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1

Exciton-relaxation dynamics in lead halides  

International Nuclear Information System (INIS)

We survey recent comprehensive studies of exciton relaxation in the crystals of lead halides. The luminescence and electron-spin-resonance studies have revealed that excitons in lead bromide spontaneously dissociate and both electrons and holes get self-trapped individually. Similar relaxation has been also clarified in lead chloride. The electron-hole separation is ascribed to repulsive correlation via acoustic phonons. Besides, on the basis of the temperature profiles of self-trapped states, we discuss the origin of luminescence components which are mainly induced under one-photon excitation into the exciton band in lead fluoride, lead chloride, and lead bromide

2003-05-01

2

Exciton-relaxation dynamics in lead halides  

Energy Technology Data Exchange (ETDEWEB)

We survey recent comprehensive studies of exciton relaxation in the crystals of lead halides. The luminescence and electron-spin-resonance studies have revealed that excitons in lead bromide spontaneously dissociate and both electrons and holes get self-trapped individually. Similar relaxation has been also clarified in lead chloride. The electron-hole separation is ascribed to repulsive correlation via acoustic phonons. Besides, on the basis of the temperature profiles of self-trapped states, we discuss the origin of luminescence components which are mainly induced under one-photon excitation into the exciton band in lead fluoride, lead chloride, and lead bromide.

Iwanaga, Masanobu E-mail: iwanaga@phys.h.kyoto-u.ac.jp; Hayashi, Tetsusuke

2003-05-01

3

Exciton-relaxation dynamics in lead halides  

CERN Document Server

We survey recent comprehensive studies of exciton relaxation in the crystals of lead halides. The luminescence and electron-spin-resonance studies have revealed that excitons in lead bromide spontaneously dissociate and both electrons and holes get self-trapped individually. Similar relaxation has been also clarified in lead chloride. The electron-hole separation is ascribed to repulsive correlation via acoustic phonons. Besides, on the basis of the temperature profiles of self-trapped states, we discuss the origin of luminescence components which are mainly induced under one-photon excitation into the exciton band in lead fluoride, lead chloride, and lead bromide.

Iwanaga, M; Iwanaga, Masanobu; Hayashi, Tetsusuke

2003-01-01

4

Excitonic Bands in the Photoconductivity Spectra of Some Organic-Inorganic Hybrid Compounds Based on Metal Halide Units  

Science.gov (United States)

The photoconductivity (PC) spectra of the compounds CH3NH3PbI3, [CH3NH3][CH3C6H4CH2NH3]2Pb2I7, [CH3C6H4CH2NH3]2PbI4, [H2NC(I)=NH2]3PbI5 as well as the spectra of similar compounds based on C10H21SC(NH2)2, H3N(CH2)6NH3, C14H9CH2SC(NH2)2, SnI4, PbClxI4-x and PbBrxI4-x are described. The position, intensity and shape of the PC bands depend on the dimensionality (or size) of the inorganic network as well as on the nature of metal halide and the organic groups. Excitonic features are discussed.

Papavassiliou, G. C.; Mousdis, G. A.; Koutselas, I. B.; Papaioannou, G. J.

2001-08-01

5

Electronic band structure, phonons, and exciton binding energies of halide perovskites CsSnCl3, CsSnBr3, and CsSnI3  

Science.gov (United States)

The halide perovskites CsSnX3, with X= Cl, Br, I, are investigated using quasiparticle self-consistent GW electronic structure calculations. These materials are found to have an “inverted” band structure from most semiconductors with a nondegenerate s-like valence band maximum (VBM) and triply degenerate p-like conduction band minimum (CBM). The small hole effective mass results in high hole mobility, in agreement with recent reports for CsSnI3. The relatively small band gap changes from Cl to Br to I result from the intra-atomic Sn s and Sn p characters of the VBM and CBM, respectively. The latter is also responsible for the high oscillator strength of the optical transition in these direct-gap semiconductors and hence a strong luminescence and absorption. The band gap change with lattice constant is also anomalous. It increases with increasing lattice constant, and this results from the decreasing valence band width due to the decreased Sn s with anion p interaction. It leads to an anomalous temperature dependence of the gap. The changes in band gap in different lower-symmetry crystallographic phases is studied. The exciton binding energy of the free exciton, estimated from the Wannier-Mott exciton theory and the calculated dielectric constants and effective masses, is found to be two orders of magnitude smaller than previously claimed in literature, or of the order of 0.1 meV. The photoluminescence peak previously assigned to the free exciton is instead ascribed to an acceptor bound exciton. The phonons at the ? point are calculated as well as the related enhancement of the dielectric constants.

Huang, Ling-yi; Lambrecht, Walter R. L.

2013-10-01

6

Excitons versus free charges in organo-lead tri-halide perovskites  

Science.gov (United States)

Excitonic solar cells, within which bound electron-hole pairs have a central role in energy harvesting, have represented a hot field of research over the last two decades due to the compelling prospect of low-cost solar energy. However, in such cells, exciton dissociation and charge collection occur with significant losses in energy, essentially due to poor charge screening. Organic–inorganic perovskites show promise for overcoming such limitations. Here, we use optical spectroscopy to estimate the exciton binding energy in the mixed-halide crystal to be in the range of 50?meV. We show that such a value is consistent with almost full ionization of the exciton population under photovoltaic cell operating conditions. However, increasing the total photoexcitation density, excitonic species become dominant, widening the perspective of this material for a host of optoelectronic applications.

D’Innocenzo, Valerio; Grancini, Giulia; Alcocer, Marcelo J. P.; Kandada, Ajay Ram Srimath; Stranks, Samuel D.; Lee, Michael M.; Lanzani, Guglielmo; Snaith, Henry J.; Petrozza, Annamaria

2014-04-01

7

Optical properties of halide and oxide compounds including the excitonic effects  

Science.gov (United States)

We have studied the optical properties of alkali halide and alkaline-earth oxide compounds including the excitonic effects by using the newly developed bootstrap kernel approximation for the exchange-correlation kernel of the Time-Dependent Density Functional Theory (TD-DFT) implemented in Full-Potential Linearized Augmented Plane Wave (FP-LAPW) method in the elk code. The bootstrap calculations are computationally less expensive and give results the same quality as the Bethe-Salpeter equation. We found improved results when compared to normal Density Functional Theory calculations, and observed results are comparable with the experiments. The lower energy peak of imaginary part of dielectric spectra shifts to lower energy regions as we move from MgO to BaO indicating the decrease in the band gap of these compounds from MgO to BaO. In all the studied compounds, the lower energy peak of the imaginary part of dielectric function is due to the transition from halogen p or oxide p states to metal derived s/d states.

Shwetha, G.; Kanchana, V.

2014-04-01

8

Excitonic energy transport in wide-band inorganic scintillators  

International Nuclear Information System (INIS)

The known experimental data on the energy transport in alkali-halide scintillators and rare gas crystals with a similar electronic structure can be explained only by taking into account the vibrational levels of a two-site self-trapped exciton (excimer quasi-molecule). In a crystal, these vibrational levels turn to narrow excitonic subbands with a very large effective mass which provides a possibility for a two-site exciton to be localized in shallow potential wells produced by impurity centers and lattice defects. A very high rate of the excitonic energy transfer, observed for alkali-halide at a low temperature, is explained by a coherent directional motion of two-site excitons with a velocity close to the sound velocity in the crystal. These two mechanisms, in their combination, provide an efficient energy transfer from the host crystal to a weak impurity or radiation defects by self-trapped two-site excitons formed after thermal relaxation of photoproduced electronic excitations

2002-06-21

9

The influence of low temperature uniaxial stress on self trapped excitons' structure in alkali halide crystals  

International Nuclear Information System (INIS)

Full text: According to contemporary conceptions the alkali halide crystals (AHC) intrinsic luminescence can be classified by three different configurations of self trapped exciton (STE) with a core of molecular formation (Hal2-): central symmetric (I-type, on) exciton is Vk center localized in its field with electron without mass center removal of electron and hole subsystems of Hal2- ion. According to mass center removal (Vk - center) STE are subdivided into configurations of II-type (weak off - weak asymmetric) and Ill-type (strong off - strong asymmetric). The structure of all STE types (I, II, III) for FCC is orientated on crystallographic direction and for BCC on . In this connection uniaxial stress influence on different STE configurations in AHC at their radiative relaxation is investigated. With an increase of uniaxial stress degree at 100 K in KI crystal Ex luminescence is converted into ?-luminescence, and or RbI crystal the situation is vice versa - ?- luminescence is converted into Ex luminescence. Usually ?-luminescence for many crystals (for example, KCl, RbCl, RbBr) appears in visible spectral region with a large Stokes shift - SR =0.6. But in some crystals, for example NaBr and NaI, ?- luminescence appears in ultraviolet spectral region with a relatively small Stokes shift - SR =0.25-0.3, what is typical for ?-luminescence. That means ?-luminescence in AHC can have different STE structure, as weak off and strong off-configurations (I, II, Ill-types) as distinct from ?--luminescence which can have on-configuration only (I-type). NaCl initially has symmetric structure in comparison with Rbl. That is why the uniaxial stress only intensifies without redistribution effect. The same picture is observed in NaBr crystal where there is only one STE luminescence band with symmetric configuration. Thus analyzing radiation spectra redistributions of KI and RbI crystals and also NaCl and NaBr crystals we came to conclusion: uniaxial stress effectively acts in the direction that brings asymmetric STE to symmetric configuration (strong?weak?on or III?II?I - types). If the uniaxial stress is applied on crystallographic direction then except compression there can be stretching between self trapped excitons' components. Such a situation is observed for KBr and CsI crystals. Thus at low temperature uniaxial stress in face centered AHC luminescence redistribution takes place in favor of symmetric configuration, and in volume centered vice versa - in favor of STE asymmetric configuration

2004-01-01

10

Density of states of the lowest exciton band and the exciton bandwidth in coronene single crystals  

International Nuclear Information System (INIS)

The thermo-absorption spectra of coronene single crystals were investigated over the temperature range between 193 and 287 K. The density of states and the exciton bandwidth of the lowest exciton band were obtained from the thermo-absorption spectra for the 0-1 absorption band region. The density of states function of the lowest exciton band, we observed in coronene has double peaks, as is expected for the one-dimensional model. The peak-to-peak energy separation gives the exciton a bandwidth of 490 cm-1. The bandwidth obtained (490 cm-1) is larger compared with the magnitude 370 cm-1 reported for coronene at 10 K. This difference can be explained in terms of the structural phase change that occurs at 140-180 K

2005-04-01

11

Density of states of the lowest exciton band and the exciton bandwidth in coronene single crystals  

Energy Technology Data Exchange (ETDEWEB)

The thermo-absorption spectra of coronene single crystals were investigated over the temperature range between 193 and 287 K. The density of states and the exciton bandwidth of the lowest exciton band were obtained from the thermo-absorption spectra for the 0-1 absorption band region. The density of states function of the lowest exciton band, we observed in coronene has double peaks, as is expected for the one-dimensional model. The peak-to-peak energy separation gives the exciton a bandwidth of 490 cm{sup -1}. The bandwidth obtained (490 cm{sup -1}) is larger compared with the magnitude 370 cm{sup -1} reported for coronene at 10 K. This difference can be explained in terms of the structural phase change that occurs at 140-180 K.

Totoki, R. [Department of Physics, Konan University, Okamoto, Kobe 658-8501 (Japan); Aoki-Matsumoto, T. [Department of Physics, Konan University, Okamoto, Kobe 658-8501 (Japan); Mizuno, K. [Department of Physics, Konan University, Okamoto, Kobe 658-8501 (Japan)]. E-mail: mizuno@konan-u.ac.jp

2005-04-15

12

Self-Trapped Excitons in Ionic-Covalent Silver Halide Crystals and Nanostructures: High-Frequency EPR, ESE, ENDOR and ODMR Studies  

Digital Repository Infrastructure Vision for European Research (DRIVER)

Silver halides have unique features in solid state physics because their properties are considered to be of borderline nature between ionic and covalent bonding. In AgCl, the self-trapped hole (STH) is centered and partly trapped in the cationic sublattice, forming an Ag2+ ion inside of a (AgCl6)4? complex as a result of the Jahn–Teller distortion. The STH in AgCl can capture an electron from the conduction band forming the self-trapped exciton (STE). Recent results of a study of STE by m...

Baranov, P. G.; Romanov, N. G.; Poluektov, O. G.; Schmidt, J.

2010-01-01

13

The enhancement effect of the lattice barrier of nonradiative decay of excitons at non-axial stress of alkali halides  

International Nuclear Information System (INIS)

Full text: According to the modern notions the formation processes of radiation defects and the luminescence of alkali halides (AH) are explained by the non-radiative and radiative relaxations of electronic excitons through various excitonic slates. In AH the luminescence of self-trapped excitons (STE) at 80 K has a small quantum efficiency, but its intensity sharply increases at elastic deformation when in contrast to STE luminescence the formation process of radiation defects decreases. This effect was interpreted by the increasing of exciton self-trapping probability in the regular lattice sites wi:b radiative annihilation. The enhancement of the STE luminescence intensity due to possible decreasing of the non-radiative decay efficiency of excitons with creation of radiation detects can be estimated by registration of the temperature dependence of luminescence of deformed AH at 80 K. The basic regularity of the intrinsic luminescence of AH are studied in less detail than the impurity luminescence. For the first time we studied the influence of the elastic deformation on the temperature quenching of the STE emission. The STE luminescence can be considered as a local interaction of excitons with lattice vibrations in analogy with the impurity luminescence according to the Mott model. At such an approach by determining the dependence of X-luminescence on temperature can be judged on the probability of the non-radiative annihilation of excitons and from the slope of straightforward lg(1/?-1)-(1/T) can be determined the activation energy required for the creation of primary radiation defects before (Q) and at (Q') elastic lattice deformation. Experimental results show that for crystals in which the luminescence efficiency close to unity (? ?1, CsI) or zero (??0.1, KCl) before and at the elastic deformation, the activation energies for non-radiative transitions of STE are not essentially different (Q?Q'). For other crystals (KI, RbI, KBr. NaCl) at their elastic deformation the activation energies for non-radiative annihilation of excitons are sharply increased (Q'>Q). The enhancement of the STE luminescence intensity in the presence of the elastic lattice deformation, caused by the decreasing of the non radiative decay efficiency of excitons or by the reduction of a barrier for exciton self-trapping or by the increasing of the probability of tunneling luminescence between the radiation defects are discussed

2001-06-04

14

Temperature Behavior of Exciton Absorption Bands in PbI2 Layer Crystals  

Directory of Open Access Journals (Sweden)

Full Text Available The influence of bending waves on the warm-up behavior of exciton absorption bands in layer crystals has been investigated. The effective mass of the current carriers in the layer semiconductor PbJ2 has been computed and used to obtain the values of the exciton-phonon interaction function by pseudopotential method energy spectra calculations. It was shown that the different signs of the warm-up dynamics of an exciton absorption peak shift and existence of inversion points is related with the concurrent influence of two exciton energy relaxation mechanisms – on both the bending waves and the lattice phonons.

V. Kramar

2002-06-01

15

First-Principles Method of Propagation of Tightly Bound Excitons: Verifying the Exciton Band Structure of LiF with Inelastic x-Ray Scattering  

Science.gov (United States)

We propose a simple first-principles method to describe the propagation of tightly bound excitons. By viewing the exciton as a composite object (an effective Frenkel exciton in Wannier orbitals), we define an exciton kinetic kernel to encapsulate the exciton propagation and decay for all binding energies. Applied to prototypical LiF, our approach produces three exciton bands, which we verified quantitatively via inelastic x-ray scattering. The proposed real-space picture is computationally inexpensive and thus enables study of the full exciton dynamics, even in the presence of surfaces and impurity scattering. It also provides an intuitive understanding to facilitate practical exciton engineering in semiconductors, strongly correlated oxides, and their nanostructures.

Lee, Chi-Cheng; Chen, Xiaoqian M.; Gan, Yu; Yeh, Chen-Lin; Hsueh, H. C.; Abbamonte, Peter; Ku, Wei

2013-10-01

16

Negative-band-gap quantum dots: Gap collapse, intrinsic surface states, excitonic response, and excitonic insulator phase  

Science.gov (United States)

The spectrum of quantum dots made from semiconductors such as HgTe and HgS changes from negative gap to positive gap with decreasing size. Furthermore, intrinsic surface states, which are not related to dangling bonds, appear in the negative-gap regime. We investigate theoretically the evolution of the spectrum of HgS quantum dots with decreasing size and show how states evolve from a negative gap to a positive gap as confinement is increased. The lowest confined electron level evolves into an intrinsic surface state with increasing size and, thus, is not derived directly from a bulk HgS band. Due to strong band mixing in narrow-gap semiconductors, spacing between confined levels decreases more slowly with increasing size than for quantum dots made from wide-gap semiconductors. Moreover, dielectric screening becomes nearly metallic as the gap closes. As a consequence, confinement energies dominate exciton binding energies for all dot sizes up to the gap closure. Excitons remain in the strong confinement limit as size increases until the gap closes. Nonetheless, the exciton binding exceeds the single-particle gap for sizes near gap closure, opening up the possibility of an excitonic insulator phase in quantum dots not possible in positive-gap quantum dots. Signatures in the quantum-dot optical response for gap collapse and surface states are identified.

Malkova, Natalia; Bryant, Garnett W.

2010-10-01

17

1D exciton band and exciton-phonon interaction in a single polymer chain  

CERN Document Server

The excitonic luminescence of an isolated polydiacetylene polymer chain in its monomer matrix is studied by micro-photoluminescence. These chains behave as perfect 1D excitonic systems with the expected 1/sqrt{E} density of states between 5 and 50 K. The temperature dependence of the homogeneous width is quantitatively explained by interaction with longitudinal acoustic phonons of the crystal in the range of temperature explored.

Dubin, F

2002-01-01

18

Exciton Dynamics within the Band-Edge Manifold States: The Onset of an Acoustic Phonon Bottleneck  

Digital Repository Infrastructure Vision for European Research (DRIVER)

Exciton dynamics within the band-edge state manifold of CdSe/ZnS and CdSe/CdS quantum dots (QDs) have been investigated. Low-temperature time-resolved photoluminescence (PL) experiments demonstrate that exciton relaxation is mediated by LO phonons, whereas an acoustic phonon bottleneck is observed for splitting energies lower than the optical phonon energy. This has important implications since the main source affecting exciton dephasing is considered to be a spin-flip process. Our results co...

2012-01-01

19

Near band-gap photoluminescence of InN due to Mahan excitons  

International Nuclear Information System (INIS)

The luminescence spectrum of degenerately n-type doped hexagonal InN extends to energies significantly higher than the renormalized band edge of the material. The line shape cannot be explained by simple band-filling. Reflectivity spectra reveal at low temperatures a strongly enhanced intensity at the Fermi energy which we ascribe to a Fermi edge singularity of the electrons interacting with holes to form so-called Mahan excitons. High k-values of the holes recombining with Fermi edge electrons are provided by localization at acceptors. A weak binding energy for a Mahan exciton is expected due to screening of the electron liquid in the conduction band. The concept of Mahan excitons allows us to understand the observed near band gap photoluminescence spectra (copyright 2009 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)

2009-06-01

20

Near band-gap photoluminescence of InN due to Mahan excitons  

Energy Technology Data Exchange (ETDEWEB)

The luminescence spectrum of degenerately n-type doped hexagonal InN extends to energies significantly higher than the renormalized band edge of the material. The line shape cannot be explained by simple band-filling. Reflectivity spectra reveal at low temperatures a strongly enhanced intensity at the Fermi energy which we ascribe to a Fermi edge singularity of the electrons interacting with holes to form so-called Mahan excitons. High k-values of the holes recombining with Fermi edge electrons are provided by localization at acceptors. A weak binding energy for a Mahan exciton is expected due to screening of the electron liquid in the conduction band. The concept of Mahan excitons allows us to understand the observed near band gap photoluminescence spectra (copyright 2009 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)

Feneberg, Martin; Daeubler, Juergen; Thonke, Klaus; Sauer, Rolf [Institut fuer Halbleiterphysik, Universitaet Ulm, 89069 Ulm (Germany); Schley, Pascal; Goldhahn, Ruediger [Institut fuer Physik, TU Ilmenau, PF 100565, 98684 Ilmenau (Germany)

2009-06-15

 
 
 
 
21

Coherent infrared multidimensional spectra of the OH stretching band in liquid water simulated by direct nonlinear exciton propagation  

Digital Repository Infrastructure Vision for European Research (DRIVER)

The two-dimensional vibrational response of the disordered strongly fluctuating OH exciton band in liquid water is investigated using a new simulation protocol. The direct nonlinear exciton propagation generalizes the nonlinear exciton equations to include nonadiabatic time dependent Hamiltonian and transition dipole fluctuations. The excitonic picture is retained and the large cancellation between Liouville pathways is built-in from the outset. The sensitivity of the photon echo and double-q...

Falvo, Cyril; Palmieri, Benoit; Mukamel, Shaul

2009-01-01

22

Excitonic and band-band transitions of Cu2ZnSiS4 determined from reflectivity spectra  

Science.gov (United States)

Exciton spectra of Cu2ZnSiS4 single crystals are investigated by reflection spectroscopy at 10 and 300 K for light polarized perpendicular (E?c) and parallel (E?c) to the optical axis. The parameters of the excitons and dielectric constant are determined. The free carriers effective masses have been estimated. The room temperature reflectivity spectra at photon energies higher than the fundamental band gap in the polarization ??? and E?? were measured and related to the electronic band structure of Cu2ZnSiS4.

Guc, M.; Levcenko, S.; Dermenji, L.; Gurieva, G.; Schorr, S.; Syrbu, N. N.; Arushanov, E.

2014-07-01

23

Exciton dynamics within the band-edge manifold states: the onset of an acoustic phonon bottleneck.  

Science.gov (United States)

Exciton dynamics within the band-edge state manifold of CdSe/ZnS and CdSe/CdS quantum dots (QDs) have been investigated. Low-temperature time-resolved photoluminescence (PL) experiments demonstrate that exciton relaxation is mediated by LO phonons, whereas an acoustic phonon bottleneck is observed for splitting energies lower than the optical phonon energy. This has important implications since the main source affecting exciton dephasing is considered to be a spin-flip process. Our results concur with recent observations of long exciton dephasing times in CdSe/CdS QDs and show a way to engineer nanoparticles with enhanced coherence time, a prerequisite for their use in quantum optical applications. PMID:23016932

Rainò, Gabriele; Moreels, Iwan; Hassinen, Antti; Stöferle, Thilo; Hens, Zeger; Mahrt, Rainer F

2012-10-10

24

Excitonic absorption edge in alkali halides and the Urbach-Martienssen rule studied by three-photon difference-frequency generation  

Science.gov (United States)

Three-photon difference-frequency generation (TP-DFG) is used to excite resonances on the lower exciton-polariton branch in alkali halides. The line shape of the resonances is investigated in KI and RbI for temperatures between 1.5 and 800 K. A drastic variation of the linewidth ranging from 0.2 to about 50 meV is observed as a function of temperature and polariton energy. The experimental results are interpreted in the framework of polariton response theory, which links the shape of the TP-DFG resonance to the complex dielectric function. From the measured positions and widths of the resonance peaks, accurate data on dispersion and absorption are obtained for the region of the exponential tail of the excitonic absorption edge. The method allows us to determine the parameters of the Urbach-Martienssen rule by measurements on large single crystals.

Fröhlich, D.; Köhler, P.; Nieswand, W.; Rappen, T.; Mohler, E.

1991-05-01

25

Band Structure Parameters and Fermi Resonances of Exciton-Polaritons in CsI and CsBr under Hydrostatic Pressure  

Energy Technology Data Exchange (ETDEWEB)

Most alkali halides crystallize in the fcc sodium chloride structure. In contrast, with the exception of CsF, the Cs-halides form the simple cubic cesium chloride (CsCl) structure at ambient conditions and they have a substantially different electronic structure than other alkali halides; in particular, they have several nearly degenerate electronic levels near the Brillouin zone center. Highly resolved Three-Photon Spectroscopy (TPS) measurements allow direct observation of the near band edge structure and, in the case of CsI, probe more states than one-photon techniques. A number of interesting phenomena, among them level repulsion (Fermi resonance), occur as these levels are tuned through one another by application of hydrostatic pressure. To the best of our knowledge, this has been observed for CsBr for the first time. Doubling the photon energy range compared to a previous publication [see Yoo et al. PRL 84, 3875 (2000)] allows direct observation of the n=1, 2 and 3 exciton-polariton members of the {Lambda}{sub 8}{sup -}-{Lambda}{sub 6}{sup +} transition in CsI and lets us establish unambiguous values for the bandgap (6.139 eV), binding energy (0.265 eV) and their pressure dependence up to 7 kbar. Similarly to CsI, the CsBr linewidth of the lowest {Lambda}{sub 4}{sup -} polariton (A) decreases upon compression.

Lipp, M J; Yoo, C H; Strachan, D; Daniels, W B

2005-11-29

26

Investigation of the excitonic luminescence band of CdTe solar cells by photoluminescence and photoluminescence excitation spectroscopy  

International Nuclear Information System (INIS)

The excitonic luminescence band of polycrystalline cadmium telluride layers has been investigated by Photoluminescence (PL) and Photoluminescence excitation spectroscopy (PLE). CdTe was deposited by means of close space sublimation and the samples were activated by different chlorine containing compounds, i.e. cadmium chloride, hydrochloric acid, and sodium chloride as well as by simple air activation or received no post deposition treatment. In the PL spectra, four different peaks within the excitonic luminescence band were resolved. These include the free-exciton peak and two transitions of excitons bound to defects. Furthermore, free excitons and band to band transitions were detected by means of PLE. The PL and PLE spectra are discussed with respect to the post deposition treatments.

2011-08-31

27

Band gaps and structural properties of graphene halides and their derivates: A hybrid functional study with localized orbital basis sets  

CERN Multimedia

DFT calculations of the electronic structure of graphane and stoichiometrically halogenated graphene derivatives (fluorographene and other analogous graphene halides) show (i) localized orbital basis sets can be successfully and effectively used for such 2D materials; (ii) several functionals predict that the band gap of graphane is greater than that of fluorographene, whereas HSE06 gives the opposite trend; (iii) HSE06 functional predicts quite good values of band gaps w.r.t benchmark theoretical and experimental data; (iv) the zero band gap of graphene is opened by hydrogenation and halogenation and strongly depends on the chemical composition of mixed graphene halides; (v) the stability of graphene halides decreases sharply with increasing size of the halogen atom - fluorographene is stable, whereas graphene iodide spontaneously decomposes. In terms of band gap and stability, the C2FBr, and C2HBr derivatives seem to be promising materials, e.g., for (opto)electronics applications, because their band gaps a...

Karlický, František; Otyepka, Michal; 10.1063/1.4736998

2012-01-01

28

Switchable S = 1/2 and J = 1/2 Rashba bands in ferroelectric halide perovskites.  

Science.gov (United States)

The Rashba effect is spin degeneracy lift originated from spin-orbit coupling under inversion symmetry breaking and has been intensively studied for spintronics applications. However, easily implementable methods and corresponding materials for directional controls of Rashba splitting are still lacking. Here, we propose organic-inorganic hybrid metal halide perovskites as 3D Rashba systems driven by bulk ferroelectricity. In these materials, it is shown that the helical direction of the angular momentum texture in the Rashba band can be controlled by external electric fields via ferroelectric switching. Our tight-binding analysis and first-principles calculations indicate that [Formula: see text] and [Formula: see text] Rashba bands directly coupled to ferroelectric polarization emerge at the valence and conduction band edges, respectively. The coexistence of two contrasting Rashba bands having different compositions of the spin and orbital angular momentum is a distinctive feature of these materials. With recent experimental evidence for the ferroelectric response, the halide perovskites will be, to our knowledge, the first practical realization of the ferroelectric-coupled Rashba effect, suggesting novel applications to spintronic devices. PMID:24785294

Kim, Minsung; Im, Jino; Freeman, Arthur J; Ihm, Jisoon; Jin, Hosub

2014-05-13

29

Excitonic characteristics in direct wide-band-gap CuScO2 epitaxial thin films  

Science.gov (United States)

Thin films of a delafossite compound CuScO2 were grown on spinel MgAl2O4 (111) substrates, yielding in highly crystalline and (0001)-oriented epitaxial structures. Absorption spectra at 20 K revealed a sharp exciton resonance at 3.97 eV, which persisted up to 300 K. Its direct transition band gap at 20 K and exciton binding energies were determined to be about 4.35 and 380 meV, both of which are considerably larger than those of ZnO. In view of its capability of naturally layered structure and p-type doping, this compound will be interesting for exciton physics as well as implementation of heterostructured devices.

Hiraga, H.; Makino, T.; Fukumura, T.; Ohtomo, A.; Kawasaki, M.

2009-11-01

30

Exciton spectra and energy band structure of Cu2ZnSiSe4  

International Nuclear Information System (INIS)

Highlights: • Reflection spectra of Cu2ZnSiSe4 were studied for E ? c and E || c light polarizations. • Four excitonic series are revealed in the reflection spectra at 10 K. • Model of exciton dispersion and the presence of a dead-layer. • Exciton Rydberg energies and free carriers effective masses were calculated. • Reflectivity for E ? c and E || c were analyzed in the region 3–6 eV at 300 K. -- Abstract: Exciton spectra are studied in Cu2ZnSiSe4 single crystals at 10 and 300 K by means of reflection spectroscopy. The exciton parameters, dielectric constant and free carriers effective masses are deduced from experimental spectra by calculations in the framework of a model taking into account the spatial dispersion and the presence of a dead-layer. The structure found in the reflectivity was analyzed and related to the theoretical electronic band structure of close related Cu2ZnSiS4 semiconductor

2014-02-25

31

Decay of Wannier-Mott excitons interacting with acoustic phonon in semiconductors with a degenerate valence band  

International Nuclear Information System (INIS)

Decay probabilities of light and heavy excitons interacting with acoustic phonons in cubic semiconductors with a degenerate valence band are calculated. The numerical results for GaAs showed that the decay probability of the light exciton is much greater than that of the heavy one. (author). 10 refs, 1 fig

1987-01-01

32

Radiative Lifetime of Wannier—Mott Excitons in Nanoclusters of Semiconductors with Direct- and Indirect Band Structures  

Science.gov (United States)

We calculate the radiative lifetime of Wannier—Mott excitons in potential wells formed by nanoclusters of narrow-band semiconductors in wide-band materials. Examples of semiconductor nanoclusters with both direct and indirect band structures (InAs and diamond, respectively) are analyzed. It is shown that in the case of nanoclusters of direct-band semiconductors the radiative lifetime of the Wannier—Mott excitons in them is determined by recombination of the electrons and holes, which form the excitons, without participation of phonons, and increases significantly as the cluster size decreases. In the case of nanoclusters of indirect-band semiconductors, the radiative lifetime of the Wannier—Mott excitons in them is determined by recombination of the electrons and holes, which form the excitons, with simultaneous radiation of phonons and is weakly dependent on the cluster size. The effect of the increase in the radiative lifetime of the Wannier—Mott excitons in nanoclusters of direct-band semiconductors with a decrease in their dimensions can be used in various exciton experiments, which require comparatively great time resources.

Kukushkin, V. A.

2013-12-01

33

Influence of Coulomb-induced band couplings on linear excitonic absorption spectra of semiconducting carbon nanotubes  

Science.gov (United States)

Starting from the extended Su-Schrieffer-Heeger model, multiband semiconductor Bloch equations are formulated in momentum space and applied to the analysis of the linear optical response of semiconducting carbon nanotubes (SCNTs). This formalism includes the coupling of electron-hole pair excitations between different valence and conduction bands, originating from the electron-hole Coulomb attraction. The influence of these couplings, which are referred to as nondiagonal interband Coulomb interaction (NDI-CI), on the linear excitonic absorption spectra is investigated and discussed for light fields polarized parallel to the tube direction. The results show that the intervalley NDI-CI leads to a significant increase of the band gap and a decrease of the exciton binding energy that results in a blueshift of the lowest-frequency excitonic absorption peak. The strength of these effects depends on the symmetry of the SCNT. Furthermore, for zigzag SCNTs with higher symmetry other nonintervalley NDI-CI terms also affect the spectral positions of excitonic absorption peaks.

Liu, Hong; Schumacher, Stefan; Meier, Torsten

2014-04-01

34

Electronic properties of metal induced gap states at insulator/metal interfaces -- dependence on the alkali halide and the possibility of excitonic mechanism of superconductivity  

CERN Multimedia

Motivated from the experimental observation of metal induced gap states (MIGS) at insulator/metal interfaces by Kiguchi {\\it et al.} [Phys. Rev. Lett. {\\bf 90}, 196803 (2003)], we have theoretically investigated the electronic properties of MIGS at interfaces between various alkali halides and a metal represented by a jellium with the first-principles density functional method. We have found that, on top of the usual evanescent state, MIGS generally have a long tail on halogen sites with a $p_z$-like character, whose penetration depth ($\\lambda$) is as large as half the lattice constant of bulk alkali halides. This implies that $\\lambda$, while little dependent on the carrier density in the jellium, is dominated by the lattice constant (hence by energy gap) of the alkali halide, where $\\lambda_{\\rm LiF} < \\lambda_{\\rm LiCl} < \\lambda_{\\rm LiI}$. We also propose a possibility of the MIGS working favorably for the exciton-mediated superconductivity.

Arita, R; Kuroki, K; Aoki, H; Arita, Ryotaro; Tanida, Yoshiaki; Kuroki, Kazuhiko; Aoki, Hideo

2003-01-01

35

The additional contribution caused by Coulomb interaction to the exciton dispersion in multiple quantum wells and superlattices for direct band gap cubic semiconductors  

International Nuclear Information System (INIS)

The exciton dispersion in multiple quantum wells and superlattices is studied for direct band gap cubic semiconductors in the three-band-model. The Coulomb interaction between the light and heavy exciton states in different wells is calculated. This leads to a direction dependence and split of the exciton dispersion. (author). 21 refs

1992-01-01

36

Self-Trapped Excitons in Ionic-Covalent Silver Halide Crystals and Nanostructures: High-Frequency EPR, ESE, ENDOR and ODMR Studies.  

Science.gov (United States)

Silver halides have unique features in solid state physics because their properties are considered to be of borderline nature between ionic and covalent bonding. In AgCl, the self-trapped hole (STH) is centered and partly trapped in the cationic sublattice, forming an Ag(2+) ion inside of a (AgCl(6))(4-) complex as a result of the Jahn-Teller distortion. The STH in AgCl can capture an electron from the conduction band forming the self-trapped exciton (STE). Recent results of a study of STE by means of high-frequency electron paramagnetic resonance, electron spin echo, electron-nuclear double resonance (ENDOR) and optically detected magnetic resonance (ODMR) are reviewed. The properties of the STE in AgCl crystals, such as exchange coupling, the ordering of the triplet and singlet sublevels, the dynamical properties of the singlet and triplet states, and the hyperfine interaction with the Ag and Cl (Br) nuclei are discussed. Direct information about the spatial distribution of the wave function of STE unpaired electrons was obtained by ENDOR. From a comparison with the results of an ENDOR study of the shallow electron center and STH, it is concluded that the electron is mainly contained in a hydrogen-like 1s orbital with a Bohr radius of 15.1 ± 0.6 Å, but near its center the electron density reflects the charge distribution of the hole. The hole of the STE is virtually identical to an isolated STH center. For AgCl nanocrystals embedded into the KCl crystalline matrix, the anisotropy of the g-factor of STE and STH was found to be substantially reduced compared with that of bulk AgCl crystals, which can be explained by a considerable suppression of the Jahn-Teller effect in nanoparticles. A study of ODMR in AgBr nanocrystals in KBr revealed spatial confinement effects and allowed estimating the nanocrystal size from the shape of the ODMR spectra. PMID:21151483

Baranov, P G; Romanov, N G; Poluektov, O G; Schmidt, J

2010-12-01

37

Exciton spectra and energy band structure of CuAlS{sub 2} crystals  

Energy Technology Data Exchange (ETDEWEB)

Three exciton series are investigated in the reflectivity spectra of CuAlS{sub 2} crystals at the temperature of 10 K. The n{sup A}=1 ({omega}{sub T}=3.543 eV, {omega}{sub L}= 3.546 eV) and n{sup A}=2 (3.565 eV) lines of the {Gamma}{sub 4} (A-series) excitons are observed in the E-parallel c polarization. In the E-perpendicular c polarization, the {Gamma}{sub 5} excitons (n{sup B}=1 at 3.668 eV and n{sup B}=2 at 3.686 eV) of the B-series, and n{sup C}=1 at 3.813 eV of the C-series are observed. The parameters of the excitons and the more exact values of the {Gamma}{sub 7} - {Gamma}{sub 6}, {Gamma}{sub 6} - {Gamma}{sub 6}, and {Gamma}{sub 7} - {Gamma}{sub 6} energy gaps are determined. The crystal field and spin-orbit splitting of the valence band is calculated. The electron (m{sub c1}{sup *}) and hole (m{sub V1}*, m{sub V2}*, and m{sub V3}*) effective masses have been estimated.

Syrbu, N.N. [Technical University of Moldova, 168 Stefan cel Mare avenue, 2004 Chisinau (Moldova, Republic of); Korzun, B.V.; Fadzeyeva, A.A.; Mianzelen, R.R. [Institute of Physics of Solids and Semiconductors, P. Brovki 17, Minsk 220072 (Belarus); Ursaki, V.V., E-mail: ursaki@yahoo.co [Institute of Applied Physics, Academy of Sciences of Moldova, 5 Academy street, 2028 Chisinau (Moldova, Republic of); Galbic, I. [Technical University of Moldova, 168 Stefan cel Mare avenue, 2004 Chisinau (Moldova, Republic of)

2010-08-15

38

Exciton spectra and energy band structure of CuGaSe{sub 2} single crystals  

Energy Technology Data Exchange (ETDEWEB)

Photoreflectivity, wavelength modulation spectroscopy and photoluminescence measurements of CuGaSe{sub 2} are used to determine the exciton and band parameters as well as the energy band structure of CuGaSe{sub 2} at photon energies higher than the fundamental band gap. The spectral dependences of the real {epsilon}{sub 1} and imaginary {epsilon}{sub 2} components of the complex dielectric function {epsilon}(E) = {epsilon}{sub 1} (E) + i{epsilon}{sub 2}(E) are calculated using the Kramers-Kronig relations. As a result, the energy band structure of CuGaSe{sub 2} at photon energies higher than the fundamental band gap is derived from the analysis of the structures observed both in the {epsilon} ({omega}) and in the wavelength modulated reflectivity spectra.

Levchenko, S; Tezlevan, V E; Arushanov, E [Institute of Applied Physics, Academy of Sciences of Moldova, Chisinau, MD 2028 (Moldova, Republic of); Syrbu, N N [Laboratory of Optoelectronics, Technical University of Moldova, 168, Bul. Stefan cel Mare, Chisinau, MD-2012 (Moldova, Republic of); Merino, J M; Leon, M [Universidad Autonoma de Madrid, Departamento Fisica Aplicada, C-XII, 28049 Madrid (Spain)], E-mail: maximo.leon@uam.es

2008-03-07

39

Band gaps and structural properties of graphene halides and their derivates: a hybrid functional study with localized orbital basis sets.  

Science.gov (United States)

Density functional theory calculations of the electronic structure of graphane and stoichiometrically halogenated graphene derivatives (fluorographene and other analogous graphene halides) show: (i) localized orbital basis sets can be successfully and effectively used for such two-dimensional materials; (ii) several functionals predict that the band gap of graphane is greater than that of fluorographene, whereas HSE06 gives the opposite trend; (iii) HSE06 functional predicts quite good values of band gaps with respect to benchmark theoretical and experimental data; (iv) the zero band gap of graphene is opened by hydrogenation and halogenation and strongly depends on the chemical composition of mixed graphene halides; (v) the stability of graphene halides decreases sharply with increasing size of the halogen atom--fluorographene is stable, whereas graphene iodide spontaneously decomposes. In terms of band gap and stability, the C(2)FBr and C(2)HBr derivatives seem to be promising materials, e.g., for (opto)electronics applications, because their band gaps are similar to those of conventional semiconductors, and they are expected to be stable under ambient conditions. The results indicate that other fluorinated compounds (C(a)H(b)F(c) and C(a)F(b)Y(c), Y = Cl, Br, I) are stable insulators. PMID:22830726

Karlický, František; Zbo?il, Radek; Otyepka, Michal

2012-07-21

40

Exciton binding energies and the valence-band offset in mixed type-I--type-II strained-layer superlattices  

International Nuclear Information System (INIS)

Strained CdTe-Cd1-xZnxTe superlattices are of mixed type: electrons and heavy holes are confined to the CdTe layers (type I) while light holes are confined to the Cd1-xZnxTe layers (type II). In this paper we calculate the exciton binding energy (EBE) as a function of superlattice period for both type-I (spatially direct) and type-II (spatially indirect) excitons. For the heavy-hole (type-I) exciton the binding energy is larger than the bulk value, and varies only slowly with the period down to small periods, where the exciton acquires a three-dimensional character and our calculation breaks down. For the light-hole (type-II) exciton the binding energy at large period is much smaller, due to the spatial separation of electron and hole. As the period decreases, the binding energy increases steadily to reach its bulk value for vanishingly small period. Given the EBE's, we can fit the already published data on the exciton transition energies with a single adjustable parameter, the ''average valence-band offset'' (averaged over the heavy and light holes). This is the algebraic sum of the chemical-offset and the hydrostatic-strain contribution, and is found to be (2±4)% of the difference in band gap between the barrier and well. This value lies in the range predicted theoretically

1992-07-15

 
 
 
 
41

Line shape of the low-energy tail of exciton absorption in molecular crystals  

Science.gov (United States)

The influence of crystal defects and phonons on the line shape of the low-energy tail of exciton absorption is calculated for molecular crystals with the lower limit of their exciton band at k-->=0. The exciton Hamiltonian for a crystal with lattice defects is derived from that for a perfect crystal with lattice vibrations. Calculation of the line shape is done assuming that the individual lattice defects have independent effects on the unperturbed exciton band. Three examples of the molecular exciton energy bands are considered: (1) one-dimensional tight-binding exciton band, (2) the exciton bands with Hubbard's model of density of states, and (3) exciton bands with density of states independent of the exciton energies. At very low temperatures where the influence of the defects is dominant, we have found the asymmetric low-energy tail due to defects, acting as shallow traps, in all the three examples. The observed asymmetric line shape in 1,2-dibromonaphthalene single crystals is, therefore, attributed to such shallow traps and analogous to the Urbach-Martienssen tail observed in alkali halides. As the temperature increases and phonons become more active, the line shape becomes symmetric due to the dominant influence of thermal broadening.

Singh, Jai

1981-02-01

42

Exciton spectra and energy band structure of Cu{sub 2}ZnSiSe{sub 4}  

Energy Technology Data Exchange (ETDEWEB)

Highlights: • Reflection spectra of Cu{sub 2}ZnSiSe{sub 4} were studied for E ? c and E || c light polarizations. • Four excitonic series are revealed in the reflection spectra at 10 K. • Model of exciton dispersion and the presence of a dead-layer. • Exciton Rydberg energies and free carriers effective masses were calculated. • Reflectivity for E ? c and E || c were analyzed in the region 3–6 eV at 300 K. -- Abstract: Exciton spectra are studied in Cu{sub 2}ZnSiSe{sub 4} single crystals at 10 and 300 K by means of reflection spectroscopy. The exciton parameters, dielectric constant and free carriers effective masses are deduced from experimental spectra by calculations in the framework of a model taking into account the spatial dispersion and the presence of a dead-layer. The structure found in the reflectivity was analyzed and related to the theoretical electronic band structure of close related Cu{sub 2}ZnSiS{sub 4} semiconductor.

Guc, M., E-mail: gmax@phys.asm.md [Institute of Applied Physics, Academy of Sciences of Moldova, Academiei Str. 5, Chisinau MD 2028, Republic of Moldova (Moldova, Republic of); Levcenko, S. [Helmholtz Zentrum Berlin für Materialien und Energie GmbH, Hahn-Meitner-Platz 1, D-14109 Berlin (Germany); Dermenji, L. [Institute of Applied Physics, Academy of Sciences of Moldova, Academiei Str. 5, Chisinau MD 2028, Republic of Moldova (Moldova, Republic of); Gurieva, G. [Helmholtz Zentrum Berlin für Materialien und Energie GmbH, Hahn-Meitner-Platz 1, D-14109 Berlin (Germany); Schorr, S. [Helmholtz Zentrum Berlin für Materialien und Energie GmbH, Hahn-Meitner-Platz 1, D-14109 Berlin (Germany); Free University Berlin, Institute of Geological Sciences, Malteserstr. 74-100, Berlin (Germany); Syrbu, N.N. [Technical University of Moldova, Chisinau MD-2004, Republic of Moldova (Moldova, Republic of); Arushanov, E. [Institute of Applied Physics, Academy of Sciences of Moldova, Academiei Str. 5, Chisinau MD 2028, Republic of Moldova (Moldova, Republic of)

2014-02-25

43

Fine structure and energy spectrum of exciton in direct band gap cubic semiconductors with degenerate valence bands  

International Nuclear Information System (INIS)

The influence of the cubic structure on the energy spectrum of direct exciton is investigated, using the new method suggested by Nguyen Van Hieu and co-workers. Explicit expressions of the exciton energy levels 1S, 2S and 2P are derived. A comparison with the experiments and the other theory is done for ZnSe. (author). 10 refs, 1 fig., 2 tabs

1987-01-01

44

Research Update: Physical and electrical characteristics of lead halide perovskites for solar cell applications  

Directory of Open Access Journals (Sweden)

Full Text Available The field of thin-film photovoltaics has been recently enriched by the introduction of lead halide perovskites as absorber materials, which allow low-cost synthesis of solar cells with efficiencies exceeding 16%. The exact impact of the perovskite crystal structure and composition on the optoelectronic properties of the material are not fully understood. Our progress report highlights the knowledge gained about lead halide perovskites with a focus on physical and optoelectronic properties. We discuss the crystal and band structure of perovskite materials currently implemented in solar cells and the impact of the crystal properties on ferroelectricity, ambipolarity, and the properties of excitons.

Simon A. Bretschneider

2014-04-01

45

Verification of ?7 symmetry assignment for the top valence band of ZnO by magneto-optical studies of the free A exciton state  

International Nuclear Information System (INIS)

Circularly-polarized and angular-resolved magneto-photoluminescence spectroscopy was carried out to study the free A exciton 1S state in wurtzite ZnO at 5 K. The ? 7 symmetry of the top valence band symmetry is confirmed according to the unique selection rules of Zeeman splitting lines. The out-of-plane component B? of the magnetic field, which is parallel to ZnO's c-axis, leads to linear Zeeman splitting of both the dipole-allowed ? 5 exciton state and the weakly allowed ? 1 /? 2 exciton states. The in-plane field B?, which is perpendicular to the c-axis, increases the oscillator strength of the weak ? 1 /? 2 states by forming a mixed exciton state. For the ? 7 symmetry, the lower energy Zeeman splitting peak of the weak ? 1 /? 2 can only be ?+ polarization. (paper)

2013-03-01

46

Band shape heterogeneity of the low-energy chlorophylls of CP29: absence of mixed binding sites and excitonic interactions.  

Science.gov (United States)

A number of spectroscopic characteristics of three almost isoenergetic, red-shifted chlorophylls (chls) in the PS II antenna complex CP29 are investigated with the aim of (i) determining whether their band shapes are substantially identical or not, (ii) addressing the topical problem of whether they are involved in excitonic interactions with other chls, and (iii) establishing whether their binding sites may be defined as "mixed" with respect to their capacity to bind chls a and b. The three chls A2-CHL612, A3-CHL613, and B3-CHL614 were analyzed after in vitro apoprotein-pigment reconstitution using the CP29 coding sequence from Arabidopsis thaliana for both the wild-type and mutant complexes. Difference spectra thermal broadening analyses indicated that the half-bandwidths varied between 12 and 15 nm (at room temperature), due mainly to differences in the optical reorganization energy (25-40 cm(-1)). Moreover, only the A2 chl displayed an intense vibrational band in the 300-600 cm(-1) interval from the 0-0 transition. We conclude that within the red absorbing (approximately 680 nm) antenna chls of a single chl-protein complex a marked spectral band shape heterogeneity exists. By analysis of the absorption and circular dichroism spectra no evidence was found of significantly strong excitonic interactions. The single gene mutation of the A3 and B3 binding sites causes absorption changes in both the long wavelength chl a absorbing region and in the chl b spectral region. This has previously been observed and was attributed to "mixed" chl a/b binding sites [Bassi, R., Croce, R., Cugini, D., and Sandona, D. (1999) Proc. Natl. Acad. Sci. U.S.A. 96,10056-10061]. This interpretation, while in principle not being unreasonable, is shown to be incorrect for these two chls. PMID:20047285

Belgio, Erica; Casazza, Anna Paola; Zucchelli, Giuseppe; Garlaschi, Flavio M; Jennings, Robert C

2010-02-01

47

Growth mechanisms of silver halide clusters from the molecule to the colloidal particle  

Energy Technology Data Exchange (ETDEWEB)

Studies of growing silver halide clusters, from the molecular level to colloidal-size particles, have been undertaken and correlations between particle size and physical properties have been investigated. The production of halide ions by dissociative electron attachment following pulse radiolysis of methylene halide solutions was utilized to instantaneously produce homogeneous, supersaturated solutions of halide and silver ions. The growth of silver halide particles in these solutions has been examined by conductance, absorption spectroscopy, and light scattering measurements. Conductivity measurements show that the initial reaction between the ions occurs at a diffusion-controlled rate and allow determination of the stability constants of molecular silver halide species. In solutions containing excess silver ions, particle growth of silver iodide and silver bromide occurs via a diffusion-controlled aggregation mechanism during the first seconds. The growth rate is reduced significantly, due to electrostatic repulsion between particles, when the concentration of halide ions is almost equal to or greater than the stoichiometric concentration of silver ions. The absorbance of silver iodide suspensions produced in this manner has also been measured. The initial absorbance, due to AgI molecules, occurs at wavelengths of 280 nm and below. The lowest energy exciton band appears {approximately} 50 {mu}s after initiation of the reaction. This band initially peaks at around 320 nm and shifts toward 425 nm (the wavelength of exciton absorption in bulk silver iodide). The surface potential at the interface evolves at about the same rate as the bulk properties of the material. For the mostly dissociated AgCl molecules growth by addition of ions to a small number of particles of critical size was observed.

Hayes, D.; Schmidt, K.H.; Meisel, D. (Argonne National Lab., IL (USA))

1989-08-10

48

Band edge emission enhancement by quadrupole surface plasmon-exciton coupling using direct-contact Ag/ZnO nanospheres  

Science.gov (United States)

Periodic Ag nanoball (NB) arrays on ZnO hollow nanosphere (HNS) supporting structures were fabricated in a large area by a laser irradiation method. The optimized laser power and spherical supporting structure of ZnO with a certain size and separation were employed to aggregate a sputtering-deposited Ag nano-film into an ordered, large-area, and two dimensional Ag NB array. A significant band edge (BE) emission enhancement of ZnO HNSs was achieved on this Ag NB/ZnO HNS hybrid structure and the mechanism was revealed by further experimental and theoretical analyses. With successfully fabricating the direct-contact structure of a Ag NB on the top of each ZnO HNS, the highly localized quadrupole mode surface plasmon resonance (SPR), realized on the metal NBs in the ultraviolet region, can effectively improve the BE emission of ZnO through strong coupling with the excitons of ZnO. Compared with the dipole mode SPR, the quadrupole mode SPR is insensitive to the metal nanoparticle's size and has a resonance frequency in the BE region of the wide band gap materials, hence, it can be potentially applied in related optoelectronic devices.Periodic Ag nanoball (NB) arrays on ZnO hollow nanosphere (HNS) supporting structures were fabricated in a large area by a laser irradiation method. The optimized laser power and spherical supporting structure of ZnO with a certain size and separation were employed to aggregate a sputtering-deposited Ag nano-film into an ordered, large-area, and two dimensional Ag NB array. A significant band edge (BE) emission enhancement of ZnO HNSs was achieved on this Ag NB/ZnO HNS hybrid structure and the mechanism was revealed by further experimental and theoretical analyses. With successfully fabricating the direct-contact structure of a Ag NB on the top of each ZnO HNS, the highly localized quadrupole mode surface plasmon resonance (SPR), realized on the metal NBs in the ultraviolet region, can effectively improve the BE emission of ZnO through strong coupling with the excitons of ZnO. Compared with the dipole mode SPR, the quadrupole mode SPR is insensitive to the metal nanoparticle's size and has a resonance frequency in the BE region of the wide band gap materials, hence, it can be potentially applied in related optoelectronic devices. Electronic supplementary information (ESI) available. See DOI: 10.1039/c2nr32906a

Zang, Yashu; He, Xu; Li, Jing; Yin, Jun; Li, Kongyi; Yue, Chuang; Wu, Zhiming; Wu, Suntao; Kang, Junyong

2012-12-01

49

Band-structure calculations of noble-gas and alkali halide solids using accurate Kohn-Sham potentials with self-interaction correction  

Energy Technology Data Exchange (ETDEWEB)

The optimized-effective-potential (OEP) method and a method developed recently by Krieger, Li, and Iafrate (KLI) are applied to the band-structure calculations of noble-gas and alkali halide solids employing the self-interaction-corrected (SIC) local-spin-density (LSD) approximation for the exchange-correlation energy functional. The resulting band gaps from both calculations are found to be in fair agreement with the experimental values. The discrepancies are typically within a few percent with results that are nearly the same as those of previously published orbital-dependent multipotential SIC calculations, whereas the LSD results underestimate the band gaps by as much as 40%. As in the LSD---and it is believed to be the case even for the exact Kohn-Sham potential---both the OEP and KLI predict valence-band widths which are narrower than those of experiment. In all cases, the KLI method yields essentially the same results as the OEP.

Li, Y.; Krieger, J.B. (Department of Physics, Brooklyn College, City University of New York, Brooklyn, New York 11210 (United States)); Norman, M.R. (Material Sciences Division, Argonne National Laboratory, Argonne, Illinois 60439 (United States)); Iafrate, G.J. (U.S. Army Research Office, Research Triangle Park, North Carolina 27709-2211 (United States))

1991-11-15

50

Manifestations of charged lattice defects in excitonic luminescence and control over properties of wide-band scintillation crystals  

International Nuclear Information System (INIS)

A fast energy transfer from the host crystal to impurity centres and charged lattice defects (e.g., vacancies in an ionic crystal) is due to the two-site exciton motion on high vibrational levels. At low temperatures this motion occurs in a coherent directional way with the sound velocity whereas a small separation between vibrational levels provides an easy exciton trapping by shallow potential wells near impurity centres. Charged defects have a much larger trapping section than isovalent impurity centres and manifest themselves in luminescence at a concentration smaller by two orders of magnitude

2003-09-01

51

Long-time Luminescence Kinetics of Localized excitons and conduction Band Edges Smearing in ZnSe(1-c)Tec Solid Solutions  

DEFF Research Database (Denmark)

It is shown that the integrated luminescence intensity of localized excitons in solid solutions ZnSe(1 - c)Tec has a component slowly decaying with time. After the excitation above the mobility threshold, the long-time intensity decreases exponentially, with a fractional exponent changing from a value corresponding to the critical index of anomalous diffusion to the index of normal diffusion as the temperature increases from 5 to 80 K. This change allows estimation of the energy scale for the fluctuation tail of the conduction band. (C) 2000 MAIK "Nauka / Interperiodica".

Vadim, Lyssenko; Hvam, Jørn Märcher

2000-01-01

52

Ab initio many-body effects in TiSe2: A possible excitonic insulator scenario from GW band-shape renormalization  

Science.gov (United States)

We present both theoretical ab-initio results within the Hedin's GW approximation and experimental angle-resolved photoemission and scanning tunneling spectroscopy measurements on TiSe2. With respect to the density-functional Kohn-Sham metallic picture, the many-body GW self-energy leads to a ?0.2-eV band-gap insulator consistent with our STS spectra at 5 K. The highest valence and the lowest conduction bands are strongly renormalized, with a loss of k2 parabolic dispersion toward a k4 shape. In particular, GW moves the top of valence moved toward a circle of points away from ?, arising in a Mexican hat shape commonly associated with an excitonic insulator. Our calculations are in good agreement with experiment.

Cazzaniga, M.; Cercellier, H.; Holzmann, M.; Monney, C.; Aebi, P.; Onida, G.; Olevano, V.

2012-05-01

53

Excitons in weakly disordered semiconductors  

International Nuclear Information System (INIS)

The effect of a random potential which is weak relative to the exciton's binding energy on the dynamics of direct and indirect Wannier excitons in semiconducting alloys (three-dimensional) and in quantum wells and superlattices (two-dimensional) is discussed. The potential fluctuations broaden the direct exciton line into a band in which excitons of sufficiently low energy are localized, being separated from the delocalized ones by an effective mobility edge. The author describes some of the ways by which one can detect the localization of direct excitons, in particular the resonant Rayleigh scattering and photon echo techniques. In the quantum-well case the mobility edge is found to be at the position predicted by classical percolation theory, and the possible significance of this result is discussed. In the case of indirect excitons, in both alloys and superlattices, the most pronounced effect of the disorder is on the decay kinetics, which are strongly affected by localization

1987-05-01

54

Band offsets and excitonic binding energies in In'xGa1-xP/GaAs heterostructures by photoluminescence spectroscopy  

International Nuclear Information System (INIS)

In this paper we report on the optical properties of disordered InxGa1-x P/GaAs (x = 0.54, 0.48, and 0.43) heterostructures grown by atomic layer molecular beam epitaxy (ALMBE). The samples, containing seven quantum wells (QWs), were investigated by means of photoluminescence (PL) and photoluminescence excitation (PLE) along with a mathematical model developed into the framework of a fractional dimensional space. A type I band offset has been deduced for all the investigated samples. For the sample with x = 0.54 and 0.48 (barrier lattice matched to GaAs) a band offset ratio about Ec /Ev = 20/80 has been estimated, whereas for the sample with x = 0.43 a value of ?Ev /?Ev = 30/70 was found. This means values of ?Ec = 0.155 eV and ?Ev = 0.360 for the sample with x = 0.43, and ?Ec = 0.085 eV and ?Ev = 0.340 for the sample with x = 0.54. These results point out the different degree of confinement for electrons and holes. We have also observed an increasing of the exciton binding energy (Eb) when the QW thickness (Lw) is reduced. The highest obtained values for Eb were 13.6 meV (x = 0.43), 11.3 meV (x = 0.48), and 10.7 meV (x = 0.54) for Lw ranging between 2 and 3 nm. The minimum values obtained for the exciton Bohr radius and the fractional dimension were 5.5 nm and 2.5, respectively, corresponding to a QW with Lw = 2.2 nm for the sample with x = 0.43. The most important consequences of all these results will be extensively discussed. (Full text)

2008-12-06

55

Coherent motion and anomalous transport properties of exciton and hole polarons with intrinsic vibrational structure  

International Nuclear Information System (INIS)

It is shown that intrinsic vibrational degrees of freedom, inherent in two-atom exciton and hole polarons, drastically affect their transport properties in wide-band dielectrics (rare-gas solids and alkali halides). A fast excitonic energy transport and a comparatively high hole mobility, experimentally observed and attributed to two-site polarons tightly bound with the lattice, cannot be explained by the conventional theory of small-radius polarons that predicts their negligibly weak diffusion, exponentially small in the ratio of the binding energy to temperature. The theory, developed below with allowance for the intrinsic vibrational structure of two-site polarons describes qualitatively a large relevant set of experimental data which seem anomalous from the viewpoint of the conventional theory

2003-03-01

56

Exciton dynamics in 121 angstrom GaAs and GaAsP QWs having different valence band structures arising from built-in biaxial stress  

International Nuclear Information System (INIS)

Exciton dynamics in various 121 angstrom single quantum wells (QWs): a AlGaAs/GaAs QW and two AlGaAs/GaAsP QWs, under different built-in biaxial tension, has been investigated using time resolved photoluminescence (PL) spectroscopy at 5 K. Heavy-hole (hh) and light-hole (lh) exciton formation times from free electron-hole pair, hh (lh) exciton to lh (hh) exciton inter-subband relaxation times, exciton localization times to interface islands, and localized exciton annihilation decay times in the strained and nonstrained QWs have been determined by fitting the PL time profiles at the lowest emission energy with an analytical solution for the localized exciton population profile obtained by solving six level rate equations

1992-03-23

57

Band-edge exciton fine structure of small, nearly spherical colloidal CdSe/ZnS quantum dots.  

Science.gov (United States)

The exciton fine structure of small (2-3.5 nm) wurtzite (WZ) and zincblende (ZB) CdSe quantum dots (Qdots) has been investigated by means of nanosecond and picosecond time-resolved photoluminescence spectroscopy, at temperatures ranging from 5 K to room temperature. For both crystal structures, we observe a similar dark-bright energy level splitting of 2.4-5 meV, with a larger splitting corresponding to smaller Qdots. In addition, spectrally resolved streak camera images collected at 5 K reveal the presence of a third state, split from the lower dark-bright manifold by 30-70 meV, again independently of the crystal structure of the Qdots. The data thus reveal that small WZ and ZB CdSe Qdots are optically indistinguishable. This contrasts with theoretical calculations within the effective-mass approximation, which, in the limit of spherical Qdots, yield a different fine structure for both. However, experimental and theoretical results converge when taking the Qdot shape into account. With transmission electron microscopy, we determined that our Qdots are prolate, with an aspect ratio of 1.15:1. Incorporating this value into our calculations, we obtain a similar fine structure for both WZ and ZB Qdots. Moreover, the opposite sign of the crystal field and shape anisotropy in CdSe suggests that the lowest energy level in small CdSe Qdots has an angular momentum projection F = 0, in contrast with (perfectly) spherical Qdots, where the lowest level corresponds to the dark ±2 state. From the experimental and theoretical data we conclude that shape anisotropy and exchange interactions dominate over the crystal field anisotropy-induced splitting in this size range. PMID:21961786

Moreels, Iwan; Rainò, Gabriele; Gomes, Raquel; Hens, Zeger; Stöferle, Thilo; Mahrt, Rainer F

2011-10-25

58

Defect production in alkali halide crystals by monochromatic x and uv radiation  

International Nuclear Information System (INIS)

The spectral dependence of the F-center production efficiency of several alkali halides was observed in diverse parts of the electromagnetic spectrum. Using low flux monochromatic sources, this efficiency was measured in the hard x-ray region at the K-edge of bromine in KBr and in the near-ultraviolet part of the spectrum corresponding to the exciton region of the alkali halide. A significant increase in the production efficiency was observed at the K-edge, indicating a new F-center production mechanism. In the ultraviolet, maxima in the F-center production efficiency were observed in the long wavelength tail of the first exciton peak and even in the region of the alpha band, providing the first direct measurements of the F-center production efficiency in this region. These studies were made possible by the development of an extremely sensitive new method of detecting F-centers in ultralow concentrations, which utilizes the luminescent properties of these centers. This laser-induced luminescence method enables detection of concentrations of F-centers that are five orders of magnitude smaller than can be detected by optical absorption measurements. Thus, a new regime of F-center production, the early part of the so-called stage I region, could be studied; it was observed to be linear for about four orders of magnitude

1985-01-01

59

The J- and H-bands of dye aggregate spectra: Analysis of the coherent exciton scattering (CES) approximation  

CERN Document Server

The validity of the CES approximation is investigated by comparison with direct diagonalisation of a model vibronic Hamiltonian of $N$ identical monomers interacting electronically. Even for quite short aggregates (N\\gtrsim 6) the CES approximation is shown to give results in agreement with direct diagonalisation, for all coupling strengths, except that of intermediate positive coupling (the H-band region). However, previously excellent agreement of CES calculations and measured spectra in the H-band region was obtained [A. Eisfeld, J. S. Briggs, Chem. Phys. 324, 376]. This is shown to arise from use of the measured monomer spectrum which includes implicitly dissipative effects not present in the model calculation.

Roden, Jan; Briggs, John S; 10.1016/j.chemphys.2008.07.001

2010-01-01

60

Two-photon transitions to exciton polaritons  

International Nuclear Information System (INIS)

A semiclassical theory for the creation of excitonic polariton states by two-photon absorption, via an intermediate exciton state, is given. A band model has been introduced which gives the dominant contribution to this process. A numerical calculation is found to be in good agreement with a recent observation in CuCl. (author)

1979-01-01

 
 
 
 
61

Photoluminescence due to inelastic exciton-exciton scattering in ZnMgO-alloy thin film  

Science.gov (United States)

We studied the photoluminescence of ZnMgO thin film, grown by the radiofrequency sputtering method, as a function of excitation intensity and temperature. As the excitation intensity increases, a nonlinear emission band caused by the radiative recombination of the inelastic exciton-exciton scattering was detected at low temperature. We found that the inelastic exciton-exciton scattering process can only persist up to T ~ 260 K. The nonlinear emission band observed at room temperature is due to the radiative recombination of the electron-hole plasma.

Chia, C. H.; Chen, J. N.; Hu, Y. M.

2011-09-01

62

Photoluminescence due to inelastic exciton-exciton scattering in ZnMgO-alloy thin film  

Energy Technology Data Exchange (ETDEWEB)

We studied the photoluminescence of ZnMgO thin film, grown by the radiofrequency sputtering method, as a function of excitation intensity and temperature. As the excitation intensity increases, a nonlinear emission band caused by the radiative recombination of the inelastic exciton-exciton scattering was detected at low temperature. We found that the inelastic exciton-exciton scattering process can only persist up to T {approx} 260 K. The nonlinear emission band observed at room temperature is due to the radiative recombination of the electron-hole plasma.

Chia, C. H.; Chen, J. N.; Hu, Y. M. [Department of Applied Physics, National University of Kaohsiung, Kaohsiung 81148, Taiwan (China)

2011-09-26

63

Photoluminescence due to inelastic exciton-exciton scattering in ZnMgO-alloy thin film  

International Nuclear Information System (INIS)

We studied the photoluminescence of ZnMgO thin film, grown by the radiofrequency sputtering method, as a function of excitation intensity and temperature. As the excitation intensity increases, a nonlinear emission band caused by the radiative recombination of the inelastic exciton-exciton scattering was detected at low temperature. We found that the inelastic exciton-exciton scattering process can only persist up to T ? 260 K. The nonlinear emission band observed at room temperature is due to the radiative recombination of the electron-hole plasma.

2011-09-26

64

Multiband theory of quantum-dot quantum wells: Dark excitons, bright excitons, and charge separation in heteronanostructures  

Digital Repository Infrastructure Vision for European Research (DRIVER)

Electron, hole, and exciton states of multishell CdS/HgS/CdS quantum-dot quantum well nanocrystals are determined by use of a multiband theory that includes valence-band mixing, modeled with a 6-band Luttinger-Kohn Hamiltonian, and nonparabolicity of the conduction band. The multiband theory correctly describes the recently observed dark-exciton ground state and the lowest, optically active, bright-exciton states. Charge separation in pair states is identified. Previous sing...

Jaskolski, W.; Bryant, Garnett W.

1997-01-01

65

Exciton-impurity luminescence from noncrystalline xenon-argon clusters  

CERN Document Server

For the first time in binary mixtures of solid rare gases exciton-impurity luminescence is observed from a xenon-argon system containing argon as impurity. An exciton-impurity emission band is registered for binary clusters with the structure of multilayer icosahedron. The optical transition occurs from an energy level lying very close to the lowest level of volume excitons in bulk xenon samples. The results demonstrate the possibility of probing excitonic levels in noncrystalline condensed media.

Danylchenko, O G; Kovalenko, S I; Libin, M Yu; Samovarov, V N; Vakula, V L

2009-01-01

66

Multiline Operation of Mercury Halide Lasers,  

Science.gov (United States)

Emission spectrum of the (B-X) band system of HgX-radicals (X=Chlorine, Bromine, Iodine) has been observed by passing electrical discharge through flowing vapors of mercury halides. The emission intensity of the most intense band of the (B-X) system of th...

M. Kushawaha M. Mahmood

1988-01-01

67

A new luminescence due to an exciton-exciton collision process in lead iodide induced by two-photon absorption  

Energy Technology Data Exchange (ETDEWEB)

Photoluminescence properties of highly excited lead iodide have been investigated through two-photon absorption for band-to-band transitions at low temperature and a new luminescence band is observed. The intensity of the new luminescence band increases as 4th power of the excitation intensity and the peak energy is about 23 meV lower than the lowest exciton energy, whose value is almost equal to the energy difference between the n=1 and 2 excitonic states of lead iodide. These results indicate that the new luminescence band observed originates from an exciton-exciton collision process, in which one of the two excitons scatters into the n=2 excitonic state and the other radiatively recombines.

Tanaka, K. [Department of Physics, Graduate School of Engineering, Yokohama National University, 79-5 Tokiwadai, Hodogaya, Yokohama 240-8501 (Japan); Hosoya, T. [Department of Physics, Graduate School of Engineering, Yokohama National University, 79-5 Tokiwadai, Hodogaya, Yokohama 240-8501 (Japan); Fukaya, R. [Department of Physics, Graduate School of Engineering, Yokohama National University, 79-5 Tokiwadai, Hodogaya, Yokohama 240-8501 (Japan); Takeda, J. [Department of Physics, Graduate School of Engineering, Yokohama National University, 79-5 Tokiwadai, Hodogaya, Yokohama 240-8501 (Japan)]. E-mail: jun@ynu.ac.jp

2007-01-15

68

Optical second harmonic generation from Wannier excitons  

DEFF Research Database (Denmark)

Excitonic effects in the linear optical response of semiconductors are well known and the subject of countless experimental and theoretical studies. For the technologically important second-order nonlinear response, however, description of excitonic effects has proved to be difficult. In this work, a simplified three-band Wannier exciton model of cubic semiconductors is applied and a closed form expression for the complex second harmonic response function including broadening is derived. Our calculated spectra are found to be in excellent agreement with the measured response near the band edge. In addition, a very substantial enhancement of the nonlinear response is predicted for the transparency region.

Pedersen, Thomas Garm; Cornean, Horia

2007-01-01

69

Ultraviolet laser ablation of halides and oxides  

International Nuclear Information System (INIS)

We compare and contrast recent measurements of the behavior of ions and excited ions desorbed from samples of alkali halides and oxide ferroelectrics by an excimer laser at 308 nm wavelength. At the intensities used in these experiments, the density of local electronic excitation is low in the halides and high in the ferroelectrics, corresponding to two- and one-photon band-to-band transitions, respectively. The observed desorption yields and changes in the sample surfaces are discussed in terms of the density of electronic excitation, the relative strengths of electron-lattice coupling, and the role of thermal relaxation processes in the two materials. (orig.)

1992-03-01

70

Quantum Dynamics and Spectroscopy of Excitons in Molecular Aggregates  

CERN Multimedia

The theoretical description and the properties of Frenkel excitons in non-covalently bonded molecular aggregates are reviewed from a multi-exciton perspective of dissipative quantum dynamics. First, the photophysical and quantum chemical characterization of the monomeric dye building blocks is discussed, including the important aspect of electron-vibrational coupling within the Huang-Rhys model. Supplementing the model by the Coulombic interactions between monomers, the description of aggregates in terms of excitonic or vibrational-excitonic bands follows. Besides of giving rise to complex absorption and emission line shapes, exciton-vibrational interaction is responsible for energy and phase relaxation and thereby limits the size of coherent excitations in larger aggregates. Throughout, emphasis is put on the electronic three-level model as a minimum requirement to describe nonlinear spectroscopies including effects of two-exciton states such as excited state absorption and exciton-exciton annihilation. The ...

Kühn, Oliver

2011-01-01

71

Surface-Plasmon Assisted Exciton Transport in 1D Nanostructures  

Science.gov (United States)

We consider effect of coupling between exciton propagating in a 1D-nanostructure (e.g., carbon nanotube) and localized surface plasmon modes induced by a metal nanoparticle located in close proximity to the nanostructure. Both regimes of weak and strong exciton-plasmon couplings are taken into account leading to the dressed exciton and plasmon states. In this representation, the dynamics of the dressed excitons is mapped on the impurity scattering problem. The analysis of the scattering matrix indicates that the surface-plasmon modes lead to the exciton intraband scattering and possibility to form localized states within the exciton band gap. Surface plasmon induced exciton radiation pattern and the radiative and non-radiative decay rates are calculated and their dependence on the exciton-plasmon coupling is analyzed.

Cherqui, Charles; Dunlap, David; Piryatinski, Andrei

2013-03-01

72

Bright and dark excitons in semiconductor carbon nanotubes  

Energy Technology Data Exchange (ETDEWEB)

We report electronic structure calculations of finite-length semiconducting carbon nanotubes using the time dependent density functional theory (TD-DFT) and the time dependent Hartree Fock (TD-HF) approach coupled with semiempirical AM1 and ZINDO Hamiltonians. We specifically focus on the energy splitting, relative ordering, and localization properties of the optically active (bright) and optically forbidden (dark) states from the lowest excitonic band of the nanotubes. These excitonic states are very important in competing radiative and non-radiative processes in these systems. Our analysis of excitonic transition density matrices demonstrates that pure DFT functionals overdelocalize excitons making an electron-hole pair unbound; consequently, excitonic features are not presented in this method. In contrast, the pure HF and A111 calculations overbind excitons inaccurately predicting the lowest energy state as a bright exciton. Changing AM1 with ZINDO Hamiltonian in TD-HF calculations, predicts the bright exciton as the second state after the dark one. However, in contrast to AM1 calculations, the diameter dependence of the excitation energies obtained by ZINDO does not follow the experimental trends. Finally, the TD-DFT approach incorporating hybrid functions with a moderate portion of the long-range HF exchange, such as B3LYP, has the most generality and predictive capacity providing a sufficiently accurate description of excitonic structure in finite-size nanotubes. These methods characterize four important lower exciton bands. The lowest state is dark, the upper band is bright, and the two other dark and nearly degenerate excitons lie in-between. Although the calculated energy splittings between the lowest dark and the bright excitons are relatively large ({approx}0.1 eV), the dense excitonic manifold below the bright exciton allows for fast non-radiative relaxation leasing to the fast population of the lowest dark exciton. This rationalizes the low luminescence efficiency in nanotubes.

Tretiak, Sergei [Los Alamos National Laboratory

2008-01-01

73

Exciton exciton scattering in GaAs/AlAs lateral superlattice  

Science.gov (United States)

We perform time integrated degenerate four-wave mixing (DFWM) measurements to investigate exciton dephasing in a GaAs/AlAs lateral superlattice (LSL) with a tunable band gap modulation. The excitons are resonantly excited by femtosecond laser pulses. Under optimal conditions of growth layer alignment, the ground state excitons in the LSL are 1D. When the LSL growth is highly tilted, the 1D confining potential vanishes and the LSL effectively becomes a 2D system. The DFWM measurements for the 1D and 2D excitons show that the exciton-exciton scattering strength is significantly reduced in the 1D case. We find that disorder in the LSL cannot fully explain the observed large reduction. It is likely that reduction in dimensionality plays an important role in this.

Venu Gopal, A.; Vengurlekar, A. S.; Laruelle, F.; Etienne, B.; Shah, J.

2000-07-01

74

Theory of core excitons  

International Nuclear Information System (INIS)

The observation of core excitons with binding energies much larger than those of the valence excitons in the same material has posed a long-standing theoretical problem. A proposed solution to this problem is presented, and Frenkel excitons and Wannier excitons are shown to coexist naturally in a single material

1980-08-29

75

Exciton Dynamics in soluble Poly( p-phenylene-vinylene): Towards an Ultrafast Excitonic Switch  

Science.gov (United States)

We have applied a variety of ps transient and cw optical techniques to elucidate the dynamics, absorption, and emission properties of excitons in soluble derivatives of poly( p-phenylene vinylene) neat films and dilute solutions. We found that the photogenerated singlet excitons in both films and solutions are characterized by strong stimulated emission and two photoinduced absorption bands. We demonstrate that these bands can be used to form an ultrafast optical switch in the near IR spectral range with variable switching times.

Frolov, S. V.; Liess, M.; Lane, P. A.; Gellermann, W.; Vardeny, Z. V.; Ozaki, M.; Yoshino, K.

1997-06-01

76

Intermediate Exciton-Phonon Coupling in Tetracene  

Science.gov (United States)

Polarized absorption spectra of tetracene (naphthacene) single crystals have been investigated between 300 and 13 K, with particular emphasis on the b-polarized spectrum. The low energy tails of the a- and b-polarized first absorption bands are expressed by the Urbach rule with the steepness coefficients ?a'{=}1.37± 0.12, ?b'{=}1.58± 0.13, with which possibility of self-trapping of excitons is discussed along with the dimensionality of tetracene crystals. The exciton-phonon coupling constants are found to be ga{=}1.09± 0.1 and gb{=}0.95± 0.08, consistent with luminescence experiments which show that excitons in tetracene are self-trapped very shallowly. The line halfwidth, the oscillator strength of the first and the second absorption bands, refractive indices, and surface exciton structure are briefly discussed.

Mizuno, Kenichi; Matsui, Atsuo; Sloan, G. J.

1984-08-01

77

Polarizable Optical Bistability of Frenkel Excitons  

Directory of Open Access Journals (Sweden)

Full Text Available Optical spectra of molecular crystals with two molecules in their elementary cell are investigated. Depending on the polarization angle one or two exciton excitation bands can be realized in the spectrum. The bands are separated by the terminal frequency interval. The exciton absorption bands genesis and the peculiarities of the function of their form at the change of the laser radiation polarization have been analyzed and the regularity of arising bistable states in organic benzol-type crystals has been established.

O. Derevyanchuk

2003-06-01

78

Photofragmentation of metal halides  

International Nuclear Information System (INIS)

The author deals with photodissociation of molecules of alkali halides. It is shown that the total absorption cross section consists of two contributions arising from transitions to excited states of total electronic angular momentum ?=0"+ and ?=1. From the inversion of the absorption continua potential energy curves of the excited states can be constructed in the Franck-Condon region. It is found that for all alkali halides the 0"+ state is higher in energy than the ?=1 state. Extensive studies are reported on three thallium halides, TlI, TlBr and TlCl at various wavelengths covering the near ultraviolet region. (Auth.)

1980-01-01

79

Photochemistry of Phenyl Halides  

Digital Repository Infrastructure Vision for European Research (DRIVER)

We have studied fundamental aspects of photo-induced dissociation kinetics and dynamics in several phenyl halides. By combining femtosecond pump-probe measurements with ab initio calculations we are able to account for several observations. In mixed phenyl halides, the dissociation kinetics is found to be dependent on the nature, the number, and the position of the substituents, and also on the excitation wavelength. A surprisingly large reduction in the dissociation time constant, compared t...

Karlsson, Daniel

2008-01-01

80

Exciton-exciton interactions in CdWO4 irradiated by intense femtosecond vacuum ultraviolet pulses  

International Nuclear Information System (INIS)

Exciton-exciton interaction is experimentally revealed and quantitatively analyzed in a wide band-gap scintillator material CdWO4. Under high-intensity femtosecond vacuum ultraviolet excitation, the CdWO4 luminescence is quenched, while its decay becomes essentially nonexponential. We propose an analytical model, which successfully reproduces the decay kinetics recorded in a wide range of excitation densities. The dipole-dipole interaction between excitons leading to their nonradiative decay is shown to be the main cause of a nonproportional response common for many scintillators.

2009-06-15

 
 
 
 
81

Elastic exciton-exciton scattering in photoexcited carbon nanotubes.  

Science.gov (United States)

We report on original nonlinear spectral hole-burning experiments in single wall carbon nanotubes that bring evidence of pure dephasing induced by exciton-exciton scattering. We show that the collision-induced broadening in carbon nanotubes is controlled by exciton-exciton scattering as for Wannier excitons in inorganic semiconductors, while the population relaxation is driven by exciton-exciton annihilation as for Frenkel excitons in organic materials. We demonstrate that this singular behavior originates from the intrinsic one-dimensionality of excitons in carbon nanotubes, which display unique hybrid features of organic and inorganic systems. PMID:22026798

Nguyen, D T; Voisin, C; Roussignol, Ph; Roquelet, C; Lauret, J S; Cassabois, G

2011-09-16

82

Surface photovoltage in exciton absorption range in CdS  

Science.gov (United States)

The high resolution, intrinsic spectra of surface photovoltage are reported for semiconducting n-type CdS single crystals. At reduced temperatures (120-160 K) the spectra exhibit three sharp maxima due to A, B and C free exciton transitions. Energy positions of these lines and valence band parameters (spin-orbit and crystal field splittings) estimated from surface photovoltage are in good agreement with values obtained by other methods. The excitonic transitions are very sensitive to surface treatment, i.e. polishing, etching, background illumination and surface doping. The mechanism of direct interaction of free excitons with surface states is proposed to explain exciton lines in surface photovoltage.

Morawski, A.; Banisch, R.; Lagowski, J.

1977-01-01

83

Observation of exciton luminescence from icosahedral xenon-argon clusters  

International Nuclear Information System (INIS)

For the first time in binary mixtures of solid rare gases, the exciton-impurity luminescence is observed from the xenon-argon system containing argon as an impurity. The intense exciton-impurity emission band is observed in binary clusters with a structure of multilayer icosahedron. The optical transition occurs from the energy level lying very close to the lowest level of volume excitons in bulk xenon samples. The results obtained show the potential possibilities of probing the excitonic levels in disordered condensed media.

2009-12-01

84

Optical properties and structural phase transitions of lead-halide based inorganic-organic 3D and 2D perovskite semiconductors under high pressure  

Science.gov (United States)

Optical absorption, photoluminescence and Raman scattering of lead-halide based inorganic-organic perovskite semiconductors were measured under quasi-hydrostatic pressure at room temperature. For the 3D perovskite semiconductor, (CH3NH3)PbBr3, the free exciton photoluminescence band exhibits red-shifts with pressure, and jumps to a higher energy by 0.07 eV at 0.8 GPa, which is associated with a phase transition from a cubic to an orthorhombic structure confirmed by Raman scattering. Above the phase transition pressure, the exciton band shows blue-shifts with further increasing pressure, and eventually disappears above 4.7 GPa. The results are compared with those for the 2D perovskite semiconductor, (C4H9NH3)2PbI4. First principles pseudopotential calculations were performed to investigate changes in octahedral distortion and electronic band structures with pressure. The calculations have explained the origins of the intriguing changes in the electronic states with pressure in view of bonding characters between atomic orbitals in octahedra.

Matsuishi, K.; Ishihara, T.; Onari, S.; Chang, Y. H.; Park, C. H.

2004-11-01

85

Excitonic Photoluminescence in Semiconductor Quantum Wells: Plasma versus Excitons  

CERN Multimedia

Time-resolved photoluminescence spectra after nonresonant excitation show a distinct 1s resonance, independent of the existence of bound excitons. A microscopic analysis identifies excitonic and electron-hole plasma contributions. For low temperatures and low densities the excitonic emission is extremely sensitive to even minute optically active exciton populations making it possible to extract a phase diagram for incoherent excitonic populations.

Chatterjee, S; Mosor, S; Khitrova, G; Gibbs, H M; Hoyer, W; Kira, M; Koch, S W; Prineas, J P; Stolz, H

2003-01-01

86

Indirect optical absorption and origin of the emission from ?-FeSi2 nanoparticles: Bound exciton (0.809 eV) and band to acceptor impurity (0.795 eV) transitions  

Science.gov (United States)

We investigated the optical absorption of the fundamental band edge and the origin of the emission from ?-FeSi2 nanoparticles synthesized by ion-beam-induced epitaxial crystallization of Fe+ implanted SiO2/Si(100) followed by thermal annealing. From micro-Raman scattering and transmission electron microscopy measurements it was possible to attest the formation of strained ?-FeSi2 nanoparticles and its structural quality. The optical absorption near the fundamental gap edge of ?-FeSi2 nanoparticles evaluated by spectroscopic ellipsometry showed a step structure characteristic of an indirect fundamental gap material. Photoluminescence spectroscopy measurements at each synthesis stage revealed complex emissions in the 0.7-0.9 eV spectral region, with different intensities and morphologies strongly dependent on thermal treatment temperature. Spectral deconvolution into four transition lines at 0.795, 0.809, 0.851, and 0.873 eV was performed. We concluded that the emission at 0.795 eV may be related to a radiative direct transition from the direct conduction band to an acceptor level and that the emission at 0.809 eV derives from a recombination of an indirect bound exciton to this acceptor level of ?-FeSi2. Emissions 0.851 and 0.873 eV were confirmed to be typical dislocation-related photoluminescence centers in Si. From the energy balance we determined the fundamental indirect and direct band gap energies to be 0.856 and 0.867 eV, respectively. An illustrative energy band diagram derived from a proposed model to explain the possible transition processes involved is presented.

Lang, R.; Amaral, L.; Meneses, E. A.

2010-05-01

87

Indirect optical absorption and origin of the emission from ?-FeSi2 nanoparticles: Bound exciton (0.809 eV) and band to acceptor impurity (0.795 eV) transitions  

International Nuclear Information System (INIS)

We investigated the optical absorption of the fundamental band edge and the origin of the emission from ?-FeSi2 nanoparticles synthesized by ion-beam-induced epitaxial crystallization of Fe+ implanted SiO2/Si(100) followed by thermal annealing. From micro-Raman scattering and transmission electron microscopy measurements it was possible to attest the formation of strained ?-FeSi2 nanoparticles and its structural quality. The optical absorption near the fundamental gap edge of ?-FeSi2 nanoparticles evaluated by spectroscopic ellipsometry showed a step structure characteristic of an indirect fundamental gap material. Photoluminescence spectroscopy measurements at each synthesis stage revealed complex emissions in the 0.7-0.9 eV spectral region, with different intensities and morphologies strongly dependent on thermal treatment temperature. Spectral deconvolution into four transition lines at 0.795, 0.809, 0.851, and 0.873 eV was performed. We concluded that the emission at 0.795 eV may be related to a radiative direct transition from the direct conduction band to an acceptor level and that the emission at 0.809 eV derives from a recombination of an indirect bound exciton to this acceptor level of ?-FeSi2. Emissions 0.851 and 0.873 eV were confirmed to be typical dislocation-related photoluminescence centers in Si. From the energy balance we determined the fundamental indirect and direct band gap energies to be 0.856 and 0.867 eV, respectively. An illustrative energy band diagram derived from a proposed model to explain the possible transition processes involved is presented.

2010-05-15

88

Collisional dephasing of excitons  

Science.gov (United States)

We studied experimentally and theoretically the dependence of optical exciton dephasing on exciton concentration. The measurements were made with the help of recently developed technique^1 in GaAs/AlGaAs quantum wells in the region of concentrations up to 3×10^10 cm-2. The measured dephasing rate of circularly polarized exciton is nearly concentration independent while the dephasing rate of linearly polarized excitons grows linearly with the concentration. The different behavior of circular and linear excitons is explained by pair exciton interactions where total excion spin is conserved. The exchange interaction of excitons with only +1 or only -1 circular polarization does not change their phase and leads only only -1 circular polarization does not change their phase and leads only to a renormalization of their energy. The interaction of linear excitons (i.e. coherent superposition of bright +1 and -1 excitons ) leads to creation of dark excitons with spins +2 and -2. While the sum of the phases of the two dark excitons is fixed by the phase of bright excitons the difference of their phases is arbitrary. This and the inverse process lead to the dephasing of bright excitons. Theoretical estimate of the dephasing time based on the recently calculated interaction matrix elements^2 gives a good agreement with the experiment. 1. X.Marie, P.Le Jeune, T.Amand, M.Brousseau, J.Barrau, M.Paillard, and R.Planel, Phys.Rev.Lett. 79, 3222 (1997). 2. S. Ben-Tabou de-Leon, B. Laikhtman, cond-mat/0010247.

Laikhtman, Boris; Marie, Xavier; Amand, Thierry

2001-03-01

89

Excited states of bound excitons in wurtzite-type semiconductors  

International Nuclear Information System (INIS)

Excitons bound to neutral impurities are studied with respect to excited states by means of excitation spectroscopy in CdS single crystals at 1.8 K. The symmetry of the four excited states of the exciton bound to neutral donor (placed energetically below the free exciton) can be deduced from the observed dependence of their optical transitions on the direction of the wave vector and the polarization. Theoretical considerations taking into account the complex valence band structure of wurtzite-type semiconductors lead to an explanation of these states in terms of excited (p- and d-like) hole states of the bound exciton. With respect to the exciton bound to neutral acceptor only the well-known I/sub 1B/-states are found experimentally arising from the incorporation of a hole from the lower B-subvalence band in the complex. (author)

1983-09-01

90

Exciton Spin Currents: Theory  

Digital Repository Infrastructure Vision for European Research (DRIVER)

An indirect exciton is a bound pair of an electron and a hole confined in spatially separated semiconductor layers. Long lifetimes of indirect excitons allow them to cool down to low temperatures below the temperature of quantum degeneracy. In coherent exciton gases the spin relaxation mechanisms linked with the scattering of particles are strongly suppressed. Recent experiments evidenced spontaneous coherence and condensation, phase singularities, spatial ordering, and polarization textures ...

Kavokin, Alexey; Vladimirova, Maria; Jouault, Benoit; Liew, Timothy; Leonard, Jason; Butov, Leonid

2013-01-01

91

Ab initio many-body effects in TiSe?: A possible excitonic insulator scenario from GW band-shape renormalization  

Digital Repository Infrastructure Vision for European Research (DRIVER)

We present both theoretical ab-initio results within the Hedin's GW approximation and experimental angle-resolved photoemission and scanning tunneling spectroscopy measurements on TiSe?. With respect to the density-functional Kohn-Sham metallic picture, the many-body GW self-energy leads to a ?0.2-eV band-gap insulator consistent with our STS spectra at 5 K. The highest valence and the lowest conduction bands are strongly renormalized, with a loss of k² parabolic dispersion toward a k?...

Cazzaniga, M.; Cercellier, H.; Holzmann, M.; Monney, Claude; Aebi, Philipp; Onida, Giovanni; Olevano, V.

2012-01-01

92

Exciton optical transitions in a hexagonal boron nitride single crystal  

International Nuclear Information System (INIS)

Near band gap photoluminescence (PL) of a hexagonal boron nitride single crystal has been studied at cryogenic temperatures with synchrotron radiation excitation. The PL signal is dominated by trapped-exciton optical transitions, while the photoluminescence excitation (PLE) spectra show features assigned to free excitons. Complementary photoconductivity and PLE measurements set the band gap transition energy to 6.4 eV and the Frenkel exciton binding energy larger than 380 meV. (copyright 2011 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)

2011-06-01

93

Exciton Dynamics in soluble Poly(p-phenylene-vinylene): Towards an Ultrafast Excitonic Switch  

International Nuclear Information System (INIS)

We have applied a variety of ps transient and cw optical techniques to elucidate the dynamics, absorption, and emission properties of excitons in soluble derivatives of poly(p-phenylene vinylene) neat films and dilute solutions. We found that the photogenerated singlet excitons in both films and solutions are characterized by strong stimulated emission and two photoinduced absorption bands. We demonstrate that these bands can be used to form an ultrafast optical switch in the near IR spectral range with variable switching times. copyright 1997 The American Physical Society

1997-06-01

94

Exciton Dynamics in soluble Poly(p-phenylene-vinylene): Towards an Ultrafast Excitonic Switch  

Energy Technology Data Exchange (ETDEWEB)

We have applied a variety of ps transient and cw optical techniques to elucidate the dynamics, absorption, and emission properties of excitons in soluble derivatives of poly(p-phenylene vinylene) neat films and dilute solutions. We found that the photogenerated singlet excitons in both films and solutions are characterized by strong stimulated emission and {ital two} photoinduced absorption bands. We demonstrate that these bands can be used to form an ultrafast optical switch in the near IR spectral range with variable switching times. {copyright} {ital 1997} {ital The American Physical Society}

Frolov, S.V.; Liess, M.; Lane, P.A.; Gellermann, W.; Vardeny, Z.V. [University of Utah, Physics Department, Salt Lake City, Utah 84112 (United States); Ozaki, M.; Yoshino, K. [Electronic Engineering Department, Osaka University, Yamada-Oka 2-1, Suita (Japan)

1997-06-01

95

Metallic Halide Thiourea Compounds.  

Science.gov (United States)

Divalant metal halide-thiorea compounds are effective as extreme pressure additives in various lubricating compositions which include synthetic and mineral oil base lubricants. An intermediate N,N' or N,N,N'N', substituted thiourea is first synthesized an...

J. Ryer P. M. Kerschner W. C. Bradbury

1965-01-01

96

Exciton laser rate equations  

Directory of Open Access Journals (Sweden)

Full Text Available The rate equations of the exciton laser in the system of interacting excitons have been obtained and the inverted population conditions and generation have been derived. The possibility of creating radically new gamma-ray laser has been shown.

Garkavenko A. S.

2011-08-01

97

Kinetic theory of exciton-exciton annihilation  

Science.gov (United States)

Weakly excited states of dye aggregates and supramolecular complexes can be characterized by single or two exciton states. Stronger excitation results in the presence of multiple excited molecules, and complex processes of internal energy transfer dynamics take place. The direct consideration of all excited states is limited to systems with a few molecules only. Therefore, an approach is used based on transition operators among the molecular states of interest and resulting in a dynamic theory for excitation energy transfer in strongly excited molecular systems. As a first application of this theory a detailed description of exciton-exciton annihilation is given. The obtained novel nonlinear theory is related to the standard description. Possible further approximation schemes in the offered theoretical framework are discussed.

May, Volkhard

2014-02-01

98

Optical properties of halide and oxide scintillators  

Energy Technology Data Exchange (ETDEWEB)

Knowledge of optical properties of scintillators is important both for the optimization of scintillator systems and the development of new materials particularly ceramic scintillators. Recent theoretical developments, especially new density functionals that enable accurate prediction of band gaps, have made it possible to perform quantitative calculations of the optical properties of scintillator materials. We used these techniques to obtain optical properties of a large number of high light output halide scintillators. These calculations showed that many halide scintillators have remarkably little optical anisotropy and may be good candidates for development as ceramic scintillators. These include materials such as CaI{sub 2}:Eu{sup 2+} that have very high light output and other favorable properties but are difficult to develop due to crystal growth issues. We review some of our recent results and present new results for CeCl{sub 3}.

Singh, David J [ORNL

2011-01-01

99

The photolysis of acetylene halides reaction with oxygen  

Energy Technology Data Exchange (ETDEWEB)

The photolysis of chloro- and bromo-acetylene reactions at low temperature with Hg lamp are carried out. There is no new absorption band observed after in-situ photolysis of acetylene halides in Ar matrix. With the presence of oxygen, several new bands appeared. The new peaks at 2138 and 2342 cm{sup -1} were assigned as CO and CO{sub 2}, respectively. Further work is essential for the assignment of the other new bands.

Hwang, Mei-Lee [Kaohsiung Polytechnic Institute (Taiwan, Province of China); Kuo, Yu-Ping [Chia Nan Junior College of Pharmacy (Taiwan, Province of China)

1996-12-31

100

The signs of the energy dispersion of one-dimensional excitons in Tb(OH)3 utilizing time-resolved spectroscopy  

International Nuclear Information System (INIS)

The signs of the energy dispersion of one-dimensional excitons in Tb(OH)3 are determined from the time-resolved spectrum of the exciton band-to-band luminescence by monitoring the time evolution of the different wavevector states after excitation of the k approximately 0 excitons with a pulsed tuneable dye laser. (author)

1976-01-01

 
 
 
 
101

Excitons on a glome  

CERN Document Server

We introduce a new model to study excitons confined in a three-dimensional space, which provides a unified treatment of Frenkel and Wannier-Mott excitons. The model consists of an exciton trapped on the 3-sphere (i.e. the surface of a four-dimensional ball), sometimes known as a glome. We show that one can determine, for particular values of the dielectric constant $\\epsilon$, the closed-form expression of the exact wave function. We use the exact wave function of the lowest bound state for $\\epsilon=2$ to introduce an intermediate regime which gives satisfactory agreement with exact diagonalization results for a wide range of $\\epsilon$ values.

Loos, Pierre-François

2011-01-01

102

Narrow band exciton coupled with acoustical anharmonic phonons: application to the vibrational energy flow in a lattice of H-bonded peptide units  

Science.gov (United States)

A time-convolutionless master equation is established for describing the transport properties of amide-I vibrons coupled with acoustic phonons in a lattice of H-bonded peptide units. Within the non-adiabatic weak coupling limit, it is shown that the vibron dynamics strongly depends on the nature of the phonons and two distinct mechanisms have been identified. Harmonic phonons, which support spatial correlations over an infinite length scale, induce a fast dephasing-rephasing mechanism in the short time limit. Consequently, the vibron keeps its wavelike nature and a coherent vibrational energy flow takes place whatever the temperature. By contrast, anharmonic phonons carry spatial correlations over a finite length scale, only. As a result, the rephasing process no longer compensates the dephasing mechanism so that dephasing-limited band motion occurs. It gives rise to the incoherent diffusion of the vibron characterized by a diffusion coefficient whose temperature dependence scales as 1/T?. In the weak anharmonicity limit, the exponent ? is about 2. It becomes smaller than unity in the strong anharmonicity limit, indicating that the diffusion coefficient behaves as a slowly decaying function of the temperature.

Pouthier, Vincent

2009-05-01

103

Narrow band exciton coupled with acoustical anharmonic phonons: application to the vibrational energy flow in a lattice of H-bonded peptide units  

International Nuclear Information System (INIS)

A time-convolutionless master equation is established for describing the transport properties of amide-I vibrons coupled with acoustic phonons in a lattice of H-bonded peptide units. Within the non-adiabatic weak coupling limit, it is shown that the vibron dynamics strongly depends on the nature of the phonons and two distinct mechanisms have been identified. Harmonic phonons, which support spatial correlations over an infinite length scale, induce a fast dephasing-rephasing mechanism in the short time limit. Consequently, the vibron keeps its wavelike nature and a coherent vibrational energy flow takes place whatever the temperature. By contrast, anharmonic phonons carry spatial correlations over a finite length scale, only. As a result, the rephasing process no longer compensates the dephasing mechanism so that dephasing-limited band motion occurs. It gives rise to the incoherent diffusion of the vibron characterized by a diffusion coefficient whose temperature dependence scales as 1/T?. In the weak anharmonicity limit, the exponent ? is about 2. It becomes smaller than unity in the strong anharmonicity limit, indicating that the diffusion coefficient behaves as a slowly decaying function of the temperature.

2009-05-06

104

Narrow band exciton coupled with acoustical anharmonic phonons: application to the vibrational energy flow in a lattice of H-bonded peptide units  

Energy Technology Data Exchange (ETDEWEB)

A time-convolutionless master equation is established for describing the transport properties of amide-I vibrons coupled with acoustic phonons in a lattice of H-bonded peptide units. Within the non-adiabatic weak coupling limit, it is shown that the vibron dynamics strongly depends on the nature of the phonons and two distinct mechanisms have been identified. Harmonic phonons, which support spatial correlations over an infinite length scale, induce a fast dephasing-rephasing mechanism in the short time limit. Consequently, the vibron keeps its wavelike nature and a coherent vibrational energy flow takes place whatever the temperature. By contrast, anharmonic phonons carry spatial correlations over a finite length scale, only. As a result, the rephasing process no longer compensates the dephasing mechanism so that dephasing-limited band motion occurs. It gives rise to the incoherent diffusion of the vibron characterized by a diffusion coefficient whose temperature dependence scales as 1/T{sup {alpha}}. In the weak anharmonicity limit, the exponent {alpha} is about 2. It becomes smaller than unity in the strong anharmonicity limit, indicating that the diffusion coefficient behaves as a slowly decaying function of the temperature.

Pouthier, Vincent [Institut UTINAM, UMR CNRS 6213, Universite de Franche-Comte, F-25030 Besancon Cedex (France)], E-mail: vincent.pouthier@univ-fcomte.fr

2009-05-06

105

One-dimensional continuum and exciton states in quantum wires  

CERN Document Server

High-quality T-shaped quantum wires are fabricated by cleaved-edge overgrowth with the molecular beam epitaxy on the interface improved by a growth-interrupt high-temperature anneal. Characterization by micro-photoluminescence (PL) and PL excitation (PLE) spectroscopy at 5 K reveals high uniformity, a sharp spectral width, and a small Stokes shift of one-dimensional (1-D) excitons. The PLE spectrum for 1-D states shows a large peak of ground-state excitons and a small absorption band ascribed to 1-D continuum states with an onset at 11 meV above the exciton peak.

Akiyama, H; Yoshita, M; Pinczuk, A; West, K W; Akiyama, Hidefumi; Pfeiffer, Loren N.; Yoshita, Masahiro; Pinczuk, Aron; West, Ken W.

2002-01-01

106

Photolytic ?-elimination of organic halides from organotellurium (IV) halides  

International Nuclear Information System (INIS)

Irradiation of organotellurium (IV) halides in benzene affords the corresponding organic halide by ?-elimination, the carbon-halogen bond being formed at ipso-position and in some case retentively. The reaction is not affected by the presence or absence of oxygen and t-butoxy radical. (author)

1980-01-01

107

Phase diagram of the excitonic insulator  

International Nuclear Information System (INIS)

Motivated by recent experiments, which give strong evidence for an excitonic insulating phase in TmSe0.45Te0.55, we developed a scheme to quantitatively construct, for generic two-band models, the phase diagram of an excitonic insulator. As a first application of our approach, we calculated the phase diagram for an effective mass two-band model with long-range Coulomb interaction. The shielded potential approximation is used to derive a generalized gap equation controlling for positive (negative) energy gaps the transition from a semi-conducting (semi-metallic) phase to an insulating phase. Numerical results, obtained within the quasi-static approximation, show a steeple-like phase diagram in contrast to long-standing expectations

2006-05-01

108

Excitonic dynamical Franz-Keldysh effect.  

Digital Repository Infrastructure Vision for European Research (DRIVER)

The dynamical Franz-Keldysh effect is exposed by exploring near-band-gap absorption in the presence of intense THz electric fields. It bridges the gap between the de Franz-Keldysh effect and multiphoton absorption and competes with the THz ac Stark effect in shifting the energy of the excitonic resonance. A theoretical model which includes the strong THz field nonperturbatively via a nonequilibrium Green functions technique is able to describe the dynamical Franz-Keldysh effect in the presenc...

Nordstrøm, K. B.; Johnsen, Kristinn; Allen, S. J.; Jauho, Antti-pekka; Birnir, B.; Kono, J.; Noda, T.; Akiyama, H.; Sakaki, H.

2006-01-01

109

Optical response and excitons in gapped graphene  

Digital Repository Infrastructure Vision for European Research (DRIVER)

Graphene can be rendered semiconducting via energy gaps introduced in a variety of ways, e.g., coupling to substrates, electrical biasing, or nanostructuring. To describe and compare different realizations of gapped graphene we propose a simple two-band model in which a "mass" term is responsible for the gap. The optical conductivity predicted for this model is obtained as a simple closed-form expression. In addition, analytical estimates for the binding energy of excitons are derived and the...

Pedersen, Thomas Garm; Jauho, Antti-pekka; Pedersen, K.

2009-01-01

110

Synthetic Alkali Halides  

Science.gov (United States)

This complex experimental investigation uses alkali halides (NaCl, KCl, and mixtures of both) to simulate the melting of alkali feldspars which melt at too high of temperatures to work with in lab. Three hypotheses are tested: It is possible to crystallize alkali-chloride salts from a magma with any composition between NaCl and KCl. Because K+ and Na+ do not have the same ionic size, the atomic spacing in alkali chlorides will vary systematically with composition. Alkali chlorides are equally stable at high (just below liquidus) and low (subsolidus)temperatures. This project takes more than one class period, depending on how many students are in the class, because there will be lines at the scales, oven, and XRD. It is advisable to introduce the lab in class and have students complete various parts on their own time. There are three main parts. Part one: Synthesize all alkali halide compositions at high temperature (hopefully above the solvus. Part two: Put grown crystals back in an oven at lower temperature to see if they will unmix. Part three: Write a report evaluating and interpreting all results, relevant graphs, and the above three hypothesis.

Perkins, Dexter

111

STRAIN INDUCED EFFECTS OF DISLOCATIONS IN CdS:SHIFT OF EXCITONIC LINES, PHOTOVOLTAIC EFFECT  

Digital Repository Infrastructure Vision for European Research (DRIVER)

The calculation of the energy displacement of excitonic lines due to elastic strain of DSB have been performed for wurzite type crystals using the deformation potential theory. It is shown that nonlinear dependence of the position of valence bands on lattice strain gives rise to a different behavior of excitonic lines for various types of dislocation. The results of calculations are applied for the explanation of the characteristics of the free exciton reflection spectra and the spectra of ph...

Vyvenko, O.

1983-01-01

112

Nonradiative Relaxation Processes of Excitons in ?-ZnP 2 Studied by Photocalorimetric Spectroscopy  

Science.gov (United States)

Nonradiative relaxation processes of excitons have been investigated by means of photocalorimetric (PC) spectroscopy in the exciton resonance region of ?-ZnP2 by detecting temperature increase of the crystal under excitation with a Ti:sapphire laser at 4.5 K. For singlet excitons, power of the light absorbed by the crystal is converted to that of the thermal heating of the crystal almost at a constant rate as high as about 80% in the energy range from the 1s longitudinal exciton energy E L to the band gap energy E g. The heat generation, however, is largely suppressed in the 1s reflection band, where the exciton luminescence is also decreased in intensity, which suggests the presence of other optical process in this region such as resonant Rayleigh scattering. From comparison of the PC spectrum with excitation spectrum for the singlet exciton luminescence, it is found that a ratio of the PC signal to the luminescence intensity increases at the exciton resonances n=1, 2, 3 and 4. This fact indicates that the rate of the nonradiative decay of excitons relative to the emission of the luminescence is high at the exciton resonances. The PC spectrum for triplet excitons is also presented.

Arimoto, Osamu; Umemoto, Shigeo; Nakamura, Kaizo

2000-11-01

113

Curved nanowire structures and exciton binding energies  

DEFF Research Database (Denmark)

Growth of quantum-confined semiconductor structures is a complicated process that may lead to imperfect and complex shapes as well as geometrical nonuniformities when comparing a large number of intended identical structures. On the other hand, the possibility of tuning the shape and size of nanostructures allows for extra optimization degrees when considering electronic and optical properties in various applications. This calls for a better understanding of size and shape effects. In the present work, we express the one-band Schrödinger equation in curved coordinates convenient for determining eigenstates of curved quantum-wire and quantum-dash structures with large aspect ratios. Firstly, we use this formulation to solve the problem of single-electron and single-hole states in curved nanowires. Secondly, exciton states for the curved quantum-wire Hamiltonian problem are found by expanding exciton eigenstates on a product of single-particle eigenstates. A simple result is found for the Coulomb matrix elements of an arbitrarily curved structure as long as the radius-of-curvature is much larger than the cross-sectional dimensions. We use this general result to compute the groundstate exciton binding energy of a bent nanowire as a function of the bending radius-of-curvature. It is demonstrated that the groundstate exciton binding energy increases by 40 meV as the radius-of-curvature changes from 20 to 2 nm while keeping the total length (and volume) of the nanowire constant.

Willatzen, Morten; Lassen, B

2009-01-01

114

Exciton emissions in alkali cyanides  

International Nuclear Information System (INIS)

The emissions of Alkali Cyanides X irradiated at low temperature were measured. In addition to the molecular (Frenkel Type) exciton emissions, another emitting centre was found and tentatively assigned to a charge transfer self trapped exciton. The nature of the molecular exciton emitting state is discussed. (Author)

1979-01-01

115

Exciton states and tunneling in semimagnetic asymmetric double quantum wells  

Energy Technology Data Exchange (ETDEWEB)

Exciton level structure and interwell relaxation are studied in Cd(Mn,Mg)Te-based asymmetric double quantum wells (ADQWs) by a steady-state optical spectroscopy in magnetic fields up to B = 10 T. The as grown heterostructures with CdTe QWs and nonmagnetic interwell CdMgTe barrier were subjected to a rapid temperature annealing to introduce Mn and Mg atoms from opposite barriers inside the QWs which results in a formation of the ADQW with completely different magnetic field behavior of the intrawell excitons. The giant Zeeman effect in the QW with magnetic Mn ions gives rise to a crossing of the ground exciton levels in two QWs at B{sub C}{proportional_to} 3-6 T which is accompanied by a reverse of the interwell tunneling direction. In a single-particle picture the exciton tunneling is forbidden at B < 1 T as supported by calculations. Experimentally, nevertheless, a very efficient interwell relaxation of excitons is found at resonant excitation in the whole magnetic field range, regardless of the tunneling direction, emphasizing importance of excitonic correlations in the interwell tunneling. At nonresonant excitation an unexpectedly slow relaxation of the {sigma}{sup -}-polarized excitons from the nonmagnetic QW to the {sigma}{sup +}-polarized ground state in the semimagnetic QW is observed at B > B{sub C}, giving rise to a nonequilibrium distribution of excitons in ADQW. A strong dependence of the total circular polarization degree on the hh-lh splitting {delta}{sub hh-lh} in the nonmagnetic QW is found and attributed to the spin dependent interwell tunneling controlled by an exciton spin relaxation. Different charge-transfer mechanisms are analyzed in details and an elastic scattering due to a strong disorder is suggested as the main tunneling mechanism with the underlying influence of the valence band-mixing. (Abstract Copyright [2010], Wiley Periodicals, Inc.)

Zaitsev, S.V.; Brichkin, A.S. [Institute of Solid State Physics, Russian Academy of Sciences, Chernogolovka (Russian Federation); Tarakanov, Yu.A. [A.F. Ioffe Physico-Technical Institute, Russian Academy of Science, St. Petersburg (Russian Federation); Bacher, G. [Lehrstuhl Werkstoffe der Elektrotechnik, Universitaet Duisburg-Essen, Duisburg (Germany)

2010-02-15

116

Huge excitonic effects in layered hexagonal boron nitride  

Digital Repository Infrastructure Vision for European Research (DRIVER)

The calculated quasiparticle band structure of bulk hexagonal boron nitride using the all-electron GW approximation shows that this compound is an indirect-band-gap semiconductor. The solution of the Bethe-Salpeter equation for the electron-hole two-particle Green function has been used to compute its optical spectra and the results are found in excellent agreement with available experimental data. A detailed analysis is made for the excitonic structures within the band gap ...

Arnaud, B.; Lebe?gue, S.; Rabiller, P.; Alouani, M.

2005-01-01

117

Configurations of excitons in BaF{sub 2}-LaF{sub 3} crystals  

Energy Technology Data Exchange (ETDEWEB)

Exciton luminescence was investigated in La-doped BaF{sub 2} crystals. Two luminescence bands at 4.26 and 3.82 eV related to excitons have been observed. The excitation spectrum of the 4.26 eV band of BaF{sub 2}-LaF{sub 3} was similar to that of the exciton emission band of pure BaF{sub 2} crystals. The excitation spectrum of the 3.82 eV band shows a new band within the 8-9 eV region. The band becomes most prominent for BaF{sub 2}-3 wt% LaF{sub 3} and has a maximum at 8.7 eV at 290 K. The decay time for the 4.26 eV emission band of BaF{sub 2}-3% LaF{sub 3} considerably increased with decreasing temperature like the exciton decay time for pure BaF{sub 2}, where the decay time of unperturbed excitons grows to 121 {mu} s at 17 K. In contrast to this, the decay time of the 3.82 eV band increases from 4.1 {mu} s at 290 K to 6.2 {mu} s at 15 K. The significant reduction of the triplet-singlet decay time can be associated with strong interaction of excited electrons with lanthanum ions. It was concluded that the 4.26 eV emission band is associated with weakly perturbed excitons, while the 3.82 eV band is associated with strongly perturbed excitons. The possible spatial configurations of excitons are discussed. (author)

Radzhabov, E.; Nepomnyashikh, A.; Egranov, A. [Vinogradov Institute of Geochemistry, Russian Academy of Sciences, Irkutsk (Russian Federation)

2002-08-12

118

Exciton and Biexciton Fine Structure in Single Elongated Islands Grown on a Vicinal Surface  

Science.gov (United States)

We report a microphotoluminescence study of the exciton and the biexciton localized in very elongated islands formed by well-width fluctuations in a thin CdTe/CdMgTe quantum well grown on a vicinal surface. The electron-hole exchange interaction in a local reduced symmetry splits the exciton states. The resulting transitions are linearly polarized along the two orthogonal principal axes of the island. The valence band mixing induced by the elongated shape of the potential leads to a strong polarization anisotropy and to the observation of dark exciton states under magnetic field. The biexciton-exciton transition reproduces all the fine structure of the exciton state including the transition of the biexciton to the dark exciton state.

Besombes, L.; Kheng, K.; Martrou, D.

2000-07-01

119

Photoluminescence-linewidth-derived exciton mass for InGaAsN alloys  

Energy Technology Data Exchange (ETDEWEB)

The authors report a measurement of the variation of the value of the linewidth of an excitonic transition in InGaAsN alloys (1 and 2% nitrogen) as a function of hydrostatic pressure using photoluminescence spectroscopy. The samples were grown by metal-organic chemical vapor deposition and the photoluminescence measurements were performed a 4K. The authors find that the value of the excitonic linewidth increases as a function of pressure until about 100 kbars after which it tends to saturate. This change in the excitonic linewidth is used to derive the pressure variation of the reduced mass of the exciton using a theoretical formalism which is based on the premise that the broadening of the excitonic transition is caused primarily by compositional fluctuations in a completely disordered alloy. The variation of the excitonic reduced mass thus derived is compared with that recently determined using a first-principles band structure calculation based on local density approximation.

JONES,ERIC D.; ALLERMAN,ANDREW A.; KURTZ,STEVEN R.; MODINE,NORMAND A.; BAJAJ,K.K.; TOZER,S.T.; WEI,XING

2000-01-27

120

Singlet exciton fission photovoltaics.  

Science.gov (United States)

Singlet exciton fission, a process that generates two excitons from a single photon, is perhaps the most efficient of the various multiexciton-generation processes studied to date, offering the potential to increase the efficiency of solar devices. But its unique characteristic, splitting a photogenerated singlet exciton into two dark triplet states, means that the empty absorption region between the singlet and triplet excitons must be filled by adding another material that captures low-energy photons. This has required the development of specialized device architectures. In this Account, we review work to develop devices that harness the theoretical benefits of singlet exciton fission. First, we discuss singlet fission in the archetypal material, pentacene. Pentacene-based photovoltaic devices typically show high external and internal quantum efficiencies. They have enabled researchers to characterize fission, including yield and the impact of competing loss processes, within functional devices. We review in situ probes of singlet fission that modulate the photocurrent using a magnetic field. We also summarize studies of the dissociation of triplet excitons into charge at the pentacene-buckyball (C60) donor-acceptor interface. Multiple independent measurements confirm that pentacene triplet excitons can dissociate at the C60 interface despite their relatively low energy. Because triplet excitons produced by singlet fission each have no more than half the energy of the original photoexcitation, they limit the potential open circuit voltage within a solar cell. Thus, if singlet fission is to increase the overall efficiency of a solar cell and not just double the photocurrent at the cost of halving the voltage, it is necessary to also harvest photons in the absorption gap between the singlet and triplet energies of the singlet fission material. We review two device architectures that attempt this using long-wavelength materials: a three-layer structure that uses long- and short-wavelength donors and an acceptor and a simpler, two-layer combination of a singlet-fission donor and a long-wavelength acceptor. An example of the trilayer structure is singlet fission in tetracene with copper phthalocyanine inserted at the C60 interface. The bilayer approach includes pentacene photovoltaic cells with an acceptor of infrared-absorbing lead sulfide or lead selenide nanocrystals. Lead selenide nanocrystals appear to be the most promising acceptors, exhibiting efficient triplet exciton dissociation and high power conversion efficiency. Finally, we review architectures that use singlet fission materials to sensitize other absorbers, thereby effectively converting conventional donor materials to singlet fission dyes. In these devices, photoexcitation occurs in a particular molecule and then energy is transferred to a singlet fission dye where the fission occurs. For example, rubrene inserted between a donor and an acceptor decouples the ability to perform singlet fission from other major photovoltaic properties such as light absorption. PMID:23611026

Lee, Jiye; Jadhav, Priya; Reusswig, Philip D; Yost, Shane R; Thompson, Nicholas J; Congreve, Daniel N; Hontz, Eric; Van Voorhis, Troy; Baldo, Marc A

2013-06-18

 
 
 
 
121

OPTICAL ABSORPTION SPECTRA OF LIQUID ALKALI HALIDES-F-CENTER GENERATION AT VERY HIGH TEMPERATURES  

Digital Repository Infrastructure Vision for European Research (DRIVER)

We have measured optical absorption coefficients of liquid alkali halides such as NaCl, KCl and CsCl up to very high temperatures and found that there appears a new absorption band originated from F-center even in "pure" alkali halides by raising temperature, which is known to be generated usually by the addition of small amount of alkali metals to liquid alkali halides. The measurements have been performed using sapphire cell, the use of which is necessary to avoid a chemical reactio...

1991-01-01

122

Triplet exciton dissociation in singlet exciton fission photovoltaics  

Energy Technology Data Exchange (ETDEWEB)

Triplet exciton dissociation in singlet exciton fission devices with three classes of acceptors are characterized: fullerenes, perylene diimides, and PbS and PbSe colloidal nanocrystals. Using photocurrent spectroscopy and a magnetic field probe it is found that colloidal PbSe nanocrystals are the most promising acceptors, capable of efficient triplet exciton dissociation and long wavelength absorption. (Copyright copyright 2012 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim)

Jadhav, Priya J.; Mohanty, Aseema; Bulovic, Vladimir; Baldo, Marc A. [Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, MA (United States); Brown, Patrick R. [Department of Physics, Massachusetts Institute of Technology, Cambridge, MA (United States); Thompson, Nicholas [Department of Materials Science, Massachusetts Institute of Technology, Cambridge, MA (United States); Wunsch, Benjamin [School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, GA (United States); Yost, Shane R.; Hontz, Eric; Van Voorhis, Troy; Bawendi, Moungi G. [Department of Chemistry, Massachusetts Institute of Technology, Cambridge, MA (United States)

2012-12-04

123

Actinide halide complexes  

Energy Technology Data Exchange (ETDEWEB)

A compound of the formula MX.sub.n L.sub.m wherein M is a metal atom selected from the group consisting of thorium, plutonium, neptunium or americium, X is a halide atom, n is an integer selected from the group of three or four, L is a coordinating ligand selected from the group consisting of aprotic Lewis bases having an oxygen-, nitrogen-, sulfur-, or phosphorus-donor, and m is an integer selected from the group of three or four for monodentate ligands or is the integer two for bidentate ligands, where the sum of n+m equals seven or eight for monodentate ligands or five or six for bidentate ligands, a compound of the formula MX.sub.n wherein M, X, and n are as previously defined, and a process of preparing such actinide metal compounds including admixing the actinide metal in an aprotic Lewis base as a coordinating solvent in the presence of a halogen-containing oxidant, are provided.

Avens, Larry R. (Los Alamos, NM); Zwick, Bill D. (Santa Fe, NM); Sattelberger, Alfred P. (Los Alamos, NM); Clark, David L. (Los Alamos, NM); Watkin, John G. (Los Alamos, NM)

1992-01-01

124

Dynamic coherent exciton condensates in a semiconductor planar microcavity  

Science.gov (United States)

We observed a coherent exciton state with a long decay time ˜1000 ps in a semiconductor planar microcavity structure. The lifetime of cavity polariton condensates previously reported has been limited to ˜10 ps. The sample consists of InGaAs quantum wells positioned near anti-nodes of the photon field in a GaAs ?-cavity sandwiched by GaAs/AlAs-based Bragg mirrors. Under a pulsed excitation above the stop-band of the Bragg mirrors (excess energy >150 meV), spatially coherent exciton emissions were observed to last for ˜1 ns. Conventional dynamic exciton-polariton condensates with a ˜10 ps lifetime were observed under a near-resonant (excess energy ˜ 6 meV) ps pulsed excitation. Dynamics of spatial coherence, energy relaxation, and spin polarization were characterized by time-resolved spectroscopies, including double-slit and polarimetry experiments. The fluctuation of excitonic emissions was characterized by a photon-correlation measurement. The existence of such a long-lived coherent exciton state is attributed to formation of dark excitons under an excitation with significant excessive energy.

Lai, Chih-Wei; Zin Latt, Kyaw; Li, Yi-Shan; Lin, Sheng-Di

2011-03-01

125

Excitons in disordered polymers  

CERN Document Server

We investigate the effects of disorder on Frenkel excitons in disordered conjugated polymers with allowed rotations about single bonds. In these materials, the principal effect of the disorder is to modify the transfer integrals appearing in the exciton Hamiltonian by changing the angle of rotation between single-bonded segments in a random manner. It is assumed the integrals have the form Jk,k+1 cos(qk-qk+1) where Jk,k+1 is taken to be a constant and qk denotes the orientation angle of the kth planar segment. Two types of disorder can be present: segmental disorder which is characterized by infrequent, large fluctuations in qk-qk+1 and a worm-like disorder marked by repeated, small fluctuations in the angular difference that are governed by a Gaussian distribution. We calculate the density of states and the localization lengths for the exciton modes in chains of 5x10^6 segments by mode counting techniques and the optical absorption spectra by direct matrix diagonalization. Both types of disorder are investig...

Avgin, I

2005-01-01

126

Electrostatic traps for dipolar excitons  

CERN Document Server

We consider the design of two-dimensional electrostatic traps for dipolar indirect excitons. We show that the exciton's dipole-dipole interaction, combined with the in-plane electric fields that arise due to the trap geometry, constrain the maximal density and lifetime of trapped excitons. We derive an analytic estimate of these values and determine their dependence on the trap geometry, thus suggesting the optimal design for high density trapping.

Rapaport, R; Simon, S; Mitrofanov, O; Pfeiffer, L N; Platzman, P M; Rapaport, Ronen; Chen, Gang; Simon, Steve; Mitrofanov, Oleg; Pfeiffer, Loren

2005-01-01

127

Dynamics of exciton creation and decay processes in composition - disordered InGaN thin films  

International Nuclear Information System (INIS)

In the GaN-based ternary alloys, InGaN crystals have been recognized as key materials for e-h plasmas-exciton dynamics, because of large exciton binding energies (24.8 meV in GaN). We report investigations of creating and recombination dynamics of excitons in commercially important InxGa1-xN composition range from x = 0.1 to 0.18 in which nanoscale indium composition fluctuation occurs and formation of indium rich clusters acting as quantum dots (QD) can be expected. Three MOCVD grown samples having x = 0.1; 0.14 and 0.18 have been investigated using 3D picosecond transient PL spectroscopy. It has been found that the band to band photo excitation at 15 K in whole composition range results in creating of complex luminescence band represented by three close overlapping Gaussian shape single exciton bands. All three exciton bands show fast decay time constants in a picosecond range. For all the samples the PL intensity dependence on excitation pulse power for each exciton band is different. An increase of the excitation pulse power density results in the linear growth of the band at the higher photon energy side. The next energy bands with lower photon energy show correspondingly quadratic and cubic dependences on laser pulse energy and are caused by the formation of biexcitons and triexcitons. Fast decay kinetics and excitation of multi-excitonic complexes are evident for important role of quantum dots by quantum capture of excitons in the considered composition range.

2011-06-23

128

Coacervation of Aqueous Solutions of Tetraalkylammonium Halides.  

Science.gov (United States)

The coacervation of aqueous solutions of tatraalkylammonium halides in the presence of not of inorganic halides and acids has been studied, considering thermodynamic and spectroscopic aspects. The importance of dispersion forces as well as forces resultin...

A. Mugnier de Trobriand

1979-01-01

129

Helium irradiation of alkali halides  

International Nuclear Information System (INIS)

An investigation has been made of radiation damage in alkali halide crystals induced by heavy bombardment of 1 MeV helium ions. The channeling technique has been employed and three processes have been monitored simultaneously to explain the apparent reduction of damage at high doses. It is shown that the initial rise and fall of the back-scattered yield from different alkali halides can be related to the Pooley mechanism coupled with the aggregation of interstitials to form dislocation loops through the intermediate stage of clusters

1979-01-01

130

Excitonic signatures in the optical response of single-wall carbon nanotubes  

Energy Technology Data Exchange (ETDEWEB)

The optical properties of single-wall carbon nanotubes (SWNTs) are dominated by the excitonic character of the transitions even at room temperature. The very peculiar properties of these excitons arise from both the one-dimensional (1D) nature of carbon nanotubes and from the electronic properties of graphene from which nanotubes are made. We first propose a brief qualitative review of the structure of the excitonic manifold and emphasize the role of dark states. We describe recent experimental investigations of this excitonic structure by means of temperature dependent PL measurements. We investigate the case of upper sub-bands and show that high-order optical transitions remain excitonic for large diameter nanotubes. A careful investigation of Rayleigh scattering spectra at the single nanotube level reveals clear exciton-phonon side-bands and Lorentzian line profiles for all semi-conducting nanotubes. In contrast, metallic nanotubes show an ambivalent behavior which is related to the reduced excitonic binding energy. Schematic of the exciton manifold in single-wall carbon nanotubes. (Copyright copyright 2012 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim)

Voisin, Christophe; Berger, Sebastien; Cassabois, Guillaume; Roussignol, Philippe [Laboratoire Pierre Aigrain, Ecole Normale Superieure, CNRS UMR8551, UPMC, Universite Paris Diderot, 24 rue Lhomond, 75005 Paris (France); Berciaud, Stephane [IPCMS, UMR 7504, CNRS Universite de Strasbourg, 23 rue du Loess, 67034 Strasbourg (France); Yan, Hugen; Hone, James; Heinz, Tony F. [Physics, Mechanical Engineering and Electrical Engineering Departments, Columbia University, New York, NY (United States); Lauret, Jean-Sebastien [Laboratoire de Photonique Quantique et Moleculaire, Ecole Normale Superieure de Cachan, CNRS UMR 8537, Institut Dalembert, 61 Avenue Wilson, Cachan (France)

2012-05-15

131

Dynamics of high-density excitons in PbI{sub 2} studied by two-photon absorption  

Energy Technology Data Exchange (ETDEWEB)

Relaxation and diffusion processes of high-density excitons in layered-type semiconductor PbI{sub 2} have been investigated through femtosecond transient grating and time-resolved luminescence measurements under two-photon absorption. We observed two luminescence bands at low temperature; one is due to an exciton-exciton collision process, while the other originates from localized excitonic states. By utilizing the transient grating spectroscopy, the lifetime and diffusion coefficient of the high-density excitons in PbI{sub 2} are estimated to be 72 ps and 4.6 cm{sup 2}/s, respectively. The lifetime observed is almost equal to the values of the decay time of the luminescence due to the exciton-exciton collision process and the rise time of the luminescence due to the localized excitons. This strongly shows that the decay of the exciton-exciton collision process is dominated by the exciton trapping to the localized states arising from potential fluctuations due to imperfection of crystal.

Hosoya, Toshihiko [Department of Physics, Graduate School of Engineering, Yokohama National University 79-5 Tokiwadai, Hodogaya, Yokohama 240-8501 (Japan); Tanaka, Keita [Department of Physics, Graduate School of Engineering, Yokohama National University 79-5 Tokiwadai, Hodogaya, Yokohama 240-8501 (Japan); Fukaya, Ryo [Department of Physics, Graduate School of Engineering, Yokohama National University 79-5 Tokiwadai, Hodogaya, Yokohama 240-8501 (Japan); Takeda, Jun [Department of Physics, Graduate School of Engineering, Yokohama National University 79-5 Tokiwadai, Hodogaya, Yokohama 240-8501 (Japan)]. E-mail: jun@ynu.ac.jp

2006-07-15

132

Exciton Mediated Superconductivity in PrOs4Sb12  

Digital Repository Infrastructure Vision for European Research (DRIVER)

The most important character of the exotic superconductor PrOs4Sb12 is the existence of low-lying excitations (excitons) with a finite energy gap and it appears as the magnetic field-induced order above 4.5 T. We focus on the a_u conduction band, which hybridizes with a Pr 4f^2 state strongly, coupled to the excitons. It results in an attractive interaction between the conduction electrons. The symmetry of the superconducting order parameter is determined by dispersion relat...

Matsumoto, Masashige; Koga, Mikito

2005-01-01

133

Excitonic surface polaritons in luminescence from ZnTe crystals  

Energy Technology Data Exchange (ETDEWEB)

The form and structure of reflection and exciton-polariton luminescence spectra of ZnTe crystals are studied in the region of the ground (n = 1) exciton state. The longitudinal-transverse splitting magnitude ..delta..E/sub LT/ is determined from the shape of the reflection spectra. A detected doublet structure of an emission band from the lower polariton branch is associated with the k-linear term. The evolution of bulk and surface polariton luminescence spectra versus temperature and wavelength of the exciting light is investigated.

Brodin, M.S.; Bandura, V.M.; Matsko, M.G. (AN Ukrainskoj SSR, Kiev. Inst. Fiziki)

1984-10-01

134

Excitonic surface polaritons in luminescence from ZnTe crystals  

International Nuclear Information System (INIS)

The form and structure of reflection and exciton-polariton luminescence spectra of ZnTe crystals are studied in the region of the ground (n = 1) exciton state. The longitudinal-transverse splitting magnitude ?E/sub LT/ is determined from the shape of the reflection spectra. A detected doublet structure of an emission band from the lower polariton branch is associated with the k-linear term. The evolution of bulk and surface polariton luminescence spectra versus temperature and wavelength of the exciting light is investigated. (author)

1984-10-01

135

Understanding excitons using spherical geometry  

Energy Technology Data Exchange (ETDEWEB)

Using spherical geometry, we introduce a novel model to study excitons confined in a three-dimensional space, which offers unparalleled mathematical simplicity while retaining much of the key physics. This new model consists of an exciton trapped on the 3-sphere (i.e. the surface of a four-dimensional ball), and provides a unified treatment of Frenkel and Wannier–Mott excitons. Moreover, we show that one can determine, for particular values of the dielectric constant ?, the closed-form expression of the exact wave function. We use the exact wave function of the lowest bound state for ?=2 to introduce an intermediate regime which gives satisfactory agreement with the exact results for a wide range of ? values. -- Highlights: ? We introduce a novel model to study excitons. ? This provides a unified treatment of Frenkel and Wannier–Mott excitons. ? We determine the exact wave functions for particular values of the dielectric constant.

Loos, Pierre-François, E-mail: loos@rsc.anu.edu.au [Research School of Chemistry, Australian National University, Canberra ACT 0200 (Australia)

2012-05-21

136

Kinematics of Cold Excitons in the Laser Induced Exciton Trap  

Science.gov (United States)

We have recently demonstrated laser induced trapping of indirect excitons in coupled quantum wells [1]. An important advantage of the laser induced exciton trapping is the possibility of controlling the trap in-situ by varying the laser intensity in space and time. Moreover, the excitons at the trap center are cold since they are far from the hot area of the laser excitation. Here, we report the studies of exciton kinetics in the laser induced traps. In particular, the kinetics of the trap formation and the exciton collection to the trap center when the excitation is switched on, as well as the kinetics of the exciton cloud when the excitation and trap are switched off have been measured by ultrafast imaging spectroscopy. The dynamics of the degenerate Bose gas of excitons in the trap was measured and analyzed for the first time. [1] A.T. Hammack, M. Griswold, L.V. Butov, L.E. Smallwood, A.L. Ivanov, and A.C. Gossard, Phys. Rev. Lett. 96, 227402 (2006).

Hammack, A. T.; Butov, L. V.; Smallwood, L. E.; Mouchliadis, L.; Ivanov, A. L.; Gossard, A. C.

2007-03-01

137

FIELD DEPENDENCE OF THE BOUND STATE OF Cr3+ EXCITON ACCOMPANIED WITH Yb3+ SPIN FLIP IN YbCrO3  

Digital Repository Infrastructure Vision for European Research (DRIVER)

In YbCrO3, the Cr3+ exciton -Yb3+ spin flip combined excitation (R' band) appears in the neighborhood of the Cr3+ exciton lines. We found that a bound state appears on the lowest energy side of the R' band and has a strong dependence on external magnetic fields.

Kojima, N.; Tsujikawa, I.

1988-01-01

138

Ab initio many-body calculation of excitons in solid Ne and Ar  

CERN Multimedia

Absorption spectra, exciton energy levels and wave functions for solid Ne and Ar have been calculated from first principles using many-body techniques. Electronic band structures of Ne and Ar were calculated using the GW approximation. Exciton states were calculated by diagonalizing an exciton Hamiltonian derived from the particle-hole Green function, whose equation of motion is the Bethe-Salpeter equation. Singlet and triplet exciton series up to n=5 for Ne and n=3 for Ar were obtained. Binding energies and longitudinal-transverse splittings of n=1 excitons are in excellent agreement with experiment. Plots of correlated electron-hole wave functions show that the electron-hole complex is delocalised over roughly 7 a.u. in solid Ar.

Galamic-Mulaomerovic, S

2004-01-01

139

Photoluminescence properties of exciton-exciton scattering in a dilute nitride GaAs{sub 1-x}N{sub x} epitaxial film  

Energy Technology Data Exchange (ETDEWEB)

We report on the photoluminescence (PL) properties of a dilute nitride GaAs{sub 1-x}N{sub x} epitaxial film with x =0.008 grown on a (001) GaAs substrate under high-density-excitation conditions. The band-gap energy of the GaAs{sub 0.992}N{sub 0.008} film is 1.34 eV at 10 K, which was confirmed by photoreflectance spectroscopy; namely, GaAs{sub 0.992}N{sub 0.008}belongs to a category of narrow-gap semiconductors. It has been found that a PL band appears with a threshold-like nature. At the threshold excitation power for the appearance of the PL band, the PL-peak energy is lower than the energy of the fundamental exciton by the exciton binding energy. The results described above indicate that the PL band originates from exciton-exciton scattering, the so-called P emission, which is typically observed in wide-gap semiconductors with large exciton binding energies. Furthermore, we have confirmed the existence of optical gain in the energy region of the P band with use of a variable-stripe-length method. (copyright 2011 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)

Nakayama, Masaaki; Hashimoto, Jun; Maeda, Yuta [Department of Applied Physics, Graduate School of Engineering, Osaka City University, Sugimoto, Sumiyoshi-ku, Osaka 558-8585 (Japan)

2011-02-15

140

Exciton Effects in Optical Absorption of Boron-Nitride Nanotubes  

CERN Document Server

Exciton effects are studied in single-wall boron-nitride (BN) nanotubes. Linear absorption spectra are calculated with changing the chiral index of the zigzag nanotubes. We consider the extended Hubbard model with atomic energies at the boron and nitrogen sites. Exciton effects are calculated using the configuration interaction technique. The Coulomb interaction dependence of the band gap, the lowest exciton energy, and the binding energy of the exciton are discussed. The optical gap of the (5,0) nanotube is about 6 eV at the onsite interaction U=2t with the hopping integral t=1.2 eV. The binding energy of the exciton is 0.50 eV for these parameters. This energy agrees well with that of other theoretical investigations. We find that the energy gap and the binding energy are almost independent of the geometries of the nanotubes. This novel property is in contrast with that of the carbon nanotubes which show metallic and semiconducting properties depending on the chiral index.

Harigaya, Kikuo

2007-01-01

 
 
 
 
141

Exciton Dynamics in Semiconductor Nanocrystals.  

Science.gov (United States)

This review article provides an overview of recent advances in the study and understanding of dynamics of excitons in semiconductor nanocrystals (NCs) or quantum dots (QDs). Emphasis is placed on the relationship between exciton dynamics and optical properties, both linear and nonlinear. We also focus on the unique aspects of exciton dynamics in semiconductor NCs as compared to those in bulk crystals. Various experimental techniques for probing exciton dynamics, particularly time-resolved laser methods, are reviewed. Relevant models and computational studies are also briefly presented. By comparing different materials systems, a unifying picture is proposed to account for the major dynamic features of excitons in semiconductor QDs. While the specific dynamic processes involved are material-dependent, key processes can be identified for all the materials that include electronic dephasing, intraband relaxation, trapping, and interband recombination of free and trapped charge carriers (electron and hole). Exciton dynamics play a critical role in the fundamental properties and functionalities of nanomaterials of interest for a variety of applications including optical detectors, solar energy conversion, lasers, and sensors. A better understanding of exciton dynamics in nanomaterials is thus important both fundamentally and technologically. PMID:23625792

Wheeler, Damon A; Zhang, Jin Z

2013-04-26

142

Superconductivity and Excitonic Charge Order  

Science.gov (United States)

It was suggested four decades ago that excitons as well as phonons could mediate superconductivity and that the temperature limits usually imposed by phonons could thus be avoided. In practice this form of excitonic pairing turned out to be elusive, because phonon softening typically causes a structural instability to occur before excitonic superconductivity has a chance to arise. Upon suppression of this CDW order however, superconductivity once again has an opportunity to materialise, as has recently been observed in for example pure TiSe2 under pressure. It is unclear what role is played by the excitons in such an environment of critical structural fluctuations, and whether they can have any effect on the pairing or indeed TC. Here we introduce a theoretical model to study the ways in which SC, CDW and excitonic order compete, coexist and even cooperate. Applying the model to TiSe2, we show that the hitherto elusive mechanism driving its CDW transition is a combination of excitonic and Jahn-Teller effects, and that under pressure it is likely to display an unusual type of superconductivity mediated by combinations of excitons and phonons.

van Wezel, Jasper; Nahai-Williamson, Paul; Saxena, Siddarth

2010-03-01

143

Electron stimulated desorption thresholds for excited atoms desorbed from alkali-halides  

International Nuclear Information System (INIS)

We have investigated the thresholds for the generation of desorbed excited alkali atoms under electron bombardment for LiF, KCl and KBr in the energy range from 10 to 120 eV, and from room temperature up to 4500C. The dependence of the yield of excited atoms on the electron energy shows features which are in good agreement with the excitation energies of alkali core-excitons in the alkali-halide crystal. This suggests the production of an alkali core-exciton as an initial step for the desorption of excited alkali metal atoms under electron bombardment. The dependence of the yield of excited alkali atoms on either the temperature of the crystal or the electron density in the irradiated surface area gives additional support for that model. (author)

1988-09-19

144

Organic halide electroreduction on silver  

Energy Technology Data Exchange (ETDEWEB)

Silver, whose extraordinary electrolytically properties for organic halide reduction have been recently evidenced, has been used as cathode material for systematic preparative electrolyses in membrane-divided cells. To better elucidate the substrate role on the remarkable positive shift of reduction potentials, and on the cage effect i. e. the promotion of intermolecular reaction on adsorbed intermediates, three halide substrate patterns are here compared in terms of both voltammetric characterization and preparative electroreduction products: aliphatic halides (adamantanes), aromatic halides (phenols) and anomeric glycosyl halides. The preparative electroreductions result mainly in dimerization in the case of glycosyl halides, in H {yields} Br substitution in the case of bromophenols, in dimerization + substitution in the case of haloadamantanes. The product analysis, both at the end of the reaction and at intermediate times, allows discussing the reaction pathways in terms of intermediate stability and of active surface accessibility. The possibility of complete dehalogenation on a wider substrate variety with remarkably lower energy consumption and almost quantitative current yields makes the process potentially very interesting for environmental purposes. [Italian] L'argento, di cui sono state recentemente evidenziate straordinarie proprieta' elettrocatalitiche per la riduzione degli alogenuri organici, e' stato utilizzato come materiale catodico per sistematiche elettrolisi preparative in celle a membrana. Per mettere in risalto il ruolo del substrato organico sul notevole anticipo del potenziale di riduzione e sull'effetto gabbia, ovvero la promozione di reazioni intermolecolari su intermedi adsorbiti, vengono qui confrontate, in termini sia di caratterizzazione voltammetrica sia di prodotti di elettroriduzioni preparative, tre tipologie di alogenuri: alifatici (adamantani), aromatici (fenoli) e glicosidici. Le elettroriduzioni preparative danno luogo principalmente a dimerizzazione nel caso di glicosilalogenuri, a H {yields} Br sostituzione nel caso di bromofenoli, a dimerizzazione + sostituzione nel caso degli alogenoadamantani. L'analisi dei prodotti, sia al termine della reazione, sia a tempi intermedi e di accessibilita' della superficie attiva. La possibilita' di completa dealogenazione di una piu' vasta categoria di substrati con notevole riduzione del consumo energetico e rendimento di corrente pressoche' unitario conferisce al processo un potenziale notevole interesse ambientale.

Fiori, G.; Mussini, P.; Rondinini, S.; Vertova, A. [Milan Univ., Milan (Italy). Dept. of Physical Chemistry and Electrochemistry

2001-04-01

145

Low temperature exciton-exciton annihilation in amphi-PIPE J-aggregates  

Digital Repository Infrastructure Vision for European Research (DRIVER)

The mobility of optically excited excitons on J-aggregates can be demonstrated by the phenomena of exciton-exciton annihilation. In this intensity-dependent process the collision of two excitons results in their annihilation and hence in a shortening of the mean excitation lifetime. By measuring the intensity-dependent fluorescent lifetime in contrast to the predicted immobilization of the excitons at low temperature we could prove the excellent mobility of the excitons at a temperature (4K)...

Spitz, C.; Daehne, S.

2006-01-01

146

Excitons into one-axis crystals of zinc phosphide (Zn3P2  

Directory of Open Access Journals (Sweden)

Full Text Available Theoretical study of excitons spectra is offered in this report as for Zn3P2 crystals. Spectra are got in the zero approach of the theory of perturbations with consideration of both the anisotropy of the dispersion law and the selection rules. The existence of two exciton series was found, which corresponds to two valence bands (hh, lh and the conductivity band (c. It is noteworthy that anisotropy of the dispersion law plus the existence of crystalline packets (layers normal to the main optical axis, both will permit the consideration of two-dimensional excitons too. The high temperature displaying of these 2D-exciton effects is not eliminated even into bulk crystals. The calculated values of the binding energies as well as the oscillator's strength for the optical transitions are given for a volume (3D and for two-dimensional (2D excitons. The model of energy exciton transitions and four-level scheme of stimulated exciton radiation for receiving laser effect are offered.

D.M. Stepanchikov

2009-01-01

147

Creation of free excitons in solid krypton investigated by time-resolved luminescence spectroscopy  

International Nuclear Information System (INIS)

The creation and relaxation of secondary excitons in solid Kr was investigated using energy-and time-resolved luminescence spectroscopy in the vacuum ultraviolet region. The spectrally selected emission of the free exciton (FE) was used as a probe for an investigation of the different exciton creation processes. Delayed FE creation via electron-hole recombination and 'prompt' (in terms of the time-resolution of the experiment) creation of excitons were separated. The 'prompt' creation of a FE appears in the region above threshold energy Eth, which is equal to the sum of the band gap energy and the free exciton energy. 'Prompt' creation of excitons above Eth is ascribed to a superposition of two processes: (i) creation of the electronic polaron complex (one-step process) and (ii) inelastic scattering of photoelectrons described in the framework of the multiple-parabolic-branch band model (two-step process). In addition, the ratio spectrum of the time-integrated FE and self-trapped exciton (STE) emission was analysed. The behaviour of the ratio spectrum is a proof that electron-hole recombination leads to STE states through FE states as precursors

2003-04-02

148

Creation of free excitons in solid krypton investigated by time-resolved luminescence spectroscopy  

Energy Technology Data Exchange (ETDEWEB)

The creation and relaxation of secondary excitons in solid Kr was investigated using energy-and time-resolved luminescence spectroscopy in the vacuum ultraviolet region. The spectrally selected emission of the free exciton (FE) was used as a probe for an investigation of the different exciton creation processes. Delayed FE creation via electron-hole recombination and 'prompt' (in terms of the time-resolution of the experiment) creation of excitons were separated. The 'prompt' creation of a FE appears in the region above threshold energy E{sub th}, which is equal to the sum of the band gap energy and the free exciton energy. 'Prompt' creation of excitons above E{sub th} is ascribed to a superposition of two processes: (i) creation of the electronic polaron complex (one-step process) and (ii) inelastic scattering of photoelectrons described in the framework of the multiple-parabolic-branch band model (two-step process). In addition, the ratio spectrum of the time-integrated FE and self-trapped exciton (STE) emission was analysed. The behaviour of the ratio spectrum is a proof that electron-hole recombination leads to STE states through FE states as precursors.

Kisand, Vambola [Institute of Physics, University of Tartu, Riia 142, 51014 Tartu (Estonia); Kirm, Marco [Institut fuer Experimentalphysik der Universitaet Hamburg, Luruper Chaussee 149, 22761 Hamburg (Germany); Negodin, Evgeni [Institut fuer Experimentalphysik der Universitaet Hamburg, Luruper Chaussee 149, 22761 Hamburg (Germany); Sombrowski, Elke [DESY, Notkestrasse 85, 22603 Hamburg (Germany); Steeg, Barbara [DESY, Notkestrasse 85, 22603 Hamburg (Germany); Vielhauer, Sebastian [Institut fuer Experimentalphysik der Universitaet Hamburg, Luruper Chaussee 149, 22761 Hamburg (Germany); Zimmerer, Georg [Institut fuer Experimentalphysik der Universitaet Hamburg, Luruper Chaussee 149, 22761 Hamburg (Germany)

2003-04-02

149

Hot trion and excited exciton states of single InGaAs/GaAs quantum dots  

International Nuclear Information System (INIS)

For a detailed understanding of the electronic structure of self-organized quantum dots (QDs) high-resolution measurements that reveal the spectrum of excited states are essential. Here, polarized photoluminescence excitation (PLE) spectra of the positive trion and the exciton of a number of MBE-grown single InGaAs/GaAs QDs were recorded and compared to results of 8-band k.p calculations. The exciton and trion absorption spectra comprise two characteristic parts. ?E=60 meV above the ground state energy exists a quasi continuous band of absorption features. The corresponding transitions involve hybrid states of QDs and wetting layer. In our study we concentrate on the regime below ?E=60 meV where the PLE spectrum of the exciton consists of well separated sharp resonances, corresponding to absorptions into excited exciton states, and a broad resonance around 35 meV due to exciton-phonon coupling. Upon adding a positive charge carrier, the PLE spectrum has a more complex structure due to the singlet-triplet splitting of the hot trion states. The sharp resonances, of both trion and exciton show clear dependence on linearly or circularly polarized excitation and detection. Moreover the spectra of different dots, detected on the ground state energy of the same excitonic complex, reveal substantial similarities

2008-02-25

150

Optical absorption in degenerately doped semiconductors: Mott transition or Mahan excitons?  

Science.gov (United States)

Electron doping turns semiconductors conductive even when they have wide fundamental band gaps. The degenerate electron gas in the lowest conduction-band states, e.g., of a transparent conducting oxide, drastically modifies the Coulomb interaction between the electrons and, hence, the optical properties close to the absorption edge. We describe these effects by developing an ab initio technique which captures also the Pauli blocking and the Fermi-edge singularity at the optical-absorption onset, that occur in addition to quasiparticle and excitonic effects. We answer the question whether free carriers induce an excitonic Mott transition or trigger the evolution of Wannier-Mott excitons into Mahan excitons. The prototypical n-type zinc oxide is studied as an example. PMID:22182110

Schleife, André; Rödl, Claudia; Fuchs, Frank; Hannewald, Karsten; Bechstedt, Friedhelm

2011-12-01

151

Optical nonlinearity and bistability in the bound exciton energy range of CdS  

International Nuclear Information System (INIS)

Under high excitation conditions thick CdS samples show pronounced broad-band nonlinear transmission in the bound exciton region and up to a wavelength of about 515 nm at cryo-temperatures. This behavior is only explainable in a model based on impurity neutralization and bound exciton creation. The suitability of these nonlinearities to yield optical bistability will be shown. Bistable operation is investigated in dependence of crystal thickness, impurity concentration, excitation density, wavelength, and temperature. A strong correlation to acceptor-bound exciton generation is obtained, and the explanation of this bistable operation fits well with that of the above mentioned transmission behavior. (author)

1988-12-01

152

Slow light enhanced singlet exciton fission solar cells with a 126% yield of electrons per photon  

Science.gov (United States)

Singlet exciton fission generates two triplet excitons per absorbed photon. It promises to increase the power extracted from sunlight without increasing the number of photovoltaic junctions in a solar cell. We demonstrate solar cells with an external quantum efficiency of 126% by enhancing absorption in thin films of the singlet exciton fission material pentacene. The device structure exploits the long photon dwell time at the band edge of a distributed Bragg reflector to achieve enhancement over a broad range of angles. Measuring the reflected light from the solar cell establishes a lower bound of 137% for the internal quantum efficiency.

Thompson, Nicholas J.; Congreve, Daniel N.; Goldberg, David; Menon, Vinod M.; Baldo, Marc A.

2013-12-01

153

THz-manipulation of excitonic polarization in (GaIn)As/GaAs quantum wells  

International Nuclear Information System (INIS)

We investigate the interaction of strong single-cycle THz-pulses with a coherent excitonic polarization. The initial bleaching is superimposed by a splitting of the 1s exciton resonance and eventually is accompanied by the emergence of side-bands on both the high and low energy sides of the former resonance. A quantum-mechanical many-body analysis associates these observations with the transition from excitationinduced bleaching and the regime of excitonic Rabi flopping to multi-THz-photon ionization and hence the population of states with high orbital quantum numbers. (copyright 2013 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)

2013-09-01

154

THz-manipulation of excitonic polarization in (GaIn)As/GaAs quantum wells  

Energy Technology Data Exchange (ETDEWEB)

We investigate the interaction of strong single-cycle THz-pulses with a coherent excitonic polarization. The initial bleaching is superimposed by a splitting of the 1s exciton resonance and eventually is accompanied by the emergence of side-bands on both the high and low energy sides of the former resonance. A quantum-mechanical many-body analysis associates these observations with the transition from excitationinduced bleaching and the regime of excitonic Rabi flopping to multi-THz-photon ionization and hence the population of states with high orbital quantum numbers. (copyright 2013 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)

Ewers, B.; Koester, N.S.; Woscholski, R.; Koch, M.; Chatterjee, S.; Klettke, A.C.; Kira, M.; Koch, S.W. [Faculty of Physics and Materials Science Center, Philipps-Universitaet Marburg (Germany); Khitrova, G.; Gibbs, H.M. [College of Optical Sciences, The University of Arizona, Tucson, AZ (United States)

2013-09-15

155

Transient hot-phonon-to-exciton spectroscopy in organic molecular semiconductors  

Science.gov (United States)

The interaction dynamics between excitons and intramolecular vibrations is investigated in ?-sexithiophene crystals using transient pump-probe spectroscopy. The ultrafast fission of vibrons, i.e., exciton states bound to intramolecular vibrations, predicted in the 1960s by Davydov and Rasbha, is observed. The fission process is monitored by stimulating the optical transitions from the outgoing scattered states, the hot ground state C-C stretching vibrations, to the lowest exciton bands. The vibrational energy is dissipated in 1.5±0.3 ps.

Cordella, F.; Orru, R.; Loi, M. A.; Mura, A.; Bongiovanni, G.

2003-09-01

156

Slow light enhanced singlet exciton fission solar cells with a 126% yield of electrons per photon  

International Nuclear Information System (INIS)

Singlet exciton fission generates two triplet excitons per absorbed photon. It promises to increase the power extracted from sunlight without increasing the number of photovoltaic junctions in a solar cell. We demonstrate solar cells with an external quantum efficiency of 126% by enhancing absorption in thin films of the singlet exciton fission material pentacene. The device structure exploits the long photon dwell time at the band edge of a distributed Bragg reflector to achieve enhancement over a broad range of angles. Measuring the reflected light from the solar cell establishes a lower bound of 137% for the internal quantum efficiency

2013-12-23

157

Spin relaxation of excitons in strained InGaAs/GaAs quantum wells  

Digital Repository Infrastructure Vision for European Research (DRIVER)

We have investigated the exciton spin dynamics in strained InxGa1-x As/GaAs (x ? 0.2) quantum wells. The variation of the initial polarization with the excitation energy is well correlated with the increasing mixing of the valence band states. The time behavior of the depolarization curves clearly demonstrates the prime importance of the hole and exciton spin relaxation processes.

1993-01-01

158

Exciton Self-Trapping in Two-Dimensional System of (C 2H 5NH 3) 2CdCl 4 Single Crystal  

Science.gov (United States)

The exciton-phonon interaction in a two-dimensional (C2H5NH3)2CdCl4 single crystal was studied by investigating the low energy tail of the first exciton absorption band in the temperature range of 100 440 K. The tail spectrum for different temperatures obeys the Urbach rule with small steepness parameters. From the analysis of those tails, it is determined that the steepness coefficient ?0 is 0.60 and the exciton-phonon coupling constant g is 2.1. These indicate that the exciton-phonon interaction in (C2H5NH3)2CdCl4 is strong and the free exciton relaxes to the self-trapped exciton state. The electronic nature of the excitonic transition is discussed on the basis of the results in the present work.

Ohnishi, Akimasa; Tanaka, Ken-ichi; Yoshinari, Takehisa

1999-01-01

159

Excitonic splittings in jet-cooled molecular dimers.  

Science.gov (United States)

In more than 60 years of research on molecular excitons, there has been extensive theoretical work but few experimental investigations have rigorously tested the predictions of exciton coupling theories. In centrosymmetric doubly H-bonded molecular dimers with identical chromophores, the S0-->S1 electronic transition dipole moments of the monomers combine in a parallel and antiparallel fashion, giving the S0-S1 and S0-->S2 transitions of the dimer. One of these is strictly symmetry-forbidden and the other fully allowed. Minimal perturbations such as 12C/13C or H/D isotopic substitution lift the symmetry restrictions sufficiently to render both transitions allowed. The excitonic (Davydov) splitting can then be measured as the energy difference between the respective vibrationless O0(0) bands. We have measured the mass-specific vibronic spectra of the centrosymmetric H-bonded dimers (2-pyridone)2 and (2-aminopyridine)2 that are supersonically cooled to a few K and isolated in molecular beams, using two-color resonant two-photon ionization spectroscopy. Comparison of the all-12C- and 13C- isotopomer spectra yield excitonic splittings of delta(exp) = 43.5 and 10.5 cm(-1), respectively. The corresponding splittings calculated by high-level ab initio methods (RI-CC2/aug-cc-pVTZ) are 20 to 50 times larger. These purely electronic ab initio exciton splittings need to be reduced ('quenched') by vibronic coupling to the optically active vibrational modes. Only after quenching are the experimentally observed exciton splittings correctly reproduced. PMID:21678767

Ottiger, Philipp; Leutwyler, Samuel

2011-01-01

160

Ordered "excitonic" phase with flat bands and a gap increasing with decreasing doping as consequence of electronic topological transition in 2D system on a square lattice Applications to high-$T_{c}$ cuprates  

CERN Document Server

We study the ordered "excitonic" phase which develops around electronic topological transition (ETT) in a 2D electron system on a square lattice. We consider the case of hoping between more than nearest neighbours for which the quantum critical point (QCP) associated with ETT (at \\delta=\\delta_c, n=1-\\delta is electron concentration) is quite unusual. We show that the amplitude of the order parameter and of the gap in the electron spectrum increase with increasing the distance from the QCP, \\delta_c - \\delta, unlike the ordinary case when they decrease when going away from the point which is a motor for instability. The electron spectrum is characterized by a gap on Fermi level which opens at "hot spot" and extends until the saddle point (SP) whatever is a doping concentration. On the other hand it has a characteristic flat shape in a proximity of SP. The existence of the gap around SP, its increasing with decreasing doping, the flat shape of the spectrum and the angle dependence of the gap have a striking si...

Kiselev, M N; Pfeuty, P

2000-01-01

 
 
 
 
161

Linear and nonlinear optical properties of free excitons in CdS  

International Nuclear Information System (INIS)

The authors report here an accurate measurement done at cryogenic temperature of both the linear and nonlinear transmission characteristics of a very high-quality optical platelet of CdS. From an expression for the contribution of the various excitons to the dielectric constant of the material, the index of refraction and the absorption coefficient can be extracted. From a fit to the experimental transmission data, the free exciton dipole dephasing time, sample thickness, background absorption coefficient, transverse exciton frequency, and impurity concentration can all be extracted. Knowledge of these parameters is very important to understand pulse propagation near the band gap and to optimize the performance of bistable devices. The authors obtain a very good fit to the experimental data. The transverse A exciton frequency is measured to be (20589 +. 1)cm/sup -1/. A study of the temperature dependence of the broadening of the A free exciton reveals that the A free exciton dephasing time below the resonance is temperature independent from 2 to 50 K and is of the order of 50 ps. This time is directly related to polariton-impurity and polariton-defect scattering. Acoustic or LO phonons do not contribute to the polariton dephasing time at these temperatures. This is a direct manifestation of the exciton-polariton bottleneck. Above the A exciton resonance, the authors observe a temperature dependence for the polariton dephasing time which they attribute to phonon absorption and emission. The measurements indicate that even 50-60 cm/sup -1/ below the free exciton resonance, the major contribution to the absorption coefficient comes from free exciton tail absorption

1986-01-01

162

SINGLE-SITE APPROXIMATION VIA IDEAL CRYSTAL GREEN'S FUNCTION : XANES OF ALKALI HALIDES  

Digital Repository Infrastructure Vision for European Research (DRIVER)

The ideal crystal Green's function method is applied to XANES problem. This scheme allows to treat simultaneously two important aspects of the problem : band effects (multiple scattering) and core hole potential. The calculated K-spectra of alkali halides agree well with the experimental ones.

Gegusin, I.; Datsyuk, V.

1986-01-01

163

Excitons and Resonant Inelastic X-Ray Scattering in Graphite  

International Nuclear Information System (INIS)

X-ray resonant inelastic scattering in graphite is analyzed including the effect of a screened core-hole Coulomb potential in the intermediate state. Numerical calculations of the scattered intensity are reported. Evidence for the presence of excitonic states in the observed K-edge spectra is provided, thus identifying a major drawback in the use of resonant scattering as a probe of electronic band structures. copyright 1997 The American Physical Society

1997-04-01

164

Synthesis of aryl halides via organoborane chemistry  

Energy Technology Data Exchange (ETDEWEB)

A method for the rapid synthesis of a variety of substituted aryl halides by the reaction of organoboranes with halide ions in the presence of chloramine-T is described in detail. The products were purified by column chromatography on silica gel using a mixture of petroleum ether-ethyl acetate as eluent.

Kabalka, G.W.; Sastry, K.A.R.; Sastry, U.; Somayaji, V.

1982-01-01

165

New layered manganese oxide halides.  

Science.gov (United States)

The first layered manganese(III) oxide chlorides, Sr2MnO3Cl and Sr4Mn3O8-yCl2, have been synthesised; Sr2MnO3Cl adopts a K2NiF4 type structure with sheets of MnO5 square based pyramids linked through oxygen and separated by SrCl layers; it is the end member of a new family of Ruddlesden-Popper type manganese oxide halides which includes the three-layer member Sr4Mn3O8-yCl2 also reported herein. PMID:12120392

Knee, Christopher S; Weller, Mark T

2002-02-01

166

On the exciton model for ion-beam damage: The example of TiO2  

International Nuclear Information System (INIS)

The non-radiative exciton decay model recently developed to account for swift-ion-beam damage to LiNbO3 is, here, discussed within a general physical perspective, taking previous work on alkali halides as a reference. Some general rules for the validity of excitonic models have been put forward, allowing one to predict the irradiation behaviour of other materials. As a new example of application, some preliminary data on the generation and growth of uniform amorphous layers induced by irradiation with Br at 13 MeV and 25 MeV have been performed on rutile (TiO2). In addition sub-threshold irradiations with Br ions at 9 MeV have been carried out. Defects generation is observed as a result. This effect is explained with the exciton model. Experiments are in the electronic excitation regime and use moderate fluences in the range of 6 x 1012 to 1.5 x 1014 cm-2. The results show similar features to those found for LiNbO3 and are, in principle, consistent with a non-radiative exciton decay model.

2010-10-01

167

Optical Absorption in Band-Edge Region of ( CH3NH3)3Bi2I9 Single Crystals  

Science.gov (United States)

Optical absorption spectra were measured around the band edge in ( CH3NH3)3Bi2I9 single crystals. In the very thin crystals, an intrinsic exciton band appears clearly at 2.51 eV at low temperatures, and the absorption peak remains even at room temperature reflecting a large binding energy of more than 300 meV. The exciton band does not show a discontinuous shift near the temperature where the methylammonium ions induce a structural phase transition. This fact indicates that the exciton states are independent of the methylammonium ions. The origin of the exciton band is examined on the basis of the annealing effect on the absorption bands in vacuum-evaporated films. The lower energy tail of the exciton band obeys the Urbach rule between 78 and 350 K. The detailed analysis by the Urbach rule reveals that the exciton-phonon interaction in the present system is relatively strong.

Kawai, Taketoshi; Ishii, Atsuko; Kitamura, Takako; Shimanuki, Shigetaka; Iwata, Makoto; Ishibashi, Yoshihiro

1996-05-01

168

Optical generation of excitonic valley coherence in monolayer WSe2  

Science.gov (United States)

As a consequence of degeneracies arising from crystal symmetries, it is possible for electron states at band-edges (`valleys') to have additional spin-like quantum numbers. An important question is whether coherent manipulation can be performed on such valley pseudospins, analogous to that implemented using true spin, in the quest for quantum technologies. Here, we show that valley coherence can be generated and detected. Because excitons in a single valley emit circularly polarized photons, linear polarization can only be generated through recombination of an exciton in a coherent superposition of the two valley states. Using monolayer semiconductor WSe2 devices, we first establish the circularly polarized optical selection rules for addressing individual valley excitons and trions. We then demonstrate coherence between valley excitons through the observation of linearly polarized luminescence, whose orientation coincides with that of the linearly polarized excitation, for any given polarization angle. In contrast, the corresponding photoluminescence from trions is not observed to be linearly polarized, consistent with the expectation that the emitted photon polarization is entangled with valley pseudospin. The ability to address coherence, in addition to valley polarization, is a step forward towards achieving quantum manipulation of the valley index necessary for coherent valleytronics.

Jones, Aaron M.; Yu, Hongyi; Ghimire, Nirmal J.; Wu, Sanfeng; Aivazian, Grant; Ross, Jason S.; Zhao, Bo; Yan, Jiaqiang; Mandrus, David G.; Xiao, Di; Yao, Wang; Xu, Xiaodong

2013-09-01

169

Curved nanowire structures and exciton binding energies.  

Science.gov (United States)

Growth of quantum-confined semiconductor structures is a complicated process that may lead to imperfect and complex shapes as well as geometrical nonuniformities when comparing a large number of intended identical structures. On the other hand, the possibility of tuning the shape and size of nanostructures allows for extra optimization degrees when considering electronic and optical properties in various applications. This calls for a better understanding of size and shape effects. In the present work, we express the one-band Schrödinger equation in curved coordinates convenient for determining eigenstates of curved quantum-wire and quantum-dash structures with large aspect ratios. Firstly, we use this formulation to solve the problem of single-electron and single-hole states in curved nanowires. Secondly, exciton states for the curved quantum-wire Hamiltonian problem are found by expanding exciton eigenstates on a product of single-particle eigenstates. A simple result is found for the Coulomb matrix elements of an arbitrarily curved structure as long as the radius-of-curvature is much larger than the cross-sectional dimensions. We use this general result to compute the groundstate exciton binding energy of a bent nanowire as a function of the bending radius-of-curvature. It is demonstrated that the groundstate exciton binding energy increases by 40 meV as the radius-of-curvature changes from 20 to 2 nm while keeping the total length (and volume) of the nanowire constant. PMID:21825527

Willatzen, M; Lassen, B

2009-05-20

170

The electronic structure of the F-center in alkali-halides-The Bethe cluster - lattice  

International Nuclear Information System (INIS)

The electronic structure of the F-center in alkali-halides with the NaCl structure has been studied using the Bethe Cluster lattice method. The central cluster has been taken as constituted by the vacancy and the nearest- and second-neighbors to it, respectively cations and anions. The optical transitions have been calculated and compared to experimental data on the location of the peak of the F-absorption band. The agreement obtained indicates that this method may be used to study properties of this defect in alkali halides. (Author)

1977-01-01

171

Electronic properties of metal-induced gap states formed at alkali-halide/metal interfaces  

CERN Document Server

The spatial distribution and site- distribution of metal induced gap states (MIGS) are studied by thickness dependent near edge x-ray absorption fine structure (NEXAFS) and comparing the cation and anion edge NEXAFS. The thickness dependent NEXAFS shows that the decay length of MIGS depends on rather an alkali halide than a metal, and it is larger for alkali halides with smaller band gap energy. By comparing the Cl edge and K edge NEXAFS for KCl/Cu(001), MIGS are found to be states localizing at anion sites.

Kiguchi, M; Ikeda, S; Saiki, K; Kiguchi, Manabu; Yoshikawa, Genki; Ikeda, Susumu; Saiki, Koichiro

2005-01-01

172

A Cluster-Bethe lattice treatment for the F-center in alkali-halides  

International Nuclear Information System (INIS)

The electronic structure of the F-center in alkali-halides with the NaCl structure has been studied using the Cluster-Bethe lattice method. The central cluster has been taken as constituted by the vacancy and the nearest- and second- neighbors to it, respectively, cations and anions. The optical transitions have been calculated and compared to experimental data on the location of the peak of the F-absorption band. The agreement obtained indicates that this method may be used to study properties of this defect in alkali halides

1977-12-01

173

Simple exciton in intermediate magnetic field  

International Nuclear Information System (INIS)

The energy levels, both discrete and resonant of a simple, anisotropic exciton in external magnetic field are calculated. Obtained results are used to describe excitonic spectra of CdMnTe mixed crystals reported earlier, for which giant spin splittings offer an exceptional possibility to observe simple excitons. (author)

1982-03-01

174

Luminescence evidence for bulk and surface excitons in free xenon clusters  

CERN Document Server

Cathodoluminescence spectra of free xenon clusters produced by condensation of xenon-argon gas mixtures in supersonic jets expanding into vacuum were studied. By varying initial experimental parameters, including xenon concentration, we could obtain clusters with a xenon core (300-3500 atoms) covered by an argon outer shell as well as shell-free xenon clusters (about 1500 atoms). The cluster size and temperature (about 40 K for both cases) were measured electronographically. Luminescence bands evidencing the existence of bulk and surface excitons were detected for shell-free xenon clusters. The emission from bulk excitons in small clusters is supposed to be due to processes of their multiple elastic reflections from the xenon-vacuum interface. A presence of an argon shell causes extinction of the excitonic bands. In addition, some new bands were found which have no analogs for bulk xenon cryosamples.

Danylchenko, O G; Kovalenko, S I; Libin, M Yu; Samovarov, V N; Vakula, V L

2007-01-01

175

Luminescence evidence for bulk and surface excitons in free xenon clusters  

International Nuclear Information System (INIS)

The cathodoluminescence spectra of free xenon clusters produced by condensation of xenon-argon gas mixtures in supersonic jets expanding into vacuum were studied. By varying the initial experimental parameters, including the xenon concentration, we could obtain clusters with a xenon core (300-3500 atoms) covered by an argon outer shell as well as shell-free xenon clusters (?1500 atoms). The cluster size and temperature (?40 K for both cases) were measured electronographically. Luminescence bands evidencing the existence of bulk and surface excitons were detected for shell-free xenon clusters. The emission from bulk excitons in small clusters is supposed to be due to processes of their multiple elastic reflections from the xenon-vacuum interface. The presence of an argon shell causes extinction of the excitonic bands. In addition, some bands were found which have no analogs for bulk xenon cryosamples

2007-10-01

176

Electrically excited mercury halide laser  

Energy Technology Data Exchange (ETDEWEB)

An improved electric-discharge-excited mercury halide dissociation laser operable on the (B-x) transitions in HgCl, HgBr and HgI at 558, 502 and 443 nm respectively is described. The laser discharge cell is elongated and made from temperature-resistant silicon-glass laminate or any other suitable material and includes separate ceramic crucibles for containing mercuric dihalide crystals. A pair of electrodes, each having external terminals, extend along the linear axis of the cell in parallel relationship with the linear axis and each other. Ultraviolet discharge means is also provided for preionizing a buffer gas of helium to which nitrogen has been added. The improvement comprises the addition of about 10% nitrogen to the buffer gas of helium which is admitted to the laser cell prior to excitation. The addition of nitrogen may act to selectively remove the terminal levels of the mercury halide laser transitions, thereby permitting more efficient extraction of the optical energy from the laser media. Thus improvements in both efficiency and output energy are obtained. The laser cavity is completed by a fully and a partially reflective surface for the wavelength of operation. The gases are preionized and excited by pulsed electrical energy from separate sources.

Burnham, R. L.

1980-10-28

177

Growth of silver halides from the molecule to the crystal. Pulse radiolysis study  

Energy Technology Data Exchange (ETDEWEB)

Halide ions were produced in situ by pulse radiolysis, via electron transfer to dihalomethanes from solvated electrons or hydrogen atoms, and were then used to generate silver halide molecules and larger aggregates. This evolution of silver halide aggregates was studied for the case of silver iodide on the time scale of 10/sup /minus/6/-10/sup 2/ s. Conductivity detection allows determination of the rate of formation of the first AgI molecule. Spectrophotometric detection of the growth of the particles from the stage of the molecular species to colloidal particles of bulk electronic properties is then possible. When the aggregates thus formed approach sizes of ca. 50/degrees/ /angstrom/, their detection by light scattering provides an independent method of size determination. In the region where such measurements are possible, the sizes determined by light scattering agree with sizes calculated assuming confinement excitons in small particles and their electron-hole coulomb screening. A similar approach is suggested for growth studies of other insoluble materials. 13 references, 3 figures.

Schmidt, K.H.; Patel, R.; Meisel, D.

1988-07-20

178

Long excitonic spin relaxation in InAs/AlAs quantum dots  

Energy Technology Data Exchange (ETDEWEB)

We investigated the excitonic spin relaxation in self-assembled InAs quantum dots, embedded in an AlAs matrix, by means of time-resolved, circularly polarized photoluminescence. The atomic-like electronic structure of self-assembled quantum dots suppresses not only elastic but also inelastic processes of spin relaxation mechanisms, thus increasing the exciton spin relaxation time {tau}{sub s}. The experimental determination of {tau}{sub s} is typically inhibited by the short exciton lifetime of a few nanoseconds. However, the novel InAs/AlAs quantum dot system, characterized by the lowest electron state at the direct {gamma} or indirect X conduction band minima, exhibits exciton lifetimes of several milliseconds. At a temperature of 1.8 K the exciton spin relaxation time {tau}{sub s} is in the range of 100{mu}s. It strongly depends on the optical excitation density, temperature and quantum dot size. Differences between the direct and indirect bandgap transitions have been observed in the temporal evolution of the non-equilibrium exciton spin-orientation induced by an external magnetic field or excitation with circularly polarized light. Spectral and temperature dependencies of photoluminescence kinetics are explained in terms of an electron redistribution between long-lived indirect and short-lived direct states of the quantum dot conduction band.

Brinkmann, Birgit; Debus, J.; Dunker, D.; Yakovlev, D.R.; Bayer, M. [Experimentelle Physik II, TU Dortmund (Germany); Shamirzaev, T.S. [Institute of Semiconductor Physics, Novosibirsk (Russian Federation)

2010-07-01

179

Exciton-polariton gap solitons in two-dimensional lattices.  

Science.gov (United States)

We report on the two-dimensional gap-soliton nature of exciton-polariton macroscopic coherent phases (PMCP) in a square lattice with a tunable amplitude. The resonantly excited PMCP forms close to the negative mass M point of the lattice band structure with energy within the lattice band gap and its wave function localized within a few lattice periods. The PMCPs are well described as gap solitons resulting from the interplay between repulsive polariton-polariton interactions and effective attractive forces due to the negative mass. The solitonic nature accounts for the reduction of the PMCP coherence length and optical excitation threshold with increasing lattice amplitude. PMID:24138259

Cerda-Méndez, E A; Sarkar, D; Krizhanovskii, D N; Gavrilov, S S; Biermann, K; Skolnick, M S; Santos, P V

2013-10-01

180

CaPd3O4 as an excitonic insulator  

Science.gov (United States)

The electronic energy band structure for cubic palladium oxides CaPd3O4 in the NaPt3O4 structure has been calculated by the full-potential linearized augmented-plane-wave method within the local-density approximation. CaPd3O4 is almost semiconducting, but has a very small number of carriers. We propose that this compound is a good candidate for being an excitonic insulator, similar to CaB6. Moreover, the bands near the Fermi level have one-dimensional character.

Hase, Izumi; Nishihara, Yoshikazu

2000-11-01

 
 
 
 
181

Excitonic emission of CuInS2 crystals using confocal microscopy system  

International Nuclear Information System (INIS)

Photoluminescence (PL) spectra in the band-edge region on bulk single-crystals of CuInS2 grown by the traveling heater method have been investigated using a confocal microscopy system. The observed PL spectra are separated into two Lorentzian peaks which are assigned to be A and B free excitons, by the analysis of the excitation intensity dependence of the emissions. Consequently, we present the behaviour of B free exciton within a wide range of temperatures. The time-resolved emissions of A free exciton have also been examined. The decay of the emissions is analyzed using a double exponential curve. Fast and slow components are attributed to nonradiative relaxation and radiative recombination, respectively. The decay-time constant of the slow component corresponds to the radiative lifetime of A free exciton and is obtained over the wide temperature region until 300 K. (copyright 2013 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)

2013-08-01

182

Size dependence of exciton-phonon coupling in sol-gel ZnMgO powders  

Science.gov (United States)

We found that the exciton-phonon coupling in the Zn1-xMgxO powders (0.01 <= x <= 0.07) is greatly influenced by the crystalline-size. Two well-resolved photoluminescence (PL) bands due to recombination of free exciton and its longitudinal optical (LO)-phonon replicas enable us to analyze the relative intensities among free excitons, one-LO-phonon replicas, and two-LO-phonon replicas. As crystalline size increases, a larger enhancement of the PL-intensity ratio of a free exciton to its LO-phonon replicas was found compared to that of two LO-phonon replica to one-LO-phonon replica.

Chia, C. H.; Chen, J. N.; Han, T. C.; Chiou, J. W.; Lin, Y. C.; Hsu, W. L.; Chou, W. C.

2011-03-01

183

Pressure-Induced Transformation of Excitons in AgBr1-xClx  

Science.gov (United States)

Pressure dependence of photoluminescence and absorption spectra in AgBr1-xClx (x{=}1.0, 0.6 and 0.15) is investigated, using a diamond anvil cell with N2 as pressure medium in order to generate good quality of hydrostatic pressure at 1.5 K. Pressure coefficients of self-trapped exciton states in AgCl, AgBr0.4Cl0.6 and that of free exciton state in AgBr0.85Cl0.15 are determined. After initial self-trapped exciton disappears at high pressures, new emission bands are found to appear in AgCl and AgBr0.4Cl0.6. These phenomena are ascribed to pressure-induced phase transition of excitons without accompanying a change of crystal structures.

Yokoyama, Teruo; Kobayashi, Michihiro

1985-06-01

184

Low-temperature magnetic circular dichroism spectra of thin layers of copper and silver halides  

Energy Technology Data Exchange (ETDEWEB)

The electronic and magnetic circular dichroism spectra of thin layers of Cu/sup 1/ and Ag/sup 1/ halides have beem measured at low ( about10 K) temperatures. The med spectra show clear A-terms which, in the case of the silver halides and for one band of cuprous iodide, have unexpected negative signs. A qualitative theoretical interpretation of these signs has been attempted by considering halogen-p and metals/d atomic orbital mixing within the framework of a simple, tight-binding model. Reasonable agreement of theory with experiment has been achieved for the positive copper halide A-terms, but it has not proved possible to obtain by calculation the experimentally observed negative A-term sign of the silver salts and cuprous iodide.

Grinter, R.; Harding, M.J.; Morris, G.R.

1986-02-01

185

Low-temperature magnetic circular dichroism spectra of thin layers of copper and silver halides  

International Nuclear Information System (INIS)

The electronic and magnetic circular dichroism spectra of thin layers of Cu"1 and Ag"1 halides have beem measured at low ( about10 K) temperatures. The med spectra show clear A-terms which, in the case of the silver halides and for one band of cuprous iodide, have unexpected negative signs. A qualitative theoretical interpretation of these signs has been attempted by considering halogen-p and metals/d atomic orbital mixing within the framework of a simple, tight-binding model. Reasonable agreement of theory with experiment has been achieved for the positive copper halide A-terms, but it has not proved possible to obtain by calculation the experimentally observed negative A-term sign of the silver salts and cuprous iodide

1986-01-01

186

Expanded fluid mercury, a ferroelectric excitonic insulator  

International Nuclear Information System (INIS)

We relate the metal-nonmetal transition in expanded liquid mercury to the dieletric transition in dense mercury vapor. We propose that the high-temperature, high-pressure fluid is an excitonic insulator phase. Condensation of Frenkel excitons is shown to occur at the dieletric transition. The phase diagram of excitonic insulator explains naturally both the metal-nonmetal and the dieletric transitions. The permanent dipole moments of the condensed excitons are estimated to order as a ferroelectric phase. The critical exponents of the liquid-gas transition are therefore classical within the ferroelectric excitonic insulator phase, crossing over 3d Ising at the ferroelectric-paraelectric transition. (The authors)

1984-03-02

187

Excitons in a mirror: Formation of "optical bilayers" using MoS2 monolayers on gold substrates  

Science.gov (United States)

We report coupling of excitons in monolayers of molybdenum disulphide to their mirror image in an underlying gold substrate. Excitons at the direct band gap are little affected by the substrate whereas strongly bound C-excitons associated with a van-Hove singularity change drastically. On quartz substrates only one C-exciton is visible (in the blue) but on gold substrates a strong red-shifted extra resonance in the green is seen. Exciton coupling to its image leads to formation of a "mirror biexciton" with enhanced binding energy. Estimates of this energy shift in an emitter-gold system match experiments well. The absorption spectrum of MoS2 on gold thus resembles a bilayer of MoS2 which has been created by optical coupling. Additional top-mirrors produce an "optical bulk."

Mertens, Jan; Shi, Yumeng; Molina-Sánchez, Alejandro; Wirtz, Ludger; Yang, Hui Ying; Baumberg, Jeremy J.

2014-05-01

188

Structure of polyvalent metal halide melts  

International Nuclear Information System (INIS)

A short review is given of recent progress in determining and understanding the structure of molten halide salts involving polyvalent metal ions. It covers the following three main topics: (i) melting mechanisms and types of liquid structure for pure polyvalent-metal chlorides; (ii) geometry and stability of local coordination for polyvalent metal ions in molten mixtures of their halides with alkali halides; and (iii) structure breaking and electron localization on addition of metal to the melt. (author). 28 refs, 3 figs, 1 tab

1990-01-01

189

Magnetic exciton dispersion in praseodymium  

DEFF Research Database (Denmark)

Measurements of the dispersion of magnetic excitons have been made in a single crystal of praseodymium metal using inelastic neutron scattering. A preliminary analysis of the data yields the first detailed information about the exchange interactions and the crystal field splittings in the light rare-earth metals.

Rainford, B. D.; Houmann, Jens Christian Gylden

1971-01-01

190

Exciton-photon interaction in a quantum dot embedded in a photonic microcavity  

CERN Document Server

We present a detailed analysis of exciton-photon interaction in a microcavity made out of a photonic crystal slab. Here we have analyzed a disk-like quantum dot where an exciton is formed. Excitonic eigen-functions in addition to their eigen-energies are found through direct matrix diagonalization, while wave functions corresponding to unbound electron and hole are chosen as the basis set for this procedure. In order to evaluate these wave functions precisely, we have used Luttinger Hamiltonian in the case of hole while ignoring bands adjacent to conduction band for electron states. After analyzing Excitonic states, a photonic crystal based microcavity with a relatively high quality factor mode has been proposed and its lattice constant has been adjusted to obtain the prescribed resonant frequency. We use finite-difference time-domain method in order to simulate our cavity with sufficient precision. Finally, we formulate the coupling constants for exciton-photon interaction both where intra-band and inter-ban...

Sodagar, Majid; Eftekharian, Amin; Khorasani, Sina

2008-01-01

191

Excitonic Effects on Optical Absorption Spectra of Doped Graphene  

CERN Document Server

We have performed first-principles calculations to study optical absorption spectra of doped graphene with many-electron effects included. Both self-energy corrections and electron-hole interactions are reduced due to the enhanced screening in doped graphene. However, self-energy corrections and excitonic effects nearly cancel each other, making the prominent optical absorption peak fixed around 4.5 eV under different doping conditions. On the other hand, an unexpected increase of the optical absorbance is observed within the infrared and visible-light frequency regime (1 ~ 3 eV). Our analysis shows that a combining effect from the band filling and electron-hole interactions results in such an enhanced excitonic effect on the optical absorption. These unique variations of the optical absorption of doped graphene are of importance to understand relevant experiments and design optoelectronic applications.

Yang, Li

2011-01-01

192

On excitons and other gap states in boron carbide  

International Nuclear Information System (INIS)

The excitons in boron carbide recently independently proved to be present by luminescence measurements (by Schmechel and co-workers) and by x-ray Raman scattering investigations in connection with ab initio calculations (by Feng and co-workers) are discussed, taking the actual structure of boron carbide within the homogeneity range (B4.3C-B?11C) into account. The excitonic levels at 1.560 and 1.5695 eV obtained from the luminescence spectrum are attributed to the central B atom in the C-B-C and the C-B-B chains respectively. Considering the transition energies related to the different gap states obtained by optical and electrical measurements for energies below the interband transition of 2.09 eV, an actualized energy band scheme of boron carbide is presented

2006-11-29

193

Low temperature exciton-exciton annihilation in amphi-PIPE J-aggregates  

Directory of Open Access Journals (Sweden)

Full Text Available The mobility of optically excited excitons on J-aggregates can be demonstrated by the phenomena of exciton-exciton annihilation. In this intensity-dependent process the collision of two excitons results in their annihilation and hence in a shortening of the mean excitation lifetime. By measuring the intensity-dependent fluorescent lifetime in contrast to the predicted immobilization of the excitons at low temperature we could prove the excellent mobility of the excitons at a temperature (4K, which is far below their expected freezing point.

S. Daehne

2006-11-01

194

Dark excitons due to direct Coulomb interactions in silicon quantum dots  

International Nuclear Information System (INIS)

Electron-hole exchange interactions can lead to spin-forbidden ''dark'' excitons in direct-gap quantum dots. Here, we explore an alternative mechanism for creating optically forbidden excitons. In a large spherical quantum dot made of a diamond-structure semiconductor, the symmetry of the valence band maximum (VBM) is t2. The symmetry of the conduction band minimum (CBM) in direct-gap material is a1, but for indirect-gap systems the symmetry could be (depending on size) a1, e, or t2. In the latter cases, the resulting manifold of excitonic states contains several symmetries derived from the symmetries of the VBM and CBM (e.g., t2xt2=A1+E+T1+T2 or t2xe=T1+T2). Only the T2 exciton is optically active or ''bright,'' while the others A1, E, and T1 are ''dark.'' The question is which is lower in energy, the dark or bright. Using pseudopotential calculations of the single-particle states of Si quantum dots and a direct evaluation of the screened electron-hole Coulomb interaction, we find that, when the CBM symmetry is t2, the direct electron-hole Coulomb interaction lowers the energy of the dark excitons relative to the bright T2 exciton. Thus, the lowest energy exciton is forbidden, even without an electron-hole exchange interaction. We find that our dark-bright excitonic splitting agrees well with experimental data of Calcott et al., Kovalev et al., and Brongersma et al. Our excitonic transition energies agree well with the recent experiment of Wolkin et al. In addition, and contradicting simplified models, we find that Coulomb correlations are more important for small dots than for intermediate sized ones. We describe the full excitonic spectrum of Si quantum dots by using a many-body expansion that includes both Coulomb and exchange electron hole terms. We present the predicted excitonic spectra. (c) 2000 The American Physical Society

2000-05-15

195

Luminescence of excitons and Eu2+vc- centres in KI:Eu2+ crystals  

Science.gov (United States)

Luminescence characteristics of KI:Eu2+ crystals are studied at 11.5-300 K under excitation in the 3.0-6.3 eV energy range. Besides the known 2.88 eV emission of Eu2+vc- centers, the new emission bands peaking at 3.75 eV, 3.3 eV, 3.06 eV, and 2.38 eV are found. The bands, located in the 3.1-4.3 eV energy range and strongly distorted due to their overlap with the Eu2+vc- absorption bands, are ascribed to the self-trapped exciton emission. The 3.06 eV and 2.88 eV bands are ascribed to the electronic transitions from two close levels of the splitted T2g excited state of an Eu2+ ion. The 2.38 eV emission is ascribed to the radiative decay of an exciton localized near an Eu2+ ion. It is assumed that the appearance of the self-trapped exciton emission under excitation in the Eu2+-related absorption bands is connected with the radiative decay of excitons, which are formed at the recombination of electrons and holes, produced in the crystal as a result of the photostimulated charge transfer from the Eu2+-perturbed I- states.

Aceves, R.; Piters, T. M.; Zazubovich, S.

2005-08-01

196

Electron scattering from alkali halide molecules  

International Nuclear Information System (INIS)

The low energy electron scattering from molecules belonging to alkali halide matrix is studied. The modified Born-Eikonal Series method is employed to calculate rotational excitation cross sections. The curves are plotted for the incident electron energy versus total cross section for each column of alkali halide matrix. The conclusion is drawn regarding dependence of the total cross section with the electron-negativity difference of the molecules. (author)

1983-04-01

197

Time-Resolved Photoluminescence of Excitons in HgI2  

Science.gov (United States)

The time-resolved photoluminescence (TRPL) of red HgI2 single crystal has been measured to determine the carrier lifetimes and to reveal the energy relaxation of excitons. Sharp near-bandgap luminescence lines due to free and bound excitons are observed at 530 nm, and a broad luminescence band appears at 630 nm at low temperatures. TRPL experiments of the near-bandgap luminescence have revealed that the luminescence comprise fast (30 to 200 ps) and slow (100 to 400 ps) decay components, showing several relaxation processes in free and bound exciton annihilation. TRPL of the broad band at 630 nm has shown that the luminescence is ascribed to the radiative recombination of donor-acceptor (DA) pairs.

Ohno, N.; Wen, X. M.

2001-08-01

198

Binding Energy and Lifetime of Excitons in InxGa1-xAs/GaAs Quantum Wells  

DEFF Research Database (Denmark)

We report a systematic study of exciton binding energies and lifetimes in InGaAs/GaAs quantum wells. The experimental binding energies have been deduced from photoluminescence excitation measurements taking into account the contribution of the 2s state of the exciton and the line broadening. The experimental results have been compared with accurate calculations in a four-band model, where exciton energies take into account the polaron correction. The theory accounts for all the experimental observations and provides a good quantitative agreement with the experimental values.

Borri, Paola

1997-01-01

199

A minimal model for excitons within time-dependent density-functional theory.  

Science.gov (United States)

The accurate description of the optical spectra of insulators and semiconductors remains an important challenge for time-dependent density-functional theory (TDDFT). Evidence has been given in the literature that TDDFT can produce bound as well as continuum excitons for specific systems, but there are still many unresolved basic questions concerning the role of dynamical exchange and correlation (xc). In particular, the roles of the long spatial range and the frequency dependence of the xc kernel f(xc) for excitonic binding are still not very well explored. We present a minimal model for excitons in TDDFT, consisting of two bands from a one-dimensional (1D) Kronig-Penney model and simple approximate xc kernels, providing an easily accessible model system for studying excitonic effects in TDDFT. For the 1D model system, it is found that adiabatic xc kernels can produce at most two bound excitons, confirming that the long spatial range of f(xc) is not a necessary condition. It is shown how the Wannier model, featuring an effective electron-hole interaction, emerges from TDDFT. The collective, many-body nature of excitons is explicitly demonstrated. PMID:22779671

Yang, Zeng-hui; Li, Yonghui; Ullrich, Carsten A

2012-07-01

200

Scattering of Hot Excitons in Quantum Wells  

CERN Multimedia

The scattering probabilities of hot excitons in narrow quantum wells (QWs)are obtained. The exciton-phonon matrix element is considered by using an envelope function Hamiltonian approach in the strong quantization limit where the QW width is smaller than the exciton bulk Bohr radius. The Fröhlich-like interaction is taken into account and the contribution of the confined and interface modes to the scattering probability are calculated as a function of quantum well width, electron and hole effective masses, and in-plane center-of-mass kinetic energy. Inter- and intra-subband excitonic transitions are discussed in term of the phonon scalar potential selectionrules. It is shown that even parity electrostatic potential states for confined and interface modes give the main contribution to the excitonic scattering rate. The consequences of exciton relaxation and scattering probability present for the multiphonon resonance Raman scattering in narrow QWs are briefly discussed.

Lua, M; Trallero-Giner, C; De Leon-Perez, F; Lua, Meng; Birman, Joseph L.; Trallero-Giner, Carlos; Leon-Perez, Fernando de

1999-01-01

 
 
 
 
201

ZnSe/ZnTe(shell/shell) radial quantum-wire heterostructures: the excitonic properties  

Energy Technology Data Exchange (ETDEWEB)

The ground-state characteristics of spatially indirect excitons trapped in radially heteronanostructured type-II band alignment ZnSe/ZnTe nanotubes as functions of the magnetic field for nanotubes with a radial size both smaller and larger than the effective Bohr radius are theoretically investigated. In the former case, dominated by the net kinetic energy of the electron and hole, the magnetic field modifies the exciton spectrum through the well-known Zeeman splitting, intra-orbital-state Aharonov-Bohm oscillations and inter-orbital-state crossovers occurring in very strong magnetic field strengths. However, in the latter case, dominated by the electron-hole Coulomb attraction, the magnetic field adjusts the exciton lines only by means of the Zeeman splitting and inter-orbital-state transitions happening in typical magnetic fields. As a result, the angular momentum transitions occurr at lower magnetic fields when the radial size of the nanotube is increased. Most importantly, another consequence is the substantially unusual exciton oscillator strength in such heteronanostructures. It is shown that when the exciton is optically active, due to the full cylindrical symmetry of the problem, the exciton oscillator strength shows undamped oscillations. This effect is associated with the periodic redistribution of the exciton density as the magnetic field is varied. Also, the magnitude of the magnetically induced excitonic persistent current is decreased with increasing radial size of the nanotube. This study may provide a platform to investigate new photonic quantum interference as well as polarization-sensitive photodetector and photovoltaic devices based on the Aharonov-Bohm effect. (fast track communication)

Bagheri, Mehran, E-mail: mh-bagheri@cc.sbu.ac.i [Condensed Matter Group, Laser and Plasma Research Institute, Shahid Beheshti University, G. C., Evin, Tehran 19835-63113 (Iran, Islamic Republic of)

2010-07-14

202

Exciton correlations within an electron gas  

Digital Repository Infrastructure Vision for European Research (DRIVER)

I report on the effect of a moderate excess electron population on nonlinearities in modulation-doped CdTe quantum wells. I show that the electron population does not qualitatively affect the nature of correlations between excitons. In this respect, I bring strong evidence of the existence of unbound and bound (stable) two-exciton states in the presence of electrons and charged excitons (trions). In time-resolved pump and probe experiment, they lead to the observation of electromagnetically i...

Berney, Jean

2007-01-01

203

EXCITON DYNAMICS IN ORGANIC MOLECULAR CRYSTALS  

Digital Repository Infrastructure Vision for European Research (DRIVER)

Dynamical behavior of Frenkel excitons in aromatic hydrocarbon crystals, pyrene, ?-perylene, ?-perylene, and tetracene are overviewed based on the published references and in terms of the self-trap depth. Then pressure-induced instability in exciton states (the change in the self-trap depth) in ?-perylene and anthracene is demonstrated and discussed. Finally a quasi-free exciton state is suggested to be the origin of the luminescence in anthracene at room temperature.

Matsui, A.; Mizuno, K.; Kobayashi, M.

1985-01-01

204

Generating Excitonic Supercurrent in Quantum Hall Bilayers  

Science.gov (United States)

Among the many examples of Bose condensation considered in physics, exciton condensation has maintained special interest because of controversy about condensate properties. Although ideal condensates can support an exciton supercurrent, it has not been clear how such a current could be induced or detected. We discuss the circuit conditions required to induce a steady-state counterflow superfluid. In addition, we will discuss interpretations of tunnel, drag and counterflow experiments in quantum Hall exciton condensates.

Su, Jung-Jung; Pereg-Barnea, Tami; MacDonald, Allan H.

2009-03-01

205

Ballistic spin transport in exciton gases  

Digital Repository Infrastructure Vision for European Research (DRIVER)

Traditional spintronics relies on spin transport by charge carriers, such as electrons in semiconductor crystals. The challenges for the realization of long-range electron spin transport include rapid spin relaxation due to electron scattering. Scattering and, in turn, spin relaxation can be effectively suppressed in excitonic devices where the spin currents are carried by electrically neutral bosonic quasiparticles: excitons or exciton-polaritons. They can form coherent quantum liquids that ...

Kavokin, Alexey; Vladimirova, Maria; Jouault, Benoit; Liew, Timothy; Leonard, Jason; Butov, Leonid

2013-01-01

206

Exciton line broadening in solid solutions with degenerated valent zone  

International Nuclear Information System (INIS)

It is shown, that exciton line broadening in solid solutions with degenerated valent zone in the case, when the exciton translation mass is negative, is irregularly large and determined by exciton scattering in composition fluctuations with large momentum transfer

1987-03-25

207

Transient photoluminescence enhancement as a probe of the structure of impurity-trapped excitons in CaF$_2$:Yb$^{2+}$  

CERN Multimedia

We demonstrate a direct measurement of the energy levels of impurity-trapped excitons in CaF$_2$:Yb$^{2+}$. The radically different radiative decay rates of the lowest exciton state and higher excited states enable the generation of a transient photoluminescence enhancement measured via a two-step excitation process. We observe sharp transitions arising from changes of state of localized electrons, broad bands associated with changes of state of delocalized electrons, and broad bands arising from trap liberation.

Reid, Michael F; Wells, Jon-Paul R; Berden, Giel; Meijerink, Andries; Salkeld, Alex; Duan, Chang-Kui

2011-01-01

208

Excitons in the rare gas solids  

International Nuclear Information System (INIS)

Excitons play a prominent role in the chemistry and physics of condensed matter. Excitons in the rare gas solids, the prototypical van der Waals insulators, will be the focus of the remainder of this report. The goal here is to investigate the controversies surrounding the description of excitons in insulators and, therefore the simplest class of these solids, namely the rare gas solids, is chosen as the exemplary system. Specific problems associated with molecular crystals are, therefore, avoided and only the salient features of excitons are thus considered. 47 refs., 9 figs., 4 tabs

1988-01-01

209

Wannier excitons in the ferromagnetic semiconductors EuO and EuS  

International Nuclear Information System (INIS)

Fine structures in the thermoreflectance (TR) spectra of EuO and EuS in the ultraviolet region are ascribed for the first time to Wannier excitons in magnetic semiconductors. The characteristic temperature-induced splittings of the spectra of EuO and EuS at the ferromagnetic spin-ordering temperatures are successfully analyzed in terms of the exciton and the spin-polarization effect of the relevant one-electron bands, an s-like conduction and p-like valence bands, at the GAMMA point. The magnetic-spin-exchange energies, J/subc/ and J/subv/, between the localized Eu-4f electrons and the band electrons, the spin-orbit interaction energy lambda for the valence band, and the electron-hole spin-exchange energy ? for the exciton are estimated. It is demonstrated that the TR measurements open a new aspect of the optical study of magnetic-semiconductor excitons which become an ideal probe for directly detecting the interaction between the localized-magnetic-spin system and the nonlocalized-Bloch-electron system in the magnetic semiconductors

1975-09-15

210

Comment on 'Topological stability of the half-vortices in spinor exciton-polariton condensates'  

International Nuclear Information System (INIS)

We show that the conclusions of recent paper by Flayac et al. [Phys. Rev. B 81, 045318 (2010)] concerning the stability of half-quantum vortices are misleading. We demonstrate the existence of static half-quantum vortices in exciton-polariton condensates and calculate the warping of their texture produced by transverse-electric-transverse-magnetic splitting of polariton band.

2010-09-15

211

PES of higher temperature vapors: Lithium halide monomers and dimers  

International Nuclear Information System (INIS)

The He (I) photoelectron spectra of ''pure'' monomers and dimers of the lithium halides have been obtained by using a double-oven technique to vary the dimer:monomer ratio. The lithium halide monomer spectra conform nicely to a model based on spin--orbit and ligand field interactions previously introduced. The first ionization potentials of the dimers are in each case higher than in the corresponding monomers. These conclusions are also deduced in this paper by ionic model calculations and molecular calculations of the X-? type, using the discrete variational method. The X-? calculations enable us to assign tentatively the dimer valence bands. These calculations have been extended to trimer and tetramer structures and indicate an oscillation in the first ionization potential in the series encompassing monomer through tetramer. Some experimental verification exists up to trimer. The oscillatory behavior is also obtained in a crude ionic model calculation. An experimental method using photoionization mass spectrometry is suggested for the study of larger clusters and is tested for Na2I2

1979-09-15

212

Electronic structure, lattice energies and Born exponents for alkali halides from first principles  

Directory of Open Access Journals (Sweden)

Full Text Available First principles calculations based on DFT have been performed on crystals of halides (X = F, Cl, Br and I of alkali metals (M = Li, Na, K, Rb and Cs. The calculated lattice energies (U0 are in good agreement with the experimental lattice enthalpies. A new exact formalism is proposed to determine the Born exponent (n for ionic solids. The values of the Born exponent calculated through this ab-initio technique is in good agreement with previous empirically derived results. Band Structure calculations reveal that these compounds are wide-gap insulators that explains their optical transparency. Projected density of states (PDOS calculations reveal that alkali halides with small cations and large anions, have small band gaps due to charge transfer from X ? M. This explains the onset of covalency in ionic solids, which is popularly known as the Fajans Rule.

C. R. Gopikrishnan

2012-02-01

213

Spin-dependent exciton-exciton interaction potential in two- and three-dimensional structure semiconductors under excitation  

International Nuclear Information System (INIS)

Analytical expressions of the exciton-exciton interaction potentials have been approximately derived in both 2D and 3D structure materials exhibiting explicit dependences on exciton momentum difference, momentum transfer, electron-hole effective mass ratio and two-exciton state spin symmetry. Numerical calculations show that the character of the exciton-exciton interaction is determined by all of the above-mentioned dependences. (author). 32 refs, 7 figs

1990-01-01

214

Synthesis and vibrational absorption spectra of scandium and yttrium halide tetrahydrofuranates  

International Nuclear Information System (INIS)

Halides of MHal3x3THF type (M=Sc, Y; Hal=Cl, Br) solvated with tetrahydrofuran are prepared by HgCl2 effect on metallic scandium, yttrium or by HgBr2 effect on metallic scandium in tetrahydrofuran (THF). Infrared absorption spectra of the prepared compounds in the range of 200-3500 cm-1 are studied. Empiric attribution of the absorption bands, observed in the infrared spectra is carried out

1982-01-01

215

Growth and Studies of Halides doped Zinc Tris-Thiourea Sulphate(HZTS) Crystals  

Science.gov (United States)

Single crystals of Sodium chloride and Potassium iodide (Halides) doped Zinc tris-thiourea sulphate (ZTS) were grown from low temperature solution growth technique by slow evaporation method using water as solvent. The powder X-Ray diffraction pattern were recorded and indexed. The UV transmittance spectrum has been recorded. The optical band gap was estimated using Taucís plot. The TGA/DTA studies show the thermal properties of the crystals.

Suveetha, P.; Sathya, T.; Sudha, S.; Raj, M. B. Jessie

2012-10-01

216

Triplet-Exciton Generation Mechanism in a New Soluble (Red-Phase) Polydiacetylene  

Science.gov (United States)

The time scale of covalent state relaxation and singlet fission into triplet pairs is determined in a solution of isolated polydiacetylene chains showing a remarkably sharp excitonic transition by applying the transient transmission difference spectroscopy technique with sub-10 fs optical pulses. Photoexcitation into the band of ionic states is followed by a branching within the first 100 fs which populates the band of covalent states. The latter start to dissociate into triplet excitations, completing the process in about 300 fs.

Lanzani, G.; Cerullo, G.; Zavelani-Rossi, M.; de Silvestri, S.; Comoretto, Davide; Musso, Gianfranco; Dellepiane, Giovanna

2001-10-01

217

Evidence for an excitonic insulator phase in 1T-TiSe$_{2}$  

Digital Repository Infrastructure Vision for European Research (DRIVER)

We present a new high-resolution angle-resolved photoemission study of 1\\textit{T}-TiSe$_{2}$ in both, its room-temperature, normal phase and its low-temperature, charge-density wave phase. At low temperature the photoemission spectra are strongly modified, with large band renormalisations at high-symmetry points of the Brillouin zone and a very large transfer of spectral weight to backfolded bands. A theoretical calculation of the spectral function for an excitonic insulato...

Cercellier, H.; Monney, C.; Clerc, F.; Battaglia, C.; Despont, L.; Garnier, M. G.; Beck, H.; Patthey, L.; Berger, H.; Aebi, P.

2007-01-01

218

Excitons in monocline zinc diphosphide. A-exciton series and Fano factor  

International Nuclear Information System (INIS)

The spectra of the A-exciton series absorption by the various wave vector directions and radiation polarization states are studied in the monocline zinc diphosphide - ?-ZnP2. The oscillators forces, which are subjected to the Fn ? n-3 dependence of the S-type, characteristic for the exciton states, are determined for the observed transitions. It is assumed, that the A-exciton series is conditioned partially by the permitted dipole transitions of the orthoexciton into the nS-state. The Fano antiresonances originate in the absorption spectra or the resonances of the A-exciton series by availability of the E radiation vector constituent on the crystallographic axis c. They are conditioned by the configuration interaction of the series A discrete exciton states with the exciton-phonon spectrum continuum, conditioned by the indirect transitions into the 1S-zone of the singlet exciton with the phonon emission

2003-07-01

219

On the exciton model for ion-beam damage: The example of TiO{sub 2}  

Energy Technology Data Exchange (ETDEWEB)

The non-radiative exciton decay model recently developed to account for swift-ion-beam damage to LiNbO{sub 3} is, here, discussed within a general physical perspective, taking previous work on alkali halides as a reference. Some general rules for the validity of excitonic models have been put forward, allowing one to predict the irradiation behaviour of other materials. As a new example of application, some preliminary data on the generation and growth of uniform amorphous layers induced by irradiation with Br at 13 MeV and 25 MeV have been performed on rutile (TiO{sub 2}). In addition sub-threshold irradiations with Br ions at 9 MeV have been carried out. Defects generation is observed as a result. This effect is explained with the exciton model. Experiments are in the electronic excitation regime and use moderate fluences in the range of 6 x 10{sup 12} to 1.5 x 10{sup 14} cm{sup -2}. The results show similar features to those found for LiNbO{sub 3} and are, in principle, consistent with a non-radiative exciton decay model.

Rivera, A. [Instituto de Microelectronica de Madrid, (CNM-CSIC), Isaac Newton 8, E-28760 Tres Cantos (Spain); Crespillo, M.L. [Centro de Microanalisis de Materiales (CMAM), Universidad Autonoma de Madrid (UAM), Cantoblanco, E-28049 Madrid (Spain); Olivares, J. [Centro de Microanalisis de Materiales (CMAM), Universidad Autonoma de Madrid (UAM), Cantoblanco, E-28049 Madrid (Spain); Instituto de Optica, Consejo Superior de Investigaciones Cientificas (CSIC), C/Serrano 121, E-28006 Madrid (Spain); Sanz, R. [Instituto de Ciencia de Materiales de Madrid (ICMM-CSIC), Cantoblanco, E-28049 Madrid (Spain); Jensen, J. [Thin Film Physics Division, Department of Physics, Chemistry and Biology - IFM, Linkoeping University, SE 581 83, Linkoeping (Sweden); Agullo-Lopez, F., E-mail: fal@uam.e [Centro de Microanalisis de Materiales (CMAM), Universidad Autonoma de Madrid (UAM), Cantoblanco, E-28049 Madrid (Spain); Departamento de Fisica de Materiales, Universidad Autonoma de Madrid (UAM), Cantoblanco, E-28049 Madrid (Spain)

2010-10-01

220

Interaction of halides in ternary mutual systems in melt  

International Nuclear Information System (INIS)

The character of halides interaction in ternary mutual systems of diagonal type is analyzed. It is shown that the direction of exchange reaction and equilibrium state in the melt are determined by a halide solubility ratio. The interaction parameter (mixing heat) of the halides is calculated. Difficulty soluble salts in the melt are noted to behave as weak electrolytes

2001-01-01

 
 
 
 
221

Condensation of indirect excitons in coupled AlAs/GaAs quantum wells  

Digital Repository Infrastructure Vision for European Research (DRIVER)

The photoluminescence (PL) of excitons confined in an electric field tunable coupled AlAs/GaAs quantum well has been investigated at T[MATH]350 mK and magnetic field H[MATH]14 T. In the indirect regime when electrons and holes are separated both in real- and in k-space, magnetic field was found to result in (i) a strong change of both the PL intensity and decay time which is attributed to the anomalies in the exciton transport and (ii) an appearance of a huge broad band noise in the PL intens...

1993-01-01

222

Exciton self-trapping in tetrafluoro-dimethyl-aminoacridine single crystals  

Science.gov (United States)

The UV-visible optical spectra of 1,2,3,4-tetrafluoro-7-(N,N)dimethyl-amino-acridine single crystals are reported. The results are discussed on the basis of the molecular transitions and crystal packing in the framework of the theory of molecular excitons under a fluctuating potential field due to dynamic disorder. A strong local geometry distortion is demonstrated by applying the Urbach rule to the absorption tails, which is the amplitude of the local potential fluctuation being larger than the intermolecular transfer energy. The lineshape and linewidth of the emission band and its temperature dependence give further evidence of exciton self-trapping.

Tavazzi, S.; Miozzo, L.; Papagni, A.; Raimondo, L.; Silvestri, L.; Spearman, P.; Camposeo, A.; Polo, M.; Pisignano, D.

2007-06-01

223

Phase Transition of Trapped Nuclear Exciton of Long-lived Rhodium Mossbauer States  

CERN Multimedia

We report two experimental observations from the long-lived rhodium Mossbauer emissions by means of the time- and energy-resolved spectroscopy. Firstly, phase transitions in time evolution indicated by characteristic spectral profiles reveal the aggregate exciton orderings of inverted nuclei at room temperature. Depending on the exciton density in polycrystal, six different phases featured by multiple ionizations are identified. Secondly, broad-band background emissions are discovered and are believed to be ascribed to branching channels of gamma radiation. More than half gamma radiation goes to these anisotropic channels. Cascade scheme to catalyze the E3 multipolar nuclear transition is thus suggested.

Xia, Bing

2007-01-01

224

First-principles study of ?-ray detector materials in perovskite halides  

Science.gov (United States)

In an effort to search for good ?-ray detector materials, perovskite halide compounds containing heavy elements were investigated. Despite the three-dimensional network of the corner shared octahedra and the extended nature of the outermost shell, its strong ionic character leads to a large band gap, which is one of the essential criteria for ?-ray detector materials. Thus, considering high density and high atomic number, these pervoskite halides are possible candidate for ?-ray detector materials. We performed first-principles calculations to investigate electronic structures and thermodynamic properties of intrinsic defects in the selected perovskite halide, CsPbBr3. The screened-exchange local density approximation scheme was employed to correct the underestimation of the band gap in the LDA method. As a result, the calculated band gap of CsPbBr3 is found to be suitable for ?-ray detection. Furthermore, defect formation energy calculations allow us to predict thermodynamic and electronic properties of possible intrinsic defects, which affect detector efficiency and energy resolution.

Im, Jino; Jin, Hosub; Stoumpos, Constantinos C.; Chung, Duck Young; Liu, Zhifu; Peters, John A.; Wessels, Bruce W.; Kanatzidis, Mercouri G.; Freeman, Arthur J.

2013-03-01

225

Radiative recombination of excitons in amorphous semiconductors  

International Nuclear Information System (INIS)

A theory for calculating the radiative lifetime of excitons in amorphous semiconductors is presented. Four possibilities of excitonic radiative recombination are considered and the corresponding rates are derived at thermal equilibrium. The radiative lifetime is calculated from the inverse of the maximum rate for all the four possibilities. Results agree very well with experiments

2005-04-01

226

Enhanced Quantum Efficiency From Hybrid Cesium Halide/Copper Photocathode  

Energy Technology Data Exchange (ETDEWEB)

The quantum efficiency of Cu is found to increase dramatically when coated by a CsI film and then irradiated by a UV laser. Over three orders of magnitude quantum efficiency enhancement at 266 nm is observed in CsI/Cu(100), indicating potential application in future photocathode devices. Upon laser irradiation, a large work function reduction to a value less than 2 eV is also observed, significantly greater than for similarly treated CsBr/Cu(100). The initial QE enhancement, prior to laser irradiation, is attributed to interface interaction, surface cleanliness and the intrinsic properties of the Cs halide film. Further QE enhancement following activation is attributed to formation of inter-band states and Cs metal accumulation at the interface induced by laser irradiation.

Kong, Lingmei; Joly, Alan G.; Droubay, Timothy C.; Gong, Yu; Hess, Wayne P.

2014-04-28

227

Vibrational spectroscopic studies of metal(II) halide benzimidazole  

Science.gov (United States)

Infrared spectra (4000-200 cm -1) are reported for metal halide(II) benzimidazole complexes of the following stoichiometries: M(benz)X 2 [M=Cd, Cu; X=Cl, Br; benz=benzimidazole], Co(benz) 2, and Co(benz) 2X 2 [X=Cl, Br, I]. Vibrational assignments are given for all the observed bands. The analysis of the vibrational spectra indicates that there are some structure-spectra correlations. For a given series of isomorphous complexes the sum of the difference between the values of the vibrational modes of uncoordinated benzimidazole and coordinated to metal ion benzimidazole was found to increase in the order of the second ionization potentials of metals.

Yurdakul, ?.; Kurt, M.

2003-05-01

228

Enhanced quantum efficiency from hybrid cesium halide/copper photocathodes  

Science.gov (United States)

The quantum efficiency (QE) of Cu is found to increase dramatically when coated by a CsI film and then irradiated by a UV laser. Over three orders of magnitude quantum efficiency enhancement at 266 nm is observed in CsI/Cu(100), indicating potential application in future photocathode devices. Upon laser irradiation, a large work function reduction to a value less than 2 eV is also observed, significantly greater than for similarly treated CsBr/Cu(100). The initial QE enhancement, prior to laser irradiation, is attributed to interface interaction and the intrinsic properties of the Cs halide film. Further QE enhancement following activation is attributed to formation of inter-band states and Cs metal accumulation at the interface induced by laser irradiation.

Kong, Lingmei; Joly, Alan G.; Droubay, Timothy C.; Gong, Yu; Hess, Wayne P.

2014-04-01

229

Infrared evanescent field sensing with quantum cascade lasers and planar silver halide waveguides.  

Science.gov (United States)

We demonstrate the first midinfrared evanescent field absorption measurements with an InGaAs/AlInAs/InP distributed feedback (DFB) quantum cascade laser (QCL) light source operated at room temperature coupled to a free-standing, thin-film, planar, silver halide waveguide. Two different analytes, each matched to the emission frequency of a QCL, were investigated to verify the potential of this technique. The emission of a 1650 cm(-1) QCL overlaps with the amide absorption band of urea, which was deposited from methanol solution, forming urea crystals at the waveguide surface after solvent evaporation. Solid urea was detected down to 80.7 microg of precipitate at the waveguide surface. The emission frequency of a 974 cm(-1) QCL overlaps with the CH3-C absorption feature of acetic anhydride. Solutions of acetic anhydride in acetonitrile have been detected down to a volume of 0.01 microL (10.8 microg) of acetic anhydride solution after deposition at the planar waveguide (PWG) surface. Free-standing, thin-film, planar, silver halide waveguides were produced by press-tapering heated, cylindrical, silver halide fiber segments to create waveguides with a thickness of 300-190 microm, a width of 3 mm, and a length of 35 mm. In addition, Fourier transform infrared (FT-IR) evanescent field absorption measurements with planar silver halide waveguides and transmission absorption QCL measurements verify the obtained results. PMID:16013852

Charlton, Christy; Katzir, Abraham; Mizaikoff, Boris

2005-07-15

230

The structure and dynamics of molecular excitons.  

Science.gov (United States)

The photophysical behavior of organic semiconductors is governed by their excitonic states. In this review, I classify the three different exciton types (Frenkel singlet, Frenkel triplet, and charge transfer) typically encountered in organic semiconductors. Experimental challenges that arise in the study of solid-state organic systems are discussed. The steady-state spectroscopy of intermolecular delocalized Frenkel excitons is described, using crystalline tetracene as an example. I consider the problem of a localized exciton diffusing in a disordered matrix in detail, and experimental results on conjugated polymers and model systems suggest that energetic disorder leads to subdiffusive motion. Multiexciton processes such as singlet fission and triplet fusion are described, emphasizing the role of spin state coherence and magnetic fields in studying singlet ? triplet pair interconversion. Singlet fission provides an example of how all three types of excitons (triplet, singlet, and charge transfer) may interact to produce useful phenomena for applications such as solar energy conversion. PMID:24313684

Bardeen, Christopher J

2014-01-01

231

Experimental search for excitonic superconductivity  

International Nuclear Information System (INIS)

A search for excitonic superconductivity in a thin-film structure of the type proposed by Ginsberg and Allender, Bray, and Bardeen was undertaken. Ultrathin layers of Pb, In, and Tl were deposited on PbTe, and layers of Pb and In were deposited on Te. Substrate preparation and film deposition were accomplished in an ultrahigh-vacuum system with integral cryostat, and were monitored by low-energy electron diffraction and Auger electron spectroscopy. In situ measurement of crystalline epitaxial Pb films as thin as 10--15 A deposited on PbTe showed a strong film-substrate interaction, however, no excitonic enhancement of T/subc/ was observed. A simple proximity-effect model is presented to explain the observed decrease in T/subc/ for the thinner films, and the influence of the substrate on the superconductivity of ultrathin films is discussed. Several aspects of these experiments involving the transition temperatures of In and Tl films are unexplained

1976-06-01

232

Exciton absorption spectrum of thin Ag sub 2 ZnI sub 4  

CERN Multimedia

In Ag sub 2 ZnI sub 4 compound thin films one investigated into the electron spectrum of absorption within 3-6 eV photon energy range. The boundary of interband absorption is determined to correspond to the direct permitted transitions with E sub g = 3.7 eV forbidden gap width. A strong exciton band at E = 3.625 eV (80 K) GAMMA half width temperature run of which within 80-390 K range is governed by exciton-phonon interaction typical for quasi-single-dimensional excitons, is adjacent to the absorption boundary. At T <= 390 K one observes a bend in E(T) and GAMMA(T) dependences associated with generation of the Frenkel defects and followed by transfer of Ag ions to the interstices and vacancies of the compound crystalline lattice

Yunakova, O N; Kovalenko, E N

2002-01-01

233

Exciton effects in optical absorption spectra of boron-nitride (BN) nanotubes  

CERN Document Server

Exciton effects are studied in single-wall boron-nitride nanotubes. The Coulomb interaction dependence of the band gap, the optical gap, and the binding energy of excitons are discussed. The optical gap of the (5,0) nanotube is about 6eV at the onsite interaction U=2t with the hopping integral t=1.1eV. The binding energy of the exciton is 0.50eV for these parameters. This energy agrees well with that of other theoretical investigations. We find that the energy gap and the binding energy are almost independent of the geometries of nanotubes. This novel property is in contrast with that of the carbon nanotubes which show metallic and semiconducting properties depending on the chiralities.

Harigaya, Kikuo

2008-01-01

234

Correlation effects in the optical spectra of porphyrin oligomer chains: exciton confinement and length dependence.  

Science.gov (United States)

Excited states of ethylene-linked free-base porphyrin oligomers and polymer are studied using many-body perturbation theory (MBPT) within the GW approximation and the Bethe-Salpeter equation. Trends in the electronic levels with oligomer length are analysed and the correct long-range behaviour in the band gap is obtained. High polarizabilities and strong redshifts in the optical absorption peaks are predicted in agreement with observations on other strongly conjugated oligoporphyrins. We explain these trends by means of spatial and spectral analyses of the exciton character. Although Wannier-Mott and charge-transfer excitons are identified in the optical spectra, the strongest polarizabilities are actually associated with small, tightly bound excitons (Frenkel-like), in contrast to expectations. Furthermore, the common procedure of extrapolating polymer properties from oligomer calculations is examined from a MBPT perspective. PMID:23320688

Hogan, Conor; Palummo, Maurizia; Gierschner, Johannes; Rubio, Angel

2013-01-14

235

Semiconducting monolayer materials as a tunable platform for excitonic solar cells.  

Science.gov (United States)

The recent advent of two-dimensional monolayer materials with tunable optical properties and high carrier mobility offers renewed opportunities for efficient, ultrathin excitonic solar cells alternative to those based on conjugated polymer and small molecule donors. Using first-principles density functional theory and many-body calculations, we demonstrate that monolayers of hexagonal BN and graphene (CBN) combined with commonly used acceptors such as PCBM fullerene or semiconducting carbon nanotubes can provide excitonic solar cells with tunable absorber gap, donor-acceptor interface band alignment, and power conversion efficiency, as well as novel device architectures. For the case of CBN-PCBM devices, we predict power conversion efficiency limits in the 10-20% range depending on the CBN monolayer structure. Our results demonstrate the possibility of using monolayer materials in tunable, efficient, ultrathin solar cells in which unexplored exciton and carrier transport regimes are at play. PMID:23062107

Bernardi, Marco; Palummo, Maurizia; Grossman, Jeffrey C

2012-11-27

236

Effect of uniaxial stress on free and bismuth-bound excitons in InP  

International Nuclear Information System (INIS)

The reduction of the shear deformation potentials of holes bound to the isoelectronic impurity Bi in InP is determined by piezoluminescence. It is compared with the corresponding reduction for holes bound to the Coulomb-type acceptors C and Zn. The theory for an effective mass acceptor describes well the cases of C and Zn. However, additional effects as local strain and Stark fields must be involved in the case of Bi leading to an extremely large reduction of the deformation potentials. No change in binding energy with applied stress as well as no exchange splitting of the Bi-bound exciton can be detected within experimental accuracy. The stress dependence of the free exciton reflectance reveals values for the band deformation potentials and a value of 0.07 meV for the exchange splitting of the free exciton in InP. (author)

1979-04-01

237

Quasi-particle energies and excitonic effects in bilayer of hexagonal boron nitride  

Science.gov (United States)

First-principles calculations based on the density functional theory (DFT), GW and Bethe–Salpeter equations are performed on the bilayer of hexagonal boron nitride (bilayer h-BN) to investigate the electronic structure and optical properties. Due to the quantum confinement effect and the less efficient electronic screening, the value of direct (indirect) band gap increases from a value of 4.45 eV (4.37 eV) within the density functional theory to a value of 6.74 eV (6.67 eV) within the GW many-body Green?s function theory. The calculated optical absorption spectra are dominated by exciton states with a binding energy about of 1.67 eV. The enhanced excitonic effects in bilayer h-BN have the potential to be used in optoelectronic and excitonic devices.

Ashhadi, Mojtaba; Ketabi, Seyed Ahmad

2014-06-01

238

Magneto-Optical Properties of Bound Excitons in ZnO  

CERN Multimedia

We present results of magneto-optical measurements and theoretical analysis of shallow bound exciton complexes in bulk ZnO. Polarization and angular dependencies of magneto-photoluminescence spectra at 5 T suggest that the upper valence band has $\\Gamma_7$ symmetry. Nitrogen doping leads to the formation of an acceptor center that compensates shallow donors. This is confirmed by the observation of excitons bound to ionized donors in nitrogen doped ZnO. The strongest transition in the ZnO:N ($I_9$ transition) is associated with a donor bound exciton. This conclusion is based on its thermalization behavior in temperature-dependent magneto-transmission measurements and is supported by comparison of the thermalization properties of the $I_9$ and $I_4$ emission lines in temperature-dependent magneto-photoluminescence investigations.

Rodina, A V; Dworzak, M; Haboeck, U; Hoffmann, A; Zeuner, A; Alves, H R; Hofmann, D M; Meyer, B K

2003-01-01

239

Spontaneous exciton condensation in 1T-TiSe2 : BCS-like approach  

Science.gov (United States)

Recently we found strong evidence in favor of a BCS-like condensation of excitons in 1T-TiSe2 [Cercellier , Phys. Rev. Lett. 99, 146403 (2007)]. Theoretical photoemission intensity maps have been generated by the spectral function calculated within the exciton condensate phase model and set against experimental angle-resolved photoemission spectroscopy data. The scope of this paper is to present the detailed calculations in the framework of this model. They represent an extension of the original excitonic insulator phase model of Jérome [Phys. Rev. 158, 462 (1967)] to three dimensional and anisotropic band dispersions. A detailed analysis of its properties and comparison with experiment is presented. Finally, the disagreement with density-functional theory is discussed.

Monney, C.; Cercellier, H.; Clerc, F.; Battaglia, C.; Schwier, E. F.; Didiot, C.; Garnier, M. G.; Beck, H.; Aebi, P.; Berger, H.; Forró, L.; Patthey, L.

2009-01-01

240

Bound Exciton Luminescence in Shock Compressed GaP:S and GaP:N  

Energy Technology Data Exchange (ETDEWEB)

Photoluminescence (PL) spectra of bound excitons were measured in uniaxially strained GaP by performing shock-wave experiments at liquid nitrogen temperatures. GaP samples doped with sulfur or nitrogen were compressed up to 3 GPa when subjected to uniaxial strains along the [100] crystallographic orientation. PL lines from shallow sulfur donors redshifted upon compression, tracking the reduction in the indirect band gap. PL lines related to the isoelectronic NN1 pairs, in contrast, exhibited splitting and nonlinear blueshift. An empirical approach was used to model the NN1 behavior. It was shown that the splitting pattern is consistent with the previously proposed symmetry of NN1 defects and nonlinearities resulting from the reduction in the exciton binding energy. At high stresses, the NN1 lines disappeared due to the ionization of bound excitons.

Grivickas, P.; McCluskey, M. D.; Gupta, Y. M.; Zhang, Y.; Geisz, J. F.

2009-01-01

 
 
 
 
241

Enhanced surface-excitonic emission in ZnO/Al2O3 core-shell nanowires  

International Nuclear Information System (INIS)

We report the influence of an Al2O3 shell on the photoluminescence emission of ZnO nanowires. At room temperature, the spectrum of the core-shell nanowires shows a strong reduction of the relative intensity of the green defect emission with respect to the near-band-edge emission. At 5 K an increase of the relative intensity of the surface exciton band with respect to the donor-bound exciton emission is observed. Annealing the core-shell nanowires at 500 deg. C does not increase the green defect luminescence at 5 K. We propose a model explaining the spectral changes

2008-07-30

242

Multi-wavelength metal halide lasers  

Energy Technology Data Exchange (ETDEWEB)

This patent describes a laser, emitting energy at selected, characteristic wavelengths within the visible and near infra-red regions of the spectrum, by dissociation of selected metal dihalides. The laser consists of: (a) enclosure means; (b) at least two metal halides from group II-B of the Periodic Table, present within the enclosure means; (c) an inert buffer gas, present within the enclosure means; (d) a heat source, for vaporization of the metal halides, capable of maintaining the enclosure means at a temperature of at least about 370/sup 0/C; (e) a source of pulsed dissociative energy, located in proximity to the enclosure means and in communication therewith, for effecting dissociation of the vaporized metal halide; (f) pre-ionization means, located in proximity to the enclosure means and in communication therewith, for effecting ionization of the vaporized metal halides in concert with and during the pulsed dissociation; and (g) an optical cavity disposed about the enclosure means, comprising a reflective surface and a partially reflective surface, each aligned to permit pulsed emission of a portion of the laser energy.

Eden, J.G.; McCown, A.W.; Ediger, M.N.; Greene, D.P.

1986-08-19

243

Temperature dependence of compressibility of alkali halides  

International Nuclear Information System (INIS)

Calculations are done to compute the temperature dependence of the isothermal compressibility for alkali halides at room temperature and atmospheric pressure. All the calculated data are tabulated together with other relevant parameters needed for the purpose. The results obtained are in good agreement with the experimental data taken from the literature

1981-08-01

244

Ultrafast exciton dynamics in Type II ZnTe-ZnSe colloidal quantum dots.  

Science.gov (United States)

Ultrafast transient absorption spectroscopy is used to investigate the exciton dynamics of Type II ZnTe-ZnSe core-shell colloidal quantum dots. Surface-trapping is shown to occur within a few picosecond for hot electrons and with a few 10s of picoseconds for electrons cooled to the band-edge, and is the dominant process in the decay of the band-edge bleach for well-stirred samples pumped at moderate powers. The surface-trapped electrons produce a broad photo-induced absorption that spectrally overlaps with the band-edge, distorting and partially cancelling out the bleach feature. At high pump powers and for unstirred samples, these surface-trapped electrons can survive sufficiently long within the pumped volume to accumulate under repeated excitation of the sample, resulting in the formation of an additional exciton decay channel. PMID:22964845

Cadirci, M; Stubbs, S K; Fairclough, S M; Tyrrell, E J; Watt, A A R; Smith, J M; Binks, D J

2012-10-21

245

Excitonic spectrum of the ZnO/ZnMgO quantum wells  

International Nuclear Information System (INIS)

Excitonic spectrum of the wurtzite ZnO/Zn1?xMgxO quantum wells with a width on the order of or larger than the Bohr radius of the exciton has been studied; the quantum wells have been grown by the method of molecular beam epitaxy (with plasma-assisted activation of oxygen) on substrates of sapphire (0001). Low-temperature (25 K) spectra of photoluminescence excitation (PLE) have been experimentally measured, making it possible to resolve the peaks of exciton absorption in the quantum well. The spectrum of excitons in the quantum well is theoretically determined as a result of numerical solution of the Schrödinger equation by the variational method. The value of elastic stresses in the structure (used in calculations) has been determined from theoretical simulation of measured spectra of optical reflection. A comparison of experimental data with the results of calculations makes it possible to relate the observed features in the PLE spectra to excitons, including the lower level of dimensional quantization for electrons and two first levels of holes for the A and B valence bands of the wurtzite crystal. The values of the electron and hole masses in ZnO are refined, and the value of the built-in electric field introduced by spontaneous and piezoelectric polarizations is estimated.

2011-06-01

246

Excitonic spectrum of the ZnO/ZnMgO quantum wells  

Energy Technology Data Exchange (ETDEWEB)

Excitonic spectrum of the wurtzite ZnO/Zn{sub 1-x}Mg{sub x}O quantum wells with a width on the order of or larger than the Bohr radius of the exciton has been studied; the quantum wells have been grown by the method of molecular beam epitaxy (with plasma-assisted activation of oxygen) on substrates of sapphire (0001). Low-temperature (25 K) spectra of photoluminescence excitation (PLE) have been experimentally measured, making it possible to resolve the peaks of exciton absorption in the quantum well. The spectrum of excitons in the quantum well is theoretically determined as a result of numerical solution of the Schroedinger equation by the variational method. The value of elastic stresses in the structure (used in calculations) has been determined from theoretical simulation of measured spectra of optical reflection. A comparison of experimental data with the results of calculations makes it possible to relate the observed features in the PLE spectra to excitons, including the lower level of dimensional quantization for electrons and two first levels of holes for the A and B valence bands of the wurtzite crystal. The values of the electron and hole masses in ZnO are refined, and the value of the built-in electric field introduced by spontaneous and piezoelectric polarizations is estimated.

Bobrov, M. A., E-mail: largaseal@gmail.com; Toropov, A. A.; Ivanov, S. V. [Russian Academy of Sciences, Ioffe Physical Technical Institute (Russian Federation); El-Shaer, A.; Bakin, A.; Waag, A. [TU Braunschweig, Institute of Semiconductor Technology (Germany)

2011-06-15

247

Ionic alkali halide XUV laser feasibility study  

Energy Technology Data Exchange (ETDEWEB)

The objective of this work is to assess the feasibility of a select set of ionic alkali halide XUV laser concepts by obtaining the relevant kinetic and spectroscopic parameters required for a proof-of-principle and conceptual design. The proposed lasers operate in the 80--200 nm spectral region and do not require input from outside radiation sources for their operation. Frequency up-conversion and frequency mixing techniques and therefore not considered in the work to be described. An experimental and theoretical study of a new type of laser operating in the extreme ultraviolet wavelength region has been conducted. The lasing species are singly ionized alkali halide molecules such as Rb{sup 2+}F{sub {minus}}, Rb{sup 2+}Br{sup {minus}} and Cs{sup 2+}F{sup {minus}}. These species are similar in electronic structure to the rare gas halide excimers, such as XeF and Krf, except that the ionic molecules emit at wavelengths of 80--200 nm, much shorter than the conventional rare-gas halide excimer laser. The radiative lifetime of these molecules are typically near 1 ns, which is about an order of magnitude shorter than that for rare-gas halide systems. The values of the cross section for stimulated emission are on the order of 1 {times} 10{sup {minus}16}cm{sup 2}. Because of the fundamental similarity to existing UV lasers, these systems show promise as a high power, efficient XUV lasers. 55 refs., 50 figs., 5 tabs.

Yang, T.T.; Gylys, V.T.; Bower, R.D.; Harris, D.G.; Blauer, J.A.; Turner, C.E.; Hindy, R.N.

1989-11-10

248

FTIR Studies of Internal Water Molecules of Bacteriorhodopsin: Structural Analysis of Halide-bound D85S and D212N Mutants in the Schiff Base Region  

Science.gov (United States)

Bacteriorhodopsin (BR), a membrane protein found in Halobacterium salinarum, functions as a light-driven proton pump. The Schiff base region has a quadropolar structure with positive charges located at the protonated Schiff base and Arg82, and counterbalancing negative charges located at Asp85 and Asp212 (Figure 1A). It is known that BR lacks a proton-pumping activity if Asp85 or Asp212 is neutralized by mutation. On the other hand, binding of C1- brings different effects for pumping functions in mutants at D85 and D212 position. While C1--bound D85T and D85S pump C1-, photovoltage measurements suggested that C1--bound D212N pumps protons at low pH. In this study, we measured low-temperature FTIR spectra of D85S and D212N containing various halides to compare the halide binding site of both proteins. In the case of D85S, the N-D stretching vibrations of the Schiff base were halide-dependent. This result suggests that the halide is a hydrogen-bond acceptor of the Schiff base, being consistent with the X-ray crystal structure. On the other hand, no halide dependence was observed for vibrational bands of the retinal skeleton and the Schiff base in the D212N mutant. This result suggests that the halide does not form a hydrogen bond with the Schiff base directly, unlike the mutation at D85 position. Halide-dependent water bands in the Schiff base region also differ between D85S and D212N. From these results, halide binding site of both proteins and role of two negative charges in BR will be discussed.

Shibata, Mikihiro; Kandori, Hideki

2007-12-01

249

Moving zero-gap Wannier-Mott excitons in graphene  

Digital Repository Infrastructure Vision for European Research (DRIVER)

We demonstrate the possibility of existence of indirect moving Wannier-Mott excitons in graphene. Electron-hole binding is conditioned by the trigonal warping of conic energy spectrum. The binding energies are found for the lowest exciton states. These energies essentially depend on the value and direction of exciton momentum and vanish when the exciton momentum tends to the conic points. The ways to observe the exciton states are discussed. The opportunity of experimental o...

Mahmoodian, M. M.; Entin, M. V.

2012-01-01

250

Excitons in two coupled conjugated polymer chains  

CERN Document Server

We have studied the exciton states in two coupled conjugated polymer chains which are modeled individually by the Su-Schrieffer-Heeger Hamiltonian and coupled by an interchain electron-transfer term. Both the intra- and inter-chain long range Coulomb interactions are taken into account. The properties of the lowest symmetric and antisymmetric exciton states are extensively discussed for both the parallel and anti-parallel ordering between these two chains. It is found that, for these two kinds of ordering, the features of excitons are quite different. Possible implications for the experiment of luminescent polymers are also addressed.

Yu, Z G; Rao, X S; Sun, X; Bishop, A R

1996-01-01

251

EXCITONS AS A NEW QUANTUM SYSTEM  

Digital Repository Infrastructure Vision for European Research (DRIVER)

In a first part, a brief review of some excitonic properties is presented, with the accent put on their relevance with respect to the possibility of Bose-Einstein condensation. In a second part, recent experimental evidence is described, which supports the idea that excitonic particles may form a highly quantum fluid. In Cu20, the analysis of the exciton decay spectrum shows a gradual evolution of the gas from a classical regime at low densities up to a strongly degenerate one at high densiti...

1980-01-01

252

Excitonic quantum efficiency of GaN  

Energy Technology Data Exchange (ETDEWEB)

We report on investigations of the excitonic quantum efficiency in GaN in dependence of different buffer layer thicknesses and residual oxygen content in the crystal. The quantum efficiency of the free excitons rises with increasing buffer layer thickness and decreasing residual oxygen content. The influence of oxygen on the quantum efficiency is stronger than that of the buffer layer thickness. In general, the observed quantum efficiencies are below 20% indicating the strong impact of nonradiative relaxation and recombination processes in the excitonic range. (orig.) 8 refs.

Goeldner, A.; Eckey, L.; Hoffmann, A. [Technische Univ. Berlin (Germany). Inst. fuer Festkoerperphysik; Gil, B.; Briot, O. [Montpellier-2 Univ., 34 (France). Groupe d`Etudes des Semiconducteurs

1998-08-01

253

Stimulated Scattering of Indirect Excitons in Coupled Quantum Wells: Signature of a Degenerate Bose-Gas of Excitons  

International Nuclear Information System (INIS)

We observe and analyze strongly nonlinear photoluminescence kinetics of indirect excitons in GaAs/AlGaAs coupled quantum wells at low bath temperatures, ?50 mK . The long recombination lifetime of indirect excitons promotes accumulation of these Bose particles in the lowest energy states and allows the photoexcited excitons to cool down to temperatures where the dilute 2D gas of indirect excitons becomes statistically degenerate. Our main result - a strong enhancement of the exciton scattering rate to the low-energy states with increasing concentration of the indirect excitonsemdashreveals bosonic stimulation of exciton scattering, which is a signature of a degenerate Bose-gas of excitons

2001-06-11

254

Exciton-exciton interaction engineering in coupled GaN quantum dots  

Digital Repository Infrastructure Vision for European Research (DRIVER)

We present a fully three-dimensional study of the multiexciton optical response of vertically coupled GaN-based quantum dots via a direct-diagonalization approach. The proposed analysis is crucial in understanding the fundamental properties of few-particle/exciton interactions and, more important, may play an essential role in the design/optimization of semiconductor-based quantum information processing schemes. In particular, we focus on the interdot exciton-exciton couplin...

Rinaldis, Sergio; D Amico, Irene; Rossi, Fausto

2002-01-01

255

Oriented Silver Halide Nanocrystals Embedded in Crystalline Alkali Halide Matrix as Studied by EPR and ODMR.  

Science.gov (United States)

Shallow electron centers self-trapped holes and self-trapped excitons in small AgCl crystals embedded in KCl matrix were investigated by optical spectroscopy optically detected magnetic resonance (ODMR), and electron paramagnetic resonance (EPR) technique...

A. G. Badalyan N. G. Romanov P. G. Baranov R. A. Babunts V. A. Khramtsov

2000-01-01

256

Optical nutation in the exciton range of spectrum  

International Nuclear Information System (INIS)

Optical nutation in the exciton range of spectrum is studied in the mean field approximation taking into account exciton-photon and elastic exciton-exciton interactions. It is shown that the features of nutation development are determined by the initial exciton and photon densities, the resonance detuning, the nonlinearity parameter, and the initial phase difference. For nonzero initial exciton and photon concentrations, three regimes of temporal evolution of excitons and photons exist: periodic conversion of excitons to photons and vice versa, aperiodic conversion of photons to excitons, and the rest regime. In the rest regime, the initial exciton and photon densities are nonzero and do not change with time. The oscillation amplitudes and periods of particle densities determined by the system parameters are found. The exciton self-trapping and photon trapping appearing in the system at threshold values of the nonlinearity parameter were predicted. As this parameter increases, the oscillation amplitudes of the exciton and photon densities sharply change at the critical value of the nonlinearity parameter. These two phenomena are shown to be caused by the elastic exciton-exciton interaction, resulting in the dynamic concentration shift of the exciton level

2013-08-01

257

Optical nutation in the exciton range of spectrum  

Energy Technology Data Exchange (ETDEWEB)

Optical nutation in the exciton range of spectrum is studied in the mean field approximation taking into account exciton-photon and elastic exciton-exciton interactions. It is shown that the features of nutation development are determined by the initial exciton and photon densities, the resonance detuning, the nonlinearity parameter, and the initial phase difference. For nonzero initial exciton and photon concentrations, three regimes of temporal evolution of excitons and photons exist: periodic conversion of excitons to photons and vice versa, aperiodic conversion of photons to excitons, and the rest regime. In the rest regime, the initial exciton and photon densities are nonzero and do not change with time. The oscillation amplitudes and periods of particle densities determined by the system parameters are found. The exciton self-trapping and photon trapping appearing in the system at threshold values of the nonlinearity parameter were predicted. As this parameter increases, the oscillation amplitudes of the exciton and photon densities sharply change at the critical value of the nonlinearity parameter. These two phenomena are shown to be caused by the elastic exciton-exciton interaction, resulting in the dynamic concentration shift of the exciton level.

Khadzhi, P. I. [Academy of Sciences of Moldova, Institute of Applied Physics (Moldova, Republic of); Vasiliev, V. V., E-mail: vasscorp@mail.ru [Taras Shevchenko Transnistria State University (Moldova, Republic of)

2013-08-15

258

Singlet exciton fission in solution  

Science.gov (United States)

Singlet exciton fission, the spin-conserving process that produces two triplet excited states from one photoexcited singlet state, is a means to circumvent the Shockley-Queisser limit in single-junction solar cells. Although the process through which singlet fission occurs is not well characterized, some local order is thought to be necessary for intermolecular coupling. Here, we report a triplet yield of 200% and triplet formation rates approaching the diffusion limit in solutions of bis(triisopropylsilylethynyl (TIPS)) pentacene. We observe a transient bound excimer intermediate, formed by the collision of one photoexcited and one ground-state TIPS-pentacene molecule. The intermediate breaks up when the two triplets separate to each TIPS-pentacene molecule. This efficient system is a model for future singlet-fission materials and for disordered device components that produce cascades of excited states from sunlight.

Walker, Brian J.; Musser, Andrew J.; Beljonne, David; Friend, Richard H.

2013-12-01

259

Optical absorption in degenerately doped semiconductors: Mott transition or Mahan excitons?  

Science.gov (United States)

In the exploration of material properties, parameter-free calculations are a modern, sophisticated complement to cutting-edge experimental techniques. Ab-initio calculations are now capable of providing a deep understanding of the interesting physics underlying the electronic structure and optical absorption, e.g., of the transparent conductive oxides. Due to electron doping, these materials are conductive even though they have wide fundamental band gaps. The degenerate electron gas in the lowest conduction-band states drastically modifies the Coulomb interaction between the electrons and, hence, the optical properties close to the absorption edge. We describe these effects by developing an ab-initio technique which captures also the Pauli blocking and the Fermi-edge singularity at the optical absorption onset, that occur in addition to quasiparticle and excitonic effects. We answer the question whether free carriers induce an excitonic Mott transition or trigger the evolution of Wannier-Mott excitons into Mahan excitons. The prototypical n-type zinc oxide is studied as an example.

Schleife, André.; Rödl, Claudia; Hannewald, Karsten; Bechstedt, Friedhelm

2012-02-01

260

Laser control of dissipative two-exciton dynamics in molecular aggregates  

International Nuclear Information System (INIS)

There are two types of two-photon transitions in molecular aggregates: non-local excitations of two monomers and local double excitations to some higher excited intra-monomer electronic state. As a consequence of the inter-monomer Coulomb interaction, these different excitation states are coupled to each other. Higher excited intra-monomer states are rather short-lived owing to the efficient internal conversion of electronic into vibrational energy. Combining both the processes leads to annihilation of an electronic excitation state, which is a major loss channel for establishing high excitation densities in molecular aggregates. Applying theoretical pulse optimization techniques to a Frenkel exciton model, it is shown that the dynamics of two-exciton states in linear aggregates (dimer to tetramer) can be influenced by ultrafast-shaped laser pulses. In particular, we studied the extent to which the decay of the two-exciton population by inter-band transitions can be transiently suppressed. Intra-band dynamics is described by a dissipative hierarchy equation approach, which takes into account strong exciton–vibrational coupling in the non-Markovian regime. (paper)

2012-10-01

 
 
 
 
261

The electronic structure of F-centers in allkali halide crystals  

International Nuclear Information System (INIS)

A method is presented for estimating the number of bound states of different orbital angular momentum quantum number for an F-centre residing in an alkali halide crystal through the introduction of a spherically symmetric potential well of finite depth. The method is illustrated through its application to the F-centre in sodium chloride, and the results for F-centres in various alkali halide lattices are tabulated. The results provide a good account of the experimental features of the F-centre including the K-band which we suggest is due to transitions from the 2p to excited levels. Values for the effective range of penetration of the defect electron with the neighbouring cation in different crystals are determined by requiring that the well depth be independent of orbital angular momentum quantum number. The technique is applied to estimate the Madelung constant and Madelung potential for the host crystal. (Author)

1982-01-01

262

Ultraviolet excitation and stimulated emission in cryogenic rare-gas halide solutions  

Science.gov (United States)

Cryogenic rare-gas halide solutions, excited optically with radiation at 351, 248 and 193 nm, exhibit fluorescence bands corresponding to rare-gas halide dimer and trimer species. For liquid Ar, Kr, and Xe hosts, these emissions display a systematic trend of wavelength shifts and state lifetimes. Due to their strong dipolar character, these excimers radiate in the liquid at wavelengths which are considerably red shifted1 with respect to the gas phase values, interaction. In the case of XeF@f|, stimulated emission has been observed on the B1/2-->X1/2 transition at 404 nm. Energies of ~110 ?J have been measured in 5 nsec pulses.

Jara, H.; Shahidi, M.; Pummer, H.; Egger, H.; Rhodes, C. K.

1986-08-01

263

Quantum Hall signatures of dipolar Mahan excitons  

Science.gov (United States)

We explore the photoluminescence of spatially indirect, dipolar Mahan excitons in a gated double quantum well diode containing a mesoscopic electrostatic trap for neutral dipolar excitons at low temperatures down to 250 mK and in quantizing magnetic fields. Mahan excitons in the surrounding of the trap, consisting of individual holes interacting with a degenerate two-dimensional electron system confined in one of the quantum wells, exhibit strong quantum Hall signatures at integer filling factors and related anomalies around filling factor ?=(2)/(3),(3)/(5), and (1)/(2), reflecting the formation of composite fermions. Interactions across the trap perimeter are found to influence the energy of the confined neutral dipolar excitons by the presence of the quantum Hall effects in the two-dimensional electron system surrounding the trap.

Schinner, G. J.; Repp, J.; Kowalik-Seidl, K.; Schubert, E.; Stallhofer, M. P.; Rai, A. K.; Reuter, D.; Wieck, A. D.; Govorov, A. O.; Holleitner, A. W.; Kotthaus, J. P.

2013-01-01

264

Giant Rydberg Excitons in Cuprous Oxide  

CERN Document Server

Highly excited atoms with an electron moved into a level with large principal quantum number are fascinating hydrogen-like objects. The giant extension of these Rydberg atoms leads to huge interaction effects. Monitoring these interactions has provided novel insights into molecular and condensed matter physics problems on a single quantum level. Excitons, the fundamental optical excitations in semiconductors consisting of a negatively charged electron and a positively charged hole, are the condensed matter analogues of hydrogen. Highly excited excitons with extensions similar to Rydberg atoms are attractive because they may be placed and moved in a crystal with high precision using microscopic potential landscapes. Their interaction may allow formation of ordered exciton phases or sensing of elementary excitations in the surrounding, also on a quantum level. Here we demonstrate the existence of Rydberg excitons in cuprous oxide, Cu2O, with principal quantum numbers as large as n=25 . These states have giant w...

Kazimierczuk, Tomasz; Scheel, Stefan; Stolz, Heinrich; Bayer, Manfred

2014-01-01

265

Excitonic spectra of orientationally disordered molecular aggregates  

CERN Multimedia

We study the linear optical response of Frenkel exciton in a disordered molecular chain. The coherent-potential approximation (CPA) is applied to a situation when both single-molecule excitation energies and transition dipole moments are affected by disorder. The scheme allows to calculate the exciton density of states and linear absorption spectra, as well as the exciton coherence length that reveals itself in the linear optics. We present a detailed analysis of the low-disorder limit of the theory. In particular, the asymptotic formulas relating the absorption linewidth and the exciton coherence length in the resonance region to the disorder strength are derived. Accounting simultaneously for both types of disorder they recover the well-established form when no disorder in the transition dipoles is present. The theory is shown to agree well with the exact numerical diagonalization for arbitrary strength of orientational disorder.

Balagurov, D B; Agranovich, V M

2003-01-01

266

Exciton in closed and opened quantum dot  

Directory of Open Access Journals (Sweden)

Full Text Available The theory of exciton spectrum in spherically symmetric states for the three- shell closed spherical quantum dot is proposed. The evolution of the exciton spectrum while varying the outer well thickness from zero (stationary spectrum of single closed spherical quantum dot to infinity (quasistationary spectrum of a single open spherical quantum dot is investigated. The mechanism of damping (semiwidth of quasistationary states due to the redistribution over the energy levels of probability of exciton location in the space of two inner shells of nanosystem is studied. It is shown that the three shell closed spherical quantum dot of a rather big thickness of the outer well quite sufficiently and exactly reflects the basic properties of the quasistationary exciton spectrum in a single open spherical quantum dot.

M.V.Tkach

2007-01-01

267

Exciton Hierarchies in Gapped Carbon Nanotubes  

Energy Technology Data Exchange (ETDEWEB)

We present evidence that the strong electron-electron (e-e) interactions in gapped carbon nanotubes lead to finite hierarchies of excitons within a given nanotube subband. We study these hierarchies by employing a field theoretic reduction of the gapped carbon nanotube permitting e-e interactions to be treated exactly. We analyze this reduction by employing a Wilsonian-like numerical renormalization group. We are so able to determine the gap ratios of the one-photon excitons as a function of the effective strength of interactions. We also determine within the same subband the gaps of the two-photon excitons, the single particle gaps, as well as a subset of the dark excitons. The strong e-e interactions in addition lead to strongly renormalized dispersion relations where the consequences of spin-charge separation can be readily observed.

Konik, R.M.

2011-04-01

268

Exciton dissociation in solid-state cathodoluminescence  

Energy Technology Data Exchange (ETDEWEB)

In the solid-state cathodoluminescence (SSCL), the spectrum is characterized by the appearance of short wavelength emission (405 nm). By comparing the short wavelength and long wavelength emission (580 nm) peaks at varies electric fields, it can be concluded that the exciton dissociation is the reason for the systematic variation of the spectrum. In this work, the exciton dissociation is investigated by using the combination of experimental and theoretical method. The following results can be obtained: the theoretical model is reasonable, and can account for the observations; the electric field is the main factor of exciton dissociation under lower electric field, with increasing electric field, both electric field and direct impact can result in exciton dissociation.

Huang Jinzhao, E-mail: jzhuangjz@hotmail.co [School of Science, University of Jinan, Jinan 250022, Shandong Province (China); Xu Zheng; Zhao Suling [Key Laboratory of Luminescence and Optical Information (Beijing Jiaotong University), Ministry of Education, Beijing 100044 (China); Zhang Zhong; Wang Peiji; Zheng Zhongshan [School of Science, University of Jinan, Jinan 250022, Shandong Province (China)

2009-05-01

269

Exciton spectra of orthorhombic indium bromide  

International Nuclear Information System (INIS)

Absorption and reflection spectra of InBr single crystal are measured at 2 K with polarized light. Dichroic exciton peaks are observed in the reflection spectrum around 2.33 eV. Binding energy of the direct exciton for E parallel c is estimated as (12 +- 1) meV. Indirect absorption edge is confirmed for E parallel c by using a wavelength modulation technique. The indirect exciton gap is determined to be 2.129 eV. Absorption components associated with five different phonon energies are resolved, one of which is presumably due to a two-phonon process. The binding energy of the indirect exciton is estimated to be (6.9 +- 0.4) meV. The indirect gap increases rapidly with increasing temperature. (author)

1982-02-01

270

Polarization-dependent photoluminescence-excitation spectra of one-dimensional exciton and continuum states in T-shaped quantum wires  

CERN Document Server

We measured polarization-dependent photoluminescence-excitation spectra of highly uniform T-shaped quantum wires at 5 K. We attribute one peak to the 1D-exciton ground state and the continuous absorption band to 1D continuum states. These had similar polarization-dependences. We also observed some other peaks above the 1D-exciton ground state and attribute them to exciton states consisting of excited hole subbands. These results show good agreement with a model calculation of a single electron-hole pair in T-shaped geometry with exact diagonalizations of the Coulomb interaction.

Itoh, H; Yoshita, M; Akiyama, H; Pfeiffer, L N; West, K W; Szymanska, M H; Littlewood, P B; Itoh, Hirotake; Hayamizu, Yuhei; Yoshita, Masahiro; Akiyama, Hidefumi; Pfeiffer, Loren N.; West, Ken W.; Szymanska, Marzena H.; Littlewood, Peter B.

2003-01-01

271

Hydrostatic pressure and growth-direction magnetic field effects on the exciton states in coupled GaAs-(Ga, Al)As quantum wells  

International Nuclear Information System (INIS)

The effects of hydrostatic pressure and growth-direction applied magnetic fields on the exciton dispersion and in-plane effective mass in coupled GaAs-(Ga, Al)As quantum wells are investigated. Calculations for spatially direct and indirect excitons were performed within the variational procedure in the effective-mass and nondegenerate parabolic band approximations and by taking into account the coupling between the exciton centre-of-mass momentum and its internal structure. The pressure coefficient is also obtained as a function of both the hydrostatic pressure and growth-direction applied magnetic field

2007-06-27

272

Features of the formation kinetics of excitonic complexes in the quantum wells of undoped GaAs/AlGaAs structures under above-carrier excitation  

International Nuclear Information System (INIS)

The spectra of the low-temperature photoluminescence and photoluminescence excitation (PLE) of the GaAs/AlGaAs (x = 0.05) structures with small quantum wells (QW) are studied. It is shown that the PLE spectra of free excitons and exciton complexes (trions) consist of wide bands, which alternate in the antiphase, the increase in the complex intensity is accompanied by decrease in the intensity of the excitons irradiation. The PLE spectra type is connected with peculiarities of the kinetics of capture of the over barrier electrons and holes in the QW and its dependence on the initial energy of the photoexcited electron-hole pairs

2002-02-01

273

Förster Coupling in Nanoparticle Excitonic Circuits  

Digital Repository Infrastructure Vision for European Research (DRIVER)

Exciton transport in semiconductor nanoparticles underlies recent experiments on electrically controlled nanostructures and proposals for new artificial light-harvesting systems. In this work, we develop a novel method for the numerical evaluation of the Förster matrix element, based on a three-dimensional real space grid and the self-consistent solution of the mesoscopic exciton in a macroscopic dielectric environment. This method enables the study of the role of the nanoparticle shape, spa...

Rebentrost, Patrick; Stopa, Michael; Aspuru-guzik, Ala?n

2010-01-01

274

Trapping of Cold Excitons with Laser Light  

CERN Document Server

Optical trapping and manipulation of neutral particles has led to a variety of experiments from stretching DNA-molecules to trapping and cooling of neutral atoms. An exciting recent outgrowth of the technique is an experimental implementation of atom Bose-Einstein condensation. In this paper, we propose and demonstrate laser induced trapping for a new system--a gas of excitons in quantum well structures. We report on the trapping of a highly degenerate Bose gas of excitons in laser induced traps.

Hammack, A T; Butov, L V; Smallwood, L E; Ivanov, A L; Gossard, A C

2006-01-01

275

Direct observation of C60 exciton  

International Nuclear Information System (INIS)

Photoexcitation of pure C60 thin films by picosecond absorption spectroscopy reveals a transient species with absorption maxima at 540 nm and 660 nm which is assigned to the exciton. Considering the dynamic behaviour of this species and luminescence properties of solid C60, it is concluded that a singlet exciton is observed both in its free (?1/21/2=250 ns) forms. (orig.)

1994-03-01

276

Probing the excitonic emission of ZnO nanoparticles using UV-VUV excitations  

Energy Technology Data Exchange (ETDEWEB)

This study deals with the influence of the excitation (UV-lamp, UV-laser and VUV synchrotron radiation) on the 3.31 eV band of ZnO microcrystals and of variously treated nanoparticles. The nanoparticles are synthesized in ultra high vacuum condition and their stoichiometry and crystallinity can be controlled. This provides an efficient way to probe the influence of these factors on the excitonic emission. The energy and intensity of the excitation have a strong influence on the excitonic luminescence and particularly on the 3.31 eV emission band. The result of these experiments are used to probe the origins of this band which is found to be not linked to any surface phenomena. Indeed, the only way to fully explain our results is to consider that the 3.31 eV band involve the superposition of two emissions features: the first due to acceptor defects and the other originates form the LO phononic repliqua of the free exciton.

Tainoff, D., E-mail: dimitri.tainoff@lpmcn.univ-lyon1.f [LPMCN Universite de Lyon, Lyon, F-69003 (France); Universite Lyon 1, CNRS, UMR5586, Villeurbanne F-69622 (France); Masenelli, B.; Melinon, P. [LPMCN Universite de Lyon, Lyon, F-69003 (France); Universite Lyon 1, CNRS, UMR5586, Villeurbanne F-69622 (France); Belsky, A.; Ledoux, G.; Amans, D.; Dujardin, C. [LPCML Universite de Lyon, Lyon, F-69003 (France); Universite Lyon 1, CNRS, UMR5620, Villeurbanne F-69622 (France); Fedorov, N.; Martin, P. [CELIA, Universite Bordeaux 1, CNRS, CEA, 351, Cours de la Liberation 33405 Talence cedex (France)

2009-12-15

277

Physical theory of excitons in conducting polymers.  

Science.gov (United States)

In this tutorial review, we cover the solid state physics approach to electronic and optical properties of conducting polymers. We attempt to bring together languages and advantages of the solid state theory for polymers and of the quantum chemistry for monomers. We consider polymers as generic one-dimensional semiconductors with features of strongly correlated electronic systems. Our model combines the long range electron-hole Coulomb attraction with a specific effect of strong intra-monomer electronic correlations, which results in effective intra-monomer electron-hole repulsion. Our approach allows to go beyond the single-chain picture and to compare excitons for polymers in solutions and in films. The approach helps connecting such different questions as shallow singlet and deep triplet excitons, stronger binding of interchain excitons in films, crossings of excitons' branches, 1/N energies shifts in oligomers. We describe a strong suppression of the luminescence from free charge carriers by long-range Coulomb interactions. Main attention is devoted to the most requested in applications phenyl based polymers. The specifics of the benzene ring monomer give rise to existence of three possible types of excitons: Wannier-Mott, Frenkel and intermediate ones. We discuss experimental manifestations of various excitons and of their transformations. We touch effects of the time-resolved self-trapping by libron modes leading to formation of torsion polarons. PMID:20517580

Brazovskii, Serguei; Kirova, Natasha

2010-07-01

278

Diffusion of a Plasmon-Exciton Polaron  

Science.gov (United States)

We consider the motion of an exciton constrained to a quasi-one-dimensional geometry in the vicinity of a metal interface. For weak coupling, the metal causes damping of the center of mass motion, leading to a decrease in the exciton diffusion constant. This can be modeled as non-contact dielectric friction between an oscillating dipole and a substrate, where the frictional force is related to the response of the metal through the fluctuation dissipation theorem [1]. When the exciton frequency is in the neighborhood of the plasma resonance, the interaction can no longer be described by linear response theory, for the exciton and plasmon form a quasiparticle, an exciton-plasmon polaron. We calculate the transmission and reflection coefficients for the exciton-plasmon polaron in the neighborhood of a metal interface, as well as the diffusion rate and radiative lifetime versus coupling strength.[4pt] [1] Seppe Kuehn, John A. Marohn, and Roger F. Loring, 110(30) J. Phys. Chem. B, (2006) 1425

Cherqui, Charles; Dunlap, David; Piryatinski, Andrei

2012-02-01

279

Excitonic nonlinearities in single-wall carbon nanotubes  

Energy Technology Data Exchange (ETDEWEB)

Excitons are composite bosons that allow a fair description of the optical properties in solid state systems. The quantum confinement in nanostructures enhances the excitonic effects and impacts the exciton-exciton interactions, which tailor the performances of classical and quantum optoelectronic devices, such as lasers or single-photon emitters. The excitonic nonlinearities exhibit significant differences between organic and inorganic compounds. Tightly bound Frenkel excitons in molecular crystals are for instance affected by an efficient exciton-exciton annihilation (EEA). This Auger process also governs the population relaxation dynamics in carbon nanotubes that share many physical properties with organic materials. Here, we show that this similarity breaks down for the excitonic decoherence in carbon nanotubes. Original nonlinear spectral-hole burning experiments bring evidence of pure dephasing induced by exciton-exciton scattering (EES) in the k-space. This mechanism controls the exciton collision-induced broadening, as for Wannier excitons in inorganic semiconductors. We demonstrate that this singular behavior originates from the intrinsic one-dimensionality of excitons in carbon nanotubes, which display unique hybrid features of organic and inorganic systems. (Copyright copyright 2012 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim)

Nguyen, D.T.; Voisin, C.; Roussignol, P. [Laboratoire Pierre Aigrain, Ecole Normale Superieure, UPMC, Universite Paris Diderot, CNRS UMR8551, Paris (France); Roquelet, C.; Lauret, J.S. [Laboratoire de Photonique Quantique et Moleculaire de l' Ecole Normale Superieure de Cachan (France); Cassabois, G. [Laboratoire Pierre Aigrain, Ecole Normale Superieure, UPMC, Universite Paris Diderot, CNRS UMR8551, Paris (France); Laboratoire Charles Coulomb, UMR5221, Universite Montpellier 2, Montpellier (France); CNRS, Laboratoire Charles Coulomb, UMR5221, Montpellier (France)

2012-05-15

280

Halide electroadsorption on single crystal surfaces  

Energy Technology Data Exchange (ETDEWEB)

The structure and phase behavior of halides have been investigated on single crystals of Ag and Au using synchrotron x-ray scattering techniques. The adlayer coverages are potential dependent. For all halides studied the authors found that with increasing potential, at a critical potential, a disordered adlayer transforms into an ordered structure. Often these ordered phases are incommensurate and exhibit potential-dependent lateral separations (electrocompression). The authors have analyzed the electrocompression in terms of a model which includes lateral interactions and partial charge. A continuous compression is not observed for Br on Ag(100). Rather, they find that the adsorption is site-specific (lattice gas) in both the ordered and disordered phases. The coverage increases with increasing potential and at a critical potential the disordered phase transforms to a well-ordered commensurate structure.

Ocko, B.M. [Brookhaven National Lab., Upton, NY (United States). Dept. of Physics; Wandlowski, T. [Univ. of Ulm (Germany). Dept. of Electrochemistry

1997-07-01

 
 
 
 
281

Lanthanide-halide based humidity indicators  

Energy Technology Data Exchange (ETDEWEB)

The present invention discloses a lanthanide-halide based humidity indicator and method of producing such indicator. The color of the present invention indicates the humidity of an atmosphere to which it is exposed. For example, impregnating an adsorbent support such as silica gel with an aqueous solution of the europium-containing reagent solution described herein, and dehydrating the support to dryness forms a substance with a yellow color. When this substance is exposed to a humid atmosphere the water vapor from the air is adsorbed into the coating on the pore surface of the silica gel. As the water content of the coating increases, the visual color of the coated silica gel changes from yellow to white. The color change is due to the water combining with the lanthanide-halide complex on the pores of the gel.

Beitz, James V. (Hinsdale, IL); Williams, Clayton W. (Chicago, IL)

2008-01-01

282

Electron localization in alkali-halide clusters  

International Nuclear Information System (INIS)

The quantum path-integral molecular-dynamics method was applied to explore the structure, energetics, and dynamics of an excess electron interacting with an alkali-halide cluster. Four distinct modes of electron localization were established, which depend on the cluster composition, size, and structure; they involve an internal F-defect, an external surface state, dissociative detachment of an alkali atom, and structural isometrization induced by electron attachment

1985-04-22

283

Analysis of the isotope effect of high-Tc ceramic YBa2-xLaxCu3O7 using the excitonic-enhancement model  

Science.gov (United States)

The excitonic-enhancement model (EEM) is based on the simultaneous excitonic and ``generalized'' Cooper-pair condensation of a charge-unbalanced system that is described by a two-band model consisting of a valence and a conduction band separated by a band gap G. A detailed numerical analysis of the consequences of the EEM with respect to shift of Tc due to the oxygen isotope effect on the La-doped Y-Ba-Cu-O system is carried out. The results are found to be in excellent agreement with the recent experimental data of Bornemann and Morris, covering a Tc range of 38.3-92.3 K.

Wong, K. W.; Fung, P. C. W.; Yeung, H. Y.; Kwok, W. Y.

1992-06-01

284

Computational screening of mixed metal halide ammines  

DEFF Research Database (Denmark)

Metal halide ammines, e.g. Mg(NH3)6Cl2 and Sr(NH3)8Cl2, can reversibly store ammonia, with high volumetric hydrogen storage capacities. The storage in the halide ammines is very safe, and the salts are therefore highly relevant as a carbon-free energy carrier in future transportation infrastructure. In this project we are searching for improved mixed materials with optimal desorption temperatures and kinetics, optimally releasing all ammonia in one step. We apply Density Functional Theory, DFT, calculations on mixed compounds selected by a Genetic Algorithm (GA), relying on biological principles of natural selection. The GA is evolving from an initial (random) population and selecting those with highest fitness, a function based on e.g. stability, release temperature and storage capacity. The search space includes all alkaline, alkaline earth, 3d and 4d metals and the four lightest halides. In total the search spaces consists of millions combinations, which makes a GA ideal, to reduce the number of necessarycalculations. We are screening for a one step release from either a hexa or octa ammine, and we have found promising candidates, which will be further investigated ? both computationally and experimentally.

Jensen, Peter Bjerre; Lysgaard, Steen

285

Complexes in polyvalent metal - Alkali halide melts  

International Nuclear Information System (INIS)

Experimental evidence is available in the literature on the local coordination of divalent and trivalent metal ions by halogens in some 140 liquid mixtures of their halides with alkali halides. After brief reference to classification criteria for main types of local coordination, we focus on statistical mechanical models that we are developing for Al-alkali halide mixtures. Specifically, we discuss theoretically the equilibrium between (AlF6)3- and (AlF4)- complexes in mixtures of AlF3 and NaF as a function of composition in the NaF-rich region, the effect of the alkali counterion on this equilibrium, the possible role of (AlF5)2- as an intermediate species in molten cryolite, and the origin of the different complexing behaviours of Al-alkali fluorides and chlorides. We also present a theoretical scenario for processes of structure breaking and electron localization in molten cryolite under addition of sodium metal. (author). 26 refs, 2 tabs

1991-01-01

286

Photoexcitation, photoionisation and photofragmentation of molecular rubidium halides  

Energy Technology Data Exchange (ETDEWEB)

Photoionisation and photofragmentation of molecular rubidium halides following photoexcitation in the vacuum ultraviolet region are investigated. For this purpose total and partial ion yield spectra are measured from the vapours of RbF, RbCl, RbBr and RbI. Two different decay channels - spectator and participator type - are studied in different rubidium halide molecules. It is demonstrated that the opening of the spectator channel takes place at different excitation energies, depending on the halide atom in the molecule.

Kisand, V. [Institute of Physics, University of Tartu, Riia 142, 51014 Tartu (Estonia); Nommiste, E. [Institute of Physics, University of Tartu, Riia 142, 51014 Tartu (Estonia)]. E-mail: ergo@fi.tartu.ee; Kukk, E. [Department of Physical Sciences, University of Oulu, P.O. Box 3000, FIN-90014 Oulu (Finland); Calo, A. [Department of Physical Sciences, University of Oulu, P.O. Box 3000, FIN-90014 Oulu (Finland); Aksela, H. [Department of Physical Sciences, University of Oulu, P.O. Box 3000, FIN-90014 Oulu (Finland); Aksela, S. [Department of Physical Sciences, University of Oulu, P.O. Box 3000, FIN-90014 Oulu (Finland)

2005-06-15

287

Process and composition for drying of gaseous hydrogen halides  

Energy Technology Data Exchange (ETDEWEB)

A process for drying a gaseous hydrogen halide of the formula HX, wherein X is selected from the group consisting of bromine, chlorine, fluorine, and iodine, to remove water impurity therefrom, comprising: contacting the water impurity-containing gaseous hydrogen halide with a scavenger including a support having associated therewith one or more members of the group consisting of: (a) an active scavenging moiety selected from one or more members of the group consisting of: (i) metal halide compounds dispersed in the support, of the formula MX.sub.y ; and (ii) metal halide pendant functional groups of the formula -MX.sub.y-1 covalently bonded to the support, wherein M is a y-valent metal, and y is an integer whose value is from 1 to 3; (b) corresponding partially or fully alkylated compounds and/or pendant functional groups, of the metal halide compounds and/or pendant functional groups of (a); wherein the alkylated compounds and/or pendant functional groups, when present, are reactive with the gaseous hydrogen halide to form the corresponding halide compounds and/or pendant functional groups of (a); and M being selected such that the heat of formation, .DELTA.H.sub.f of its hydrated halide, MX.sub.y.(H.sub.2 O).sub.n, is governed by the relationship: .DELTA.H.sub.f .gtoreq.n.times.10.1 kilocalories/mole of such hydrated halide compound wherein n is the number of water molecules bound to the metal halide in the metal halide hydrate. Also disclosed is an appertaining scavenger composition and a contacting apparatus wherein the scavenger is deployed in a bed for contacting with the water impurity-containing gaseous hydrogen halide.

Tom, Glenn M. (New Milford, CT); Brown, Duncan W. (Wilton, CT)

1989-08-01

288

Exciton-polariton emission from organic semiconductor optical waveguides  

CERN Document Server

We photo-excite slab polymer waveguides doped with J-aggregating dye molecules and measure the leaky emission from strongly coupled waveguide exciton polariton modes at room temperature. We show that the momentum of the waveguide exciton polaritons can be controlled by modifying the thickness of the excitonic waveguide. Non-resonantly pumped excitons in the slab excitonic waveguide decay into transverse electric and transverse magnetic strongly coupled exciton waveguide modes with radial symmetry. These leak to cones of light with radial and azimuthal polarizations.

Ellenbogen, Tal

2011-01-01

289

Moving zero-gap Wannier-Mott excitons in graphene  

Science.gov (United States)

We demonstrate the possibility of existence of indirect moving Wannier-Mott excitons in graphene. Electron-hole binding is conditioned by the trigonal warping of the conic energy spectrum. The binding energies are found for the lowest exciton states. These energies essentially depend on the value and direction of exciton momentum and vanish when the exciton momentum tends to the conic points. The ways to observe the exciton states are discussed. The opportunity of experimental observation of zero-gap excitons by means of external electron scattering is examined.

Mahmoodian, M. M.; Entin, M. V.

2013-05-01

290

Polarized interacting exciton gas in quantum wells and bulk semiconductors  

CERN Multimedia

We develop a theory to calculate exciton binding energies of both two- and three-dimensional spin polarized exciton gases within a mean field approach. Our method allows the analysis of recent experiments showing the importance of the polarization and intensity of the excitation light on the exciton luminescence of GaAs quantum wells. We study the breaking of the spin degeneracy observed at high exciton density (5 \\ \\ 10^{10} cm ^2). Energy level splitting betwen spin +1 and spin -1 is shown to be due to many-body inter-excitonic exchange while the spin relaxation time is controlled by intra-exciton exchange.

Fernández-Rossier, J; Muñoz, L; Viña, L

1995-01-01

291

10 CFR 431.322 - Definitions concerning metal halide lamp ballasts and fixtures.  

Science.gov (United States)

...concerning metal halide lamp ballasts and fixtures. 431.322 Section 431.322...EQUIPMENT Metal Halide Lamp Ballasts and Fixtures § 431.322 Definitions concerning metal halide lamp ballasts and fixtures. Ballast efficiency...

2010-01-01

292

75 FR 5544 - Energy Conservation Program: Energy Conservation Standards for Metal Halide Lamp Fixtures: Public...  

Science.gov (United States)

...Conservation Standards for Metal Halide Lamp Fixtures: Public Meeting and Availability of...standards for certain metal halide lamp fixtures. This document announces that the period...conservation standards for metal halide lamp fixtures and provide docket number...

2010-02-03

293

Theoretical study on exciton dynamics in dendritic systems: exciton recurrence and migration.  

Science.gov (United States)

The optical functionalities such as exciton recurrence and migration for dendritic systems, e.g., dendrimers, are investigated using the quantum master equation (QME) approach based on the ab initio molecular orbital configuration interaction (MOCI) method, which can treat both the coherent and incoherent exciton dynamics at the first principle level. Two types of phenylacetylene dendrimers, Cayley-tree dendrimer and nanostar dendrimer with anthracene core, are examined to elucidate the features of excion recurrence and migration motions in relation to their structural dependences. It is found that the nanostar dendrimer exhibits faster exciton migration from the periphery to the core than Cayley-tree dendrimer, which alternatively exhibits exciton recurrence motion among dendron parts in case of small relaxation parameters. Such strong structural dependence of exciton dynamics demonstrates the advantage of dendritic molecular systems for future applications in nano-optical and light-harvesting devices. PMID:19783953

Nakano, Masayoshi; Kishi, Ryohei; Minami, Takuya; Yoneda, Kyohei

2009-01-01

294

Theoretical Study on Exciton Dynamics in Dendritic Systems: Exciton Recurrence and Migration  

Directory of Open Access Journals (Sweden)

Full Text Available The optical functionalities such as exciton recurrence and migration for dendritic systems, e.g., dendrimers, are investigated using the quantum master equation (QME approach based on the ab initio molecular orbital configuration interaction (MOCI method, which can treat both the coherent and incoherent exciton dynamics at the first principle level. Two types of phenylacetylene dendrimers, Cayley-tree dendrimer and nanostar dendrimer with anthracene core, are examined to elucidate the features of excion recurrence and migration motions in relation to their structural dependences. It is found that the nanostar dendrimer exhibits faster exciton migration from the periphery to the core than Cayley-tree dendrimer, which alternatively exhibits exciton recurrence motion among dendron parts in case of small relaxation parameters. Such strong structural dependence of exciton dynamics demonstrates the advantage of dendritic molecular systems for future applications in nano-optical and light-harvesting devices.

Kyohei Yoneda

2009-09-01

295

Strain induced exciton fine-structure splitting and shift in bent ZnO microwires  

Science.gov (United States)

Lattice strain is a useful and economic way to tune the device performance and is commonly present in nanostructures. Here, we investigated for the first time the exciton spectra evolution in bent ZnO microwires along the radial direction via high spatial/energy resolution cathodeluminescence spectroscopy at 5.5?K. Our experiments show that the exciton peak splits into multi fine peaks towards the compressive part while retains one peak in the tensile part and the emission peak displays a continuous blue-shift from tensile to compressive edges. In combination with first-principles calculations, we show that the observed NBE emission splitting is due to the valence band splitting and the absence of peak splitting in the tensile part maybe due to the highly localized holes in the A band and the carrier density distribution across the microwire. Our studies may pave the way to design nanophotonic and electronic devices using bent ZnO nanowires.

Liao, Zhi-Min; Wu, Han-Chun; Fu, Qiang; Fu, Xuewen; Zhu, Xinli; Xu, Jun; Shvets, Igor V.; Zhang, Zhuhua; Guo, Wanlin; Leprince-Wang, Yamin; Zhao, Qing; Wu, Xiaosong; Yu, Da-Peng

2012-01-01

296

Exciton effects in the index of refraction of multiple quantum wells and superlattices  

Science.gov (United States)

Theoretical calculations of the index of refraction of multiple quantum wells and superlattices are presented. The model incorporates both the bound and continuum exciton contributions for the gamma region transitions. In addition, the electronic band structure model has both superlattice and bulk alloy properties. The results indicate that large light-hole masses, i.e., of about 0.23, produced by band mixing effects, are required to account for the experimental data. Furthermore, it is shown that superlattice effects rapidly decrease for energies greater than the confining potential barriers. Overall, the theoretical results are in very good agreement with the experimental data and show the importance of including exciton effects in the index of refraction.

Kahen, K. B.; Leburton, J. P.

1986-01-01

297

Evidence for an excitonic insulator phase in 1T-TiSe2.  

Science.gov (United States)

We present a new high-resolution angle-resolved photoemission study of 1T-TiSe2 in both its room-temperature, normal phase and its low-temperature, charge-density wave phase. At low temperature the photoemission spectra are strongly modified, with large band renormalizations at high-symmetry points of the Brillouin zone and a very large transfer of spectral weight to backfolded bands. A calculation of the theoretical spectral function for an excitonic insulator phase reproduces the experimental features with very good agreement. This gives strong evidence in favor of the excitonic insulator scenario as a driving force for the charge-density wave transition in 1T-TiSe2. PMID:17930692

Cercellier, H; Monney, C; Clerc, F; Battaglia, C; Despont, L; Garnier, M G; Beck, H; Aebi, P; Patthey, L; Berger, H; Forró, L

2007-10-01

298

The Franz-Keldysh effect revisited: Electroabsorption including interband coupling and excitonic effects  

CERN Document Server

We study the linear optical absorption of bulk semiconductors in the presence of a homogeneous constant (dc) electric field with an approach suitable for including excitonic effects while working with many-band models. The absorption coefficient is calculated from the time evolution of the interband polarization excited by an optical pulse. We apply the formalism to a numerical calculation for GaAs using a 14-band $\\textbf{k} \\cdot \\textbf{p}$ model, which allows us to properly include interband coupling, and the exchange self-energy to account for the excitonic effects due to the electron-hole interaction. The Coulomb interaction enhances the features of the absorption coefficient captured by the $\\textbf{k} \\cdot \\textbf{p}$ model. We consider the dependence of the enhancement on the strength of the dc field and the polarization of the optical field.

Duque-Gomez, Federico

2014-01-01

299

Unexpectedly slow two particle decay of ultra-dense excitons in cuprous oxide  

Science.gov (United States)

For an ultra-dense exciton gas in cuprous oxide (Cu2O), exciton-exciton interactions are the dominant cause of exciton decay. This study demonstrates that the accepted Auger recombination model overestimates the exciton decay rate following intense two photon excitation. Two exciton decay is relevant to the search for collective quantum behavior of excitons in bulk systems. These results suggest the existence of a new high density regime of exciton behavior.

Laszlo Frazer, N.; Schaller, Richard D.; Ketterson, J. B.

2013-09-01

300

Unexpectedly Slow Two Particle Decay of Ultra-Dense Excitons in Cuprous Oxide  

CERN Document Server

For an ultra-dense exciton gas in cuprous oxide (Cu$_2$O), exciton-exciton interactions are the dominant cause of exciton decay. This study demonstrates that the accepted Auger recombination model overestimates the exciton decay rate following intense two photon excitation. Two exciton decay is relevant to the search for collective quantum behavior of excitons in bulk systems. These results suggest the existence of a new high density regime of exciton behavior.

Frazer, Laszlo; Ketterson, J B

2013-01-01

 
 
 
 
301

Interband electronic Raman scattering of polarized light in cubic semiconductors with degenerate valence bands  

International Nuclear Information System (INIS)

The cross-sections are calculated for the Raman scattering processes with polarized light in cubic semiconductors where the valence band top is fourfold degenerate ?8. The exciton effect is taken into account. (author). 17 refs

1987-01-01

302

Lead Halide Perovskites and Other Metal Halide Complexes As Inorganic Capping Ligands for Colloidal Nanocrystals  

Digital Repository Infrastructure Vision for European Research (DRIVER)

Lead halide perovskites (CH3NH3PbX3, where X = I, Br) and other metal halide complexes (MX, where M = Pb, Cd, In, Zn, Fe, Bi, Sb) have been studied as inorganic capping ligands for colloidal nanocrystals. We present the methodology for the surface functionalization via ligand-exchange reactions and the effect on the optical properties of IV-VI, II-VI, and III-V semiconductor nanocrystals. In particular, we show that the Lewis acid base properties of the solvents, in addition to the solvent di...

2014-01-01

303

Luminescence due to radiative transitions between valence band and upper core band in ionic crystals (crossluminescence)  

International Nuclear Information System (INIS)

In some wide band gap halides of heavy alkali and alkaline-earth metals (BaF2, CsCl, RbF etc.) the energy separation between the valence band involving mainly the halogen electronic states and the upper cation core band is smaller than the band gap. Under the excitation generating holes in the core band radiative electronic transitions from the valence band to the core band take place giving rise to a specific kind of intrinsic emission which is proposed to call crossluminescence. Its decay time is about 1 ns and the thermal quenching starts at temperatures well above room temperature. The width of the emission spectrum practically coincides with the width of the valence band deduced from ultraviolet photoelectron spectroscopy and the shape of the spectrum depicts the electronic structure of the valence band. (author)

1987-12-01

304

Exciton Energy Transfer in Pairs of Single-Walled Carbon Nanotubes  

Digital Repository Infrastructure Vision for European Research (DRIVER)

We studied the exciton energy transfer in pairs of semiconducting nanotubes using high-resolution optical microscopy and spectroscopy on the nanoscale. Photoluminescence from large band gap nanotubes within bundles is observed with spatially varying intensities due to distance-dependent internanotube transfer. The range of efficient energy transfer is found to be limited to a few nanometers because of competing fast nonradiative relaxation responsible for low photoluminescence quantum yield.

Qian, Huihong; Georgi, Carsten; Anderson, Neil; Green, Alexander A.; Hersam, Mark C.; Novotny, Lukas; Hartschuh, Achim

2008-01-01

305

A size dependent discontinuous decay rate for the exciton emission in ZnO quantum dots.  

Science.gov (United States)

The time resolved UV-fluorescence in ZnO quantum dots has been investigated using femtosecond laser spectroscopy. The measurements were performed as a function of particle size for particles between 3 and 7 nm in diameter, which are in the quantum confined regime. A red shift in the fluorescence maximum is seen while increasing the particle size, which correlates with the shift in band gap due to quantum confinement. The energy difference between the UV-fluorescence and the band gap does, however, increase for the smaller particles. For 3.7 nm particles the fluorescence energy is 100 meV smaller than the band gap energy, whereas it is only 20 meV smaller for the largest particles. This indicates a stabilization of the excitons in the smallest particles. The lifetime of the UV fluorescence is in the picosecond time scale and interestingly, it is discontinuous with respect to particle size. For the smallest particles, the exciton emission life time reaches 30 ps, which is three times longer than that for the largest particles. This demonstrates a transition between two different mechanisms for the UV-fluorescence. We suggest that this is an effect of surface trapping and stabilization of the excitons occurring in the smallest particles but not in the larger ones. We also discuss the time scale limit for slowed hot carrier dynamics in ensembles of quantum confined ZnO particles. PMID:24658340

Jacobsson, T Jesper; Viarbitskaya, Sviatlana; Mukhtar, Emad; Edvinsson, Tomas

2014-07-21

306

Structure of luminescence spectra of UO_2"+"2 impurity ions in halide salt solid solutions  

International Nuclear Information System (INIS)

Luminescence spectra of impurity polyatomic ions of UO"2_2"+ in water and aqueous solutions of alkali-halide salts are studied in the 1.62-77 K temperature range. The emission spectra of these solutions at 77 K are characterized by a pronounced vibrational structure specified by the interaction of an electron transition with an impurity ion intramolecular vibration #betta#"1. At 1.62-4.2 K individual vibrational bands are splitted to phononless lines and structural phonon wings (FW). The PW structure is defined by an excitation of crystalline vibrations of a solution solid matrix. For the H"2O-UO"2_2"+ solution the PW maximum frequences are shown to be comparable with the main critical points of the vibration spectrum of an orientation-disorded ice crystal. In case of aqueous solutions of alkali-halide salts with UO"2_2"+ impurity ions the PW luminescence spectra are compared with the density functions of undistorted vibrations of alkali-halide salt crystals and ice

1982-10-01

307

Pairing interaction effects in exciton level densities  

International Nuclear Information System (INIS)

Recent progress in pairing corrections for exciton state-density formulas used in pre-compound nuclear reaction theories is reviewed. These correction factors are, strictly speaking, dependent on the nuclear excitation energy U and the exciton number n. A simple formula for (U,n)-dependent pairing corrections has been derived, based on the BCS pairing equations for constant single-particle spacing, for the exciton state-density formula for one kind of Fermion. It has been shown that the constant-pairing-energy correction used in standard state-density formulas, such U0 in Gilbert and Cameron, is a limiting case of the general (U,n)-dependent results. Spin cutoff factors with pairing effects were also obtained using the same theory and parameterized into an explicit (U,n)-dependent function, thereby defining a simple exciton level-density formula for applications in quantum mechanical precompound theories. Preliminary results from extending such simple pairing-interaction representations to level-density formulas for two kinds of Fermions are summarized. The results show that the ratios in the exciton level densities in the one-Fermion and two-Fermion approaches vary with both U and n, thus likely leading to differences in calculated compound to precompound ratios. However, the differences in the spin cutoff factors in the two cases are found to be rather small. 12 refs., 3 figs

1989-11-15

308

Diamagnetic Exciton Properties in Quantum Dot Molecules  

Science.gov (United States)

The magnetic properties of nanostructures like quantum dots and rings are the subject of intense research. In particular, magnetic control of coupled quantum dots (artificial molecules) has become subject of interest. The diamagnetic shift of confined excitons complexes has been used as a measured of the wave function spatial extent in semiconductor nanostructures. In weak magnetic field, the diamagnetic shift is expected to exhibit quadratic dependence. However, for exciton complexes the diamagnetic behavior is expected to exhibit more complicated features related to electron-hole asymmetry effects on Coulomb interactions. In this work we study the magnetic response of neutral and charged excitons in InAs/GaAs asymmetric artificial molecules By using a first order perturbation approach, and within the effective mass approximation, we calculate magnetic field dependent electronic structures of confined excitons and trions in vertically coupled quantum dots. These predicted regions, which show coexistence of crossing and anticrossing exciton states, because of allowing control of charge localization and polarization of emitted photons. .

Ricardo, Fino Puerto Nelson; Hanz Yecid, Ramirez; S, Camacho Angela

2012-02-01

309

Charged excitons in the quantum Hall regime  

International Nuclear Information System (INIS)

We review our recent optical experiments on two-dimensional electron systems at temperatures below 1 K and under high magnetic fields. The two-dimensional electron systems are realized in modulation-doped GaAs/AlGaAs single quantum wells. Via gate electrodes the carrier density of the two-dimensional electron systems can be tuned in a quite broad range between about 1x1010 cm-2 and 2x1011cm-2. In dilute two-dimensional electron systems, at very low electron densities, we observe the formation of negatively charged excitons in photoluminescence experiments. In this contribution we report about the observation of a dark triplet exciton, which is observable at temperatures below 1 K and for electron filling factors < 1/3, i.e., in the fractional quantum Hall regime only. In experiments where we have interested the density of two-dimensional electron systems so that an uniform two-dimensional electron system starts to form, we have found a strong energy anomaly of the charged excitons in the vicinity of filling factor 1/3. This anomaly was found to exist in a very narrow parameter range of the density and temperature, only. We propose a model where we assume that localized charged excitons and a uniform Laughlin liquid coexist. The localized charged exciton in close proximity to the Laughlin liquid leads to the creation of the fractionally-charged quasihole in the liquid, which can account for the experimentally observed anomaly. (author)

2004-06-10

310

Controlling of exciton condensate by external fields and phonon laser  

CERN Document Server

The novel method of observation and controlling of Bose-Einstein condensation in the system of spatially and momentum-space indirect excitons in coupled quantum wells using in-plane magnetic and normal electric fields is proposed. Fields are used for exciton dispersion engeneering. In the presence of in-plane magnetic field ground state of spatially indirect exciton becomes also indirect in the momentum space. Manipulation of electric field magnitude is used for tuning to resonance of transition of "dark" long-life spatially and momentum-space indirect exciton in condensate into spatially and momentum-space direct exciton in radiative zone, with phonon creation, and with subsequent recombination of the direct exciton in radiative zone. Recombination rate of indirect excitons in condensate according to proposed scheme sharply rises at resonance. As a result besides enhanced spectral narrow photoluminescence connected with recombination of direct exciton in radiative zone with specific angular dependence, almos...

Lozovik, Yu E; Lozovik, Yu.E.

2000-01-01

311

The trion as an exciton interacting with a carrier  

CERN Document Server

The X$^-$ trion is essentially an electron bound to an exciton. However, due to the composite nature of the exciton, there is no way to write an exciton-electron interaction potential. We can overcome this difficulty by using a commutation technique similar to the one we introduced for excitons interacting with excitons, which allows to take exactly into account the close-to-boson character of the excitons. From it, we can obtain the X$^-$ trion creation operator in terms of exciton and electron. We can also derive the X$^-$ trion ladder diagram between an exciton and an electron. These are the basic tools for future works on many-body effects involving trions.

Combesccot, M

2003-01-01

312

Dependence of resonance energy transfer on exciton dimensionality.  

Science.gov (United States)

We investigate the dependence of resonance energy transfer from Wannier-Mott excitons to an organic overlayer on exciton dimensionality. We exploit the excitonic potential disorder in a single quantum well to tune the balance between localized and free excitons by scaling the Boltzmann distribution of excitons through temperature. Theoretical calculations predict the experimentally observed temperature dependence of resonance energy transfer and allow us to quantify the contribution of localized and free excitons. We show that free excitons can undergo resonance energy transfer with an order of magnitude higher rate compared to localized excitons, emphasizing the potential of hybrid optoelectronic devices utilizing resonance energy transfer as a means to overcome charge transfer related limitations. PMID:22182117

Rindermann, Jan Junis; Pozina, Galia; Monemar, Bo; Hultman, Lars; Amano, Hiroshi; Lagoudakis, Pavlos G

2011-12-01

313

Visualization of exciton transport in ordered and disordered molecular solids  

Science.gov (United States)

Transport of nanoscale energy in the form of excitons is at the core of photosynthesis and the operation of a wide range of nanostructured optoelectronic devices such as solar cells, light-emitting diodes and excitonic transistors. Of particular importance is the relationship between exciton transport and nanoscale disorder, the defining characteristic of molecular and nanostructured materials. Here we report a spatial, temporal and spectral visualization of exciton transport in molecular crystals and disordered thin films. Using tetracene as an archetype molecular crystal, the imaging reveals that exciton transport occurs by random walk diffusion, with a transition to subdiffusion as excitons become trapped. By controlling the morphology of the thin film, we show that this transition to subdiffusive transport occurs at earlier times as disorder is increased. Our findings demonstrate that the mechanism of exciton transport depends strongly on the nanoscale morphology, which has wide implications for the design of excitonic materials and devices.

Akselrod, Gleb M.; Deotare, Parag B.; Thompson, Nicholas J.; Lee, Jiye; Tisdale, William A.; Baldo, Marc A.; Menon, Vinod M.; Bulovi?, Vladimir

2014-04-01

314

Observation of Two-Exciton States in Perylene Bisimide Aggregates  

Directory of Open Access Journals (Sweden)

Full Text Available The behavior of excitons on perylene bisimide aggregates is investigated at high excitation densities by femtosecond absorption spectroscopy. Indications for a significant population in the two-exciton manifold are found.

Lochbrunner S.

2013-03-01

315

Resonance Raman spectra of metal halide vapor complexes  

International Nuclear Information System (INIS)

Resonance Raman spectra of complex vapor phase compounds formed by reacting ''acidic'' gases (A_2X_6 = Al_2Cl_6, Al_2Br_6, In_2Cl_6) with metal halides have been measured. Spectra obtained from equilibrium vapor mixtures of A_2X_6 over solid MX_2 (= PdCl_2, PdBr_2, CuCl_2, CoBr_2, TiCl_2, FeCl_2, NiCl_2, PtCl_2) were a superposition of the A_2X_6-AX_3 bands and in few cases of new resonance-enhanced polarized bands due to MA_2X_8 and/or MAX_5 complexes. At temperatures above 800"0K, characteristic bands due to MX_2(g) (M = Fe, Co, Ni, Cu, Zn) and M_2X_4(g) (M = Cu) were observed. The predominant features of the PdAl_2Cl_8, CuAl_2Cl_8, and PdAl_2Br_6 spectra were three high-intensity, polarized bands which were attributed to the vibrational modes of the complex coupled to the electronic state of the central atom. The spectra of CuAlCl_5(g), CuInCl_5(g) and Cu_2Cl_4(g) species showed resonance enhancement of selective fundamentals which were attributed to vibrational modes of trigonally coordinated Cu(II). Resonance Raman spectra of U_2Cl_1_0(g) and UCl_5.AlCl_3(g) were characterized by the presence of a strong band attributed to the U-Cl/sub t/ stretching frequency. Raman band intensity measurements were carried out for the iron(III) chloride vapors and for the vapor complexes of CuAl_2Cl_8, CuInCl_5 and UCl_5.AlCl_3 using different laser powers and frequencies. The measurements suggested increasing spectroscopic temperatures and decomposition of the vapor complexes. The data are discussed in terms of the distribution of vibrational modes and the structure of the vapor species. 22 figs

1978-09-22

316

Improved value for the silicon free exciton binding energy  

Directory of Open Access Journals (Sweden)

Full Text Available The free exciton binding energy is a key parameter in silicon material and device physics. In particular, it provides the necessary link between the energy threshold for valence to conduction band optical absorption and the bandgap determining electronic properties. The long accepted low temperature binding energy value of 14.7 ± 0.4 meV is reassessed taking advantage of developments subsequent to its original determination, leading to the conclusion that this value is definitely an underestimate. Using three largely independent experimental data sets, an improved low temperature value of 15.01 ± 0.06 meV is deduced, in good agreement with the most comprehensive theoretical calculations to date.

Martin A. Green

2013-11-01

317

Improved value for the silicon free exciton binding energy  

International Nuclear Information System (INIS)

The free exciton binding energy is a key parameter in silicon material and device physics. In particular, it provides the necessary link between the energy threshold for valence to conduction band optical absorption and the bandgap determining electronic properties. The long accepted low temperature binding energy value of 14.7 ± 0.4 meV is reassessed taking advantage of developments subsequent to its original determination, leading to the conclusion that this value is definitely an underestimate. Using three largely independent experimental data sets, an improved low temperature value of 15.01 ± 0.06 meV is deduced, in good agreement with the most comprehensive theoretical calculations to date

2013-11-01

318

Exciton-Exciton Annihilation in Copper-Phthalocyanine Single-Crystal Nanowires  

Energy Technology Data Exchange (ETDEWEB)

Femtosecond one-color pump-probe spectroscopy was applied to study exciton dynamics in single-crystal copper-phthalocyanine (CuPc) nanowires grown on an opaque silicon substrate. The transient reflectance kinetics measured at different pump fluences exhibit a remarkable intensity-dependent decay behavior which accelerates significantly with increasing pump pulse intensity. All the kinetic decays can be satisfactorily described using a bi-exponential decay function with lifetimes of 22 and 204 ps, and corresponding relative amplitudes depending on the pump intensity. The accelerated decay behavior observed at high pump intensities arises from a nonlinear exciton-exciton annihilation process. While this phenomenon has been found previously in crystalline metallophthalocyanine (MPc) polymorphs such as colloidal particles and thin films, the results obtained using the CuPc nanowires are markedly distinct, namely, much longer decay times and a linear intensity dependence of the initial peak amplitudes. Despite these differences, detailed data analysis further shows that, as found for other metal-phthalocyanine polymorphs, exciton-exciton annihilation in the CuPc nanowires is one-dimensional (1D) diffusion-limited, which possibly involves intra-chain exciton diffusion along 1D molecular stacks. The significantly long-lived excitons of CuPc nanowires in comparison to those of other crystalline polymorphs make them particularly suitable for photovoltaic applications.

Ma, Yingzhong [ORNL; Xiao, Kai [ORNL; Shaw, Robert W [ORNL

2012-01-01

319

Dark excitons and Bose-Einstein condensation in strain-trapped excitons in coupled quantum wells  

Science.gov (United States)

It is possible to make excitons in coupled quantum wells with very long lifetime compared to their thermalization, allowing us to study equilibrium behavior. Using a localized stress to create traps for excitons in coupled quantum wells, we have demonstrated that at low temperature, high density, and large stress, the spatial pattern of photoluminescence (PL) from interwell excitons transitions to one with a dimmed center. This pattern emerges despite the center remaining the region of highest exciton density. This darkening is related to a strain-induced interaction between the light hole and heavy hole states. However, while this explanation provides a mechanism to explain many of the features, a few important predictions of this theory are not borne out by experiments. An alternate explanation is possible, utilizing an increasing population of dark (J=2) excitons and a separation of the dark and bright species. It has been proposed that a Bose-Einstein Condensate in this system would occur in a dark state, and this transition is consistent with the onset criteria of the pattern formation and explains how a slight bright/dark energy difference could lead to spatial separation of the species. Experiments employing a magnetic field to turn `dark' excitons slightly `bright' should allow the disambiguation of the role of dark excitons in this system. I will review this pattern formation and discuss data from experiments employing a magnetic field.

Sinclair, Nicholas; Wuenschell, Jeff; West, Kenneth; Pfeiffer, Loren; Snoke, David

2012-02-01

320

High Pressure Optical Studies of the Thallous Halides and of Charge-Transfer Complexes  

Science.gov (United States)

High pressure was used to study the insulator -to-metal transition in sulfur and the thallous halides and to study the intermolecular interactions in charge -transfer complexes. The approach to the band overlap insulator -to-metal transition was studied in three thallous halides and sulfur by optical absorption measurements of the band gap as a function of pressure. The band gap of sulfur continuously decreases with pressure up to the insulator -to-metal transition which occurs between 450 and 485 kbars. The results on the thallous halides indicate that the indirect gap decreases more rapidly than the direct gap; the closing of the indirect gap is responsible for the observed insulator -to-metal transitions. High pressure electronic and vibrational spectroscopic measurements on the solid-state complexes of HMB-TCNE were used to study the intermolecular interactions of charge -transfer complexes. The vibrational frequency shifts indicate that the degree of charge transfer increases with pressure which is independently confirmed by an increase in the molar absorptivity of the electronic charge-transfer peak. Induction and dispersion forces contribute towards a red shift of the charge-transfer peak; however, charge-transfer resonance contributes toward a blue shift and this effect is dominant for the HMB-TCNE complexes. High pressure electronic spectra were used to study the effect of intermolecular interactions on the electronic states of TCNQ and its complexes. The red shifts with pressure of the electronic spectra of TCNQ and (TCNQ)(' -) in polymer media and of crystalline TCNQ can be understood in terms of Van der Waals interactions. None of the calculations which considered intradimer distance obtained the proper behavior for either the charge-transfer of the locally excited states of the complexes. The qualitative behavior of both states can be interpreted as the effect of increased mixing of the locally excited and charge transfer states.

Jurgensen, Charles Willard

 
 
 
 
321

Non-conservation of excitons in finite molecular chain  

Energy Technology Data Exchange (ETDEWEB)

We have analyzed a linear molecular chain with exciton excitations when the number of excitons is not conserved. The dispersion law depends on two independent variables and it is surfaced in a 3D plot. The same conclusion is valid for the concentrations of excitons and exciton pairs. As it was expected, physical characteristics of the finite chain depend on spatial coordinates. All results are compared to the corresponding results of an infinite chain.

Tosic, Bratislav, E-mail: btosic@yahoo.co [Vojvodina Academy of Science and Arts, 21000 Novi Sad, Dunavska 37 (Serbia); Sajfert, Vjekoslav, E-mail: sajfertv@nadlanu.co [University of Novi Sad, Technical Faculty ' M. Pupin' , 23000 Zrenjanin, Djure Djakovica bb (Serbia); Maskovic, Ljiljana, E-mail: maskovicm@yahoo.co.u [Academy of Criminalistic and Police Studies, 11000 Belgrade, Zemun (Serbia); Bednar, Nikola, E-mail: bednar.nikola@gmail.co [University of Novi Sad, Faculty of Technical Sciences, 21000 Novi Sad, Trg Dositeja Obradovica 6 (Serbia)

2010-11-15

322

Non-conservation of excitons in finite molecular chain  

International Nuclear Information System (INIS)

We have analyzed a linear molecular chain with exciton excitations when the number of excitons is not conserved. The dispersion law depends on two independent variables and it is surfaced in a 3D plot. The same conclusion is valid for the concentrations of excitons and exciton pairs. As it was expected, physical characteristics of the finite chain depend on spatial coordinates. All results are compared to the corresponding results of an infinite chain.

2010-11-01

323

Ultrafast exciton relaxation in quasi-one-dimensional perylene derivatives  

Digital Repository Infrastructure Vision for European Research (DRIVER)

This thesis deals with exciton relaxation processes in thin polycrystalline films and matrix-isolated molecules of the perylene derivatives PTCDA (3,4,9,10-perylenetetracarboxylic dianhydride) and MePTCDI (N,N'-dimethylperylene-3,4,9,10-dicarboximide). Using femtosecond pump-probe spectroscopy, transient absorption spectra, excitonic relaxation in the lowest excited state subsequent to excitation, and exciton-exciton interaction and annihilation at high excitation densities have been addresse...

Engel, Egbert

2006-01-01

324

Exciton et polariton dans les semiconducteurs cubiques : étude de la réflectivité  

Science.gov (United States)

We present a quantitative study of the fine structure of the exciton, the polariton and the reflectivity spectrum in a cubic semiconductor under different external perturbations : uniaxial stress hydrostatic pressure and magnetic field. In the case of InP we determine the whole series of exciton parameters. Nous présentons une étude quantitative de la structure fine de l'exciton, du polariton et du spectre de réflectivité dans un semiconducteur cubique soumis à différents types de perturbation extérieure. Les perturbations considérées sont successivement la contrainte uniaxiale, la pression hydrostatique et le champ magnétique dans différents types de configuration. Nous déterminons l'ensemble des paramètres caractéristiques de l'exciton et du polariton ainsi que des électrons et des trous au voisinage des extrema des bandes de valence et de conduction. Nous avons utilisé InP comme matériau test et obtenu pour les différents paramètres de ce semiconducteur les valeurs suivantes : Eex = 5,1 ± 0,1 meV ; ? = 0,04 ± 0,02 meV ; E LT = 0,17 ± 0,02 meV ; ?Eex/Eex = 2 ± 1 % par kbar ; gc = 1,24 ; me = 0,0803 m0 ; ?1 = 5,22 ; ? 2 = 1,83 ; ?3 = 2,34 ; q = 0 ; ? = 0,89.

Chen, Y.; Gil, B.; Mathieu, H.

325

Exciton and negative trion dissociation by an external electric field in vertically coupled quantum dots  

CERN Document Server

We study the Stark effect for an exciton confined in a pair of vertically coupled quantum dots. A single-band approximation for the hole and a parabolic lateral confinement potential are adopted which allows for the separation of the lateral center-of-mass motion and consequently for an exact numerical solution of the Schr\\"odinger equation. We show that for intermediate tunnel coupling the external electric field leads to the dissociation of the exciton via an avoided crossing of bright and dark exciton energy levels which results in an atypical form of the Stark shift. The electric-field-induced dissociation of the negative trion is studied using the approximation of frozen lateral degrees of freedom. It is shown that in a symmetric system of coupled dots the trion is more stable against dissociation than the exciton. For an asymmetric system of coupled dots the trion dissociation is accompanied by a positive curvature of the recombination energy line as a function of the electric field.

Szafran, B; Peeters, F M; Bednarek, S; Adamowski, J; Partoens, B

2005-01-01

326

Exciton states in GaAs ?-doped systems under magnetic fields and hydrostatic pressure  

Energy Technology Data Exchange (ETDEWEB)

Excitons in GaAs n-type ?-doped quantum wells are studied taking into account the effects of externally applied magnetic fields as well as of hydrostatic pressure. The one-dimensional potential profile in both the conduction and valence bands is described including Hartree effects via a Thomas–Fermi-based local density approximation. The allowed uncorrelated energy levels are calculated within the effective mass and envelope function approximations by means of an expansion over an orthogonal set of infinite well eigenfunctions and a variational method is used to obtain the exciton states. The results are presented as functions of the two-dimensional doping concentration and the magnetic field strength for zero and finite values of the hydrostatic pressure. In general, it is found that the exciton binding energy is a decreasing function of the doping-density and an increasing function of the magnetic field intensity. A comparison with recent experiments on exciton-related photoluminescence in n-type ?-doped GaAs is made.

Mora-Ramos, M.E. [Facultad de Ciencias, Universidad Autónoma del Estado de Morelos, Ave. Universidad 1001, CP 62209 Cuernavaca, Morelos (Mexico); Instituto de Física, Universidad de Antioquia, AA 1226 Medellín (Colombia); Duque, C.A., E-mail: cduque@fisica.udea.edu.co [Instituto de Física, Universidad de Antioquia, AA 1226 Medellín (Colombia)

2013-04-15

327

Exciton states in GaAs ?-doped systems under magnetic fields and hydrostatic pressure  

International Nuclear Information System (INIS)

Excitons in GaAs n-type ?-doped quantum wells are studied taking into account the effects of externally applied magnetic fields as well as of hydrostatic pressure. The one-dimensional potential profile in both the conduction and valence bands is described including Hartree effects via a Thomas–Fermi-based local density approximation. The allowed uncorrelated energy levels are calculated within the effective mass and envelope function approximations by means of an expansion over an orthogonal set of infinite well eigenfunctions and a variational method is used to obtain the exciton states. The results are presented as functions of the two-dimensional doping concentration and the magnetic field strength for zero and finite values of the hydrostatic pressure. In general, it is found that the exciton binding energy is a decreasing function of the doping-density and an increasing function of the magnetic field intensity. A comparison with recent experiments on exciton-related photoluminescence in n-type ?-doped GaAs is made

2013-04-15

328

Theory of magnetic-field-dependent alloy broadening of exciton-photoluminescence linewidths in semiconductor alloys  

Energy Technology Data Exchange (ETDEWEB)

A theory is developed for the inhomogeneous photoluminescence line shape and, in particular, the linewidth of excitons due to alloy disorder in undoped semiconductor alloys in the presence of external magnetic fields. In contrast to previous theories, we find that both the linewidth and its field dependence depend not only on the exciton wave function for the electron-hole relative motion but also depend sensitively on the localization length [ital R][sub 0] of the center-of-mass wave function. In general, the line shape depends on the nature of the exciton localization. The linewidths arise from the fluctuations of the conduction- and valence-band edges in the localization region where there is significant amplitude of the total exciton wave function. The wave function for the relative motion is calculated numerically by reducing the Schroedinger equation to a difference equation at arbitrary fields, while the center-of-mass wave function is treated phenomenologically. The linewidth is calculated as a function of the magnetic field and the localization length [ital R][sub 0]. The results yield good agreement with recent experimental data from In[sub 0.48]Ga[sub 0.52]P.

Lyo, S.K. (Sandia National Laboratories, Albuquerque, New Mexico 87185 (United States))

1993-07-15

329

Proposal of structures possessing high exciton concentration  

Energy Technology Data Exchange (ETDEWEB)

The possibility of achievement of high exciton concentrations is analyzed. It is shown that high concentrations can be achieved in a three-layer thin molecular film due to the autoreduction processes taking place in it. Shortly, the appearance of high concentrations is the consequence of boundary conditions in film and of the magnitude of matrix elements of dipol-dipol interactions. The autoreduction takes place in the cases when matrix elements characterizing exciton transfer are less than statistical matrix elements. Based on numerical analysis, it was found that optical quanta concentrations of a three-layer film can achieve values of about 5x10{sup -2}. The structures possessing so high concentration do not exist in nature, thus they have to be synthesised. For the current state of nanotechnology, it is not a problem. Fortunately achieving high concentrations requires only certain ratios of relevant characteristics of the film with a two-level exciton scheme, but not their single values.

Sajfert, Vjekoslav [Technical Faculty ' Mihajlo Pupin' , Zrenjanin (Serbia)], E-mail: sajfertv@ptt.yu; Jacimovski, Stevo [Faculty of Electrotechnic Sciences, Belgrade (Serbia); Tosic, Bratislav [Vojvodina Academy of Sciences and Arts, Novi Sad (Serbia)

2008-09-15

330

Proposal of structures possessing high exciton concentration  

International Nuclear Information System (INIS)

The possibility of achievement of high exciton concentrations is analyzed. It is shown that high concentrations can be achieved in a three-layer thin molecular film due to the autoreduction processes taking place in it. Shortly, the appearance of high concentrations is the consequence of boundary conditions in film and of the magnitude of matrix elements of dipol-dipol interactions. The autoreduction takes place in the cases when matrix elements characterizing exciton transfer are less than statistical matrix elements. Based on numerical analysis, it was found that optical quanta concentrations of a three-layer film can achieve values of about 5x10-2. The structures possessing so high concentration do not exist in nature, thus they have to be synthesised. For the current state of nanotechnology, it is not a problem. Fortunately achieving high concentrations requires only certain ratios of relevant characteristics of the film with a two-level exciton scheme, but not their single values

2008-09-01

331

Radiation damage in the alkali halide crystals  

International Nuclear Information System (INIS)

A general review is given of the experimental data on radiation damage in the alkali halide crystals. A report is presented of an experimental investigation of irradiation produced interstitial dislocation loops in NaCl. These loops are found to exhibit the usual growth and coarsening behaviour during thermal annealing which operates by a glide and self-climb mechanism. It is shown that the recombination of defects in these crystals is a two stage process, and that the loss of interstitials stabilized at the loops is caused by extrinsic vacancies. The theoretical techniques used in simulating point defects in ionic crystals are described. Shell model potentials are derived for all the alkali halide crystals by fitting to bulk crystal data. The fitting is supplemented by calculations of the repulsive second neighbour interactions using methods based on the simple electron gas model. The properties of intrinsic and substitutional impurity defects are calculated. The HADES computer program is used in all the defect calculations. Finally the report returns to the problems of irradiation produced interstitial defects. The properties of H centres are discussed; their structure, formation energies, trapping at impurities and dimerization. The structure, formation energies and mobility of the intermediate and final molecular defects are then discussed. The thermodynamics of interstitial loop formation is considered for all the alklai halide crystals. The nucleation of interstitial loops in NaCl and NaBr is discussed, and the recombination of interstitial and vacancy defects. The models are found to account for all the main features of the experimental data. (author)

1975-01-01

332

Time-dependent density-functional theory of exciton-exciton correlations in the nonlinear optical response  

Science.gov (United States)

We analyze possible nonlinear exciton-exciton correlation effects in the optical response of semiconductors by using a time-dependent density-functional theory (TDDFT) approach. For this purpose we derive the nonlinear (third-order) TDDFT equation for the excitonic polarization. In this equation the nonlinear time-dependent effects are described by the time-dependent (nonadiabatic) part of the effective exciton-exciton interaction, which depends on the exchange-correlation (XC) kernel. We apply the approach to study the nonlinear optical response of a GaAs quantum well. In particular, we calculate the two-dimensional Fourier spectrum (2DFS) of the system and compare it with experimental data. We find that the memory effects play a crucial role in this response, and in particular that it is necessary to use a nonadiabatic XC kernel to describe excitonic bound states, biexcitons, which are formed due to the retarded TDDFT exciton-exciton interaction.

Turkowski, Volodymyr; Leuenberger, Michael N.

2014-02-01

333

Stimulated Scattering of Indirect Excitons in Coupled Quantum Wells: Signature of a Degenerate Bose-Gas of Excitons  

Energy Technology Data Exchange (ETDEWEB)

We observe and analyze strongly nonlinear photoluminescence kinetics of indirect excitons in GaAs/AlGaAs coupled quantum wells at low bath temperatures, {ge}50 mK . The long recombination lifetime of indirect excitons promotes accumulation of these Bose particles in the lowest energy states and allows the photoexcited excitons to cool down to temperatures where the dilute 2D gas of indirect excitons becomes statistically degenerate. Our main result{emdash}a strong enhancement of the exciton scattering rate to the low-energy states with increasing concentration of the indirect excitons{emdash}reveals bosonic stimulation of exciton scattering, which is a signature of a degenerate Bose-gas of excitons.

Butov, L. V.; Ivanov, A. L.; Imamoglu, A.; Littlewood, P. B.; Shashkin, A. A.; Dolgopolov, V. T.; Campman, K. L.; Gossard, A. C.

2001-06-11

334

Spontaneous coherence of indirect excitons in a trap  

Digital Repository Infrastructure Vision for European Research (DRIVER)

We report on the emergence of spontaneous coherence in a gas of indirect excitons in an electrostatic trap. At low temperatures, the exciton coherence length becomes much larger than the thermal de Broglie wavelength and reaches the size of the exciton cloud in the trap.

High, Alexander A.; Leonard, Jason R.; Remeika, Mikas; Butov, Leonid V.; Hanson, Micah; Gossard, Arthur C.

2011-01-01

335

Exciton-exciton interaction engineering in coupled GaN quantum dots  

CERN Document Server

We present a fully three-dimensional study of the multiexciton optical response of vertically coupled GaN-based quantum dots via a direct-diagonalization approach. The proposed analysis is crucial in understanding the fundamental properties of few-particle/exciton interactions and, more important, may play an essential role in the design/optimization of semiconductor-based quantum information processing schemes. In particular, we focus on the interdot exciton-exciton coupling, key ingredient in recently proposed all-optical quantum processors. Our analysis demonstrates that there is a large window of realistic parameters for which both biexcitonic shift and oscillator strength are compatible with such implementation schemes.

De Rinaldis, S; Rossi, F; Rinaldis, Sergio De; Amico, Irene D'; Rossi, Fausto

2002-01-01

336

Improved processing for silver halide pulse holography  

Science.gov (United States)

Using of an improved developer with optical latensification allowed to significantly increase exposure sensitivity of currently in use silver halide materials. Transmission large-scale holograms (30 X 40 cm2) of diffused objects have been recorded under pulse exposure of about 6.5 X 10-6 J/cm2 for VRP and of about 2 X 10-6 J/cm2 for Agfa-Gavaert 8E56HD, in both cases without appreciable contrast deterioration. Results of the first experiments on pulse reflection holography are also discussed.

Mikhailov, Viktor N.; Son, Jung-Young; Grinevitskaya, Olga V.; Lee, Hyuk-Su; Choi, Y.-J.

1996-04-01

337

Arsenic-bound excitons in diamond  

Science.gov (United States)

A set of new excitonic recombinations is observed in arsenic-implanted diamond. It is composed of two groups of emissions at 5.355/5.361 eV and at 5.215/5.220/5.227 eV. They are respectively attributed to the no-phonon and transverse-optical phonon-assisted recombinations of excitons bound to neutral arsenic donors. From the Haynes rule, an ionization energy of 0.41 eV is deduced for arsenic in diamond, which shows that arsenic is a shallower donor than phosphorus (0.6 eV), in agreement with theory.

Barjon, J.; Jomard, F.; Morata, S.

2014-01-01

338

Detecting an exciton crystal by statistical means  

Science.gov (United States)

We investigate an ensemble of excitons in a coupled quantum well excited via an applied laser field. Using an effective disordered quantum Ising model, we perform a numerical simulation of the experimental procedure and calculate the probability distribution function P(M) to create M excitons as well as their correlation function. It shows clear evidence of the existence of two phases corresponding to a liquid and a crystal phase. We demonstrate that not only the correlation function but also the distribution P(M) is very well suited to monitor this transition.

Breyel, D.; Soller, H.; Schmidt, T. L.; Komnik, A.

2014-05-01

339

Switching exciton pulses through conical intersections  

CERN Document Server

Exciton pulses transport excitation and entanglement adiabatically through Rydberg aggregates, assemblies of highly excited light atoms, which are set into directed motion by resonant dipole-dipole interaction. Here, we demonstrate the coherent splitting of such pulses as well as the spatial segregation of electronic excitation and atomic motion. Both mechanisms exploit local non-adiabatic effects at a conical intersection, turning them from a decoherence source into an asset. The intersection provides a sensitive knob controlling the propagation direction and coherence properties of exciton pulses.

Leonhardt, K; Rost, J -M

2013-01-01

340

Heat pumping with optically driven excitons  

CERN Multimedia

We present a theoretical study showing that an optically driven excitonic two-level system in a solid state environment acts as a heat pump by means of repeated phonon emission or absorption events. We derive a master equation for the combined phonon bath and two-level system dynamics and analyze the direction and rate of energy transfer as a function of the externally accessible driving parameters. We discover that if the driving laser is detuned from the exciton transition, cooling the phonon environment becomes possible.

Gauger, Erik M

2010-01-01

 
 
 
 
341

Spin cutoff parameter for the exciton model  

International Nuclear Information System (INIS)

In order to estimate the angular momentum dependence of the state density for the exciton model, the spin cutoff parameter for a noninteracting harmonic oscillator shell model (HOSM) is computed. A simple but useful estimate is obtained. A simpler estimation based on the level density formula of Williams is also given by introducing the ''approximation of restricted population'' (ARP). The spin cutoff parameters for HOSM and ARP are functions of not only the exciton number nx but also of excitation energy Ex. The ARP reproduces the average behavior of HOSM. ARP provides an improved version of the formula for the spin cutoff parameter based on the equi-distance model. (author)

1986-01-01

342

Configurational coordinate diagram of Ni{sup 2+} doped silver halide crystals, as determined by optical and luminescence measurements  

Energy Technology Data Exchange (ETDEWEB)

Optical and luminescence measurements were carried out on Ni{sup 2+} doped silver halide crystals. The configurational coordinates of the ground and the first excited states made it possible to explain the important optical properties, such as the Stokes shift between the ground and excited states, the broadening of the bands with temperature and the thermal quenching. Determining the configurational coordinate diagram of the ground and first excited states is the first step for the design and fabrication of solid state and fiber lasers which are based on Ni{sup 2+} doped silver halides and which operate in the middle infrared. - Highlights: Black-Right-Pointing-Pointer Optical and luminescence measurements were carried out on Ni{sup 2+} doped silver halide crystals. Black-Right-Pointing-Pointer Configurational coordinates of the ground and the first excited state were calculated. Black-Right-Pointing-Pointer Optical properties of Ni{sup 2+} doped silver halide crystals can be obtained from the configurational coordinate diagram. Black-Right-Pointing-Pointer No emission signal was found above 220 K.

Zakosky-Neuberger, I., E-mail: yifat_z2@hotmail.com [Raymond and Beverly Sackler Faculty of Exact Sciences, School of Physics and Astronomy, Tel-Aviv University, Tel-Aviv 69978 (Israel); Shafir, I.; Nagli, L.; Katzir, A. [Raymond and Beverly Sackler Faculty of Exact Sciences, School of Physics and Astronomy, Tel-Aviv University, Tel-Aviv 69978 (Israel)

2012-08-15

343

Interface Excitons in Krmnen Clusters : The Role of Electron Affinity in the Formation of Electronic Structure  

CERN Document Server

The formation of the electronic structure of small Kr_m clusters (mDelta_1i . Kr clusters comprising up to 70 atoms do not exhibit bulk absorption bands. We suggest that this is due to the penetration of the interface excitons into the Kr_m cluster volume, because of the negative electron affinity of surrounding Ne atoms. From the energy shift of the interface absorption bands with cluster size an unexpectedly large penetration depth of delta_1i =7.0+/-0.1 A is estimated, which can be explained by the interplay between the electron affinities of the guest and the host cluster.

Kanaev, A; Edery, F; Laarmann, T; Möller, T

2008-01-01

344

Multiphonon resonant Raman scattering in the semimagnetic semiconductor Cd sub 1 sub - sub x Mn sub x Te: Froehlich and deformation potential exciton-phonon interaction  

CERN Document Server

A theory describing multiphonon resonant Raman scattering (MPRRS) processes in wide-gap diluted magnetic semiconductors is presented, with Cd sub 1 sub - sub x Mn sub x Te as an example. The incident radiation frequency omega sub l is taken above the fundamental absorption region. The photoexcited electron and hole make real transitions through the LO phonon, when one considers Froehlich (F) and deformation potential (DP) interactions. The strong exchange interaction, typical of these materials, leads to a large spin splitting of the exciton states in the magnetic field. Neglecting Landau quantization, this Zeeman splitting gives rise to the formation of eight bands (two conduction and six valence ones) and ten different exciton states according to the polarization of the incident light. Explicit expressions for the MPRRS intensity of second and third order, the indirect creation and annihilation probabilities, the exciton lifetime, and the probabilities of transition between different exciton states and diff...

Riera, R; Marin, J L; Bergues, J M; Campoy, G

2003-01-01

345

Radiative recombination of excitons in the transition-metal dichalcogenides MoS{sub 2}:Cl{sub 2} and WS{sub 2}:Br{sub 2}  

Energy Technology Data Exchange (ETDEWEB)

The comparative study of the excitonic radiative recombination in 2H-MoS{sub 2}:Cl{sub 2} and 2H-WS{sub 2}:Br{sub 2} layered crystals is presented. The steady-state emission spectra of both the investigated materials include two distinct regions. The short-wavelength region consisting of sharp lines is produced by bound excitons, related to the halogen molecules intercalated in the layered semiconductors during the growth process. The second spectral region-a vibronic broad band identical for both types of crystals, is attributed to the deep radiative centres, caused by the intrinsic defects of the host lattices. The atypical temperature increase of the excitonic radiative decay time observed for 2H-WS{sub 2}:Br{sub 2} in the range T=2-7K is related to the presence of a long lifetime sublevel, placed 0.3meV above the lowest excitonic state.

Charron, L. [Department of Physics, University of Ottawa, Ont., K1N 6N5 (Canada); Dumchenko, D. [Institute of Applied Physics, Academy of Sciences of Moldova, Chisinau, MD-2028 (Moldova, Republic of); Fortin, E. [Department of Physics, University of Ottawa, Ont., K1N 6N5 (Canada); Gherman, C. [Institute of Applied Physics, Academy of Sciences of Moldova, Chisinau, MD-2028 (Moldova, Republic of); Kulyuk, L. [Institute of Applied Physics, Academy of Sciences of Moldova, Chisinau, MD-2028 (Moldova, Republic of)]. E-mail: kulyuk@phys.asm.md.ca

2005-04-15

346

Free and bound exciton emission in CuInSe{sub 2} and CuGaSe{sub 2} single crystals  

Energy Technology Data Exchange (ETDEWEB)

The free-exciton series with n=1,2 in high perfect CuInSe{sub 2} and CuGaSe{sub 2} single crystals have been observed in photoluminescence. The exciton binding energy and direct energy gap are estimated as 9.5 meV and 1.7305 eV at 4.2 K for CuGaSe{sub 2}. New values are determined for the band gap energy E{sub g}=1.0459 eV and for the excitonic binding energy 5.1 meV in CuInSe{sub 2} at 4.2 K. The temperature dependence of the n=1 excitonic line between 4.2 and 300 K have been studied

Mudryi, A.V.; Bodnar, I.V.; Gremenok, V.F.; Victorov, I.A.; Patuk, A.I.; Shakin, I.A. [Institute of Physics of Solids and Semiconductors, Academy of Sciences of Belarus, P.Brovki Str. 17, 220072 Minsk (Belarus)

1998-06-10

347

Model of mixed Frenkel and charge-transfer excitons in donor acceptor molecular crystals: investigation of vibronic spectra  

Science.gov (United States)

The mixing of Frenkel excitons (FEs) and charge-transfer excitons (CTEs) in a molecular stack of regularly arranged donor (D) and acceptor (A) molecules is considered a model case and its vibronic line shapes have been calculated for several parameter sets. The two types of excitons (FE and CTE) are coupled linearly and quadratically with one vibrational mode of the D molecule (or of the A molecule). Using the methods of canonical transformation and of Green's functions (at T=0), as well as the vibronic approach which is applicable in the case of a narrow exciton band, the linear optical susceptibility is calculated for the three spectral regions: (a) excitonic, (b) one-phonon vibronics, and (c) two-phonon vibronics. As the study is directed to centrosymmetrical stacks, the cases of mixing of gerade excitons and of ungerade excitons have been treated separately in the calculation of the linear absorption coefficients. Because until now experimental observations of FE CTE mixing in DA charge-transfer systems are absent, the numerical calculations have been performed for hypothetical sets of parameters which include: the parameters of CTEs in DA stacks (like anthracene PMDA) and the parameters of FE CTE mixing in a one-component stack (like that of PTCDA). The simulations establish the main features of the excitonic and vibronic spectra in the case of FE CTE mixing, namely (i) the mutual influence on the positions and on the absorption intensities of all terms of the vibronic progressions stemming from FE and CTE levels; (ii) in the case of vibration of an A molecule (if the FE is assumed to be an excited electronic state of the D molecule), only one vibronic progression is manifest and the vibronic levels closer to the FE will be most enhanced; (iii) the redistribution of the absorption intensities depends on the sign of the mixing constant and may be so strong that the terms of the two vibronic progressions could have comparable absorption; (iv) spectral lines of different shape correspond to the bound and unbound exciton phonon states; and (v) in the case of mixing of gerade excitons its possible impact on the absorption of the ungerade CTE-combination connected with the noncentral part k?0 of the Brillouin zone was established. The simulation of the FE CTE mixing can be useful in the assignment of the linear absorption spectra and in the description of the FE CTE vibration coupling.

Lalov, I. J.; Warns, C.; Reineker, P.

2008-08-01

348

Model of mixed Frenkel and charge-transfer excitons in donor-acceptor molecular crystals: investigation of vibronic spectra  

International Nuclear Information System (INIS)

The mixing of Frenkel excitons (FEs) and charge-transfer excitons (CTEs) in a molecular stack of regularly arranged donor (D) and acceptor (A) molecules is considered a model case and its vibronic line shapes have been calculated for several parameter sets. The two types of excitons (FE and CTE) are coupled linearly and quadratically with one vibrational mode of the D molecule (or of the A molecule). Using the methods of canonical transformation and of Green's functions (at T=0), as well as the vibronic approach which is applicable in the case of a narrow exciton band, the linear optical susceptibility is calculated for the three spectral regions: (a) excitonic, (b) one-phonon vibronics, and (c) two-phonon vibronics. As the study is directed to centrosymmetrical stacks, the cases of mixing of gerade excitons and of ungerade excitons have been treated separately in the calculation of the linear absorption coefficients. Because until now experimental observations of FE-CTE mixing in DA charge-transfer systems are absent, the numerical calculations have been performed for hypothetical sets of parameters which include: the parameters of CTEs in DA stacks (like anthracene-PMDA) and the parameters of FE-CTE mixing in a one-component stack (like that of PTCDA). The simulations establish the main features of the excitonic and vibronic spectra in the case of FE-CTE mixing, namely (i) the mutual influence on the positions and on the absorption intensities of all terms of the vibronic progressions stemming from FE and CTE levels; (ii) in the case of vibration of an A molecule (if the FE is assumed to be an excited electronic state of the D molecule), only one vibronic progression is manifest and the vibronic levels closer to the FE will be most enhanced; (iii) the redistribution of the absorption intensities depends on the sign of the mixing constant and may be so strong that the terms of the two vibronic progressions could have comparable absorption; (iv) spectral lines of different shape correspond to the bound and unbound exciton-phonon states; and (v) in the case of mixing of gerade excitons its possible impact on the absorption of the ungerade CTE-combination connected with the noncentral part k?0 of the Brillouin zone was established. The simulation of the FE-CTE mixing can be useful in the assignment of the linear absorption spectra and in the description of the FE-CTE-vibration coupling

2008-08-01

349

Peculiarities of the determination of shallow impurity concentrations in semiconductors from the analysis of exciton luminescence spectra  

CERN Document Server

An analysis was made of the applicability limits of the method for the determination of the content of shallow acceptors and donors in semiconductors from the ratio of the low-temperature (T = 1.8-4.2 K) luminescence intensities of exciton bands, in particular, induces by radiative annihilation of excitons bound to acceptors (donors) and free excitons. It is shown that correct data about the concentrations of shallow acceptors and donors as well as data on changes in their content as a result of various treatments may be obtained if the occupancy of the defects in question by holes and electrons does not depend on the excitation intensity or external treatments. A way to check the fulfillment of criteria for the method application is suggested. An example is given is given of the method application for determination of thermally stimulated changes in the concentration of shallow acceptors and donors in gallium arsenide

Glinchuk, K D

2002-01-01

350

Exceptionally Slow Rise in Differential Reflectivity Spectra of Excitons in GaN Effect of Excitation-induced Dephasing  

CERN Document Server

Femtosecond pump-probe (PP) differential reflectivity spectroscopy (DRS) and four-wave mixing (FWM) experiments were performed simultaneously to study the initial temporal dynamics of the exciton line-shapes in GaN epilayers. Beats between the A-B excitons were found \\textit{only for positive time delay} in both PP and FWM experiments. The rise time at negative time delay for the differential reflection spectra was much slower than the FWM signal or PP differential transmission spectroscopy (DTS) at the exciton resonance. A numerical solution of a six band semiconductor Bloch equation model including nonlinearities at the Hartree-Fock level shows that this slow rise in the DRS results from excitation induced dephasing (EID), that is, the strong density dependence of the dephasing time which changes with the laser excitation energy.

Jho, Y D; Fischer, A J; Song, J J; Kenrow, J; Sayed, K E; Stanton, C J

2003-01-01

351

Defects induced melting in alkali halides  

International Nuclear Information System (INIS)

In the present paper we study the pressure dependence of melting of NaCl and CsCl crystals. A formulation has been presented for the pressure dependence of melting temperature on the basis of the vacancy model using the expression for the pressure dependence of the volume of Schottky defects from the Roy-Roy equation of state. Values of pressure derivatives of melting temperature have been calculated at elevated pressures to determine the rate of change of melting temperature with increase in pressures using the data of vacancy formation energy and effective volume of Schottky defects. The vacancy model revised in the present study takes into account the variation of bulk modulus with pressure, whereas in the Ksiazek and Gorecki model, it was treated constant. Results for pressure derivative of melting temperature are calculated for the solids under study. The melting curves have also been obtained and found to compare well with results based on molecular dynamics simulation and experimental data reported in recent literature. -- Research Highlights: ? We have studied the pressure dependence of melting in alkali halides on the basis of the vacancy model. ? We have taken into account the pressure dependence of volume of Schottky defects. ? The pressure derivatives of melting temperature are calculated for the alkali halide solids. ? The results have been found to compare well with the corresponding experimental data. ? This work will be useful in the analysis of thermal equation of state for solids.

2011-02-01

352

Nonboson treatment of excitonic nonlinearity in optically excited media  

International Nuclear Information System (INIS)

The present article shortly reviews some recent results in the study of excitonic nonlinearity in optically excited media using a nonboson treatment for many-exciton systems. After a brief discussion of the exciton nonbosonity the closed commutation relations are given for exciton operators which hold for any exciton density and type. The nonboson treatment is then applied to the problems of intrinsic optical bistability and nonlinear polariton yielding quite interesting and new effects, e.g. new shapes of hysteresis loops of intrinsic optical bistability or anomalies of polariton dispersion. (author). 71 refs, 4 figs

1990-01-01

353

Excitonic optical bistability in n-type doped semiconductors  

International Nuclear Information System (INIS)

A resonant monochromatic pump laser generates coherent excitons in an n-type doped semiconductor. Both exciton-exciton and exciton-donor interactions come into play. The former interaction can give rise to the appearance of optical bistability which is heavily influenced by the latter one. When optical bistability occurs at a fixed laser frequency both its holding intensity and hysteresis loop size are shown to decrease with increasing donor concentration. Two possibilities are suggested for experimentally determining one of the two parameters of the system - the exciton-donor coupling constant and the donor concentration, if the other parameter is known beforehand. (author). 36 refs, 2 figs

1991-01-01

354

Inversion of exciton level splitting in quantum dots  

CERN Document Server

The demonstration of degeneracy of the exciton spin states is an important step towards the production of entangled photons pairs from the biexciton cascade. We measure the fine structure of exciton and biexciton states for a large number of single InAs quantum dots in a GaAs matrix; the energetic splitting of the horizontally and vertically polarised components of the exciton doublet is shown to decrease as the exciton confinement decreases, crucially passing through zero and changing sign. Thermal annealing is shown to reduce the exciton confinement, thereby increasing the number of dots with splitting close to zero.

Young, R J; Shields, A J; Atkinson, P; Cooper, K; Ritchie, D A; Groom, K M; Tartakovskii, A I; Skolnick, M S

2005-01-01

355

Theoretical Study on Exciton Dynamics in Dendritic Systems: Exciton Recurrence and Migration  

Digital Repository Infrastructure Vision for European Research (DRIVER)

The optical functionalities such as exciton recurrence and migration for dendritic systems, e.g., dendrimers, are investigated using the quantum master equation (QME) approach based on the ab initio molecular orbital configuration interaction (MOCI) method, which can treat both the coherent and incoherent exciton dynamics at the first principle level. Two types of phenylacetylene dendrimers, Cayley-tree dendrimer and nanostar dendrimer with anthracene core, are examined to elucidate the featu...

Masayoshi Nakano; Ryohei Kishi; Takuya Minami; Kyohei Yoneda

2009-01-01

356

Recombination of radiation defects in alkali halide crystals at low temperature  

International Nuclear Information System (INIS)

The complex investigation of the ionic-electronic processes of the charge transport and thermally stimulated luminescence of colour centres (I, ?, F, F', Vsub(K) and H) in X-irradiated KBr, NaCl, NaI and KCl crystals at 4.2 K has been carried out. The experiment has been performed on X-irradiated KBr at 4.2 K by a complex of thermally stimulated methods. A number of substages of temperature annealing of different defects was studied. The effect of electron tunneling ''recharge'' in the excited defect pair (F-H) during photostimulation in the F-band of KBr is observed which results in the charged Frenkel defect pair formation (?-I). The conclusion is drawn that the tunneling ''recharge'' of the non-excited F-H pair is unlikely. It was observed that the Frenkel defect (I, ?, ''H-Vsub(K)'', F, H') accumulation efficiencies increases by 10% or more if the X-irradiation at 4.6 K is carried out under an applied DC field of 3 kV/cm. Two competitive mechanism of the primary generation of the anion Frenkel defects are discussed. The formation of the self-trapped excitons and their luminescence as a result of the H and F centers direct recombination could be noneffective. It is found that anion exciton dissociation into the electron- hole pair is a process competitive to the exciton decay into the Frenkel defect pair

1981-01-01

357

Free exciton luminescence in ZnCdSe solid solutions, ZnSe epitaxial films and ZnSe1-xSx/ZnSe1-ySy superlattices  

International Nuclear Information System (INIS)

We studied the luminescence of bulk Zn1-xCdxSe solid solutions epitaxial ZnSe films and ZnSe1-xSx/ZnSe1-ySy superlattices at T 2-80 K. The analysis of the bands of exciton radiative annihilation with simultaneous emission of 1 or 2 LO phonons shows that the excitons in the studied samples can be considered as free particles with thermalized distribution of kinetic energies. In ZnSe1-xSx/ZnSe1-ySy superlattices a three-dimensional character of exciton motion was established. The periodic potential of superlattice perturbs the exciton wave vector and induces additional elastic scattering of excitons. (author)

1996-05-27

358

Interplay of Cu and oxygen vacancy in optical transitions and screening of excitons in ZnO:Cu films  

Science.gov (United States)

We study room temperature optics and electronic structures of ZnO:Cu films as a function of Cu concentration using a combination of spectroscopic ellipsometry, photoluminescence, and ultraviolet-visible absorption spectroscopy. Mid-gap optical states, interband transitions, and excitons are observed and distinguishable. We argue that the mid-gap states are originated from interactions of Cu and oxygen vacancy (Vo). They are located below conduction band (Zn4s) and above valence band (O2p) promoting strong green emission and narrowing optical band gap. Excitonic states are screened and its intensities decrease upon Cu doping. Our results show the importance of Cu and Vo driving the electronic structures and optical transitions in ZnO:Cu films.

Darma, Yudi; Seng Herng, Tun; Marlina, Resti; Fauziah, Resti; Ding, Jun; Rusydi, Andrivo

2014-02-01

359

Engineering the spin-flip limited exciton dephasing in colloidal CdSe/CdS quantum dots.  

Science.gov (United States)

We have measured the intrinsic exciton dephasing in high-quality zinc blende CdSe/CdS colloidal quantum dots in the temperature range from 5 to 170 K using a sensitive three-beam photon echo technique in heterodyne detection, which is not affected by spectral diffusion. Pure dephasing via acoustic phonons dominates the initial dynamics, followed by an exponential zero-phonon line dephasing. From the temperature dependence of the zero-phonon line dephasing, the exciton lifetime, and the exciton thermalization within its fine structure, we show that the zero-phonon line dephasing of the lowest bright state originates from the phonon-assisted spin-flip to dark exciton states. Importantly, we can control the dephasing by tailoring the exciton fine structure through its dependence on the dot core size and shell thickness, as expected from the spin-flip mechanism. By reducing the electron-hole exchange interaction with increasing core size and delocalization of the electron wave function in the quasi-type-II core/shell band alignment, we find the longest zero-phonon line dephasing time of ?110 ps at 5 K in dots with the largest core diameter (5.7 nm) and the thickest CdSe shell (9 monolayers) in the series studied. PMID:22564176

Accanto, Nicolò; Masia, Francesco; Moreels, Iwan; Hens, Zeger; Langbein, Wolfgang; Borri, Paola

2012-06-26

360

Singlet exciton fission in a hexacene derivative.  

Science.gov (United States)

Hexacene, an acene with six benzene rings, is notable for its exceptionally small triplet energy, around one third of the singlet energy. Herein, singlet fission, i.e., conversion of a singlet exciton into two triplets, is demonstrated in a thin film of hexacene derivative, employing both transient absorption spectroscopy and magnetic field effects on photocurrent. PMID:23293054

Lee, Jiye; Bruzek, Matthew J; Thompson, Nicholas J; Sfeir, Matthew Y; Anthony, John E; Baldo, Marc A

2013-03-13

 
 
 
 
361

Effective models for excitons in carbon nanotubes  

DEFF Research Database (Denmark)

We analyse the low lying spectrum of a model of excitons in carbon nanotubes. Consider two particles with opposite charges and a Coulomb self-interaction, placed on an infinitely long cylinder. If the cylinder radius becomes small, the low lying spectrum of their relative motion is well described by a one-dimensional effective Hamiltonian which is exactly solvable. Udgivelsesdato: FEB

Cornean, Horia; Duclos, Pierre

2007-01-01

362

Spin dynamics of isoelectronic bound excitons in ZnO  

Science.gov (United States)

Time-resolved optical spin orientation is employed to study spin dynamics of I* and I1* excitons bound to isoelectronic centers in bulk ZnO. It is found that spin orientation at the exciton ground state can be generated using resonant excitation via a higher lying exciton state located at about 4 meV from the ground state. Based on the performed rate equation analysis of the measured spin dynamics, characteristic times of subsequent hole, electron, and direct exciton spin flips in the exciton ground state are determined as being ?hs = 0.4 ns, ?es? 15 ns, and ?ehs? 15 ns, respectively. This relatively slow spin relaxation of the isoelectronic bound excitons is attributed to combined effects of (i) weak e-h exchange interaction, (ii) restriction of the exciton movement due to its binding at the isoelectronic center, and (iii) suppressed spin-orbit coupling for the tightly bound hole.

Chen, S. L.; Chen, W. M.; Buyanova, I. A.

2014-06-01

363

Laser Control of Dissipative Two-Exciton Dynamics in Molecular Aggregates  

CERN Document Server

There are two types of two-photon transitions in molecular aggregates, that is, non-local excitations of two monomers and local double excitations to some higher excited intra-monomer electronic state. As a consequence of the inter-monomer Coulomb interaction these different excitation states can be converted into each other. Higher excited intra-monomer states are rather short-lived due to efficient internal conversion of electronic into vibrational energy. Combining both processes leads to the annihilation of an electronic excitation state, which is a major loss channel for establishing high excitation densities in molecular aggregates. Applying theoretical pulse optimization techniques to a Frenkel exciton model it is shown that the dynamics of two-exciton states in linear aggregates (dimer to tetramer) can be influenced by ultrafast shaped laser pulses. In particular, it is studied to what extent the decay of the two-exciton population by inter-band transitions can be transiently suppressed. Intra-band dy...

Yan, Yun-an

2012-01-01

364

Heavy n-doping: Wannier-Mott and Mahan excitons in ZnO  

Energy Technology Data Exchange (ETDEWEB)

Transparent conductive oxides such as ZnO are highly interesting within the modern field of optoelectronics since they have large fundamental band gaps while intentional as well as unintentional n-doping renders them conductive. However, the free electrons in the material form a degenerate electron gas which occupies the lowest conduction-band states and whose effect on the optical properties is unknown. In addition to the Pauli blocking of the lowest optical transitions, the degenerate electron gas significantly influences the screening of the electron-hole interaction. We generalize the solution of the Bethe-Salpeter equation for the polarization function to investigate both of these aspects as well as their interplay with the excitonic effects for n-ZnO. We introduce k-dependent occupation numbers to account for the Pauli blocking. The additional screening due to the free electrons is taken into account by means of a Thomas-Fermi approach. Our approach essentially captures the involved physics, hence, we observe a Mahan exciton at the absorption edge - in perfect agreement with a measured result. We show that due to the strong decrease of the binding energy and the oscillator strength with an increasing free-electron concentration in the material an excitonic Mott transition is barely observable.

Schleife, Andre; Roedl, Claudia; Hannewald, Karsten; Bechstedt, Friedhelm [Institut fuer Festkoerpertheorie und -optik and European Theoretical Spectroscopy Facility, Friedrich-Schiller-Universitaet Jena, 07743 Jena (Germany)

2011-07-01

365

Pouring Excitons into a Two-Dimensional Bucket: Equilibration of Indirect Excitons in an In-Plane Harmonic Potential  

CERN Document Server

We have trapped a gas of long-lifetime, high-mobility excitons in an in-plane harmonic potential. Trapping is an important step toward the goal of a controlled Bose-Einstein condensate of excitons. We show that the repulsive interaction between the excitons plays a dominant role in the behavior of the excitons, in contrast to the weak interactions in atomic gases. We show that under proper conditions the excitons thermalize in the trap to a well-defined equilibrium spatial distribution.

Vörös, Z; Pfeiffer, L; West, K

2006-01-01

366

Exciton diffusion in monolayer and bulk MoSe2  

Science.gov (United States)

The exciton dynamics in monolayer and bulk MoSe2 samples are studied by transient absorption microscopy with a high spatiotemporal resolution. Excitons are injected with a point-like spatial distribution using a tightly focused femtosecond pulse. The spatiotemporal dynamics of these excitons are monitored by measuring transient absorption of a time-delayed and spatially scanned probe pulse. We obtain the exciton diffusion coefficients of 12 +/- 3 and 19 +/- 2 cm2 s-1 and exciton lifetimes of 130 +/- 20 and 210 +/- 10 ps in the monolayer and bulk samples, respectively. These values are useful for understanding excitons and their interactions with the environment in these structures and potential applications of MoSe2 in optoelectronics and electronics.The exciton dynamics in monolayer and bulk MoSe2 samples are studied by transient absorption microscopy with a high spatiotemporal resolution. Excitons are injected with a point-like spatial distribution using a tightly focused femtosecond pulse. The spatiotemporal dynamics of these excitons are monitored by measuring transient absorption of a time-delayed and spatially scanned probe pulse. We obtain the exciton diffusion coefficients of 12 +/- 3 and 19 +/- 2 cm2 s-1 and exciton lifetimes of 130 +/- 20 and 210 +/- 10 ps in the monolayer and bulk samples, respectively. These values are useful for understanding excitons and their interactions with the environment in these structures and potential applications of MoSe2 in optoelectronics and electronics. Electronic supplementary information (ESI) available: Relationship between the differential reflection, the differential absorption, and the exciton density; drift diffusion model and fits to the exciton density profiles. See DOI: 10.1039/c3nr06863c

Kumar, Nardeep; Cui, Qiannan; Ceballos, Frank; He, Dawei; Wang, Yongsheng; Zhao, Hui

2014-04-01

367

Process and composition for drying of gaseous hydrogen halides  

Energy Technology Data Exchange (ETDEWEB)

This patent describes a scavenger, having utility for drying a gaseous hydrogen halide of the formula HX, wherein X is selected from the group consisting of bromine, chlorine, fluorine, and iodine, to remove water impurity therefrom and produce an essentially completely water-free gaseous hydrogen halide effluent characterized by residual water concentration of below 0.1 part per million by volume (ppm), the scavenger comprising: a support; and associated with the support an active scavenging moiety selected from one or more members of the group consisting of: metal halide compounds dispersed in the support, of the formula MX{sub y}; metal halide pendant functional groups, of the formula {minus}MX{sub y{minus}1}, covalently bonded to the support. This patent describes an apparatus for drying a gaseous hydrogen halide of the formula HX, wherein X is selected from the group consisting of bromine, chlorine, fluorine, and iodine, to remove water impurity therefrom, comprising: a vessel containing a bed of a scavenger according to the above claim; means for introducing the water impurity-containing gaseous hydrogen halide to the vessel for passage through the bed therein; and means for discharging water impurity-depleted gaseous hydrogen halide from the vessel.

Tom, G.M.; Brown, D.W.

1989-08-01

368

Nickel-Catalyzed Borylation of Halides and Pseudo-Halides with Tetrahydroxydiboron [B2(OH)4  

Science.gov (United States)

Arylboronic acids are gaining increased importance as reagents and target structures in a variety of useful applications. Recently, the palladium-catalyzed synthesis of arylboronic acids employing the atom economical tetrahydroxydiboron (BBA) reagent has been reported. The high cost associated with palladium, combined with several limitations of both palladium and copper-catalyzed processes, prompted us to develop an alternative method. Thus, the nickel-catalyzed borylation of aryl and heteroaryl halides and pseudo-halides using tetrahydroxydiboron (BBA) has been formulated. The reaction proved to be widely functional group tolerant and applicable to a number of heterocyclic systems. To the best of our knowledge, the examples presented here represent the only effective Ni-catalyzed Miyaura borylations conducted at room temperature.

Molander, Gary A.; Cavalcanti, Livia N.; Garcia-Garcia, Carolina

2013-01-01

369

On the equivalence of two approaches in the exciton-polariton theory  

International Nuclear Information System (INIS)

The polariton effect in the optical processes involving photons with energies near that of an exciton is investigated by the Bogolubov diagonalization and the Green function approaches in a simple model of the direct band gap semiconductor with the electrical dipole allowed transition. To take into account the non-resonant terms of the interaction Hamiltonian of the photon-exciton system the Green function approach derived by Nguyen Van Hieu is presented with the use of Green's function matrix technique analogous to that suggested by Nambu in the theory of superconductivity. It is shown that with the suitable choice of the phase factors the renormalization constants are equal to the diagonalization coefficients. The disperson of polaritons and the matrix elements of processes with the participation of polaritons are identically calculated by both methods. However the Green function approach has an advantage in including the damping effect of polaritons. (author)

1983-01-01

370

Excitonic luminescence of ZnSe single crystals doped with Au  

Energy Technology Data Exchange (ETDEWEB)

Photoluminescence spectra of ZnSe single crystals doped with Au during a long high-temperature treatment of as-grown crystals in Zn+Au or Se+Au melts are investigated in the temperature range from 83 to 300 K. The Au-doping from Zn+Au melt leads to the formation of both simple defects (Au{sub i} donors and Au{sub Zn} acceptors) and acceptor associates (Au{sub Zn}-Au{sub i}). The edge luminescence is attributed to radiative annihilation of Au{sub i} and V{sub Se} donor-bound excitons. The edge spectra of the crystals doped with Au from Se+Au melt contain the band ascribed to radiative annihilation of Au{sub Zn} acceptor-bound excitons.

Nedeoglo, N.D. [Department of Physics, State University of Moldova, A. Mateevich str. 60, MD 2009 Kishinau (Moldova, Republic of)]. E-mail: nedeoglo@usm.md; Avdonin, A.N. [Department of Physics, State University of Moldova, A. Mateevich str. 60, MD 2009 Kishinau (Moldova, Republic of); Ivanova, G.N. [Department of Physics, State University of Moldova, A. Mateevich str. 60, MD 2009 Kishinau (Moldova, Republic of); Nedeoglo, D.D. [Department of Physics, State University of Moldova, A. Mateevich str. 60, MD 2009 Kishinau (Moldova, Republic of); Kolibaba, G.V. [Department of Physics, State University of Moldova, A. Mateevich str. 60, MD 2009 Kishinau (Moldova, Republic of); Sirkeli, V.P. [Department of Physics, State University of Moldova, A. Mateevich str. 60, MD 2009 Kishinau (Moldova, Republic of)

2005-04-15

371

Theory of excitonic second-harmonic generation in monolayer MoS2  

Science.gov (United States)

Recent experimental results have demonstrated the ability of monolayer MoS2 to efficiently generate second harmonic fields with susceptibilities between 0.1 and 100 nm/V. However, few theoretical calculations exist with which to interpret these findings. In particular, it is of interest to theoretically estimate the modulus of the second harmonic response since experimental reports on this differ by almost three orders of magnitude. Here, we present calculations of the second harmonic response based on a tight-binding band structure and implementation of excitons in a Bethe-Salpeter framework. We compare directly with recent experimental findings demonstrating a good agreement with the excitonic theory regarding, e.g., peak position. Furthermore, we predict an off-resonance susceptibility on the order of 0.1 nm/V, while on-resonance values rise to 4 nm/V.

Trolle, Mads L.; Seifert, Gotthard; Pedersen, Thomas G.

2014-06-01

372

Temperature Evolution of Excitonic Absorptions in Cd(1-x)Zn(x)Te Materials  

Science.gov (United States)

The studies consist of measuring the frequency dependent transmittance (T) and reflectance (R) above and below the optical band-gap in the UV/Visible and infrared frequency ranges for Cd(l-x),Zn(x),Te materials for x=0 and x=0.04. Measurements were also done in the temperature range from 5 to 300 K. The results show that the optical gap near 1.49 eV at 300 K increases to 1.62 eV at 5 K. Finally, we observe sharp absorption peaks near this gap energy at low temperatures. The close proximity of these peaks to the optical transition threshold suggests that they originate from the creation of bound electron-hole pairs or excitons. The decay of these excitonic absorptions may contribute to a photoluminescence and transient background response of these back-illuminated HgCdTe CCD detectors.

Quijada, Manuel A.; Henry, Ross

2007-01-01

373

Mechanoluminescence response to the plastic flow of coloured alkali halide crystals  

International Nuclear Information System (INIS)

The present paper reports the luminescence induced by plastic deformation of coloured alkali halide crystals using pressure steps. When pressure is applied onto a ?-irradiated alkali halide crystal, then initially the mechanoluminescence (ML) intensity increases with time, attains a peak value and later on it decreases with time. The ML of diminished intensity also appears during the release of applied pressure. The intensity Im corresponding to the peak of ML intensity versus time curve and the total ML intensity IT increase with increase in value of the applied pressure. The time tm corresponding to the ML peak slightly decreases with the applied pressure. After tm, initially the ML intensity decreases at a fast rate and later on it decreases at a slow rate. The decay time of the fast decrease in the ML intensity is equal to the pinning time of dislocations and the decay time for the slow decrease of ML intensity is equal to the diffusion time of holes towards the F-centres. The ML intensity increases with the density of F-centres and it is optimum for a particular temperature of the crystals. The ML spectra of coloured alkali halide crystals are similar to the thermoluminescence and afterglow spectra. The peak ML intensity and the total ML intensity increase drastically with the applied pressure following power law, whereby the pressure dependence of the ML intensity is related to the work-hardening exponent of the crystals. The ML also appears during the release of the applied pressure because of the movement of dislocation segments and movements of dislocation lines blocked under pressed condition. On the basis of the model based on the mechanical interaction between dislocation and F-centres, expressions are derived for the ML intensity, which are able to explain different characteristics of the ML. From the measurements of the plastico ML induced by the application of loads on ?-irradiated alkali halide crystals, the pinning time of dislocations, diffusion time of holes towards F-centres, the energy gap Ea between the bottom of acceptor dislocation band and the energy level of interacting F-centres, and work-hardening exponent of the crystals can be determined. As in the elastic region the strain increases linearly with stress, the ML intensity also increases linearly with stress, however, as in the plastic region, the strain increases drastically with stress and follows power law, the ML intensity also increases drastically with stress and follows power law. Thus, the ML is intimately related to the plastic flow of alkali halide crystals.

2010-02-01

374

The effects of crystallographic orientation and strain on the properties of excitonic emission from wurtzite InGaN/GaN quantum wells  

International Nuclear Information System (INIS)

We have examined in detail crystal orientation effects on the properties of excitonic emission from wurtzite InxGa1-xN/GaN quantum wells (QWs) with piezoelectric polarization using exciton binding and transition energy calculations based on a single-band effective-mass theory. We show numerical results for the bandgaps, effective heavy-hole masses, piezoelectric polarizations and fields, exciton wavefunctions, exciton binding and transition energies and radiative lifetimes of excitonic emission as a function of the QW crystallographic growth planes. Band-edge and effective-mass parameters for a continuum of GaN crystallographic orientations, on which InGaN/GaN QWs are grown, were obtained from In-composition- and strain-dependent k·p calculation for wurtzite InxGa1-xN, using the 6x6 k·p Hamiltonian in appropriate {hkil} representations. We have performed calculations for a continuum of technologically relevant QW growth planes {hh-bar0l} oriented at various angles ? relative to the (0001) c-plane. The excitonic ground- and first-excited-state energies and wavefunctions were calculated using an effective potential method. A strong reduction of average in-plane heavy-hole effective mass and normal to the plane piezoelectric polarization and field is observed as ? varies from ? = 0 deg. (i.e. the c-axis direction) to ? = 49.5 deg., where the piezoelectric polarization and electric field reverse their orientation with respect to the plane of the QW. The decrease of the electric field in the InGaN/GaN QW growth direction leads to an increased exciton transition energy and oscillator strength, which results in the increase of the exciton binding energy and decrease of the excitonic radiative lifetime. For angles ?>49.5 deg. only small variations on the order of ?10% in the exciton binding and transition energies and excitonic radiative lifetime are observed for narrow In0.12Ga0.88N/GaN QWs that have widths less than ? 3.5 nm. The average in-plane heavy-hole effective mass reaches its minimum for ? = 90 deg., i.e. m-plane {11-bar00} growth. These results indicate that InGaN/GaN QW structures grown on non-(0001)-oriented planes in a wide variety of angles 49.5 deg. ???90 deg. can be used for optimized operation of optoelectronic devices

2008-05-28

375

Impurity trapped exciton states related to rare earth ions in crystals under high hydrostatic pressure  

Energy Technology Data Exchange (ETDEWEB)

Emission related to rare earth ions in solids takes place usually due to 4f{sup n} {yields} 4f{sup n} and 4f{sup n-1}5d{sup 1} {yields} 4f{sup n} internal transitions. In the case of band to band excitation the effective energy transfer from the host to optically active impurity is required. Among other processes one of the possibilities is capturing of the electron at excited state and hole at the ground state of impurity. Localization of electron or hole at the dopand site creates a long range Coulomb potential that attracts the second carrier which then occupies the localized Rydberg-like states. Such a system can be considered as impurity trapped exciton. Usually impurity trapped exciton is a short living phenomenon which decays non-radiatively leaving the impurity ion in the excited state. However, in several compounds doped with Eu{sup 2+} the impurity trapped exciton states become stable and contribute to the radiative processes though anomalous luminescence that appears apart of the 4f{sup 7} {yields} 4f{sup 7} and 4f{sup 7}5d{sup 1} {yields} 5f{sup 7} emission. In this contribution pressure effect on energies of the 4f{sup n-1}5d{sup 1}{yields}5f{sup n} transitions in Ln doped oxides and fluorides as well as influence of pressure on the energy of impurity trapped exciton states is discussed. The latest results on high pressure investigations of luminescence related to Pr{sup 3+}, and Eu{sup 2+} in different lattices are reviewed.

Grinberg, M., E-mail: fizmgr@univ.gda.pl; Mahlik, S. [University of Gdan' sk, Institute of Experimental Physics (Poland)

2013-01-15

376

Study of Ammonium Halides by Neutron Spectrometry  

International Nuclear Information System (INIS)

The scattering of slow neutrons from the ammonium halides NH4Cl, ND4Cl, and NH4Br has been studied at several temperatures with a view to elucidate the nature of the specific heat anomaly associated with these substances. According to Pauling the anomaly is due to the onset of free rotation of the ammonium ion, whereas according to Frenkel the anomaly is due to an order-disorder process. The present measurements rule out the possibility of free rotation thus confirming the Frenkel hypothesis. Values of the torsional frequencies and barrier heights for rotation have been derived and the relation of neutron scattering data to infra-red absorption. Raman scattering and NMR results are discussed. (author)

1963-01-01

377

Excitons, polarons, and laser action in poly(p-phenylene vinylene) films  

Science.gov (United States)

We have used a multitude of linear and nonlinear cw optical spectroscopies to study the optical properties of water precursor poly(p-phenylene vinylene) (PPV) thin films. These spectroscopies include absorption, photoluminescence, photoinduced absorption and their respective optically detected magnetic resonance, and electroabsorption spectroscopy. We have studied singlet and triplet excitons, polarons, and laser action in PPV films. We found that the lowest-lying absorption band is excitonic in origin. It consists of two absorption components due to a bimodal distribution of the polymer chain conjugation lengths. Electroabsorption spectroscopy unambiguously shows the positions of the lowest-lying odd parity exciton 1Bu at 2.59 eV and two of the higher-lying even-parity excitons, namely, mAg at 3.4 eV and kAg at 3.7 eV. From these exciton energies we obtained a lower bound for the exciton binding energy in PPV, Eb(min)=E(mAg)-E(1Bu)=0.8 eV. The quantum efficiency spectrum for triplet exciton photogeneration consists of two contributions; the intersystem crossing and, at higher energies, singlet fission. From the onset of the singlet fission process at ESF=2ET, we could estimate the energy of the lowest-lying triplet exciton, 1 3Bu, at 1.55 eV, with a singlet-triplet splitting as large as 0.9 eV. From photoinduced absorption spectroscopy we measured the triplet-triplet transition, T-->T*, to be 1.45 eV. The quantum efficiency spectrum for polaron photogeneration also consists of two contributions: one extrinsic and the other intrinsic. The latter shows a monotonically increasing function of energy with an energy onset at 3.3 eV. The intrinsic photogeneration process is analyzed with a model of free-electron-hole pair photogeneration, which separate more effectively at higher energy. The carrier generation quantum yield at 3.65 eV is estimated to be 0.5%. The quantum efficiency for photoluminescence, on the other hand, shows one single step-function process, with an onset at 2.4 eV. The photoluminescence spectrum is well structured, showing five phonon side bands with 190 meV separation. We have also studied laser action in PPV thin films and microcavities such as microrings and microdisks. The effective gain spectrum is calculated and the estimated threshold excitation intensity for laser action for the 0-1 transition is found to be in good agreement with the data, with an estimated exciton density of 2×1018 cm-3. Lasing from microring devices shows several narrow waveguide modes, with intermode spacing of 0.45 nm that corresponds to an effective mode refractive index, neff=1.45. The spectral width of the laser modes is resolution limited and gives a lower estimate of the cavity quality factor, Q. For microrings we found Q>5000, which is limited by self-absorption in the polymer film.

Österbacka, R.; Wohlgenannt, M.; Shkunov, M.; Chinn, D.; Vardeny, Z. V.

2003-05-01

378

Different responses of localized and extended excitons to exciton - exciton scattering manifested in excitation density-dependent photoluminescence excitation spectra  

International Nuclear Information System (INIS)

Excitation density-dependent photoluminescence excitation (PLE) measurements are performed for a disordered two-dimensional (2D) In0.40Ga0.60As/GaAs(311)B quantum dot superlattice (QDSL) containing localized and extended states separated by a mobility edge. For low excitation densities, the PLE spectrum is independent of excitation density. In the extended state region, it takes the shape of the absorption spectrum of the 2D QDSL. With increasing excitation density, the PLE spectral shape is dramatically changed. For excitation densities at which the state filling can be excluded, it is found that the different responses of localized and extended excitons to exciton - exciton scattering are responsible for the observed change in the PLE spectrum. A qualitative explanation for the evolution of the PLE spectrum is presented based on the excitation density-dependent capture, relaxation and recombination times obtained previously in time-resolved photoluminescence (TRPL) experiments. The mobility edge extracted from the modification of the PLE spectrum is in good agreement with that determined by TRPL measurements. To show the effect of coupling strength, a comparison of the excitation density-dependent PLE spectra is made between the 2D QDSL and a weekly coupled quantum dot array. It further confirms the existence of highly extended states in the 2D QDSL. [copyright] 2001 American Institute of Physics

2001-06-01

379

High average brightness rare-gas halide laser technology  

Energy Technology Data Exchange (ETDEWEB)

The authors describe several high repetition rate (100-1000Hz) discharge pumped rare-gas halide laser systems having near diffraction-limited divergence, resulting in unusually high average brightness.

Moody, S.E.; Mullaney, G.J.; Grossman, W.; Cassady, P.E.; Byron, S.

1982-01-01

380

Measurement of cadmium vapor pressure in the metal halide arc  

International Nuclear Information System (INIS)

Spectral diagnostics of operating arcs are used to determine the core densities of cadmium as an additive in high pressure mercury and metal halide (NaI, ScI_3, ThI_4) arcs. Based on the cold spot temperatures of the arc tube (700"0C to 800"0C), the total charge of cadmium would be expected to be vaporized, which the data substantiate when only mercury and cadmium are present; however, the presence of the metal halides suppresses the cadmium vapor pressure significantly. Absorbance measurements in closed cells containing various amounts of cadmium and metal halides point to a chemical interaction between cadmium and the metal halides which affects the cadmium vapor pressure

1984-01-01

 
 
 
 
381

Optimized spatial frequency response in silver halide sensitized gelatin  

Digital Repository Infrastructure Vision for European Research (DRIVER)

Silver halide sensitized gelatin processing is optimized to increase the spatial frequency response in Agfa-Gevaert 8E75 HD emulsion; therefore a diffraction efficiency of 55% in reflection gratings of 5000 lines/mm is achieved.

Fimia Gil, Antonio; Pascual Villalobos, Inmaculada; Bele?ndez Va?zquez, Augusto

1991-01-01

382

Crystallographic and fluorescence studies of the interaction of haloalkane dehalogenase with halide ions. Studies with halide compounds reveal a halide binding site in the active site.  

Science.gov (United States)

Haloalkane dehalogenase from Xanthobacter autotrophicus GJ10 catalyzes the conversion of 1,2-dichloroethane to 2-chloroethanol and chloride without use of oxygen or cofactors. The active site is situated in an internal cavity, which is accessible from the solvent, even in the crystal. Crystal structures of the dehalogenase enzyme complexed with iodoacetamide, chloroacetamide, iodide, and chloride at pH 6.2 and 8.2 revealed a halide binding site between the ring NH's of two tryptophan residues, Trp-125 and Trp-175, located in the active site. The halide ion lies on the intersection of the planes of the rings of the tryptophans. The binding of iodide and chloride to haloalkane dehalogenase caused a strong decrease in protein fluorescence. The decrease could be fitted to a modified form of the Stern-Volmer equation, indicating the presence of fluorophors of different accessibilities. Halide binding was much stronger at pH 6.0 than at pH 8.2. Assuming ligand binding to Trp-125 and Trp-175 as the sole cause of fluorescence quenching, dissociation constants at pH 6.0 with chloride and iodide were calculated to be 0.49 +/- 0.04 and 0.074 +/- 0.007 mM, respectively. Detailed structural investigation showed that the halide binding site probably stabilizes the halide product as well as the negatively charged transition state occurring during the formation of the covalent intermediate. PMID:8369276

Verschueren, K H; Kingma, J; Rozeboom, H J; Kalk, K H; Janssen, D B; Dijkstra, B W

1993-09-01

383

Optical properties of MgZnO alloys: Excitons and exciton-phonon complexes  

International Nuclear Information System (INIS)

The characteristics of the excitonic absorption and emission around the fundamental bandgap of wurtzite MgxZn1-xO grown on c-plane sapphire substrates by plasma assisted molecular beam epitaxy with Mg contents between x = 0 and x = 0.23 are studied using spectroscopic ellipsometry and photoluminescence (PL) measurements. The ellipsometric data were analyzed using a multilayer model yielding the dielectric function (DF). The imaginary part of the DF for the alloys exhibits a pronounced feature which is attributed to exciton-phonon coupling (EPC) similar to the previously reported results for ZnO. Thus, in order to determine reliable transition energies, the spectral dependence is analyzed by a model which includes free excitonic lines, the exciton continuum, and the enhanced absorption due to EPC. A line shape analysis of the temperature-dependent PL spectra yielded in particular the emission-related free excitonic transition energies, which are compared to the results from the DF line-shape analysis. The PL linewidth is discussed within the framework of an alloy disorder model.

2011-07-01

384

Mott transition of excitons in GaAs-GaAlAs quantum wells  

International Nuclear Information System (INIS)

We investigate the breakup of bound electron–hole pairs, known as Mott transition of excitons, in GaAs-GaAlAs quantum wells with increasing excitation, comparing two different theoretical approaches. Firstly, a thermodynamic approach is used to investigate the ionization equilibrium between electrons, holes and excitons, where the abrupt jump of the degree of ionization from 0 to 1 indicates the Mott density. It is extended to a self-consistent quasi-particle approximation (QPA) for the carrier properties, including dynamical screening of the Coulomb interaction between carriers. Secondly, a spectral approach based on the semiconductor Bloch equations within linear optical response is used, considering the quasi-particle (QP) properties of carriers and the dynamical screening between electron–hole pairs. While the first is effectively a one-particle approach, in the second the whole two-particle spectrum is analyzed. Within the thermodynamic approach, a simple criterion for the Mott transition can be given: namely, if the sum of chemical potentials of carriers, reflecting the effective shrinkage of the band edge, crosses the exciton energy with increasing excitation. We demonstrate that this simple picture cannot be maintained in the two-particle approach. Here, a compact quantity, which describes the behavior of the band edge, does not exist. In fact, the behavior of the single states in the spectrum is generated by the interplay of dynamical screening in the interband self-energy and the effective interaction of the electron–hole pairs. Moreover, the band edge cannot be clearly resolved, since it is merged with excited exciton states (e.g. 2s state), which show up only for densities far below the Mott density. Instead of a Mott density, only a density range can be given, where the Mott transition appears. We demonstrate that a small damping as a prerequisite for the validation of the extended QPA in the thermodynamic approach breaks down, analyzing (i) the dephasing processes with increasing excitation, (ii) the strong increase of the excitonic linewidth and (iii) comparing with the lifetime of carriers in the QP description. (paper)

2012-09-01

385

Moments of the phonon spectra in alkali halides  

International Nuclear Information System (INIS)

The results of a systematic calculation of the moments of the phonon spectra for 20 alkali halides are reported. The relevant phonon spectra needed are taken from the Green's function compilations by Haridasan et al. The calculated moments show good agreement with the available experimental data for sodium, potasium and caesium halides. The moments deduced from Debye-Waller factor data and from specific heat data are mutually consistent. A critical inter-comparison of these moments in these crystals is made. (author)

1976-06-01

386

Triplet exciton diffusion and delayed interfacial charge separation in a TiO{sub 2}/PdTPPC bilayer: Monte Carlo simulations  

Energy Technology Data Exchange (ETDEWEB)

Nanosecond photoexcitation of a bilayer of smooth anatase TiO{sub 2} coated with palladium tetrakis(4-carboxyphenyl)porphyrin (PdTPPC) results in a delayed, after-pulse growth of the conductivity over many microseconds as monitored by time-resolved microwave conductivity. This phenomenon is attributed to the slow diffusion of PdTPPC triplet excitons followed by electron injection into the TiO{sub 2} conduction band. The temporal form and intensity dependence of this process have been simulated by Monte Carlo (MC) calculations of exciton diffusion and exciton-exciton annihilation. Good agreement between the experimental results and MC simulations are obtained using a triplet exciton diffusion coefficient D{sub E} = 8 x 10{sup -11} m{sup 2}/s, exciton lifetime {rho}{sub E}{>=}10 {mu}s, effective triplet-triplet annihilation distance R{sub a} = 1.5 nm and interfacial electron injection efficiency {theta}{sub inj} = 0.44. (Author)

Kroeze, Jessica E.; Savenije, Tom J.; Candeias, Luis P.; Warman, John M.; Siebbeles, Laurens D.A. [Delft Univ. of Technology, Radiation Chemistry Dept., Delft (Netherlands)

2005-01-15

387

Triplet exciton diffusion and delayed interfacial charge separation in a TiO{sub 2}/PdTPPC bilayer. Monte Carlo simulations  

Energy Technology Data Exchange (ETDEWEB)

Nanosecond photoexcitation of a bilayer of smooth anatase TiO{sub 2} coated with palladium tetrakis(4-carboxyphenyl)porphyrin (PdTPPC) results in a delayed, after-pulse growth of the conductivity over many microseconds as monitored by time-resolved microwave conductivity. This phenomenon is attributed to the slow diffusion of PdTPPC triplet excitons followed by electron injection into the TiO{sub 2} conduction band. The temporal form and intensity dependence of this process have been simulated by Monte Carlo (MC) calculations of exciton diffusion and exciton-exciton annihilation. Good agreement between the experimental results and MC simulations are obtained using a triplet exciton diffusion coefficient D{sub E}=8x10{sup -11}m{sup 2}/s, exciton lifetime t{sub E}>=10{mu}s, effective triplet-triplet annihilation distance R{sub a}=1.5nm and interfacial electron injection efficiency f{sub inj}=0.44.

Kroeze, Jessica E.; Savenije, Tom J.; Candeias, Luis P.; Warman, John M.; Siebbeles, Laurens D.A. [Radiation Chemistry Department, IRI, Delft University of Technology, Mekelweg 15, 2629 JB Delft (Netherlands)

2005-01-15

388

Microcavity controlled coupling of excitonic qubits  

CERN Document Server

Controlled non-local energy and coherence transfer enables light harvesting in photosynthesis and non-local logical operations in quantum computing. The most relevant mechanism of coherent coupling of distant qubits is coupling via the electromagnetic field. Here, we demonstrate the controlled coherent coupling of spatially separated excitonic qubits via the photon mode of a solid state microresonator. This is revealed by two-dimensional spectroscopy of the sample's coherent response, a sensitive and selective probe of the coherent coupling. The experimental results are quantitatively described by a rigorous theory of the cavity mediated coupling within a cluster of quantum dots excitons. Having demonstrated this mechanism, it can be used in extended coupling channels - sculptured, for instance, in photonic crystal cavities - to enable a long-range, non-local wiring up of individual emitters in solids.

Albert, F; Kasprzak, J; Strauß, M; Schneider, C; Höfling, S; Kamp, M; Forchel, A; Reitzenstein, S; Muljarov, E A; Langbein, W

2012-01-01

389

Semiconductors under strong excitation and exciton condensation  

International Nuclear Information System (INIS)

We report on research activities of the laboratory of C. Benoit a la Guillaume during the late sixties. In the first part we then have carrier injection in semiconductors and in particular CdS via electron bombardment, leading to laser effect. The spatial separation of spontaneous and stimulated emission allows us to study the optical gain as a function of injection rate and to characterise interaction processes between hot electrons and excitons. The second part is devoted to the condensation of excitons in a liquid electron-hole plasma. This phenomenon occurs at low temperature in Ge. It is characterized by an excitation threshold and a thermodynamical study of this original phase transition was made possible by the photoluminescence signatures of the two phases in quasi equilibrium. (orig.)

1995-06-01

390

Frenkel-exciton-polaritons in organic microcavities  

Scientific Electronic Library Online (English)

Full Text Available SciELO Brazil | Language: English Abstract in english We study organic microcavities in the strong coupling regime. The cavity exciton-polariton dispersion relations and quantum states are derived in using a microscopic theory. We consider the two cases of anisotropic organic crystals with one and two molecules per unit cell. In general, the cavity exc [...] iton-polaritons are a coherent superposition of both cavity mode polarizations and of both Davydov exciton branches. The obtained polarization mixing is in contrast to the case of typical inorganic semiconductor cavities in which no mixing between the TM and TE polarizations occurs. By applying the quasi-mode approximation, we derive the transmission, reflection, and absorption coefficients for high quality organic cavities.

Hashem, Zoubi; G. C., La Rocca.

391

Synthesis of methyl halides from biomass using engineered microbes.  

Science.gov (United States)

Methyl halides are used as agricultural fumigants and are precursor molecules that can be catalytically converted to chemicals and fuels. Plants and microorganisms naturally produce methyl halides, but these organisms produce very low yields or are not amenable to industrial production. A single methyl halide transferase (MHT) enzyme transfers the methyl group from the ubiquitous metabolite S-adenoyl methionine (SAM) to a halide ion. Using a synthetic metagenomic approach, we chemically synthesized all 89 putative MHT genes from plants, fungi, bacteria, and unidentified organisms present in the NCBI sequence database. The set was screened in Escherichia coli to identify the rates of CH(3)Cl, CH(3)Br, and CH(3)I production, with 56% of the library active on chloride, 85% on bromide, and 69% on iodide. Expression of the highest activity MHT and subsequent engineering in Saccharomyces cerevisiae results in productivity of 190 mg/L-h from glucose and sucrose. Using a symbiotic co-culture of the engineered yeast and the cellulolytic bacterium Actinotalea fermentans, we are able to achieve methyl halide production from unprocessed switchgrass (Panicum virgatum), corn stover, sugar cane bagasse, and poplar (Populus sp.). These results demonstrate the potential of producing methyl halides from non-food agricultural resources. PMID:19378995

Bayer, Travis S; Widmaier, Daniel M; Temme, Karsten; Mirsky, Ethan A; Santi, Daniel V; Voigt, Christopher A

2009-05-13

392

Singlet exciton fission in nanostructured organic solar cells.  

Science.gov (United States)

Singlet exciton fission is an efficient multiexciton generation process in organic molecules. But two concerns must be satisfied before it can be exploited in low-cost solution-processed organic solar cells. Fission must be combined with longer wavelength absorption in a structure that can potentially surpass the single junction limit, and its efficiency must be demonstrated in nanoscale domains within blended devices. Here, we report organic solar cells comprised of tetracene, copper phthalocyanine, and the buckyball C(60). Short wavelength light generates singlet excitons in tetracene. These are subsequently split into two triplet excitons and transported through the phthalocyanine. In addition, the phthalocyanine absorbs photons below the singlet exciton energy of tetracene. To test tetracene in nanostructured blends, we fabricate coevaporated bulk heterojunctions and multilayer heterojunctions of tetracene and C(60). We measure a singlet fission efficiency of (71 ± 18)%, demonstrating that exciton fission can efficiently compete with exciton dissociation on the nanoscale. PMID:21355536

Jadhav, Priya J; Mohanty, Aseema; Sussman, Jason; Lee, Jiye; Baldo, Marc A

2011-04-13

393

Modulational instability and solitons in excitonic semiconductor waveguides  

CERN Document Server

Nonlinear light propagation in a single-mode micron-size waveguide made of semiconducting excitonic material has been theoretically studied in terms of exciton-polaritons by using an analysis based on macroscopic fields. When a light pulse is spectrally centered in the vicinity of the ground-state Wannier exciton resonance, it interacts with the medium nonlinearly. This optical cubic nonlinearity is caused by the repulsive exciton-exciton interactions in the semiconductor, and at resonance it is orders of magnitude larger than the Kerr nonlinearity (e.g., in silica). We demonstrate that a very strong and unconventional modulational instability takes place, which has not been previously reported. After reducing the problem to a single nonlinear Schr\\"odinger-like equation, we also explore the formation of solitary waves both inside and outside the polaritonic gap and find evidence of spectral broadening. A realistic physical model of the excitonic waveguide structure is proposed.

Smyrnov, Oleksii A; Malzer, Stefan

2011-01-01

394

Q-deformed description of excitons and associated physical results  

Energy Technology Data Exchange (ETDEWEB)

We consider excitons in a quantum dot as q-deformed systems. The interaction of some excitonic systems with one cavity mode is considered. The dynamics of the system is obtained by diagonalizing the total Hamiltonian, and the emission spectrum of a quantum dot is derived. The physical consequences of a q-deformed exciton on the emission spectrum of a quantum dot are given. It is shown that when the exciton system deviates from Bose statistics, the emission spectra will become multi-peak. With our investigation we try to find the origin of the q-deformation of the exciton. The optical response of excitons, which is affected by the nonlinear nature of q-deformed systems, up to the second order of approximation is calculated and the absorption spectrum of the system is given.

Harouni, M Bagheri; Roknizadeh, R; Naderi, M H [Quantum Optics Group, Department of Physics, Faculty of Science, University of Isfahan, Hezar Jerib Str., 81746-73441, Isfahan (Iran, Islamic Republic of)], E-mail: m-baghreri@phys.ui.ac.ir, E-mail: rokni@sci.ui.ac.ir, E-mail: mhnaderi@phys.ui.ac.ir

2009-05-14

395

Molecular weight dependence of exciton diffusion in poly(3-hexylthiophene)  

DEFF Research Database (Denmark)

A joint experimental and theoretical study of singlet exciton diffusion in spin-coated poly(3-hexylthiophene) (P3HT) films and its dependence on molecular weight is presented. The results show that exciton diffusion is fast along the co-facial Ï?â??Ï? aggregates of polymer chromophores and about 100 times slower in the lateral direction between aggregates. Exciton hopping between aggregates is found to show a subtle dependence on interchain coupling, aggregate size, and Boltzmann statistics. Additionally, a clear correlation is observed between the effective exciton diffusion coefficient, the degree of aggregation of chromophores, and exciton delocalization along the polymer chain, which suggests that exciton diffusion length can be enhanced by tailored synthesis and processing conditions.

Masri, Zarifi; Ruseckas, Arvydas

2013-01-01

396

Excitons in small dielectric crystals: spectral distribution of oscillator strength  

International Nuclear Information System (INIS)

Excitonic states with small wave vectors, agreed with light by spatial phase and responsible for the formation of optical and luminescence properties, are considered for a dielectric crystal with an allowed excitonic transition. Spectroscopic manifestations of such excitons in a cubic-symmetry crystal are traced depending on the ratio of crystal size L to light wavelength ?. In a large crystal, the exciton energy spectrum is known to consist of two degenerate levels related to exciton polarization parallel and perpendicular to its wave vector. The inward part of this longitudinal-transverse splitting interval, making no contribution to optical characteristics at L/???, begins to play a growing role with a decrease of L/?. The energy distribution of oscillator strength inside the longitudinal-transverse splitting interval was obtained vs. L/?; it was found that at L/?1 excitonic states inside this interval intercept a significant part of oscillator strength and participate essentially in the formation of optical properties

2004-11-22

397

Exciton transfer dynamics in a trimer system  

International Nuclear Information System (INIS)

We study exciton transfer dynamics in a trimer system by investigating excitation transfer probability (ETP). We calculate ETP in the zero-temperature limit and theoretically predict the environment-assisted quantum critical effect, in which ETP exhibits a sudden change at the critical point of quantum phase transition for the trimer. In particular, we find that the steady-state ETP can be observed in the presence of the environment interaction. (general)

2013-01-01

398

Charge and exciton transport through molecular wires  

CERN Document Server

As functional elements in opto-electronic devices approach the singlemolecule limit, conducting organic molecular wires are the appropriateinterconnects that enable transport of charges and charge-like particles such as excitons within the device. Reproducible syntheses and athorough understanding of the underlying principles are therefore indispensable for applications like even smaller transistors, molecularmachines and light-harvesting materials. Bringing together experiment and theory to enable applications in real-life devices, this handbookand ready reference provides ess

Siebbeles, Laurens D A

2011-01-01

399

Excitones confinados en puntos cuánticos esferoidales prolatos  

Directory of Open Access Journals (Sweden)

Full Text Available Se usa el método variacional para resolver en forma aproximada la ecuación de onda de Schrodinger asociada a un exciton de Wannier-Mott confinado en un punto cuántico de forma esferoidal. Se analiza el efecto que el confinamento tiene sobre la energ?a del estado base del par electrón-hueco atrapado en una cristalita con esta geometr?a, considerando paredes tanto impenetrables como penetrables, según convenga, imponiendo las condiciones adecuadas en la frontera. Los resultados de este estudio, que se muestran a continuación, se comparan con los obtenidos por otros autores mediante métodos más sofisticados, as? como con experimentos realizados con cristalitas de CdS contenidas en materiales de diferente composición. En el caso de una barrera de potencial finita, se predice un tamaño cr?tico de la cristalita a partir del cual el exciton se desconfina, lo cual contrasta con el modelo de barrera de potencial infinita en donde el exciton nunca puede escapar de la región donde se encuentra.

J.L. Marín

2004-01-01

400

Microcavity controlled coupling of excitonic qubits.  

Science.gov (United States)

Controlled non-local energy and coherence transfer enables light harvesting in photosynthesis and non-local logical operations in quantum computing. This process is intuitively pictured by a pair of mechanical oscillators, coupled by a spring, allowing for a reversible exchange of excitation. On a microscopic level, the most relevant mechanism of coherent coupling of distant quantum bits--like trapped ions, superconducting qubits or excitons confined in semiconductor quantum dots--is coupling via the electromagnetic field. Here we demonstrate the controlled coherent coupling of spatially separated quantum dots via the photon mode of a solid state microresonator using the strong exciton-photon coupling regime. This is enabled by two-dimensional spectroscopy of the sample's coherent response, a sensitive probe of the coherent coupling. The results are quantitatively understood in a rigorous description of the cavity-mediated coupling of the quantum dot excitons. This mechanism can be used, for instance in photonic crystal cavity networks, to enable a long-range, non-local coherent coupling. PMID:23612288

Albert, F; Sivalertporn, K; Kasprzak, J; Strauß, M; Schneider, C; Höfling, S; Kamp, M; Forchel, A; Reitzenstein, S; Muljarov, E A; Langbein, W

2013-01-01

 
 
 
 
401

Microcavity controlled coupling of excitonic qubits  

Science.gov (United States)

Controlled non-local energy and coherence transfer enables light harvesting in photosynthesis and non-local logical operations in quantum computing. This process is intuitively pictured by a pair of mechanical oscillators, coupled by a spring, allowing for a reversible exchange of excitation. On a microscopic level, the most relevant mechanism of coherent coupling of distant quantum bits—like trapped ions, superconducting qubits or excitons confined in semiconductor quantum dots—is coupling via the electromagnetic field. Here we demonstrate the controlled coherent coupling of spatially separated quantum dots via the photon mode of a solid state microresonator using the strong exciton–photon coupling regime. This is enabled by two-dimensional spectroscopy of the sample’s coherent response, a sensitive probe of the coherent coupling. The results are quantitatively understood in a rigorous description of the cavity-mediated coupling of the quantum dot excitons. This mechanism can be used, for instance in photonic crystal cavity networks, to enable a long-range, non-local coherent coupling.

Albert, F.; Sivalertporn, K.; Kasprzak, J.; Strauss, M.; Schneider, C.; Hofling, S.; Kamp, M.; Forchel, A.; Reitzenstein, S.; Muljarov, E.A.; Langbein, W.

2013-01-01

402

Long-range excitons in conjugated polymers with ring torsions  

CERN Document Server

Ring torsion effects on optical excitation properties in poly(para-phenylene) (PPP) and polyaniline (PAN) are investigated by the intermediate exciton formalism. Long-range excitons are characterized, and the long-range component of the oscillator strengths is calculated. We find that ring torsions affect the long-range excitons in PAN more easily than in PPP, due to the larger torsion angle of PAN and the large number of bonds whose hopping integrals are modulated by torsions.

Harigaya, K

1997-01-01

403

Effective bosonic hamiltonian for excitons a too naive concept  

CERN Document Server

Excitons, being made of two fermions, may appear from far as bosons. Their close-to-boson character is however quite tricky to handle properly. Using our commutation technique especially designed to deal with interacting close-to-boson particles, we here calculate the exact expansion in Coulomb interaction of theexciton-exciton correlations, and show that a naive effective bosonic hamiltonian for excitons cannot produce these X-X correlations correctly.

Combescot, M

2002-01-01

404

Exciton condensates in semiconductor quantum wells emit coherent light  

CERN Multimedia

We show that a quasi-two dimensional condensate of optically active excitons emits coherent light even in the absence of population inversion. This allows an unambiguous and clear experimental detection of the condensed phase. We prove that, due to the exciton-photon coupling, quantum and thermal fluctuations do not destroy condensation at finite temperature. Suitable conditions to achieve condensation are temperatures of a few K for typical exciton densities, and the use of a pulsed, and preferably circularly polarized, laser.

Fernández-Rossier, J; Merlin, R

1998-01-01

405

Photoexcitation of the triplet exciton in single wall carbon nanotubes  

Digital Repository Infrastructure Vision for European Research (DRIVER)

The carbon nanotube photoexcitation spectrum is dominated by excitonic transitions, rather than interband transitions between continuum states. There are eight distinct excitonic transitions (four singlet and four triplet), each with two-fold degeneracy. Because the triplet excitons are spin polarized with electron and hole spins both pointing in the same direction, they are optically inactive, and optical spectroscopy has revealed no evidence for their existence. Here, we show that by the i...

Santos, Tiffany S.; Moodera, Jagadeesh; Mohite, Aditya D.; Alphenaar, Bruce W.

2009-01-01

406

Internal field induced exciton binding energy and the optical transition in a strained Mg based II–VI quantum well  

International Nuclear Information System (INIS)

Binding energy of an exciton in a wurtzite ZnO/Zn1?xMgxO strained quantum well is investigated theoretically in which the strong built-in electric field due to the spontaneous and piezoelectric polarizations is included. Numerical calculations are performed using variational procedure within the single band effective mass approximation by varying the Mg composition in the barrier. The exciton oscillator strength and the exciton lifetime for radiative recombination as functions of well width and Mg content have been computed. The internal field induced interband emission energy of strained ZnO/Zn1?xMgxO well is investigated with the various structural parameters. The total optical absorption coefficients and the changes of refractive index as a function of normalized photon energy in the presence of built-in internal field are analyzed. The result shows that the strong built-in electric field has influence on the oscillator strength and the recombination life time of the exciton. The optical absorption coefficients and the refractive index changes strongly depend on Mg composition. The occurred blue shift of the resonant peak due to the incorporation of Mg ions will give the information about the variation of two energy levels in the quantum well. -- Highlights: • Binding energy of an exciton in a wurtzite ZnO/Zn1?xMgxO strained quantum well is investigated. • The built-in internal fields due to the spontaneous and piezoelectric polarizations are included. • The oscillator strength and the exciton lifetime for radiative recombination are computed. • The internal field induced transition energy of strained ZnO/Zn1?xMgxO well is investigated. • The results show that the nonlinear optical properties strongly depend on Mg composition

2013-11-01

407

Internal field induced exciton binding energy and the optical transition in a strained Mg based II–VI quantum well  

Energy Technology Data Exchange (ETDEWEB)

Binding energy of an exciton in a wurtzite ZnO/Zn{sub 1?x}Mg{sub x}O strained quantum well is investigated theoretically in which the strong built-in electric field due to the spontaneous and piezoelectric polarizations is included. Numerical calculations are performed using variational procedure within the single band effective mass approximation by varying the Mg composition in the barrier. The exciton oscillator strength and the exciton lifetime for radiative recombination as functions of well width and Mg content have been computed. The internal field induced interband emission energy of strained ZnO/Zn{sub 1?x}Mg{sub x}O well is investigated with the various structural parameters. The total optical absorption coefficients and the changes of refractive index as a function of normalized photon energy in the presence of built-in internal field are analyzed. The result shows that the strong built-in electric field has influence on the oscillator strength and the recombination life time of the exciton. The optical absorption coefficients and the refractive index changes strongly depend on Mg composition. The occurred blue shift of the resonant peak due to the incorporation of Mg ions will give the information about the variation of two energy levels in the quantum well. -- Highlights: • Binding energy of an exciton in a wurtzite ZnO/Zn{sub 1?x}Mg{sub x}O strained quantum well is investigated. • The built-in internal fields due to the spontaneous and piezoelectric polarizations are included. • The oscillator strength and the exciton lifetime for radiative recombination are computed. • The internal field induced transition energy of strained ZnO/Zn{sub 1?x}Mg{sub x}O well is investigated. • The results show that the nonlinear optical properties strongly depend on Mg composition.

Elangovan, P. [Department of Physics, Maamallan Institute of Technology, Chennai 602105 (India); John Peter, A., E-mail: a.john.peter@gmail.com [Department of Physics, Government Arts College, Melur 625 106. Madurai (India); Kyoo Yoo, Chang [Center for Environmental Studies/Green Energy Center, Deptartment of Environmental Science and Engineering, College of Engineering, Kyung Hee University, Seocheon-dong 1, Giheung-gu, Yongin-Si, Gyeonggi-Do, 446-701 (Korea, Republic of)

2013-11-15

408

Phonon-Induced Dephasing of Excitons in Semiconductor Quantum Dots: Multiple Exciton Generation, Fission, and Luminescence  

Science.gov (United States)

Phonon-induced dephasing processes that govern optical line widths, multiple exciton (ME) generation (MEG), and ME fission (MEF) in semiconductor quantum dots (QDs) are investigated by ab initio molecular dynamics simulation. Using Si QDs as an example, we propose that MEF occurs by phonon-induced dephasing and, for the first time, estimate its time scale to be 100 fs. In contrast, luminescence and MEG dephasing times are all sub-10 fs. Generally, dephasing is faster for higher-energy and higher-order excitons and increased temperatures. MEF is slow because it is facilitated only by low-frequency acoustic modes. Luminescence and MEG couple to both acoustic and optical modes of the QD, as well as ligand vibrations. The detailed atomistic simulation of the dephasing processes advances understanding of exciton dynamics in QDs and other nanoscale materials.

Madrid, Angeline; Kim, Hyeon-Deuk; Habenicht, Bradley; Prezhdo, Oleg

2010-03-01

409

Triplet diffusion in singlet exciton fission sensitized pentacene solar cells  

Science.gov (United States)

Singlet fission sensitized photovoltaics have the potential to surpass the Shockley-Queisser limit for a single-junction structure. We investigate the dynamics of triplet excitons resulting from singlet fission in pentacene and their ionization at a C60 heterojunction. We model the generation and diffusion of excitons to predict the spectral response. We find the triplet diffusion length in polycrystalline pentacene to be 40 nm. Poly(3-hexylthiophene) between the electrode and pentacene works both to confine triplet excitons and also to transfer photogenerated singlet excitons into pentacene with 30% efficiency. The lower bound for the singlet fission quantum efficiency in pentacene is 180 +/- 15%.

Tabachnyk, Maxim; Ehrler, Bruno; Bayliss, Sam; Friend, Richard H.; Greenham, Neil C.

2013-10-01

410

Pattern formation of indirect excitons in coupled quantum wells  

CERN Document Server

Using a nonlinear Schr\\"odinger equation including short-range two-body attraction and three-body repulsion, we investigate the spatial distribution of indirect excitons in semiconductor coupled quantum wells. The results obtained can interpret the experimental phenomenon that annular exciton cloud first contracts then expands when the number of confined excitons is increased, as observed by Lai {\\em et al.} [Lai $et al.$, Science \\textbf{303}, 503 (2004)]. In particular, the model reconciles the patterns of exciton rings reported by Butov {\\em et al.} [Butov $et al.$, Nature \\textbf{418}, 751 (2002)]. At higher densities, the model predicts much richer patterns, which could be tested by future experiments.

Liu, C S; Wu, W C

2006-01-01

411

Dark and Bright Excitonic States in Nitride Quantum Dots  

CERN Document Server

Formation of excitonic states in quantum dots of nitride based III-V semiconductors GaN and AlN including coulomb and exchange interactions are investigated. Dark exciton formation is found to occur for both GaN quantum dots(QDs) with wurtzite structure having positive crystal field splitting and GaN and AlN QDs with zero crystal field splitting with a transition from dark to bright exciton at about 40{\\AA}. In wurtzite AlN QDs with negative crystal field splitting the splitting between the dark and bright excitonic states is very small and vanishes at about 15{\\AA}.

Bagga, A; Ghosh, S; Bagga, Anjana; Ghosh, Subhasis

2004-01-01

412

Exciton molecule in semiconductors by two-photon absorption  

International Nuclear Information System (INIS)

Direct creation of bi-exciton states by two-photon absorption in direct gap semiconductors is investigated theoretically. A numerical application to the case of CuCl shows that the two-photon absorption coefficient for bi-excitonic transitions is larger than that for two-photon interband transitions by three orders of magnitude. It becomes comparable to that for one-photon excitonic transitions for available laser intensities. The main contribution to this enhancement of the absorption coefficient for the transitions to the bi-exciton states is found to be from the resonance effect

1976-01-01

413

Exciton management in organic photovoltaic multidonor energy cascades.  

Science.gov (United States)

Multilayer donor regions in organic photovoltaics show improved power conversion efficiency when arranged in decreasing exciton energy order from the anode to the acceptor interface. These so-called "energy cascades" drive exciton transfer from the anode to the dissociating interface while reducing exciton quenching and allowing improved overlap with the solar spectrum. Here we investigate the relative importance of exciton transfer and blocking in a donor cascade employing diphenyltetracene (D1), rubrene (D2), and tetraphenyldibenzoperiflanthene (D3) whose optical gaps monotonically decrease from D1 to D3. In this structure, D1 blocks excitons from quenching at the anode, D2 accepts transfer of excitons from D1 and blocks excitons at the interface between D2 and D3, and D3 contributes the most to the photocurrent due to its strong absorption at visible wavelengths, while also determining the open circuit voltage. We observe singlet exciton Förster transfer from D1 to D2 to D3 consistent with cascade operation. The power conversion efficiency of the optimized cascade OPV with a C60 acceptor layer is 7.1 ± 0.4%, which is significantly higher than bilayer devices made with only the individual donors. We develop a quantitative model to identify the dominant exciton processes that govern the photocurrent generation in multilayer organic structures. PMID:24702468

Griffith, Olga L; Forrest, Stephen R

2014-05-14

414

Lateral superlattices as voltage-controlled traps for excitons  

CERN Document Server

We demonstrate the localisation of quantum well excitons in a periodic array of linear traps using photoluminescence experiments. The excitonic traps are induced by applying spatially alternating external voltages via interdigitated metal gates. The localisation originates from the periodical modulation of the strength of the quantum-confined Stark effect in the plane of the quantum well. In our experiments, the trap depth is easily tuned by the voltages applied to the interdigitated gates. Furthermore, we find that a perpendicular magnetic field reduces the exciton diffusion length. In short-period lateral superlattices, we observe a magnetic-field-induced stabilisation of excitons in the presence of strong in-plane electric fields.

Zimmermann, S; Hansen, W; Kotthaus, J P; Bichler, M; Wegscheider, W

1997-01-01

415

Intermediate type excitons in Schottky barriers of A{sup 3}B{sup 6} layer semiconductors and UV photodetectors  

Energy Technology Data Exchange (ETDEWEB)

Photoelectric and photovoltaic spectra of Schottky barrier (SB) structures of InSe, GaSe and GaS layered semiconductors (LS) are investigated at quantum energies from the band edge excitons of corresponding materials up to 6.5eV. Spectral dependences of photoconductivity (PC) of photo resistors and barrier structures are strongly different at the quantum energies corresponding to the intermediate type excitons (ITE) observed in these semiconductors. It was suggested that high UV photoconductivity of A{sup 3}B{sup 6} LS is due to existence of high mobility light carriers in the depth of the band structure. It is shown that SB of semitransparent Au-InSe is high sensitive photo detector in UV region of spectra. (copyright 2006 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)

Alekperov, O.Z.; Guseinov, N.M.; Nadjafov, A.I. [Insitute of Physics of National Academy of Sinces of Azerbaijan, H. Javid av. 33, 1133 Baku (Azerbaijan)

2006-09-15

416

Relativistic quasiparticle self-consistent electronic structure of hybrid halide perovskite photovoltaic absorbers  

Science.gov (United States)

Solar cells based on a light absorbing layer of the organometal halide perovskite CH3NH3PbI3 have recently surpassed 15% conversion efficiency, though how these materials work remains largely unknown. We analyze the electronic structure and optical properties within the quasiparticle self-consistent GW approximation. While this compound bears some similarity to conventional sp semiconductors, it also displays unique features. Quasiparticle self-consistency is essential for an accurate description of the band structure: Band gaps are much larger than what is predicted by the local-density approximation (LDA) or GW based on the LDA. Valence band dispersions are modified in a very unusual manner. In addition, spin-orbit coupling strongly modifies the band structure and gives rise to unconventional dispersion relations and a Dresselhaus splitting at the band edges. The average hole mass is small, which partially accounts for the long diffusion lengths observed. The surface ionization potential (work function) is calculated to be 5.7 eV with respect to the vacuum level, explaining efficient carrier transfer to TiO2 and Au electrical contacts.

Brivio, Federico; Butler, Keith T.; Walsh, Aron; van Schilfgaarde, Mark

2014-04-01

417

Exciton-exciton correlations revealed by two-quantum, two-dimensional fourier transform optical spectroscopy.  

Science.gov (United States)

The Coulomb correlations between photoexcited charged particles in materials such as photosynthetic complexes, conjugated polymer systems, J-aggregates, and bulk or nanostructured semiconductors produce a hierarchy of collective electronic excitations, for example, excitons, and biexcitons, which may be harnessed for applications in quantum optics, light-harvesting, or quantum information technologies. These excitations represent correlations among successively greater numbers of electrons and holes, and their associated multiple-quantum coherences could reveal detailed information about complex many-body interactions and dynamics. However, unlike single-quantum coherences involving excitons, multiple-quantum coherences do not radiate; consequently, they have largely eluded direct observation and characterization. In this Account, we present a novel optical technique, two-quantum, two-dimensional Fourier transform optical spectroscopy (2Q 2D FTOPT), which allows direct observation of the dynamics of multiple exciton states that reflect the correlations of their constituent electrons and holes. The approach is based on closely analogous methods in NMR, in which multiple phase-coherent fields are used to drive successive transitions such that multiple-quantum coherences can be accessed and probed. In 2Q 2D FTOPT, a spatiotemporal femtosecond pulse-shaping technique has been used to overcome the challenge of control over multiple, noncollinear, phase-coherent optical fields in experimental geometries used to isolate selected signal contributions through wavevector matching. We present results from a prototype GaAs quantum well system, which reveal distinct coherences of biexcitons that are formed from two identical excitons or from two excitons that have holes in different spin sublevels ("heavy-hole" and "light-hole" excitons). The biexciton binding energies and dephasing dynamics are determined, and changes in the dephasing rates as a function of the excitation density are observed, revealing still higher order correlations due to exciton-biexciton interactions. Two-quantum coherences due to four-particle correlations that do not involve bound biexciton states but that influence the exciton properties are also observed and characterized. The 2Q 2D FTOPT technique allows many-body interactions that cannot be treated with a mean-field approximation to be studied in detail; the pulse-shaping approach simplifies greatly what would have otherwise been daunting measurements. This spectroscopic tool might soon offer insight into specific applications, for example, in detailing the interactions that affect how electronic energy moves within the strata of organic photovoltaic cells. PMID:19691277

Stone, Katherine W; Turner, Daniel B; Gundogdu, Kenan; Cundiff, Steven T; Nelson, Keith A

2009-09-15

418

Collection of indirect excitons in a diamond-shaped electrostatic trap  

Digital Repository Infrastructure Vision for European Research (DRIVER)

We report on the principle and realization of a new trap for excitons -- the diamond electrostatic trap -- which uses a single electrode to create a confining potential for excitons. We also create elevated diamond traps which permit evaporative cooling of the exciton gas. We observe collection of excitons towards the trap center with increasing exciton density. This effect is due to screening of disorder in the trap by the excitons. As a result, the diamond trap behaves as ...

High, A. A.; Thomas, A. K.; Grosso, G.; Remeika, M.; Hammack, A. T.; Meyertholen, A. D.; Fogler, M. M.; Butov, L. V.; Hanson, M.; Gossard, A. C.

2009-01-01

419

Excitonic energy level structure and pigment-protein interactions in the recombinant water-soluble chlorophyll protein. II. Spectral hole-burning experiments.  

Science.gov (United States)

Persistent spectral hole burning at 4.5 K has been used to investigate the excitonic energy level structure and the excited state dynamics of the recombinant class-IIa water-soluble chlorophyll-binding protein (WSCP) from cauliflower. The hole-burned spectra are composed of four main features: (i) a narrow zero-phonon hole (ZPH) at the burn wavelength, (ii) a number of vibrational ZPHs, (iii) a broad low-energy hole at ~665 and ~683 nm for chlorophyll b- and chlorophyll a-WSCP, respectively, and (iv) a second satellite hole at ~658 and ~673 nm for chlorophyll b- and chlorophyll a-WSCP, respectively. The doublet of broad satellite holes is assigned to an excitonically coupled chlorophyll dimer. The lower-energy holes at ~665 and ~683 nm for chlorophyll b- and chlorophyll a-WSCP, respectively, represent the lower exciton states. Taking into account the parameters of electron-phonon coupling, the lower exciton state can be assigned as the fluorescence origin. The lower exciton state is populated by two processes: (i) exciton relaxation from the higher exciton state and (ii) vibrational relaxation within the lower exciton state. Assuming identical site energies for the two excitonically coupled chlorophyll molecules, the dipole-dipole interaction energy J is directly determined to be 85 and 100 cm(-1) for chlorophyll b- and chlorophyll a-WSCP, respectively, based on the positions of the satellite holes. The Gaussian low-energy absorption band identified by constant fluence hole burning at 4.5 K has a width of ~150 cm(-1) and peaks at 664.9 and 682.7 nm for chlorophyll b- and chlorophyll a-WSCP, respectively. The action spectrum is broader and blue-shifted compared to the fluorescent lower exciton state. This finding can be explained by a slow protein relaxation between energetically inequivalent conformational substates within the lowest exciton state in agreement with the results of Schmitt et al. (J. Phys. Chem. B2008, 112, 13951). PMID:21417356

Pieper, J; Rätsep, M; Trostmann, I; Schmitt, F-J; Theiss, C; Paulsen, H; Eichler, H J; Freiberg, A; Renger, G

2011-04-14

420

Excitonic recombination in superstoichiometric nanocrystalline TiO2 grown by cluster precursors at room temperature.  

Science.gov (United States)

Unprecedented room temperature excitonic emissions are achieved from TiO(2) nanocrystals synthesized at 300 K by supersonic cluster beams. Transmission electron microscopy studies show the crystalline nature of the nanoparticles (NPs) with a diameter ranging from 5 to 30 nm. All the samples show mixed rutile and anatase phases as confirmed by Raman spectroscopy. XPS core level analyses evidence an O/Ti ratio of the as-grown nanoparticles of 2.30 ± 0.04. Two room temperature cathodoluminescence excitonic peaks observed at 3.16 and 3.25 eV are ascribed to the coexistence of rutile and anatase crystallographic phases respectively. Subsequent thermal treatments at 450 °C cause the complete quenching of the UV excitonic emissions and result in a more conventional broad visible band centered at 2.5 eV. HRTEM and XPS studies reveal that, after annealing, the NPs remain single crystals in nature with an O/Ti ratio of 2.20 ± 0.04. These results suggest a correlation between the emission properties and the oxygen concentration of our NPs. The achieved ability to tune the optical properties of TiO(2) nanoparticles is very promising for sensing and energy applications. PMID:22426007

Detto, Francesca; Armani, Nicola; Lazzarini, Laura; Toccoli, Tullio; Verucchi, Roberto; Aversa, Lucrezia; Nardi, Marco Vittorio; Rossi, Barbara; Salviati, Giancarlo; Iannotta, Salvatore

2012-04-28