing the microscopic steady-state phase diagram of the iron pnictides, showing that superconductivity is enhanced with respect to the competing antiferromagnetic phase.

Comparison of three empirical force fields for phonon calculations in CdSe quantum dots

Energy Technology Data Exchange (ETDEWEB)

Kelley, Anne Myers [Chemistry and Chemical Biology, University of California, Merced, 5200 North Lake Road, Merced, California 95343 (United States)

2016-06-07

Three empirical interatomic force fields are parametrized using structural, elastic, and phonon dispersion data for bulk CdSe and their predictions are then compared for the structures and phonons of CdSe quantum dots having average diameters of ~2.8 and ~5.2 nm (~410 and ~2630 atoms, respectively). The three force fields include one that contains only two-body interactions (Lennard-Jones plus Coulomb), a Tersoff-type force field that contains both two-body and three-body interactions but no Coulombic terms, and a Stillinger-Weber type force field that contains Coulombic interactions plus two-body and three-body terms. While all three force fields predict nearly identical peak frequencies for the strongly Raman-active “longitudinal optical” phonon in the quantum dots, the predictions for the width of the Raman peak, the peak frequency and width of the infrared absorption peak, and the degree of disorder in the structure are very different. The three force fields also give very different predictions for the variation in phonon frequency with radial position (core versus surface). The Stillinger-Weber plus Coulomb type force field gives the best overall agreement with available experimental data.

Phonon properties of americium phosphide

Energy Technology Data Exchange (ETDEWEB)

Arya, B. S., E-mail: bsarya13@yahoo.com [Department of Physics, Govt. Narmada P G College, Hoshangabad -461001 (India); Aynyas, Mahendra [Department of Physics, C. S. A. Govt. P. G. College Sehore-46601 (India); Sanyal, S. P. [Department of Physics, Barkatullah University, Bhopal-462026 (India)

2016-05-23

Phonon properties of AmP have been studied by using breathing shell models (BSM) which includes breathing motion of electrons of the Am atoms due to f-d hybridization. The phonon dispersion curves, specific heat calculated from present model. The calculated phonon dispersion curves of AmP are presented follow the same trend as observed in uranium phosphide. We discuss the significance of this approach in predicting the phonon dispersion curves of these compounds and examine the role of electron-phonon interaction.

Peierls transition with acoustic phonons and twist deformation in carbon nanotubes

NARCIS (Netherlands)

Figge, M. T.; Mostovoy, M. V.; Knöster, J.

1999-01-01

Submitted to: Phys. Rev. Lett. Abstract: We consider the Peierls instability due to the interaction of electrons with both acoustic and optical phonons. We suggest that such a transition takes place in carbon nanotubes with small radius. The topological excitations and the temperature dependence of

Infrared optical responses of wurtzite In{sub x}Ga{sub 1−x}N thin films with porous surface morphology

Energy Technology Data Exchange (ETDEWEB)

Yew, P., E-mail: paulinevcu@hotmail.com [Institute of Nano-Optoelectronics Research and Technology (INOR), Universiti Sains Malaysia, 11800 Minden, Penang (Malaysia); School of Physics, Universiti Sains Malaysia, 11800 Minden, Penang (Malaysia); Lee, S.C. [School of Physics, Universiti Sains Malaysia, 11800 Minden, Penang (Malaysia); Centre of Excellence for Pre-University Studies, INTI International College Penang, Laureate International University, 1-Z, Lebuh Bukit Jambul, 11900 Penang (Malaysia); Ng, S.S. [Institute of Nano-Optoelectronics Research and Technology (INOR), Universiti Sains Malaysia, 11800 Minden, Penang (Malaysia); Hassan, H. Abu [School of Physics, Universiti Sains Malaysia, 11800 Minden, Penang (Malaysia); Chen, W.L. [Department of Electronic Engineering, National Changhua University of Education, 500, Taiwan, ROC (China); Osipowicz, T.; Ren, M.Q. [Centre for Ion Beam Applications (CIBA), Department of Physics, National University of Singapore, Singapore 119260 (Singapore)

2016-03-31

Room temperature infrared (IR) optical responses of wurtzite indium gallium nitride (In{sub x}Ga{sub 1−x}N) in the composition range of 0.174 ≤ x ≤ 0.883 were investigated by the polarized IR reflectance spectroscopy. Analyses of the amplitudes of oscillation fringes in the non-reststrahlen region revealed that the high frequency dielectric constants of the samples were unusually smaller than the values predicted from the Clausius–Mossotti relation. This odd behavior was attributed to the porous surface morphology of the In{sub x}Ga{sub 1−x}N samples. The E{sub 1} optical phonon modes of the In{sub x}Ga{sub 1−x}N were deduced from the composition dependent reststrahlen features. The obtained values were compared to those calculated through the modified random element iso-displacement (MREI) model. The deviation between the measured data and the MREI model prediction were explained in detail from the aspects of strain, thermal expansion and anharmonic phonon-coupling. Finally, it was found that the large discrepancy of the E{sub 1}(LO) mode is mainly attributed to the effects of the longitudinal phonon–plasmon coupling. - Highlights: • Composition dependence of E{sub 1} modes of In{sub x}Ga{sub 1−x}N alloys (x = 0.174 to 0.883) • Dependence of porous morphology on infrared (IR) optical properties • Longitudinal phonon–plasmon coupling obscures determination of E{sub 1}(LO) mode.

Coherent generation of symmetry-forbidden phonons by light-induced electron-phonon interactions in magnetite

Science.gov (United States)

Borroni, S.; Baldini, E.; Katukuri, V. M.; Mann, A.; Parlinski, K.; Legut, D.; Arrell, C.; van Mourik, F.; Teyssier, J.; Kozlowski, A.; Piekarz, P.; Yazyev, O. V.; Oleś, A. M.; Lorenzana, J.; Carbone, F.

2017-09-01

Symmetry breaking across phase transitions often causes changes in selection rules and emergence of optical modes which can be detected via spectroscopic techniques or generated coherently in pump-probe experiments. In second-order or weakly first-order transitions, fluctuations of the ordering field are present above the ordering temperature, giving rise to intriguing precursor phenomena, such as critical opalescence. Here, we demonstrate that in magnetite (Fe3O4 ) light excitation couples to the critical fluctuations of the charge order and coherently generates structural modes of the ordered phase above the critical temperature of the Verwey transition. Our findings are obtained by detecting coherent oscillations of the optical constants through ultrafast broadband spectroscopy and analyzing their dependence on temperature. To unveil the coupling between the structural modes and the electronic excitations, at the origin of the Verwey transition, we combine our results from pump-probe experiments with spontaneous Raman scattering data and theoretical calculations of both the phonon dispersion curves and the optical constants. Our methodology represents an effective tool to study the real-time dynamics of critical fluctuations across phase transitions.

Phonon operators in deformed nuclei

International Nuclear Information System (INIS)

Soloviev, V.G.

1981-01-01

For the description of the excited states in deformed nuclei new phonon operators are introduced, which depend on the sign of the angular momentum projection onto the symmetry axis of a deformed nucleus. In the calculations with new phonons the Pauli principle is correctly taken into account in the two-phonon components of the wave functions. There is a difference in comparison with the calculation with phonons independent of the sign of the angular momentum projection. The new phonons should be used in deformed nuclei if the Pauli principle is consistently taken into account and in the calculations with the excited state wave functions having the components with more than one phonon operator [ru

Surface phonons on Bi2Sr2CaCu2O8+δ

International Nuclear Information System (INIS)

Phelps, R.B.; Akavoor, P.; Kesmodel, L.L.; Demuth, J.E.; Mitzi, D.B.

1993-01-01

We report measurements of surface optical phonons on Bi 2 Sr 2 CaCu 2 O 8+δ with high-resolution electron-energy-loss spectroscopy (HREELS). In addition to peaks near 50 and 80 meV (403 and 645 cm -1 ), which have been previously observed, our loss spectra exhibit a peak at 26 meV (210 cm -1 ). Loss spectra were measured at temperatures from 45 to 146 K, and the temperature dependence of the peaks was found to be weak. The 50 and 80 meV peaks shift to lower frequency by ∼1.5 meV over this temperature range. All three peaks are attributed to surface optical phonons. The identification of particular bulk modes corresponding to the surface modes observed with HREELS is discussed

Electron-phonon interactions and the phonon anomaly in β-phase NiTi

International Nuclear Information System (INIS)

Zhao, G.L.; Harmon, B.N.

1993-01-01

The electronic structure of β-phase NiTi has been calculated using a first-principles linear-combination-of-atomic-orbitals method. The resulting band structure was fitted with a nonorthogonal tight-binding Hamiltonian from which electron-phonon matrix elements were evaluated. The soft phonon near Q 0 =(2/3, 2) / (3 ,0)π/a, which is responsible for the premartensitic phase transition in β-phase NiTi, is found to arise from the strong electron-phonon coupling of nested electronic states on the Fermi surface. Thermal vibrations and changes in electronic occupation cause a smearing of the nested features, which in turn cause a hardening of the phonon anomaly

Quasiparticle-phonon nuclear model

International Nuclear Information System (INIS)

Soloviev, V.G.

1977-01-01

The general assumptions of the quasiparticle-phonon model of complex nuclei are given. The choice of the model hamiltonian as an average field and residual forces is discussed. The phonon description and quasiparticle-phonon interaction are presented. The system of basic equations and their approximate solutions are obtained. The approximation is chosen so as to obtain the most correct description of few-quasiparticle components rather than of the whole wave function. The method of strenght functions is presented, which plays a decisive role in practical realization of the quasiparticle-phonon model for the description of some properties of complex nuclei. The range of applicability of the quasiparticle-phonon nuclear model is determined as few-quasiparticle components of the wave functions at low, intermediate and high excitation energies averaged in a certain energy interval

Phonon Self-Energy Corrections to Nonzero Wave-Vector Phonon Modes in Single-Layer Graphene

Science.gov (United States)

Araujo, P. T.; Mafra, D. L.; Sato, K.; Saito, R.; Kong, J.; Dresselhaus, M. S.

2012-07-01

Phonon self-energy corrections have mostly been studied theoretically and experimentally for phonon modes with zone-center (q=0) wave vectors. Here, gate-modulated Raman scattering is used to study phonons of a single layer of graphene originating from a double-resonant Raman process with q≠0. The observed phonon renormalization effects are different from what is observed for the zone-center q=0 case. To explain our experimental findings, we explored the phonon self-energy for the phonons with nonzero wave vectors (q≠0) in single-layer graphene in which the frequencies and decay widths are expected to behave oppositely to the behavior observed in the corresponding zone-center q=0 processes. Within this framework, we resolve the identification of the phonon modes contributing to the G⋆ Raman feature at 2450cm-1 to include the iTO+LA combination modes with q≠0 and also the 2iTO overtone modes with q=0, showing both to be associated with wave vectors near the high symmetry point K in the Brillouin zone.

Geometrical and band-structure effects on phonon-limited hole mobility in rectangular cross-sectional germanium nanowires

International Nuclear Information System (INIS)

Tanaka, H.; Mori, S.; Morioka, N.; Suda, J.; Kimoto, T.

2014-01-01

We calculated the phonon-limited hole mobility in rectangular cross-sectional [001], [110], [111], and [112]-oriented germanium nanowires, and the hole transport characteristics were investigated. A tight-binding approximation was used for holes, and phonons were described by a valence force field model. Then, scattering probability of holes by phonons was calculated taking account of hole-phonon interaction atomistically, and the linearized Boltzmann's transport equation was solved to calculate the hole mobility at low longitudinal field. The dependence of the hole mobility on nanowire geometry was analyzed in terms of the valence band structure of germanium nanowires, and it was found that the dependence was qualitatively reproduced by considering an average effective mass and the density of states of holes. The calculation revealed that [110] germanium nanowires with large height along the [001] direction show high hole mobility. Germanium nanowires with this geometry are also expected to exhibit high electron mobility in our previous work, and thus they are promising for complementary metal-oxide-semiconductor (CMOS) applications

Vacuum phonon tunneling.

Science.gov (United States)

Altfeder, Igor; Voevodin, Andrey A; Roy, Ajit K

2010-10-15

Field-induced phonon tunneling, a previously unknown mechanism of interfacial thermal transport, has been revealed by ultrahigh vacuum inelastic scanning tunneling microscopy (STM). Using thermally broadened Fermi-Dirac distribution in the STM tip as in situ atomic-scale thermometer we found that thermal vibrations of the last tip atom are effectively transmitted to sample surface despite few angstroms wide vacuum gap. We show that phonon tunneling is driven by interfacial electric field and thermally vibrating image charges, and its rate is enhanced by surface electron-phonon interaction.

The longitudinal optical conductivity in bilayer graphene and other two-dimensional systems

Energy Technology Data Exchange (ETDEWEB)

Yang, C.H., E-mail: chyang@nuist.edu.cn [School of Physics and Optoelectronic Engineering, Nanjing University of Information Science and Technology, Nanjing 210044 (China); Ao, Z.M., E-mail: zhimin.ao@uts.edu.au [Centre for Clean Energy Technology, School of Chemistry and Forensic Science, University of Technology, Sydney ,PO Box 123, Broadway, Sydney, NSW 2007 (Australia); Wei, X.F. [West Anhui University, Luan 237012 (China); Jiang, J.J. [Department of Physics, Sanjing College, Nanjing 210012 (China)

2015-01-15

The longitudinal optical conductivity in bilayer graphene is calculated using the dielectric function by defining the density operator theoretically, while the effect of the broadening width determined by the scattering sources on the optical conductivity is also investigated. Some features, such as chirality, energy dispersion and density of state (DOS) in bilayer graphene, are similar to those in monolayer graphene and a traditional two-dimensional electron gas (2DEG). Therefore, in this paper, the bilayer graphene optical conductivity is compared with the results in these two systems. The analytical and numerical results show that the optical conductivity per graphene layer is almost a constant and close to e{sup 2}/(4ℏ), which agrees with the experimental results.

Density functional theory studies on theelectronic, structural, phonon dynamicaland thermo-stability properties of bicarbonates MHCO3, M D Li, Na, K

Energy Technology Data Exchange (ETDEWEB)

Duan, Yuhua; Zhang, Bo; Sorescu, Dan C.; Johnson, Karl; Majzoub, Eric H; Luebke, David R.

2012-07-01

The structural, electronic, phonon dispersion and thermodynamic properties of MHCO3 (M D Li, Na, K) solids were investigated using density functional theory. The calculated bulk properties for both their ambient and the high-pressure phases are in good agreement with available experimental measurements. Solid phase LiHCO3 has not yet been observed experimentally. We have predicted several possible crystal structures for LiHCO3 using crystallographic database searching and prototype electrostatic ground state modeling. Our total energy and phonon free energy .FPH/ calculations predict that LiHCO3 will be stable under suitable conditions of temperature and partial pressures of CO2 and H2O. Our calculations indicate that the HCO 3 groups in LiHCO3 and NaHCO3 form an infinite chain structure through O H O hydrogen bonds. In contrast, the HCO 3 anions form dimers, .HCO 3 /2, connected through double hydrogen bonds in all phases of KHCO3. Based on density functional perturbation theory, the Born effective charge tensor of each atom type was obtained for all phases of the bicarbonates. Their phonon dispersions with the longitudinal optical–transverse optical splitting were also investigated. Based on lattice phonon dynamics study, the infrared spectra and the thermodynamic properties of these bicarbonates were obtained. Over the temperature range 0–900 K, the FPH and the entropies (S) of MHCO3 (M D Li, Na, K) systems vary as FPH.LiHCO3/ > FPH.NaHCO3/ > FPH.KHCO3/ and S.KHCO3/ > S.NaHCO3/ > S.LiHCO3/, respectively, in agreement with the available experimental data. Analysis of the predicted thermodynamics of the CO2 capture reactions indicates that the carbonate/bicarbonate transition reactions for Na and K could be used for CO2 capture technology, in agreement with experiments.

Real-Time Observation of Exciton-Phonon Coupling Dynamics in Self-Assembled Hybrid Perovskite Quantum Wells.

Science.gov (United States)

Ni, Limeng; Huynh, Uyen; Cheminal, Alexandre; Thomas, Tudor H; Shivanna, Ravichandran; Hinrichsen, Ture F; Ahmad, Shahab; Sadhanala, Aditya; Rao, Akshay

2017-11-28

Self-assembled hybrid perovskite quantum wells have attracted attention due to their tunable emission properties, ease of fabrication, and device integration. However, the dynamics of excitons in these materials, especially how they couple to phonons, remains an open question. Here, we investigate two widely used materials, namely, butylammonium lead iodide (CH 3 (CH 2 ) 3 NH 3 ) 2 PbI 4 and hexylammonium lead iodide (CH 3 (CH 2 ) 5 NH 3 ) 2 PbI 4 , both of which exhibit broad photoluminescence tails at room temperature. We performed femtosecond vibrational spectroscopy to obtain a real-time picture of the exciton-phonon interaction and directly identified the vibrational modes that couple to excitons. We show that the choice of the organic cation controls which vibrational modes the exciton couples to. In butylammonium lead iodide, excitons dominantly couple to a 100 cm -1 phonon mode, whereas in hexylammonium lead iodide, excitons interact with phonons with frequencies of 88 and 137 cm -1 . Using the determined optical phonon energies, we analyzed photoluminescence broadening mechanisms. At low temperatures (photoluminescence line shape observed in hybrid perovskite quantum wells and provide insights into the mechanism of exciton-phonon coupling in these materials.

Phonon self-energy corrections to non-zero wavevector phonon modes in single-layer graphene

Science.gov (United States)

Araujo, Paulo; Mafra, Daniela; Sato, Kentaro; Saito, Richiiro; Kong, Jing; Dresselhaus, Mildred

2012-02-01

Phonon self-energy corrections have mostly been studied theoretically and experimentally for phonon modes with zone-center (q = 0) wave-vectors. Here, gate-modulated Raman scattering is used to study phonons of a single layer of graphene (1LG) in the frequency range from 2350 to 2750 cm-1, which shows the G* and the G'-band features originating from a double-resonant Raman process with q 0. The observed phonon renormalization effects are different from what is observed for the zone-center q = 0 case. To explain our experimental findings, we explored the phonon self-energy for the phonons with non-zero wave-vectors (q 0) in 1LG in which the frequencies and decay widths are expected to behave oppositely to the behavior observed in the corresponding zone-center q = 0 processes. Within this framework, we resolve the identification of the phonon modes contributing to the G* Raman feature at 2450 cm-1 to include the iTO+LA combination modes with q 0 and the 2iTO overtone modes with q = 0, showing both to be associated with wave-vectors near the high symmetry point K in the Brillouin zone.

Surface dependent behaviour of CdS LO-phonon mode

International Nuclear Information System (INIS)

Molina-Contreras, J R; Medina-Gutierrez, C; Frausto-Reyes, C; Trejo-Vazquez, R; Villalobos-Pina, F J; Romo-Luevano, G; Calixto, S

2007-01-01

In this paper, we develop a sensitive optical method to monitor the surface roughness in the investigation of surfaces. By applying this method to measure the RMS surface roughness of various surfaces, we found RMS values which are comparable to those obtained by atomic force microscopy measurements. In addition, we present a simple empirical model to calculate the RMS surface roughness which shows very good agreement with the surface roughness measurements taken by the method reported in this paper. Finally, the application of our method to the study of the LO-phonon mode of CdS suggests that its intensity is dominated by the surface roughness. This roughness dependent behaviour of the CdS LO-phonon mode is experimentally confirmed by using an excitation wavelength near its E 0 transition

Phonon and magnon scattering of Bi{sub 2}Fe{sub 4}O{sub 9} ceramic

Energy Technology Data Exchange (ETDEWEB)

Sharma, Poorva, E-mail: vdinesh33@rediffmail.com, E-mail: vdinesh33@rediffmail.com; Kumar, Ashwini, E-mail: vdinesh33@rediffmail.com, E-mail: vdinesh33@rediffmail.com; Varshney, Dinesh, E-mail: vdinesh33@rediffmail.com, E-mail: vdinesh33@rediffmail.com [School of Physics, Vigyan Bhawan, Devi Ahilya University, Khandwa Road Campus, Indore-452001 (India)

2014-04-24

We report the phonon structure of Bi{sub 2}Fe{sub 4}O{sub 9} ceramics as synthesized by solid-state reaction route. Rietveld refined X-ray diffraction patterns confirmed the formation of single-phase perovskite structure and all the peaks of Bi{sub 2}Fe{sub 4}O{sub 9} perfectly indexed to the orthorhombic (space group Pbam). Raman scattering measurements identifies 12A{sub g}+1B{sub 2g}+1B{sub 3g} Raman active optical phonon modes. Apart from phonon scattering, mode at 470 cm{sup −1} is observed which is due to magnon scattering. The P-E loop infers paraelectric nature of Bi{sub 2}Fe{sub 4}O{sub 9}.

A first principle study for the comparison of phonon dispersion of armchair carbon and silicon nanotubes

International Nuclear Information System (INIS)

Chandel, Surjeet Kumar; Kumar, Arun; Bharti, Ankush; Sharma, Raman

2015-01-01

Using first principles density functional theoretical calculations, the present paper reports a systematic study of phonon dispersion curves in pristine carbon (CNT) and silicon nanotubes (SiNT) having chirality (6,6) in the armchair configuration. Some of the phonon modes are found to have negative frequencies which leads to instability of the systems under study. The number of phonon branches has been found to be thrice as much as the number of atoms. The frequency of the higher optical bands varies from 1690 to 1957 cm −1 for CNT(6,6) while it is 596 to 658 cm −1 for SiNT

On-chip plasmonic cavity-enhanced spontaneous emission rate at the zero-phonon line

DEFF Research Database (Denmark)

Siampour, Hamidreza; Kumar, Shailesh; Bozhevolnyi, Sergey I.

Highly confined surface plasmon polariton (SPP) modes can be utilized to enhance light-matter interaction at the single emitter level of quantum optical systems [1-4]. Dielectric-loaded SPP waveguides (DLSPPWs) confine SPPs laterally with relatively low propagation loss, enabling to benefit both ...... and an up to 42-fold spontaneous emission rate enhancement at the zero-phonon line (a ∼7-fold resonance enhancement in addition to a ∼6-fold broadband enhancement) is achieved, revealing the potential of our approach for on-chip realization of quantum-optical networks....... from a large Purcell factor and from a large radiative efficiency (low quenching rates) [1, 2]. In this work, we present a DLSPPW-based Bragg cavity resonator to direct emission from a single diamond nitrogen vacancy (NV) center into the zero-phonon line (Fig. 1). A quality factor of ∼70 for the cavity...

Phonon number measurements using single photon opto-mechanics

International Nuclear Information System (INIS)

Basiri-Esfahani, S; Akram, U; Milburn, G J

2012-01-01

We describe a system composed of two coupled optical cavity modes with a coupling modulated by a bulk mechanical resonator. In addition, one of the cavity modes is irreversibly coupled to a single photon source. Our scheme is an opto-mechanical realization of the Jaynes–Cummings model where the qubit is a dual rail optical qubit while the bosonic degree of freedom is a matter degree of freedom realized as the bulk mechanical excitation. We show the possibility of engineering phonon number states of the mechanical oscillator in such a system by computing the conditional state of the mechanics after successive photon counting measurements. (paper)

Ballistic phonon transport in holey silicon.

Science.gov (United States)

Lee, Jaeho; Lim, Jongwoo; Yang, Peidong

2015-05-13

When the size of semiconductors is smaller than the phonon mean free path, phonons can carry heat with no internal scattering. Ballistic phonon transport has received attention for both theoretical and practical aspects because Fourier's law of heat conduction breaks down and the heat dissipation in nanoscale transistors becomes unpredictable in the ballistic regime. While recent experiments demonstrate room-temperature evidence of ballistic phonon transport in various nanomaterials, the thermal conductivity data for silicon in the length scale of 10-100 nm is still not available due to experimental challenges. Here we show ballistic phonon transport prevails in the cross-plane direction of holey silicon from 35 to 200 nm. The thermal conductivity scales linearly with the length (thickness) even though the lateral dimension (neck) is as narrow as 20 nm. We assess the impact of long-wavelength phonons and predict a transition from ballistic to diffusive regime using scaling models. Our results support strong persistence of long-wavelength phonons in nanostructures and are useful for controlling phonon transport for thermoelectrics and potential phononic applications.

Far-infrared phonon spectroscopy of Pb1-xMn xTe layers grown by molecular beam epitaxy

International Nuclear Information System (INIS)

Romcevic, N.; Nadolny, A.J.; Romcevic, M.; Story, T.; Taliashvili, B.; Milutinovic, A.; Trajic, J.; Lusakowska, E.; Vasiljevic-Radovic, D.; Domukhovski, V.; Osinniy, V.; Hadzic, B.; Dziawa, P.

2007-01-01

In this paper we used far-infrared spectroscopy, reflection high energy electron diffraction (RHEED), X-ray diffraction and atomic force microscopy (AFM) to investigate structural and optical properties of Pb 1-x Mn x Te layers grown by molecular beam epitaxy (MBE). A numerical model for calculating the reflectivity coefficient for complex systems which include films, buffer layer and substrate has been applied. The infrared reflectivity spectra consist of Pb 1-x Mn x Te phonons, which exhibit intermediate one-two mode behavior, and MnTe phonons. A good agreement between calculated and experimental spectra is achieved. We registered the local distribution of Mn impurities depending on substrate type. For films growth on BaF 2 substrate we registered the orthorhombic local structure of MnTe clusters, while in the case of KCl substrate this structure is cubic. The Pb 1-x Mn x Te long wavelength optical phonons were described by the modified Genzel's model

Phonon dispersion in vanadium

International Nuclear Information System (INIS)

Ivanov, A.S.; Rumiantsev, A.Yu.

1999-01-01

Complete text of publication follows. Phonon dispersion curves in Vanadium metal are investigated by neutron inelastic scattering using three-axis spectrometers. Due to extremely low coherent scattering amplitude of neutrons in natural isotope mixture of vanadium the phonon frequencies could be determined in the energy range below about 15 meV. Several phonon groups were measured with the polarised neutron scattering set-up. It is demonstrated that the intensity of coherent inelastic scattering observed in the non-spin-flip channel vanishes in the spin-flip channel. The phonon density of states is measured on a single crystal keeping the momentum transfer equal to a vector of reciprocal lattice where the coherent inelastic scattering is suppressed. Phonon dispersion curves in vanadium, as measured by neutron and earlier by X-ray scattering, are described in frames of a charge-fluctuation model involving monopolar and dipolar degrees of freedom. The model parameters are compared for different transition metals with body-centred cubic-structure. (author)

Electron-phonon interaction and scattering in Si and Ge: Implications for phonon engineering

International Nuclear Information System (INIS)

Tandon, Nandan; Albrecht, J. D.; Ram-Mohan, L. R.

2015-01-01

We report ab-initio results for electron-phonon (e-ph) coupling and display the existence of a large variation in the coupling parameter as a function of electron and phonon dispersion. This variation is observed for all phonon modes in Si and Ge, and we show this for representative cases where the initial electron states are at the band gap edges. Using these e-ph matrix elements, which include all possible phonon modes and electron bands within a relevant energy range, we evaluate the imaginary part of the electron self-energy in order to obtain the associated scattering rates. The temperature dependence is seen through calculations of the scattering rates at 0 K and 300 K. The results provide a basis for understanding the impacts of phonon scattering vs. orientation and geometry in the design of devices, and in analysis of transport phenomena. This provides an additional tool for engineering the transfer of energy from carriers to the lattice

Origin of the "waterfall" effect in phonon dispersion of relaxor perovskites.

Science.gov (United States)

Hlinka, J; Kamba, S; Petzelt, J; Kulda, J; Randall, C A; Zhang, S J

2003-09-05

We have undertaken an inelastic neutron scattering study of the perovskite relaxor ferroelectric Pb(Zn(1/3)Nb(2/3))O3 with 8% PbTiO3 (PZN-8%PT) in order to elucidate the origin of the previously reported unusual kink on the low frequency transverse phonon dispersion curve (known as the "waterfall effect"). We show that its position (q(wf)) depends on the choice of the Brillouin zone and that the relation of q(wf) to the size of the polar nanoregions is highly improbable. The waterfall phenomenon is explained in the framework of a simple model of coupled damped harmonic oscillators representing the acoustic and optic phonon branches.

Phonon assisted electronic transition in telluric acid ammonium phosphate single crystals

Science.gov (United States)

El-Muraikhi, M.; Kassem, M. E.; Al-Houty, L.

The effect of gamma-irradiation on the absorption optical spectra of telluric acid ammonium phosphate single crystals (TAAP) has been studied, in the wave length of 200-600 nm, for samples irradiated by various doses up to 10 Mrad. The results show that the electron phonon coupling constant increases with the irradiation dose.

Nonlinear electron-phonon heat exchange

International Nuclear Information System (INIS)

Woods, L.M.; Mahan, G.D.

1998-01-01

A calculation of the energy exchange between phonons and electrons is done for a metal at very low temperatures. We consider the energy exchange due to two-phonon processes. Second-order processes are expected to be important at temperatures less than 1 K. We include two different second-order processes: (i) the Compton-like scattering of phonons, and (ii) the electron-dual-phonon scattering from the second-order electron-phonon interaction. It is found that the Compton-like process contains a singular energy denominator. The singularity is removed by introducing quasiparticle damping. For pure metals we find that the energy exchange depends upon the lifetime of the electrons and it is proportional to the temperature of the lattice as T L 8 . The same calculation is performed for the electron-dual-phonon scattering and it is found that the temperature dependence is T L 9 . The results can be applied to quantum dot refrigerators. copyright 1998 The American Physical Society

Self-consistency in the phonon space of the particle-phonon coupling model

Science.gov (United States)

Tselyaev, V.; Lyutorovich, N.; Speth, J.; Reinhard, P.-G.

2018-04-01

In the paper the nonlinear generalization of the time blocking approximation (TBA) is presented. The TBA is one of the versions of the extended random-phase approximation (RPA) developed within the Green-function method and the particle-phonon coupling model. In the generalized version of the TBA the self-consistency principle is extended onto the phonon space of the model. The numerical examples show that this nonlinear version of the TBA leads to the convergence of results with respect to enlarging the phonon space of the model.

Enhanced creation of dispersive monolayer phonons in Xe/Pt(111) by inelastic helium atom scattering at low energies

DEFF Research Database (Denmark)

Hansen, Flemming Yssing; Bruch, Ludwig Walter

2007-01-01

Conditions likely to lead to enhanced inelastic atomic scattering that creates shear horizontal (SH) and longitudinal acoustic (LA) monolayer phonons are identified, specifically examining the inelastic scattering of He-4 atoms by a monolayer solid of Xe/Pt(111) at incident energies of 2-25 meV. ...

Investigation of proton damage in III-V semiconductors by optical spectroscopy

Energy Technology Data Exchange (ETDEWEB)

Yaccuzzi, E.; Giudici, P. [Departamento Energía Solar, Centro Atómico Constituyentes, Av. Gral. Paz 1499, 1650 San Martín (Argentina); CONICET, Godoy Cruz 2290 (C1425FQB), CABA (Argentina); Khachadorian, S.; Strittmatter, A.; Hoffmann, A. [Institut für Festkörperphysik, Technische Universität Berlin, Hardenbergstr. 36, 10623 Berlin (Germany); Suárez, S. [Laboratorio de Colisiones Atómicas, Centro Atómico Bariloche, E. Bustillo 9500, 8400 Bariloche (Argentina); CONICET, Godoy Cruz 2290 (C1425FQB), CABA (Argentina); Reinoso, M. [Departamento Física Experimental, Centro Atómico Constituyentes, Av. Gral. Paz 1499, 1650 San Martín (Argentina); CONICET, Godoy Cruz 2290 (C1425FQB), CABA (Argentina); Goñi, A. R. [ICREA, Passeig Lluís Companys 23, 08010 Barcelona (Spain); Institut de Ciència de Materials de Barcelona (ICMAB-CSIC), Campus UAB, 08193 Bellaterra (Spain)

2016-06-21

We studied the damage produced by 2 MeV proton radiation on epitaxially grown InGaP/GaAs structure by means of spatially resolved Raman and photoluminescence (PL) spectroscopy. The irradiation was performed parallel to the sample surface in order to determine the proton penetration range in both compounds. An increase in the intensity of longitudinal optical phonons and a decrease in the luminescence were observed. We associate these changes with the creation of defects in the damaged region, also responsible for the observed change of the carrier concentration in the GaAs layer, determined by the shift of the phonon-plasmon coupled mode frequency. From the spatially resolved profile of the PL and phonon intensities, we obtained the proton range in both materials and we compared them with stopping and range of ions in matter simulations. The comparison between the experimentally obtained proton range and simulations shows a very good agreement for GaAs but a discrepancy of 20% for InGaP. This discrepancy can be explained in terms of limitations of the model to simulate the electronic orbitals and bonding structure of the simulated compound. In order to overcome this limitation, we propose an increase in 40% in the electronic stopping power for InGaP.

Electron-phonon contribution to the phonon and excited electron (hole) linewidths in bulk Pd

International Nuclear Information System (INIS)

Sklyadneva, I Yu; Leonardo, A; Echenique, P M; Eremeev, S V; Chulkov, E V

2006-01-01

We present an ab initio study of the electron-phonon (e-ph) coupling and its contribution to the phonon linewidths and to the lifetime broadening of excited electron and hole states in bulk Pd. The calculations, based on density-functional theory, were carried out using a linear-response approach in the plane-wave pseudopotential representation. The obtained results for the Eliashberg spectral function α 2 F(ω), e-ph coupling constant λ, and the contribution to the lifetime broadening, Γ e-ph , show strong dependence on both the energy and momentum of an electron (hole) state. The calculation of phonon linewidths gives, in agreement with experimental observations, an anomalously large broadening for the transverse phonon mode T 1 in the Σ direction. In addition, this mode is found to contribute most strongly to the electron-phonon scattering processes on the Fermi surface

Phonon lineshapes in atom-surface scattering

Energy Technology Data Exchange (ETDEWEB)

MartInez-Casado, R [Department of Chemistry, Imperial College London, South Kensington, London SW7 2AZ (United Kingdom); Sanz, A S; Miret-Artes, S [Instituto de Fisica Fundamental, Consejo Superior de Investigaciones CientIficas, Serrano 123, E-28006 Madrid (Spain)

2010-08-04

Phonon lineshapes in atom-surface scattering are obtained from a simple stochastic model based on the so-called Caldeira-Leggett Hamiltonian. In this single-bath model, the excited phonon resulting from a creation or annihilation event is coupled to a thermal bath consisting of an infinite number of harmonic oscillators, namely the bath phonons. The diagonalization of the corresponding Hamiltonian leads to a renormalization of the phonon frequencies in terms of the phonon friction or damping coefficient. Moreover, when there are adsorbates on the surface, this single-bath model can be extended to a two-bath model accounting for the effect induced by the adsorbates on the phonon lineshapes as well as their corresponding lineshapes.

Isotopic effects on phonon anharmonicity in layered van der Waals crystals: Isotopically pure hexagonal boron nitride

Science.gov (United States)

Cuscó, Ramon; Artús, Luis; Edgar, James H.; Liu, Song; Cassabois, Guillaume; Gil, Bernard

2018-04-01

Hexagonal boron nitride (h -BN) is a layered crystal that is attracting a great deal of attention as a promising material for nanophotonic applications. The strong optical anisotropy of this crystal is key to exploit polaritonic modes for manipulating light-matter interactions in 2D materials. h -BN has also great potential for solid-state neutron detection and neutron imaging devices, given the exceptionally high thermal neutron capture cross section of the boron-10 isotope. A good knowledge of phonons in layered crystals is essential for harnessing long-lived phonon-polariton modes for nanophotonic applications and may prove valuable for developing solid-state 10BN neutron detectors with improved device architectures and higher detection efficiencies. Although phonons in graphene and isoelectronic materials with a similar hexagonal layer structure have been studied, the effect of isotopic substitution on the phonons of such lamellar compounds has not been addressed yet. Here we present a Raman scattering study of the in-plane high-energy Raman active mode on isotopically enriched single-crystal h -BN. Phonon frequency and lifetime are measured in the 80-600-K temperature range for 10B-enriched, 11B-enriched, and natural composition high quality crystals. Their temperature dependence is explained in the light of perturbation theory calculations of the phonon self-energy. The effects of crystal anisotropy, isotopic disorder, and anharmonic phonon-decay channels are investigated in detail. The isotopic-induced changes in the phonon density of states are shown to enhance three-phonon anharmonic decay channels in 10B-enriched crystals, opening the possibility of isotope tuning of the anharmonic phonon decay processes.

Spacetime representation of topological phononics

Science.gov (United States)

Deymier, Pierre A.; Runge, Keith; Lucas, Pierre; Vasseur, Jérôme O.

2018-05-01

Non-conventional topology of elastic waves arises from breaking symmetry of phononic structures either intrinsically through internal resonances or extrinsically via application of external stimuli. We develop a spacetime representation based on twistor theory of an intrinsic topological elastic structure composed of a harmonic chain attached to a rigid substrate. Elastic waves in this structure obey the Klein–Gordon and Dirac equations and possesses spinorial character. We demonstrate the mapping between straight line trajectories of these elastic waves in spacetime and the twistor complex space. The twistor representation of these Dirac phonons is related to their topological and fermion-like properties. The second topological phononic structure is an extrinsic structure composed of a one-dimensional elastic medium subjected to a moving superlattice. We report an analogy between the elastic behavior of this time-dependent superlattice, the scalar quantum field theory and general relativity of two types of exotic particle excitations, namely temporal Dirac phonons and temporal ghost (tachyonic) phonons. These phonons live on separate sides of a two-dimensional frequency space and are delimited by ghost lines reminiscent of the conventional light cone. Both phonon types exhibit spinorial amplitudes that can be measured by mapping the particle behavior to the band structure of elastic waves.

Correction of the exciton Bohr radius in monolayer transition metal dichalcogenides

Science.gov (United States)

Li, Run-Ze; Dong, Xi-Ying; Li, Zhi-Qing; Wang, Zi-Wu

2018-07-01

We theoretically investigate the correction of exciton Bohr radius in monolayer transition metal dichalcogenides (TMDCs) on different polar substrates arising from the exciton-optical phonon coupling, in which both the intrinsic longitudinal optical phonon and surface optical phonon modes couple with the exciton are taken into account. We find that the exciton Bohr radius is enlarged markedly due to these coupling. Moreover, it can be changed on a large scale by modulating the polarizability of polar substrate and the internal distance between the monolayer TMDCs and polar substrate. Theoretical result provides a potential explanation for the variation of the exciton Bohr radius in experimental measurement.

Glass-like phonon scattering from a spontaneous nanostructure in AgSbTe2.

Science.gov (United States)

Ma, J; Delaire, O; May, A F; Carlton, C E; McGuire, M A; VanBebber, L H; Abernathy, D L; Ehlers, G; Hong, Tao; Huq, A; Tian, Wei; Keppens, V M; Shao-Horn, Y; Sales, B C

2013-06-01

Materials with very low thermal conductivity are of great interest for both thermoelectric and optical phase-change applications. Synthetic nanostructuring is most promising for suppressing thermal conductivity through phonon scattering, but challenges remain in producing bulk samples. In crystalline AgSbTe2 we show that a spontaneously forming nanostructure leads to a suppression of thermal conductivity to a glass-like level. Our mapping of the phonon mean free paths provides a novel bottom-up microscopic account of thermal conductivity and also reveals intrinsic anisotropies associated with the nanostructure. Ground-state degeneracy in AgSbTe2 leads to the natural formation of nanoscale domains with different orderings on the cation sublattice, and correlated atomic displacements, which efficiently scatter phonons. This mechanism is general and suggests a new avenue for the nanoscale engineering of materials to achieve low thermal conductivities for efficient thermoelectric converters and phase-change memory devices.

Electron hopping and optic phonons in Eu3S4

International Nuclear Information System (INIS)

Guentherodt, G.

1981-01-01

Raman scattering on single crystals of Eu 3 S 4 does not show the allowed q=o phonon modes in the cubic phase and exhibits no new modes in the distorted low temperature phase (T 2- ions. This mode does not show any anomaly near the charge order -disorder phase transition Tsub(t)=186 K. Temperature tunable spin fluctuations associated with the temperature activated Eu 2+ → Eu 3+ electron hopping are detected in the scattering intensity, superimposed on the usual thermal spin disorder. (author)

Au{sup 3+} ion implantation on FTO coated glasses: Effect on structural, electrical, optical and phonon properties

Energy Technology Data Exchange (ETDEWEB)

Sahu, Bindu; Dey, Ranajit; Bajpai, P.K., E-mail: bajpai.pk1@gmail.com

2017-06-01

Highlights: • Effects of 11.00 MeV Au{sup 3+} ions implanted in FTO coated (thickness ≈300 nm) silicate glasses at varying fluence. • Metal clustering near the surface and subsurface region below glass-FTO interface changes electrical and optical properties significantly. • Ion implantation does not affect the crystalline structure of the coated films; however, the tetragonal distortion increases with increasing ion fluence. • Significant surface reconstruction takes place with ion beam fluence; The average roughness also decreases with increasing fluence. • The sheet resistivity increases with increasing fluence. • Raman analysis also corroborates the re-crystallization process inducing due to ion implantation. • Optical properties of the implanted surfaces changes significantly. - Abstract: Effects of 11.00 MeV Au{sup 3+} ions implanted in FTO coated (thickness ≈300 nm) silicate glasses on structural, electrical optical and phonon behavior have been explored. It has been observed that metal clustering near the surface and sub-surface region below glass-FTO interface changes electrical and optical properties significantly. Ion implantation does not affect the crystalline structure of the coated films; however, the unit cell volume decreases with increase in fluence and the tetragonal distortion (c/a ratio) also decreases systematically in the implanted samples. The sheet resistivity of the films increases from 11 × 10{sup −5} ohm-cm (in pristine) to 7.5 × 10{sup −4} ohm-cm for highest ion beam fluence ≈10{sup 15} ions/cm{sup 2}. The optical absorption decreases with increasing fluence whereas, the optical transmittance as well as reflectance increases with increasing fluence. The Raman spectra are observed at ∼530 cm{sup −1} and ∼1103 cm{sup −1} in pristine sample. The broad band at 530 cm{sup −1} shifts towards higher wave number in the irradiated samples. This may be correlated with increased disorder and strain relaxation in

Sound and heat revolutions in phononics

Science.gov (United States)

Maldovan, Martin

2013-11-01

The phonon is the physical particle representing mechanical vibration and is responsible for the transmission of everyday sound and heat. Understanding and controlling the phononic properties of materials provides opportunities to thermally insulate buildings, reduce environmental noise, transform waste heat into electricity and develop earthquake protection. Here I review recent progress and the development of new ideas and devices that make use of phononic properties to control both sound and heat. Advances in sonic and thermal diodes, optomechanical crystals, acoustic and thermal cloaking, hypersonic phononic crystals, thermoelectrics, and thermocrystals herald the next technological revolution in phononics.

Carrier-phonon interaction in semiconductor quantum dots

Energy Technology Data Exchange (ETDEWEB)

Seebeck, Jan

2009-03-10

In recent years semiconductor quantum dots have been studied extensively due to their wide range of possible applications, predominantly for light sources. For successful applications, efficient carrier scattering processes as well as a detailed understanding of the optical properties are of central importance. The aims of this thesis are theoretical investigations of carrier scattering processes in InGaAs/GaAs quantum dots on a quantum-kinetic basis. A consistent treatment of quasi-particle renormalizations and carrier kinetics for non-equilibrium conditions is presented, using the framework of non-equilibrium Green's functions. The focus of our investigations is the interaction of carriers with LO phonons. Important for the understanding of the scattering mechanism are the corresponding quasi-particle properties. Starting from a detailed study of quantum-dot polarons, scattering and dephasing processes are discussed for different temperature regimes. The inclusion of polaron and memory effects turns out to be essential for the description of the carrier kinetics in quantum-dot systems. They give rise to efficient scattering channels and the obtained results are in agreement with recent experiments. Furthermore, a consistent treatment of the carrier-LO-phonon and the carrier-carrier interaction is presented for the optical response of semiconductor quantum dots, both giving rise to equally important contributions to the dephasing. Beside the conventional GaAs material system, currently GaN based light sources are of high topical interest due to their wide range of possible emission frequencies. In this material additionally intrinsic properties like piezoelectric fields and strong band-mixing effects have to be considered. For the description of the optical properties of InN/GaN quantum dots a procedure is presented, where the material properties obtained from an atomistic tight-binding approach are combined with a many-body theory for non

The Role of C-axis Polarized Phonons in High Temperature Superconductors

International Nuclear Information System (INIS)

Timusk, T.; Homes, C. C.; Reichardt, W.

1995-01-01

We report on the optical conductivity of c-axis phonons in YBa 2 Cu 3 O 7-σ as a function of doping and temperature. At room temperature the frequencies and strengths of the modes are in good agreement with results from shell models based on neutron scattering. We discuss the apical oxygen mode which becomes asymmetric in underdoped materials and argue, with Burns, that the Au mode shifts from 570 cm -1 to 610 cm -1 for the two-fold coordinated copper sites in the chain layer in oxygen depleted materials. At low temperature there is a large transfer of c-axis phonon oscillator strength from O(4) apical and O (2, 3,) plane bending modes, to a very broad at 400 cm -1

Optical study of Dirac fermions and related phonon anomalies in the antiferromagnetic compound CaFeAsF

Science.gov (United States)

Xu, B.; Xiao, H.; Gao, B.; Ma, Y. H.; Mu, G.; Marsik, P.; Sheveleva, E.; Lyzwa, F.; Dai, Y. M.; Lobo, R. P. S. M.; Bernhard, C.

2018-05-01

We performed optical studies on CaFeAsF single crystals, a parent compound of the 1111-type iron-based superconductors that undergoes a structural phase transition from tetragonal to orthorhombic at Ts=121 K and a magnetic one to a spin density wave (SDW) state at TN=110 K. In the low-temperature optical conductivity spectrum, after the subtraction of a narrow Drude peak, we observe a pronounced singularity around 300 cm-1 that separates two regions of quasilinear conductivity. We outline that these characteristic absorption features are signatures of Dirac fermions, similar to what was previously reported for the BaFe2As2 system [Z.-G. Chen et al., Phys. Rev. Lett. 119, 096401 (2017), 10.1103/PhysRevLett.119.096401]. In support of this interpretation, we show that for the latter system this singular feature disappears rapidly upon electron and hole doping, as expected if it arises from a van Hove singularity in between two Dirac cones. Finally, we show that one of the infrared-active phonon modes (the Fe-As mode at 250 cm-1) develops a strongly asymmetric line shape in the SDW state and note that this behavior can be explained in terms of a strong coupling with the Dirac fermions.

On the interplay between phonon-boundary scattering and phonon-point-defect scattering in SiGe thin films

Science.gov (United States)

Iskandar, A.; Abou-Khalil, A.; Kazan, M.; Kassem, W.; Volz, S.

2015-03-01

This paper provides theoretical understanding of the interplay between the scattering of phonons by the boundaries and point-defects in SiGe thin films. It also provides a tool for the design of SiGe-based high-efficiency thermoelectric devices. The contributions of the alloy composition, grain size, and film thickness to the phonon scattering rate are described by a model for the thermal conductivity based on the single-mode relaxation time approximation. The exact Boltzmann equation including spatial dependence of phonon distribution function is solved to yield an expression for the rate at which phonons scatter by the thin film boundaries in the presence of the other phonon scattering mechanisms. The rates at which phonons scatter via normal and resistive three-phonon processes are calculated by using perturbation theories with taking into account dispersion of confined acoustic phonons in a two dimensional structure. The vibrational parameters of the model are deduced from the dispersion of confined acoustic phonons as functions of temperature and crystallographic direction. The accuracy of the model is demonstrated with reference to recent experimental investigations regarding the thermal conductivity of single-crystal and polycrystalline SiGe films. The paper describes the strength of each of the phonon scattering mechanisms in the full temperature range. Furthermore, it predicts the alloy composition and film thickness that lead to minimum thermal conductivity in a single-crystal SiGe film, and the alloy composition and grain size that lead to minimum thermal conductivity in a polycrystalline SiGe film.

Experimental Study of Electron and Phonon Dynamics in Nanoscale Materials by Ultrafast Laser Time-Domain Spectroscopy

Science.gov (United States)

Shen, Xiaohan

less than 100 nm was observed. The longitudinal acoustic phonon transport in silicon (Si) nanorod with confined diameter and length was investigated. The guided phonon modes in Si nanorod with different frequencies and wave vectors were observed. The mean-free-path of the guided phonons in Si nanorod was found to be larger than the effective phonon mean-free-path in Si film, because of the limited phonon scattering channels in Si nanorod. The phonon density of states and dispersion relation strongly depend on the size and boundary conditions of nanorod. Our work demonstrates the possibility of modifying the phonon transport properties in nanoscale materials by designing the size and boundary conditions, hence the control of thermal conductivity. In addition, the periodicity effect of nanostructures on acoustic phonon transport was investigated in silicon dioxide (SiO2) nanorod arrays. The lattice modes and mechanical eigenmodes were observed, and the pitch effect on lattice modes was discussed. A narrowband acoustic phonon spectroscopic technique with tunable frequency and spectral width throughout GHz frequency range has been developed to investigate the frequency-dependent acoustic phonon transport in nanoscale materials. The quadratic frequency dependence of acoustic attenuation of SiO2 and indium tin oxide (ITO) thin films was observed, and the acoustic attenuation of ITO was found to be larger than SiO2. Moreover, the acoustic control on mechanical resonance of nanoscale materials using the narrowband acoustic phonon source was demonstrated in tungsten thin film.

Phonon Scattering and Confinement in Crystalline Films

Science.gov (United States)

Parrish, Kevin D.

. Removing the isotropic assumption, leading to the formulation of modal ray-tracing, corrects the mean free path distribution. The effect of phonon line-of-sight is investigated in nanoporous silicon films using free path sampling. When the line-of-sight is cut off there is a distinct change in thermal conductivity versus porosity. By analyzing the free paths of an obstructed phonon mode, it is concluded that the trend change is due to a hard upper limit on the free paths that can exist due to the nanopore geometry in the material. The transient grating technique is an optical contact-less laser based experiment for measuring the in-plane thermal diffusivity of thin films and membranes. The theory of operation and physical setup of a transient grating experiment is detailed. The procedure for extracting the thermal diffusivity from the raw experimental signal is improved upon by removing arbitrary user choice in the fitting parameters used and constructing a parameterless error minimizing procedure. The thermal conductivity of ultra-thin argon films modeled with the Lennard-Jones potential is calculated from both the Monte Carlo free path sampling technique and from explicit reduced dimensionality lattice dynamics calculations. In these ultra-thin films, the phonon properties are altered in more than a perturbative manner, referred to as the confinement regime. The free path sampling technique, which is a perturbative method, is compared to a reduced dimensionality lattice dynamics calculation where the entire film thickness is taken as the unit cell. Divergence in thermal conductivity magnitude and trend is found at few unit cell thick argon films. Although the phonon group velocities and lifetimes are affected, it is found that alterations to the phonon density of states are the primary cause of the deviation in thermal conductivity in the confinement regime.

Low frequency phononic band structures in two-dimensional arc-shaped phononic crystals

International Nuclear Information System (INIS)

Xu, Zhenlong; Wu, Fugen; Guo, Zhongning

2012-01-01

The low frequency phononic band structures of two-dimensional arc-shaped phononic crystals (APCs) were studied by the transfer matrix method in cylindrical coordinates. The results showed the first phononic band gaps (PBGs) of APCs from zero Hz with low modes. Locally resonant (LR) gaps were obtained with higher-order rotation symmetry, due to LR frequencies corresponding to the speeds of acoustic waves in the materials. These properties can be efficiently used in a structure for low frequencies that are forbidden, or in a device that permits a narrow window of frequencies. -- Highlights: ► We report a new class of quasi-periodic hetero-structures, arc-shaped phononic crystals (APCs). ► The results show the first PBGs start with zero Hz with low modes. ► Locally resonant (LR) gaps were obtained with higher-order rotation symmetry, due to LR frequencies corresponding to the speeds of acoustic waves in the materials.

Collective two-phonon states in deformed nuclei

International Nuclear Information System (INIS)

Solov'ev, V.G.; Shirikova, N.Y.

1982-01-01

The Pauli principle in the two-phonon components of the wave functions is taken into account within the framework of the quasiparticle-phonon model of the nucleus with phonon operators depending on the sign of the projection of the angular momentum. The centroid energies of collective two-phonon states in even-even deformed nuclei are calculated and it is shown that the inclusion of the Pauli principle shifts them by 1--3 MeV to higher energies. The shifts of the three-phonon poles due to the inclusion of the Pauli principle in the three-phonon components of the wave functions are calculated. Strong fragmentation of collective two-phonon states whose energy centroids are 3--5 MeV should be expected. It is concluded that collective two-phonon states need not exist in deformed nuclei. The situation with the 168 Er nucleus and the Th and U isotopes is analyzed

Acoustic Longitudinal Field NIF Optic Feature Detection Map Using Time-Reversal & MUSIC

Energy Technology Data Exchange (ETDEWEB)

Lehman, S K

2006-02-09

We developed an ultrasonic longitudinal field time-reversal and MUltiple SIgnal Classification (MUSIC) based detection algorithm for identifying and mapping flaws in fused silica NIF optics. The algorithm requires a fully multistatic data set, that is one with multiple, independently operated, spatially diverse transducers, each transmitter of which, in succession, launches a pulse into the optic and the scattered signal measured and recorded at every receiver. We have successfully localized engineered ''defects'' larger than 1 mm in an optic. We confirmed detection and localization of 3 mm and 5 mm features in experimental data, and a 0.5 mm in simulated data with sufficiently high signal-to-noise ratio. We present the theory, experimental results, and simulated results.

Anisotropy and phonon modes from analysis of the dielectric function tensor and the inverse dielectric function tensor of monoclinic yttrium orthosilicate

Science.gov (United States)

Mock, A.; Korlacki, R.; Knight, S.; Schubert, M.

2018-04-01

We determine the frequency dependence of the four independent Cartesian tensor elements of the dielectric function for monoclinic symmetry Y2SiO5 using generalized spectroscopic ellipsometry from 40-1200 cm-1. Three different crystal cuts, each perpendicular to a principle axis, are investigated. We apply our recently described augmentation of lattice anharmonicity onto the eigendielectric displacement vector summation approach [A. Mock et al., Phys. Rev. B 95, 165202 (2017), 10.1103/PhysRevB.95.165202], and we present and demonstrate the application of an eigendielectric displacement loss vector summation approach with anharmonic broadening. We obtain an excellent match between all measured and model-calculated dielectric function tensor elements and all dielectric loss function tensor elements. We obtain 23 Au and 22 Bu symmetry long-wavelength active transverse and longitudinal optical mode parameters including their eigenvector orientation within the monoclinic lattice. We perform density functional theory calculations and obtain 23 Au symmetry and 22 Bu transverse and longitudinal optical mode parameters and their orientation within the monoclinic lattice. We compare our results from ellipsometry and density functional theory and find excellent agreement. We also determine the static and above reststrahlen spectral range dielectric tensor values and find a recently derived generalization of the Lyddane-Sachs-Teller relation for polar phonons in monoclinic symmetry materials satisfied [M. Schubert, Phys Rev. Lett. 117, 215502 (2016), 10.1103/PhysRevLett.117.215502].

Neutron-Phonon Interaction Studies in Copper, Zinc and Magnesium Single Crystals

International Nuclear Information System (INIS)

Maliszewski, E.; Sosnowski, J.; Blinowski, K.; Kozubowski, J.; Padlo, L.; Sledziewska, D.

1963-01-01

The phonon dispersion relations in copper single crystals has been studied by means of a triple-axis crystal neutron spectrometer. In the [100] direction the transversal branch, not reported in the papers of Cabie and Jacrot, has been found. This branch fits well to the recent data of sound velocity; however, it differs partly from the X-ray results of Jacobsen. For the longitudinal branch in the [100] direction the dispersion curve obtained by Cribier and Jacrot is lying well above the Jacobsen's curve, and the experimental points reported in the present paper support the results of Cribier and Jacrot. The phonon dispersion relations in zinc and magnesium single crystals has been studied using the cold neutron method and by means of a triple-axis crystal neutron spectrometer as well. The scattering surfaces in the [1010] plane were traced, the AT and AL branches found and the phonon dispersion relations in the [001] and [010] directions obtained. The results have been compared with those obtained by Johnson with X-rays. In the [001] direction the present results fit well lo Johnson's foe the AL branch. In the [010] direction for the AT branch a large discrepancy has been found between Johnson's and the present results. Some explanation of this discrepancy is given. Similar measurements in the same directions in magnesium single crystals are under way and will be reported. (author) [fr

Two-phonon bound states in imperfect crystals

International Nuclear Information System (INIS)

Behera, S.N.; Samsur, Sk.

1980-01-01

The question of the occurrence of two-phonon bound states in imperfect crystals is investigated. It is shown that the anharmonicity mediated two-phonon bound state which is present in perfect crystals gets modified due to the presence of impurities. Moreover, the possibility of the occurrence of a purely impurity mediated two-phonon bound state is demonstrated. The bound state frequencies are calculated using the simple Einstein oscillator model for the host phonons. The two-phonon density of states for the imperfect crystal thus obtained has peaks at the combination and difference frequencies of two host phonons besides the peaks at the bound state frequencies. For a perfect crystal the theory predicts a single peak at the two-phonon bound state frequency in conformity with experimental observations and other theoretical calculations. Experimental data on the two-phonon infrared absorption and Raman scattering from mixed crystals of Gasub(1-c)Alsub(c)P and Gesub(1-c)Sisub(c) are analysed to provide evidence in support of impurity-mediated two-phonon bound states. The relevance of the zero frequency (difference spectrum) peak to the central peak, observed in structural phase transitions, is conjectured. (author)

Controlling competing orders via nonequilibrium acoustic phonons: Emergence of anisotropic effective electronic temperature

Science.gov (United States)

Schütt, Michael; Orth, Peter P.; Levchenko, Alex; Fernandes, Rafael M.

2018-01-01

Ultrafast perturbations offer a unique tool to manipulate correlated systems due to their ability to promote transient behaviors with no equilibrium counterpart. A widely employed strategy is the excitation of coherent optical phonons, as they can cause significant changes in the electronic structure and interactions on short time scales. One of the issues, however, is the inevitable heating that accompanies these resonant excitations. Here, we explore a promising alternative route: the nonequilibrium excitation of acoustic phonons, which, due to their low excitation energies, generally lead to less heating. We demonstrate that driving acoustic phonons leads to the remarkable phenomenon of a momentum-dependent effective temperature, by which electronic states at different regions of the Fermi surface are subject to distinct local temperatures. Such an anisotropic effective electronic temperature can have a profound effect on the delicate balance between competing ordered states in unconventional superconductors, opening a so far unexplored avenue to control correlated phases.

Splash, pop, sizzle: Information processing with phononic computing

Directory of Open Access Journals (Sweden)

Sophia R. Sklan

2015-05-01

Full Text Available Phonons, the quanta of mechanical vibration, are important to the transport of heat and sound in solid materials. Recent advances in the fundamental control of phonons (phononics have brought into prominence the potential role of phonons in information processing. In this review, the many directions of realizing phononic computing and information processing are examined. Given the relative similarity of vibrational transport at different length scales, the related fields of acoustic, phononic, and thermal information processing are all included, as are quantum and classical computer implementations. Connections are made between the fundamental questions in phonon transport and phononic control and the device level approach to diodes, transistors, memory, and logic.

Squeezed Phonons: Modulating Quantum Fluctuations of Atomic Displacements.

Science.gov (United States)

Hu, Xuedong; Nori, Franco

1997-03-01

We have studied phonon squeezed states and also put forward several proposals for their generation(On phonon parametric process, X. Hu and F. Nori, Phys. Rev. Lett. 76), 2294 (1996); on polariton mechanism, X. Hu and F. Nori, Phys. Rev. B 53, 2419 (1996); on second-order Raman scattering, X. Hu and F. Nori, preprint.. Here, we compare the relative merits and limitations of these approaches, including several factors that will limit the amount of phonon squeezing. In particular, we investigate the effect of the initial thermal states on the phonon modes. Using a model for the phonon density matrix, we also study the mixing of the phonon squeezed states with thermal states, which describes the decay of the phonon coherence. Finally, we calculate the maximum possible squeezing from a phonon parametric process limited by phonon decay.

Analysis of Lattice Thermal Conductivity of Si Considering the Effect of Phonon Dispersion on Three-phonon Processes

Science.gov (United States)

He, Ping; Li, Zhijian

2001-03-01

In this work we present the new relaxation time expressions considering the detailed information of the phonon dispersion. For the three-phonon processes, it is found that only limited types of three-phonon processes are allowed to occur and the attenuation of phonon that conduct heat varies roughly with the fifth power of frequency. By using these expressions, the data of thermal conductivity of bulk silicon is well fitted. And further, the data for thin films of single crystal silicon which cannot be well fitted by the widely used model that proposed by Holland is also well fitted using the new expressions for three-phonon processes and parameters got at the previous step.

Birefringent phononic structures

Directory of Open Access Journals (Sweden)

I. E. Psarobas

2014-12-01

Full Text Available Within the framework of elastic anisotropy, caused in a phononic crystal due to low crystallographic symmetry, we adopt a model structure, already introduced in the case of photonic metamaterials, and by analogy, we study the effect of birefringence and acoustical activity in a phononic crystal. In particular, we investigate its low-frequency behavior and comment on the factors which determine chirality by reference to this model.

Ab initio phonon thermal transport in monolayer InSe, GaSe, GaS, and alloys

Energy Technology Data Exchange (ETDEWEB)

Pandey, Tribhuwan; Parker, David S.; Lindsay, Lucas

2017-10-17

We compare vibrational properties and phonon thermal conductivities (κ) of monolayer InSe, GaSe and GaS systems using density functional theory and Peierls-Boltzmann transport methods. In going from InSe to GaSe to GaS, system mass decreases giving both increasing acoustic phonon velocities and decreasing scattering of these heat-carrying modes with optic phonons, ultimately giving κInSe< κGaSe< κGaS. This behavior is demonstrated by correlating the scattering phase space limited by fundamental conservation conditions with mode scattering rates and phonon dispersions for each material. We also show that, unlike flat monolayer systems such as graphene, thermal transport is governed by in-plane vibrations in InSe, GaSe and GaS, similar to buckled monolayer materials such as silicene. Alloying of InSe, GaSe and GaS systems provides an effective method for modulating their κ through intrinsic vibrational modifications and phonon scattering from mass disorder giving reductions ~2-3.5 times. This disorder also suppresses phonon mean free paths in the alloy systems compared to those in their crystalline counterparts. This work provides fundamental insights of lattice thermal transport from basic vibrational properties for an interesting set of two-dimensional materials.

Optical Orientation of Mn2+ Ions in GaAs in Weak Longitudinal Magnetic Fields

Science.gov (United States)

Akimov, I. A.; Dzhioev, R. I.; Korenev, V. L.; Kusrayev, Yu. G.; Sapega, V. F.; Yakovlev, D. R.; Bayer, M.

2011-04-01

We report on optical orientation of Mn2+ ions in bulk GaAs subject to weak longitudinal magnetic fields (B≤100mT). A manganese spin polarization of 25% is directly evaluated by using spin-flip Raman scattering. The dynamical Mn2+ polarization occurs due to the s-d exchange interaction with optically oriented conduction band electrons. Time-resolved photoluminescence reveals a nontrivial electron spin dynamics, where the oriented Mn2+ ions tend to stabilize the electron spins.

Study of Phonon Dispersion Relations in Cuprous Oxide by Inelastic Neutron Scattering

DEFF Research Database (Denmark)

Beg, M. M.; Shapiro, S. M.

1976-01-01

Phonon dispersion relations in Cu2O have been studied at 20°C using inelastic neutron scattering. Seven acoustic branches and twelve optical branches have been studied in detail in the three symmetry directions [00ζ], [ζζ0], and [ζζζ] of the cubic lattice. Four of the six zone-center phonons have...... been observed and the assignments and energies are confirmed as Γ25=87±2 cm-1, Γ12′=105±3 cm-1, Γ15=146±1 cm-1, and Γ2′≈347 cm-1. The dispersion relations agree only qualitatively with the rigid-ion-model calculations. It is suggested that more detailed calculations may be performed in the light...

Experimental study of surface plasmon-phonon polaritons in GaAs-based microstructures

Science.gov (United States)

Galimov, A. I.; Shalygin, V. A.; Moldavskaya, M. D.; Panevin, V. Yu; Melentyev, G. A.; Artemyev, A. A.; Firsov, D. A.; Vorobjev, L. E.; Klimko, G. V.; Usikova, A. A.; Komissarova, T. A.; Sedova, I. V.; Ivanov, S. V.

2018-03-01

Optical properties of a heavily-doped GaAs epitaxial layer with a regular grating at its surface have been experimentally investigated in the terahertz spectral range. Reflectivity spectra for the layer with a profiled surface drastically differ from those for the as-grown epilayer with a planar surface. For s-polarized radiation, this difference is totally caused by the electromagnetic wave diffraction at the grating. For p-polarized radiation, additional resonant dips arise due to excitation of surface plasmon-phonon polaritons. Terahertz radiation emission under significant electron heating in an applied pulsed electric field has also been studied. Polarization measurements revealed pronounced peaks related to surface plasmon-phonon polariton resonances of the first and second order in the emission spectra.

Thermoelectric power of YBa2Cu3O7-δ: Phonon drag and multiband conduction

International Nuclear Information System (INIS)

Cohn, J.L.; Wolf, S.A.; Selvamanickam, V.; Salama, K.

1991-01-01

We report measurements of the a-b-plane thermopower (S) on crystalline YBa 2 Cu 3 O 7-δ (δ≤0.16) for temperatures below 310 K. Much sharper features in the T dependence of S are observed than have been previously reported. Both the temperature and oxygen-doping dependences of S, which have caused confusion, are consistently accounted for by conventional metallic theory for the first time. Strong phonon drag and two carrier species are indicated, with holes on the planes and electrons on the chains. An anomalous, positive component to S, observed for T 2 planes and optical-mode phonons

Electron and phonon spectra in La2-xSrxCuI4+δ

International Nuclear Information System (INIS)

Nomerovannaya, L.V.; Makhnev, A.A.; Malyuk, A.N.; Bolotin, G.A.; Shtrapenin, G.L.; Ignatenkov, A.N.

1995-01-01

Ellipsometric measurements of optical constants and measurements of reflection spectra of La 2-x Sr x CuI 4+δ monocrystals were carried out. Variation of peculiarities of electron and phonon spectra at strontium doping was followed. Formulae to calculate ε dielectric permittivity tensor component on the ground of ellipsometric measurements for tetragonal and orthorhombic crystals are given. Effect of superstoichiometric oxygen content on anisotropy of La 2 CuO 4+δ optical properties was studied. 18 refs., 5 figs., 1 tab

Directional asymmetry of the nonlinear wave phenomena in a three-dimensional granular phononic crystal under gravity.

Science.gov (United States)

Merkel, A; Tournat, V; Gusev, V

2014-08-01

We report the experimental observation of the gravity-induced asymmetry for the nonlinear transformation of acoustic waves in a noncohesive granular phononic crystal. Because of the gravity, the contact precompression increases with depth inducing space variations of not only the linear and nonlinear elastic moduli but also of the acoustic wave dissipation. We show experimentally and explain theoretically that, in contrast to symmetric propagation of linear waves, the amplitude of the nonlinearly self-demodulated wave depends on whether the propagation of the waves is in the direction of the gravity or in the opposite direction. Among the observed nonlinear processes, we report frequency mixing of the two transverse-rotational modes belonging to the optical band of vibrations and propagating with negative phase velocities, which results in the excitation of a longitudinal wave belonging to the acoustic band of vibrations and propagating with positive phase velocity. We show that the measurements of the gravity-induced asymmetry in the nonlinear acoustic phenomena can be used to compare the in-depth distributions of the contact nonlinearity and of acoustic absorption.

Topological chiral phonons in center-stacked bilayer triangle lattices

Science.gov (United States)

Xu, Xifang; Zhang, Wei; Wang, Jiaojiao; Zhang, Lifa

2018-06-01

Since chiral phonons were found in an asymmetric two-dimensional hexagonal lattice, there has been growing interest in the study of phonon chirality, which were experimentally verified very recently in monolayer tungsten diselenide (2018 Science 359 579). In this work, we find chiral phonons with nontrivial topology in center-stacked bilayer triangle lattices. At the Brillouin-zone corners, (), circularly polarized phonons and nonzero phonon Berry curvature are observed. Moreover, we find that the phonon chirality remain robust with changing sublattice mass ratio and interlayer coupling. The chiral phonons at the valleys are demonstrated in doubler-layer sodium chloride along the [1 1 1] direction. We believe that the findings on topological chiral phonons in triangle lattices will give guidance in the study of chiral phonons in real materials and promote the phononic applications.

Enhanced photoelastic modulation in silica phononic crystal cavities

Science.gov (United States)

Kim, Ingi; Iwamoto, Satoshi; Arakawa, Yasuhiko

2018-04-01

The enhanced photoelastic modulation in quasi-one-dimensional (1D) phononic crystal (PnC) cavities made of fused silica is experimentally demonstrated. A confined acoustic wave in the cavity can induce a large birefringence through the photoelastic effect and enable larger optical modulation amplitude at the same acoustic power. We observe a phase retardation of ∼26 mrad of light passing through the cavity when the exciting acoustic frequency is tuned to the cavity mode resonance of ∼500 kHz at 2.5 V. In the present experiment, a 16-fold enhancement of retardation in the PnC cavity is demonstrated compared with that in a bar-shaped silica structure. Spatially resolved optical retardation measurement reveals that the large retardation is realized only around the cavity reflecting the localized nature of the acoustic cavity mode. The enhanced interactions between acoustic waves and light can be utilized to improve the performance of acousto-optic devices such as photoelastic modulators.

Waveguiding in supported phononic crystal plates

International Nuclear Information System (INIS)

Vasseur, J; Hladky-Hennion, A-C; Deymier, P; Djafari-Rouhani, B; Duval, F; Dubus, B; Pennec, Y

2007-01-01

We investigate, with the help of the finite element method, the existence of absolute band gaps in the band structure of a free-standing phononic crystal plate and of a phononic crystal slab deposited on a substrate. The two-dimensional phononic crystal is constituted by a square array of holes drilled in an active piezoelectric (PZT5A or AlN) matrix. For both matrix materials, an absolute band gap occurs in the band structure of the free-standing plate provided the thickness of the plate is on the order of magnitude of the lattice parameter. When the plate is deposited on a Si substrate, the absolute band gap still remains when the matrix of the phononic crystal is made of PZT5A. The AlN phononic crystal plate losses its gap when supported by the Si substrate. In the case of the PZT5A matrix, we also study the possibility of localized modes associated with a linear defect created by removing one row of air holes in the deposited phononic crystal plate

Coalescence towards exceptional contours in synthetic phononic media

DEFF Research Database (Denmark)

Christensen, Johan

2016-01-01

Parity-time symmetric media, also referred to as synthetic media, have been devised in many optical systems with the ground breaking potential to create non-reciprocal structures and one-way cloaks of invisibility. Here we demonstrate a feasible approach for the case of sound where gain and loss...... are induced via the acousto-electric effect in electrically biased piezoelectric semiconductors. We study how wave attenuation and amplification can be tuned, and when combined, can give rise to phononic synthetic media with unidirectional suppressed reflectance, a feature directly applicable to evading sonar...

Quasiparticles, phonons and beyond. Enlargement the basis of quasiparticle-phonon model

International Nuclear Information System (INIS)

Stoyanov, Ch.

2000-01-01

The version of Quasiparticle-Phonon Model (QPM) which accounts up to three-phonons is discussed. The new basis is used to study the low-lying isovector mode and the low-energy E1 transitions forbidden in the ideal boson picture. The coupling to the continuum is incorporated in the formalism of QPM. The phenomenon of trapping of states is studied in the case of high-lying states with large angular momentum. (author)

Longitudinal versus transversal excitation in doped graded-index polymer optical fibers

Science.gov (United States)

Illarramendi, M. A.; Arrue, J.; Ayesta, I.; Jiménez, F.; Zubia, J.; Bikandi, I.; Tagaya, A.; Koike, Y.

2014-03-01

In this work we perform a detailed experimental and theoretical analysis of the properties of amplified spontaneous emission (ASE) in a rhodamine-6G-doped graded-index polymer optical fiber when the fiber is pumped either longitudinally or transversally with respect to the fiber axis. The dependence of the ASE threshold and efficiency on fiber length has been compared for both schemes of excitation. A theoretical model for longitudinal excitation has been carried out by means of the laser rate equations as functions of time, distance traveled by light and wavelength. The analysis takes into account that the fiber is a typical graded-index POF in which the radial distributions of light power density and dye concentration are not uniform. The theoretical calculations agree satisfactorily with the experimental results. The photodegradation of the ASE intensity has also been measured for both pumping schemes.

Thickness-dependent coherent phonon frequency in ultrathin FeSe/SrTiO₃ films

Energy Technology Data Exchange (ETDEWEB)

Yang, Shuolong [SLAC National Accelerator Lab., Menlo Park, CA (United States); Stanford Univ., Stanford, CA (United States); Sobota, Jonathan A. [SLAC National Accelerator Lab., Menlo Park, CA (United States); Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States); Leuenberger, Dominik [SLAC National Accelerator Lab., Menlo Park, CA (United States); Stanford Univ., Stanford, CA (United States); Kemper, Alexander F. [Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States); Lee, James J. [SLAC National Accelerator Lab., Menlo Park, CA (United States); Stanford Univ., Stanford, CA (United States); Schmitt, Felix T. [SLAC National Accelerator Lab., Menlo Park, CA (United States); Stanford Univ., Stanford, CA (United States); Li, Wei [SLAC National Accelerator Lab., Menlo Park, CA (United States); Stanford Univ., Stanford, CA (United States); Moore, Rob G. [SLAC National Accelerator Lab., Menlo Park, CA (United States); Stanford Univ., Stanford, CA (United States); Kirchmann, Patrick S. [SLAC National Accelerator Lab., Menlo Park, CA (United States); Shen, Zhi -Xun [SLAC National Accelerator Lab., Menlo Park, CA (United States); Stanford Univ., Stanford, CA (United States)

2015-06-01

Ultrathin FeSe films grown on SrTiO₃ substrates are a recent milestone in atomic material engineering due to their important role in understanding unconventional superconductivity in Fe-based materials. By using femtosecond time- and angle-resolved photoelectron spectroscopy, we study phonon frequencies in ultrathin FeSe/SrTiO₃ films grown by molecular beam epitaxy. After optical excitation, we observe periodic modulations of the photoelectron spectrum as a function of pump–probe delay for 1-unit-cell, 3-unit-cell, and 60-unit-cell thick FeSe films. The frequencies of the coherent intensity oscillations increase from 5.00 ± 0.02 to 5.25 ± 0.02 THz with increasing film thickness. By comparing with previous works, we attribute this mode to the Se A_1g phonon. The dominant mechanism for the phonon softening in 1-unit-cell thick FeSe films is a substrate-induced lattice strain. Results demonstrate an abrupt phonon renormalization due to a lattice mismatch between the ultrathin film and the substrate.

Polyaniline/carbon nanotube/CdS quantum dot composites with enhanced optical and electrical properties

Science.gov (United States)

Goswami, Mrinmoy; Ghosh, Ranajit; Maruyama, Takahiro; Meikap, Ajit Kumar

2016-02-01

A new kind of polyaniline/carbon nanotube/CdS quantum dot composites have been developed via in-situ polymerization of aniline monomer in the presence of dispersed CdS quantum dots (size: 2.7-4.8 nm) and multi-walled carbon nanotubes (CNT), which exhibits enhanced optical and electrical properties. The existences of 1st order, 2nd order, and 3rd order longitudinal optical phonon modes, strongly indicate the high quality of synthesized CdS quantum dots. The occurrence of red shift of free exciton energy in photoluminescence is due to size dependent quantum confinement effect of CdS. The conductivity of the composites (for example PANI/CNT/CdS (2 wt.% CdS)) is increased by about 7 of magnitude compared to that of pure PANI indicating a charge transfer between CNT and polymer via CdS quantum dots. This advanced material has a great potential for high-performance of electro-optical applications.

Longitudinal Lorentz force on a subwavelength-diameter optical fiber

International Nuclear Information System (INIS)

Yu Huakang; Fang Wei; Gu Fuxing; Yang Zongyin; Tong Limin; Qiu Min

2011-01-01

We analyze the longitudinal Lorentz forces that a propagating continuous-wave light exerts on a subwavelength-diameter optical fiber. Our theoretical results show that, during the propagating process, the guided light exerts no net time-averaged force on the fiber. Via numerical simulation, we find a significant overall pull force of 0.4 pN/mW acting on a 450-nm-diam fiber tip at a wavelength of 980 nm due to the scattering of the end face and a calculated force distribution reveals the feature of a near-field accumulation. Our results may be helpful to the configuration of optomechanical components or devices based on these fibers.

Mutual interactions of phonons, rotons, and gravity

Science.gov (United States)

Nicolis, Alberto; Penco, Riccardo

2018-04-01

We introduce an effective point-particle action for generic particles living in a zero-temperature superfluid. This action describes the motion of the particles in the medium at equilibrium as well as their couplings to sound waves and generic fluid flows. While we place the emphasis on elementary excitations such as phonons and rotons, our formalism applies also to macroscopic objects such as vortex rings and rigid bodies interacting with long-wavelength fluid modes. Within our approach, we reproduce phonon decay and phonon-phonon scattering as predicted using a purely field-theoretic description of phonons. We also correct classic results by Landau and Khalatnikov on roton-phonon scattering. Finally, we discuss how phonons and rotons couple to gravity, and show that the former tend to float while the latter tend to sink but with rather peculiar trajectories. Our formalism can be easily extended to include (general) relativistic effects and couplings to additional matter fields. As such, it can be relevant in contexts as diverse as neutron star physics and light dark matter detection.

Dominant phonon wave vectors and strain-induced splitting of the 2D Raman mode of graphene

Science.gov (United States)

Narula, Rohit; Bonini, Nicola; Marzari, Nicola; Reich, Stephanie

2012-03-01

The dominant phonon wave vectors q* probed by the 2D Raman mode of pristine and uniaxially strained graphene are determined via a combination of ab initio calculations and a full two-dimensional integration of the transition matrix. We show that q* are highly anisotropic and rotate about K with the polarizer and analyzer condition relative to the lattice. The corresponding phonon-mediated electronic transitions show a finite component along K-Γ that sensitively determines q*. We invalidate the notion of “inner” and “outer” processes. The characteristic splitting of the 2D mode of graphene under uniaxial tensile strain and given polarizer and analyzer setting is correctly predicted only if the strain-induced distortion and red-shift of the in-plane transverse optical (iTO) phonon dispersion as well as the changes in the electronic band structure are taken into account.

Phonon broadening in high entropy alloys

Science.gov (United States)

Körmann, Fritz; Ikeda, Yuji; Grabowski, Blazej; Sluiter, Marcel H. F.

2017-09-01

Refractory high entropy alloys feature outstanding properties making them a promising materials class for next-generation high-temperature applications. At high temperatures, materials properties are strongly affected by lattice vibrations (phonons). Phonons critically influence thermal stability, thermodynamic and elastic properties, as well as thermal conductivity. In contrast to perfect crystals and ordered alloys, the inherently present mass and force constant fluctuations in multi-component random alloys (high entropy alloys) can induce significant phonon scattering and broadening. Despite their importance, phonon scattering and broadening have so far only scarcely been investigated for high entropy alloys. We tackle this challenge from a theoretical perspective and employ ab initio calculations to systematically study the impact of force constant and mass fluctuations on the phonon spectral functions of 12 body-centered cubic random alloys, from binaries up to 5-component high entropy alloys, addressing the key question of how chemical complexity impacts phonons. We find that it is crucial to include both mass and force constant fluctuations. If one or the other is neglected, qualitatively wrong results can be obtained such as artificial phonon band gaps. We analyze how the results obtained for the phonons translate into thermodynamically integrated quantities, specifically the vibrational entropy. Changes in the vibrational entropy with increasing the number of elements can be as large as changes in the configurational entropy and are thus important for phase stability considerations. The set of studied alloys includes MoTa, MoTaNb, MoTaNbW, MoTaNbWV, VW, VWNb, VWTa, VWNbTa, VTaNbTi, VWNbTaTi, HfZrNb, HfMoTaTiZr.

Temperature dependence of the optical absorption spectra of InP/ZnS quantum dots

Science.gov (United States)

Savchenko, S. S.; Vokhmintsev, A. S.; Weinstein, I. A.

2017-03-01

The optical-absorption spectra of InP/ZnS (core/shell) quantum dots have been studied in a broad temperature range of T = 6.5-296 K. Using the second-order derivative spectrophotometry technique, the energies of optical transitions at room temperature were found to be E 1 = 2.60 ± 0.02 eV (for the first peak of excitonic absorption in the InP core) and E 2 = 4.70 ± 0.02 eV (for processes in the ZnS shell). The experimental curve of E 1( T) has been approximated for the first time in the framework of a linear model and in terms of the Fan's formula. It is established that the temperature dependence of E 1 is determined by the interaction of excitons and longitudinal acoustic phonons with hω = 15 meV.

Electromagnetic decay of two-phonon states

International Nuclear Information System (INIS)

Catara, F.; Chomaz, Ph.; Van Giai, N.; Paris-11 Univ., 91 - Orsay

1991-01-01

The electromagnetic decay of two-phonon states corresponding to the multi-excitation of giant resonances is studied. The calculations are performed within a boson expansion approach and the elementary modes are constructed in random phase approximation (RPA). The rates for direct transition of two-phonon states to the ground state turn out to be not negligibly smaller than those from the (single) giant resonances. The former transitions are accompanied by a γ-ray whose energy is equal to the sum of the two phonon energies. Thus the detection of such high energy γ-rays could provide a signature of the excitation of two-phonon states. (author) 9 refs., 3 tabs

Phonon dispersion relation of uranium nitrate above and below the Neel temperature

International Nuclear Information System (INIS)

Dolling, G.; Holden, T.M.; Evensson, E.C.; Buyers, W.J.L.; Lander, G.H.

1977-01-01

Neutron coherent inelastic scattering measurements have been made of the phonon dispersion relation of uranium nitride both above and below the Neel temperature T/sub N/ = 50 K. Within the precision of the measurements, about 1% in frequency and 10% in line width and in scattered neutron intensity, no significant changes in these phonon properties were observed as a function of temperature other than those arising from population factor changes and a small stiffening of the lattice as the temperature decreases. At 4.2 K, two acoustic and two optic branches have been determined for each of the [001], [110] and [111] directions. The optic mode measurements revealed (a) a 20% variation in frequency across the Brillouin zone and (b) an interesting disposition of the LO and TO modes, such that nu/sub LO/ > nu/sub TO/ along [001] and [110], while the reverse is true along the [111] directions. Within the experimental resolution, the LO and TO modes are degenerate near q = 0. We have been unable to obtain any satisfactory description of these results on the basis of conventional theoretical treatments (e.g. rigid-ion or shell models). Other possible interpretations of the results are discussed

Phonon dispersion relation of uranium nitride above and below the Neel temperature

International Nuclear Information System (INIS)

Dolling, G.; Holden, T.M.; Svensson, E.C.; Buyers, W.J.L.; Lander, G.H.

1977-01-01

Neutron coherent inelastic scattering measurements have been made of the phonon dispersion relation of uranium nitride both above and below the Neel temperature T N = 50 K. Within the precision of the measurements, about 1% in frequency and 10% in line width and in scattered neutron intensity, no significant changes in these phonon properties were observed as a function of temperature other than those arising from population factor changes and a small stiffening of the lattice as the temperature decreases. At 4.2 K, two acoustic and two optic branches have been determined for each of the [001], [110] and [111] directions. The optic mode measurements revealed (a) a 20% variation in frequency across the Brillouin zone and (b) and interesting disposition of the LO and TO modes, such that ν LO > ν TO along [001] and [11-], while the reverse is true along the [111] directions. Within the experimental resolution, the LO and TO modes are degenerate near q = 0. We have been unable to obtain any satisfactory description of these results on the basis of conventional theoretical treatments (e.g. rigid-ion or shell models). Other possible interpretations of the results are discussed. (author)

Electron-phonon relaxation and excited electron distribution in gallium nitride

Energy Technology Data Exchange (ETDEWEB)

Zhukov, V. P. [Institute of Solid State Chemistry, Urals Branch of the Russian Academy of Sciences, Pervomayskaya st. 91, Yekaterinburg (Russian Federation); Donostia International Physics Center (DIPC), P. Manuel de Lardizabal 4, 20018 San Sebastian (Spain); Tyuterev, V. G., E-mail: valtyut00@mail.ru [Donostia International Physics Center (DIPC), P. Manuel de Lardizabal 4, 20018 San Sebastian (Spain); Tomsk State Pedagogical University, Kievskaya st. 60, Tomsk (Russian Federation); Tomsk State University, Lenin st. 36, Tomsk (Russian Federation); Chulkov, E. V. [Donostia International Physics Center (DIPC), P. Manuel de Lardizabal 4, 20018 San Sebastian (Spain); Tomsk State University, Lenin st. 36, Tomsk (Russian Federation); Departamento de Fisica de Materiales, Facultad de Ciencias Qumicas, UPV/EHU and Centro de Fisica de Materiales CFM-MPC and Centro Mixto CSIC-UPV/EHU, Apdo. 1072, 20080 San Sebastian (Spain); Echenique, P. M. [Donostia International Physics Center (DIPC), P. Manuel de Lardizabal 4, 20018 San Sebastian (Spain); Departamento de Fisica de Materiales, Facultad de Ciencias Qumicas, UPV/EHU and Centro de Fisica de Materiales CFM-MPC and Centro Mixto CSIC-UPV/EHU, Apdo. 1072, 20080 San Sebastian (Spain)

2016-08-28

We develop a theory of energy relaxation in semiconductors and insulators highly excited by the long-acting external irradiation. We derive the equation for the non-equilibrium distribution function of excited electrons. The solution for this function breaks up into the sum of two contributions. The low-energy contribution is concentrated in a narrow range near the bottom of the conduction band. It has the typical form of a Fermi distribution with an effective temperature and chemical potential. The effective temperature and chemical potential in this low-energy term are determined by the intensity of carriers' generation, the speed of electron-phonon relaxation, rates of inter-band recombination, and electron capture on the defects. In addition, there is a substantial high-energy correction. This high-energy “tail” largely covers the conduction band. The shape of the high-energy “tail” strongly depends on the rate of electron-phonon relaxation but does not depend on the rates of recombination and trapping. We apply the theory to the calculation of a non-equilibrium distribution of electrons in an irradiated GaN. Probabilities of optical excitations from the valence to conduction band and electron-phonon coupling probabilities in GaN were calculated by the density functional perturbation theory. Our calculation of both parts of distribution function in gallium nitride shows that when the speed of the electron-phonon scattering is comparable with the rate of recombination and trapping then the contribution of the non-Fermi “tail” is comparable with that of the low-energy Fermi-like component. So the high-energy contribution can essentially affect the charge transport in the irradiated and highly doped semiconductors.

Anisotropic Electron-Photon and Electron-Phonon Interactions in Black Phosphorus.

Science.gov (United States)

Ling, Xi; Huang, Shengxi; Hasdeo, Eddwi H; Liang, Liangbo; Parkin, William M; Tatsumi, Yuki; Nugraha, Ahmad R T; Puretzky, Alexander A; Das, Paul Masih; Sumpter, Bobby G; Geohegan, David B; Kong, Jing; Saito, Riichiro; Drndic, Marija; Meunier, Vincent; Dresselhaus, Mildred S

2016-04-13

Orthorhombic black phosphorus (BP) and other layered materials, such as gallium telluride (GaTe) and tin selenide (SnSe), stand out among two-dimensional (2D) materials owing to their anisotropic in-plane structure. This anisotropy adds a new dimension to the properties of 2D materials and stimulates the development of angle-resolved photonics and electronics. However, understanding the effect of anisotropy has remained unsatisfactory to date, as shown by a number of inconsistencies in the recent literature. We use angle-resolved absorption and Raman spectroscopies to investigate the role of anisotropy on the electron-photon and electron-phonon interactions in BP. We highlight, both experimentally and theoretically, a nontrivial dependence between anisotropy and flake thickness and photon and phonon energies. We show that once understood, the anisotropic optical absorption appears to be a reliable and simple way to identify the crystalline orientation of BP, which cannot be determined from Raman spectroscopy without the explicit consideration of excitation wavelength and flake thickness, as commonly used previously.

Analytical approach to phonons and electron-phonon interactions in single-walled zigzag carbon nanotubes

Energy Technology Data Exchange (ETDEWEB)

Kandemir, B S; Keskin, M [Department of Physics, Faculty of Sciences, Ankara University, 06100 Tandogan, Ankara (Turkey)

2008-08-13

In this paper, exact analytical expressions for the entire phonon spectra in single-walled carbon nanotubes with zigzag geometry are presented by using a new approach, originally developed by Kandemir and Altanhan. This approach is based on the concept of construction of a classical lattice Hamiltonian of single-walled carbon nanotubes, wherein the nearest and next nearest neighbor and bond bending interactions are all included, then its quantization and finally diagonalization of the resulting second quantized Hamiltonian. Furthermore, within this context, explicit analytical expressions for the relevant electron-phonon interaction coefficients are also investigated for single-walled carbon nanotubes having this geometry, by the phonon modulation of the hopping interaction.

Analytical approach to phonons and electron-phonon interactions in single-walled zigzag carbon nanotubes

International Nuclear Information System (INIS)

Kandemir, B S; Keskin, M

2008-01-01

In this paper, exact analytical expressions for the entire phonon spectra in single-walled carbon nanotubes with zigzag geometry are presented by using a new approach, originally developed by Kandemir and Altanhan. This approach is based on the concept of construction of a classical lattice Hamiltonian of single-walled carbon nanotubes, wherein the nearest and next nearest neighbor and bond bending interactions are all included, then its quantization and finally diagonalization of the resulting second quantized Hamiltonian. Furthermore, within this context, explicit analytical expressions for the relevant electron-phonon interaction coefficients are also investigated for single-walled carbon nanotubes having this geometry, by the phonon modulation of the hopping interaction

Time and angle resolved phonon absorption in the fractional quantum hall regime

International Nuclear Information System (INIS)

Devitt, A.M.

2000-09-01

The work described in this thesis is a study of the phonon absorption by a two-dimensional electron system (2DES) in the fractional quantum Hall regime and also at ν = 1/2. The fractional quantum Hall effect arises in 2DES's in high magnetic fields and is characterised by the quantisation of the transverse or Hall resistance and the vanishing longitudinal conductivity. The filling factor denotes the number of filled Landau levels and the quantum Hall effect occurs when this ratio is at certain rational odd denominator filling factors. The phenomenology of the effect arises due to the existence of an energy gap between the ground state and the lowest excited state. This energy gap is characterised by a deep minimum, or minima, at finite in-plane wavevector. Acoustic phonon absorption is expected to probe the energy gap at wavevectors close to or at the minimum in the dispersion curve. The experiments reported here incorporate the use of a thin film heater to produce a pulse of phonons of which a fraction are absorbed by the 2DES. A fast amplifier and signal averaging board enable detection of small signals due to absorption of phonons. The technique used allows time resolution of the phonon signal which typically takes place over a period of 10 μs or so. The time resolution enables different phonon modes to be studied. By altering the position of the heater relative to the 2DES angular resolution is also possible. The phonon absorption at several different filling factors has been investigated and the energy gaps found are in reasonable agreement with theoretical predictions. The absorption at ν 1/2 has also been investigated. Here the composite fermions are expected to have a well defined Fermi wavevector. The absorption at ν = 1/2 and the fractional quantum Hall states is found to be qualitatively and quantitatively different. We see that the change in electron temperature atν = 1/2 is much less than at ν = 1/3 due to the larger heat capacity. At ν = 1

One-dimensional hypersonic phononic crystals.

Science.gov (United States)

Gomopoulos, N; Maschke, D; Koh, C Y; Thomas, E L; Tremel, W; Butt, H-J; Fytas, G

2010-03-10

We report experimental observation of a normal incidence phononic band gap in one-dimensional periodic (SiO(2)/poly(methyl methacrylate)) multilayer film at gigahertz frequencies using Brillouin spectroscopy. The band gap to midgap ratio of 0.30 occurs for elastic wave propagation along the periodicity direction, whereas for inplane propagation the system displays an effective medium behavior. The phononic properties are well captured by numerical simulations. The porosity in the silica layers presents a structural scaffold for the introduction of secondary active media for potential coupling between phonons and other excitations, such as photons and electrons.

Coherent helix vacancy phonon and its ultrafast dynamics waning in topological Dirac semimetal C d3A s2

Science.gov (United States)

Sun, Fei; Wu, Q.; Wu, Y. L.; Zhao, H.; Yi, C. J.; Tian, Y. C.; Liu, H. W.; Shi, Y. G.; Ding, H.; Dai, X.; Richard, P.; Zhao, Jimin

2017-06-01

We report an ultrafast lattice dynamics investigation of the topological Dirac semimetal C d3A s2 . A coherent phonon beating among three evenly spaced A1 g optical phonon modes (of frequencies 1.80, 1.96, and 2.11 THz, respectively) is unambiguously observed. The two side modes originate from the counter helixes composing Cd vacancies. Significantly, such helix vacancy-induced phonon (HVP) modes experience prominent extra waning in their ultrafast dynamics as temperature increases, which is immune to the central mode. Above 200 K, the HVP becomes inactive, which may potentially affect the topological properties. Our results in the lattice degree of freedom suggest the indispensable role of temperature in considering topological properties of such quantum materials.

Coherent coupling between radio frequency, optical, and acoustic waves in piezo-optomechanical circuits

Science.gov (United States)

Balram, Krishna C.; Davanço, Marcelo I.; Song, Jin Dong; Srinivasan, Kartik

2016-01-01

Optomechanical cavities have been studied for applications ranging from sensing to quantum information science. Here, we develop a platform for nanoscale cavity optomechanical circuits in which optomechanical cavities supporting co-localized 1550 nm photons and 2.4 GHz phonons are combined with photonic and phononic waveguides. Working in GaAs facilitates manipulation of the localized mechanical mode either with a radio frequency (RF) field through the piezo-electric effect, which produces acoustic waves that are routed and coupled to the optomechanical cavity by phononic crystal waveguides, or optically through the strong photoelastic effect. Along with mechanical state preparation and sensitive readout, we use this to demonstrate an acoustic wave interference effect, similar to atomic coherent population trapping, in which RF-driven coherent mechanical motion is cancelled by optically-driven motion. Manipulating cavity optomechanical systems with equal facility through both photonic and phononic channels enables new architectures for signal transduction between the optical, electrical, and mechanical domains. PMID:27446234

Anisotropic phonon coupling in the relaxor ferroelectric (Na1/2Bi1/2)TiO3 near its high-temperature phase transition

Science.gov (United States)

Cai, Ling; Toulouse, Jean; Luo, Haosu; Tian, Wei

2014-08-01

The lead free relaxor Na1/2Bi1/2TiO3 (NBT) undergoes a structural cubic-to-tetragonal transition near 800 K which is caused by the cooperative rotations of O6 octahedra. These rotations are also accompanied by the displacements of the cations and the formation of the polar nanodomains (PNDs) that are responsible for the characteristic dielectric dispersion of relaxor ferroelectrics. Because of their intrinsic properties, spontaneous polarization, and lack of inversion symmetry, these PNDs are also piezoelectric and can mediate an interaction between polarization and strain or couple the optic and acoustic phonons. Because PNDs introduce a local tetragonal symmetry, the phonon coupling they mediate is found to be anisotropic. In this paper we present inelastic neutron scattering results on coupled transverse acoustic (TA) and transverse optic (TO) phonons in the [110] and [001] directions and across the cubic-tetragonal phase transition at TC˜800 K. The phonon spectra are analyzed using a mode coupling model. In the [110] direction, as in other relaxors and some ferroelectric perovskites, a precipitous drop of the TO phonon into the TA branch or "waterfall" is observed at a certain qwf˜0.14 r.l.u. In the [001] direction, the highly overdamped line shape can be fitted with closely positioned bare mode energies which are largely overlapping along the dispersion curves. Two competing lattice coupling mechanism are proposed to explain these observations.

High mobility In0.75Ga0.25As quantum wells in an InAs phonon lattice

Science.gov (United States)

Chen, C.; Holmes, S. N.; Farrer, I.; Beere, H. E.; Ritchie, D. A.

2018-03-01

InGaAs based devices are great complements to silicon for CMOS, as they provide an increased carrier saturation velocity, lower operating voltage and reduced power dissipation (International technology roadmap for semiconductors (www.itrs2.net)). In this work we show that In0.75Ga0.25As quantum wells with a high mobility, 15 000 to 20 000 cm2 V-1 s-1 at ambient temperature, show an InAs-like phonon with an energy of 28.8 meV, frequency of 232 cm-1 that dominates the polar-optical mode scattering from  ˜70 K to 300 K. The measured optical phonon frequency is insensitive to the carrier density modulated with a surface gate or LED illumination. We model the electron scattering mechanisms as a function of temperature and identify mechanisms that limit the electron mobility in In0.75Ga0.25As quantum wells. Background impurity scattering starts to dominate for temperatures  <100 K. In the high mobility In0.75Ga0.25As quantum well, GaAs-like phonons do not couple to the electron gas unlike the case of In0.53Ga0.47As quantum wells.

Effect of Holstein phonons on the electronic properties of graphene

OpenAIRE

Stauber, T.; Peres, N. M. R.

2007-01-01

We obtain the self-energy of the electronic propagator due to the presence of Holstein polarons within the first Born approximation. This leads to a renormalization of the Fermi velocity of one percent. We further compute the optical conductivity of the system at the Dirac point and at finite doping within the Kubo-formula. We argue that the effects due to Holstein phonons are negligible and that the Boltzmann approach which does not include inter-band transition and can thus not treat optica...

Situation with collective two-phonon states in deformed nuclei

International Nuclear Information System (INIS)

Soloviev, V.G.; Shirikova, N.Yu.

1982-01-01

Within the quasiparticle-phonon nuclear model with the operators of phonons depending on the sign of the angular momentum projection, the Pauli principle is taken into account in the two-phonon components of the wave functions. The centroid energies of the collective two-phonon states in even-even deformed nuclei are calculated. It is shown that the inclusion of the Pauli principle leads to their shift by 1-3 MeV towards high energies. The shifts of three-phonon poles due to the Pauli principle are calculated in the three-phonon components of the wave functions. The collective two-phonon states, the centroid energies of which are 3-5 MeV, are expected to be strongly fragmented. The conclusion is confirmed that the collective two-phonon states should not exist in deformed nuclei. The situation in 168 Er and in the 228 Th isotopes is analysed

Theory of Raman scattering in coupled electron-phonon systems

Science.gov (United States)

Itai, K.

1992-01-01

The Raman spectrum is calculated for a coupled conduction-electron-phonon system in the zero-momentum-transfer limit. The Raman scattering is due to electron-hole excitations and phonons as well. The phonons of those branches that contribute to the electron self-energy and the correction of the electron-phonon vertex are assumed to have flat energy dispersion (the Einstein phonons). The effect of electron-impurity scattering is also incorporated. Both the electron-phonon interaction and the electron-impurity interaction cause the fluctuation of the electron distribution between different parts of the Fermi surface, which results in overdamped zero-sound modes of various symmetries. The scattering cross section is obtained by solving the Bethe-Salpeter equation. The spectrum shows a lower threshold at the smallest Einstein phonon energy when only the electron-phonon interaction is taken into consideration. When impurities are also taken into consideration, the threshold disappears.

A holographic perspective on phonons and pseudo-phonons

Energy Technology Data Exchange (ETDEWEB)

Amoretti, Andrea [Institute of Theoretical Physics and Astrophysics, University of Würzburg,97074 Würzburg (Germany); Physique Théorique et Mathématique and International Solvay Institutes,Université Libre de Bruxelles,C.P. 231, 1050 Brussels (Belgium); Areán, Daniel [Max-Planck-Institut für Physik (Werner-Heisenberg-Institut),Föhringer Ring 6, D-80805, Munich (Germany); Argurio, Riccardo [Physique Théorique et Mathématique and International Solvay Institutes,Université Libre de Bruxelles,C.P. 231, 1050 Brussels (Belgium); Musso, Daniele [Departamento de Física de Partículas, Universidade de Santiago de Compostelaand Instituto Galego de Física de Altas Enerxías (IGFAE),E-15782, Santiago de Compostela (Spain); Zayas, Leopoldo A. Pando [Michigan Center for Theoretical Physics, Department of Physics, University of Michigan,Ann Arbor, MI 48109 (United States)

2017-05-10

We analyze the concomitant spontaneous breaking of translation and conformal symmetries by introducing in a CFT a complex scalar operator that acquires a spatially dependent expectation value. The model, inspired by the holographic Q-lattice, provides a privileged setup to study the emergence of phonons from a spontaneous translational symmetry breaking in a conformal field theory and offers valuable hints for the treatment of phonons in QFT at large. We first analyze the Ward identity structure by means of standard QFT techniques, considering both spontaneous and explicit symmetry breaking. Next, by implementing holographic renormalization, we show that the same set of Ward identities holds in the holographic Q-lattice. Eventually, relying on the holographic and QFT results, we study the correlators realizing the symmetry breaking pattern and how they encode information about the low-energy spectrum.

Phonon optimized interatomic potential for aluminum

Directory of Open Access Journals (Sweden)

Murali Gopal Muraleedharan

2017-12-01

Full Text Available We address the problem of generating a phonon optimized interatomic potential (POP for aluminum. The POP methodology, which has already been shown to work for semiconductors such as silicon and germanium, uses an evolutionary strategy based on a genetic algorithm (GA to optimize the free parameters in an empirical interatomic potential (EIP. For aluminum, we used the Vashishta functional form. The training data set was generated ab initio, consisting of forces, energy vs. volume, stresses, and harmonic and cubic force constants obtained from density functional theory (DFT calculations. Existing potentials for aluminum, such as the embedded atom method (EAM and charge-optimized many-body (COMB3 potential, show larger errors when the EIP forces are compared with those predicted by DFT, and thus they are not particularly well suited for reproducing phonon properties. Using a comprehensive Vashishta functional form, which involves short and long-ranged interactions, as well as three-body terms, we were able to better capture interactions that reproduce phonon properties accurately. Furthermore, the Vashishta potential is flexible enough to be extended to Al2O3 and the interface between Al-Al2O3, which is technologically important for combustion of solid Al nano powders. The POP developed here is tested for accuracy by comparing phonon thermal conductivity accumulation plots, density of states, and dispersion relations with DFT results. It is shown to perform well in molecular dynamics (MD simulations as well, where the phonon thermal conductivity is calculated via the Green-Kubo relation. The results are within 10% of the values obtained by solving the Boltzmann transport equation (BTE, employing Fermi’s Golden Rule to predict the phonon-phonon relaxation times.

Phonon optimized interatomic potential for aluminum

Science.gov (United States)

Muraleedharan, Murali Gopal; Rohskopf, Andrew; Yang, Vigor; Henry, Asegun

2017-12-01

We address the problem of generating a phonon optimized interatomic potential (POP) for aluminum. The POP methodology, which has already been shown to work for semiconductors such as silicon and germanium, uses an evolutionary strategy based on a genetic algorithm (GA) to optimize the free parameters in an empirical interatomic potential (EIP). For aluminum, we used the Vashishta functional form. The training data set was generated ab initio, consisting of forces, energy vs. volume, stresses, and harmonic and cubic force constants obtained from density functional theory (DFT) calculations. Existing potentials for aluminum, such as the embedded atom method (EAM) and charge-optimized many-body (COMB3) potential, show larger errors when the EIP forces are compared with those predicted by DFT, and thus they are not particularly well suited for reproducing phonon properties. Using a comprehensive Vashishta functional form, which involves short and long-ranged interactions, as well as three-body terms, we were able to better capture interactions that reproduce phonon properties accurately. Furthermore, the Vashishta potential is flexible enough to be extended to Al2O3 and the interface between Al-Al2O3, which is technologically important for combustion of solid Al nano powders. The POP developed here is tested for accuracy by comparing phonon thermal conductivity accumulation plots, density of states, and dispersion relations with DFT results. It is shown to perform well in molecular dynamics (MD) simulations as well, where the phonon thermal conductivity is calculated via the Green-Kubo relation. The results are within 10% of the values obtained by solving the Boltzmann transport equation (BTE), employing Fermi's Golden Rule to predict the phonon-phonon relaxation times.

Electron-phonon coupling in one dimension

International Nuclear Information System (INIS)

Apostol, M.; Baldea, I.

1981-08-01

The Ward identity is derived for the electron-phonon coupling in one dimension and the spectrum of elementary excitations is calculated by assuming that the Fermi distribution is not strongly distorted by interaction. The electron-phonon vertex is renormalized in the case of the forward scattering and Migdal's theorem is discussed. A model is proposed for the giant Kohn anomaly. The dip in the phonon spectrum is obtained and found to be in agreement with the experimental data for KCP. (author)

Quasiparticle--phonon model of the nucleus. V. Odd spherical nuclei

International Nuclear Information System (INIS)

Vdovin, A.I.; Voronov, V.V.; Solov'ev, V.G.; Stoyanov, C.

1985-01-01

The formalism of the quasiparticle--phonon model of the nucleus for odd spherical nuclei is presented. The exact commutation relations of the quasiparticle and phonon operators together with the anharmonic corrections for the phonon excitations are taken into account in the derivation of equations for the energies and structure coefficients of the wave functions of excited states, which include quasiparticle--phonon and quasiparticle--two-phonon components. The influence of various physical effects and of the dimension of the phonon basis on the fragmentation of the single-quasiparticle and quasiparticle-phonon states is investigated

Engineering the hypersonic phononic band gap of hybrid Bragg stacks.

Science.gov (United States)

Schneider, Dirk; Liaqat, Faroha; El Boudouti, El Houssaine; El Hassouani, Youssef; Djafari-Rouhani, Bahram; Tremel, Wolfgang; Butt, Hans-Jürgen; Fytas, George

2012-06-13

We report on the full control of phononic band diagrams for periodic stacks of alternating layers of poly(methyl methacrylate) and porous silica combining Brillouin light scattering spectroscopy and theoretical calculations. These structures exhibit large and robust on-axis band gaps determined by the longitudinal sound velocities, densities, and spacing ratio. A facile tuning of the gap width is realized at oblique incidence utilizing the vector nature of the elastic wave propagation. Off-axis propagation involves sagittal waves in the individual layers, allowing access to shear moduli at nanoscale. The full theoretical description discerns the most important features of the hypersonic one-dimensional crystals forward to a detailed understanding, a precondition to engineer dispersion relations in such structures.

Observation of magnon-phonon interaction at short wavelengths

International Nuclear Information System (INIS)

Dolling, G.; Cowley, R.A.

1966-01-01

Measurements have been made of the magnon and phonon dispersion relations in uranium dioxide at 9 o K. These measurements provide evidence of a strong interaction between the magnon and phonon excitations and enable a value to be deduced for the coupling constant. The interaction of long-wavelength magnons in ferromagnetic materials has been studied previously with ultrasonic techniques; however, inelastic scattering of slow neutrons enables both the magnon and phonon dispersion relations to be determined for short wavelengths. In those magnetic materials which have been studied by earlier workers, the magnons and phonons either interacted with one another very weakly or else their frequencies were very different. The results could then be understood without introducing any magnon-phonon interaction. In this note we report measurements of both the magnon and the phonon spectra of antiferromagnetic uranium dioxide, which lead to a magnon-phonon coupling constant of 9.6 ± 1.6 o K. Since the Neel temperature is 30.8 o K, this coupling constant is of a similar magnitude to the direct magnetic interactions. (author)

LDRD final report on theory and exploration of quantum-dot optical nonlinearities and coherences

International Nuclear Information System (INIS)

Chow, Weng Wah

2008-01-01

A microscopic theory for investigating quantum-dot optical properties was developed. The theory incorporated advances on various aspects of quantum-dot physics developed at Sandia and elsewhere. Important components are a non-Markovian treatment of polarization dephasing due to carrier-carrier scattering (developed at Sandia) and a nonperturbative treatment within a polaron picture of the scattering of carriers by longitudinal-optical phonons (developed at Bremen University). A computer code was also developed that provides a detailed accounting of electronic structure influences and a consistent treatment of many-body effects, the latter via the incorporation of results from the microscopic theory. This code was used to explore quantum coherence physics in a quantum-dot system. The investigation furthers the understanding of the underlying differences between atomic quantum coherence and semiconductor quantum coherence, and helps improve the potential of using quantum coherences in quantum computing, coherent control and high-resolution spectroscopy

Electron-Mediated Phonon-Phonon Coupling Drives the Vibrational Relaxation of CO on Cu(100)

Science.gov (United States)

Novko, D.; Alducin, M.; Juaristi, J. I.

2018-04-01

We bring forth a consistent theory for the electron-mediated vibrational intermode coupling that clarifies the microscopic mechanism behind the vibrational relaxation of adsorbates on metal surfaces. Our analysis points out the inability of state-of-the-art nonadiabatic theories to quantitatively reproduce the experimental linewidth of the CO internal stretch mode on Cu(100) and it emphasizes the crucial role of the electron-mediated phonon-phonon coupling in this regard. The results demonstrate a strong electron-mediated coupling between the internal stretch and low-energy CO modes, but also a significant role of surface motion. Our nonadiabatic theory is also able to explain the temperature dependence of the internal stretch phonon linewidth, thus far considered a sign of the direct anharmonic coupling.

Phonon anomalies and electron-phonon coupling of metal surfaces and thin films; Phononenanomalien und Elektron-Phonon-Kopplung an Metalloberflaechen und duennen Schichten

Energy Technology Data Exchange (ETDEWEB)

Flach, B.

2000-01-01

This thesis has two topics: One is the investigation of an adsorbate induced phonon anomaly on W(110) and Mo{sub 1-x}Re{sub x}(110) (x = 5, 15, 25%) with inelastic helium atom scattering (HAS). The other one is the study of the growth, morphology and dynamics of ultra-thin lithium films deposited on W(110). In 1992 a giant phonon anomaly was found by J. Luedecke on the hydrogen saturated W(110) and Mo(110) surfaces. The anomaly consists of a deep and sharp indentation in the phonon dispersion curves in which the phonon energy nearly drops to zero ({omega}{sub 1}). In addition, a small and broad dip in the surface Rayleigh mode is observed ({omega}{sub 2}). The anomaly appears in the anti {gamma}-H- as well as in the anti {gamma}-S-direction of the surface Brillouin zone (SBZ). Since its first discovery, numerous other experimental and theoretical studies have followed. In the present work the effects is reinvestigated and experimental parameters, such as the crystal temperature and the incident energy, were changed in order to study their influence on the anomalous phonon behavior. In the case of H/Mo(110) the substrate was changed as well by alloying with small amounts of rhenium. In the present experiments a strong crystal temperature dependence of the {omega}{sub 2}-branch was found which leads to lower energies at the 'dip' for smaller temperatures, while the {omega}{sub 1}-anomaly remains unchanged. Such behavior agrees well with the picture that the {omega}{sub 2}-branch is due to a Kohn anomaly. (orig.)

Electron–phonon superconductivity in YIn3

International Nuclear Information System (INIS)

Billington, D; Llewellyn-Jones, T M; Maroso, G; Dugdale, S B

2013-01-01

First-principles calculations of the electron–phonon coupling were performed on the cubic intermetallic compound YIn 3 . The electron–phonon coupling constant was found to be λ ep = 0.42. Using the Allen–Dynes formula with a Coulomb pseudopotential of μ* = 0.10, a T c of approximately 0.77 K is obtained which is reasonably consistent with the experimentally observed temperature (between 0.8 and 1.1 K). The results indicate that conventional electron–phonon coupling is capable of producing the superconductivity in this compound. (paper)

Electron-phonon superconductivity in YIn3

Science.gov (United States)

Billington, D.; Llewellyn-Jones, T. M.; Maroso, G.; Dugdale, S. B.

2013-08-01

First-principles calculations of the electron-phonon coupling were performed on the cubic intermetallic compound YIn3. The electron-phonon coupling constant was found to be λep = 0.42. Using the Allen-Dynes formula with a Coulomb pseudopotential of μ* = 0.10, a Tc of approximately 0.77 K is obtained which is reasonably consistent with the experimentally observed temperature (between 0.8 and 1.1 K). The results indicate that conventional electron-phonon coupling is capable of producing the superconductivity in this compound.

Nonlinear optical effects in pure and N-doped semiconductors

International Nuclear Information System (INIS)

Donlagic, N.S.

2000-01-01

Over the last decades, the nonlinear optical properties of condensed matter systems have been an attractive and fruitful field of research. While the linear response functions of solids provide information about the elementary excitations of the systems, nonlinear optical experiments give insight into the dynamics of the fundamental many-body processes which are initiated by the external excitations. Stimulated by the experimental results, new theoretical concepts and methods have been developed in order to relate the observed phenomena to the microscopic properties of the investigated materials. The present work deals with the study of the nonlinear dynamics of the optical interband polarization in pure and n-doped semiconductors.In the first part of the thesis, the relaxation behavior of optically excited electron-hole pairs in a one-dimensional semiconductor, which are coupled to longitudinal optical phonons with an initial lattice temperature T>0, is studied with the help of quantum kinetic equations. Apart from Hartree-Fock-like Coulomb contributions, these equations contain additional Coulomb terms, the so-called vertex corrections, by which the influence of the electron-electron interaction on the electron-phonon scattering processes is taken into account. The numerical studies indicate that the vertex corrections are essential for a correct description of the excitonic dynamics.In the second part of the thesis, the attention is shifted to the characteristics of the optical response of a one-dimensional n-doped two-band semiconductor whose conduction band has been linearized with respect to the two Fermi points. Due to the linearization it is possible to calculate the linear and nonlinear response functions of the interacting electron system exactly. These response functions are then used in order to determine the linear absorption spectrum and the time-integrated signal of a degenerated four-wave-mixing experiment. It is shown that the well-known features

Features of the non-collinear one-phonon anomalous light scattering controlled by elastic waves with elevated linear losses: potentials for multi-frequency parallel spectrum analysis of radio-wave signals.

Science.gov (United States)

Shcherbakov, Alexandre S; Arellanes, Adan Omar

2017-12-01

During subsequent development of the recently proposed multi-frequency parallel spectrometer for precise spectrum analysis of wideband radio-wave signals, we study potentials of new acousto-optical cells exploiting selected crystalline materials at the limits of their capabilities. Characterizing these wide-aperture cells is non-trivial due to new features inherent in the chosen regime of an advanced non-collinear one-phonon anomalous light scattering by elastic waves with significantly elevated acoustic losses. These features can be observed simpler in uniaxial, tetragonal, and trigonal crystals possessing linear acoustic attenuation. We demonstrate that formerly studied additional degree of freedom, revealed initially for multi-phonon regimes of acousto-optical interaction, can be identified within the one-phonon geometry as well and exploited for designing new cells. We clarify the role of varying the central acoustic frequency and acoustic attenuation using the identified degree of freedom. Therewith, we are strongly restricted by a linear regime of acousto-optical interaction to avoid the origin of multi-phonon processes within carrying out a multi-frequency parallel spectrum analysis of radio-wave signals. Proof-of-principle experiments confirm the developed approaches and illustrate their applicability to innovative technique for an advanced spectrum analysis of wideband radio-wave signals with the improved resolution in an extended frequency range.

Interplay between electron-phonon and electron-electron interactions

International Nuclear Information System (INIS)

Roesch, O.; Gunnarsson, O.; Han, J.E.; Crespi, V.H.

2005-01-01

We discuss the interplay between electron-electron and electron-phonon interactions for alkali-doped fullerides and high temperature superconductors. Due to the similarity of the electron and phonon energy scales, retardation effects are small for fullerides. This raises questions about the origin of superconductivity, since retardation effects are believed to be crucial for reducing effects of the Coulomb repulsion in conventional superconductors. We demonstrate that by treating the electron-electron and electron-phonon interactions on an equal footing, superconductivity can be understood in terms of a local pairing. The Jahn-Teller character of the important phonons in fullerides plays a crucial role for this result. To describe effects of phonons in cuprates, we derive a t-J model with phonons from the three-band model. Using exact diagonalization for small clusters, we find that the anomalous softening of the half-breathing phonon as well as its doping dependence can be explained. By comparing the solution of the t-J model with the Hartree-Fock approximation for the three-band model, we address results obtained in the local-density approximation for cuprates. We find that genuine many-body results, due to the interplay between the electron-electron and electron-phonon interactions, play an important role for the the results in the t-J model. (copyright 2005 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)

Remarkable reduction of thermal conductivity in phosphorene phononic crystal

International Nuclear Information System (INIS)

Xu, Wen; Zhang, Gang

2016-01-01

Phosphorene has received much attention due to its interesting physical and chemical properties, and its potential applications such as thermoelectricity. In thermoelectric applications, low thermal conductivity is essential for achieving a high figure of merit. In this work, we propose to reduce the thermal conductivity of phosphorene by adopting the phononic crystal structure, phosphorene nanomesh. With equilibrium molecular dynamics simulations, we find that the thermal conductivity is remarkably reduced in the phononic crystal. Our analysis shows that the reduction is due to the depressed phonon group velocities induced by Brillouin zone folding, and the reduced phonon lifetimes in the phononic crystal. Interestingly, it is found that the anisotropy ratio of thermal conductivity could be tuned by the ‘non-square’ pores in the phononic crystal, as the phonon group velocities in the direction with larger projection of pores is more severely suppressed, leading to greater reduction of thermal conductivity in this direction. Our work provides deep insight into thermal transport in phononic crystals and proposes a new strategy to reduce the thermal conductivity of monolayer phosphorene. (paper)

Phonon localization transition in relaxor ferroelectric PZN-5%PT

International Nuclear Information System (INIS)

Manley, Michael E.; Christianson, Andrew D.; Abernathy, Douglas L.; Sahul, Raffi

2017-01-01

Relaxor ferroelectric behavior occurs in many disordered ferroelectric materials but is not well understood at the atomic level. Recent experiments and theoretical arguments indicate that Anderson localization of phonons instigates relaxor behavior by driving the formation of polar nanoregions (PNRs). Here, we use inelastic neutron scattering to observe phonon localization in relaxor ferroelectric PZN-5%PT (0.95[Pb(Zn 1/3 Nb 2/3 )O 3 ]–0.05PbTiO 3 ) and detect additional features of the localization process. In the lead, up to phonon localization on cooling, the local resonant modes that drive phonon localization increase in number. The increase in resonant scattering centers is attributed to a known increase in the number of locally off centered Pb atoms on cooling. The transition to phonon localization occurs when these random scattering centers increase to a concentration where the Ioffe-Regel criterion is satisfied for localizing the phonon. Finally, we also model the effects of damped mode coupling on the observed phonons and phonon localization structure.

Projection-reduction method applied to deriving non-linear optical conductivity for an electron-impurity system

Directory of Open Access Journals (Sweden)

Nam Lyong Kang

2013-07-01

Full Text Available The projection-reduction method introduced by the present authors is known to give a validated theory for optical transitions in the systems of electrons interacting with phonons. In this work, using this method, we derive the linear and first order nonlinear optical conductivites for an electron-impurity system and examine whether the expressions faithfully satisfy the quantum mechanical philosophy, in the same way as for the electron-phonon systems. The result shows that the Fermi distribution function for electrons, energy denominators, and electron-impurity coupling factors are contained properly in organized manners along with absorption of photons for each electron transition process in the final expressions. Furthermore, the result is shown to be represented properly by schematic diagrams, as in the formulation of electron-phonon interaction. Therefore, in conclusion, we claim that this method can be applied in modeling optical transitions of electrons interacting with both impurities and phonons.

NATO Advanced Study Institute on Nonequilibrium Phonon Dynamics

CERN Document Server

1985-01-01

Phonons are always present in the solid state even at an absolute temperature of 0 K where zero point vibrations still abound. Moreover, phonons interact with all other excitations of the solid state and, thereby, influence most of its properties. Historically experimental information on phonon transport came from measurements of thermal conductivity. Over the past two decades much more, and much more detailed, information on phonon transport and on many of the inherent phonon interaction processes have come to light from experiments which use nonequilibrium phonons to study their dynamics. The resultant research field has most recently blossomed with the development of ever more sophisticated experimental and theoretical methods which can be applied to it. In fact, the field is moving so rapidly that new members of the research community have difficulties in keeping up to date. This NATO Advanced Study Institute (ASI) was organized with the objective of overcoming the information barrier between those expert...

Phononic crystals and elastodynamics: Some relevant points

Directory of Open Access Journals (Sweden)

N. Aravantinos-Zafiris

2014-12-01

Full Text Available In the present paper we review briefly some of the first works on wave propagation in phononic crystals emphasizing the conditions for the creation of acoustic band-gaps and the role of resonances to the band-gap creation. We show that useful conclusions in the analysis of phononic band gap structures can be drawn by considering the mathematical similarities of the basic classical wave equation (Helmholtz equation with Schrödinger equation and by employing basic solid state physics concepts and conclusions regarding electronic waves. In the second part of the paper we demonstrate the potential of phononic systems to be used as elastic metamaterials. This is done by demonstrating negative refraction in phononic crystals and subwavelength waveguiding in a linear chain of elastic inclusions, and by proposing a novel structure with close to pentamode behavior. Finally the potential of phononic structures to be used in liquid sensor applications is discussed and demonstrated.

Acoustic phonon dispersion of CoSi2

International Nuclear Information System (INIS)

Weiss, L.; Rumyantsev, A.Yu.; Ivanov, A.S.

1985-01-01

The acoustical phonon dispersion curves of CoSi 2 are measured at room temperature along the main symmetry directions by means of coherent one-phonon scattering of thermal neutrons. The dispersion curves are compared with those of Ge, Si, and the fluorite structure types as CaF 2 and UO 2 . From the slope of the phonon dispersion curves at the GAMMA-point the elastic constants have been obtained

Optical coherence tomography evaluation of retinal nerve fiber layer in longitudinally extensive transverse myelitis

Directory of Open Access Journals (Sweden)

Frederico C. Moura

2011-02-01

Full Text Available OBJECTIVE: To compare optical coherence tomography (OCT measurements on the retinal nerve fiber layer (RNFL of healthy controls and patients with longitudinally extensive transverse myelitis (LETM without previous optic neuritis. METHOD: Twenty-six eyes from 26 patients with LETM and 26 control eyes were subjected to automated perimetry and OCT for comparison of RNFL measurements. RESULTS: The mean deviation values from perimetry were significantly lower in patients with LETM than in controls (p<0.0001. RNFL measurements in the nasal quadrant and in the 3-o'clock segment were significantly smaller in LETM eyes than in controls. (p=0.04 and p=0.006, respectively. No significantly differences in other RNFL measurements were found. CONCLUSION: Patients with LETM may present localized RNFL loss, particularly on the nasal side of the optic disc, associated with slight visual field defects, even in the absence of previous episodes of optic neuritis. These findings emphasize the fact that patients with LETM may experience attacks of subclinical optic nerve damage.

Thermal transport in phononic crystals: The role of zone folding effect

Science.gov (United States)

Dechaumphai, Edward; Chen, Renkun

2012-04-01

Recent experiments [Yu et al., Nature Nanotech 5, 718 (2010); Tang et al., Nano Lett. 10, 4279 (2010); Hopkins etal., Nano Lett. 11, 107(2011)] on silicon based nanoscale phononic crystals demonstrated substantially reduced thermal conductivity compared to bulk Si, which cannot be explained by incoherent phonon boundary scattering within the Boltzmann Transport Equation (BTE). In this paper, partial coherent treatment of phonons, where phonons are regarded as either wave or particles depending on their frequencies, was considered. Phonons with mean free path smaller than the characteristic size of phononic crystals are treated as particles and the transport in this regime is modeled by BTE with phonon boundary scattering taken into account. On the other hand, phonons with mean free path longer than the characteristic size are treated as waves. In this regime, phonon dispersion relations are computed using the Finite Difference Time Domain (FDTD) method and are found to be modified due to the zone folding effect. The new phonon spectra are then used to compute phonon group velocity and density of states for thermal conductivity modeling. Our partial coherent model agrees well with the recent experimental results on in-plane thermal conductivity of phononic crystals. Our study highlights the importance of zone folding effect on thermal transport in phononic crystals.

Effect of thermal phonons on the superconducting transition temperature

International Nuclear Information System (INIS)

Leavens, C.R.; Talbot, E.

1983-01-01

There is no consensus in the literature on whether or not thermal phonons depress the superconducting transition temperature T/sub c/. In this paper it is shown by accurate numerical solution of the real-frequency Eliashberg equations for the pairing self-energy phi and renormalization function Z that thermal phonons in the kernel for phi raise T/sub c/ but those in Z lower it by a larger amount so that the net effect is to depress T/sub c/. (A previous calculation which ignored the effect of thermal phonons in phi overestimated the suppression of T/sub c/ by at least a factor of 3.) It is shown how to switch off the thermal phonons in the imaginary-frequency Eliashberg equations, exactly for Z and approximately for phi. The real-frequency and approximate imaginary-frequency results for the depression of T/sub c/ by thermal phonons are in very satisfactory agreement. Thermal phonons are found to depress the transition temperature of Nb 3 Sn by only 2%. It is estimated that the suppression of T/sub c/ by thermal phonons saturates at about 50% in the limit of very strong electron-phonon coupling

Phonon cross-plane transport and thermal boundary resistance: effect of heat source size and thermal boundary resistance on phonon characteristics

Science.gov (United States)

Ali, H.; Yilbas, B. S.

2016-09-01

Phonon cross-plane transport across silicon and diamond thin films pair is considered, and thermal boundary resistance across the films pair interface is examined incorporating the cut-off mismatch and diffusive mismatch models. In the cut-off mismatch model, phonon frequency mismatch for each acoustic branch is incorporated across the interface of the silicon and diamond films pair in line with the dispersion relations of both films. The frequency-dependent and transient solution of the Boltzmann transport equation is presented, and the equilibrium phonon intensity ratios at the silicon and diamond film edges are predicted across the interface for each phonon acoustic branch. Temperature disturbance across the edges of the films pair is incorporated to assess the phonon transport characteristics due to cut-off and diffusive mismatch models across the interface. The effect of heat source size, which is allocated at high-temperature (301 K) edge of the silicon film, on the phonon transport characteristics at the films pair interface is also investigated. It is found that cut-off mismatch model predicts higher values of the thermal boundary resistance across the films pair interface as compared to that of the diffusive mismatch model. The ratio of equilibrium phonon intensity due to the cut-off mismatch over the diffusive mismatch models remains >1 at the silicon edge, while it becomes <1 at the diamond edge for all acoustic branches.

Investigation of the Full Spectrum Phonon Lifetime in Thin Silicon Films from the Bulk Spectral Phonon Mean-Free-Path Distribution by Using Kinetic Theory

Energy Technology Data Exchange (ETDEWEB)

Jin, Jae Sik [Chosun College of Science and Technology, Gwangju (Korea, Republic of)

2017-03-15

Phonon dynamics in nanostructure is critically important to thermoelectric and optoelectronic devices because it determines the transport and other crucial properties. However, accurately evaluating the phonon lifetimes is extremely difficult. This study reports on the development of a new semi-empirical method to estimate the full-spectrum phonon lifetimes in thin silicon films at room temperature based on the experimental data on the phonon mean-free-path spectrum in bulk silicon and a phenomenological consideration of phonon transport in thin films. The bulk of this work describes the theory and the validation; then, we discuss the trend of the phonon lifetimes in thin silicon films when their thicknesses decrease.

Computational modeling of geometry dependent phonon transport in silicon nanostructures

Science.gov (United States)

Cheney, Drew A.

Recent experiments have demonstrated that thermal properties of semiconductor nanostructures depend on nanostructure boundary geometry. Phonons are quantized mechanical vibrations that are the dominant carrier of heat in semiconductor materials and their aggregate behavior determine a nanostructure's thermal performance. Phonon-geometry scattering processes as well as waveguiding effects which result from coherent phonon interference are responsible for the shape dependence of thermal transport in these systems. Nanoscale phonon-geometry interactions provide a mechanism by which nanostructure geometry may be used to create materials with targeted thermal properties. However, the ability to manipulate material thermal properties via controlling nanostructure geometry is contingent upon first obtaining increased theoretical understanding of fundamental geometry induced phonon scattering processes and having robust analytical and computational models capable of exploring the nanostructure design space, simulating the phonon scattering events, and linking the behavior of individual phonon modes to overall thermal behavior. The overall goal of this research is to predict and analyze the effect of nanostructure geometry on thermal transport. To this end, a harmonic lattice-dynamics based atomistic computational modeling tool was created to calculate phonon spectra and modal phonon transmission coefficients in geometrically irregular nanostructures. The computational tool is used to evaluate the accuracy and regimes of applicability of alternative computational techniques based upon continuum elastic wave theory. The model is also used to investigate phonon transmission and thermal conductance in diameter modulated silicon nanowires. Motivated by the complexity of the transmission results, a simplified model based upon long wavelength beam theory was derived and helps explain geometry induced phonon scattering of low frequency nanowire phonon modes.

Redshift of A 1(longitudinal optical) mode for GaN crystals under strong electric field

Science.gov (United States)

Gu, Hong; Wu, Kaijie; Zheng, Shunan; Shi, Lin; Zhang, Min; Liu, Zhenghui; Liu, Xinke; Wang, Jianfeng; Zhou, Taofei; Xu, Ke

2018-01-01

We investigated the property of GaN crystals under a strong electric field. The Raman spectra of GaN were measured using an ultraviolet laser, and a remarkable redshift of the A 1(LO) mode was observed. The role of the surface depletion layer was discussed, and the interrelation between the electric field and phonons was revealed. First-principles calculations indicated that, in particular, the phonons that vibrate along the [0001] direction are strongly influenced by the electric field. This effect was confirmed by a surface photovoltage experiment. The results revealed the origin of the redshift and presented the phonon property of GaN under a strong electric field.

Optical, structural, and nuclear scientific studies of AlGaN with high Al composition

Science.gov (United States)

Lin, Tse Yang; Chung, Yee Ling; Li, Lin; Yao, Shude; Lee, Y. C.; Feng, Zhe Chuan; Ferguson, Ian T.; Lu, Weijie

2010-08-01

AlGaN epilayers with higher Al-compositions were grown by Metalorganic Chemical Vapor Deposition (MOCVD) on (0001) sapphire. Trimethylgallium (TMGa), trimethylaluminium (TMAl) and NH3 were used as the source precursors for Ga, Al, and N, respectively. A 25 nm AlN nucleation layer was first grown at low-temperature of 590 °C at 300 Torr. Followed, AlxGa1-xN layers were grown at 1080 °C on low-temperature AlN nucleation layers. The heterostructures were characterized by a series of techniques, including x-ray diffraction (XRD), Rutherford backscattering (RBS), photoluminescence (PL), scanning electron microscopy (SEM) and Raman scattering. Precise Al compositions were determined through XRD, RBS, and SEM combined measurements. Room Temperature Raman Scattering spectra shows three major bands from AlGaN alloys, which are AlN-like, A1 longitudinal optical (LO) phonon modes, and E2 transverse optical (TO) band, respectively, plus several peak comes from the substrate. Raman spectral line shape analysis lead to an optical determination of the electrical property free carrier concentration of AlGaN. The optical properties of AlGaN with high Al composition were presented here.

Isotope effects on the optical spectra of semiconductors

Science.gov (United States)

Cardona, Manuel; Thewalt, M. L. W.

2005-10-01

Since the end of the cold war, macroscopic amounts of separated stable isotopes of most elements have been available “off the shelf” at affordable prices. Using these materials, single crystals of many semiconductors have been grown and the dependence of their physical properties on isotopic composition has been investigated. The most conspicuous effects observed have to do with the dependence of phonon frequencies and linewidths on isotopic composition. These affect the electronic properties of solids through the mechanism of electron-phonon interaction, in particular, in the corresponding optical excitation spectra and energy gaps. This review contains a brief introduction to the history, availability, and characterization of stable isotopes, including their many applications in science and technology. It is followed by a concise discussion of the effects of isotopic composition on the vibrational spectra, including the influence of average isotopic masses and isotopic disorder on the phonons. The final sections deal with the effects of electron-phonon interaction on energy gaps, the concomitant effects on the luminescence spectra of free and bound excitons, with particular emphasis on silicon, and the effects of isotopic composition of the host material on the optical transitions between the bound states of hydrogenic impurities.

Generation and detection of high-energy phonons by superconducting junctions

International Nuclear Information System (INIS)

Singer, I.L.

1976-01-01

Superconducting tunnel junctions are used to investigate the dynamics of energy exchange that takes place in superconductors driven out of equilibrium. In a Sn junction biased at a voltage V much greater than 2Δ(Sn)/e, the tunneling current sustains a continual energy exchange amongst the quasiparticles, phonons, and Cooper pairs. Repeatedly, high-energy quasiparticles decay, emitting phonons; and phonons with energy greater than 2Δ(Sn) break pairs, producing quasiparticles. The phonon-induced component of the current is recovered by synchronously detecting the full tunneling current with respect to a small modulation current in the generator. Sharp onsets observed at intervals of the gap energies require that the escaping phonons are produced by the direct decay of the injected quasiparticles and are not merely the high-energy tail of the thermalized phonons. Both primary and secondary phonons can be abserved distinctly. Theoretical transconductance curves have been computed. The experimental and theoretical curves are in good qualitative agreement. A more detailed comparison suggests that the escape rate of high-energy phonons depends on the energy of the phonons. The dependence of the observed transconductance signal on the temperature and the total junction thickness suggests that the presence of quasiparticles plays a major role in the escape of high-energy phonons. The dependence on temperature can be fitted to exp(b/kT), 0.74 less than b less than 1.05 MeV. It is speculated that the excitation energy is first transported across the superconductor and then carried out of the film by the phonons. It is concluded that high-energy phonons are a sensitive probe of the very reabsorption effects that make their escape so unlikely, and analysis of the detected phonons rich details of the behavior of superconductors removed from equilibrium

Infrared reflectivity of the solid solutions LaNi1-xFexO3 (0.00≤x≤1.00)

International Nuclear Information System (INIS)

Massa, N.E.; Falcon, H.; Salva, H.; Carbonio, R.E.

1997-01-01

We report temperature-dependent far- and midinfrared reflectivity spectra of LaNi (1-x) Fe x O 3 solid solutions that span the passage from LaFeO 3 , a room-temperature antiferromagnetic insulator, to LaNiO 3 , a known metal oxide. Light Ni doping creates defects that induce extra bands assigned to electronic transitions within the insulating gap. An incipient Drude term emerges in the reflectivity spectrum of LaNi 0.39 Fe 0.61 O 3 together with subbands that contribute to the electronic background. At these concentrations the dielectric response shows a picture in which the spectral weigh switches over toward far-infrared frequencies while phonon features develop strong antiresonances near longitudinal-optical modes. Further increment of carriers produces phonon screening and the development of a reflectivity tail that extends beyond 1 eV. We assign extra-non-Drude terms in the 700 endash 4000cm -1 frequency region to transitions due to intrinsic defects. While the increment in reflectivity at far-infrared frequencies is evident for Fe concentrations well above the insulator-metal transition (x∼0.30), the spectral features of a metal oxide, with phonons mostly screened, are found for x=0.23. These metallic spectra show an absorption dip at ∼650cm -1 that is traced to the perovskite symmetric stretching longitudinal mode. Electron-phonon interactions are thus present in solid solutions even when the numbers of effective carriers are those of a metal. This characterization is also supported by the observation of weak reflectivity dips in LaNiO 3 that have a direct correspondence to longitudinal-optical mode frequencies of the insulating phases of our series. We infer that strong electron-phonon interactions play a role in the conductivity of those solid solutions and are likely related to polaron formation and carrier phonon-assisted hopping motion. (Abstract Truncated)

Non-equilibrium Green's function calculation for GaN-based terahertz-quantum cascade laser structures

Science.gov (United States)

Yasuda, H.; Kubis, T.; Hosako, I.; Hirakawa, K.

2012-04-01

We theoretically investigated GaN-based resonant phonon terahertz-quantum cascade laser (QCL) structures for possible high-temperature operation by using the non-equilibrium Green's function method. It was found that the GaN-based THz-QCL structures do not necessarily have a gain sufficient for lasing, even though the thermal backfilling and the thermally activated phonon scattering are effectively suppressed. The main reason for this is the broadening of the subband levels caused by a very strong interaction between electrons and longitudinal optical (LO) phonons in GaN.

Infrared reflectance measurement for InN thin film characterization

International Nuclear Information System (INIS)

Fukui, K.; Kugumiya, Y.; Nakagawa, N.; Yamamoto, A.

2006-01-01

Infrared reflectance measurements of a series of InN thin films have been performed and attempt to derive carrier concentration and other physical constants for InN thin film characterization. Fitting calculations are performed by use of the dielectric function equation based on phonon-plasmon coupling model. Longitudinal and transverse optical phonon frequencies, plasma frequency and their damping parameters can be derived from fitting. From those results, electrical and phonon properties of InN and characterization of films are discussed. (copyright 2006 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)

Theoretical modelling of electron transport in InAs/GaAs quantum dot superlattices

International Nuclear Information System (INIS)

Vukmirovic, Nenad; Ikonic, Zoran; Savic, Ivana; Indjin, Dragan; Harrison, Paul

2006-01-01

A theoretical model describing the electron transport in InAs/GaAs quantum dot infrared photodetectors, modelled as ideal quantum dot superlattices, is presented. The carrier wave functions and energy levels were evaluated using the strain dependent 8-band k.p Hamiltonian and used to calculate all intra- and inter-period transition rates due to interaction with phonons and electromagnetic radiation. The interaction with longitudinal acoustic phonons and electromagnetic radiation was treated perturbatively within the framework of Fermi's golden rule, while the interaction with longitudinal optical phonons was considered taking into account their strong coupling to electrons. The populations of energy levels were then found from a system of rate equations, and the electron current in the superlattice was subsequently extracted. (copyright 2006 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)

Positron diffusion in zinc-blende-type semiconductors

International Nuclear Information System (INIS)

Singh, R.P.; Singh, M.

1992-01-01

A theory of positron diffusion in the presence of positron scattering with acoustic, piezoelectric-acoustic, and optical phonons in zinc-blende-type semiconductors is developed. The velocity-velocity correlation function is used to calculate the diffusion coefficient and the Green's function is used to calculate the correlation function. In this formulation the relaxation time automatically appears through the imaginary part of the self-energy. The effect of screening is included within the Thomas-Fermi approximation. The theory is used to calculate the energy relaxation time and diffusion length of positrons in GaAs. It is shown that the transverse and longitudinal phonons contribute almost the same amount towards the relaxation time in the piezoelectric-acoustic-phonon scattering of positrons. It is also found that the contribution of piezoelectric-acoustic-phonon scattering is about 10% to that of acoustic-phonon scattering at low temperatures. We obtained a fairly good agreement between our theory and experimental results available to us. It is found that optical-phonon scattering plays an important role in getting good agreement between theory and experiment at high temperatures

Lattice dynamics and electron/phonon interactions in epitaxial transition-metal nitrides

Science.gov (United States)

Mei, Antonio Rodolph Bighetti

increasing cation mass; optical mode energies remain approximately constant for the TM nitrides, but are significantly lower for the RE nitride due to a lower interatomic force constant. Optical/acoustic peak-intensity ratios are 1.15+/-0.1 for all four nitrides, indicating similar electron/phonon coupling strengths alphatr(h o) for both modes. Elastic constants are determined for single-crystal stoichiometric NaCl-structure VN(001), VN(011), and VN(111) epitaxial layers grown by magnetically-unbalanced reactive magnetron sputter deposition on 001-, 011-, and 111-oriented MgO substrates at 430 °C. The relaxed lattice parameter ao = 0.4134+/-0.0004 nm, obtained from high-resolution reciprocal space maps, and the mass density rho = 6.1 g/cm3, determined from the combination of Rutherford backscattering spectroscopy and film thickness measurements, of the VN layers are both in good agreement with reported values for bulk crystals. Sub-picosecond ultrasonic optical pump/probe techniques are used to generate and detect VN longitudinal sound waves with measured velocities v001 = 9.8+/-0.3, v011 = 9.1+/-0.3, and v111 = 9.1+/-0.3 km/s. The VN c11 elastic constant is determined from the sound wave velocity measurements as 585+/-30 GPa; the c44 elastic constant, 126+/-3 GPa, is obtained from surface acoustic wave measurements. From the combination of c11, c44, vhkl, and rho, the VN c 12 elastic constant is 178+/-33 GPa, the VN elastic anisotropy A = 0.62, the isotropic Poisson ratio nu = 0.29, and the anisotropic Poisson ratios nu001 = 0.23, nu011 = 0.30, and nu 111 = 0.29. The elastic stability criteria requires cubic crystals to resist [001] and [011] shears as well as isotropic compression or, equivalently, for G001 = (c11 -- c12)/2 > 0, G 011 = c44 > 0, and B = (c11 + 2c12)/3 > 0, in which G001 and G011 are directional shear moduli and B is the bulk modulus. Thus, NaCl-structure VN is elastically stable at room temperature. Structural phase transitions in epitaxial stoichiometric

Basic equations of the quasiparticle-phonon nuclear model with the effects due to the Pauli principle and the phonon ground state correlations

International Nuclear Information System (INIS)

Nguyen Dinh Dang; Voronov, V.V.

1983-01-01

A system of basic equations of the quasiparticle-phonon model is obtained for energies and a structure of excited states described by the wave functions containing one- and two-phonon components. The effects due to the Pauli principle for two-phonon components and the phonon ground state correlations of a spherical nucleus are taken here into account. The quantitative estimations of these effects are given by a simplified scheme. The relation between these equations with the results from other theoretical approaches is discussed

Nanomesh phononic structures for low thermal conductivity and thermoelectric energy conversion materials

Science.gov (United States)

Yu, Jen-Kan; Mitrovic, Slobodan; Heath, James R.

2016-08-16

A nanomesh phononic structure includes: a sheet including a first material, the sheet having a plurality of phononic-sized features spaced apart at a phononic pitch, the phononic pitch being smaller than or equal to twice a maximum phonon mean free path of the first material and the phononic size being smaller than or equal to the maximum phonon mean free path of the first material.

``New'' energy states lead to phonon-less optoelectronic properties in nanostructured silicon

Science.gov (United States)

Singh, Vivek; Yu, Yixuan; Korgel, Brian; Nagpal, Prashant

2014-03-01

Silicon is arguably one of the most important technological material for electronic applications. However, indirect bandgap of silicon semiconductor has prevented optoelectronic applications due to phonon assistance required for photon light absorption/emission. Here we show, that previously unexplored surface states in nanostructured silicon can couple with quantum-confined energy levels, leading to phonon-less exciton-recombination and photoluminescence. We demonstrate size dependence (2.4 - 8.3 nm) of this coupling observed in small uniform silicon nanocrystallites, or quantum-dots, by direct measurements of their electronic density of states and low temperature measurements. To enhance the optical absorption of the these silicon quantum-dots, we utilize generation of resonant surface plasmon polariton waves, which leads to several fold increase in observed spectrally-resolved photocurrent near the quantum-confined bandedge states. Therefore, these enhanced light emission and absorption enhancement can have important implications for applications of nanostructured silicon for optoelectronic applications in photovoltaics and LEDs.

Phonon squeezed states: quantum noise reduction in solids

Science.gov (United States)

Hu, Xuedong; Nori, Franco

1999-03-01

This article discusses quantum fluctuation properties of a crystal lattice, and in particular, phonon squeezed states. Squeezed states of phonons allow a reduction in the quantum fluctuations of the atomic displacements to below the zero-point quantum noise level of coherent phonon states. Here we discuss our studies of both continuous-wave and impulsive second-order Raman scattering mechanisms. The later approach was used to experimentally suppress (by one part in a million) fluctuations in phonons. We calculate the expectation values and fluctuations of both the atomic displacement and the lattice amplitude operators, as well as the effects of the phonon squeezed states on macroscopically measurable quantities, such as changes in the dielectric constant. These results are compared with recent experiments. Further information, including preprints and animations, are available in http://www-personal.engin.umich.edu/∼nori/squeezed.html.

Resonant Magnon-Phonon Polaritons in a Ferrimagnet

Science.gov (United States)

2000-09-29

UNCLASSIFIED Defense Technical Information Center Compilation Part Notice ADPO 11604 TITLE: Resonant Magnon -Phonon Polaritons in a Ferrimagnet...part numbers comprise the compilation report: ADP011588 thru ADP011680 UNCLASSIFIED 75 Resonant Magnon -Phonon Polaritons in a Ferrimagnet I. E...susceptibilities X"aa and X’m << X’m appear, where 77 xem - DPx igEo0 i_ Xxy - hy- C1 (0)2 _ 00t2) 4= -7• 4 3. Phonon and magnon polaritons We solve the

A comprehensive phononics of phonon assisted energy transfer in the Yb3+ aided upconversion luminescence of Tm3+ and Ho3+ in solids

International Nuclear Information System (INIS)

Debnath, Radhaballabh; Bose, Saptasree

2015-01-01

The theory of phonon assisted energy transfer is being widely used to explain the Yb 3+ ion aided normal and upconversion emission of various rare earth ions in different Yb 3+ co-doped solids. The reported phonon dynamics in many of these studies are either incomplete or erroneous. Here we report Yb 3+ aided upconversion luminescence properties of Tm 3+ and Ho 3+ in (Yb 3+ /Tm 3+ ) and (Yb 3+ /Ho 3+ ) co-doped two BaO–tellurite glasses and explain their phononics in the light of Dexter's theory by proposing a comprehensive scheme. The approach is valid for other systems of different phonon structures. - Highlights: • Yb 3+ aided upconversion luminescence properties of Tm 3+ and Ho 3+ in (Yb 3+ /Tm 3+ ) and (Yb 3+ /Ho 3+ ) co-doped two BaO–tellurite glasses, are reported. • Phonon assisted energy transfer in these systems are explained in the light of Dexter's theory by proposing a comprehensive scheme of phononics. • The approach is valid for other systems of different phonon structures

Structure, phonons and related properties in zinc-IV-nitride (IV = silicon, germanium, tin), scandium nitride, and rare-earth nitrides

Science.gov (United States)

Paudel, Tula R.

This thesis presents a study of the phonons and related properties in two sets of nitride compounds, whose properties are until now relatively poorly known. The Zn-IV-N2 group of compounds with the group IV elements Si, Ge and Sn, form a series analogous to the well known III-N nitride series with group III element Al, Ga, In. Structurally, they can be derived by doubling the period of III-V compounds in the plane in two directions and replacing the group-III elements with Zn and a group-IV element in a particular ordered pattern. Even though they are similar to the well-known III-V nitride compounds, the study of the properties of these materials is in its early stages. The phonons in these materials and their relation to the phonons in the corresponding group-III nitrides are of fundamental interest. They are also of practical interest because the phonon related spectra such as infrared absorption and Raman spectroscopy are sensitive to the structural quality of the material and can thus be used to quantify the degree of crystalline perfection of real samples. First-principles calculations of the phonons and related ground state properties of these compounds were carried out using Density Functional Perturbation Theory (DFPT) with the Local Density Approximation (LDA) for exchange and correlation and using a pseudopotential plane wave implementation which was developed by several authors over the last decades. The main focus of our study is on the phonons at the center of the Brillouin zone because the latter are most directly related to commonly used spectroscopies to probe the vibrations in a solid: infrared reflectivity and Raman spectroscopy. For a semiconducting or insulating compound, a splitting occurs between transverse and longitudinal phonons at the Gamma-point because of the long-range nature of electrostatic forces. The concepts required to handle this problem are reviewed. Our discussion emphasizes how the various quantities required are related to

First-principles calculation of the polarization-dependent force driving the Eg mode in bismuth under optical excitation.

Science.gov (United States)

Murray, Eamonn; Fahy, Stephen

2014-03-01

Using first principles electronic structure methods, we calculate the induced force on the Eg (zone centre transverse optical) phonon mode in bismuth immediately after absorption of polarized light. When radiation with polarization perpendicular to the c-axis is absorbed in bismuth, the distribution of excited electrons and holes breaks the three-fold rotational symmetry and leads to a net force on the atoms in the direction perpendicular to the axis. We calculate the initial excited electronic distribution as a function of photon energy and polarization and find the resulting transverse and longitudinal forces experienced by the atoms. Using the measured, temperature-dependent rate of decay of the transverse force[2], we predict the approximate amplitude of induced atomic motion in the Eg mode as a function of temperature and optical fluence. This work is supported by Science Foundation Ireland and a Marie Curie International Incoming Fellowship.

Detecting phonon blockade with photons

International Nuclear Information System (INIS)

Didier, Nicolas; Pugnetti, Stefano; Fazio, Rosario; Blanter, Yaroslav M.

2011-01-01

Measuring the quantum dynamics of a mechanical system, when few phonons are involved, remains a challenge. We show that a superconducting microwave resonator linearly coupled to the mechanical mode constitutes a very powerful probe for this scope. This new coupling can be much stronger than the usual radiation pressure interaction by adjusting a gate voltage. We focus on the detection of phonon blockade, showing that it can be observed by measuring the statistics of the light in the cavity. The underlying reason is the formation of an entangled state between the two resonators. Our scheme realizes a phonotonic Josephson junction, giving rise to coherent oscillations between phonons and photons as well as a self-trapping regime for a coupling smaller than a critical value. The transition from the self-trapping to the oscillating regime is also induced dynamically by dissipation.

A Numerical Study on Phonon Spectral Contributions to Thermal Conduction in Silicon-on-Insulator Transistor Using Electron-Phonon Interaction Model

Energy Technology Data Exchange (ETDEWEB)

Kang, Hyung-sun; Koh, Young Ha; Jin, Jae Sik [Chosun College of Science and Technology, Gwangju (Korea, Republic of)

2017-06-15

The aim of this study is to understand the phonon transfer characteristics of a silicon thin film transistor. For this purpose, the Joule heating mechanism was considered through the electron-phonon interaction model whose validation has been done. The phonon transport characteristics were investigated in terms of phonon mean free path for the variations in the device power and silicon layer thickness from 41 nm to 177 nm. The results may be used for developing the thermal design strategy for achieving reliability and efficiency of the silicon-on-insulator (SOI) transistor, further, they will increase the understanding of heat conduction in SOI systems, which are very important in the semiconductor industry and the nano-fabrication technology.

Theory of phonon properties in doped and undoped CuO nanoparticles

Energy Technology Data Exchange (ETDEWEB)

Bahoosh, S.G. [Institute of Physics, Martin-Luther-University, D-06099 Halle (Germany); Apostolov, A.T. [University of Architecture, Civil Engineering and Geodesy Faculty of Hydrotechnics, Department of Physics, 1, Hristo Smirnenski Blvd., 1046 Sofia (Bulgaria); Apostolova, I.N. [University of Forestry, Faculty of Forest Industry, 10, Kl. Ohridsky Blvd., 1756 Sofia (Bulgaria); Wesselinowa, J.M., E-mail: julia@phys.uni-sofia.bg [University of Sofia, Department of Physics, 5 J. Bouchier Blvd., 1164 Sofia (Bulgaria)

2012-07-02

We have studied the phonon properties of CuO nanoparticles and have shown the importance of the anharmonic spin–phonon interaction. The Raman peaks of CuO nanoparticles shift to lower frequency and become broader as the particle size decreases in comparison with those of bulk CuO crystals owing to size effects. By doping with different ions, in dependence of their radius compared to the host ionic radius the phonon energies ω could be reduced or enhanced. The phonon damping is always enhanced through the ion doping effects. -- Highlights: ► The phonon properties of CuO nanoparticles are studied using a miscroscopic model. ► The phonon energy decreases whereas the damping increases with decreasing of particle size. ► It is shown the importance of the anharmonic spin–phonon interaction. ► By doping with RE-ions the phonon energy is reduced, whereas with TM-ions it is enhanced. ► The phonon damping is always enhanced through the ion doping effects.

Thermal transport across metal–insulator interface via electron–phonon interaction

International Nuclear Information System (INIS)

Zhang, Lifa; Wang, Jian-Sheng; Li, Baowen; Lü, Jing-Tao

2013-01-01

The thermal transport across a metal–insulator interface can be characterized by electron–phonon interaction through which an electron lead is coupled to a phonon lead if phonon–phonon coupling at the interface is very weak. We investigate the thermal conductance and rectification between the electron part and the phonon part using the nonequilibrium Green’s function method. It is found that the thermal conductance has a nonmonotonic behavior as a function of average temperature or the coupling strength between the phonon leads in the metal part and the insulator part. The metal–insulator interface shows a clear thermal rectification effect, which can be reversed by a change in average temperature or the electron–phonon coupling. (paper)

Topological phononic insulator with robust pseudospin-dependent transport

Science.gov (United States)

Xia, Bai-Zhan; Liu, Ting-Ting; Huang, Guo-Liang; Dai, Hong-Qing; Jiao, Jun-Rui; Zang, Xian-Guo; Yu, De-Jie; Zheng, Sheng-Jie; Liu, Jian

2017-09-01

Topological phononic states, which facilitate unique acoustic transport around defects and disorders, have significantly revolutionized our scientific cognition of acoustic systems. Here, by introducing a zone folding mechanism, we realize the topological phase transition in a double Dirac cone of the rotatable triangular phononic crystal with C3 v symmetry. We then investigate the distinct topological edge states on two types of interfaces of our phononic insulators. The first one is a zigzag interface which simultaneously possesses a symmetric mode and an antisymmetric mode. Hybridization of the two modes leads to a robust pseudospin-dependent one-way propagation. The second one is a linear interface with a symmetric mode or an antisymmetric mode. The type of mode is dependent on the topological phase transition of the phononic insulators. Based on the rotatability of triangular phononic crystals, we consider several complicated contours defined by the topological zigzag interfaces. Along these contours, the acoustic waves can unimpededly transmit without backscattering. Our research develops a route for the exploration of the topological phenomena in experiments and provides an excellent framework for freely steering the acoustic backscattering-immune propagation within topological phononic structures.

Electron-phonon interaction on an Al(001) surface

International Nuclear Information System (INIS)

Sklyadneva, I Yu; Chulkov, E V; Echenique, P M

2008-01-01

We report an ab initio study of the electron-phonon (e-ph) interaction and its contribution to the lifetime broadening of excited hole (electron) surface states on Al(001). The calculations based on density-functional theory were carried out using a linear response approach in the plane-wave pseudopotential representation. The obtained results show that both the electron-phonon coupling and the linewidth experience a weak variation with the energy and momentum position of a hole (electron) surface state in the energy band. An analysis of different contributions to the e-ph coupling reveals that bulk phonon modes turn out to be more involved in the scattering processes of excited electrons and holes than surface phonon modes. It is also shown that the role of the e-ph coupling in the broadening of the Rayleigh surface phonon mode is insignificant compared to anharmonic effects

Optical spectra of Zn{sub 1-x}Be{sub x}Te mixed crystals determined by IR-VIS-UV ellipsometry and photoluminescence measurements

Energy Technology Data Exchange (ETDEWEB)

Wronkowska, A.A., E-mail: aleksandra.wronkowska@utp.edu.p [Institute of Mathematics and Physics, University of Technology and Life Sciences, S. Kaliskiego 7, PL-85796 Bydgoszcz (Poland); Arwin, H. [Department of Physics, Chemistry and Biology, Linkoeping University, SE-58183 Linkoeping (Sweden); Firszt, F.; Legowski, S. [Institute of Physics, Nicholas Copernicus University, Grudziadzka 5, PL-87100 Torun (Poland); Wronkowski, A.; Skowronski, L. [Institute of Mathematics and Physics, University of Technology and Life Sciences, S. Kaliskiego 7, PL-85796 Bydgoszcz (Poland)

2011-02-28

Spectroscopic ellipsometry in the photon energy range from 0.04 eV to 6.50 eV is used for investigation of the optical response of Zn{sub 1-x}Be{sub x}Te crystals grown by a high-pressure Bridgman method in the composition range x {<=} 0.12. Infrared spectra display absorption bands centred between 411 cm{sup -1} and 420 cm{sup -1} associated with BeTe-type optical phonon modes. The positions of the transverse-optical and longitudinal-optical phonon modes have been found by modelling the line shape of the complex dielectric functions, {epsilon}-tilde and Im(-{epsilon}-tilde{sup -1}), using a classical damped Lorentzian oscillator approach. Ellipsometric measurements in the VIS-UV range allow determination of the fundamental energy-gap (E{sub 0}) and the higher threshold energies (E{sub 1}, E{sub 1} + {Delta}{sub 1}, E{sub 2}) originating from the band edge and spin-orbit splitting critical points. We have found that the Be content x = 0.12 causes an increase of the fundamental energy gap about 0.15 eV at room temperature when compared to the E{sub 0} = 2.23 eV of ZnTe crystal at the same temperature. Photoluminescence spectra were measured in the temperature range from 30 K to room temperature. Luminescence at temperature T > 200 K is very weak. The peak positions of the exciton emission lines agree well with the E{sub 0} band-gaps derived from ellipsometric data if corrected for their temperature dependence.

Improved longitudinal magneto-optic Kerr effect signal contrast from nanomagnets with dielectric coatings

Science.gov (United States)

Holiday, L. F.; Gibson, U. J.

2006-12-01

We report on the use of dielectric coatings to improve the contrast of longitudinal magneto-optic Kerr effect signals from submicron magnetic structures. Electron-beam lithography was used to define disks in 22 nm thick Ni films deposited on Si substrates. The structures were measured in four configurations: as-deposited, through a fused silica prism using index-matching fluid, coated with ZnS, and using a prism on top of the ZnS layer. The modified samples show up to 20 times improvement in the MOKE contrast due to admittance matching to the magnetic material and suppression of the substrate reflectance. The behavior is successfully predicted by a model that includes the magneto-optic response of the nickel layer and accounts for the fraction of the beam intercepted by the magnetic structure.

Microscopic theory of longitudinal sound velocity in CDW and SDW ordered cuprate systems

International Nuclear Information System (INIS)

Rout, G.C.; Panda, S.K.

2011-01-01

Research highlights: → Reported the study of the interplay of the CDW and SDW interactions in the high-Tc cuprates. → The longitudinal velocity of sound is studied in the under-doped region. → The velocity of sound exhibits suppression in both the CDW and SDW phases. → Strong electron-phonon interaction is observed in normal phases. - Abstract: We address here the self-consistent calculation of the spin density wave and the charge density wave gap parameters for high-T c cuprates on the basis of the Hubbard model. In order to describe the experimental observations for the velocity of sound, we consider the phonon coupling to the conduction band in the harmonic approximation and then the expression for the temperature dependent velocity of sound is calculated from the real part of the phonon Green's function. The effects of the electron-phonon coupling, the frequency of the sound wave, the hole doping concentration, the CDW coupling and the SDW coupling parameters on the sound velocity are investigated in the pure CDW phase as well as in the co-existence phase of the CDW and SDW states. The results are discussed to explain the experimental observations.

Microscopic theory of longitudinal sound velocity in CDW and SDW ordered cuprate systems

Energy Technology Data Exchange (ETDEWEB)

Rout, G.C., E-mail: gcr@iopb.res.i [Condensed Matter Physics Group, PG Dept. of Applied Physics and Ballistics, FM University, Balasore 756 019 (India); Panda, S K [KD Science College, Pochilima, Hinjilicut 761 101, Ganjam, Orissa (India)

2011-02-15

Research highlights: {yields} Reported the study of the interplay of the CDW and SDW interactions in the high-Tc cuprates. {yields} The longitudinal velocity of sound is studied in the under-doped region. {yields} The velocity of sound exhibits suppression in both the CDW and SDW phases. {yields} Strong electron-phonon interaction is observed in normal phases. - Abstract: We address here the self-consistent calculation of the spin density wave and the charge density wave gap parameters for high-T{sub c} cuprates on the basis of the Hubbard model. In order to describe the experimental observations for the velocity of sound, we consider the phonon coupling to the conduction band in the harmonic approximation and then the expression for the temperature dependent velocity of sound is calculated from the real part of the phonon Green's function. The effects of the electron-phonon coupling, the frequency of the sound wave, the hole doping concentration, the CDW coupling and the SDW coupling parameters on the sound velocity are investigated in the pure CDW phase as well as in the co-existence phase of the CDW and SDW states. The results are discussed to explain the experimental observations.

Microscopic theory of longitudinal sound velocity in CDW and SDW ordered cuprate systems

Energy Technology Data Exchange (ETDEWEB)

Rout, G.C., E-mail: gcr@iopb.res.i [Condensed Matter Physics Group, PG Dept. of Applied Physics and Ballistics, FM University, Balasore 756 019 (India); Panda, S.K. [KD Science College, Pochilima, Hinjilicut 761 101, Ganjam, Orissa (India)

2011-02-15

Research highlights: {yields} Reported the study of the interplay of the CDW and SDW interactions in the high-Tc cuprates. {yields} The longitudinal velocity of sound is studied in the under-doped region. {yields} The velocity of sound exhibits suppression in both the CDW and SDW phases. {yields} Strong electron-phonon interaction is observed in normal phases. - Abstract: We address here the self-consistent calculation of the spin density wave and the charge density wave gap parameters for high-T{sub c} cuprates on the basis of the Hubbard model. In order to describe the experimental observations for the velocity of sound, we consider the phonon coupling to the conduction band in the harmonic approximation and then the expression for the temperature dependent velocity of sound is calculated from the real part of the phonon Green's function. The effects of the electron-phonon coupling, the frequency of the sound wave, the hole doping concentration, the CDW coupling and the SDW coupling parameters on the sound velocity are investigated in the pure CDW phase as well as in the co-existence phase of the CDW and SDW states. The results are discussed to explain the experimental observations.

Toward stimulated interaction of surface phonon polaritons

Energy Technology Data Exchange (ETDEWEB)

Kong, B. D.; Trew, R. J.; Kim, K. W., E-mail: kwk@ncsu.edu [Department of Electrical and Computer Engineering, North Carolina State University, Raleigh, North Carolina 27695-7911 (United States)

2013-12-21

Thermal emission spectra mediated by surface phonon polariton are examined by using a theoretical model that accounts for generation processes. Specifically, the acoustic phonon fusion mechanism is introduced to remedy theoretical deficiencies of the near thermal equilibrium treatments. The model clarifies the thermal excitation mechanism of surface phonon polaritons and the energy transfer path under non-zero energy flow. When applied to GaAs and SiC semi-infinite surfaces, the nonequilibrium model predicts that the temperature dependence of the quasi-monochromatic peak can exhibit distinctly different characteristics of either sharp increase or slow saturation depending on the materials, which is in direct contrast with the estimate made by the near-equilibrium model. The proposed theoretical tool can accurately analyze the nonequilibrium steady states, potentially paving a pathway to demonstrate stimulated interaction/emission of thermally excited surface phonon polaritons.

Influence of the Pauli principle on the two-phonon states

International Nuclear Information System (INIS)

Djolos, R.V.; Molina, J.L.; Soloviev, V.G.

1979-01-01

It is shown that the commutation relations between quasiparticles forming phonons can correctly be taken into account within the quasiparticle-phonon nuclear model. The case of the even-even deformed nuclei is studied. Exact and approximate secular equations are obtained. The corrections arising due to the Pauli principle are shown to be large for the two-phonon components of the wave functions, when the phonons are identical. The influence of the Pauli principle on the energies of the two-phonon states and radiative strength functions requires further investigation [ru

Diamond family of colloidal supercrystals as phononic metamaterials

Science.gov (United States)

Aryana, Kiumars; Zanjani, Mehdi B.

2018-05-01

Colloidal crystals provide a versatile platform for designing phononic metamaterials with exciting applications for sound and heat management. New advances in the synthesis and self-assembly of anisotropic building blocks such as colloidal clusters have expanded the library of available micro- and nano-scale ordered multicomponent structures. Diamond-like supercrystals formed by such clusters and spherical particles are notable examples that include a rich family of crystal symmetries such as diamond, double diamond, zinc-blende, and MgCu2. This work investigates the design of phononic supercrystals by predicting and analyzing phonon transport properties. In addition to size variation and structural diversity, these supercrystals encapsulate different sub-lattice types within one structure. Computational models are used to calculate the effect of various parameters on the phononic spectrum of diamond-like supercrystals. The results show that structures with relatively small or large filling factors (f > 0.65 or f f > 0.45). The double diamond and zinc-blende structures render the largest bandgap size compared to the other supercrystals studied in this paper. Additionally, this article discusses the effect of incorporating various configurations of sub-lattices by selecting different material compositions for the building blocks. The results suggest that, for the same structure, there exist multiple phononic variants with drastically different band structures. This study provides a valuable insight for evaluating novel colloidal supercrystals for phononic applications and guides the future experimental work for the synthesis of colloidal structures with desired phononic behavior.

Phonon emission in a degenerate semiconductor at low lattice temperatures

International Nuclear Information System (INIS)

Midday, S.; Nag, S.; Bhattacharya, D.P.

2015-01-01

The characteristics of phonon growth in a degenerate semiconductor at low lattice temperatures have been studied for inelastic interaction of non-equilibrium electrons with the intravalley acoustic phonons. The energy of the phonon and the full form of the phonon distribution are taken into account. The results reveal significant changes in the growth characteristics compared to the same for a non-degenerate material

Optical Nonlinearities and Ultrafast Carrier Dynamics in Semiconductor Quantum Dots

Energy Technology Data Exchange (ETDEWEB)

Klimov, V.; McBranch, D.; Schwarz, C.

1998-08-10

Low-dimensional semiconductors have attracted great interest due to the potential for tailoring their linear and nonlinear optical properties over a wide-range. Semiconductor nanocrystals (NC's) represent a class of quasi-zero-dimensional objects or quantum dots. Due to quantum cordhement and a large surface-to-volume ratio, the linear and nonlinear optical properties, and the carrier dynamics in NC's are significantly different horn those in bulk materials. napping at surface states can lead to a fast depopulation of quantized states, accompanied by charge separation and generation of local fields which significantly modifies the nonlinear optical response in NC's. 3D carrier confinement also has a drastic effect on the energy relaxation dynamics. In strongly confined NC's, the energy-level spacing can greatly exceed typical phonon energies. This has been expected to significantly inhibit phonon-related mechanisms for energy losses, an effect referred to as a phonon bottleneck. It has been suggested recently that the phonon bottleneck in 3D-confined systems can be removed due to enhanced role of Auger-type interactions. In this paper we report femtosecond (fs) studies of ultrafast optical nonlinearities, and energy relaxation and trap ping dynamics in three types of quantum-dot systems: semiconductor NC/glass composites made by high temperature precipitation, ion-implanted NC's, and colloidal NC'S. Comparison of ultrafast data for different samples allows us to separate effects being intrinsic to quantum dots from those related to lattice imperfections and interface properties.

Frictional drag between quantum wells mediated by phonon exchange

DEFF Research Database (Denmark)

Bønsager, M.C.; Flensberg, Karsten; Hu, Ben Yu-Kuang

1998-01-01

We use the Kubo formalism to evaluate the contribution of acoustic-phonon exchange to the frictional drag between nearby two-dimensional electron systems. In the case of free phonons, we find a divergent drag rate (tau(D)(-l)). However, tau(D)(-l) becomes finite when phonon scattering from either...

Phonon-based scalable platform for chip-scale quantum computing

Directory of Open Access Journals (Sweden)

Charles M. Reinke

2016-12-01

Full Text Available We present a scalable phonon-based quantum computer on a phononic crystal platform. Practical schemes involve selective placement of a single acceptor atom in the peak of the strain field in a high-Q phononic crystal cavity that enables coupling of the phonon modes to the energy levels of the atom. We show theoretical optimization of the cavity design and coupling waveguide, along with estimated performance figures of the coupled system. A qubit can be created by entangling a phonon at the resonance frequency of the cavity with the atom states. Qubits based on this half-sound, half-matter quasi-particle, called a phoniton, may outcompete other quantum architectures in terms of combined emission rate, coherence lifetime, and fabrication demands.

The phonon-polariton spectrum of one-dimensional Rudin-Shapiro photonic superlattices with uniaxial polar materials

Science.gov (United States)

Gómez-Urrea, H. A.; Duque, C. A.; Mora-Ramos, M. E.

2015-11-01

The properties of the optical-phonon-associated polaritonic modes that appear under oblique light incidence in 1D superlattices made of photonic materials are studied. The investigated systems result from the periodic repetition of quasiregular Rudin-Shapiro (RS) multilayer units. It is assume that the structure consists of both passive non-dispersive layers of constant refraction index and active layers of uniaxial polar materials. In particular, we consider III-V wurtzite nitrides. The optical axis of these polaritonic materials is taken along the growth direction. Maxwell equations are solved using the transfer matrix technique for all admissible values of the incidence angle.

Phonon-assisted tunneling and its dependence on pressure

International Nuclear Information System (INIS)

Roy, P.N.; Singh, A.P.; Thakur, B.N.

1999-01-01

First the mechanism of phonon-assisted tunneling has been investigated. The indirect tunnel current density has been computed after taking the amplitude of the time dependent perturbation as the energy of the lattice vibration. Later the pressure dependence of the phonon-assisted tunnel current has been computed using Payne's expression for the dependence of phonon frequency on pressure. Very good qualitative agreements are obtained between predicted and observed characteristics. (author)

One and two-phonon processes of the spin-flip relaxation in quantum dots: Spin-phonon coupling mechanism

Science.gov (United States)

Wang, Zi-Wu; Li, Shu-Shen

2012-07-01

We investigate the spin-flip relaxation in quantum dots using a non-radiation transition approach based on the descriptions for the electron-phonon deformation potential and Fröhlich interaction in the Pavlov-Firsov spin-phonon Hamiltonian. We give the comparisons of the electron relaxations with and without spin-flip assisted by one and two-phonon processes. Calculations are performed for the dependence of the relaxation time on the external magnetic field, the temperature and the energy separation between the Zeeman sublevels of the ground and first-excited state. We find that the electron relaxation time of the spin-flip process is more longer by three orders of magnitudes than that of no spin-flip process.

Effects of strain on phonon interactions and phase nucleation in several semiconductor and nano particle systems

Science.gov (United States)

Tallman, Robert E.

Raman scattering is utilized to explore the effects of applied pressure and strain on anharmonic phonon interactions and nucleation of structural transitions in several bulk and nanoparticle semiconductor systems. The systems investigated are bulk ZnS and ZnSe in several isotopic compositions, InP/CdS core/shell nanoparticles exhibiting confined and surface optical Raman modes, and amorphous selenium films undergoing photo-induced crystallization. The anharmonic decay of long-wavelength optical modes into two-phonon acoustic combinations modes is studied in 64Zn32S, 64Zn34S, natZnatS bulk crystals by measuring the TO(Gamma) Raman line-shape as a function of applied hydrostatic pressure. The experiments are carried out at room temperature and 16K for pressures up to 150 kbars using diamond-anvil cells. The most striking effects occur in 68Zn32S where the TO(Gamma) peak narrows by a factor of 10 and increases in intensity at pressures for which the TO(Gamma) frequency has been tuned into a gap in the two-phonon density of states (DOS). In all the isotopic compositions, the observed phonon decay processes can be adequately explained by a second order perturbation treatment of the anharmonic coupling between TO(Gamma) and TA + LA combinations at various critical points, combined with an adiabatic bond-charge model for the phonon DOS and the known mode Gruneisen parameters. Bulk ZnSe crystals exhibit very different behavior. Here we find that anharmonic decay alone can not explain the excessive (˜ 60 cm-1 ) broadening in the TO(Gamma) Raman peak observed as the pressure approaches to within 50kbar of the ZB -> B1 phase transition (at P ˜ 137 kbar). Rather the broadening appears to arise from antecedent nucleation of structural changes within nanoscopic domains, with the mechanism for line-shape changes being mode mixing via localization and disorder instead of anharmonicity. To sort out these contributions, pressure experiments on natural ZnSe and on isotopically pure

Depth profile of strain and composition in Si/Ge dot multilayers by microscopic phonon Raman spectroscopy

International Nuclear Information System (INIS)

Tan, P.H.; Bougeard, D.; Abstreiter, G.; Brunner, K.

2005-01-01

We characterized strain and Ge content depending on depth in a self-assembled Si/Ge dot multilayer by scanning a microscopic Raman probe at a (110) cleavage plane. The multilayer structure was deposited by molecular-beam epitaxy on a (001) Si substrate and consisted of 80 periods, each of them composed by 25 nm Si spacers and 8 monolayer Ge forming laterally and vertically uncorrelated islands with a height of 2 nm and a lateral diameter of about 20 nm. An average biaxial strain of -3.5% within the core regions of islands is determined from the splitting of longitudinal and transversal optical Ge-Ge phonon modes observed in polarized Raman measurements. The absolute mode frequencies further enable analysis of a Ge content of 0.82. The analyzed strain and composition of islands are nearly independent from depths below the sample surface. This indicates well-controlled deposition parameters and negligible intermixing during deposition of subsequent layers. These Raman results are in agreement with x-ray diffraction data. Small, local Raman frequency shifts were observed and discussed with respect to partial elastic strain relaxation of the multilayer stack after cleavage, undefined Raman-scattering geometries at the sample edge, and local heating by the laser probe

Nonlinear optical diagnostic of semimagnetic semiconductors Pb1-xYb xX (X = S, Se, Te)

International Nuclear Information System (INIS)

Nouneh, K.; Kityk, I.V.; Viennois, R.; Benet, S.; Charar, S.; Plucinski, K.J.

2007-01-01

Nonlinear optical measurements were performed to elucidate the influence of magnetic ions on the behavior of charge carriers in magnetic semiconductors-Pb 1-x Yb x X (X = S, Se, Te at x = 1-3%). It was shown that nonlinear optical methods could be used as sensitive tools for investigations of electron-phonon anharmonicity near low-temperature semiconductor-insulator phase transitions. There exists a difference between surface and bulk-like contributions to the nonlinear optical effects. It was shown that only low-temperature Two Photon Absorption (TPA) oscillator may be related to the number of the electron-phonon anharmonic modes responsible for the observed phase transformation. The explanation of the anomalous temperature dependences is given in accordance with dipole momentum's behaviors determined by low-temperature spin-spin interactions and by electron-phonon anharmonic interactions. We have discovered that low-temperature dependence of specific heat of Pb 1-x R x Te (R = Yb, Pr with x = 3% and 1.6%, respectively) exhibits a non-magnetic order caused by large electron-phonon contributions and structural disorder effects

Phonon shake-up satellites in x-ray absorption: an operator approach

International Nuclear Information System (INIS)

Bryant, G.W.

1980-01-01

The phonon shake-up that occurs when the linear and quadratic phonon potentials both change during x-ray absorption is considered. Full account of all quadratic terms and the competition between linear and quadratic shake-up effects is made. Many previous studies of quadratic phonon shake-up have used a wavefunction approach. The phonon matrix elements have been determined by explicit evaluation of the overlap integrals. However, an equations of motion approach is used to transform the time evolution operator to a form that allows an exact evaluation of the phonon matrix elements needed to describe the spectra. This theory is used to determine the strengths of the phonon shake-up satellites in x-ray absorption spectra at zero temperature. An exact expression is obtained for the strength of each satellite. During quadratic shake-up, two phonon transitions and phonon frequency shifts occur. Both effects significantly change the strength of a a satellite from that predicted for linear shake-up alone. Inclusion of the two phonon transitions enhances the high-energy satellites. Inclusion of the frequency shifts can either broaden the spectra or increase the strength of the zero phonon lines depending on the sign of the frequency shift. (author)

Effects of the electron-phonon coupling activation in collision cascades

Energy Technology Data Exchange (ETDEWEB)

Zarkadoula, Eva, E-mail: zarkadoulae@ornl.gov [Materials Science & Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831 (United States); Samolyuk, German [Materials Science & Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831 (United States); Weber, William J. [Materials Science & Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831 (United States); Department of Materials Science & Engineering, University of Tennessee, Knoxville, TN 37996 (United States)

2017-07-15

Using the two-temperature (2T-MD) model in molecular dynamics simulations, we investigate the condition of switching the electronic stopping term off when the electron-phonon coupling is activated in the damage production due to 50 keV Ni ion cascades in Ni and equiatomic NiFe. Additionally, we investigate the effect of the electron-phonon coupling activation time in the damage production. We find that the switching condition has negligible effect in the produced damage, while the choice of the activation time of the electron-phonon coupling can affect the amount of surviving damage. - Highlights: •The electron-phonon interactions in irradiation affect the energy dissipation. •The resulting damage depends on the electron-phonon interaction activation time. •The electronic stopping acts on the ions before the electron-phonon interactions.

Phonon response of some heavy Fermion systems in dynamic limit

Science.gov (United States)

Sahoo, Jitendra; Shadangi, Namita; Nayak, Pratibindhya

2017-05-01

The phonon excitation spectrum of some Heavy Fermion (HF) systems in the presence of electron-phonon interaction is studied in the dynamic limit (ω≠0). The renormalized excitation phonon frequencies (ω˜ = ω/ω0) are evaluated through Periodic Anderson Model (PAM) in the presence of electron-phonon interaction using Zubarev-type double time temperature-dependent Green function. The calculated renormalized phonon energy is analyzed through the plots of (ω˜ = ω/ω0) against temperature for different system parameters like effective coupling strength ‘g’ and the position of f-level ‘d’. The observed behavior is analyzed and found to agree with the general features of HF systems found in experiments. Further, it is observed that in finite but small q-values the propagating phonons harden and change to localized peaks.

Search for the 3-phonon state of 40Ca

International Nuclear Information System (INIS)

Fallot, M.

2002-09-01

We study collective vibrational states of the nucleus: giant resonances and multiphonon states. It has been shown that multiphonon states, which are built with several superimposed giant resonances, can be excited in inelastic heavy ion scattering near the grazing angle. No three photon states have been observed until now. An experiment has been performed at GANIL, aiming at the observation of the 3-phonon state built with the giant quadrupole resonance (GQR) in 40 Ca, with the reaction 40 Ca + 40 Ca at 50 A.Me.V. The ejectile was identified in the SPEG spectrometer. Light charged particles were detected in 240 CsI scintillators of the INDRA 4π array. The analysis confirms the previous results about the GQR and the 2-phonon state in 40 Ca. For the first time, we have measured an important direct decay branch of the GQR by alpha particles. Applying the so-called 'missing energy method' to events containing three protons measured in coincidence with the ejectile, we observe a direct decay branch revealing the presence of a 3-phonon state in the excitation energy region expected for the triple GQR. Dynamical processes are also studied in the inelastic channel, emphasizing a recently discovered mechanism named towing-mode. We observe for the first time the towing-mode of alpha particles. The energies of multiphonon states in 40 Ca and 208 Pb have been computed microscopically including some anharmonicities via boson mapping methods. The basis of the calculation has been extended to the 3-phonon states. Our results show large anharmonicities (several MeV), due to the coupling of 3-phonon states to 2-phonon states. The extension of the basis to 4-phonon states has been performed for the first time. The inclusion of the 4 phonon states in the calculation did not affect the previous observations concerning the 2-phonon states. Preliminary results on the anharmonicities of the 3-phonon states are presented. (author)

Theory of electron-phonon-dislon interacting system—toward a quantized theory of dislocations

Science.gov (United States)

Li, Mingda; Tsurimaki, Yoichiro; Meng, Qingping; Andrejevic, Nina; Zhu, Yimei; Mahan, Gerald D.; Chen, Gang

2018-02-01

We provide a comprehensive theoretical framework to study how crystal dislocations influence the functional properties of materials, based on the idea of a quantized dislocation, namely a ‘dislon’. In contrast to previous work on dislons which focused on exotic phenomenology, here we focus on their theoretical structure and computational power. We first provide a pedagogical introduction that explains the necessity and benefits of taking the dislon approach and why the dislon Hamiltonian takes its current form. Then, we study the electron-dislocation and phonon-dislocation scattering problems using the dislon formalism. Both the effective electron and phonon theories are derived, from which the role of dislocations on electronic and phononic transport properties is computed. Compared with traditional dislocation scattering studies, which are intrinsically single-particle, low-order perturbation and classical quenched defect in nature, the dislon theory not only allows easy incorporation of quantum many-body effects such as electron correlation, electron-phonon interaction, and higher-order scattering events, but also allows proper consideration of the dislocation’s long-range strain field and dynamic aspects on equal footing for arbitrary types of straight-line dislocations. This means that instead of developing individual models for specific dislocation scattering problems, the dislon theory allows for the calculation of electronic structure and electrical transport, thermal transport, optical and superconducting properties, etc, under one unified theory. Furthermore, the dislon theory has another advantage over empirical models in that it requires no fitting parameters. The dislon theory could serve as a major computational tool to understand the role of dislocations on multiple materials’ functional properties at an unprecedented level of clarity, and may have wide applications in dislocated energy materials.

A new class of tunable hypersonic phononic crystals based on polymer-tethered colloids.

Science.gov (United States)

Alonso-Redondo, E; Schmitt, M; Urbach, Z; Hui, C M; Sainidou, R; Rembert, P; Matyjaszewski, K; Bockstaller, M R; Fytas, G

2015-09-22

The design and engineering of hybrid materials exhibiting tailored phononic band gaps are fundamentally relevant to innovative material technologies in areas ranging from acoustics to thermo-optic devices. Phononic hybridization gaps, originating from the anti-crossing between local resonant and propagating modes, have attracted particular interest because of their relative robustness to structural disorder and the associated benefit to 'manufacturability'. Although hybridization gap materials are well known, their economic fabrication and efficient control of the gap frequency have remained elusive because of the limited property variability and expensive fabrication methodologies. Here we report a new strategy to realize hybridization gap materials by harnessing the 'anisotropic elasticity' across the particle-polymer interface in densely polymer-tethered colloidal particles. Theoretical and Brillouin scattering analysis confirm both the robustness to disorder and the tunability of the resulting hybridization gap and provide guidelines for the economic synthesis of new materials with deliberately controlled gap position and width frequencies.

A new class of tunable hypersonic phononic crystals based on polymer-tethered colloids

Science.gov (United States)

Alonso-Redondo, E.; Schmitt, M.; Urbach, Z.; Hui, C. M.; Sainidou, R.; Rembert, P.; Matyjaszewski, K.; Bockstaller, M. R.; Fytas, G.

2015-09-01

The design and engineering of hybrid materials exhibiting tailored phononic band gaps are fundamentally relevant to innovative material technologies in areas ranging from acoustics to thermo-optic devices. Phononic hybridization gaps, originating from the anti-crossing between local resonant and propagating modes, have attracted particular interest because of their relative robustness to structural disorder and the associated benefit to `manufacturability'. Although hybridization gap materials are well known, their economic fabrication and efficient control of the gap frequency have remained elusive because of the limited property variability and expensive fabrication methodologies. Here we report a new strategy to realize hybridization gap materials by harnessing the `anisotropic elasticity' across the particle-polymer interface in densely polymer-tethered colloidal particles. Theoretical and Brillouin scattering analysis confirm both the robustness to disorder and the tunability of the resulting hybridization gap and provide guidelines for the economic synthesis of new materials with deliberately controlled gap position and width frequencies.

Controllable photon and phonon localization in optomechanical Lieb lattices.

Science.gov (United States)

Wan, Liang-Liang; Lü, Xin-You; Gao, Jin-Hua; Wu, Ying

2017-07-24

The Lieb lattice featuring flat band is not only important in strongly-correlated many-body physics, but also can be utilized to inspire new quantum devices. Here we propose an optomechanical Lieb lattice, where the flat-band physics of photon-phonon polaritons is demonstrated. The tunability of the band structure of the optomechanical arrays allows one to obtain an approximate photon or phonon flat band as well as the transition between them. This ultimately leads to the result that the controllable photon or phonon localization could be realized by the path interference effects. This study offers an alternative approach to explore the exotic photon and phonon many-body effects, which has potential applications in the future hybrid-photon-phonon quantum network and engineering new type solid-state quantum devices.

Thermal rectification based on phonon hydrodynamics and thermomass theory

Directory of Open Access Journals (Sweden)

Dong Yuan

2016-06-01

Full Text Available The thermal diode is the fundamental device for phononics. There are various mechanisms for thermal rectification, e.g. different temperature dependent thermal conductivity of two ends, asymmetric interfacial resistance, and nonlocal behavior of phonon transport in asymmetric structures. The phonon hydrodynamics and thermomass theory treat the heat conduction in a fluidic viewpoint. The phonon gas flowing through the media is characterized by the balance equation of momentum, like the Navier-Stokes equation for fluid mechanics. Generalized heat conduction law thereby contains the spatial acceleration (convection term and the viscous (Laplacian term. The viscous term predicts the size dependent thermal conductivity. Rectification appears due to the MFP supersession of phonons. The convection term also predicts rectification because of the inertia effect, like a gas passing through a nozzle or diffuser.

Resonant tunneling in a pulsed phonon field

DEFF Research Database (Denmark)

Kral, P.; Jauho, Antti-Pekka

1999-01-01

, The nonequilibrium spectral function for the resonance displays the formation and decay of the phonon sidebands on ultrashort time scales. The time-dependent tunneling current through the individual phonon satellites reflects this quasiparticle formation by oscillations, whose time scale is set by the frequency...

Electron-phonon interaction in Chevrel-phase compounds

International Nuclear Information System (INIS)

Rainer, D.; Pobell, F.

1981-03-01

Experiments on the electron-phonon interaction in Chevrel-phase compounds (CPC) and a theoretical discussion of their results are presented. The authors particularly discuss measurements of the isotope effect of the transition temperature in Mo 6 Se 8 and SnMo 6 S 8 and tunneling spectroscopy experiments on Cu 1 . 8 Mo 6 S 8 and PbMo 6 S 8 . These investigations have been performed to get information about the strength of the electron-phonon interaction in CPC, and about the question whether there are phonon modes which couple particularly strongly to the electrons in these compounds. (orig./GSCH)

Theoretical modelling of electron transport in InAs/GaAs quantum dot superlattices

Energy Technology Data Exchange (ETDEWEB)

Vukmirovic, Nenad; Ikonic, Zoran; Savic, Ivana; Indjin, Dragan; Harrison, Paul [School of Electronic and Electrical Engineering, University of Leeds, Leeds LS2 9JT (United Kingdom)

2006-07-01

A theoretical model describing the electron transport in InAs/GaAs quantum dot infrared photodetectors, modelled as ideal quantum dot superlattices, is presented. The carrier wave functions and energy levels were evaluated using the strain dependent 8-band k.p Hamiltonian and used to calculate all intra- and inter-period transition rates due to interaction with phonons and electromagnetic radiation. The interaction with longitudinal acoustic phonons and electromagnetic radiation was treated perturbatively within the framework of Fermi's golden rule, while the interaction with longitudinal optical phonons was considered taking into account their strong coupling to electrons. The populations of energy levels were then found from a system of rate equations, and the electron current in the superlattice was subsequently extracted. (copyright 2006 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)

Search for the 3-phonon state of {sup 40}Ca; Recherche de l'etat a trois phonons dans le {sup 40}Ca

Energy Technology Data Exchange (ETDEWEB)

Fallot, M

2002-09-01

We study collective vibrational states of the nucleus: giant resonances and multiphonon states. It has been shown that multiphonon states, which are built with several superimposed giant resonances, can be excited in inelastic heavy ion scattering near the grazing angle. No three photon states have been observed until now. An experiment has been performed at GANIL, aiming at the observation of the 3-phonon state built with the giant quadrupole resonance (GQR) in {sup 40}Ca, with the reaction {sup 40}Ca + {sup 40}Ca at 50 A.Me.V. The ejectile was identified in the SPEG spectrometer. Light charged particles were detected in 240 CsI scintillators of the INDRA 4{pi} array. The analysis confirms the previous results about the GQR and the 2-phonon state in {sup 40}Ca. For the first time, we have measured an important direct decay branch of the GQR by alpha particles. Applying the so-called 'missing energy method' to events containing three protons measured in coincidence with the ejectile, we observe a direct decay branch revealing the presence of a 3-phonon state in the excitation energy region expected for the triple GQR. Dynamical processes are also studied in the inelastic channel, emphasizing a recently discovered mechanism named towing-mode. We observe for the first time the towing-mode of alpha particles. The energies of multiphonon states in {sup 40}Ca and {sup 208}Pb have been computed microscopically including some anharmonicities via boson mapping methods. The basis of the calculation has been extended to the 3-phonon states. Our results show large anharmonicities (several MeV), due to the coupling of 3-phonon states to 2-phonon states. The extension of the basis to 4-phonon states has been performed for the first time. The inclusion of the 4 phonon states in the calculation did not affect the previous observations concerning the 2-phonon states. Preliminary results on the anharmonicities of the 3-phonon states are presented. (author)

Phonon replica dynamics in high quality GaN epilayers and AlGaN/GaN quantum wells

Energy Technology Data Exchange (ETDEWEB)

Alderighi, D.; Vinattieri, A.; Colocci, M. [Ist. Nazionale Fisica della Materia, Firenze (Italy); Dipt. di Fisica and LENS, Firenze (Italy); Bogani, F. [Ist. Nazionale Fisica della Materia, Firenze (Italy); Dipt. di Energetica, Firenze (Italy); Gottardo, S. [Dipt. di Fisica and LENS, Firenze (Italy); Grandjean, N.; Massies, J. [Centre de Recherche sur l' Hetero-Epitaxie et ses Applications, CNRS, Valbonne (France)

2001-01-01

We present an experimental study of the exciton and phonon replica dynamics in high quality GaN epilayers and AlGaN/GaN quantum wells (QW) by means of picosecond time-resolved photoluminescence (PL) measurements. A non-exponential decay is observed both at the zero phonon line (ZPL) and at the n = 1 LO replica. Time-resolved spectra unambiguously assign the replica to the free exciton A recombination. Optical migration effects are detected both in the epilayer and the QWs samples and disappear as the temperature increases up to 60-90 K. Even though the sample quality is comparable to state-of-the-art samples, localization effects dominate the exciton dynamics at low temperature in the studied GaN based structures. (orig.)

Hydrodynamic states of phonons in insulators

Directory of Open Access Journals (Sweden)

S.A. Sokolovsky

2012-12-01

Full Text Available The Chapman-Enskog method is generalized for accounting the effect of kinetic modes on hydrodynamic evolution. Hydrodynamic states of phonon system of insulators have been studied in a small drift velocity approximation. For simplicity, the investigation was carried out for crystals of the cubic class symmetry. It has been found that in phonon hydrodynamics, local equilibrium is violated even in the approximation linear in velocity. This is due to the absence of phonon momentum conservation law that leads to a drift velocity relaxation. Phonon hydrodynamic equations which take dissipative processes into account have been obtained. The results were compared with the standard theory based on the local equilibrium validity. Integral equations have been obtained for calculating the objects of the theory (including viscosity and heat conductivity. It has been shown that in low temperature limit, these equations are solvable by iterations. Steady states of the system have been considered and an expression for steady state heat conductivity has been obtained. It coincides with the famous result by Akhiezer in the leading low temperature approximation. It has been established that temperature distribution in the steady state of insulator satisfies a condition of heat source absence.

Kohn anomaly in phonon driven superconductors

International Nuclear Information System (INIS)

Das, M P; Chaudhury, R

2014-01-01

Anomalies often occur in the physical world. Sometimes quite unexpectedly anomalies may give rise to new insight to an unrecognized phenomenon. In this paper we shall discuss about Kohn anomaly in a conventional phonon-driven superconductor by using a microscopic approach. Recently Aynajian et al.'s experiment showed a striking feature; the energy of phonon at a particular wave-vector is almost exactly equal to twice the energy of the superconducting gap. Although the phonon mechanism of superconductivity is well known for many conventional superconductors, as has been noted by Scalapino, the new experimental results reveal a genuine puzzle. In our recent work we have presented a detailed theoretical analysis with the help of microscopic calculations to unravel this mystery. We probe this aspect of phonon behaviour from the properties of electronic polarizability function in the superconducting phase of a Fermi liquid metal, leading to the appearance of a Kohn singularity. We show the crossover to the standard Kohn anomaly of the normal phase for temperatures above the transition temperature. Our analysis provides a nearly complete explanation of this new experimentally discovered phenomenon. This report is a shorter version of our recent work in JPCM.

Generation of acoustic phonons from quasi-two-dimensional hole gas

International Nuclear Information System (INIS)

Singh, J.; Oh, I.K.

2002-01-01

Full text: Generation of phonons from two dimensional electron and hole gases in quantum wells has attracted much attraction recently. The mechanism of phonon emission plays an important role in the phonon spectroscopy which enables us to study the angular and polarization dependence of phonon emission. The acoustic phonon emission from a quasi-two-dimensional hole gas (2DHG) in quantum wells is influenced by the anisotropic factors in the valence band structure, screening, elastic property, etc. The anisotropy in the valence band structure gives rise to anisotropic effective mass and deformation potential and that in the elastic constants leads to anisotropic sound velocity. Piezoelectric coupling in non-centrosymmetric materials such as GaAs is also anisotropic. In this paper, considering the anisotropy in the effective mass, deformation potential, piezoelectric coupling and screening effect, we present a theory to study the angular and polarization dependence of acoustic phonon emission from a quasi-2DHG in quantum wells. The theory is finally applied to calculate the rate of acoustic phonon emission in GaAs quantum wells