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1

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

2

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

3

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

4

Band filling with free charge carriers in organometal halide perovskites  

Science.gov (United States)

The unique and promising properties of semiconducting organometal halide perovskites have brought these materials to the forefront of solar energy research. Here, we present new insights into the excited-state properties of CH3NH3PbI3 thin films through femtosecond transient absorption spectroscopy measurements. The photoinduced bleach recovery at 760?nm reveals that band-edge recombination follows second-order kinetics, indicating that the dominant relaxation pathway is via recombination of free electrons and holes. Additionally, charge accumulation in the perovskite films leads to an increase in the intrinsic bandgap that follows the Burstein-Moss band filling model. Both the recombination mechanism and the band-edge shift are studied as a function of the photogenerated carrier density and serve to elucidate the behaviour of charge carriers in hybrid perovskites. These results offer insights into the intrinsic photophysics of semiconducting organometal halide perovskites with direct implications for photovoltaic and optoelectronic applications.

Manser, Joseph S.; Kamat, Prashant V.

2014-09-01

5

Plasmonic band gap engineering of plasmon-exciton coupling.  

Science.gov (United States)

Controlling plasmon-exciton coupling through band gap engineering of plasmonic crystals is demonstrated in the Kretschmann configuration. When the flat metal surface is textured with a sinusoidal grating only in one direction, using laser interference lithography, it exhibits a plasmonic band gap because of the Bragg scattering of surface plasmon polaritons on the plasmonic crystals. The contrast of the grating profile determines the observed width of the plasmonic band gap and hence allows engineering of the plasmonic band gap. In this work, resonant coupling between the molecular resonance of a J-aggregate dye and the plasmonic resonance of a textured metal film is extensively studied through plasmonic band gap engineering. Polarization dependent spectroscopic reflection measurements probe the spectral overlap occurring between the molecular resonance and the plasmonic resonance. The results indicate that plasmon-exciton interaction is attenuated in the band gap region along the grating direction. PMID:25360962

Karademir, Ertugrul; Balci, Sinan; Kocabas, Coskun; Aydinli, Atilla

2014-10-01

6

Exciton band structure in bacterial peripheral light-harvesting complexes.  

Science.gov (United States)

The variability of the exciton spectra of bacteriochlorophyll molecules in light-harvesting (LH) complexes of photosynthetic bacteria ensures the excitation energy funneling trend toward the reaction center. The decisive shift of the energies is achieved due to exciton spectra formation caused by the resonance interaction between the pigments. The possibility to resolve the upper Davydov sub-band corresponding to the B850 ring and, thus, to estimate the exciton bandwidth by analyzing the temperature dependence of the steady-state absorption spectra of the LH2 complexes is demonstrated. For this purpose a self-modeling curve resolution approach was applied for analysis of the temperature dependence of the absorption spectra of LH2 complexes from the photosynthetic bacteria Rhodobacter (Rba.) sphaeroides and Rhodoblastus (Rbl.) acidophilus. Estimations of the intradimer resonance interaction values as follows directly from obtained estimations of the exciton bandwidths at room temperature give 385 and 397 cm(-1) for the LH2 complexes from the photosynthetic bacteria Rba. sphaeroides and Rhl. acidophilus, respectively. At 4 K the corresponding couplings are slightly higher (391 and 435 cm(-1), respectively). The retained exciton bandwidth at physiological conditions supports the decisive role of the exciton coherence determining light absorption in bacterial light-harvesting antenna complexes. PMID:22480241

Trinkunas, Gediminas; Zerlauskiene, Oksana; Urbonien?, Vidita; Chmeliov, Jevgenij; Gall, Andrew; Robert, Bruno; Valkunas, Leonas

2012-05-01

7

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 station 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

8

Exciton binding energies and band gaps in GaN bulk crystals  

OpenAIRE

Photoluminescence and two-photon measurements were performed on hexagonal bulk GaN. From photoluminescence we have obtained the energies of free IS excitons, from two-photon spectroscopy the energies of 2P excitons. These results together allow an accurate determination of exciton binding energies and band gaps © 1998 Elsevier Science B.V. All rights reserved.

Reimann, K.; Steube, M.; Frohlich, D.; Clarke, Sj

1998-01-01

9

Relaxation processes of the triplet state of self-trapped excitons in alkali-halide crystals  

OpenAIRE

The spin lattice relaxation, radiative and microwave excitation processes of the triplet self-trapped exciton have been studied in alkali chlorides and bromides by measuring the transient response of the X-ray excited luminescence to resonant pulse microwave. The model is applied to magnetic circular polarization data. The symmetry of dominant exciton-phonon coupling are discussed.

Mori, Y.; Jaccard, C.; Von Weid, J.; Aegerter, M.

1980-01-01

10

Comparative analysis of spectral characteristics of triplet autolocalized excitons and F2 centers in alkaline-halide crystal  

International Nuclear Information System (INIS)

The comparative analysis of the spectral characteristics of the autolocalized excitons (AE) and F2-centers in the states with the similar spin multiplicity is carried out. The criterion of separating the triplet-triplet (T-T) absorption transitions in the AE electron and hole components in any alkaline-halide crystal is proposed on the basis of the above analysis. The conclusion is made, that the spectral position of the T-T transitions in the AE electron and hole components is influenced not by the AE hole component spatial position but by the presence, in particular, of the nonuniformity in the form of the homologous cation and anion admixture in the AE near coordination spheres

11

Charge transfer band of the benzyl radical--halide ion formed by dissociative electron attachment to benzyl halides in a rigid organic matrix  

International Nuclear Information System (INIS)

In order to elucidate the anomalous features previously reported on electronic spectra of the benzyl radical generated from benzyl chloride, the spectra were studied by the fluorescence spectrophotometric method for benzyl radicals generated from benzyl fluoride, chloride, bromide, and iodide in a 3-methylhexane matrix by either uv or ? irradiation at 770K. When benzyl radicals were generated by dissociative electron attachment to the benzyl halides in the ?-irradiated matrix, the halide ions close to the benzyl radicals were found to cause a red-shift of the spectra of the radicals, a change in the vibrational structure (especially an increase in the 0 to 0 band intensity of the fluorescence spectrum), and also the appearance of CT bands due to complexes between the halide ion and the benzyl radical at 265, 362, 413 and 500 nm for F-, Cl-, Br-, and I-, respectively. The dependence of the CT transition energy upon the halide ions agrees well with that expected from the ionization energy of the donors (the electron affinity of the halogen atoms) and the solvation energy of the halide ions in the matrix. For the benzyl radical from benzyl chloride, the effects of the chloride ion were found to be much less in 2-methyltetrahydrofuran matrix, and could not be observed at all in an ethanol matrix. Halide ions also caused a decrease in the fluorescence lifetime of the benzyl radicalical

12

Specific features of the temperature quenching of luminescence of self-trapped excitons in alkali halide crystals under low-temperature deformation  

Science.gov (United States)

The activation energy of temperature quenching of luminescence of self-trapped excitons in alkali halide crystals subjected to low-temperature uniaxial deformation is evaluated experimentally. It is found that an increase in the activation energy is observed in the following series of crystals: KBr ? NaCl ? KI ? NaBr ? CsBr ? RbI. The effect of enhancement of intrinsic luminescence of alkali halide crystals due to the lowering of the symmetry of the crystal lattice under low-temperature uniaxial deformation is interpreted by analyzing the observed increase in the activation energy that characterizes the height of the potential barrier separating channels of radiative and nonradiative decay (with the formation of radiation defects) of self-trapped excitons.

Shunkeev, K.; Sarmukhanov, E.; Barmina, A.; Myasnikova, L.; Sagimbaeva, Sh.; Shunkeev, S.

2008-10-01

13

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

14

Steric engineering of metal-halide perovskites with tunable optical band gaps  

Science.gov (United States)

Owing to their high energy-conversion efficiency and inexpensive fabrication routes, solar cells based on metal-organic halide perovskites have rapidly gained prominence as a disruptive technology. An attractive feature of perovskite absorbers is the possibility of tailoring their properties by changing the elemental composition through the chemical precursors. In this context, rational in silico design represents a powerful tool for mapping the vast materials landscape and accelerating discovery. Here we show that the optical band gap of metal-halide perovskites, a key design parameter for solar cells, strongly correlates with a simple structural feature, the largest metal–halide–metal bond angle. Using this descriptor we suggest continuous tunability of the optical gap from the mid-infrared to the visible. Precise band gap engineering is achieved by controlling the bond angles through the steric size of the molecular cation. On the basis of these design principles we predict novel low-gap perovskites for optimum photovoltaic efficiency, and we demonstrate the concept of band gap modulation by synthesising and characterising novel mixed-cation perovskites.

Filip, Marina R.; Eperon, Giles E.; Snaith, Henry J.; Giustino, Feliciano

2014-12-01

15

Phase diagram of exciton condensate in doped two-band Hubbard model  

OpenAIRE

Using the dynamical mean-field approximation we investigate formation of excitonic condensate in the two-band Hubbard model in the vicinity of the spin-state transition. With temperature and band filling as the control parameters we realize all symmetry allowed spin-triplet excitonic phases, some exhibiting a ferromagnetic polarization. While the transitions are first-order at low temperatures, at elevated temperatures continuous transitions are found that give rise to a mul...

Kunes, Jan

2014-01-01

16

Phase diagram of exciton condensate in doped two-band Hubbard model  

Science.gov (United States)

Using the dynamical mean-field approximation we investigate the formation of excitonic condensate in the two-band Hubbard model in the vicinity of the spin-state transition. With temperature and band filling as the control parameters we realize all symmetry allowed spin-triplet excitonic phases, some exhibiting a ferromagnetic polarization. While the transitions are first order at low temperatures, at elevated temperatures continuous transitions are found that give rise to a multicritical point. Rapid but continuous transition between ferromagnetic and nonmagnetic excitonic phases allows switching of uniform magnetization by small changes of chemical potential.

Kuneš, Jan

2014-12-01

17

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

CERN Document Server

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

18

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 and 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

2014-01-01

19

Theory of two-photon absorption by exciton states in cubic semiconductors with degenerate valence bands  

International Nuclear Information System (INIS)

The coefficient of the absorption of two polarized photons is calculated for direct band gap semiconductors with degenerate valence bands. Wannier-Mott exciton states are included in both the intermediate and final states. Numerical calculations are performed for ZnSe and are compared with Sondergeld's experimental and theoretical results. (author). 11 refs, 2 tabs

20

Optically decomposed near-band-edge structure and excitonic transitions in Ga2S3  

Science.gov (United States)

The band-edge structure and band gap are key parameters for a functional chalcogenide semiconductor to its applications in optoelectronics, nanoelectronics, and photonics devices. Here, we firstly demonstrate the complete study of experimental band-edge structure and excitonic transitions of monoclinic digallium trisulfide (Ga2S3) using photoluminescence (PL), thermoreflectance (TR), and optical absorption measurements at low and room temperatures. According to the experimental results of optical measurements, three band-edge transitions of EA = 3.052 eV, EB = 3.240 eV, and EC = 3.328 eV are respectively determined and they are proven to construct the main band-edge structure of Ga2S3. Distinctly optical-anisotropic behaviors by orientation- and polarization-dependent TR measurements are, respectively, relevant to distinguish the origins of the EA, EB, and EC transitions. The results indicated that the three band-edge transitions are coming from different origins. Low-temperature PL results show defect emissions, bound-exciton and free-exciton luminescences in the radiation spectra of Ga2S3. The below-band-edge transitions are respectively characterized. On the basis of experimental analyses, the optical property of near-band-edge structure and excitonic transitions in the monoclinic Ga2S3 crystal is revealed.

Ho, Ching-Hwa; Chen, Hsin-Hung

2014-08-01

21

Nature of the narrow optical band in H*-aggregates: Dozy-chaos–exciton coupling  

Energy Technology Data Exchange (ETDEWEB)

Dozy chaos emerges as a combined effect of the collective chaotic motion of electrons and nuclei, and their chaotic electromagnetic interactions in the transient state of molecules experiencing quantum transitions. Following earlier discussions of the well-known Brönsted relations for proton-transfer reactions; the temperature-dependent electron transfer in Langmuir–Blodgett films; the shape of the optical bands of polymethine dye monomers, their dimers, and J-aggregates, this paper reports one more application of the dozy-chaos theory of molecular quantum transitions. The qualitative and quantitative explanations for shape of a narrow and blue-shifted optical absorption band in H{sup *}-aggregates is given on the basis of the dozy-chaos theory by taking into account the dozy-chaos–exciton coupling effect. It is emphasized that in the H{sup *}-aggregate chromophore (dimer of cyclic bis-thiacarbocyanines) there is a competition between two Frenkel exciton transitions through the chaotic reorganization motion of nuclear environment. As a result, the highly organized quantum transition to the upper exciton state becomes an exciton-induced source of dozy chaos for the low organized transition to the lower exciton state. This manifests itself in appearing the narrow peak and broad wing in the optical spectrum pattern of H{sup *}-aggregates. A similar enhancement in the H{sup *}-effect caused by the strengthening of the exciton coupling in H{sup *}-dimers, which could be achieved by synthesizing tertiary and quarternary thiacarbocyanine monomers, is predicted.

Egorov, Vladimir V., E-mail: egorov@photonics.ru [Photochemistry Center, Russian Academy of Sciences, Moscow, 119421 (Russian Federation)

2014-07-15

22

Nature of the narrow optical band in H*-aggregates: Dozy-chaos–exciton coupling  

International Nuclear Information System (INIS)

Dozy chaos emerges as a combined effect of the collective chaotic motion of electrons and nuclei, and their chaotic electromagnetic interactions in the transient state of molecules experiencing quantum transitions. Following earlier discussions of the well-known Brönsted relations for proton-transfer reactions; the temperature-dependent electron transfer in Langmuir–Blodgett films; the shape of the optical bands of polymethine dye monomers, their dimers, and J-aggregates, this paper reports one more application of the dozy-chaos theory of molecular quantum transitions. The qualitative and quantitative explanations for shape of a narrow and blue-shifted optical absorption band in H*-aggregates is given on the basis of the dozy-chaos theory by taking into account the dozy-chaos–exciton coupling effect. It is emphasized that in the H*-aggregate chromophore (dimer of cyclic bis-thiacarbocyanines) there is a competition between two Frenkel exciton transitions through the chaotic reorganization motion of nuclear environment. As a result, the highly organized quantum transition to the upper exciton state becomes an exciton-induced source of dozy chaos for the low organized transition to the lower exciton state. This manifests itself in appearing the narrow peak and broad wing in the optical spectrum pattern of H*-aggregates. A similar enhancement in the H*-effect caused by the strengthening of the exciton coupling in H*-dimers, which could be achieved by synthesizing tertiary and quarternary thiacarbocyanine monomers, is predicted

23

Optically decomposed near-band-edge structure and excitonic transitions in Ga2S3  

OpenAIRE

The band-edge structure and band gap are key parameters for a functional chalcogenide semiconductor to its applications in optoelectronics, nanoelectronics, and photonics devices. Here, we firstly demonstrate the complete study of experimental band-edge structure and excitonic transitions of monoclinic digallium trisulfide (Ga2S3) using photoluminescence (PL), thermoreflectance (TR), and optical absorption measurements at low and room temperatures. According to the experimental results of opt...

Ho, Ching-hwa; Chen, Hsin-hung

2014-01-01

24

Piezoreflectance Study of Band-Edge Excitons of ReS2:Au  

Science.gov (United States)

The optical properties of Au-doped rhenium disulphide (ReS2:Au) have been investigated by temperature- and polarization-dependent piezoreflectance (PzR) measurements. In comparison with the undoped ReS2, ReS2:Au shows not only the main excitonic transitions of E1ex, E2ex, E3ex, and E4ex near the direct band edge but also two extra transitions EAex and EBex owing to the doping effects. From temperature-dependent PzR spectra in the temperature range from 40 to 300 K, the least-squares fits are used to extract the parameters that describe the temperature variations of the band-edge excitonic transitions. The excitonic transitions show a strong polarization dependence in the near-infrared region. The polarization-dependent PzR measurement in the range between 0 and 180° was used to characterize the unique polarization property of ReS2:Au, and identify the origin of the excitonic transitions. However, the variation in the amplitude of PzR excitonic transitions with different polarization characteristics follows the Malus rule.

Zheng, Jing Yao; Lin, Der Yuh; Huang, Ying Sheng

2009-05-01

25

Low energy electron scattering by alkali halides  

International Nuclear Information System (INIS)

The energy distribution curves (EDC's) of the electrons scattered by the evaporated films of twenty kinds of alkali halides are obtained by impact with low energy electrons of 10-30 eV. Stationary bands observed in the EDC's are interpreted as the reflection of the high density of state levels in the conduction band. Energy loss structures due to the valence electron excitation are also observed in the EDC's. Furthermore the structures observed in the yield spectra are found to relate with exciton formation, band-to-band transition, plasmon and double excitation of valence electrons. (auth.)

26

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

27

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

28

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

29

The initial production of defects in alkali halides: F and H centre production by non-radiative decay of the self-trapped exciton  

International Nuclear Information System (INIS)

Radiation damage in KCl can be produced by the decay of a self-trapped exciton into an F centre and an H centre. Calculations are presented of the energies of the states involved for various stages in the evolution of the damage. These lead to important conclusions about the very rapid damage process, and support strongly Itoh and Saidoh's suggestion (J.Physique;34:C19:101(1973)) that damage proceeds through an excited hole state. The results also help in understanding the prompt decay of F and H pairs at low temperatures, the thermal annihilation of F and H centres, the effects of optical excitation of the self-trapped exciton, and some of the trends within the alkali halides. The calculations use a self-consistent semi-empirical molecular-orbital method, here the CNDO method as implemented in the MOSES code. A large cluster of ions is used (either 42 or 57 ions) plus long-range Madelung terms. The ion positions were obtained from separate lattice-relaxation calculations with the HADES code. The choice of CNDO parameters and the adequacy of the method were checked by a number of separate predictions. These include the energy ? luminescence, where the 2.33 eV predicted is very close to the 2.31 eV observed. (author)

30

Direct determination of indium antimonide energy band parameters from diamagnetic exciton spectra  

International Nuclear Information System (INIS)

Diamagnetic exciton spectra of InSb crystals derived from transmission measurements carried out on thin unstressed samples at 1.8 K and in magnetic fields up to 80 kOe are used for a direct determination of parameters of the c-and v-bands. Detailed identification and precise binding energy calculations permit one to derive the cyclotron frequencies of the electron and the holes, and the spin splittings. The m*/sub c/(epsilon/sub c/) and g*/sub c/(epsilon/sub c/) relationships thus obtained exhibit good agreement with Kane's model up to energies close to epsilon/sub c/ approximately epsilon/sub g/. Light holes also reveal a strong nonparabolicity characterized in the linear-in-k4 approximation by the coefficient p/sub lh/ = 0.47 meV-1. An analysis of the splitting of two light-hole long-wavelength transitions including the diamagnetic exciton binding energies and the data on the c-band obtained permits one to calculate the v-band parameter set. (author)

31

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

32

Excitonic spectra and band structure of CdGa2Se4 birefractive crystals  

International Nuclear Information System (INIS)

We report on the intersection of spectral dependences of refractive indices no and ne at the wavelengths 546 nm (?0) and 450 nm (?01) in CdGa2Se4 single crystals. The value of difference ?n=ne?no is equal to zero at the wavelengths involved. When placed between two crossed polarizers, the crystals of CdGa2Se4 exhibit a transmission band at the wavelength of ?0=546 nm (300 K). The ground and excited states of three excitonic series (A, B and C) were found out at 13 K in CdGa2Se4 crystals, and other parameters of excitons and bands were determined. In the ? point of Brillouin zone the effective mass of electrons mc is equal to 0.14m0, and the effective masses of holes mv2 and mv3 are equal to 0.76m0 and 0.94m0, respectively. The hole mass mv1 depends upon the direction of wave vector k: at polarization E?c, k?a the mass mv1=1.15m0, and at polarization E?c, k?b mv1=0.84m0. The values of valence bands splitting in the center of Brillouin zone by the crystal field (?cf=49 meV) and spin–orbital interaction (?so=351 meV) were determined. The optical functions n, k, ?1 and ?2 in polarizations E?c and E?c for the energy diapason from 3 to 6 eV were calculated from the reflectivity spectra by Kramers–Kronig analysis. The evidenced features are discussed on the basis of recent theoretical calculations of the band structure of CdGa2Se4 crystals

33

Study of the point defect creation and of the excitonic luminescence in alkali halides irradiated by swift heavy ions  

International Nuclear Information System (INIS)

The aim of this experimental thesis is to study the excitonic mechanisms and of the defect creation, in NaCl and KBr, under dense electronic excitations induced by swift heavy ion irradiations. In the first part, we present the main features of the interaction of swift heavy ions with solid targets, and after we review the well known radiolytic processes of the defect creation during X-ray irradiation. In the second chapter, we describe our experimental set-up. In the chapter III, we present our results of the in-situ optical absorption measurements. This results show that defect creation is less sensitive to the temperature than during a classical irradiation. Besides, we observe new mechanisms concerning the defect aggregation. In the chapter IV, we present the results of excitonic luminescence induced by swift by swift heavy ions. We observe that the luminescence yields only change with the highest electronic stopping power. In the chapter V, we perform thermal spike and luminescence yields calculations and we compare the numerical results to the experiments presented in the chapter IV. (author). 121 refs., 65 figs., 30 tabs

34

Excitonic Side Bands of Inner-Shell Excitations in Rare Gas Solids  

Science.gov (United States)

Valence-exciton luminescence under inner-shell excitation of the rare gas solids Xe, Kr, and Ar has been measured using time-resolved photoluminescence. Two different processes for exciton creation can be distinguished: creation of ``prompt'' excitons immediately after excitation (within the experimental time resolution), and creation of ``delayed'' excitons through electron-hole recombination. The decay structure of the exciton emission in the range of inner-shell excitation is characterized by the coexistence of the two processes. Time-resolved excitation spectra near the 2p edge in Ar, the 3d edge in Kr, and the 4d edge in Xe are discussed. The process of prompt exciton creation is strongly enhanced above an excitation threshold at the energy position of the ionization limit of the core state plus the energy of the valence free exciton.

Vielhauer, S.; Kirm, M.; Kisand, V.; Negodin, E.; Sombrowski, E.; Steeg, B.; Zimmerer, G.

35

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

36

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

37

Interaction of wide band gap single crystals with 248 nm excimer laser radiation. XII. The emission of negative atomic ions from alkali halides  

International Nuclear Information System (INIS)

Many wide band gap materials yield charged and neutral emissions when exposed to sub-band-gap laser radiation at power densities below the threshold for optical breakdown and plume formation. In this work, we report the observation of negative alkali ions from several alkali halides under comparable conditions. We observe no evidence for negative halogen ions, in spite of the high electron affinities of the halogens. Significantly, the positive and negative alkali ions show a high degree of spatial and temporal overlap. A detailed study of all the relevant particle emissions from potassium chloride (KCl) suggests that K- is formed by the sequential attachment of two electrons to K+

38

Alternation of band gap and localization of excitons in InGaNAs nanostructures with low nitrogen content  

International Nuclear Information System (INIS)

Continuous wave photoluminescence (cw PL) spectroscopy has been used to study the optical properties of a set of InGaNAs epilayers and single quantum wells with nitrogen concentration less than a few per cent at different temperatures and different excitation powers. We found that nitrogen has a critical role on the emission light of InGaNAs nanostructures and the recombination mechanism. The incorporation of a few per cent of nitrogen leads to shrinkage of the InGaNAs band gap. The physical origin of such band gap reduction has been investigated both experimentally and theoretically by using a band anticrossing model. We have found that localization of excitons that have been caused by incorporation of a few per cent of nitrogen in these structures is the main explanation of such anomalous behavior observed in the low-temperature photoluminescence spectra of these nanostructures. The localization energies of carriers have been evaluated by studying the variation of the quantum well (QW) emission versus temperature, and it was found that the localization energy increases with increasing nitrogen composition. Our data also show that, with increasing excitation intensity, the PL peak position moves to higher energies (blue shift) due to the filling of localized states and capture centers for excitons by photo-generated carriers

39

Alternation of band gap and localization of excitons in InGaNAs nanostructures with low nitrogen content  

Energy Technology Data Exchange (ETDEWEB)

Continuous wave photoluminescence (cw PL) spectroscopy has been used to study the optical properties of a set of InGaNAs epilayers and single quantum wells with nitrogen concentration less than a few per cent at different temperatures and different excitation powers. We found that nitrogen has a critical role on the emission light of InGaNAs nanostructures and the recombination mechanism. The incorporation of a few per cent of nitrogen leads to shrinkage of the InGaNAs band gap. The physical origin of such band gap reduction has been investigated both experimentally and theoretically by using a band anticrossing model. We have found that localization of excitons that have been caused by incorporation of a few per cent of nitrogen in these structures is the main explanation of such anomalous behavior observed in the low-temperature photoluminescence spectra of these nanostructures. The localization energies of carriers have been evaluated by studying the variation of the quantum well (QW) emission versus temperature, and it was found that the localization energy increases with increasing nitrogen composition. Our data also show that, with increasing excitation intensity, the PL peak position moves to higher energies (blue shift) due to the filling of localized states and capture centers for excitons by photo-generated carriers.

Gholami, M; Haratizadeh, H; Esmaeili, M; Amiri, R [Physics Department, Shahrood University of Technology, 3619995161, PO Box 316, Shahrood (Iran, Islamic Republic of); Holtz, P O [Department of Physics, Chemistry and Biology, Linkoping University, SE-581 581 83 Linkoping (Sweden); Hammar, M [Department of Microelectronics and Applied Physics, School of Information and Communication Technology, Royal Institute of Technology (KTH), Electrum 229, 16440 Kista-Stocholm (Sweden)], E-mail: mgholamim@gmail.com

2008-08-06

40

Magneto-photoluminescence of GaN\\/AlGaN quantum wells valence band reordering and excitonic binding energies  

CERN Document Server

A re-ordered valence band in GaN/AlGaN quantum wells with respect to GaN epilayers has been found as a result of the observation of an enhanced g-factor in magneto-luminescence spectra in fields up to 55 T. This has been caused by a reversal of the states in the strained AlGaN barriers thus giving different barrier heights for the different quantum well hole states. From k.p calculations in the quasi-cubic approximation, a change in the valence-band ordering will account for the observed values for the g-factors. We have also observed the well-width dependence of the in-plane extent of the excitonic wavefunction from which we infer an increase in the exciton binding energy with the reduction of the well width in general agreement with theoretical calculations of Bigenwald et al (phys. stat. sol. (b) 216, 371 (1999)) that uses a variational approach in the envelope function formalism that includes the effect of the electric field in the wells.

Shields, P A; Grandjean, N; Massies, J

2001-01-01

41

Amplitude scaling behavior of band center states of Frenkel exciton chains with correlated off-diagonal disorder  

International Nuclear Information System (INIS)

We report the amplitude scaling behavior of Frenkel exciton chains with nearest-neighbor correlated off-diagonal random interactions. The band center spectrum and its localization properties are investigated through the integrated density of states and the inverse localization length. The correlated random interactions are produced through a binary sequence similar to the interactions in spin glass chains. We produced sets of data with different interaction strength and 'wrong' sign concentrations that collapsed after scaling to the predictions of a theory developed earlier for Dirac fermions with random-varying mass. We found good agreement as the energy approaches the band center for a wide range of concentrations. We have also established the concentration dependence of the lowest order expansion coefficient of the scaling amplitudes for the correlated case. The correlation causes unusual behavior of the spectra, i.e., deviations from the Dyson-type singularity

42

Charge Transfer Excitons and Possible Exitonic Pairing in the Extended Three Band Hubbard Model  

OpenAIRE

Exact diagonalisations of the extended Hubbard model are performed. In the insulating regime it is shown that the nearest neighbour copper-oxygen repulsion, $V$, leads to Frenkel excitons in the charge transfer gap at values of $V$ of the order of copper-oxygen hybridisation, $t$. In the metallic regime it is shown that the static charge-transfer and density-density correlation functions diverge as a function of $V$, indicating a charge-tansfer instability and phase separati...

Vermeulen, C.; Barford, W.; Gagliano, Er

1995-01-01

43

Exciton interaction in non-direct-band solid solutions Insub(1-x)Gasub(x)P at high excitation levels  

International Nuclear Information System (INIS)

Cathodoluminescence spectra of GaP and Insub(1-x)Gasub(x)P solid solutions with nondirect energy band structure, produced by liquid epitaxy method, are envestigated at t=77 and 9 K. It isascertained, that in spectra of low temperature luminescence of GaP and Insub(1-x)Gasub(x)P with low background impurity concentration excition recombination at neutral donors and inherent exciton recombination with the participation of TA (h/2??=13+-/1MeV), LA(h/2??=31+-1 MeV) and TO(h/2??=43+-2 MeV) phonons predominate. The complex luminescence band, occuring at sufficiently high excitation levels and T=9 K, is ascribed to electron-holedroplet luminescence (EHD). Transformation of EHD band spectral form under transition from GaP to Insub(1-x)Gasub(x)P is ascribed to simultaneous TA- and TO-replica contribution reduction and to EHD luminescence phononless component development

44

Band gaps and optical spectra from single- and double-layer fluorographene to graphite fluoride: many-body effects and excitonic states  

International Nuclear Information System (INIS)

First-principle band gaps and optical absorption spectra of single- and double-layer fluorographene with bulk graphite fluoride are compared. The electronic properties are calculated using the many-body GW approximation and the optical spectra using the Bethe-Salpeter equation (BSE). The inclusion of electron-hole interactions is crucial for predicting low energy excitonic absorption peaks. The position of the first exciton peak is identical in single-, double- and multilayer fluorographene, which may indicate that the onset of the absorption spectra does not differ in these materials. (copyright 2014 by WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim)

45

Band gaps and optical spectra from single- and double-layer fluorographene to graphite fluoride: many-body effects and excitonic states  

Energy Technology Data Exchange (ETDEWEB)

First-principle band gaps and optical absorption spectra of single- and double-layer fluorographene with bulk graphite fluoride are compared. The electronic properties are calculated using the many-body GW approximation and the optical spectra using the Bethe-Salpeter equation (BSE). The inclusion of electron-hole interactions is crucial for predicting low energy excitonic absorption peaks. The position of the first exciton peak is identical in single-, double- and multilayer fluorographene, which may indicate that the onset of the absorption spectra does not differ in these materials. (copyright 2014 by WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim)

Karlicky, Frantisek; Otyepka, Michal [Regional Centre of Advanced Technologies and Materials, Department of Physical Chemistry, Faculty of Science, Palacky University, Olomouc (Czech Republic)

2014-10-15

46

Excitonic spectra and band structure of CdGa{sub 2}Se{sub 4} birefractive crystals  

Energy Technology Data Exchange (ETDEWEB)

We report on the intersection of spectral dependences of refractive indices n{sub o} and n{sub e} at the wavelengths 546 nm (?{sub 0}) and 450 nm (?{sub 01}) in CdGa{sub 2}Se{sub 4} single crystals. The value of difference ?n=n{sub e}?n{sub o} is equal to zero at the wavelengths involved. When placed between two crossed polarizers, the crystals of CdGa{sub 2}Se{sub 4} exhibit a transmission band at the wavelength of ?{sub 0}=546 nm (300 K). The ground and excited states of three excitonic series (A, B and C) were found out at 13 K in CdGa{sub 2}Se{sub 4} crystals, and other parameters of excitons and bands were determined. In the ? point of Brillouin zone the effective mass of electrons m{sub c} is equal to 0.14m{sub 0}, and the effective masses of holes m{sub v2} and m{sub v3} are equal to 0.76m{sub 0} and 0.94m{sub 0}, respectively. The hole mass m{sub v1} depends upon the direction of wave vector k: at polarization E?c, k?a the mass m{sub v1}=1.15m{sub 0}, and at polarization E?c, k?b m{sub v1}=0.84m{sub 0}. The values of valence bands splitting in the center of Brillouin zone by the crystal field (?{sub cf}=49 meV) and spin–orbital interaction (?{sub so}=351 meV) were determined. The optical functions n, k, ?{sub 1} and ?{sub 2} in polarizations E?c and E?c for the energy diapason from 3 to 6 eV were calculated from the reflectivity spectra by Kramers–Kronig analysis. The evidenced features are discussed on the basis of recent theoretical calculations of the band structure of CdGa{sub 2}Se{sub 4} crystals.

Syrbu, N.N., E-mail: sirbunn@yahoo.com [Technical University of Moldova, 168 Stefan cel Mare Avenue, 2004 Chisinau, Republic of Moldova (Moldova, Republic of); Stamov, I.G. [T.G. Shevchenko State University of Pridnestrovie, 25 Oktyabrya Street 107, 3300 Tiraspol, Republic of Moldova (Moldova, Republic of); Parvan, V.I. [Technical University of Moldova, 168 Stefan cel Mare Avenue, 2004 Chisinau, Republic of Moldova (Moldova, Republic of); Zalamai, V.V. [Institute of Applied Physics, Academy of Sciences of Moldova, 5 Academy Street, 2028 Chisinau, Republic of Moldova (Moldova, Republic of); Tiginyanu, I.M. [Institute of Electronic Engineering and Nanotechnologies, Academy of Sciences of Moldova, 3/3 Academy Street, 2028 Chisinau, Republic of Moldova (Moldova, Republic of)

2013-11-15

47

Excitonic properties related to valence band levels split by spin-orbit interaction in layered oxychalcogenide LaCuOCh(Ch=S,Se)  

International Nuclear Information System (INIS)

The energy splitting of valence band in oxychalcogenides LaCuOCh (Ch=S, Se) was studied through degenerative four-wave mixing (DFWM) signals excited by femtosecond laser pulses and high-resolution absorption spectra at 4 K. The DFWM signals for LaCuOS exhibited a beat structure with a period of 480 fs just at the exciton peak energy, indicating that the lowest exciton states were split by 9 meV, while the corresponding splitting in LaCuOSe was 125 meV. The spin-orbit interaction of Ch ion accompanied by the hybridization of Cu 3d orbital causes the splitting of the exciton levels

48

Optical density of states in ultradilute GaAsN alloy: Coexistence of free excitons and impurity band of localized and delocalized states  

International Nuclear Information System (INIS)

Optically active states in liquid phase epitaxy-grown ultra-dilute GaAsN are studied. The feature-rich low temperature photoluminescence spectrum has contributions from excitonic band states of the GaAsN alloy, and two types of defect states—localized and extended. The degree of delocalization for extended states both within the conduction and defect bands, characterized by the electron temperature, is found to be similar. The degree of localization in the defect band is analyzed by the strength of the phonon replicas. Stronger emission from these localized states is attributed to their giant oscillator strength.

49

Interlayer-exciton mediated three-hole-Auger-decay in the ?*-band of highly oriented pyrolytic graphite  

Science.gov (United States)

Resonant photoemission spectroscopy (resPES) is used to probe the occupied ?- and unoccupied ?*-bands of carbon thin films with particular focus on the Auger decay. Highly Oriented Pyrolytic Graphite (HOPG) is studied at the C1s edge. We find strong resonant features at 285.5 eV and 292 eV in the resPES diagram. The normal two-hole Auger decay proceeds under constant kinetic energy (45°) only in the ?*-region. In the ?*-region, however, it proceeds under 67.5° in terms of a Ebind(??) diagram. We attribute this to a multiple Auger decay with a net three hole final state. For this novel decay process we propose a model. We discuss the long lifetime of the first resonant excitation and conclude that it arises from the strong excitonic character of the first resonant state. We use HOPG as a reference system and suggest that this novel process is a tool to identify interlayer-substrate interaction of the carbon layers involved.

Richter, Matthias; Friedrich, Daniel; Schmeißer, Dieter

2014-02-01

50

Exciton-to-Carrier Conversion Processes in a Low-Band-Gap Organic Photovoltaic  

Science.gov (United States)

A bulk heterojunction (BHJ) based on a donor (D) polymer and an acceptor (A) fullerene derivative is a promising organic photovoltaic (OPV). We investigated femtosecond charge dynamics after D (at 633 nm) and A (at 400 nm) excitations in a prototypical low-band-gap and highly efficient OPV, i.e., poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b']dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl

Yonezawa, Kouhei; Kamioka, Hayato; Yasuda, Takeshi; Han, Liyuan; Moritomo, Yutaka

2013-06-01

51

Spin-flip Raman scattering on ?-X mixed excitons in indirect band-gap (In,Al)As/AlAs quantum dots  

Energy Technology Data Exchange (ETDEWEB)

We studied the fine structure of the indirect exciton in self-assembled (In,Al)As/AlAs quantum dots (QDs) by means of spin-flip Raman scattering (SFRS). The QDs are characterized by a type-I band alignment, wherein, dependent on the dot size, a crossover between the energetically lowest conduction-band states of the ?- and X-valley occurs. This ?-X mixing of the electron levels is used to optically study the indirect in momentum-space exciton. It has a long recombination lifetime and longitudinal spin relaxation time of up to several milliseconds. Using the resonant SFRS the g-factor tensors of the indirect exciton, ?-valley heavy-hole, and X-valley electron are determined. The spin-flip scattering mechanisms are based on acoustic phonon interaction in tilted magnetic field geometries. The efficiencies of the electron and heavy-hole spin scattering strongly depend on the excitation energy across the inhomogeneously broadened QD ensemble. The ?-valley electron cannot be observed because of its short lifetime and the broad dispersion of its g-factor corresponding to the strong variation in the QD sizes, which is evidenced in experiment and theory.

Kudlacik, Dennis; Debus, J.; Dunker, D.; Shamirzaev, T.S.; Bayer, M. [Experimentelle Physik 2, Technische Universitaet Dortmund, 44227 Dortmund (Germany); Sapega, V.F.; Ivchenko, E.L. [Ioffe Physical-Technical Institute, Russian Academy of Sciences, 194021 St. Petersburg (Russian Federation); Yakovlev, D.R. [Experimentelle Physik 2, Technische Universitaet Dortmund, 44227 Dortmund (Germany); Ioffe Physical-Technical Institute, Russian Academy of Sciences, 194021 St. Petersburg (Russian Federation)

2013-07-01

52

Spin-flip Raman scattering on ?-X mixed excitons in indirect band-gap (In,Al)As/AlAs quantum dots  

International Nuclear Information System (INIS)

We studied the fine structure of the indirect exciton in self-assembled (In,Al)As/AlAs quantum dots (QDs) by means of spin-flip Raman scattering (SFRS). The QDs are characterized by a type-I band alignment, wherein, dependent on the dot size, a crossover between the energetically lowest conduction-band states of the ?- and X-valley occurs. This ?-X mixing of the electron levels is used to optically study the indirect in momentum-space exciton. It has a long recombination lifetime and longitudinal spin relaxation time of up to several milliseconds. Using the resonant SFRS the g-factor tensors of the indirect exciton, ?-valley heavy-hole, and X-valley electron are determined. The spin-flip scattering mechanisms are based on acoustic phonon interaction in tilted magnetic field geometries. The efficiencies of the electron and heavy-hole spin scattering strongly depend on the excitation energy across the inhomogeneously broadened QD ensemble. The ?-valley electron cannot be observed because of its short lifetime and the broad dispersion of its g-factor corresponding to the strong variation in the QD sizes, which is evidenced in experiment and theory.

53

Observation of the exciton and Urbach band tail in low-temperature-grown GaAs using four-wave mixing spectroscopy  

Science.gov (United States)

Four-wave mixing (FWM) spectroscopy reveals clear signatures associated with the exciton, free carrier inter-band transitions, and the Urbach band tail in low-temperature-grown GaAs, providing a direct measure of the effective band gap as well as insight into the influence of disorder on the electronic structure. The ability to detect (and resolve) these contributions, in contrast to linear spectroscopy, is due to an enhanced sensitivity of FWM to the optical joint density of states and to many-body effects. Our experiments demonstrate the power of FWM for studying the near-band-edge optical properties and coherent carrier dynamics in low-temperature-grown semiconductors.

Webber, D.; Yildirim, M.; Hacquebard, L.; March, S.; Mathew, R.; Gamouras, A.; Liu, X.; Dobrowolska, M.; Furdyna, J. K.; Hall, K. C.

2014-11-01

54

Momentum dependence of the excitons in pentacene.  

Science.gov (United States)

We have carried out electron energy-loss investigations of the lowest singlet excitons in pentacene at 20 K. Our studies allow to determine the full exciton band structure in the a?, b? reciprocal lattice plane. The lowest singlet exciton can move coherently within this plane, and the resulting exciton dispersion is highly anisotropic. The analysis of the energetically following (satellite) features indicates a strong admixture of charge transfer excitations to the exciton wave function. PMID:22667582

Roth, Friedrich; Schuster, Roman; König, Andreas; Knupfer, Martin; Berger, Helmuth

2012-05-28

55

Intermediately bound excitons  

Science.gov (United States)

A possibility of intermediately bound excitons in semiconductors doped with transition metal impurities is considered. These sorts of exciton can appear in hexagonal (wurzite-type) semiconductors due to strong hybridization of the conduction band states and the impurity d states. In tetrahedral (zinc blende) semiconductors this hybridization is strongly suppressed due to a symmetry consideration. It is shown that the exciton hole in ZnS:Ni (zinc blende type) is bound by the Coulomb field of the exciton electron and may be considered within the framework of the hydrogen-like model. As for CdS:Ni (wurzite type) the orthogonalization central cell pseudopotential appears to be the leading attracting potential for the hole. These differences in the binding mechanisms account for striking differences measured experimentally in the structure of the exciton spectra and values of the g-factors of these two presumably similar systems.

Dahan, P.; Fleurov, V.; Kikoin, K.

1997-06-01

56

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

57

Enhanced Free Exciton and Direct Band-Edge Emissions at Room Temperature in Ultrathin Zn0 Films Grown on Si Nanopillars by Atomic Layer Deposition  

Science.gov (United States)

Room-temperature ultraviolet (UV) luminescence was investigated for the atomic layer deposited ZnO films, which were grown on silicon nanowires (Si-NWs) fabricated by self-masking dry etching in hydrogen-containing plasma. For films deposited at 200^o C, an intensive UV emission corresponding to free-exciton recombination ( 3.31eV) was observed with a nearly complete suppression of the defect-associated broad visible range emission peak. On the other hand, for ZnO films grown at 25^o C, albeit the appearance of the abovementioned defect-associated broad visible emission, the UV emission peak was observed to shift by 60meV to near the direct band edge (3.37 eV) recombination emission. The high resolution transmission electron microscopy (HRTEM) examinations showed that, indeed, the microstructure of the obtained ZnO films for the former case was of continuous crystalline nature, while that for the latter displayed a microstructure consisting of ZnO nanocrystals with a mean diameter of 4nm embedded in a largely amorphous matrix. The blue shift of the UV emission peak in the latter films, thus, might have been due to the effects of quantum confinement on the free-exciton recombination.

Chu, Pei-Yuan; Chang, Yuan-Ming; Shieh, Jiann; Juang, Jenh-Yih

2012-02-01

58

Exciton condensations in thin film topological insulator  

OpenAIRE

We study the many-body physics in thin film topological band insulator, where the inter-edge Coulomb interaction can lead to an exciton condensation transition. We investigate the universality class of the exciton condensation quantum critical point. With different chemical potentials and interactions, the exciton condensation can belong to z = 2 mean field, or 3d XY, or Yukawa-Higgs universality classes. The interplay between exciton condensate and the time-reversal symmetr...

Moon, Eun Gook; Xu, Cenke

2010-01-01

59

Enhanced free exciton and direct band-edge emissions at room temperature in ultrathin ZnO films grown on Si nanopillars by atomic layer deposition.  

Science.gov (United States)

Room-temperature ultraviolet (UV) luminescence was investigated for the atomic layer deposited ZnO films grown on silicon nanopillars (Si-NPs) fabricated by self-masking dry etching in hydrogen-containing plasma. For films deposited at 200 °C, an intensive UV emission corresponding to free-exciton recombination (~3.31 eV) was observed with a nearly complete suppression of the defect-associated broad visible range emission peak. On the other hand, for ZnO films grown at 25 °C, albeit the appearance of the defect-associated visible emission, the UV emission peak was observed to shift by ~60 meV to near the direct band edge (3.37 eV) recombination emission. The high-resolution transmission electron microscopy (HRTEM) showed that the ZnO films obtained at 25 °C were consisting of ZnO nanocrystals with a mean radius of 2 nm embedded in a largely amorphous matrix. Because the Bohr radius of free-exictons in bulk ZnO is ~2.3 nm, the size confinement effect may have occurred and resulted in the observed direct band edge electron-hole recombination. Additionally, the results also demonstrate order of magnitude enhancement in emission efficiency for the ZnO/Si-NP structure, as compared to that of ZnO directly deposited on Si substrate under the same conditions. PMID:21967063

Chang, Yuan-Ming; Shieh, Jiann; Chu, Pei-Yuan; Lee, Hsin-Yi; Lin, Chih-Ming; Juang, Jenh-Yih

2011-11-01

60

Multiline operation of mercury halide lasers  

Energy Technology Data Exchange (ETDEWEB)

Emission spectrum of the (B-X) band system of HgX radicals (X=chlorine, bromine, iodine) was 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 these radicals was observed to decrease in the presence of other mercury halide vapors at temperatures higher than 130C. Laser action was observed from the (B-X) transition of mercurous chloride, mercurous bromide, and mercurous iodide radicals by electrical discharge pumping of mercuric chloride, mercuric bromide and mercuric iodide vapors individually.

Kushawaha, M.; Mahmood, M.

1988-01-01

61

Magneto-photoluminescence of GaN/AlGaN quantum wells: valence band reordering and excitonic binding energies  

OpenAIRE

A re-ordered valence band in GaN/AlGaN quantum wells with respect to GaN epilayers has been found as a result of the observation of an enhanced g-factor in magneto-luminescence spectra in fields up to 55 T. This has been caused by a reversal of the states in the strained AlGaN barriers thus giving different barrier heights for the different quantum well hole states. From k.p calculations in the quasi-cubic approximation, a change in the valence-band ordering will account for...

Shields, P. A.; Nicholas, R. J.; Grandjean, N.; Massies, J.

2001-01-01

62

Auger-free luminescence due to interatomic p–d transition and self-trapped exciton luminescence in Rb2ZnCl4 crystals  

International Nuclear Information System (INIS)

It is reported that Auger-free (AF) luminescence appears with two bands at 4.5 and 6.3 eV in Rb2ZnCl4. This luminescence originates from a radiative transition of the Cl 3p valence electrons into the Zn 3d outermost-core holes. The present work is the first observation of AF luminescence due to interatomic p–d transitions in halide crystals. The appearance of two AF luminescence bands suggests the existence of two types of AF transitions following core hole creation. A largely Stokes-shifted luminescence band is also found to appear at 1.9 eV. This band has an excitation threshold at the fundamental absorption edge, and is ascribed to the radiative decay of a self-trapped exciton. - Highlights: ? Studied Auger-Free (AF) and self-trapped exciton (STE) luminescence in Rb2ZnCl4. ? Found AF luminescence due to interatomic p–d transitions in halide crystals for the first time. ? Clarified the STE luminescence in this crystal for the first time.

63

Auger-free luminescence due to interatomic p-d transition and self-trapped exciton luminescence in Rb{sub 2}ZnCl{sub 4} crystals  

Energy Technology Data Exchange (ETDEWEB)

It is reported that Auger-free (AF) luminescence appears with two bands at 4.5 and 6.3 eV in Rb{sub 2}ZnCl{sub 4}. This luminescence originates from a radiative transition of the Cl 3p valence electrons into the Zn 3d outermost-core holes. The present work is the first observation of AF luminescence due to interatomic p-d transitions in halide crystals. The appearance of two AF luminescence bands suggests the existence of two types of AF transitions following core hole creation. A largely Stokes-shifted luminescence band is also found to appear at 1.9 eV. This band has an excitation threshold at the fundamental absorption edge, and is ascribed to the radiative decay of a self-trapped exciton. - Highlights: Black-Right-Pointing-Pointer Studied Auger-Free (AF) and self-trapped exciton (STE) luminescence in Rb{sub 2}ZnCl{sub 4}. Black-Right-Pointing-Pointer Found AF luminescence due to interatomic p-d transitions in halide crystals for the first time. Black-Right-Pointing-Pointer Clarified the STE luminescence in this crystal for the first time.

Ohnishi, Akimasa, E-mail: ohnishi@sci.kj.yamagata-u.ac.jp [Department of Physics, Yamagata University, Yamagata 990-8560 (Japan); Saito, Miki; Kitaura, Mamoru [Department of Physics, Yamagata University, Yamagata 990-8560 (Japan); Itoh, Minoru [Department of Electrical and Electronic Engineering, Shinshu University, Nagano 380-8553 (Japan); Sasaki, Minoru [Department of Physics, Yamagata University, Yamagata 990-8560 (Japan)

2012-10-15

64

Exciton dispersion from first principles  

OpenAIRE

We present a scheme to calculate exciton dispersions in real materials that is based on the first-principles many-body Bethe-Salpeter equation. We assess its high level of accuracy by comparing our results for LiF with recent inelastic x-ray scattering experimental data on a wide range of energy and momentum transfer. We show its great analysis power by investigating the role of the different electron-hole interactions that determine the exciton band structure and the peculiar “exciton revi...

Gatti, Matteo; Sottile, Francesco

2013-01-01

65

Polynuclear technetium halide clusters  

International Nuclear Information System (INIS)

Development of chemistry of polynuclear technetium halide clusters in works devoted to synthesis, structure and investigation of their chemical and physical properties is considered. The role of academician V.I. Spitsyn as an initiator of investigation of polynuclear technetium halide clusters in the Institute of Physical Chemistry of Academy of Science of USSR is noted. Reactions and stability of cluster halides, their molecular and electronic structures are analyzed. Prospects of development of polynuclear technetium halide clusters chemistry as a direction being on the junction of cluster chemistry and theory of metal-metal multiple bonds are appreciated

66

Weak Exciton Scattering in Molecular Nanotubes Revealed by Double-Quantum Two-Dimensional Electronic Spectroscopy  

Science.gov (United States)

The two-exciton manifold of a double-wall cylindrical molecular aggregate is studied using a coherent third order optical technique. Experiments reveal the anharmonic character of the exciton bands. Atomistic simulations of the exciton-exciton scattering show that the excitons can be treated as weakly coupled hard-core bosons. The weak coupling stems from the extended exciton delocalization made possible by the nanotube geometry. PMID:22401120

Abramavicius, Darius; Nemeth, Alexandra; Milota, Franz; Sperling, Jaroslaw; Mukamel, Shaul; Kauffmann, Harald F.

2013-01-01

67

Effect of silicon and oxygen doping on donor bound excitons in bulk GaN  

Science.gov (United States)

Freestanding n-type intentionally doped GaN layers grown by halide vapor phase epitaxy (HVPE) were studied by transient photoluminescence (PL). Concentrations of silicon and oxygen were varied in the range between 1017 and 1018 cm-3, as confirmed by secondary ion mass spectroscopy (SIMS). We show that a reduction of the background silicon concentration by one order of magnitude compared to the background level in undoped samples can be achieved by incorporation of oxygen during the growth. A strong band gap narrowing (BGN) of ˜6 meV was observed with increasing doping in the studied samples. The low temperature PL recombination time for donor-bound excitons (DBEs) was found to depend significantly on donor concentration. A model assuming generation of DBEs by capturing of free excitons by neutral donors explains the experimental results at low temperature. From fitting the experimental DBE lifetime to the model, the donor concentration dependence for O and Si donors could be reproduced. An effective exciton capture cross-section was found to be ˜9.4 × 10-15 and 1.2 × 10-14 cm2 for silicon and oxygen donors, respectively.

Pozina, G.; Khromov, S.; Hemmingsson, C.; Hultman, L.; Monemar, B.

2011-10-01

68

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)

69

Exciton Effects in Optical Absorption of Boron-Nitride Nanotubes  

OpenAIRE

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 op...

Harigaya, Kikuo

2007-01-01

70

Spatially indirect excitons in coupled quantum wells  

Energy Technology Data Exchange (ETDEWEB)

Microscopic quantum phenomena such as interference or phase coherence between different quantum states are rarely manifest in macroscopic systems due to a lack of significant correlation between different states. An exciton system is one candidate for observation of possible quantum collective effects. In the dilute limit, excitons in semiconductors behave as bosons and are expected to undergo Bose-Einstein condensation (BEC) at a temperature several orders of magnitude higher than for atomic BEC because of their light mass. Furthermore, well-developed modern semiconductor technologies offer flexible manipulations of an exciton system. Realization of BEC in solid-state systems can thus provide new opportunities for macroscopic quantum coherence research. In semiconductor coupled quantum wells (CQW) under across-well static electric field, excitons exist as separately confined electron-hole pairs. These spatially indirect excitons exhibit a radiative recombination time much longer than their thermal relaxation time a unique feature in direct band gap semiconductor based structures. Their mutual repulsive dipole interaction further stabilizes the exciton system at low temperature and screens in-plane disorder more effectively. All these features make indirect excitons in CQW a promising system to search for quantum collective effects. Properties of indirect excitons in CQW have been analyzed and investigated extensively. The experimental results based on time-integrated or time-resolved spatially-resolved photoluminescence (PL) spectroscopy and imaging are reported in two categories. (i) Generic indirect exciton systems: general properties of indirect excitons such as the dependence of exciton energy and lifetime on electric fields and densities were examined. (ii) Quasi-two-dimensional confined exciton systems: highly statistically degenerate exciton systems containing more than tens of thousands of excitons within areas as small as (10 micrometer){sup 2} were observed. The spatial and energy distributions of optically active excitons were used as thermodynamic quantities to construct a phase diagram of the exciton system, demonstrating the existence of distinct phases. Optical and electrical properties of the CQW sample were examined thoroughly to provide deeper understanding of the formation mechanisms of these cold exciton systems. These insights offer new strategies for producing cold exciton systems, which may lead to opportunities for the realization of BEC in solid-state systems.

Lai, Chih-Wei Eddy

2004-03-01

71

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

72

Binary technetium halides  

Science.gov (United States)

In this work, the synthetic and coordination chemistry as well as the physico-chemical properties of binary technetium (Tc) chlorides, bromides, and iodides were investigated. Resulting from these studies was the discovery of five new binary Tc halide phases: alpha/beta-TcCl3, alpha/beta-TcCl 2, and TcI3, and the reinvestigation of the chemistries of TcBr3 and TcX4 (X = Cl, Br). Prior to 2009, the chemistry of binary Tc halides was poorly studied and defined by only three compounds, i.e., TcF6, TcF5, and TcCl4. Today, ten phases are known (i.e., TcF6, TcF5, TcCl4, TcBr 4, TcBr3, TcI3, alpha/beta-TcCl3 and alpha/beta-TcCl2) making the binary halide system of Tc comparable to those of its neighboring elements. Technetium binary halides were synthesized using three methods: reactions of the elements in sealed tubes, reactions of flowing HX(g) (X = Cl, Br, and I) with Tc2(O2CCH3)4Cl2, and thermal decompositions of TcX4 (X = Cl, Br) and alpha-TcCl 3 in sealed tubes under vacuum. Binary Tc halides can be found in various dimensionalities such as molecular solids (TcF6), extended chains (TcF5, TcCl4, alpha/beta-TcCl2, TcBr 3, TcI3), infinite layers (beta-TcCl3), and bidimensional networks of clusters (alpha-TcCl3); eight structure-types with varying degrees of metal-metal interactions are now known. The coordination chemistry of Tc binary halides can resemble that of the adjacent elements: molybdenum and ruthenium (beta-TcCl3, TcBr3, TcI 3), rhenium (TcF5, alpha-TcCl3), platinum (TcCl 4, TcBr4), or can be unique (alpha-TcCl2 and beta-TcCl 2) in respect to other known transition metal binary halides. Technetium binary halides display a range of interesting physical properties that are manifested from their electronic and structural configurations. The thermochemistry of binary Tc halides is extensive. These compounds can selectively volatilize, decompose, disproportionate, or convert to other phases. Ultimately, binary Tc halides may find application in the nuclear fuel cycle and as precursors in inorganic and organometallic chemistry.

Johnstone, Erik Vaughan

73

Low-temperature quenching of one-dimensional localised Frenkel excitons  

CERN Document Server

We present a theoretical analysis of low-temperature diffusion of one-dimensional Frenkel excitons that are localised by moderate on-site (diagonal) uncorrelated disorder. Exciton diffusion over the localised states is considered as an incoherent hopping process; the hopping rates being proportional to the overlap integrals of the squared exciton wave functions. The diffusion of localised excitons is probed by the exciton fluorescence quenching that is due to point traps. The trapping rate is taken to be proportional to the probability of finding an exciton at the sites with traps. We use the rate equation to calculate the temperature dependence of the exciton quenching. The interplay between hopping and spontaneous emission determines the exciton diffusion. The activation temperature of the diffusion is found to be of the order of the width of the exciton absorption band. We discuss experimental data on the fast exciton-exciton annihilation in J-aggregates at low temperatures.

Malyshev, A V; Domínguez-Adame, F

2003-01-01

74

The influence of deformation on off- and on-configuration of excitons in crystals CsI  

International Nuclear Information System (INIS)

It has been defined that in alkali halides crystals, the probability of self-trapped excitons (STE) with a radiative annihilation in a stressed field of uniaxial compression is increased. This fluming effect of own luminescence of crystals at a uniaxial deformation allows searching ?-scintillations, based on own luminescence of excitons, rather than through impurity of a luminescence, as in case of conventional scintillations. In the present work low temperature (at 80 K) influence of uniaxial compression stress on the process of electronic excitations' migration in CsI crystals and mutual transformation of two bands of the radiation of excitons of on- and off- configuration at 4.25 and 3.67 eV respectively has been studied. At X-ray and photoexcitation with an energy of quantum at 5.85 eV, corresponding to the creation of free excitons in CsI crystals, in the spectrum of the radiation at 80 K, two bands with maxima 3.67 and 4.25 eV have been registered. The intensity of the last luminescence in some measurements is dominating, and the impurity luminescence is practically imperceptible. At the photoexcitation with the energy of quantum at 6.45 eV, corresponding to the creation of divided electronic - hole pairs in crystals CsI, in a spectrum of luminescence, except the luminescence at 3.67 eV, impurity luminescence in the range of spectrum 2.0-3.4 eV are found. At such mode of photoexcitation of crystal CsI the contribution of the luminescence at 4.25 eV is small. At the luminescence at 4.25 eV is small. At low temperature uniaxial deformation (?=1-2 %) the spectra of luminescence of CsI crystal at all three modes of excitation become identical and consist of one band of luminescence at 3.67 eV. Thus, the uniaxial compression of crystal CsI brings to the redistribution of radiation intensity between 4.25 and 3.67 eV for the benefit of the last with full vanishing of the band of a radiation at 4.25 eV. It is reasonable to connect the effect of vanishing of a radiation at 4.25 eV in intense at 80 K CsI crystals with increase of potential barrier between conditions of on - and off- configurations of self-trapped excitons and it is possible to suppose that at reduction of the symmetry of lattice the on-configuration exciton (3.67 eV) is steady

75

Tuning of the valence band mixing of excitons confined in GaAs/AlGaAs quantum dots via piezoelectric-induced anisotropic strain  

Science.gov (United States)

This work presents an experimental method to tune the degree of heavy-hole (HH) and light-hole (LH) mixing of the ground state of quantum dots (QDs). A ferroelectric crystal is used to apply reversible anisotropic biaxial stress to thin nanomembranes, containing GaAs/AlGaAs QDs. The stress-induced modification of the QD anisotropy leads to a change of the relative intensity of the two emission lines produced by the recombination of neutral bright excitonic states. Such a change is ascribed to a variation of the degree of HH-LH mixing. At the same time the modified anisotropy produces a change of the excitonic fine structure splitting (FSS). Model calculations provide a qualitative insight into the relation between strain, HH-LH mixing, and the FSS in epitaxial GaAs/AlGaAs QDs.

Plumhof, J. D.; Trotta, R.; K?ápek, V.; Zallo, E.; Atkinson, P.; Kumar, S.; Rastelli, A.; Schmidt, O. G.

2013-02-01

76

Mott excitons in ferrodielectrics  

International Nuclear Information System (INIS)

Mott excitons in ferrodielectrics are investigated. It is shown that the two-particle equations for these excitons are different. These differences lead to the splitting of excitonic states. The generation of these excitons by electron beams is also discussed. (author)

77

Exciton Regeneration at Polymeric Semiconductor Heterojunctions  

CERN Document Server

Control of the band-edge offsets at heterojunctions between organic semiconductors allows efficient operation of either photovoltaic or light-emitting diodes. We investigate systems where the exciton is marginally stable against charge separation, and show via E-field-dependent time-resolved photoluminescence spectroscopy that excitons that have undergone charge separation at a heterojunction can be efficiently regenerated. This is because the charge transfer produces a geminate electron-hole pair (separation 2.2-3.1nm) which may collapse into an exciplex and then endothermically (E=100-200meV) back-transfer towards the exciton.

Morteani, A C; Herz, L M; Friend, R H; Silva, C; Morteani, Arne C.; Sreearunothai, Paiboon; Herz, Laura M.; Friend, Richard H.; Silva, Carlos

2004-01-01

78

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

79

Quantitative interpretation of the excitonic splittings in aluminum nitride  

OpenAIRE

Abstract We address the interpretation of the splitting between the ground state excitonic transition which indicates the energy of the lowest direct band gap in AlN bulk films and epilayers, and a 36–38 meV higher energy companion. We demonstrate that this splitting is consistent with the initial interpretation in terms of 1s–2s excitonic splitting by using a calculation of the exciton binding energy which includes mass anisotropy and anisotropy of the...

Gil, B.; Guizal, B.; Felbacq, D.; Bouchitte?, G.

2011-01-01

80

Biexciton excitation profile for a semiconductor with intermediately bound excitons  

International Nuclear Information System (INIS)

A method is presented for calculating the biexciton excitation profile for a semiconductor at low temperatures. The model consists of two levels (the valence and conduction bands) for cubium, with periodic boundary condition. The exciton which is discussed is intermediate between the Frenkel and Wannier type excitons. Using a formalism which is similar to Gorkov's theory of superconductivity, it is shown that excitons may interact to form charge transfer bound states. (orig.)

81

Energetics and dynamics of exciton-exciton interactions in compound colloidal semiconductor quantum dots.  

Science.gov (United States)

The energetics and dynamics of multiply excited states in single material colloidal quantum dots have already been shown to exhibit universal trends. Here we attempt to identify similar trends in exciton-exciton interactions within compound colloidal quantum dots. For this end, we thoroughly review previously available data and also present experimental data on several newly synthesized systems, focusing on core/shell nanocrystals with a type-II band alignment. A universal condition for the transition from binding to repulsion of the biexciton (type-I-type-II transition) is established in terms of the change in the exciton radiative lifetime. A scaling rule is also presented for the magnitude of exciton-exciton repulsion. In contrast, we do not identify a clear universal scaling of the non-radiative Auger recombination lifetime of the biexciton state. Finally, a perspective on future applications of engineered multiexcitonic states is presented. PMID:21229166

Deutsch, Zvicka; Avidan, Assaf; Pinkas, Iddo; Oron, Dan

2011-02-28

82

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.

83

Towards r-space Bose-Einstein condensation of photonic crystal exciton polaritons  

OpenAIRE

Coupled states of semiconductor quantum well (QW) excitons and photons in a two dimensional (2D) periodic lattice of microcavities are analyzed theoretically, revealing allowed bands and forbidden gaps in the energy spectrum of exciton polaritons. Photonic crystal exciton polaritons have spatially uniform excitonic constituent set by flat QWs, but exhibit periodic Bloch oscillations in the plane of QWs due to their photonic component. The envelope functions of photonic cryst...

Boiko, D. L.

2008-01-01

84

On the low-temperature diffusion of localized Frenkel excitons in linear molecular aggregates  

OpenAIRE

We study theoretically diffusion of one-dimensional Frenkel excitons in J-aggregates at temperatures that are smaller or of the order of the J-band width. We consider an aggregate as an open linear chain with uncorrelated on-site (diagonal) disorder that localizes the exciton at chain segments of size smaller than the full chain length. The exciton diffusion over the localization segments is considered as incoherent hopping. The diffusion is probed by the exciton fluorescenc...

Malyshev, A. V.; Malyshev, V. A.; Dominguez-adame, F.

2003-01-01

85

Low-temperature dynamics of weakly localized Frenkel excitons in disordered linar chains  

OpenAIRE

We calculate the temperature dependence of the fluorescence Stokes shift and the fluorescence decay time in linear Frenkel exciton systems resulting from the thermal redistribution of exciton population over the band states. The following factors, relevant to common experimental conditions, are accounted for in our kinetic model: (weak) localization of the exciton states by static disorder, coupling of the localized excitons to vibrations in the host medium, a possible non-e...

Bednarz, M.; Knoester, J.; Malyshev, V. A.

2003-01-01

86

The Influence of Size Effect in the Exciton Energy Spectrum of Inert Element Clusters upon the Exciton Energy Relaxation  

International Nuclear Information System (INIS)

The influence of the size effect in the exciton energy spectrum on exciton relaxation in argon, krypton, and xenon clusters is investigated by the method of VUV cathodoluminescence spectroscopy. It is found that as the average cluster size is decreased to a certain critical value Nc?160-190 atoms/cluster a blocking of the usual one-phonon channel of exciton energy relaxation occurs when the increasing distance between successive excitonic levels in the band reaches the phonon Debye energy. This leads to the appearance of another channel of relaxation (suppressed in the solid) that ends in the desorption of excited atoms and molecules from rare gas clusters

87

Influence of the size effect in the exciton energy spectrum on exciton relaxation in rare gas clusters  

Science.gov (United States)

The influence of the size effect in the exciton energy spectrum on exciton relaxation in argon, krypton, and xenon clusters is investigated by the method of VUV cathodoluminescence spectroscopy. It is found that as the average cluster size is decreased to a certain critical value Nc˜160-190 atoms/cluster a blocking of the usual one-phonon channel of exciton energy relaxation occurs when the increasing distance between successive excitonic levels in the band reaches the phonon Debye energy. This leads to the appearance of another channel of relaxation (suppressed in the solid) that ends in the desorption of excited atoms and molecules from rare gas clusters.

Verkhovtseva, É. T.; Doronin, Yu. S.; Ratner, A. M.; Bondarenko, E. A.

2006-10-01

88

Influence of Effects of Self-Polarization and Exciton-Phonon Interactions on the Exciton Energy in Lead Iodide Nanofilms  

Science.gov (United States)

In the approximation of effective masses for electronic and phononic - dielectric continuum - systems, the influence of spatial bounding, self-polarization, and exciton-phonon interactions on the exciton state in a flat double nanoheterostructure (a nanofilm) - lead iodide in a polymer matrix -is theoretically investigated for the model of a single infinitely deep quantum well. It is demonstrated that the dominating factor determining the energy of the bottom of the ground exciton band and its binding energy is spatial bounding. The relationship between two other effects depends on the nanofilm thickness, namely, the influence of the self-polarization effect in ultrathin films significantly exceeds that of exciton-phonon interaction.

Kramar, V. M.; Pugantseva, O. V.

2014-08-01

89

Halide laser glasses  

International Nuclear Information System (INIS)

Energy storage and energy extraction are of prime importance for efficient laser action and are affected by the line strengths and linewidths of optical transitions, excited-state lifetimes, nonradiative decay processes, spectroscopic inhomogeneities, nonlinear refractive index, and damage threshold. These properties are all host dependent. To illustrate this, the spectroscopic properties of Nd3+ have been measured in numerous oxide, oxyhalide, and halide glasses. A table summarizes the reported ranges of stimulated emission cross sections, peak wavelengths, linewidths, and radiative lifetimes associated with the 4F/sub 3/2/ ? 4I/sub 11/2/ lasing transition

90

Halide laser glasses  

Energy Technology Data Exchange (ETDEWEB)

Energy storage and energy extraction are of prime importance for efficient laser action and are affected by the line strengths and linewidths of optical transitions, excited-state lifetimes, nonradiative decay processes, spectroscopic inhomogeneities, nonlinear refractive index, and damage threshold. These properties are all host dependent. To illustrate this, the spectroscopic properties of Nd/sup 3 +/ have been measured in numerous oxide, oxyhalide, and halide glasses. A table summarizes the reported ranges of stimulated emission cross sections, peak wavelengths, linewidths, and radiative lifetimes associated with the /sup 4/F/sub 3/2/ ..-->.. /sup 4/I/sub 11/2/ lasing transition.

Weber, M.J.

1982-01-14

91

The radiative biexciton-exciton recommendation in wurtzite crystals  

International Nuclear Information System (INIS)

In the effective mass approximation in the two-band of semiconductor the dipole moment matrix element due to the biexciton-exciton transition is expressed by the functional of the exciton and biexciton envelopes. In comparison to previous estimations of the biexciton-exciton transition probability the detailed band structure at band extrema was taken into account and the biexciton envelope optimized by variational calculation was used in calculations. The oscillator strength ratio I2 fbiex-ex/fband-to-band is given for several values of ? = me/mh. The calculated functional I for CdS is about four times smaller compared to that obtained by Hanamura. The calculated giant oscillator strength of the biexciton-exciton recommendation in CdS is fbiex-ex = 4x102. (author)

92

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

OpenAIRE

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

93

Excitonic photoconductivity and structural defects in Cu2O crystals  

OpenAIRE

The photoconductivity spectrum of Cu2O at 4 K is studied in crystals submitted to different treatments. The role of strained zones near inclusions in the dissociation process of excitons in free carriers is pointed out. A direct correlation exists between the density of inclusions and the intensity of the photocurrent. The band to band photocurrent is submitted to the same variations as the excitonic photocurrent.

Haydar, A.; Coret, A.

1980-01-01

94

Freezing in Halide Salts  

Science.gov (United States)

The static criterion that the amplitude of the principal peak of the liquid structure factor has a constant value along the freezing line and the onset of freezing are studied from the structure factors and the static dielectric functions of halide salts interacting via the effective pair potentials through the hypernetted-chain approximation. It is observed that the criterion above is restricted to the effective charge difference. The critical value of plasma parameter at freezing is affected by the mobility and number concentration of anions and cations. The distribution of the value of the static dielectric function closest to the wave number axis in the negative region is also determined by the charge difference and the ordering of ions and related to the onset of freezing.

Akdere, ?.; Y?lmaz, M.; Kavanoz, H. B.; Ta?seven, Ç.

2008-06-01

95

Heat capacity of molten halides.  

Science.gov (United States)

The heat capacities of molten salts are very important for their practical use. Experimental investigation of this property is challenging because of the high temperatures involved and the corrosive nature of these materials. It is preferable to combine experimental investigations with empirical relationships, which allows for the evaluation of the heat capacity of molten salt mixtures. The isobaric molar heat capacities of all molten alkali and alkaline-earth halides were found to be constant for each group of salts. The value depends on the number of atoms in the salt, and the molar heat capacity per atom is constant for all molten halide salts with the exception of the lithium halides. The molar heat capacities of molten halides do not change when the anions are changed. PMID:25530462

Redkin, Alexander A; Zaikov, Yurii P; Korzun, Iraida V; Reznitskikh, Olga G; Yaroslavtseva, Tatiana V; Kumkov, Sergey I

2015-01-15

96

Isotopic mixed crystal exciton spectra in the far infrared: Naphthalene  

International Nuclear Information System (INIS)

The far infrared vibrational exciton spectra of isotopic mixed crystals of naphthalene-h8 and d8 were studied. The two observed translational phonon modes were determined to fall into the amalgamated band limit while the lowest energy B3sub(u), Asub(u) and B1sub(u) vibrational exciton bands were found to be in the separated band limit. The lowest energy B3sub(u) butterfly mode with its large (15 cm-1) exciton splitting was found to agree well with CPA calculations of mixed crystal spectra. A peak at 185 cm-1 was also assigned as a peak in the vibrational exciton density-of-states of the B3sub(u) mode. (orig.)

97

Superionicity Study from Electronic State Calculation of Metal Halide Cluster  

Science.gov (United States)

The electronic states of silver halides and sodium halides are calculated by the DV-X? cluster method to get more microscopic evidence for the p-d hybridization in noble metal halides. The calculations are carried out for the (A13B14)- (A=Ag+ and Na+, B = halogen ion) clusters. It is found that both components of anti-bonding and bonding exist in the diagram of overlap population (DOP) for AgX (X=halogen) and these two components are made up of the 4\\textit{d} band of Ag+ and the \\textit{p} band of halogen ion, which form the \\textit{p}--\\textit{d} hybridization. On the other hand, the DOP of alkali halides is occupied by almost only bonding component. It is inferred that the origin of AgI type superionic conductor is in the weakness of \\textit{p}--\\textit{d} hybridization. The hypothetical rock-salt AgI is also discussed to investigate the unstableness of rock-salt AgI.

Ono, Shinji; Kobayashi, Michisuke

2005-02-01

98

Exciton Spin Currents: Theory  

OpenAIRE

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

99

Excitons in cuprous oxide  

OpenAIRE

This work revisits the excitonic properties of Cu2O as observed with optical techniques, making use of current days experimental capabilities providing very precise measurements, with high energy and/or time resolution, using a variety of high power coherent pulsed light sources, intense magnetic fields, and cryogenic temperatures. A short overview of the theory of excitons and the excitonic Bose-Einstein condensation is presented. We discuss the ideas and results of the experiments aimed at ...

Fishman, Dmitry

2008-01-01

100

Optical response and excitons in gapped graphene  

DEFF Research Database (Denmark)

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 impact of excitons on the optical response is analyzed.

Pedersen, Thomas Garm; Jauho, Antti-Pekka

2009-01-01

101

Exciton dans un fil quantique organique  

OpenAIRE

Nous avons analysé la luminescence excitonique d'une chaîne unique et isolée d'un poly-diacétylène en matrice cristalline, la phase rouge du poly-3BCMU, par microphotoluminescence. A partir des formes de raie de micro-photoluminescence, nous avons montré que les états permis pour le centre de masse de l'exciton rouge forment une bande dans l'espace réciproque, de densité d'états parfaitement unidimensionnelle. Nous avons aussi montré que l'interaction entre l'exciton et les phonons...

Dubin, Francois

2004-01-01

102

Methyl Halide Production by Fungi  

Science.gov (United States)

Methyl chloride (CH3Cl), methyl bromide (CH3Br) and methyl iodide (CH3I) are methyl halide gases that contribute significant amounts of halogen radicals to the atmosphere. In an effort to better understand the global budget of methyl halides and their impact on the atmosphere, we need to identify the natural sources in addition to the known anthropogenic sources of these compounds. We are investigating the role of fungi in the production of methyl halides in the soils and wetlands in southern New Hampshire, USA. Previous research has shown that wood decay fungi and ectomycorrhizal fungi, which are within a group of fungi called basidiomycetes, emit methyl halides. In our study, measurements of headspace gas extracted from flasks containing fungi grown in culture demonstrate that a variety of fungi, including basidiomycetes and non-basidiomycetes, emit methyl halides. Our research sites include four ecosystems: an agricultural field, a temperate forest, a fresh water wetland, and coastal salt marshes. We have collected and isolated fungi at each site by culturing tissue samples of fruiting bodies and plant material, by using wood baits, and from the direct culture of soil. We compared the rates of methyl halide emissions from the fungi in the four ecosystems. In addition, we measured emissions from previously assayed fungal isolates after reintroducing them to sterilized soils that were collected from their original environments. Fungal biomass was determined by substrate-induced respiration (SIR). The emission rate by the fungus was determined by a linear regression of the concentration of methyl halide in the sample headspace over time divided by the fungal biomass.

Dailey, G. D.; Varner, R. K.; Blanchard, R. O.; Sive, B. C.; Crill, P. M.

2005-12-01

103

Multiple exciton generation in nano-crystals revisited: Consistent calculation of the yield based on pump-probe spectroscopy  

OpenAIRE

Multiple exciton generation (MEG) is a process in which more than one exciton is generated upon the absorption of a high energy photon, typically higher than two times the band gap, in semiconductor nanocrystals. It can be observed experimentally using time resolved spectroscopy such as the transient absorption measurements. Quantification of the MEG yield is usu- ally done by assuming that the bi-exciton signal is twice the signal from a single exciton. Herein we show that ...

Karki, Khadga J.; Ma, Fei; Zheng, Kaibo; Zidek, Karel; Mousa, Abdelrazek; Abdellah, Mohamed A.; Messing, Maria E.; Wallenberg, L. Reine; Yartsev, Arkadi; Pullerits, To?nu

2013-01-01

104

Coherently coupled exciton lasing  

OpenAIRE

Lowest reported threshold organic semiconductor VCSEL (4.9 muJ/cm[superscript 2]) is achieved when excitons coherently couple, upon sub-picosecond non-resonant optical excitation. Temperature dependence of lambda/2 n device, absent in ges lambda/n cavities, indicates excitons undergo condensate-like phase transition.

Bulovic, Vladimir; Tischler, Jonathan R.; Young, Elizabeth Renee; Nocera, Daniel G.

2009-01-01

105

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

106

Competitive hydrated positron reactions with halides  

International Nuclear Information System (INIS)

The hydrated positron, esub(aq)sup(+), reactions with halides and pseudohalides were studied by angular correlation technique. At halide concentrations below 0.02 M, the reaction of esub(aq)sup(+) with halides, x-, to form bound states [x-,e+] is diffusion controlled. On adding two halides to water containing a Ps inhibitor, the first positron transfer rate constants could be determined. For example, the reaction [Br-,e+] + I- ? Br- + [I-,e+] was studied. (Auth.)

107

Preparation of cerium halide solvate complexes  

Science.gov (United States)

Crystals of a solvated cerium(III) halide solvate complex resulted from a process of forming a paste of a cerium(III) halide in an ionic liquid, adding a solvent to the paste, removing any undissolved solid, and then cooling the liquid phase. Diffusing a solvent vapor into the liquid phase also resulted in crystals of a solvated cerium(III) halide complex.

Vasudevan, Kalyan V; Smith, Nickolaus A; Gordon, John C; McKigney, Edward A; Muenchaussen, Ross E

2013-08-06

108

Ultrafast biexciton spectroscopy in semiconductor quantum dots: evidence for early emergence of multiple-exciton generation  

Science.gov (United States)

Understanding multiple-exciton generation (MEG) in quantum dots (QDs) requires in-depth measurements of transient exciton dynamics. Because MEG typically faces competing ultrafast energy-loss intra-band relaxation, it is of central importance to investigate the emerging time-scale of the MEG kinetics. Here, we present ultrafast spectroscopic measurements of the MEG in PbS QDs via probing the ground-state biexciton transients. Specifically, we directly compare the biexciton spectra with the single-exciton ones before and after the intra-band relaxation. Early emergence of MEG is evidenced by observing transient Stark shift and quasi-instantaneous linewidth broadening, both of which take place before the intra-band relaxation. Photon-density-dependent study shows that the broadened biexciton linewidth strongly depends on the MEG-induced extra-exciton generation. Long after the intra-band relaxation, the biexciton broadening is small and the single-exciton state filling is dominant.

Choi, Younghwan; Sim, Sangwan; Lim, Seong Chu; Lee, Young Hee; Choi, Hyunyong

2013-11-01

109

Excitonic signatures in the intersubband THz absorption of optically excited semiconductor quantum wells  

Energy Technology Data Exchange (ETDEWEB)

We present a theoretical study of the intersubband absorption of an optically excited undoped semiconductor quantum well using our recently developed microscopic THz theory. In the past, intersubband transitions were often studied using doped quantum wells. In this case, the absorption follows entirely from the equilibrium carrier distribution in the lowest subband. For photoexcited quantum wells, however, there are two distinct contributions to the intersubband response. The optically generated charge carriers can be excited to a higher subband giving rise to usual band-to-band transitions. The optical pump pulse can also create an excitonic polarization which is under certain conditions converted into an incoherent exciton population. We show that the presence of such coherent or incoherent excitons yields additional excitonic THz transitions that are spectrally different from the band-to-band transitions. In my talk, I show how the intersubband absorption is influenced by excitons and for which experimental conditions one can expect to observe them in a measurement.

Golde, Daniel; Kira, Mackillo; Koch, Stephan W. [Fachbereich Physik, Philipps-Universitaet, Marburg (Germany)

2009-07-01

110

Phonon-assisted two-photon exciton transitions in semiconductors  

International Nuclear Information System (INIS)

The theory of phonon-assisted two-photon transitions to excitonic states in semiconductors has been theoretically investigated. The effects of both the nonparabolicity of the band and the degeneracy of the valence band have been taken into account. Expressions for the absorption coefficient through different band models are calculated. The numerical applications to CdI2 and GaP show that the 4-band model gives the dominant contribution which leads to a final s-exciton state. An exciton peak appears at an energy which is close to that recently observed in CdI2. The non-parabolic effect enhances the absorption coefficient by a two-order of magnitude. (author). 6 refs, 1 fig., 1 tab

111

Probing Excitonic Dark States in Single-layer Tungsten Disulfide  

CERN Document Server

Transition metal dichalcogenide (TMDC) monolayer has recently emerged as an important two-dimensional semiconductor with promising potentials for electronic and optoelectronic devices. Unlike semi-metallic graphene, layered TMDC has a sizable band gap. More interestingly, when thinned down to a monolayer, TMDC transforms from an indirect bandgap to a direct bandgap semiconductor, exhibiting a number of intriguing optical phenomena such as valley selective circular dichroism, doping dependent charged excitons, and strong photocurrent responses. However, the fundamental mechanism underlying such a strong light-matter interaction is still under intensive investigation. The observed optical resonance was initially considered to be band-to-band transitions. In contrast, first-principle calculations predicted a much larger quasiparticle band gap size and an optical response that is dominated by excitonic effects. Here, we report experimental evidence of the exciton dominance mechanism by discovering a series of exc...

Ye, Ziliang; O'Brien, Kevin; Zhu, Hanyu; Yin, Xiaobo; Wang, Yuan; Louie, Steven G; Zhang, Xiang

2014-01-01

112

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

113

CD spectrum of bacteriorhodopsin: Best evidence against exciton model  

OpenAIRE

We summarize the predictions of the exciton model that was originally proposed to explain the observed biphasic band shape of its CD spectrum in the visible region of bacteriorhodopsin (bR). It is shown that to reconcile these predictions with the observed results on the linear dichroism, the retinal isomerization time and, the retinal-retinal distance, the biphasic nature of the observed CD spectrum of bR becomes itself an evidence against the exciton model because of the uncertainty principle.

Wu, Shuguang; El-sayed, Mostafa A.

1991-01-01

114

Electron stimulated desorption of solid argon via exciton creation  

OpenAIRE

The correlation between the intrinsic luminescence of solid argon excited by slow electrons (a few eV to 100 eV) and desorption was studied. Related to exciton creation, a sharp threshold of desorption and of luminescence is observed at the same energy of the impinging electrons. The desorption rate is proportional to the exciton density at the surface. The surface nature of different luminescence bands is established. The mechanism of desorption corresponds to the mechanism of cavity formati...

Coletti, F.; Debever, J. M.; Zimmerer, G.

1984-01-01

115

Excitonic effects in oxyhalide scintillating host compounds  

Energy Technology Data Exchange (ETDEWEB)

Ab-initio calculations based on density functional theory have been performed to study the electronic, optical, mechanical, and vibrational properties of scintillator host compounds YOX (X?=?F, Cl, Br, and I). Semiempirical dispersion correction schemes are used to find the effect of van der Waals forces on these layered compounds and we found this effect to be negligible except for YOBr. Calculations of phonons and elastic constants showed that all the compounds studied here are both dynamically and mechanically stable. YOF and YOI are found to be indirect band gap insulators while YOCl and YOBr are direct band gap insulators. The band gap is found to decrease as we move from fluorine to iodine, while the calculated refractive index shows the opposite trend. As the band gap decreases on going down the periodic table from YOF to YOI, the luminescence increases. The excitonic binding energy calculated, within the effective mass approximation, is found to be more for YOF than the remaining compounds, suggesting that the excitonic effect to be more in YOF than the other compounds. The optical properties are calculated within the Time-Dependent Density Functional Theory (TDDFT) and compared with results obtained within the random phase approximation. The TDDFT calculations, using the newly developed bootstrap exchange-correlation kernel, showed significant excitonic effects in all the compounds studied here.

Shwetha, G.; Kanchana, V., E-mail: kanchana@iith.ac.in [Department of Physics, Indian Institute of Technology Hyderabad, Ordnance Factory Estate, Yeddumailaram 502 205, Telangana (India); Valsakumar, M. C. [School of Engineering Sciences and Technology (SEST), University of Hyderabad, Prof. C. R. Rao Road, Gachibowli, Hyderabad 500 046, Telangana (India)

2014-10-07

116

Excitonic effects in oxyhalide scintillating host compounds  

Science.gov (United States)

Ab-initio calculations based on density functional theory have been performed to study the electronic, optical, mechanical, and vibrational properties of scintillator host compounds YOX (X = F, Cl, Br, and I). Semiempirical dispersion correction schemes are used to find the effect of van der Waals forces on these layered compounds and we found this effect to be negligible except for YOBr. Calculations of phonons and elastic constants showed that all the compounds studied here are both dynamically and mechanically stable. YOF and YOI are found to be indirect band gap insulators while YOCl and YOBr are direct band gap insulators. The band gap is found to decrease as we move from fluorine to iodine, while the calculated refractive index shows the opposite trend. As the band gap decreases on going down the periodic table from YOF to YOI, the luminescence increases. The excitonic binding energy calculated, within the effective mass approximation, is found to be more for YOF than the remaining compounds, suggesting that the excitonic effect to be more in YOF than the other compounds. The optical properties are calculated within the Time-Dependent Density Functional Theory (TDDFT) and compared with results obtained within the random phase approximation. The TDDFT calculations, using the newly developed bootstrap exchange-correlation kernel, showed significant excitonic effects in all the compounds studied here.

Shwetha, G.; Kanchana, V.; Valsakumar, M. C.

2014-10-01

117

Investigation of change regularity of energy states of Mn2+ in halides  

International Nuclear Information System (INIS)

Data on 4E, 4A1 (4G) and 4T1 (4G) energy states of Mn2+ ion in some halides have been obtained and analyzed. With use of the dielectric theory of the chemical bond for complex crystals, several chemical bond parameters were calculated. The change regularity of the energy states of Mn2+ in halides has been studied. The results show that the covalence, the coordination number and the radius of the central ion are the main factors influencing the energy states of Mn2+ ion in halides. The relationships between these factors and the energy state 4T1 (4G), the energy difference ?E (?E=4E, 4A1 (4G)?4T1 (4G)) of Mn2+ ion in halides were established: E=2.0898+0.8618 exp (?F/0.2431); ?E=0.3201+0.9713?F. These relationships allow us to predict the position of energy state 4T1 (4G) and the energy difference ?E of Mn2+ in halides. This work can be significant for further understanding the luminescent properties of Mn2+ and can be used to develop new Mn2+-doped phosphors. - Highlights: ? Relationship between F and energy state 4T1(4G) of Mn2+ in halides was set up. ? Relationship between F and energy difference ?E of Mn2+rence ?E of Mn2+ in halides was set up. ? Site occupation of Mn2+-doped halides with two or more cations can be made clear. ? Energy state 4T1(4G) and emission band of Mn2+ in halides can be predicted.

118

Dynamical reconstruction of the exciton in LiF with inelastic x-ray scattering  

Science.gov (United States)

The absorption of light by materials proceeds through the formation of excitons, which are states in which an excited electron is bound to the valence hole it vacated. Understanding the structure and dynamics of excitons is important, for example, for developing technologies for light-emitting diodes or solar energy conversion. However, there has never been an experimental means to study the time-dependent structure of excitons directly. Here, we use causality-inverted inelastic x-ray scattering (IXS) to image the charge-transfer exciton in the prototype insulator LiF, with resolutions ?t = 20.67 as (2.067 × 10?17 s) in time and ?x = 0.533 ? (5.33 × 10?11 m) in space. Our results show that the exciton has a modulated internal structure and is coherently delocalized over two unit cells of the LiF crystal (?8 ?). This structure changes only modestly during the course of its life, which establishes it unambiguously as a Frenkel exciton and thus amenable to a simplified theoretical description. Our results resolve an old controversy about excitons in the alkali halides and demonstrate the utility of IXS for imaging attosecond electron dynamics in condensed matter. PMID:18711146

Abbamonte, Peter; Graber, Tim; Reed, James P.; Smadici, Serban; Yeh, Chen-Lin; Shukla, Abhay; Rueff, Jean-Pascal; Ku, Wei

2008-01-01

119

EXCITON SCATTERING ON SPIN WAVES IN QUASI-ONE-DIMENSIONAL ANTIFERROMAGNET  

OpenAIRE

Exciton absorption bandwidths have been measured in a quasi-one-dimensional antiferromagnet CsMnCl3.2 H2O and a three-dimensional MnF2 in magnetic fields oriented along crystal “easy” axes. In one-dimensional antiferromagnets exciton-magnon interaction is shown to result in a considerable broadening of exciton bands in the vicinity of the critical field of the spin-flop transition.

Eremenko, V.; Shapiro, V.; Kachur, I.

1988-01-01

120

Photonic Crystal Architecture for Room Temperature Equilibrium Bose-Einstein Condensation of Exciton-Polaritons  

OpenAIRE

We describe photonic crystal microcavities with very strong light-matter interaction to realize room-temperature, equilibrium, exciton-polariton Bose-Einstein condensation (BEC). This is achieved through a careful balance between strong light-trapping in a photonic band gap (PBG) and large exciton density enabled by a multiple quantum-well (QW) structure with moderate dielectric constant. This enables the formation of long-lived, dense 10~$\\mu$m - 1~cm scale cloud of exciton...

Jiang, Jian-hua; John, Sajeev

2014-01-01

121

Multiple Exciton Generation in Colloidal Nanocrystals  

Directory of Open Access Journals (Sweden)

Full Text Available In a conventional solar cell, the energy of an absorbed photon in excess of the band gap is rapidly lost as heat, and this is one of the main reasons that the theoretical efficiency is limited to ~33%. However, an alternative process, multiple exciton generation (MEG, can occur in colloidal quantum dots. Here, some or all of the excess energy is instead used to promote one or more additional electrons to the conduction band, potentially increasing the photocurrent of a solar cell and thereby its output efficiency. This review will describe the development of this field over the decade since the first experimental demonstration of multiple exciton generation, including the controversies over experimental artefacts, comparison with similar effects in bulk materials, and the underlying mechanisms. We will also describe the current state-of-the-art and outline promising directions for further development.

Charles Smith

2013-12-01

122

Phase diagram of the excitonic insulator  

CERN Document Server

Motivated by recent experiments, which give strong evidence for an excitonic insulating phase in $\\rm TmSe_{0.45}Te_{0.55}$, 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 generalised 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.

Hülsen, B; Fehske, H; Yonemitsu, K

2006-01-01

123

Dynamical process of exciton-exciton scattering in CuI thin films  

International Nuclear Information System (INIS)

We have investigated the time-resolved photoluminescence (PL) spectra of CuI thin films under intense excitation conditions in the time region up to 25 ps. In the time-integrated PL spectra, we have clearly observed the PL band originated from the inelastic scattering of excitons, so-called P emission. The time-resolved PL spectra obtained by using an optical Kerr gating method exhibit that the peak energy of the P-emission band temporally changes in a picosecond region, which reflects the variation of the effective temperature of excitonic system. In the time profile of the P-emission band, we found the following two characteristic properties. One is that the decay time hardly depends on the excitation power. The other is that the rise time becomes considerably faster with increasing excitation power. Moreover, we have found that the inverse of the rise time of the P emission exhibits an almost quadratic dependence on the excitation power

124

Dynamical process of exciton-exciton scattering in CuI thin films  

Energy Technology Data Exchange (ETDEWEB)

We have investigated the time-resolved photoluminescence (PL) spectra of CuI thin films under intense excitation conditions in the time region up to 25 ps. In the time-integrated PL spectra, we have clearly observed the PL band originated from the inelastic scattering of excitons, so-called P emission. The time-resolved PL spectra obtained by using an optical Kerr gating method exhibit that the peak energy of the P-emission band temporally changes in a picosecond region, which reflects the variation of the effective temperature of excitonic system. In the time profile of the P-emission band, we found the following two characteristic properties. One is that the decay time hardly depends on the excitation power. The other is that the rise time becomes considerably faster with increasing excitation power. Moreover, we have found that the inverse of the rise time of the P emission exhibits an almost quadratic dependence on the excitation power.

Ichida, Hideki [Venture Business Laboratory, Center for Advanced Science and Innovation, Osaka University, 2-1 Yamada-oka, Suita 565-0871 (Japan)]. E-mail: ichida@casi.osaka-u.ac.jp; Shimomura, Takeya [Department of Applied Physics, Osaka City University, 3-3-138 Sugimoto, Sumiyoshi-ku, Osaka 558-8585 (Japan); Mizoguchi, Kohji [Department of Applied Physics, Osaka City University, 3-3-138 Sugimoto, Sumiyoshi-ku, Osaka 558-8585 (Japan); Kim, Dae Gwi [Department of Applied Physics, Osaka City University, 3-3-138 Sugimoto, Sumiyoshi-ku, Osaka 558-8585 (Japan); Kanematsu, Yasuo [Venture Business Laboratory, Center for Advanced Science and Innovation, Osaka University, 2-1 Yamada-oka, Suita 565-0871 (Japan); Nakayama, Masaaki [Department of Applied Physics, Osaka City University, 3-3-138 Sugimoto, Sumiyoshi-ku, Osaka 558-8585 (Japan)

2007-01-15

125

Excitons in motion in II-VI semiconductors  

Energy Technology Data Exchange (ETDEWEB)

We have shown recently that the magnetic properties of excitons change significantly as the excitons acquire kinetic energy. In particular, the exciton magnetic moments are enhanced considerably, whilst the diamagnetism decreases. The behaviour can be investigated through spectroscopic studies of excitons confined in quantum wells of large width (greater than five times the exciton Bohr radius) and these motion-induced changes in the magnetic properties have now been observed for CdTe, ZnSe, ZnTe and GaAs. The present paper summarises these phenomena, with particular focus on CdTe and ZnSe, and shows that the changes can be accounted for by motion-induced mixing between the exciton ground and higher lying states. The mixing is caused by the {gamma}{sub 3} term in the Luttinger Hamiltonian which describes the dispersion curves for the valence band and, as a result, the form of the exciton wavefunction becomes motion-dependent. For both materials, excellent agreement is obtained between experiment and the results predicted by this mechanism. (Abstract Copyright [2010], Wiley Periodicals, Inc.)

Davies, J.J.; Smith, L.C.; Wolverson, D. [Department of Physics, University of Bath, Bath BA2 7AY (United Kingdom); Kochereshko, V.P. [A.F. Ioffe Physico-Technical Institute, RAS, 194021 St. Petersburg (Russian Federation); Cibert, J.; Mariette, H.; Boukari, H. [Institut Neel, CNRS-Universite Joseph Fourier, 38042 Grenoble Cedex 9 (France); Wiater, M.; Karczewski, G.; Wojtowicz, T. [Institute of Physics, Polish Academy of Sciences, 02-668 Warsaw (Poland); Gust, A.; Kruse, C.; Hommel, D. [Institute for Solid State Physics, Semiconductor Epitaxy Group, University of Bremen, 28334 Bremen (Germany)

2010-06-15

126

Exciton luminescence of boron nitride nanotubes and nano-arches  

Energy Technology Data Exchange (ETDEWEB)

We report photoluminescence (PL) and PL-excitation spectroscopy of BN nanotubes (nt-BN) mixed with some residual hexagonal crystalline (h-BN) starting material, and of pure h-BN microcrystalline powder. The nanotube phase exhibits a broad-band PL near 380 nm, in agreement with a published report of cathodoluminescence from a sample comprising >90% nanotubes. This emission is almost 3 eV lower in energy than unrelaxed exciton states found in recent all-electron theories of nt-BN and h-BN and about 1.4 eV lower than the lowest (perturbed dark?) exciton seen in absorption of nt-BN. This may suggest that excitons in nt-BN vibrationally relax to self-trapped states before emitting, a path found in many wide-gap solids, especially in quasi-1-dimensional forms. Exciton emission from bulk single-crystal h-BN has been shown to occur from vibrationally unrelaxed (free-exciton) states. We suggest a hypothesis in which known nano-arch reconstructions on the surface of h-BN may provide the low-dimensional environment to make exciton self-trapping on the surfaces of h-BN likely. This allows consistent interpretation of the surface-related 380 nm emission from h-BN powder within a half-nanotube self-trapped exciton hypo- thesis. (copyright 2006 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)

Berzina, B.; Trinkler, L.; Korsak, V.; Krutohvostov, R. [Institute of Solid State Physics, University of Latvia, Riga (Latvia); Carroll, D.L.; Ucer, K.B.; Williams, R.T. [Department of Physics and Center for Nanotechnology and Molecular Materials, Wake Forest University, Winston-Salem, NC 27109 (United States)

2006-11-15

127

Exciton-exciton interaction and biexciton formation in bilayer systems  

OpenAIRE

We report quantum Monte Carlo calculations of biexciton binding energies in ideal two-dimensional bilayer systems with isotropic electron and hole masses. We have also calculated exciton-exciton interaction potentials, and pair distribution functions for electrons and holes in bound biexcitons. Comparing our data with results obtained in a recent study using a model exciton-exciton potential [C. Schindler and R. Zimmermann, Phys. Rev. B \\textbf{78}, 045313 (2008)], we find a...

Lee, R. M.; Drummond, Neil; Needs, R. J.

2008-01-01

128

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

OpenAIRE

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...

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

2014-01-01

129

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 consideruantum capture of excitons in the considered composition range.

130

Huge excitonic effects in layered hexagonal boron nitride  

OpenAIRE

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

131

Exciton-exciton annihilation in organic polariton microcavities  

OpenAIRE

Sublinear intensity dependence of photoluminescence from organic exciton-polariton microcavities under non-resonant excitation in two power regimes is shown. The sublinearity is attributed to exciton-exciton annihilation, which could compete with polariton-polariton scattering in these devices.

Bulovic, Vladimir; Tischler, Jonathan R.; Young, Elizabeth Renee; Bradley, Michael Scott; Nocera, Daniel G.; Akselrod, Gleb Markovitch

2009-01-01

132

Decay of 5pCs-core excitations in halide crystals with core valence luminescence  

International Nuclear Information System (INIS)

The decay of 5pCs-core excitons and holes is studied in halide crystals with intrinsic and impurity core valence luminescence (CVL). Under excitation in the range of 5pCs(j=((3)/(2))) core exciton creation, the CVL does not appear in CsCl and CsBr crystals. However impurity CVL is observed for RbCl-CsCl and RbBr-CsBr mixed crystals under the excitation in the above mentioned region. This peculiarity is interpreted to be a result of thermal decay of core excitations and subsequent interaction of released core holes with valence electrons. The change in the intrinsic CVL decay time in CsCl-Tl crystals is ascribed to the interaction of core holes with impurity Tl ions

133

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)

134

The rate of the exciton self-trapping in KI and RbI at different temperatures  

Science.gov (United States)

The article disclosed the theory of the kinetics of excitons self-trapping in alkali halide crystals. On the example of KI and RbI crystals the time of excitons self-trapping and the length of free path before self-trapping were calculated. Also, the theory of self-trapping rate was revealed. According to the Arrhenius law the dependence of excitons self-trapping rate on the temperature and the degree of depth and uniaxial compression was analyzed. The increase of the rate of excitons self-trapping by temperature increasing was shown; while increasing the degree of compression at the full rate of strain localization decreases under uniaxial - is increasing. These data are good agreed with the experimental fact of luminescencei increasing under uniaxial compression and allow to make a conclusion about weakening of the luminescence by applying hydrostatic pressure.

Zhanturina, N.; Shunkeev, K.

2012-12-01

135

Relaxation Between Bright Optical Wannier Excitons in Perovskite Solar Absorber CH$_3$NH$_3$PbI$_3$  

OpenAIRE

We study the light-absorbing states of the mixed-halide perovskite CH$_{3}$NH$_{3}$PbI$_2$Cl and tri-iodide perovskite CH$_{3}$NH$_{3}$PbI$_3$ with density functional and many-body calculations to explain the desirable photovolatic features of these materials. The short-lived electron-hole bound states produced in this photovoltaic material are of halide to lead electron transfer character, with a Wannier-type exciton. Bethe-Salpeter (GW+BSE) calculations of the absorption c...

Nanguneri, Ravindra; Parkhill, John

2014-01-01

136

Multiscale photosynthetic exciton transfer  

CERN Document Server

Photosynthetic light harvesting provides a natural blueprint for bioengineered and biomimetic solar energy and light detection technologies. Recent evidence suggests some individual light harvesting protein complexes (LHCs) and LHC subunits efficiently transfer excitons towards chemical reaction centers (RCs) via an interplay between excitonic quantum coherence, resonant protein vibrations, and thermal decoherence. The role of coherence in vivo is unclear however, where excitons are transferred through multi-LHC/RC aggregates over distances typically large compared with intra-LHC scales. Here we assess the possibility of long-range coherent transfer in a simple chromophore network with disordered site and transfer coupling energies. Through renormalization we find that, surprisingly, decoherence is diminished at larger scales, and long-range coherence is facilitated by chromophoric clustering. Conversely, static disorder in the site energies grows with length scale, forcing localization. Our results suggest s...

Ringsmuth, A K; Stace, T M; 10.1038/nphys2332

2012-01-01

137

Excitons in disordered polymers  

OpenAIRE

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 orientat...

Avgin, I.; Huber, D. L.

2005-01-01

138

Infrared spectroscopic study of the rotation of the NH 4+ ion in ammonium halides  

OpenAIRE

Infrared correlation functions, have been obtained from the analysis of band shapes of the 1400 cm?1 bending mode of NH4Cl, NH4Br and NH4I in both the Pm3m and Fm3m phases. The NH 4 + ion seems to undergo relatively free rotation in the high temperature Fm3m phases of these halides.

Ganguly, Somnath

1980-01-01

139

An optical criterion to obtain miscible mixed crystals in alkali halides  

Directory of Open Access Journals (Sweden)

Full Text Available This work gives a novel criterion to predict the formation of alkali halide solid solutions and discusses some results obtained in the development of ternary and quaternary miscible crystalline dielectric mixtures of alkali halides. These mixtures are miscible in any concentration of their components. The miscibility of these mixed crystals is quite related to the F center through the behavior observed in the spectral position of the optical absorption F band as a function of the lattice constant of the alkali halide where the F center was formed. By inspection of an energy graph of the F band energy versus lattice constant (Mollwo-Ivey law, a set of points is observed corresponding to several pure alkali halides (such as KCl, KBr, RbCl, which gives a notion of possible mixed materials that would correspond to adjacent points and a solid solution could be obtained, meaning a single phase crystal, which result in ternary and quaternary mixed crystals. Thus, the optical absorption F band allows have a numerical criterion, based on the percentage respective of the F band energy, in order to predict possible solid solutions. We obtained experimental information using diffractograms of the mixed crystals, from which the lattice constant was obtained and compared with a theoretical calculus using a generalization of Vegard's law, finally it is discussed the case of a crystal growing, starting from five components, picking up five consecutive dots from the graph of Mollwo-Ivey's law.

R. Rodr\\u00EDguez-Mij\\u00E1ngos

2008-01-01

140

Self-trapped exciton and core-valence luminescence in BaF2 nanoparticles  

International Nuclear Information System (INIS)

The influence of the BaF2 nanoparticle size on the intensity of the self-trapped exciton luminescence and the radiative core-valence transitions is studied by the luminescence spectroscopy methods using synchrotron radiation. The decrease of the self-trapped exciton emission intensity at energies of exciting photons in the range of optical exciton creation (h? ? Eg) is less sensitive to the reduction of the nanoparticle sizes than in the case of band-to-band excitation, where excitons are formed by the recombination way. The intensity of the core-valence luminescence shows considerably weaker dependence on the nanoparticle sizes in comparison with the intensity of self-trapped exciton luminescence. The revealed regularities are explained by considering the relationship between nanoparticle size and photoelectron or photohole thermalization length as well as the size of electronic excitations

141

Self-trapped exciton and core-valence luminescence in BaF2 nanoparticles  

Science.gov (United States)

The influence of the BaF2 nanoparticle size on the intensity of the self-trapped exciton luminescence and the radiative core-valence transitions is studied by the luminescence spectroscopy methods using synchrotron radiation. The decrease of the self-trapped exciton emission intensity at energies of exciting photons in the range of optical exciton creation (h? ? Eg) is less sensitive to the reduction of the nanoparticle sizes than in the case of band-to-band excitation, where excitons are formed by the recombination way. The intensity of the core-valence luminescence shows considerably weaker dependence on the nanoparticle sizes in comparison with the intensity of self-trapped exciton luminescence. The revealed regularities are explained by considering the relationship between nanoparticle size and photoelectron or photohole thermalization length as well as the size of electronic excitations.

Vistovskyy, V. V.; Zhyshkovych, A. V.; Chornodolskyy, Ya. M.; Myagkota, O. S.; Gloskovskii, A.; Gektin, A. V.; Vasil'ev, A. N.; Rodnyi, P. A.; Voloshinovskii, A. S.

2013-11-01

142

Self-trapped exciton and core-valence luminescence in BaF{sub 2} nanoparticles  

Energy Technology Data Exchange (ETDEWEB)

The influence of the BaF{sub 2} nanoparticle size on the intensity of the self-trapped exciton luminescence and the radiative core-valence transitions is studied by the luminescence spectroscopy methods using synchrotron radiation. The decrease of the self-trapped exciton emission intensity at energies of exciting photons in the range of optical exciton creation (h? ? E{sub g}) is less sensitive to the reduction of the nanoparticle sizes than in the case of band-to-band excitation, where excitons are formed by the recombination way. The intensity of the core-valence luminescence shows considerably weaker dependence on the nanoparticle sizes in comparison with the intensity of self-trapped exciton luminescence. The revealed regularities are explained by considering the relationship between nanoparticle size and photoelectron or photohole thermalization length as well as the size of electronic excitations.

Vistovskyy, V. V., E-mail: vistvv@gmail.com; Zhyshkovych, A. V.; Chornodolskyy, Ya. M.; Voloshinovskii, A. S. [Ivan Franko National University of Lviv, 8 Kyryla i Mefodiya, 79005 Lviv (Ukraine); Myagkota, O. S. [Lviv Polytechnic National University, 12S. Bandera, 79013 Lviv (Ukraine); Gloskovskii, A. [Deutsches Elektronen-Synchrotron DESY, 22607 Hamburg (Germany); Gektin, A. V. [Institute for Scintillation Materials, NAS of Ukraine 60 Lenina Ave, 61001 Kharkiv (Ukraine); Vasil' ev, A. N. [Skobeltsyn Institute of Nuclear Physics, Lomonosov Moscow State University, 119991 Moscow (Russian Federation); Rodnyi, P. A. [Saint-Petersburg State Polytechnical University, 29, Polytekhnicheskaya, 195251 Saint-Petersburg (Russian Federation)

2013-11-21

143

Exciton formation stimulated by optically pumped electron - hole plasma in semiconductor nanostructures  

International Nuclear Information System (INIS)

A mechanism to increase the exciton binding energy in wide band-gap semiconductors due to the presence of optically pumped electron-hole plasma is proposed. These exciton states with high binding energy (>150 MeV) can exist at room temperature when the dielectric permittivity of a semiconductor approaches zero in the infrared region. Calculations for CdS show that the density of electron-hole plasma should be above 1019 cm-3 to form such excitons. There are large numbers of closely spaced energy levels of exciton states with a high binding energy in the forbidden band gap of semiconductors. We believe these excitons are involved in lasing process observed in optically pumped semiconductor nanocrystals

144

Cohesive Energy-Lattice Constant and Bulk Modulus-Lattice Constant Relationships: Alkali Halides, Ag Halides, Tl Halides  

Science.gov (United States)

In this note we present two expressions relating the cohesive energy, E(sub coh), and the zero pressure isothermal bulk modulus, B(sub 0), of the alkali halides. Ag halides and TI halides, with the nearest neighbor distances, d(sub nn). First, we show that the product E(sub coh)d(sub 0) within families of halide crystals with common crystal structure is to a good approximation constant, with maximum rms deviation of plus or minus 2%. Secondly, we demonstrate that within families of halide crystals with a common cation and common crystal structure the product B(sub 0)d(sup 3.5)(sub nn) is a good approximation constant, with maximum rms deviation of plus or minus 1.36%.

Schlosser, Herbert

1992-01-01

145

Exciton effects and nonlinear optical response in soliton lattice states of doped conjugated polymers  

OpenAIRE

Exciton effects on conjugated polymers are investigated in the soliton lattice system. We use the Su-Schrieffer-Heeger model with long-range Coulomb interactions treated by the single-excitation configuration-interaction method. The soliton band is present in the Peierls gap of the doped system. There appears a new kind of the exciton where an electron-hole pair is excited between the soliton band and the continuum states. We find that the oscillator strengths accumulate rap...

Harigaya, Kikuo; Shimoi, Yukihiro; Abe, Shuji

1995-01-01

146

Anyon exciton revisited  

OpenAIRE

We review the main results of the anyon exciton model in light of recent criticism by Wojs and Quinn. We show that the appearance of fractionally charged anyon ions at the bottom of their numerically calculated excitation spectra is an artefact caused by finite-size effects in a spherical geometry.

Parfitt, D. G. W.; Portnoi, M. E.

2003-01-01

147

Triplet exciton dynamics  

International Nuclear Information System (INIS)

Results are presented of electron spin echo experiments combined with laser flash excitation on triplet states of aromatic molecules. Some of the theoretical and experimental aspects of the photoexcited triplet state are discussed in detail and the electron spin echo spectrometers and laser systems are described. All the experiments described in this thesis were performed at liquid helium temperatures. An account is given of the ESE experiments performed on the photoexcited, non-radiative, triplet state of pentacene in napthalene. This is an example of the ESE technique being used to ascertain the zero-field splitting parameters, the populating and depopulating rates, and the orientation of the pentacene molecules in the naphthalene host. A combination of high resolution laser flash excitation and electron-spin echoes in zero-magnetic field allowed the author to observe directly k(vector)?k(vector)' exciton scattering processes in the one-dimensional triplet excitons in tetrachlorobenzene for the first time. Additional experimental data about exciton scattering is provided and a study of the orientational dependence of the spin-lattice relaxation of the triplet excitons in an external magnetic field is described. (Auth.)

148

Exciton-plasmon coupling in monolayer molybdenum disulfide  

Science.gov (United States)

Two-dimensional materials such as monolayer molybdenum disulfide (MoS2) represent a unique platform for investigating the dynamics of exciton-plasmon coupling. We report on the generation and modulation of coherent and incoherent coupled states between excitons in monolayer MoS2 and plasmons in an array of gold nanoparticle deposited onto the surface of MoS2. We study the behavior of these coherent states, termed plexcitons using a combination of photoluminescence, extinction and ultrafast spectroscopies. The close proximity of the two characteristic exciton bands of MoS2 presents multiple coherent coupling configurations, including A-or-B exciton-plasmon, and A-and-B exciton-plasmon interactions. These configurations of plexciton formation that are shown to modulate both the extinction and photoluminescence spectra of the hybrid system. This includes broadband photoluminescence and Fano-type resonances. This behavior is distinct from the spectral response of the MoS2 and plasmonic components of the system. Incoherent exciton-plasmon coupling, achieved by detuning from the plasmon extinction peaks, enhances the interaction of MoS2 with light by focusing the plasmon energy. Depending on which coupling configuration is chosen, our results show that the MoS2/plasmon hybrid systems can act as high efficiency light harvesters, broadband emitters and as tunable visible and NIR photodetectors.

Ziegler, Jed; Newaz, A. K. M.; Bolotin, Kirill; Haglund, Richard

2013-03-01

149

Hygroscopicity Evaluation of Halide Scintillators  

Energy Technology Data Exchange (ETDEWEB)

A collaborative study of relative hygroscopicity of anhydrous halide scintillators grown at various laboratories is presented. We have developed a technique to evaluate moisture sensitivity of both raw materials and grown crystals, in which the moisture absorption rate is measured using a gravimetric analysis. Degradation of the scintillation performance was investigated by recording gamma-ray spectra and monitoring the photopeak position, count rate and energy resolution. The accompanying physical degradation of the samples exposed to ambient atmosphere was photographically recorded as well. The results were compared with ben

Zhuravleva, M [The University of Tennessee; Stand, L [The University of Tennessee; Wei, H [The University of Tennessee; Hobbs, C. L. [University of Tennessee, Knoxville (UTK); Boatner, Lynn A [ORNL; Ramey, Joanne Oxendine [ORNL; Burger, Arnold [Fisk University, Nashville; Rowe, E [Fisk University, Nashville; Bhattacharya, P. [Fisk University, Nashville; Tupitsyn, E [Fisk University, Nashville; Melcher, Charles L [University of Tennessee, Knoxville (UTK)

2014-01-01

150

Ion bombardment of alkali halides  

International Nuclear Information System (INIS)

Color-center effects unique to crystals irradiated to high fluence (1016 particles/cm2) are reported for KCl and other alkali halides. These include (1) a minimum in F-center concentration at the end of the proton range and (2) an F-center distribution profile which changes with time in the dark at room temperature after irradiation. The first effect is due to annihilation of F centers by hydrogen atom capture (U-center formation) and the second is due to F-center diffusion and aggregation in a heavily damaged region of the crystal. (author)

151

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)

152

Photoluminescence decay profiles of exciton-exciton scattering in a ZnO thin film  

Energy Technology Data Exchange (ETDEWEB)

We have investigated the photoluminescence (PL) dynamics of a ZnO thin film under intense excitation conditions at 10 K. In the ZnO thin film, a PL band due to exciton-exciton scattering, the so-called P emission, appears. The optical-Kerr-gating method with the ultrashort gating time of 0.6 ps enabled us to obtain precise information of the temporal profiles including the peak energy, the bandwidth and the intensity of the P emission, at various detection energies. We have found that the decay time of the P emission gradually shortens with decreasing detection energy. The change in the decay time can be reasonably attributed to the photon-like characteristics of the lower polariton branch that is the final state of the exciton-exciton scattering. Furthermore, we have shown that the decay time of the P emission is phenomenologically determined by the group velocity of the final state although the origin of the long decay time remains to be elucidated (copyright 2011 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)

Wakaiki, Shuji [Department of Material and Life Science, Division of Advanced Science and Biotechnology, Graduate School of Engineering, Osaka University, 2-1 Yamada-oka, Suita, Osaka 565-0871 (Japan); Ichida, Hideki [Venture Business Laboratory, Center for Advanced Science and Innovation, Osaka University, 2-1 Yamada-oka, Suita, Osaka 565-0871 (Japan); Mizoguchi, Kohji [Department of Physical Science, Graduate School of Science, Osaka Prefecture University, 1-1 Gakuen, Naka-ku, Sakai, Osaka 599-8531 (Japan); Kim, DaeGwi; Nakayama, Masaaki [Department of Applied Physics, Graduate School of Engineering, Osaka City University, 3-3-138 Sugimoto, Sumiyoshi-ku, Osaka 558-8585 (Japan); Kanematsu, Yasuo [Department of Material and Life Science, Division of Advanced Science and Biotechnology, Graduate School of Engineering, Osaka University, 2-1 Yamada-oka, Suita, Osaka 565-0871 (Japan); Venture Business Laboratory, Center for Advanced Science and Innovation, Osaka University, 2-1 Yamada-oka, Suita, Osaka 565-0871 (Japan)

2011-01-15

153

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

154

On the low-temperature diffusion of localized Frenkel excitons in linear molecular aggregates  

CERN Document Server

We study theoretically diffusion of one-dimensional Frenkel excitons in J-aggregates at temperatures that are smaller or of the order of the J-band width. We consider an aggregate as an open linear chain with uncorrelated on-site (diagonal) disorder that localizes the exciton at chain segments of size smaller than the full chain length. The exciton diffusion over the localization segments is considered as incoherent hopping. The diffusion is probed by the exciton fluorescence quenching which is due to the presence of point traps in the aggregate. The rate equation for populations of the localized exciton states is used to describe the exciton diffusion and trapping. We show that there exist two regimes of the exciton diffusion at low temperatures. The first, slower one, involves only the states of the very tail of the density of states, while the second, much faster one, also involves the higher states that are close to the bottom of the exciton band. The activation energy for the first regime of diffusion is...

Malyshev, A V; Domínguez-Adame, F

2003-01-01

155

Dynamics of high-density excitons in PbI2 studied by two-photon absorption  

International Nuclear Information System (INIS)

Relaxation and diffusion processes of high-density excitons in layered-type semiconductor PbI2 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 PbI2 are estimated to be 72 ps and 4.6 cm2/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

156

Charge-transfer interaction of benzyl type radical-halide ion complexes  

International Nuclear Information System (INIS)

When a benzyl type radical is generated by the dissociative electron attachment to a halide compound in ?-irradiated organic rigid matrices at low temperature, the radical becomes complexed temporarily with the counter halide ion. The radical-ion interaction has been studied in detail by means of the fluorescence spectroscopic method. The results of the study are auto-reviewed. The most significant finding is that the radical-ion complexes generally show charge-transfer bands, whose transition energy follows the relationship, hv sub(CT) = E sub(A)(X) + E sub(S)(X-) - (E sub(A)(R.) + E sub(S)(R-)), where E sub(A) and E sub(S) refer to electron affinity and solvation energy, and X- and R. to the halide ion and the radical. Based on the relationship, the electron affinities of methyl-substituted benzyl radicals were successfully determined. (author)

157

Exciton states in cylindrical nanowires  

OpenAIRE

The exciton ground state and excited state energies are calculated for a model system of an infinitely long cylindrical wire. The effective Coulomb potential between the electron and the hole is studied as function of the wire radius. Within the adiabatic approximation, we obtain `exact' numerical results for the effective exciton potential and the lowest exciton energy levels which are fitted to simple analytical expressions. Furthermore, we investigated the influence of a ...

Slachmuylders, A. F.; Partoens, B.; Magnus, W.; Peeters, F.

2005-01-01

158

Shallow halogen vacancies in halide optoelectronic materials  

Science.gov (United States)

Halogen vacancies (VH ) are usually deep color centers (F centers) in halides and can act as major electron traps or recombination centers. The deep VH contributes to the typically poor carrier transport properties in halides. However, several halides have recently emerged as excellent optoelectronic materials, e.g., C H3N H3Pb I3 and TlBr. Both C H3N H3Pb I3 and TlBr have been found to have shallow VH , in contrast to commonly seen deep VH in halides. In this paper, several halide optoelectronic materials, i.e., C H3N H3Pb I3 , C H3N H3Sn I3 (photovoltaic materials), TlBr, and CsPbB r3 (gamma-ray detection materials) are studied to understand the material chemistry and structure that determine whether VH is a shallow or deep defect in a halide material. It is found that crystal structure and chemistry of n s2 ions both play important roles in creating shallow VH in halides such as C H3N H3Pb I3 , C H3N H3Sn I3 , and TlBr. The key to identifying halides with shallow VH is to find the right crystal structures and compounds that suppress cation orbital hybridization at VH , such as those with large cation-cation distances and low anion coordination numbers and those with crystal symmetry that prevents strong hybridization of cation dangling bond orbitals at VH . The results of this paper provide insight and guidance to identifying halides with shallow VH as good electronic and optoelectronic materials.

Shi, Hongliang; Du, Mao-Hua

2014-11-01

159

Intersubband exciton relaxation dynamics in single-walled carbon nanotubes.  

Science.gov (United States)

We study exciton (EX) dynamics in single-walled carbon nanotubes (SWNTs) included in polymethylmethacrylate by two-color pump-probe experiments with unprecedented temporal resolution. In the semiconducting SWNTs, we resolve the intersubband energy relaxation from the EX2 to the EX1 transition and find time constants of about 40 fs. The observation of a photoinduced absorption band strictly correlated to the photobleaching of the EX1 transition supports the excitonic model for primary excitations in SWNTs. We also detect in the time domain coherent oscillations due to the radial breathing modes at approximately 250 cm(-1). PMID:16090288

Manzoni, C; Gambetta, A; Menna, E; Meneghetti, M; Lanzani, G; Cerullo, G

2005-05-27

160

Optical traps for dark excitons.  

Science.gov (United States)

We propose a mechanism for optical trapping of dark excitons by linearly polarized unabsorbed standing waves, with a potential depth of the order of a few meV. Since this trapping, based on carrier exchanges with virtual excitons coupled to unabsorbed photons, equally acts on bright and dark states, Bose-Einstein condensation of excitons--which occurs in dark states--must appear as dark spots in a cloud of bright excitons, at the trap potential minima, when the temperature decreases. PMID:21668248

Combescot, Monique; Moore, Michael G; Piermarocchi, Carlo

2011-05-20

161

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

162

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

163

Electron-stimulated desorption of neutral lithium atoms from LiF due to excitation of surface excitons  

International Nuclear Information System (INIS)

We have measured electron-stimulated desorption of neutral lithium atoms from LiF at energies near the band gap of the crystal. The significant contribution to the desorption yield of neutral alkali-metal atoms due to the creation of surface excitons is resolved. These measurements also prove the theoretically expected threshold for desorption at the energy of the bulk exciton

164

Transfer of Aryl Halide to Alkyl Halide: Reductive Elimination of Alkylhalide from Alkylpalladium Halides Containing syn-?-Hydrogen Atoms.  

Science.gov (United States)

?-Hydride abstraction is a well-accepted elementary step for catalytic cycles in organometallic chemistry. It is usually anticipated that alkylpalladium halides containing syn-?-hydrogen atoms will undergo ?-hydride abstraction to afford the Heck-type products. However, this study discloses that the above general knowledge is only conditionally correct. Our experimental results demonstrate that the reductive elimination of alkylhalides from alkylpalladium halides containing syn-?-hydrogen atoms may surpass the ?-hydride abstraction or even become exclusive in certain cases. PMID:25367776

Hao, Wei; Wei, Junnian; Geng, Weizhi; Zhang, Wen-Xiong; Xi, Zhenfeng

2014-12-22

165

Structure of molten alkali halides  

International Nuclear Information System (INIS)

The results of X-ray and neutron diffraction experiments on molten alkali halides in which some data of our experiments by X-ray diffraction such as those of molten LiCl, NaCl, KCl, LiBr and KBr are included were summarized. The first peak positions in the radial distribution function in molten alkali halides by X-ray or neutron diffraction experiments are always longer than those by computer simulations and the differences of 0.1 -- 0.3 A exceed the experimental error. It seems to be due to the deformation of the electron shell. In the computer simulation, the shell model which has the spherical deformation was expected to have more closer value of the first peak position to the experimental one than the rigid ion model by taking the polarization of ions. However, no change in the first peak position was found. Therefore, the non-spherical deformation of electron shell at the point where ions are in contact with each other has to be taken into account. The neutron diffraction method with isotopic substitution has a merit of getting the partial correlation functions which are difficult to be obtained by the ordinary X-ray diffraction method and can compare directly with the results by the computer simulation. However, results of some molten salts such as KCl and CsCl by the neutron diffraction method with isotopic substitution have significant errors and there are something left to be improved for accuracy in measurements or correction of data in this method. (author)orrection of data in this method. (author)

166

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

167

Inhibition of palmito polyphenoloxidase by halide salts.  

Science.gov (United States)

The inhibitory properties of halide salts on palmito polyphenoloxidase (PPO) are described. Halide salts have the same inhibitory effect on the two forms of palmito PPO separated by hydrophobic chromatography. Fluoride and chloride ions showed a non-competitive, mixed type inhibition while bromide and iodide ions were found to be non-competitive inhibitors. A study of the Ki for the different halide salts showed that the smaller F- ion is a stronger inhibitor than I- and Br- and that Cl- has the highest Ki value. This suggests that the active site of the palmito PPO is not easily accessible. The inhibition by chloride and fluoride ion was found to be pH-dependent. The inhibitory effects of these ions increased with a decrease in pH. It is suggested that halide ions (X) could bind to either the protonated enzyme (EH) or the protonated substrate-enzyme complex (EHS) to yield inactive forms EHX and EHSX, respectively. PMID:9795866

Robert, C; Rouch, C; Cadet, F

1998-07-01

168

Stark effect of excitons in corrugated lateral surface superlattices: effect of centre-of-mass quantization  

International Nuclear Information System (INIS)

The quantum confined Stark effect (QCSE) of excitons in GaAs/AlAs corrugated lateral surface superlattices (CLSSLs) is calculated. Blue and red shifts in the exciton energies are predicted for the heavy- and light-excitons in the CLSSLs, respectively, comparing with those in the unmodulated quantum well due to the different effective hole masses in the parallel direction. Sensitive dependence of the QCSE on the hole effective mass in the parallel direction is expected because of the ''centre-of-mass'' quantization (CMQ) induced by the periodic corrugated interfaces of the CLSSLs. The effect of the CMQ on the exciton mini-bands and the localization of the excitons in the CLSSLs is discussed. (author)

169

In-situ optical transmission electron microscope study of exciton phonon replicas in ZnO nanowires by cathodoluminescence  

International Nuclear Information System (INIS)

The cathodoluminescence spectrum of single zinc oxide (ZnO) nanowires is measured by in-situ optical Transmission Electron Microscope. The coupling between exciton and longitudinal optical phonon is studied. The band edge emission varies for different excitation spots. This effect is attributed to the exciton propagation along the c axis of the nanowire. Contrary to free exciton emission, the phonon replicas are well confined in ZnO nanowire. They travel along the c axis and emit at the end surface. Bending strain increases the relative intensity of second order phonon replicas when excitons travel along the c-axis.

170

Oxidation of hydrogen halides to elemental halogens  

Science.gov (United States)

A process for oxidizing hydrogen halides having substantially no sulfur impurities by means of a catalytically active molten salt is disclosed. A mixture of the subject hydrogen halide and an oxygen bearing gas is contacted with a molten salt containing an oxidizing catalyst and alkali metal normal sulfates and pyrosulfates to produce an effluent gas stream rich in the elemental halogen and substantially free of sulfur oxide gases.

Rohrmann, Charles A. (Kennewick, WA); Fullam, Harold T. (Richland, WA)

1985-01-01

171

Multiple exciton generation and ultrafast exciton dynamics in HgTe colloidal quantum dots.  

Science.gov (United States)

The investigation of sub-nanosecond exciton dynamics in HgTe colloidal quantum dots using ultrafast transient absorption spectroscopy is reported. The transmittance change spectrum acquired immediately after pumping is dominated by a bleach blue-shifted by ~200-300 nm from the photoluminescent emission band. Comparison with a tight-binding model of the electronic structure allows this feature to be attributed to the filling of band edge states. The form of the pump-induced transmittance transients is dependent on the excitation rate and the rate of sample stirring. For moderate pumping of stirred samples, the transmittance transients are well-described by a mono-exponential decay associated with biexciton recombination, with a lifetime of 49 ± 2 ps. For samples that are strongly-pumped or unstirred, the decay becomes bi-exponential in form, indicating that trap-related recombination has become significant. We also present a new analysis that enables fractional transmittance changes to be related to band edge occupation for samples with arbitrary optical density at the pump wavelength. This allows us to identify the occurrence of multiple exciton generation, which results in a quantum yield of 1.36 ± 0.04 for a photon energy equivalent to 3.1 times the band gap, in good agreement with the results of the model. PMID:23999734

Al-Otaify, Ali; Kershaw, Stephen V; Gupta, Shuchi; Rogach, Andrey L; Allan, Guy; Delerue, Christophe; Binks, David J

2013-10-21

172

Strongly bound excitons in gapless two-dimensional structures  

Science.gov (United States)

Common wisdom asserts that bound excitons cannot form in high-dimensional (d>1) metallic structures because of their overwhelming screening and the unavoidable resonance with nearby continuous bands. Strikingly we illustrate that this prevalent assumption is not quite true. A key ingredient has been overlooked: Destructive coherent effects are capable of thwarting the formation of resonance. As an example of this general mechanism, we focus on an experimentally relevant material and predict bound excitons in twisted bilayer graphene, which is a two-dimensional gapless structure that exhibits metallic screening. The binding energies calculated by first-principles simulations are surprisingly large. The low-energy effective model reveals that these bound states are produced by a unique destructive coherence between two alike subband resonant excitons. In particular, this coherent effect is not sensitive to the screening and dimensionality, and hence may persist as a general mechanism for creating bound excitons in various metallic structures, opening the door for excitonic applications based on metallic structures.

Liang, Yufeng; Soklaski, Ryan; Huang, Shouting; Graham, Matthew W.; Havener, Robin; Park, Jiwoong; Yang, Li

2014-09-01

173

Excitons and excess electrons in nanometer size molecular polyoxotitanate clusters: electronic spectra, exciton dynamics, and surface states.  

Science.gov (United States)

The behavior of excitons and excess electrons in the confined space of a molecular polyoxotitanate cluster Ti17(?4-O)4(?3-O)16(?2-O)4(OPr(i))20 (in short Ti17) was studied using femtosecond pump-probe transient absorption, pulse radiolysis, and fluorescence spectroscopy. Due to pronounced quantum size effects, the electronic spectra of the exciton, Ti17*, and the excess electron carrying radical anion, Ti17(•-), are blue-shifted in comparison with bulk TiO2 and have maxima at 1.91 and 1.24 eV, respectively. The 0.7 eV difference in the position of the absorption maxima of Ti17* and Ti17(•-) indicates the presence of strong Coulomb interaction between the conduction band electron and the valence band hole in the ?1 nm diameter cluster. Ground state Raman spectra and the vibronic structure of the fluorescence spectrum point to the importance of the interfacial ligand modes in the stabilization and localization of the fully relaxed exciton. Four pentacoordinate Ti sites near the surface of the cluster appear to play a special role in this regard. Solvent polarity has only a minor influence on the spectral behavior of Ti17*. Exciton recombination in Ti17 is faster than in anatase nanoparticles or mesoporous films. The kinetics exhibits three components, ranging from less than 1 ps to 100 ps, which are tentatively assigned to the geminate recombination within the core of the cluster and to the decay of the surface stabilized charge transfer exciton. A persistent long-lived component with ? > 300 ps may indicate the involvement of intraband dark states, i.e., triplet excitons (3)Ti17*. PMID:23113586

Bao, Jianhua; Yu, Zhihao; Gundlach, Lars; Benedict, Jason B; Coppens, Philip; Chen, Hung Cheng; Miller, John R; Piotrowiak, Piotr

2013-04-25

174

A method for the selective hydrogenation of alkenyl halides to alkyl halides.  

Science.gov (United States)

A general method for the selective hydrogenation of alkenyl halides to alkyl halides is described. Fluoro, chloro, bromo, iodo, and gem-dihaloalkenes are viable substrates for the transformation. The selectivity of the hydrogenation is consistent with reduction by a hydrogen atom transfer pathway. PMID:24824195

King, Sandra M; Ma, Xiaoshen; Herzon, Seth B

2014-05-14

175

Low-temperature kinetics of exciton-exciton annihilation of weakly localized one-dimensional Frenkel excitons  

OpenAIRE

We present results of numerical simulations of the kinetics of exciton-exciton annihilation of weakly localized one-dimensional Frenkel excitons at low temperatures. We find that the kinetics is represented by two well-distinguished components: a fast short-time decay and a very slow long-time tail. The former arises from excitons that initially reside in states belonging to the same localization segment of the chain, while the slow component is caused by excitons created on...

Ryzhov, I. V.; Kozlov, G. G.; Malyshev, V. A.; Knoester, J.

2001-01-01

176

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

177

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

178

Excitonic Bose-Einstein condensation in Ta2NiSe5 above room temperature  

Science.gov (United States)

We show that finite temperature variational cluster approximation (VCA) calculations on an extended Falicov-Kimball model can reproduce angle-resolved photoemission spectroscopy (ARPES) results on Ta2NiSe5 across a semiconductor-to-semiconductor structural phase transition at 325 K. We demonstrate that the characteristic temperature dependence of the flat-top valence band observed by ARPES is reproduced by the VCA calculation on the realistic model for an excitonic insulator only when the strong excitonic fluctuation is taken into account. The present calculations indicate that Ta2NiSe5 falls in the Bose-Einstein condensation regime of the excitonic insulator state.

Seki, K.; Wakisaka, Y.; Kaneko, T.; Toriyama, T.; Konishi, T.; Sudayama, T.; Saini, N. L.; Arita, M.; Namatame, H.; Taniguchi, M.; Katayama, N.; Nohara, M.; Takagi, H.; Mizokawa, T.; Ohta, Y.

2014-10-01

179

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

180

Self-trapping and hot luminescence of excitons in rare gas solids  

International Nuclear Information System (INIS)

The mechanism of intrinsic luminescence of excitons in crystalline Xe and Ar is treated. The band at 9.7 eV in Ar is shown to be non-elementary. The height of the exciton self-trapping barrier in Ar is determined experimentally (epsilon = 3.5 meV). An experimental and theoretical study of hot luminescence of self-trapping excitons in rare gas crystals is carried out. The contribution of hot luminescence to the total light sum increases in the series Xe-Ar-Ne. (author)

181

Exciton localization effect in Mn-implanted GaN by photoluminescence measurements  

International Nuclear Information System (INIS)

We investigated the temperature-dependent and excitation power-dependent photoluminescence spectra of Mn-implanted (Ga,Mn)N samples with five Mn-implantation doses. The near-band-energy emission was observed and was attributed to the Mn-related exciton transition, which exhibits localized exciton behaviour resulting from alloy potential fluctuations and demonstrates a special temperature-dependence characteristic of alloy disorder. In terms of the integrated photoluminescence intensity as a function of temperature, the activation energy of the localized exciton was obtained. All these results show strong dependence on the Mn concentration of (Ga,Mn)N epilayers.

182

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

Energy Technology Data Exchange (ETDEWEB)

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.; Baldo, Marc A., E-mail: vmenon@qc.cuny.edu, E-mail: baldo@mit.edu [Energy Frontier Research Center for Excitonics, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 (United States); Goldberg, David; Menon, Vinod M., E-mail: vmenon@qc.cuny.edu, E-mail: baldo@mit.edu [Department of Physics, Queens College and Graduate Center, The City University of New York, Flushing, New York 11367 (United States)

2013-12-23

183

Exciton and interband optical transitions in hBN single crystal  

OpenAIRE

Near band gap photoluminescence (PL) of hBN single crystal has been studied at cryogenic temperatures with synchrotron radiation excitation. The PL signal is dominated by the D-series previously assigned to excitons trapped on structural defects. A much weaker S-series of self-trapped excitons at 5.778 eV and 5.804 eV has been observed using time-window PL technique. The S-series excitation spectrum shows a strong peak at 6.02 eV, assigned to free exciton absorption. Complem...

Museur, Luc; Brasse, Gurvan; Pierret, Aure?lie; Maine, Sylvain; Attal-tre?tout, Brigitte; Ducastelle, Franc?ois; Loiseau, Annick; Barjon, Julien; Watanabe, Kenji; Taniguchi, Takashi; Kanaev, Andrei?

2011-01-01

184

Excitons and charged excitons in semiconductor quantum wells  

CERN Document Server

A variational calculation of the ground-state energy of neutral excitons and of positively and negatively charged excitons (trions) confined in a single-quantum well is presented. We study the dependence of the correlation energy and of the binding energy on the well width and on the hole mass. The conditional probability distribution for positively and negatively charged excitons is obtained, providing information on the correlation and the charge distribution in the system. A comparison is made with available experimental data on trion binding energies in GaAs-, ZnSe-, and CdTe-based quantum well structures, which indicates that trions become localized with decreasing quantum well width.

Riva, C; Varga, K

2000-01-01

185

Superfluidity and collective properties of excitonic polaritons in gapped graphene in a microcavity  

OpenAIRE

We predict the formation and superfluidity of polaritons in an optical microcavity formed by excitons in gapped graphene embedded there and microcavity photons. The Rabi splitting related to the creation of an exciton in a graphene layer in the presence of the band gap is obtained. The analysis of collective excitations as well as the sound velocity is presented. We show that the superfluid density and temperature of the Kosterlitz-Thouless phase transition are decreasing fu...

Berman, Oleg L.; Kezerashvili, Roman Ya; Ziegler, Klaus

2012-01-01

186

Ultrafast biexciton spectroscopy in semiconductor quantum dots: evidence for early emergence of multiple-exciton generation  

OpenAIRE

Understanding multiple-exciton generation (MEG) in quantum dots (QDs) requires in-depth measurements of transient exciton dynamics. Because MEG typically faces competing ultrafast energy-loss intra-band relaxation, it is of central importance to investigate the emerging time-scale of the MEG kinetics. Here, we present ultrafast spectroscopic measurements of the MEG in PbS QDs via probing the ground-state biexciton transients. Specifically, we directly compare the biexciton spectra with the si...

Choi, Younghwan; Sim, Sangwan; Lim, Seong Chu; Lee, Young Hee; Choi, Hyunyong

2013-01-01

187

Thermal effects in exciton harvesting in biased one-dimensional systems  

OpenAIRE

The study of energy harvesting in chain-like structures is important due to its relevance to a variety of interesting physical systems. Harvesting is understood as the combination of exciton transport through intra-band exciton relaxation (via scattering on phonon modes) and subsequent quenching by a trap. Previously, we have shown that in the low temperature limit different harvesting scenarios as a function of the applied bias strength (slope of the energy gradient towards...

Vlaming, S. M.; Malyshev, V. A.; Knoester, J.

2007-01-01

188

Spontaneous Coherence in a Cold Exciton Gas  

OpenAIRE

Excitons, bound pairs of electrons and holes, form a model system to explore the quantum physics of cold bosons in solids. Cold exciton gases can be realized in a system of indirect excitons, which can cool down below the temperature of quantum degeneracy due to their long lifetimes. Here, we report on the measurement of spontaneous coherence in a gas of indirect excitons. We found that extended spontaneous coherence of excitons emerges in the region of the macroscopically o...

High, Alexander A.; Leonard, Jason R.; Hammack, Aaron T.; Fogler, Michael M.; Butov, Leonid V.; Kavokin, Alexey V.; Campman, Kenneth L.; Gossard, Arthur C.

2011-01-01

189

New halide clusters of technetium  

International Nuclear Information System (INIS)

The present work is devoted to the synthesis, identification, and investigation of the physicochemical properties of new halide clusters of technetium(II) which are forms in which the disproportionation products of Tc2X83- (X = Cl, Br, I) are stabilized. Comparing the results obtained in the present work with earlier data it is concluded that the cluster compounds of technetium are very inclined to undergo easy redox conversion, staggered-eclipsed conformational changes, oxidative addition of oxygen, formation of multinuclear compounds, ligand-substitution reactions, hydrolysis reactions, and hydrolysis and disproportionation with cleavage of the Tc-Tc bond. Small changes in the external synthesis conditions (an increase in temperature, the presence or absence of atmospheric oxygen, the use of organic solvents or different hydrohalic acids) can result in the stabilization of technetium clusters differing in composition and structure. It is also noted that, in contrast to the cases of rhenium and molybdenum, in the case of technetium, binuclear clusters with an order of the M-M bond equal to 3.5 or 3.0 (or even multinuclear clusters) may be more stable than the analogous compounds with a quadruple bond, depending on the conditions

190

Dimming of metal halide lamps  

Science.gov (United States)

We ran some tests on the effect of dimming of metal halide (MH) lamps upon the stability and the spectral quality of the light output. Lamps used were a new Philips lamp HPI-T 250W, a similar Philips lamp with a few thousand burning hours and a new Osram lamp HQI-T 250W/D. The ballast was a BBC type DJ 250/2KS, the starter a BAS TORGI type MZN 250 SE and the dimmer an Elstrom Control System type ERHQ-T 250. Power was derived from a Philips stabilizer, type PE 1602. Lamp output was monitored with a PAR meter. Spectra were taken at 100% and at 50% output as measured with the PAR meter. Lamps were allowed to stabilize at any setting for 30 minutes before measurements were made. Lamp manufacturers advise against dimming for fear of poor stability and intolerable changes of the spectrum. However, none of the lamps showed a decrease in stability, no flicker or wandering of the discharge, and the changes of the spectrum were not negligible, but certainly not dramatic. Lamps of either manufacture retain their white color, relative peak heights of spectral lines did shift, but no gaps in the spectrum occurred. Spectra taken at 50% with 30 minutes intervals coincided. Differences between the new and the older Philips lamp were noticeable, but not really significant.

Schurer, Kees

1994-01-01

191

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

192

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

2012-03-01

193

Exciton condensation in quantum wells. Exciton hydrodynamics. The effect of localized states  

Directory of Open Access Journals (Sweden)

Full Text Available The hydrodynamic equations for indirect excitons in the double quantum wells are studied taking into account 1 a possibility of an exciton condensed phase formation, 2 the presence of pumping, 3 finite value of the exciton lifetime, 4 exciton scattering by defects. The threshold pumping emergence of the periodical exciton density distribution is found. The role of localized and free exciton states is analyzed in the formation of emission spectra.

V.I. Sugakov

2014-09-01

194

The Remarkable Reactivity of Aryl Halides with Nucleophiles  

Science.gov (United States)

Discusses the reactivity of aryl halides with nucleophilic or basic reagents, including nucleophilic attacks on carbon, hydrogen, halogen, and arynes. Suggestions are made concerning revisions of the sections on aryl halide chemistry courses and the corresponding chapters in textbooks. (CC)

Bunnett, Joseph F.

1974-01-01

195

Synthesis of methyl halides from biomass using engineered microbes.  

OpenAIRE

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 synth...

Bayer, Ts; Widmaier, Dm; Temme, K.; Mirsky, Ea; Santi, Dv; Voigt, Ca

2009-01-01

196

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

197

Muonium centers in the alkali halides  

International Nuclear Information System (INIS)

Muonium centers (Mu) in single crystals and powdered alkali halides have been studied using the high-timing-resolution transverse field ?SR technique. Mu has been observed and its hyperfine parameter (HF) determined in every alkali halide. For the rocksalt alkali halides, the HF parameter A? shows a systematic dependence on the host lattice constant. A comparison of the Mu HF parameter with hydrogen ESR data suggests that the Mu center is the muonic analogue of the interstitial hydrogen Hi0-center. The rate of Mu diffusion can be deduced from the motional narrowing of the nuclear hyperfine interaction. KBr shows two different Mu states, a low-temperature MuI-state and a high-temperature MuII-state. (orig.)

198

Activation of glycosyl halides by halogen bonding.  

Science.gov (United States)

Halogen bonding is the formation of a non-covalent interaction between an electrophilic halogen substituent and a Lewis base, for instance, a halide. These kinds of relatively weak interactions have found applications in crystal engineering and initial applications in solution-phase chemistry are starting to appear. We report on the exploration of bis(iodoimidazolium) compounds as halogen-based Lewis acids in the activation of glycosyl halides. We show that these dicationic halogen-bond donors can be used to activate glycosyl halides if the carbohydrate core is sufficiently reactive enough. Furthermore, we provide comparison experiments which indicate that the mode of activation is indeed based on halogen bonding. This represents the first glycosylation reaction mediated by a (carbon-backbone-based) halogen-bond donor. PMID:24962953

Castelli, Riccardo; Schindler, Severin; Walter, Sebastian M; Kniep, Florian; Overkleeft, Herman S; Van der Marel, Gijsbert A; Huber, Stefan M; Codée, Jeroen D C

2014-08-01

199

Recent advances in technetium halide chemistry.  

Science.gov (United States)

Transition metal binary halides are fundamental compounds, and the study of their structure, bonding, and other properties gives chemists a better understanding of physicochemical trends across the periodic table. One transition metal whose halide chemistry is underdeveloped is technetium, the lightest radioelement. For half a century, the halide chemistry of technetium has been defined by three compounds: TcF6, TcF5, and TcCl4. The absence of Tc binary bromides and iodides in the literature was surprising considering the existence of such compounds for all of the elements surrounding technetium. The common synthetic routes that scientists use to obtain binary halides of the neighboring elements, such as sealed tube reactions between elements and flowing gas reactions between a molecular complex and HX gas (X = Cl, Br, or I), had not been reported for technetium. In this Account, we discuss how we used these routes to revisit the halide chemistry of technetium. We report seven new phases: TcBr4, TcBr3, ?/?-TcCl3, ?/?-TcCl2, and TcI3. Technetium tetrachloride and tetrabromide are isostructural to PtX4 (X = Cl or Br) and consist of infinite chains of edge-sharing TcX6 octahedra. Trivalent technetium halides are isostructural to ruthenium and molybdenum (?-TcCl3, TcBr3, and TcI3) and to rhenium (?-TcCl3). Technetium tribromide and triiodide exhibit the TiI3 structure-type and consist of infinite chains of face-sharing TcX6 (X = Br or I) octahedra. Concerning the trichlorides, ?-TcCl3 crystallizes with the AlCl3 structure-type and consists of infinite layers of edge-sharing TcCl6 octahedra, while ?-TcCl3 consists of infinite layers of Tc3Cl9 units. Both phases of technetium dichloride exhibit new structure-types that consist of infinite chains of [Tc2Cl8] units. For the technetium binary halides, we studied the metal-metal interaction by theoretical methods and magnetic measurements. The change of the electronic configuration of the metal atom from d(3) (Tc(IV)) to d(5) (Tc(II)) is accompanied by the formation of metal-metal bonds in the coordination polyhedra. There is no metal-metal interaction in TcX4, a Tc?Tc double bond is present in ?/?-TcCl3, and a Tc?Tc triple bond is present in ?/?-TcCl2. We investigated the thermal behavior of these binary halides in sealed tubes under vacuum at elevated temperature. Technetium tetrachloride decomposes stepwise to ?-TcCl3 and ?-TcCl2 at 450 °C, while ?-TcCl3 converts to ?-TcCl3 at 280 °C. The technetium dichlorides disproportionate to Tc metal and TcCl4 above ?600 °C. At 450 °C in a sealed Pyrex tube, TcBr3 decomposes to Na{[Tc6Br12]2Br}, while TcI3 decomposes to Tc metal. We have used technetium tribromide in the preparation of new divalent complexes; we expect that the other halides will also serve as starting materials for the synthesis of new compounds (e.g., complexes with a Tc3(9+) core, divalent iodide complexes, binary carbides, nitrides, and phosphides, etc.). Technetium halides may also find applications in the nuclear fuel cycle; their thermal properties could be utilized in separation processes using halide volatility. In summary, we hope that these new insights on technetium binary halides will contribute to a better understanding of the chemistry of this fascinating element. PMID:24393028

Poineau, Frederic; Johnstone, Erik V; Czerwinski, Kenneth R; Sattelberger, Alfred P

2014-02-18

200

G W quasiparticle band gap of the hybrid organic-inorganic perovskite CH3NH3PbI3 : Effect of spin-orbit interaction, semicore electrons, and self-consistency  

Science.gov (United States)

We study the quasiparticle band gap of the hybrid organic-inorganic lead halide perovskite CH3NH3PbI3 , using many-body perturbation theory based on the G W approximation. We perform a systematic analysis of the band gap sensitivity to relativistic spin-orbit effects, to the description of semicore Pb-5 d and I-4 d electrons, and to the starting Kohn-Sham eigenvalues. We find that the inclusion of semicore states increases the calculated band gap by 0.2 eV, and self-consistency on the quasiparticle eigenvalues using a scissor correction increases the band gap by 0.5 eV with respect to the G0W0 result. These findings allow us to resolve an inconsistency between previously reported G W calculations for CH3NH3PbI3 . Our most accurate band gap is 1.72 eV, and is in good agreement with the measured optical gap after considering a small excitonic shift as determined in experiments.

Filip, Marina R.; Giustino, Feliciano

2014-12-01

201

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.

202

Hyperspherical theory of anisotropic exciton  

CERN Document Server

A new approach to the theory of anisotropic exciton based on Fock transformation, i.e., on a stereographic projection of the momentum to the unit 4-dimensional (4D) sphere, is developed. Hyperspherical functions are used as a basis of the perturbation theory. The binding energies, wave functions and oscillator strengths of elongated as well as flattened excitons are obtained numerically. It is shown that with an increase of the anisotropy degree the oscillator strengths are markedly redistributed between optically active and formerly inactive states, making the latter optically active. An approximate analytical solution of the anisotropic exciton problem taking into account the angular momentum conserving terms is obtained. This solution gives the binding energies of moderately anisotropic exciton with a good accuracy and provides a useful qualitative description of the energy level evolution.

Muljarov, E A; Tikhodeev, S G; Bulatov, A E; Birman, Joseph L; 10.1063/1.1286772

2012-01-01

203

Reference spectroscopic data for hydrogen halides, Part II: The line lists  

International Nuclear Information System (INIS)

Accurate spectroscopic parameters for the hydrogen halides, namely HF, HCl, HBr, and HI, together with their deuterated isotopologues, are crucial for the quantitative study of terrestrial and planetary atmospheres, astrophysical objects, and chemical lasers. A thorough evaluation of all the hydrogen halide line parameters in previous HITRAN editions has been carried out. A new set of line lists was generated for the HITRAN2012 edition using methods described here. In total, 131,798 entries were generated for numerous pure-rotational and ro-vibrational transitions (fundamental, overtone, and hot bands) for hydrogen halides and their deuterated species in a standard HITRAN 160-character format. Data for the deuterated isotopologues have been entered into HITRAN for the first time. The calculations employ the recently developed semi-empirical dipole moment functions [Li G, et al. J Quant Spectrosc Radiat Transfer 2013;121:78–90] and very accurate analytical potential energy functions and associated functions characterizing Born–Oppenheimer breakdown effects. Line-shape parameters have also been updated using the most recent available experimental and theoretical studies. Comparison with the previous HITRAN compilation has shown significant improvements. -- Highlights: • Significant improvements of line positions and intensities of hydrogen halides. • Extended ro-vibrational range for non-LTE applications. • First time inclusion of the deuterated species, DF, DCl, DBr and DI in HITRAN. • Updated line-shape parameters

204

Time-dependent surface plasmon resonance spectroscopy of silver nanoprisms in the presence of halide ions.  

Science.gov (United States)

Herein, we find that the surface plasmon resonance (SPR) spectra of silver nanoprisms in the presence of halide ions change gradually with reaction time. The changes in the spectra correspond to the shape transformation of silver nanoprisms. There are threshold concentrations of halide ions that initiate the shape-transformation reaction. The threshold concentrations for Cl(-), Br(-), and I(-) are about 3x10(-4) M, 1x10(-6) M, and 1.5x10(-6) M, respectively. Any concentrations of the added halide ions above these thresholds can eventually etch the silver nanoprisms into nanodisks if the reaction time is long enough. The higher the concentration of the halide ions, the higher the etching rate will be. The kinetics of the shape transformation of the silver nanoprisms can be studied by recording their time-dependent surface plasmon resonance (SPR) spectra on a commercial UV/Vis-NIR spectrometer. The peak positions of in-plane dipole SPR bands of silver colloids in the presence of chloride and bromide ions can be fitted very well with the biexponential functions. We propose that the fast components of the biexponential behaviors should correlate to the truncating effect on the corners of silver nanoprisms, and the slow component should correlate to the redeposition of the truncated residues onto the basal plane of the nanoplates. PMID:20217886

Hsu, Ming-Sheng; Cao, Yi-Wei; Wang, Hsuang-Wen; Pan, Ya-Sin; Lee, Bo-Hong; Huang, Cheng-Liang

2010-06-01

205

Enhanced quantum efficiency from hybrid cesium halide/copper photocathodes  

International Nuclear Information System (INIS)

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

206

Thickness dependence of photoluminescence-decay profiles of exciton-exciton scattering in ZnO thin films  

Science.gov (United States)

We have investigated the photoluminescence (PL) dynamics of ZnO thin films under intense excitation conditions using an optical-Kerr-gating method. The PL bands originating from exciton-exciton scattering (P emission) and biexciton (M emission) have been observed at 10 K. The ultrashort gating time of 0.6 ps has enabled us to obtain precise information of the temporal profiles of the peak energies and the intensities of the P- and M-PL bands. We have found that the decay time of the P emission becomes longer with increasing film thickness, while that of the M emission is independent of the film thickness. Although the decay time of the P emission is an increasing function of the film thickness, the relation is not in proportion, which is contrary to the predicted proportionality based on a simple model of photon-like polariton propagation.

Wakaiki, Shuji; Ichida, Hideki; Kawase, Toshiki; Mizoguchi, Kohji; Kim, DaeGwi; Nakayama, Masaaki; Kanematsu, Yasuo

2013-09-01

207

Polarizabilities of the alkali halide dimers. II  

International Nuclear Information System (INIS)

We have measured the average electric dipole polarizabilities of twenty alkali halide dimers, using a molecular beam electric deflection method. This is a continuation of the earlier measurements of five alkali halide dimers [R. Kremens et al., J. Chem. Phys. 81, 1676 (1984)]. We compare our measurements with a simple combination of bond (via a ''spring'' model) and effective ionic polarizabilities, using literature values for the required molecular constants. Agreement between our measured and calculated values is reasonable, in most cases, considering the relative crudeness of the model

208

Features of radiation processes in ammonium halide crystals with rare-earth impurity  

International Nuclear Information System (INIS)

Full text: The ammonium halide crystals (NH4X, X-halogen) are a touchstone for various theories of phase transition of type the order-disorder. Being analogues alkali halide crystals, they sharply differ from them processes radiation-induced defects because of presence complex cation NH4+. In the present work the review of our researches on radiation processes in NH4X with an impurity bivalent rare-earth ions (TR2+) is resulted- Yb2+, Eu2+, Pr2+, Nd2+, Dy2+. At an irradiation in X-ray quanta NH4X-TR2+ in spectra of absorption there is a induced band-doped the band connected to radiation defects. As well as in alkali halide crystals, there are HZ-centers representing impurity-vacancy a dipole, stabilizing interstitial, and also - VKZ -centers. As against alkali halide crystals the F-centers in ammonium halide crystals are not found out neither in optical spectra, nor in spectra electron paramagnetic resonance. Because of reaction NH4++ e?NH40?NH30+H0, in ammonium halide crystals there are centers of type TR2+NH30. In X-ray luminescence spectra the luminescence band connected to transition radiation in molecules TR2+NH30. In absorption and photoconductivity spectra the paintnd photoconductivity spectra the painting centers of type TR+, luminescence Yb+Vc- are shown and the isolated centers Yb+ is not observed even at the lowest temperatures. At transition phase the order -disorder spectra of TR2+-ions in ammonium halide crystals and the radiation defects connected to them in steps change. These changes are connected to change of static interaction of an impurity with a field of ligands, parameters of electron-phonon interactions, probabilities radiationless transitions.On phase transition in spectra of thermoluminescence is revealed and the radiation-induced luminescence possessing unusual properties - is investigated by independence of his occurrence of an electronic structure of an impurity: having the recombination nature with identical energy of activation for all TR2+. The spectral structure of peak of thermoluminescence on phase transition contains two radiation bands - short-wave and long-wave. Short-wave bands are caused by transitions from the mixed 4fn-15d configuration in the basic condition 4fn. Long-wave bands are connected to radiating transitions TR2+NH30. At research of dependence of the reserved energy from a dose of an irradiation on the transition phase two stages of accumulation of defects -fast and slow -are found out. Presence slow components is connected to action of the channel of repeated capture of carriers of a charge. In work all features of radiating processes in ammonium halide crystals with an impurity bivalent rare-earth are discussed

209

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

Energy Technology Data Exchange (ETDEWEB)

We present a detailed analysis of exciton-photon interaction in a microcavity made out of a photonic crystal slab. Here we have analysed a disc-like quantum dot where an exciton is formed. Excitonic eigen functions in addition to their eigen energies are found through direct matrix diagonalization, while wavefunctions corresponding to unbound electron and hole are chosen as the basis set for this procedure. In order to evaluate these wavefunctions precisely, we have used the 6 x 6 Luttinger Hamiltonian in the case of hole while ignoring bands adjacent to the conduction band for electron states. After analysing 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 a finite-difference time-domain method in order to simulate our cavity with sufficient precision. Finally, we formulate the coupling constants for the exciton-photon interaction both where intra-band and inter-band transitions occur. By evaluating a sample coupling constant, it has been shown that the system can be in a strong-coupling regime and Rabi oscillations can occur.

Sodagar, Majid; Khoshnegar, Milad; Eftekharian, Amin; Khorasani, Sina [School of Electrical Engineering, Sharif University of Technology, PO Box 11365-9363, Tehran (Iran, Islamic Republic of)

2009-04-28

210

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

Science.gov (United States)

We present a detailed analysis of exciton-photon interaction in a microcavity made out of a photonic crystal slab. Here we have analysed a disc-like quantum dot where an exciton is formed. Excitonic eigen functions in addition to their eigen energies are found through direct matrix diagonalization, while wavefunctions corresponding to unbound electron and hole are chosen as the basis set for this procedure. In order to evaluate these wavefunctions precisely, we have used the 6 × 6 Luttinger Hamiltonian in the case of hole while ignoring bands adjacent to the conduction band for electron states. After analysing 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 a finite-difference time-domain method in order to simulate our cavity with sufficient precision. Finally, we formulate the coupling constants for the exciton-photon interaction both where intra-band and inter-band transitions occur. By evaluating a sample coupling constant, it has been shown that the system can be in a strong-coupling regime and Rabi oscillations can occur.

Sodagar, Majid; Khoshnegar, Milad; Eftekharian, Amin; Khorasani, Sina

2009-04-01

211

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

212

Vibrational spectroscopic characterization of the sulphate-halide mineral sulphohalite - Implications for evaporites  

Science.gov (United States)

The mineral sulphohalite - Na6(SO4)2FCl is a rare sodium halogen sulphate and occurs associated with evaporitic deposits. Sulphohalite formation is important in saline evaporites and in pipe scales. Sulphohalite is an anhydrous sulphate-halide with an apparent variable anion ratio of formula Na6(SO4)2FCl. Such a formula with oxyanions lends itself to vibrational spectroscopy. The Raman band at 1003 cm-1 is assigned to the (SO4)2- ?1 symmetric stretching mode. Shoulders to this band are found at 997 and 1010 cm-1. The low intensity Raman bands at 1128, 1120 and even 1132 cm-1 are attributed to the (SO4)2- ?3 antisymmetric stretching vibrations. Two symmetric sulphate stretching modes are observed indicating at least at the molecular level the non-equivalence of the sulphate ions in the sulphohalite structure. The Raman bands at 635 and 624 cm-1 are assigned to the ?4 SO42- bending modes. The ?2 (SO4)2- bending modes are observed at 460 and 494 cm-1. The observation of multiple bands supports the concept of a reduction in symmetry of the sulphate anion from Td to C3v or even C2v. No evidence of bands attributable to the halide ions was found.

Frost, Ray L.; Scholz, Ricardo; López, Andrés; Theiss, Frederick L.

2014-12-01

213

Quantum condensation in electron-hole systems: excitonic BEC-BCS crossover and biexciton crystallization  

International Nuclear Information System (INIS)

Quantum condensation of electron-hole (e-h) systems in photoexcited semiconductors is reviewed from a theoretical viewpoint, stressing the exciton Bose-Einstein condensation (BEC), the e-h BCS-type condensed state, the exciton Mott transition, and the biexciton crystallization. First, we discuss the crossover between the exciton BEC and the e-h BCS states at low temperature using the self-consistent t-matrix and local approximations, applied to the high-dimensional two-band Hubbard model with both repulsive and attractive on-site interactions. We also study the metal-insulator transition (called the 'exciton Mott transition') at zero and finite temperatures, investigated with the dynamical mean-field theory. Away from half-filling we find excitonic/biexcitonic insulating phases and the first-order transition between metallic and insulating states. Second, in a one-dimensional e-h system, we employ the exciton bosonization and renormalization-group techniques to clarify quantum orders at zero temperature. The most probable ground state exhibits the biexciton crystallization, which reflects the Tomonaga-Luttinger liquid properties, the e-h backward scattering, and the long-range Coulomb interaction. The one-dimensional e-h system is insulating even at the high-density limit, hence the exciton Mott transition never occurs at zero temperature in one dimension

214

Quantum condensation in electron-hole systems: excitonic BEC-BCS crossover and biexciton crystallization.  

Science.gov (United States)

Quantum condensation of electron-hole (e-h) systems in photoexcited semiconductors is reviewed from a theoretical viewpoint, stressing the exciton Bose-Einstein condensation (BEC), the e-h BCS-type condensed state, the exciton Mott transition, and the biexciton crystallization. First, we discuss the crossover between the exciton BEC and the e-h BCS states at low temperature using the self-consistent t-matrix and local approximations, applied to the high-dimensional two-band Hubbard model with both repulsive and attractive on-site interactions. We also study the metal-insulator transition (called the 'exciton Mott transition') at zero and finite temperatures, investigated with the dynamical mean-field theory. Away from half-filling we find excitonic/biexcitonic insulating phases and the first-order transition between metallic and insulating states. Second, in a one-dimensional e-h system, we employ the exciton bosonization and renormalization-group techniques to clarify quantum orders at zero temperature. The most probable ground state exhibits the biexciton crystallization, which reflects the Tomonaga-Luttinger liquid properties, the e-h backward scattering, and the long-range Coulomb interaction. The one-dimensional e-h system is insulating even at the high-density limit, hence the exciton Mott transition never occurs at zero temperature in one dimension. PMID:21483057

Ogawa, Tetsuo; Tomio, Yuh; Asano, Kenichi

2007-07-25

215

Excitonic splitting, delocalization, and vibronic quenching in the benzonitrile dimer.  

Science.gov (United States)

The excitonic S1/S2 state splitting and the localization/delocalization of the S1 and S2 electronic states are investigated in the benzonitrile dimer (BN)2 and its (13)C and d5 isotopomers by mass-resolved two-color resonant two-photon ionization spectroscopy in a supersonic jet, complemented by calculations. The doubly hydrogen-bonded (BN-h5)2 and (BN-d5)2 dimers are C2h symmetric with equivalent BN moieties. Only the S0 ? S2 electronic origin is observed, while the S0 ? S1 excitonic component is electric-dipole forbidden. A single (12)C/(13)C or 5-fold h5/d5 isotopic substitution reduce the dimer symmetry to Cs, so that the heteroisotopic dimers (BN)2-(h5 – h5(13)C), (BN)2-(h5 – d5), and (BN)2-(h5 – h5(13)C) exhibit both S0 ? S1 and S0 ? S2 origins. Isotope-dependent contributions ?iso to the excitonic splittings arise from the changes of the BN monomer zero-point vibrational energies; these range from ?iso((12)C/(13)C) = 3.3 cm(–1) to ?iso(h5/d5) = 155.6 cm(–)1. The analysis of the experimental S1/S2 splittings of six different isotopomeric dimers yields the S1/S2 exciton splitting ?exc = 2.1 ± 0.1 cm(–1). Since ?iso(h5/d5) ? ?exc and ?iso((12)C/(13)C) > ?exc, complete and near-complete exciton localization occurs upon (12)C/(13)C and h5/d5 substitutions, respectively, as diagnosed by the relative S0 ? S1 and S0 ? S2 origin band intensities. The S1/S2 electronic energy gap of (BN)2 calculated by the spin-component scaled approximate second-order coupled-cluster (SCS-CC2) method is ?el(calc) = 10 cm(–1). This electronic splitting is reduced by the vibronic quenching factor ?. The vibronically quenched exciton splitting ?el(calc)·? = ?vibron(calc) = 2.13 cm(–1) is in excellent agreement with the observed splitting ?exc = 2.1 cm(–1). The excitonic splittings can be converted to semiclassical exciton hopping times; the shortest hopping time is 8 ps for the homodimer (BN-h5)2, the longest is 600 ps for the (BN)2(h5 – d5) heterodimer. PMID:25353324

Balmer, Franziska A; Ottiger, Philipp; Leutwyler, Samuel

2014-11-26

216

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

217

Effect of strain on GaN exciton spectra  

Science.gov (United States)

The large majority of GaN epitaxial films have been grown on substrates which show both lattice and thermal expansion mismatch and, as a result, are significantly strained. The effect is most readily monitored by measuring exciton energies in photoluminescence spectra, and this paper compares published data on free exciton energies in a homoepitaxial film with a range of heteroepitaxial samples grown on sapphire substrates, all the films having the hexagonal (wurtzite) structure. We observe that strain results in a marked increase in separation of the B and C excitons while leaving the A, B separation relatively unaffected, and interpret this in terms of the quasi-cubic band structure model developed by Hopfield for the very similar case of CdS and other II - VI compounds. Within the limits of this model, there is an ambiguity in the interpretation of exciton energies which depends on the fact that the spin - orbit and crystal field splitting parameters are interchangeable. Consideration of strain effects removes this ambiguity and allows us to derive values of 0268-1242/11/7/009/img1 meV and 0268-1242/11/7/009/img2 meV for the unstrained homoepitaxial sample, consistent with the earlier estimates of Dingle et al based on relative intensity measurements.

Orton, J. W.

1996-07-01

218

Relative Ordering between Bright and Dark Excitons in Single-walled Carbon Nanotubes  

Science.gov (United States)

The ordering and relative energy splitting between bright and dark excitons are critical to the optical properties of single-walled carbon nanotubes (SWNTs), as they eventually determine the radiative and non-radiative recombination processes of generated carriers. In this work, we report systematic high-field magneto-optical study on the relative ordering between bright and dark excitons in SWNTs. We identified the relative energy position of the dark exciton unambiguously by brightening it in ultra-high magnetic field. The bright-dark excitonic ordering was found to depend not only on the tube structure, but also on the type of transitions. For the 1st sub-band transition, the bright exciton appears to be higher in energy than its dark counterpart for any chiral species and is robust against environmental effect. While for the 2nd sub-band, their relative ordering was found to be chirality-sensitive: the bright exciton can be either higher or lower than the dark one, depending on the specific nanotube structures. These findings provide new clues for engineering the optical and electronic properties of SWNTs.

Zhou, Weihang; Nakamura, Daisuke; Liu, Huaping; Kataura, Hiromichi; Takeyama, Shojiro

2014-11-01

219

Asymmetric intermolecular Heck reaction of aryl halides.  

Science.gov (United States)

The asymmetric intermolecular Heck reaction has been limited to aryl and vinyl triflates. Herein, we extend the reaction to aryl and vinyl bromides. Various cyclic olefins coupled with high enantioselectivity. Only bisphosphine oxides on a spiro backbone formed highly stereoselective Pd catalysts. The use of alcoholic solvents and alkylammonium salts were essential to promote halide dissociation from neutral arylpalladium complexes. PMID:24383764

Wu, Chunlin; Zhou, Jianrong Steve

2014-01-15

220

Elastic constants of alkali halides: Multipole expansion  

Science.gov (United States)

The elastic constants at zero temperature are calculated for the alkali-halide crystals. Two new terms are included: the induced quadrupole and induced octupole polarizabilities of the ions. These terms are significant, and affect the final result by 5-50%. The elastic constants are a good quantity for testing theories of ionic bonding.

Mahan, G. D.

1984-05-01

221

Reactivity of halide and pseudohalide ligands  

International Nuclear Information System (INIS)

Reactivity of halide and pseudohalide (cyanide, azide, thiocyanate, cyanate) ligands tending to form bridge bonds in transition metal (Re, Mo, W) complexes is considered. Complexes where transition metal salts are ligands of other, complex-forming ion, are described. Transformation of innerspheric pseudohalide ligands is an important way of directed synthesis of these metal coordination compounds

222

Dissociation of alkali halides under electron bombardment  

International Nuclear Information System (INIS)

The low-energy electron bombardment of alkali halides results in their dissociation and emission of halogen. The alkali metal excess on the irradiated surface is found by the electron Auger-spectroscopy, neutron-activated analysis, electron microscopy, conductivity and secondary electron emission coefficient measurements

223

Novel ultra-violet/blue optoelectronic materials and devices based on copper halides (CuHa)  

OpenAIRE

Considerable research is being carried out in the area of wide band gap semiconductor materials for light emission applications in the UV/Blue (300-400 nm) spectral range. This project explores the novel use of the Copper Halides (CuHa), specifically ?-CuCl and ?-CuBr, I–VII wide band gap mixed ionic–electronic semiconducting materials with light emitting properties suitable for novel UV/blue light applications. This project details novel research carried out towards achieving single c...

Cowley, Aidan James

2012-01-01

224

First-principles study on the electronic and optical properties of cubic ABX3 halide perovskites  

International Nuclear Information System (INIS)

The electronic properties of ABX3 type compounds in the cubic phase are systematically studied using the first-principles calculations. The chemical trend of their properties as A or B or X varies is fully investigated. The optical properties of the ABX3 compounds are also investigated. Our calculations show that taking into account the spin–orbit coupling effect is crucial for predicting the accurate band gap of these halide perovskites. We predict that CH3NH3SnBr3 is a promising material for solar cells absorber with a perfect band gap and good optical absorption.

225

Exciton-magnon structure of the optical absorption spectrum of antiferromagnetic MnPS3  

Science.gov (United States)

In the region of the A1g6(S6)?A1g4,Eg4(G4) optical transition, the absorption spectrum of the quasi-two-dimensional antiferromagnet MnPS3 has an exciton-magnon structure. It includes a pure exciton line with a wave number of 21223.5cm-1 and an exciton-magnon band with a peak at 21328cm-1 for T =1.7K. The spectra are studied at temperatures ranging from 1.7to80K and with external magnetic fields up to 60kOe. In a magnetic field oriented along the easy axis z, splitting of the exciton line is observed owing to the nonequivalence of the magnetic sublattices with respect to an external field. The behavior of the split components in the region of the spin-orientation transition (35-41kOe) is typical of second order phase transitions.

Gnatchenko, S. L.; Kachur, I. S.; Piryatinskaya, V. G.; Vysochanskii, Yu. M.; Gurzan, M. I.

2011-02-01

226

Time-domain chirally-sensitive three-pulse coherent probes of vibrational excitons in proteins  

CERN Document Server

The third order optical response of bosonic excitons is calculated using the Green's function solution of the Nonlinear Exciton Equations (NEE) which establish a quasiparticle-scattering mechanism for optical nonlinearities. Both time ordered and non ordered forms of the response function which represent time and frequency domain techniques, respectively, are derived. New components of the response tensor are predicted for isotropic ensembles of periodic chiral structures to first order in the optical wavevector. The nonlocal nonlinear response function is calculated in momentum space, where the finite exciton-exciton interaction length greatly reduces the computational effort. Applications are made to coupled anharmonic vibrations in the amide I infrared band of peptides. Chirally-sensitive and non sensitive signals for alpha helices and antiparallel beta sheets are compared.

Abramavicius, D; Abramavicius, Darius; Mukamel, Shaul

2005-01-01

227

Structure parameters and external electric field effects on exciton binding energies of CdTe/ZnTe quantum dots  

International Nuclear Information System (INIS)

We study the effects of the structure parameters of self-assembled CdTe/ZnTe quantum dots (QDs) under an electric field on the exciton binding energies due to Coulomb interaction between electrons and holes with a finite-element method based on the linear elasticity theory of solids and the eight-band k ·p Hamiltonian. The exciton binding energy is shown to decrease with increasing base width of the QD, regardless of its height. We point out that the monotonic decrease in the exciton binding energy is due to the confinement of the electron and the hole wavefunctions inside the QD. The exciton binding energy is also found to decrease as the wetting layer thickness increases, which can be attributed to the dipole-like wavefunction of the hole. The fact that the electron and the hole energies decrease parabolically and the exciton binding energy decreases with increasing electric field due to the Stark effect is demonstrated.

228

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

International Nuclear Information System (INIS)

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.”

229

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

230

Tightly bound excitons in monolayer WSe2  

OpenAIRE

Exciton binding energy and excited states in monolayers of tungsten diselenide (WSe2) are investigated using the combined linear absorption and two-photon photoluminescence excitation spectroscopy. The exciton binding energy is determined to be 0.37eV, which is about an order of magnitude larger than that in III-V semiconductor quantum wells and renders the exciton excited states observable even at room temperature. The exciton excitation spectrum with both experimentally de...

He, Keliang; Kumar, Nardeep; Zhao, Liang; Wang, Zefang; Mak, Kin Fai; Zhao, Hui; Shan, Jie

2014-01-01

231

Adiabatic preparation of a cold exciton condensate  

OpenAIRE

We propose a scheme for the controllable preparation of a cold indirect exciton condensate using dipolaritonic setup with an optical pumping. Dipolaritons are bosonic quasiparticles which arise from the coupling between cavity photon (C), direct exciton (DX), and indirect exciton (IX) modes, and appear in a double quantum well embedded in a semiconductor microcavity. Controlling the detuning between modes of the system, the limiting cases of exciton-polaritons and indirect e...

Shahnazaryan, V.; Kyriienko, O.; Shelykh, I.

2014-01-01

232

Exciton-photon correlations in bosonic condensates of exciton-polaritons  

OpenAIRE

Exciton-polaritons are mixed light-matter quasiparticles. We have developed a statistical model describing stochastic exciton-photon transitions within a condensate of exciton polaritons. We show that the exciton-photon correlator depends on the "hidden variable" which characterizes the rate of exciton-photon transformations in the condensate. We discuss implications of this effect for the quantum statistics of photons emitted by polariton lasers.

Kavokin, A. V.; Sheremet, A. S.; Shelykh, I. A.; Lagoudakis, P. G.; Rubo, Y. G.

2014-01-01

233

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

234

Exciton Dissociation Dynamics in Model Donor-Acceptor Polymer Heterojunctions: I. Energetics and Spectra  

CERN Document Server

In this paper we consider the essential electronic excited states in parallel chains of semiconducting polymers that are currently being explored for photovoltaic and light-emitting diode applications. In particular, we focus upon various type II donor-acceptor heterojunctions and explore the relation between the exciton binding energy to the band off-set in determining the device characteristic of a particular type II heterojunction material. As a general rule, when the exciton binding energy is greater than the band off-set at the heterojunction, the exciton will remain the lowest energy excited state and the junction will make an efficient light-emitting diode. On the other hand, if the off-set is greater than the exciton binding energy, either the electron or hole can be transferred from one chain to the other. Here we use a two-band exciton to predict the vibronic absorption and emission spectra of model polymer heterojunctions. Our results underscore the role of vibrational relaxation and suggest that i...

Bittner, E R; Karabunarliev, S; Bittner, Eric R.; Ramon, John Glenn Santos; Karabunarliev, Stoyan

2005-01-01

235

Excitonic polaritons in Fibonacci quasicrystals.  

Science.gov (United States)

The fabrication and characterization of light-emitting one-dimensional photonic quasicrystals based on excitonic resonances is reported. The structures consist of high-quality GaAs/AlGaAs quantum wells grown by molecular-beam epitaxy with wavelength-scale spacings satisfying a Fibonacci sequence. The polaritonic (resonant light-matter coupling) effects and light emission originate from the quantum well excitonic resonances. Measured reflectivity spectra as a function of detuning between emission and Bragg wavelength are in good agreement with excitonic polariton theory. Photoluminescence experiments show that active photonic quasicrystals, unlike photonic crystals, can be good light emitters: While their long-range order results in a stopband similar to that of photonic crystals, the lack of periodicity results in strong emission. PMID:18825174

Hendrickson, J; Richards, B C; Sweet, J; Khitrova, G; Poddubny, A N; Ivchenko, E L; Wegener, M; Gibbs, H M

2008-09-29

236

Theoretical study of excitonic complexes in semiconductors quantum wells  

International Nuclear Information System (INIS)

A physical system where indistinguishable particles interact with each other creates the possibility of studying correlation and exchange effect. The simplest system is that one with only two indistinguishable particles. In condensed matter physics, these complexes are represented by charged excitons, donors and acceptors. In quantum wells, the valence band is not parabolic, therefore, the negatively charged excitons and donors are theoretically described in a simpler way. Despite the fact that the stability of charged excitons (trions) is known since the late 50s, the first experimental observation occurred only at the early 90s in quantum well samples, where their binding energies are one order of magnitude larger due to the one dimensional carriers confinement. After this, these complexes became the subject of an intense research because the intrinsic screening of electrical interactions in semiconductor materials allows that magnetic fields that are usual in laboratories have strong effects on the trion binding energy. Another rich possibility is the study of trions as an intermediate state between the neutral exciton and the Fermi edge singularity when the excess of doping carriers is increased. In this thesis, we present a theoretical study of charged excitons and negatively charged donors in GaAs/Al0.3Ga0.7As quantum wells considering the effects of external electric and magnetic fields. We use a simple, accurate and physically clear method to describe these systems in contrast with the few and complex treatments s available in the literature. Our results show that the QW interface defects have an important role in the trion dynamics. This is in agreement with some experimental works, but it disagrees with other ones. (author)

237

Motional enhancement of the exciton magnetic moment  

International Nuclear Information System (INIS)

This paper overviews our recent results on the magneto-optics of excitons in wide quantum wells, when the well width is much larger than the exciton Bohr radius. We have demonstrated that the magnetic moment of the exciton increases by an order of magnitude due to its motion along the magnetic field. The effect was observed for a set of quantum well structures with different well widths and based on various semiconductor compounds. The reason for this effect is mixing of the exciton's center-of-mass motion and the internal motion of electron and hole in the exciton

238

Relaxation time in the exciton model  

International Nuclear Information System (INIS)

The equation for the time-dependent average number of excitons is derived making use of the system of kinetic equations of the exciton model. The relaxation time is obtained as a function of the excitation energy and of the number of nucleons with and without account of the Pauli principle for the level densities of states with fixed number of excitons. The equation is as well written for the mean-square fluctuation of the exciton number. Relation is discussed between the exciton model equations and the stochastic differential equation for the number of collisions in a medium with random scatterers whose average density is time dependent

239

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

240

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.

Orani, D.; Polimeni, A.

1997-01-01

241

Phase diagram for the exciton Mott transition in infinite-dimensional electron-hole systems  

CERN Document Server

To understand the essence of the exciton Mott transition in three-dimensional electron-hole systems, the metal-insulator transition is studied for a two-band Hubbard model in infinite dimensions with interactions of electron-electron (hole-hole) repulsion U and electron-hole attraction -U'. By using the dynamical mean-field theory, the phase diagram in the U-U' plane is obtained (which is exact in infinite dimensions) assuming that electron-hole pairs do not condense. When both electron and hole bands are half-filled, two types of insulating states appear: the Mott-Hubbard insulator for U > U' and the biexciton-like insulator for U < U'. Even when away from half-filling, we find the phase transition between the exciton- or biexciton-like insulator and a metallic state. This transition can be assigned to the exciton Mott transition, whereas the Mott-Hubbard transition is absent.

Tomio, Y; Tomio, Yuh; Ogawa, Tetsuo

2004-01-01

242

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. In this project we are searching for improved mixed materials with optimal desorption temperature and kinetics. We apply 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, 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, giving in total almost two million combinations.

Jensen, Peter Bjerre; Lysgaard, Steen

243

Excitonic polaritons in transverse magnetic fields  

Energy Technology Data Exchange (ETDEWEB)

Exciton-polariton reflectivity of wide CdTe/CdZnTe quantum-wells (with well widths exceeding by far the exciton Bohr radius) was measured in the Voigt geometry in a magnetic field. In a magnetic field, forbidden exciton lines were observed to appear for each exciton quantization level. The exciton diamagnetic shift constant was observed to decrease noticeably with increasing quantum number of the confined level. A comparison of calculated with experimental spectra reveals a growth of the exciton translational mass with increasing magnetic field. It has been shown that both the growth of mass and the decrease of the diamagnetic shift can be traced to mixing between exciton internal and translational motion states. (Abstract Copyright [2010], Wiley Periodicals, Inc.)

Loginov, D.K. [Institute of Physics, St. Petersburg State University, 198504 St. Petersburg (Russian Federation); Kochereshko, V.P. [Ioffe Physical-Technical Institute, 194021 St.-Petersburg (Russian Federation); Cox, R.T.; Besombes, L.; Mariette, H. [Institut Neel, CNRS, 38042 Grenoble (France); Davies, J.J.; Wolverson, D.; Smith, L.C. [Department of Physics, University of Bath, Bath BA2 7AY (United Kingdom)

2010-06-15

244

Flame inhibition by hydrogen halides - Some spectroscopic measurements  

Science.gov (United States)

The far-ultraviolet absorption spectrum of an air-propane diffusion flame inhibited with hydrogen halides has been studied. Plots of the absorption of light by hydrogen halides as a function of position in the flame and also as a function of the amount of hydrogen halide added to the flame have been obtained. The hydrogen halides are shown to be more stable on the fuel side of the reaction zone than they are on the air side. Thermal diffusion is seen to be important in determining the concentration distribution of the heavier hydrogen halides in diffusion flames. The relationship between the concentration distribution of the hydrogen halides in the flame and the flame inhibition mechanism is discussed.

Lerner, N. R.; Cagliostro, D. E.

1973-01-01

245

Measurement of high exciton binding energy in the monolayer transition-metal dichalcogenides WS2 and WSe2  

OpenAIRE

Monolayer transition-metal dichalcogenides are direct gap semiconductors with great promise for optoelectronic devices. Although spatial correlation of electrons and holes plays a key role, there is little experimental information on such fundamental properties as exciton binding energies and band gaps. We report here an experimental determination of exciton excited states and binding energies for monolayer WS2 and WSe2. We observe peaks in the optical reflectivity/absorptio...

Hanbicki, A. T.; Currie, M.; Kioseoglou, G.; Friedman, A. L.; Jonker, B. T.

2014-01-01

246

Spectroscopy of the two Lowest Exciton Zero-Phonon Lines in Single CdSe/ZnS Nanocrystals  

Energy Technology Data Exchange (ETDEWEB)

We study the optical properties of the lowest-energy exciton states in highly photostable individual CdSe/ZnS nanocrystals at low temperatures. We observe two sharp zero-phonon lines which we attribute to the radiative recombination from the two lowest-energy levels of the band-edge exciton fine structure. By using resonant laser excitation on these two lines we measure a spectral broadening of 10 {mu}eV over integration times of 100 ms.

Louyer, Y; Biadala, L; Tamarat, Ph; Lounis, B, E-mail: y.louyer@cpmoh.u-bordeaux1.f [Centre de Physique Moleculaire Optique et Hertzienne, University of Bordeaux and CNRS, 351 cours de la Liberation, Talence, F-33405 (France)

2010-09-01

247

Reduction of Saturated Alkyl Halides to Alkanes  

OpenAIRE

This chapter describes the hydrogenolysis of a saturated halide (I, Br, Cl, and F) to an alkane. A number of methods have been developed for these reductions, and the ones that are thought to be the most useful are described here. The methods are assessed by the type of reagent that is being used to carry out the transformation, with the emphasis being placed on reliable methods that are chemoselective and generally applicable. References to reviews are provided to facilitate an indepth view ...

Ziffle, Ve; Fletcher, Sp

2014-01-01

248

Alkali Halide Nanotubes: Structure and Stability  

OpenAIRE

Accurate density functional theory (DFT) and coupled-cluster (CCSD) calculations on a series of (LiF)n=2,36 neutral clusters suggest that nanotube structures with hexagonal and octagonal transversal cross sections show stability equal to or greater than that of the typical cubic form of large LiF crystals. The nanotube stability was further corroborated by quantum dynamic calculations at room temperature. The fact that stable nanotube structures were also found for other alkali halides (e.g.,...

Fernandez-lima, Francisco A.; Henkes, Aline Vero?nica; Da Silveira, Enio F.; Nascimento, Marco Antonio Chaer

2012-01-01

249

Solvation at nanoscale: Alkali-halides in water clusters  

International Nuclear Information System (INIS)

The solvation of alkali-halides in water clusters at nanoscale is studied by photoelectron spectroscopy using synchrotron radiation. The Na 2p, K 3p, Cl 2p, Br 3d, and I 4d core level binding energies have been measured for salt-containing water clusters. The results have been compared to those of alkali halide clusters and the dilute aqueous salt solutions. It is found that the alkali halides dissolve in small water clusters as ions.

250

Process and composition for drying of gaseous hydrogen halides  

Science.gov (United States)

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

251

Iridium-catalyzed intramolecular [4 + 2] cycloadditions of alkynyl halides  

OpenAIRE

Iridium-catalyzed intramolecular [4 + 2] cycloadditions of diene-tethered alkynyl halides were investigated by using [IrCl(cod)]2 as catalyst, and dppe was found to be the most suitable phosphine ligand for the reaction. No oxidative insertion of the iridium into the carbon–halide bond was observed, and the reactions proceeded to provide the halogenated cycloadducts in good yield (75–94%). These results are the first examples of cycloadditions of alkynyl halides using a...

Andrew Tigchelaar; William Tam

2012-01-01

252

Coordination Networks of Mercury(II) Halides and Polyether Ligand  

OpenAIRE

New crystalline mercury halide adducts with polyetheras ligands were isolated, characterized, and identified as trans-[HgIICl?(diox)?]n (1) and trans-[HgIII?(diox)]n (2). The compounds were obtained from the metal halide salts in solution of 1,4-dioxane and can be considered as “cutting-out” products from the metal halide as they show similarity in the arrangement with starting compounds.

Crochet, Aure?lien; Fromm, Katharina M.

2012-01-01

253

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

254

Large excitonic effects in monolayers of molybdenum and tungsten dichalcogenides  

Science.gov (United States)

Quasiparticle band structures and optical properties of MoS2, MoSe2, MoTe2, WS2, and WSe2 monolayers are studied using the GW approximation in conjunction with the Bethe-Salpeter equation (BSE). The inclusion of two-particle excitations in the BSE approach reveals the presence of two strongly bound excitons (A and B) below the quasiparticle absorption onset arising from vertical transitions between a spin-orbit-split valence band and the conduction band at the K point of the Brillouin zone. The transition energies for monolayer MoS2, in particular, are shown to be in excellent agreement with available absorption and photoluminescence measurements. Excitation energies for the remaining monolayers are predicted to lie in the range of 1-2 eV. Systematic trends are identified for quasiparticle band gaps, transition energies, and exciton binding energies within as well as across the Mo and W families of dichalcogenides. Overall, the results suggest that quantum confinement of carriers within monolayers can be exploited in conjunction with chemical composition to tune the optoelectronic properties of layered transition-metal dichalcogenides at the nanoscale.

Ramasubramaniam, Ashwin

2012-09-01

255

Localized excitons in fluoroperovskite LiBaF3 crystals  

Science.gov (United States)

Two radiating processes in LiBaF3 crystals, fast valence-core transitions (5.4 - 6.5 eV) and slow, so called self-trapped exciton luminescence (about 4.3 eV), are important for practical application. Here we present a study of 4.3 eV luminescence under X-ray excitation and photoexcitation as well as under photostimulation after X-irradiation of undoped and Ag-doped LiBaF3 crystals at various temperatures. It is shown that 4.3 eV luminescence appears under X-ray excitation at least from 85 K to 400 K in both undoped and doped crystals. In all samples studied the excitation spectra of 4.3 eV luminescence contain both the main exciton like band at the edge of fundamental absorption at about 10 eV and weaker band in 7.8 - 8.6 eV region. Luminescence spectrum in the 3.8 - 4.8 eV region under 7.8 - 8.6 eV excitation differs slightly from that under 10 eV excitation. Several luminescence bands in 3.8 - 4.8 eV region arises in the temperature range 85 - 230 K under photostimulation in absorption band of F-type center at 2.9 eV created previously under X-irradiation. We propose the luminescence of LiBaF3 crystals in the 3.8 - 4.8 eV region may be caused by localized excitons formed not only under excitation near the fundamental absorption but also in result of electron recombination with localized holes thermally destroyed above 230 K.

Springis, Maris; Trukhin, Anatoly N.; Tale, Ivar

2003-08-01

256

Stirring Potential for Indirect Excitons  

OpenAIRE

We demonstrate experimental proof of principle for a stirring potential for indirect excitons. The azimuthal wavelength of this stirring potential is set by the electrode periodicity, the amplitude is controlled by the applied AC voltage, and the angular velocity is controlled by the AC frequency.

Hasling, M. W.; Kuznetsova, Y. Y.; Andreakou, P.; Leonard, J. R.; Calman, E. V.; Dorow, C.; Butov, L. V.; Hanson, M.; Gossard, A. C.

2014-01-01

257

Angular momentum dependent exciton model  

International Nuclear Information System (INIS)

To consider the angular momentum and parity conservation the angular momentum (J) and parity (?) should be addressed in the master equation of the exciton model. Therefore the internal transition rates and the emission rates must be J? dependent. The angular momentum factor of the internal transition rates is given and the angular momentum conservation effect is discussed

258

Gravitational excitons from extra dimensions  

CERN Document Server

We study inhomogeneous multidimensional cosmological models with a higher dimensional space-time manifold under dimensional reduction and show that small inhomogeneous excitations of the scale factors of the internal spaces near minima of effective potentials should be observable as massive scalar particles (gravitational excitons) in the external space-time.

Günther, U

1997-01-01

259

Berry phase effect on the exciton transport and on the exciton Bose-Einstein condensate.  

Science.gov (United States)

With the exciton lifetime much extended in semiconductor quantum-well structures, the exciton transport and Bose-Einstein condensation have become a focus of research in recent years. We reveal a momentum-space gauge field in the exciton center-of-mass dynamics due to Berry phase effects. We predict a spin-dependent transport of the excitons analogous to the anomalous Hall and Nernst effects for electrons. We also predict spin-dependent circulation of a trapped exciton gas and instability in an exciton condensate in favor of vortex formation. PMID:18851231

Yao, Wang; Niu, Qian

2008-09-01

260

Motion-enhanced magnetic moments of excitons in ZnSe  

Science.gov (United States)

Recent studies of excitons in wide quantum wells have shown that the magnetic properties are strongly affected as the excitons acquire kinetic energy. In the center of mass approximation, these motion-induced changes are ascribed to mixing between the 1S exciton ground state and the higher lying nP states. The origin of the mixing is due to the dispersion curves for the valence band not being of simple parabolic form. Detailed previous studies of excitons in CdTe have resulted in excellent agreement between experiment and the predictions of this model. One consequence of the mixing is that the magnetic moment of the exciton is not simply the sum of the magnetic moments of the electron and hole, but contains motion-induced contributions, which can easily dominate the contributions from the individual charge carriers. To confirm the validity of the model, we have carried out detailed investigations of the magnetic properties of center of mass excitons in a second semiconductor, ZnSe, for which the magneto-optical properties of the individual charge carriers are completely different from those of CdTe. Excellent agreement is obtained between theory and experiment with a choice of the Luttinger parameter ?3=0.98 , in close agreement with the value determined independently by two-photon magnetoabsorption experiments. The success of the model when applied to both materials provides strong evidence that motion-induced changes in magnetism are a universal feature in zinc-blende semiconductors.

Davies, J. J.; Smith, L. C.; Wolverson, D.; Gust, A.; Kruse, C.; Hommel, D.; Kochereshko, V. P.

2010-02-01

261

Excitonic recombination in epitaxial lateral overgrown AlN on sapphire  

Science.gov (United States)

Excitonic emission in heteroepitaxially grown aluminum nitride (AlN) with reduced defect density due to the epitaxial lateral overgrowth (ELO) of patterned AlN/sapphire templates has been investigated by photoluminescence spectroscopy and compared to AlN/sapphire and homoepitaxially grown AlN. The ELO sample exhibits small linewidths of the free exciton and two different bound exciton emission bands. The free exciton emission energy is shifted by 58.5 meV with respect to unstrained homoepitaxially grown AlN attributed to compressive strain. A donor bound exciton D0X with an exciton localization energy of 13.0-13.5 meV is dominating in the photoluminescence spectra of ELO AlN/sapphire. This D0X does not show strong phonon replica and is dominant at elevated temperatures in ELO AlN/sapphire. The optical quality of heteroepitaxial AlN is significantly improved using the ELO technique and therefore suitable for high efficiency ultraviolet light emitters.

Reich, Christoph; Feneberg, Martin; Kueller, Viola; Knauer, Arne; Wernicke, Tim; Schlegel, Jessica; Frentrup, Martin; Goldhahn, Rüdiger; Weyers, Markus; Kneissl, Michael

2013-11-01

262

Recombination of free and bound excitons in GaN  

Energy Technology Data Exchange (ETDEWEB)

We report on recent optical investigations of free and bound exciton properties in bulk GaN. In order to obtain reliable data it is important to use low defect density samples of low doping. We have used thick GaN layers (of the order of 1 mm) grown by halide vapour phase epitaxy (HVPE) with a residual doping down to <10{sup 16} cm{sup -3} in this work. With such samples all polarisation geometries could also easily be exploited. The influence of the surface states on the photoluminescence (PL) experiments is analysed, it is concluded that surface recombination plays an important role for the free exciton (FE) recombination. The electronic structure of the FEs is discussed in detail, including the influence of spin-exchange and polariton effects, and compared with polarised PL spectra at 2 K. The detailed structure of excited states from the PL spectra is discussed, but further data are needed to fully explain all the peaks observed. The polarized FE spectra at room temperature allow a determination of the bandgap as 3.437 eV at 290 K, assuming an exciton binding energy of 25 meV. The PL transient of the A FE is very short (about 100 ps) for the no-phonon (NP) line interpreted as dominated by nonradiative surface recombination. The longitudinal-optical (LO) phonon replicas of the A FE exhibit a longer decay of about 1.4 ns at 2 K, suggested to represent the bulk lifetime of the FE. The corresponding decay time at 290 K is 9 ns in our samples, a value that might be affected by nonradiative recombination. The Si and O donor bound exciton (DBE) spectra with sharp NP lines at 3.4723 eV and 3.4714 eV respectively, are well resolved together with the so-called two-electron transitions (TETs) and several optical phonon replicas. The electronic structure of the DBE states including excited rotational states is discussed and compared with experiment. The well-resolved TET lines allow an accurate determination of the ground state binding energy of the Si donor as 30.4 meV and 33.2 meV for the O donor. The PL transients of the DBEs reveal a non-exponential decay for the NP lines. The DBE NP transient lineshape is assumed to be influenced by optical dispersion and scattering in the vicinity of exciton resonances, as well as by surface effects. The DBE decay time can most properly be deduced from the PL decay of the respective TETs and LO replicas, leading to values in the range of 1.1-1.8 ns. These values differ significantly from previous theoretical predictions, where values about two orders of magnitude shorter were obtained. A tentative discussion of the main observed features of acceptor bound excitons (ABEs), which are much less studied in GaN, is given. A decay time of about 0.9 ns for the shallowest 3.466 eV ABE is estimated, i.e. shorter than that for the shallow donor BEs. (copyright 2008 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)

Monemar, B.; Paskov, P.P.; Bergman, J.P. [Department of Physics, Chemistry and Biology, Linkoeping University (Sweden); Toropov, A.A.; Shubina, T.V. [Ioffe Physico-Technical Institute, St. Petersburg (Russian Federation); Malinauskas, T. [Institute of Materials Science and Applied Research, Vilnius University (Lithuania); Usui, A. [R and D Division, Furukawa Co., Ltd. Tsukuba, Ibaraki (Japan)

2008-09-15

263

Mechanism and selectivity in nickel-catalyzed cross-electrophile coupling of aryl halides with alkyl halides.  

Science.gov (United States)

The direct cross-coupling of two different electrophiles, such as an aryl halide with an alkyl halide, offers many advantages over conventional cross-coupling methods that require a carbon nucleophile. Despite its promise as a versatile synthetic strategy, a limited understanding of the mechanism and origin of cross selectivity has hindered progress in reaction development and design. Herein, we shed light on the mechanism for the nickel-catalyzed cross-electrophile coupling of aryl halides with alkyl halides and demonstrate that the selectivity arises from an unusual catalytic cycle that combines both polar and radical steps to form the new C-C bond. PMID:23952217

Biswas, Soumik; Weix, Daniel J

2013-10-30

264

Anisotropic optical properties of excitons in strain-controlled InAs quantum dots  

Science.gov (United States)

We report the optical anisotropy of excitons in self-assembled InAs quantum dots (QDs), the strain of which is controlled by tuning the lattice constant of the spacer layers between stacked QD layers. The strain dependence of the energy structure for excitons is investigated using the four-wave mixing technique. The fine-structure beat and the exciton-biexciton beat in four-wave mixing measurements clearly show that the fine-structure splitting of exciton states increases with increasing strain in the growth plane. The strain also causes significant optical anisotropy, which is observed by the angular-dependent measurements of the collinear polarization of excitation pulses. The strain dependence of the optical anisotropy is accurately reproduced by a model calculation in which the valence band mixing between heavy- and light-hole bands is taken into account. We demonstrate that the optical properties of excitons in QD ensembles are successfully controlled by tuning the lattice constant of the spacer layers.

Tahara, H.; Ogawa, Y.; Minami, F.; Akahane, K.; Sasaki, M.

2013-01-01

265

Reference spectroscopic data for hydrogen halides, Part II: The line lists  

Science.gov (United States)

Accurate spectroscopic parameters for the hydrogen halides, namely HF, HCl, HBr, and HI, together with their deuterated isotopologues, are crucial for the quantitative study of terrestrial and planetary atmospheres, astrophysical objects, and chemical lasers. A thorough evaluation of all the hydrogen halide line parameters in previous HITRAN editions has been carried out. A new set of line lists was generated for the HITRAN2012 edition using methods described here. In total, 131,798 entries were generated for numerous pure-rotational and ro-vibrational transitions (fundamental, overtone, and hot bands) for hydrogen halides and their deuterated species in a standard HITRAN 160-character format. Data for the deuterated isotopologues have been entered into HITRAN for the first time. The calculations employ the recently developed semi-empirical dipole moment functions [Li G, et al. J Quant Spectrosc Radiat Transfer 2013;121:78-90] and very accurate analytical potential energy functions and associated functions characterizing Born-Oppenheimer breakdown effects. Line-shape parameters have also been updated using the most recent available experimental and theoretical studies. Comparison with the previous HITRAN compilation has shown significant improvements.

Li, Gang; Gordon, Iouli E.; Hajigeorgiou, Photos G.; Coxon, John A.; Rothman, Laurence S.

2013-11-01

266

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

267

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)

268

Magneto-optical properties and exciton dynamics in diluted magnetic semiconductor nanostructures  

International Nuclear Information System (INIS)

Nanostructures of diluted magnetic semiconductors were fabricated to study novel magneto-optical properties that are derived from quantum confined band electrons interacting with magnetic ions. Quantum dots (QDs) of Cd0.97Mn0.03Se were grown by the self-organization on a ZnSe substrate layer. QDs of Zn0.69Cd0.23Mn0.08Se and quantum wires (QWRs) of Cd0.92Mn0.08Se and Zn0.69Cd0.23Mn0.08Se were fabricated by the electron beam lithography. A single quantum well (QW) of ZnTe/Zn0.97Mn0.03Te and double QWs of Cd0.95Mn0.05Te-CdTe were grown by molecular beam epitaxy. Magneto-optical properties and the formation and relaxation dynamics of excitons were investigated by ultrafast time-resolved photoluminescence (PL) spectroscopy. Excitons in these nanostructures were affected by the low-dimensional confinement effects and the interaction with magnetic ion spins. The exciton luminescence of the Cd0.97Mn0.03Se QDs shows the confined exciton energy due to the dot size of 4-6 nm and also shows marked increase of the exciton lifetime with increasing the magnetic field. The QDs of Zn0.69Cd0.23Mn0.08Se fabricated by the electron beam lithography display narrow exciton PL spectrum due to the uniform shape of the dots. The exciton luminescence from the QWRs of Cd0.92Mn0.92Mn0.08Se and Zn0.69Cd0.23Mn0.08Se shows the influence of the one-dimensional confinement effect for the exciton energy and the luminescence is linearly polarized parallel to the wire direction. The transient PL from the ZnTe/Zn0.97Mn0.03Te QWs displays, by the magnetic field, the level crossing of the exciton spin states of the nonmagnetic and magnetic layers and the spatial spin separation for the excitons. Cd0.95Mn0.05Te-CdTe double QWs show the injection of the spin polarized excitons from the magnetic well to the nonmagnetic QW

269

Photonic Crystal Architecture for Room-Temperature Equilibrium Bose-Einstein Condensation of Exciton Polaritons  

Science.gov (United States)

We describe photonic crystal microcavities with very strong light-matter interaction to realize room-temperature, equilibrium, exciton-polariton Bose-Einstein condensation (BEC). This goal is achieved through a careful balance between strong light trapping in a photonic band gap (PBG) and large exciton density enabled by a multiple quantum-well (QW) structure with a moderate dielectric constant. This approach enables the formation of a long-lived, dense 10-?m-1-cm- scale cloud of exciton polaritons with vacuum Rabi splitting that is roughly 7% of the bare exciton-recombination energy. We introduce a woodpile photonic crystal made of Cd0.6 Mg0.4Te with a 3D PBG of 9.2% (gap-to-central-frequency ratio) that strongly focuses a planar guided optical field on CdTe QWs in the cavity. For 3-nm QWs with 5-nm barrier width, the exciton-photon coupling can be as large as ??=55 meV (i.e., a vacuum Rabi splitting of 2??=110 meV). The exciton-recombination energy of 1.65 eV corresponds to an optical wavelength of 750 nm. For N =106 QWs embedded in the cavity, the collective exciton-photon coupling per QW (??/?N =5.4 meV) is much larger than the state-of-the-art value of 3.3 meV, for the CdTe Fabry-Pérot microcavity. The maximum BEC temperature is limited by the depth of the dispersion minimum for the lower polariton branch, over which the polariton has a small effective mass of approximately 10-5m0, where m0 is the electron mass in vacuum. By detuning the bare exciton-recombination energy above the planar guided optical mode, a larger dispersion depth is achieved, enabling room-temperature BEC. The BEC transition temperature ranges as high as 500 K when the polariton density per QW is increased to (11aB)-2, where aB?3.5 nm is the exciton Bohr radius and the exciton-cavity detuning is increased to 30 meV. A high-quality PBG can suppress exciton radiative decay and enhance the polariton lifetime to beyond 150 ps at room temperature, sufficient for thermal equilibrium BEC.

Jiang, Jian-Hua; John, Sajeev

2014-07-01

270

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

OpenAIRE

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 prope...

Duque-gomez, Federico; Sipe, J. E.

2014-01-01

271

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

OpenAIRE

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

272

Excitons in the wurtzite AlGaN/GaN quantum-well heterostructures  

OpenAIRE

We have theoretically studied exciton states and photoluminescence spectra of strained wurtzite AlGaN/GaN quantum-well heterostructures. The electron and hole energy spectra are obtained by numerically solving the Schr\\"odinger equation, both for a single-band Hamiltonian and for a non-symmetrical 6-band Hamiltonian. The deformation potential and spin-orbit interaction are taken into account. For increasing built-in field, generated by the piezoelectric polarization and by t...

Pokatilov, E. P.; Nika, D. L.; Fomin, V. M.; Devreese, J. T.; Photonics; Nanostructures, Semiconductor; Eindhoven, Tu; Netherlands, The

2008-01-01

273

Excitonic coupling of chlorophylls in the plant light-harvesting complex LHC-II.  

OpenAIRE

Manifestation and extent of excitonic interactions in the red Chl-absorption region (Q(y) band) of trimeric LHC-II were investigated using two complementary nonlinear laser-spectroscopic techniques. Nonlinear absorption of 120-fs pulses indicates an increased absorption cross section in the red wing of the Q(y) band as compared to monomeric Chl a in organic solution. Additionally, the dependence of a nonlinear polarization response on the pump-field intensity was investigated. This approach r...

Schubert, Axel; Beenken, Wichard J. D.; Stiel, Holger; Voigt, Bernd; Leupold, Dieter; Lokstein, Heiko

2002-01-01

274

Multiphonon resonant Raman scattering in the semimagnetic semiconductor Cd1-xMnxTe: Froehlich and deformation potential exciton-phonon interaction  

International Nuclear Information System (INIS)

A theory describing multiphonon resonant Raman scattering (MPRRS) processes in wide-gap diluted magnetic semiconductors is presented, with Cd1-xMnxTe as an example. The incident radiation frequency ?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 different types of exciton as a function of ?l and the external magnetic field are presented. The selection rules for all hot exciton transitions via exciton-photon interaction and F and DP exciton-phonon interactions are investigated. The exciton energies, as a function of B, the Mn concentration x, and the temperature T, are compared to a theoretical expression. Graphics for creation and annihilation probabilities, lifetime, and Raman intensity of seces, lifetime, and Raman intensity of second and third order are discussed

275

Exciton dynamics in disordered poly(p-phenylenevinylene). 2. Exciton diffusion.  

OpenAIRE

We present a first principles theory of exciton diffusion in conformationally disordered conjugated polymers. Central to our theory is that exciton transfer occurs from vibrationally relaxed states (VRSs) to local exciton ground states (LEGSs). LEGSs are determined by the diagonal and off-diagonal disorder induced by static density and torsional fluctuations, and VRSs are further localized by exciton-phonon coupling. The theory is implemented using the Frenkel-Holstein model to calculate the ...

Barford, W.; Bittner, Er; Ward, A.

2012-01-01

276

Exciton Seebeck effect in molecular systems.  

Science.gov (United States)

We investigate the exciton dynamics under temperature difference with the hierarchical equations of motion. Through a nonperturbative simulation of the transient absorption of a heterogeneous trimer model, we show that the temperature difference causes exciton population redistribution and affects the exciton transfer time. It is found that one can reproduce not only the exciton population redistribution but also the change of the exciton transfer time induced by the temperature difference with a proper tuning of the site energies of the aggregate. In this sense, there exists a site energy shift equivalence for any temperature difference in a broad range. This phenomenon is similar to the Seebeck effect as well as spin Seebeck effect and can be named as exciton Seebeck effect. PMID:25106568

Yan, Yun-An; Cai, Shaohong

2014-08-01

277

Ultrafast exciton dissociation at donor/acceptor interfaces  

Science.gov (United States)

Charge generation at donor/acceptor interface is a highly debated topic in the organic photovoltaics (OPV) community. The primary photoexcited state evolution happens in few femtosecond timescale, thus making very intriguing their full understanding. In particular charge generation is believed to occur in OPV. We demonstrate that excitons dissociation leads, on a timescale of 20-50 fs, to two byproducts: bound interfacial charge transfer states (CTS) and free charges. The branching ratio of their formation depends on the excess photon energy provided. When high energy singlet polymer states are excited, well above the optical band gap, an ultrafast hot electron transfer happens between the polymer singlet state and the interfacial hot CTS* due to the high electronic coupling between them. Hot exciton dissociation prevails then on internal energy dissipation that occurs within few hundreds of fs. By measuring the internal quantum efficiency of a prototypical device a rising trend with energy is observed, thus indicating that hot exciton dissociation effectively leads to a higher fraction of free charges.

Grancini, G.; Fazzi, D.; Binda, M.; Maiuri, M.; Petrozza, A.; Criante, L.; Perissinotto, S.; Egelhaaf, H.-J.; Brida, D.; Cerullo, G.; Lanzani, G.

2013-09-01

278

Metal halide reduction with molten sodium/potassium alloy  

International Nuclear Information System (INIS)

A method of obtaining a desired metal, selected from the group consisting of titanium, aluminium, iron, manganese, hafnium, zirconium, tantalum, vanadium, uranium and tungsten, which comprises reacting a halide of the desired metal with an alkali metal reducing agent at temperature at which the reducing agent is molten, in order to produce the desired metal and halide of the metal reducing agent

279

Regioselective Heck reaction of aliphatic olefins and aryl halides.  

Science.gov (United States)

A regioselective Heck reaction of aliphatic olefins and aryl bromides is realized at internal carbons of olefins. Methanol solvent promoted halide ionization from neutral arylpalladium halide complexes via hydrogen bonding, so as to create cationic aryl-Pd species for regioselective olefin insertion. PMID:24060852

Qin, Liena; Hirao, Hajime; Zhou, Jianrong Steve

2013-11-11

280

EFFECT OF EXCITON DIFFUSION AND REABSORPTION ON DECAY CURVE OF EXCITON LUMINESCENCE  

OpenAIRE

Well-known wavelength-dependent decay time of exciton luminescence is interpreted in a model based on exciton diffusion and reabsorption of the short wavelength of the luminescence spectrum. The decay curves of the singlet-exciton luminescence in anthracene, depending on wavelength, experimental geometry and penetration depth of incident light, are simulated very well by the model.

Nishimura, H.

1985-01-01

281

Exciton-related electroluminescence from monolayer MoS2  

OpenAIRE

Excitons in MoS2 dominate the absorption and emission properties of the two-dimensional system. Here, we study the microscopic origin of the electroluminescence from monolayer MoS2 fabricated on a heavily p-type doped silicon substrate. By comparing the photoluminescence and electroluminescence of a MoS2 diode, direct-exciton and bound-exciton related recombination processes can be identified. Auger recombination of the exciton-exciton annihilation of bound exciton emission ...

Ye, Yu; Ye, Ziliang; Gharghi, Majid; Zhu, Hanyu; Zhao, Mervin; Yin, Xiaobo; Zhang, Xiang

2013-01-01

282

On Critical Velocities in Exciton Superfluidity  

CERN Document Server

The presence of exciton phonon interactions is shown to play a key role in the exciton superfluidity. We apply the Landau criterion for an exciton-phonon condensate moving uniformly at zero temperature. It turns out that there are essentially two critical velocities in the theory. Within the range of these velocities the condensate can exist only as a bright soliton. The excitation spectrum and differential equations for the wave function of this condensate are derived.

Loutsenko, I

1997-01-01

283

Impurity effects and ferromagnetism in excitonic insulators  

OpenAIRE

Both nonmagnetic and magnetic impurity effects in spin singlet and triplet excitonic insulators were investigated. The bound state energies caused by single impurity were given. The different compositions of the bound states can be used to detect the symmetry of the excitonic insulators. In finite concentration problems, nonmagnetic impurities showed same pair-breaking effect in singlet and triplet excitonic insulators while magnetic impurities showed weaker pair-breaking ef...

Li, Jian; Hao, Ning; Wang, Yupeng

2010-01-01

284

Exciton correlations within an electron gas  

OpenAIRE

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

285

Exciton dynamics in WSe2 bilayers  

OpenAIRE

We investigate exciton dynamics in 2H-WSe2 bilayers in time-resolved photoluminescence (PL) spectroscopy. Fast PL emission times are recorded for both the direct exciton with $\\tau_{D}$ ~ 3 ps and the indirect optical transition with $\\tau_{i}$ ~ 25 ps. For temperatures between 4 to 150 K $\\tau_{i}$ remains constant. Following polarized laser excitation, we observe for the direct exciton transition at the K point of the Brillouin zone efficient optical orientation and alignm...

Wang, G.; Marie, X.; Bouet, L.; Vidal, M.; Balocchi, A.; Amand, T.; Lagarde, D.; Urbaszek, B.

2014-01-01

286

Statistical mechanics of screened spatially indirect excitons  

CERN Document Server

We study thermodynamic properties of spatially separated electron-hole plasma in double-layered systems using Green function formalism. The screening of the Coulomb interaction is considered in the framework of Thomas-Fermi approximation, and a qualitatively new mechanism of screening by indirect excitons is taken into account. The exciton density is shown to decrease sharply with increasing electron-hole separation up to one exciton Born radius. The strong mutual enhancement of screening and charge-separation effects is found.

Nikolaev, V V

2002-01-01

287

Exciton and biexciton energies in bilayer systems  

OpenAIRE

We report calculations of the energies of excitons and biexcitons in ideal two-dimensional bilayer systems within the effective-mass approximation with isotropic electron and hole masses. The exciton energies are obtained by a simple numerical integration technique, while the biexciton energies are obtained from diffusion quantum Monte Carlo calculations. The exciton binding energy decays as the inverse of the separation of the layers, while the binding energy of the biexcit...

Tan, M. Y. J.; Drummond, Neil; Needs, R. J.

2008-01-01

288

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)

289

Anion polarizability functions in alkali halide crystals  

Science.gov (United States)

Anion polarizabilities in alkali halide crystals are analysed as a function of interionic separation R . The anion polarizability is treated as a function of the anion and cation radii, with its partial derivatives approximated by those with respect to R for fixed cation and anion, respectively. With pressure derivatives of the ionic radii deduced from the crystal compressibility, assuming transferability among crystals, the polarizability derivatives with respect to ionic radius yield pressure derivatives of the polarizability that agree with experiment to within a factor of two. These results offer a useful means of predicting the pressure dependence of dielectric data.

Batana, A.; Bruno, J.; Munn, R. W.

290

Energy levels and predissociation in mercuric halides  

International Nuclear Information System (INIS)

Electron energy-loss spectra were obtained over an extended energy-loss range (approximately 5 to 15 eV) for incident electron energies of 200 eV and scattering angles of 20 in the mercuric halides HgBr2 and HgCl2. Under these conditions of impact energy and scattering angle, the energy-loss spectra correspond closely to optical absorption spectra, though the energy-loss spectra extend to much higher energies than previous photoabsorption spectra, and reveal many more excited states of these molecules than had previously been observed. This preliminary report gives raw spectra and tentative identifications of the principal features of these spectra

291

Gravitational Excitons as Dark Matter  

OpenAIRE

In earlier work it was pointed out that for warped product spacetimes the conformal (geometrical moduli) excitations of the internal compactified factor spaces should be observable as massive scalar fields in the external spacetime. Here we show that these scalar fields (gravitational excitons) describe weakly interacting particles and can be considered as dark matter component. Masses of the gravexcitons are defined by the form of the effective potential of the theory and t...

Guenther, U.; Zhuk, A.

2000-01-01

292

Gravitational excitons from extra dimensions  

OpenAIRE

Inhomogeneous multidimensional cosmological models with a higher dimensional space-time manifold are investigated under dimensional reduction. In the Einstein conformal frame, small excitations of the scale factors of the internal spaces near minima of an effective potential have a form of massive scalar fields in the external space-time. Parameters of models which ensure minima of the effective potentials are obtained for particular cases and masses of gravitational exciton...

Guenther, U.; Zhuk, A.

1997-01-01

293

Extended excitons and compact heliumlike biexcitons in type-II quantum dots.  

OpenAIRE

We have used magneto-photoluminescence measurements to establish that InP/GaAs quantum dots have a type-II (staggered) band alignment. The average excitonic Bohr radius and the binding energy are estimated to be 15nm and 1.5 meV respectively. When compared to bulk InP, the excitonic binding is weaker due to the repulsive (type-II) potential at the hetero-interface. The measurements are extended to over almost six orders of magnitude of laser excitation powers and to magnetic...

Bansal, Bhavtosh; Godefroo, Stefanie; Hayne, Manus; Medeiros-ribeiro, Gilberto; Moshchalkov, Victor

2009-01-01

294

Selective optical pumping of charged excitons in unintentionally doped InAs quantum dots  

International Nuclear Information System (INIS)

We have investigated the selective optical pumping of charged excitonic species in a sample containing quantum dots of different sizes and low areal density by photoluminescence and excitation of the photoluminescence microspectroscopy. We study the selective optical excitation of negatively charged excitons as an alternative to commonly used electrical methods. We demonstrate that under resonant excitation in impurity related bands, the selective pumping efficiency can be as high as 85% in small quantum dots having one electron shell and emitting at around 930 nm, and around 65% in big quantum dots having four electron shells and emitting at 1160 nm

295

Optical properties of light-hole excitons in GaN epilayers  

OpenAIRE

Optical properties of light-hole free exciton (FX B) in GaN epilayers were investigated by using near-resonance photoluminescence (PL) and time-resolved PL techniques. In contrast to the case of off-resonance PL where only heavy-hole free excitons (FX A) have strong response, FX B band can be well resolved in the near-resonance PL spectra. The variable-temperature near-resonance PL spectra show that the linewidth of FX B broadens faster than the FX A with increasing temperature. Moreover, the...

Xu, Sj; Ning, Jq; Che, Cm; Zhang, F.; Zheng, Cc; Zhao, Dg; Yang, H.

2010-01-01

296

Towards exciton condensation in quantum semiconductor structures  

International Nuclear Information System (INIS)

Full text: The phenomena of exciton condensation, analogous to Bose-Einstein Condensation (BEC) of bosonic atoms is a rapidly advancing field with recent observations of macroscopically ordered states in a two-dimensional system, The basic requirements of an exciton system include long radiative lifetime, repulsive interactions and low potential disorder. In this paper we present measurements of the luminescence properties of cold excitons in a range of systems including coupled quantum wells, quantum ring-like topologies in the optical Aharonov-Bohm regime and selectively patterned 2D geometries and comment on their applicability to the observation of exciton condensation. Copyright (2005) Australian Institute of Physics

297

Exciton dynamics in WSe2 bilayers  

Science.gov (United States)

We investigate exciton dynamics in WSe2 bilayers in time-resolved photoluminescence (PL) spectroscopy. Fast PL emission times are recorded for both the direct exciton with ?D ? 3 ps and the indirect optical transition with ?I ? 25 ps. For temperatures between 4 to 150 K ?I remains constant. Following polarized laser excitation, we observe for the direct exciton transition at the K point of the Brillouin zone efficient optical orientation and alignment during the short emission time ?D. The evolution of the direct exciton polarization and intensity as a function of excitation laser energy is monitored in PL excitation experiments.

Wang, G.; Marie, X.; Bouet, L.; Vidal, M.; Balocchi, A.; Amand, T.; Lagarde, D.; Urbaszek, B.

2014-11-01

298

Extraordinary exciton conductance induced by strong coupling  

CERN Document Server

We demonstrate that exciton conductance in organic materials can be enhanced by several orders of magnitude when the molecules are strongly coupled to an electromagnetic mode. Using a 1D model system, we show how the formation of a collective polaritonic mode allows excitons to bypass the disordered array of molecules and jump directly from one end of the structure to the other. This finding could have important implications in the fields of exciton transistors, heat transport, photosynthesis, and biological systems in which exciton transport plays a key role.

Feist, Johannes

2014-01-01

299

Spontaneous coherence in a cold exciton gas.  

Science.gov (United States)

If bosonic particles are cooled down below the temperature of quantum degeneracy, they can spontaneously form a coherent state in which individual matter waves synchronize and combine. Spontaneous coherence of matter waves forms the basis of a number of fundamental phenomena in physics, including superconductivity, superfluidity and Bose-Einstein condensation. Spontaneous coherence is the key characteristic of condensation in momentum space. Excitons--bound pairs of electrons and holes--form a model system to explore the quantum physics of cold bosons in solids. Cold exciton gases can be realized in a system of indirect excitons, which can cool down below the temperature of quantum degeneracy owing to their long lifetimes. Here we report measurements of spontaneous coherence in a gas of indirect excitons. We found that spontaneous coherence of excitons emerges in the region of the macroscopically ordered exciton state and in the region of vortices of linear polarization. The coherence length in these regions is much larger than in a classical gas, indicating a coherent state with a much narrower than classical exciton distribution in momentum space, characteristic of a condensate. A pattern of extended spontaneous coherence is correlated with a pattern of spontaneous polarization, revealing the properties of a multicomponent coherent state. We also observed phase singularities in the coherent exciton gas. All these phenomena emerge when the exciton gas is cooled below a few kelvin. PMID:22437498

High, A A; Leonard, J R; Hammack, A T; Fogler, M M; Butov, L V; Kavokin, A V; Campman, K L; Gossard, A C

2012-03-29

300

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

301

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

302

Nonlinear Photoluminescence in Atomically Thin Layered WSe2 Arising from Diffusion-Assisted Exciton-Exciton Annihilation  

OpenAIRE

We studied multi-exciton dynamics in monolayer WSe2 using nonlinear photoluminescence (PL) spectroscopy and Monte Carlo simulations. We observed strong nonlinear saturation behavior of exciton PL with increasing excitation power density, and long-distance exciton diffusion reaching several micrometers. We demonstrated that the diffusion-assisted exciton-exciton annihilation model accounts for the observed nonlinear PL behavior. The long-distance exciton diffusion and subsequ...

Mouri, Shinichiro; Miyauchi, Yuhei; Toh, Minglin; Zhao, Weijie; Eda, Goki; Matsuda, Kazunari

2014-01-01

303

Lattice theory of ultrafast excitonic and charge-transfer dynamics in DNA.  

Science.gov (United States)

We propose a lattice fermion model suitable for studying the ultrafast photoexcitation dynamics of ordered chains of deoxyribonucleic acid (DNA) polymers. The model includes both parallel (intrachain) and perpendicular (cross-chain) terms as well as diagonal cross-chain terms coupling neighboring bases. The general form of our Hamiltonian is borrowed from lattice fermion models of quantum chromodynamics. The band structure for this model can be determined analytically, and we use this as a basis for computing the singly excited states of the poly(dA)poly(dT) DNA duplex using configuration interaction singles. Parameters for the model are taken from various literature sources and our own ab initio calculations. Results indicate that the excited states consist of a low energy band of dark charge-separated states followed by separate bands of delocalized excitonic states which have weak mixing between the thymidine and adenosine sides of the DNA chain. We then propose a lattice exciton model based upon the transition dipole-dipole couplings between bases and compare the analytical results for the survival probability of an initially localized exciton to exact numerical results. The results herein underscore the competing role of excitonic and charge-transfer dynamics in these systems. PMID:16965121

Bittner, Eric R

2006-09-01

304

A new fundamental hydrogen defect in alkali halides  

International Nuclear Information System (INIS)

Atom hydrogen in neutral (H0) and negative (H-) form on substitutional and interstitial lattice sites gives rise to well characterized model defects in alkali-halides (U,U1,U2,U3 centers), which have been extensively investigated in the past. When studying the photo-decomposition of OH- defects, a new configuration of atomic charged hidrogen was discovered, which can be produced in large quantities in the crystal and is apparently not connected to any other impurity. This new hidrogen defect does not show any pronounced electronic absorption, but displays a single sharp local mode band (at 1114cm-1 in KCl) with a perfect isotope shift. The defect can be produced by various UV or X-ray techniques in crystais doped with OH-, Sh- or H- defects. A detailed study of its formation kinetics at low temperature shows that it is primarily formed by the reaction of a mobile CI2- crowdion (H-center) with hidrogen defects

305

Multiple exciton generation in nano-crystals revisited: Consistent calculation of the yield based on pump-probe spectroscopy  

Science.gov (United States)

Multiple exciton generation (MEG) is a process in which more than one exciton is generated upon the absorption of a high energy photon, typically higher than two times the band gap, in semiconductor nanocrystals. It can be observed experimentally using time resolved spectroscopy such as the transient absorption measurements. Quantification of the MEG yield is usually done by assuming that the bi-exciton signal is twice the signal from a single exciton. Herein we show that this assumption is not always justified and may lead to significant errors in the estimated MEG yields. We develop a methodology to determine proper scaling factors to the signals from the transient absorption experiments. Using the methodology we find modest MEG yields in lead chalcogenide nanocrystals including the nanorods. PMID:23887181

Karki, Khadga J.; Ma, Fei; Zheng, Kaibo; Zidek, Karel; Mousa, Abdelrazek; Abdellah, Mohamed A.; Messing, Maria E.; Wallenberg, L. Reine; Yartsev, Arkadi; Pullerits, Tõnu

2013-01-01

306

Multiple exciton generation in nano-crystals revisited: Consistent calculation of the yield based on pump-probe spectroscopy  

CERN Document Server

Multiple exciton generation (MEG) is a process in which more than one exciton is generated upon the absorption of a high energy photon, typically higher than two times the band gap, in semiconductor nanocrystals. It can be observed experimentally using time resolved spectroscopy such as the transient absorption measurements. Quantification of the MEG yield is usu- ally done by assuming that the bi-exciton signal is twice the signal from a single exciton. Herein we show that this assumption is not always justified and may lead to significant errors in the estimated MEG yields. We develop a methodology to determine proper scaling factors to the signals from the transient absorption experiments. Using the methodology we find modest MEG yields in lead chalcogenide nanocrystals including the nanorods.

Karki, Khadga J; Zheng, Kaibo; Zidek, Karel; Mousa, Abdelrazek; Abdellah, Mohamed A; Messing, Maria; Wallenberg, L Reine; Yartsev, Arkadi; Pullerits, Tonu

2013-01-01

307

Direct determination of free exciton binding energy from phonon-assisted luminescence spectra in GaN epilayers  

OpenAIRE

Investigation of variable-temperature longitudinal optical (LO) phonon-assisted luminescence spectra of free excitons and free electrons in heteroepitaxial GaN thin films has been conducted. It is found that thermal broadening of the LO phonon-assisted photoluminescence peaks is much slower than those of the peaks of their parents so that the first-order LO peaks of the free exciton transition and the band-to-band transition can be well resolved even at room temperature, leading to a direct d...

Xu, Sj; Liu, W.; Li, Mf

2002-01-01

308

Divergent electronic structures of isoelectronic metalloclusters: tungsten(II) halides and rhenium(III) chalcogenide halides.  

Science.gov (United States)

Same but different: DFT calculations on hexanuclear tungsten(II) halide clusters [W(6)X(8)X'(6)](2-) (X, X'=Cl, Br, I) indicate a breakdown in the isoelectronic analogy between themselves and the isostructural rhenium(III) chalcogenide clusters [Re(6)S(8)X(6)](4-) (see figure).The hexanuclear tungsten(II) halide clusters and the sulfido-halide clusters of rhenium(III) are subsets of a broad system of 24-electron metal-metal bonded assemblies that share a common structure. Tungsten(II) halide clusters and rhenium(III) sulfide clusters luminesce from triplet excited states upon ultraviolet or visible excitation; emission from both cluster series has been extensively characterized elsewhere. Reported here are density-functional theory studies of the nine permutations of [W(6)X(8)X'(6)](2-) (X, X'=Cl, Br, I). Ground-state properties including geometries, harmonic vibrational frequencies, and orbital energy-level diagrams, have been calculated. Comparison is made to the sulfide clusters of rhenium(III), of which [Re(6)S(8)Cl(6)](4-) is representative. [W(6)X(8)X'(6)](2-) and [Re(6)S(8)Cl(6)](4-) possess disparate electronic structures owing to the greater covalency of the metal-sulfur bond and hence of the [Re(6)S(8)](2+) core. Low-lying virtual orbitals are raised in energy in [Re(6)S(8)Cl(6)](4-) with the result that the LUMO+7 (or LUMO+8 in some cases) of tungsten(II) halide clusters is the LUMO of [Re(6)S(8)Cl(6)](4-) species. An inversion of the HOMO and HOMO-1 between the two cluster series also occurs. Time-dependent density-functional calculations using asymptotically correct functionals do not recapture the experimentally observed periodic trend in [W(6)X(14)](2-) luminescence (E(em) increasing in the order [W(6)Cl(14)](2-) halides. This circumstance is either a gross failure of the time-dependent formalism of DFT or it indicates extensive multistate emission in [W(6)X(8)X'(6)](2-) clusters. The inapplicability of isoelectronic analogies between clusters of Group 6 and Group 7 is emphasized. PMID:19180591

Gray, Thomas G

2009-03-01

309

Electric-field effects on two-photon transitions to Wannier excitons  

International Nuclear Information System (INIS)

The effects of an electric field, ?, on the excitonic two-photon absorption is presented for direct-gap semiconductors and insulators. Expression for the electroabsorption coefficient, ?ex(2) (?1, ?2, ?), have been calculated adopting a three-band model and allowed-transition matrix elements. The final state allows a s-exciton state. A numerical application to the ZnS crystal is given. It shows that: i) The electron-hole interaction enhances the magnitude of ?ex(2) (?1, ?2, ?) and also the electric-field-induced oscillations after the absorption edge. ii) The order of magnitude of ?ex(2) (?1, ?2, ?) decreases as ? increases. iii) Before the edge and as ? increases, the exciton peak broadens and shifts towards higher energies

310

Influence of size on the excitonic absorbance of CdSe nano crystal  

Science.gov (United States)

The size quantization effect is noticed as a shift of the transition energies toward higher values with decreasing particle size. Based on the SEM images report a curve is drawn and fitted with MATLAB programming to obtain a relation between excitonic absorbance and diameter of the nano crystal. Nonlinear fit of the curve is used to provide an empirical formula for the estimation of the excitonic absorbance as a dependent parameter of diameter of nano crystal. Some adjustable parameter is required to calculate the excitonic absorbance wavelength of CdSe nano crystal. Theoretical findings are graphically represented and compared with available experimental results. The theoretical model predicts a decrease in absorbance wavelength or blue shift in absorbance spectrum which consequently increases the band gap of CdSe nano crystal with decreasing of crystalline sizes. Results are comparable with the established experimental findings.

Sisodia, Namita; Bhadkare, Bhavana; Bhavsar, Priyanka; Sisodia, Amitesh

2013-06-01

311

Theoretical studies of excitons in type II CdSe/CdTe quantum dots  

Science.gov (United States)

We present a method for calculating exciton and bi-exciton energies in type-II colloidal quantum dots. Our methodology is based on an 8-band k · p Hamiltonian of the zinc- blend structure, which incorporates the effects of spin-orbit interaction, strain between the core and the shell and piezoelectric potentials. Exciton states are found using the configuration interaction (CI) method that explicitly includes the effects of Coulomb interaction, as well as exchange and correlation between many-electron configurations. We pay particular attention to accurate modelling of the electrostatic interaction between quasiparticles. The model includes surface polarization and self-polarization effects due to the large difference in dielectric constants at the boundary of the QD.

Miloszewski, Jacek M.; Tomi?, Stanko; Binks, David

2014-06-01

312

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.

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

2013-01-01

313

Excitons and cavity polaritons for ultracold atoms in an optical lattice  

CERN Document Server

We study the resonant electronic excitation dynamics for ultracold atoms trapped in a deep optical lattice prepared in a Mott insulator state. Excitons in these artificial crystals are similar to Frenkel excitons in Noble atom or molecular crystals. They appear when the atomic excited state line width is smaller than the exciton band width generated by dipole-dipole coupling. When the atoms are placed within a cavity the electronic excitations and the quantized cavity mode get coupled. In the collective strong coupling regime excitations form two branches of cavity polaritons with Rabi splitting larger than the atomic and the cavity line width. To demonstrate their properties we calculate the transmission, reflection, and absorption spectra for an incident weak probe field, which show resonances at the polariton frequencies.

Zoubi, Hashem

2007-01-01

314

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

315

Mott-Hubbard exciton in YTiO$_3$ probed by ellipsometry  

CERN Document Server

In the Mott-Hubbard insulator YTiO$_3$ we study optical excitations from the lower to the upper Hubbard band. Peaks in the optical conductivity at 2.55 and 4.15 eV reflect the existence of multiple upper Hubbard bands in a multi-orbital system. A strongly temperature-dependent peak at 1.95 eV is interpreted in terms of a Hubbard exciton, i.e. a charge-neutral (quasi-)bound state of a hole in the lower Hubbard band and a double occupancy in the upper one. The binding to such a Hubbard exciton may arise both due to Coulomb attraction between nearest-neighbor sites and due to a lowering of the kinetic energy in an orbitally ordered state.

Gössling, A; Roth, H; Haverkort, M W; Lorenz, T; Mydosh, J A; Müller-Hartmann, E; Grüninger, M

2006-01-01

316

Exciton correlations and input–output relations in non-equilibrium exciton superfluids  

International Nuclear Information System (INIS)

The photoluminescence (PL) measurements on photons and the transport measurements on excitons are the two types of independent and complementary detection tools to search for possible exciton superfluids in electron–hole semi-conductor bilayer systems. In fact, it was believed that the transport measurements can provide more direct evidences on superfluids than the spectroscopic measurements. It is important to establish the relations between the two kinds of measurements. In this paper, using quantum Heisenberg–Langevin equations, we establish such a connection by calculating various exciton correlation functions in the putative exciton superfluids. These correlation functions include both normal and anomalous greater, lesser, advanced, retarded, and time-ordered exciton Green functions and also various two exciton correlation functions. We also evaluate the corresponding normal and anomalous spectral weights and the Keldysh distribution functions. We stress the violations of the fluctuation and dissipation theorem among these various exciton correlation functions in the non-equilibrium exciton superfluids. We also explore the input–output relations between various exciton correlation functions and those of emitted photons such as the angle resolved photon power spectrum, phase sensitive two mode squeezing spectrum and two photon correlations. Applications to possible superfluids in the exciton–polariton systems are also mentioned. For a comparison, using conioned. For a comparison, using conventional imaginary time formalism, we also calculate all the exciton correlation functions in an equilibrium dissipative exciton superfluid in the electron–electron coupled semi-conductor bilayers at the quantum Hall regime at the total filling factor ?T=1. We stress the analogies and also important differences between the correlations functions in the two exciton superfluid systems. - Highlights: ? Establish the relations between photoluminescence and transport measurements. ? Stress the violations of the fluctuation–dissipation theorem in non-equilibrium systems. ? Explore the input–output relations between excitons and photons. ? Discuss differences between non-equilibrium systems and dissipative ones.

317

Exciton-polaritons condensate in a microwire  

Science.gov (United States)

Recently, polariton condensation has been claimed in microwires. Numerical solutions of the time-dependent Gross-Pitaevskii equation that describes the behavior of the condensate in a trap and exciton-polariton interaction, have been obtained. In this work we study theoretically exciton-polariton one dimensional condensation in several quantized states.

Kamoun, O.; Jaziri, S.

2013-12-01

318

Constraint on ?E and exciton number  

International Nuclear Information System (INIS)

It is shown that the constraint on ?E introduced by Alhassid, Levine, Karp and Steadman in their information-theoretical analysis of heavy ion collision data reflects the constraint on the exciton number nx. Arguments are based on Williams' state density formula for exciton model. Ericson's formula is also considered. (author)

319

Status of trivalent copper and charge-transfer excitons in high-TC cuprates.  

Science.gov (United States)

A chemical bonding approach based on tight-binding cluster and band calculations, taking into account on-site Coulomb repulsion (Hubbard U parameter) to differentiate doubly and singly occupied states, was applied to high- T C superconducting cuprates and related compounds. This work provides rational insight and explanations for issues such as (i) the actual oxidation number Cu (I+) for formally trivalent copper in oxides such as La 2Li 1/2Cu 1/2O 4, (ii) the dominant oxygen character of the doping holes in (CuO 2) ( n- ) planes, (iii) the Mott-Hubbard character of the insulator-to-metal transition triggered by hole doping, leading to an oxygen-to-copper charge transfer of avalanche type, (iv) the occurrence of an excitonic phase with anisotropic Frenkel-type excitons, (v) the role of Coulomb interactions between excitons and between doping holes and their exciton surroundings, and (vi) the on-time pairing of doping holes by means of an "excitonic glue". PMID:18821815

Pouchard, Michel; Doumerc, Jean-Pierre; Villesuzanne, Antoine

2008-10-01

320

Excitonic photoluminescence in symmetric coupled double quantum wells subject to an external electric field  

CERN Document Server

The effect of an external electric field F on the excitonic photoluminescence (PL) spectra of a symmetric coupled double quantum well (DQW) is investigated both theoretically and experimentally. We show that the variational method in a two-particle electron-hole wave function approximation gives a good agreement with measurements of PL on a narrow DQW in a wide interval of F including flat-band regime. The experimental data are presented for an MBE-grown DQW consisting of two 5 nm wide GaAs wells, separated by a 4 monolayers (MLs) wide pure AlAs central barrier, and sandwiched between Ga_{0.7}Al_{0.3}As layers. The bias voltage is applied along the growth direction. Spatially direct and indirect excitonic transitions are identified, and the radius of the exciton and squeezing of the exciton in the growth direction are evaluated variationally. The excitonic binding energies, recombination energies, oscillator strengths, and relative intensities of the transitions as functions of the applied field are calculate...

Soubusta, J; Hlidek, P; Zvara, M; Smrcka, L; Malzer, S; Geisselbrecht, W; Döhler, G H

1999-01-01

321

Exciton states in metallic zigzag single-walled carbon nanotubes under uniaxial strain  

Science.gov (United States)

The exciton energy spectrum and its binding energy under the uniaxial strain have been theoretically studied by using the tight-binding model in the metallic zigzag single-walled carbon nanotubes (SWNTs). It is found that the energy of M 11 - and M 22 - excitons and their binding energies increase with the increase of uniaxial strain, but the energy of M 11 + and M 22 + excitons and their binding energies decrease as the uniaxial strain increase. So, we can deduce that the splitting of M11 and M22 exciton will disappear as the uniaxial strain increases up to some degree, which is expected to be detected by the future experiment. On the other hand, it is interesting to calculate the exciton energy spectrum and binding energies of the bands that nearest to the Fermi level, with a narrow gap under the uniaxial strain. The obtained results show that they increase with increasing the uniaxial strain, based on which a supplemented tool is offered to detect the deformation degree of a metallic SWNT under uniaxial strain. In addition, we expect the results obtained here can offer some useful information for the future THz applications.

Yu, Guili; Li, Guichen; Jia, Yonglei; Tang, Gang

2014-11-01

322

Spectroscopy of localized excitons in semimagnetic semiconductors  

International Nuclear Information System (INIS)

The localization of exciton states in Cd1-xMnxTe, Cd1-xFexTe and Zn1-xMnxTe crystals is studied. The exciton reflection spectra and luminescence depending on the content (x) of Mn or Fe, intensity (I) of laser radiation exciting the photoluminescence, temperature (T) and external magnetic field magnitude (H) have been measured. The following results have been yielded: 1) As x increases a successive transition from a free exciton state to the state localized on potential fluctuations and to an exciton with the resultant formation of a magnetic polaron is observed. 2) The exciton magnetic polaron existence region and the stepwise nature of its coupling energy depending on T and different x are determined. 3) A change of I as well as superposition of H results in a temperature dependent variation of the localized energy. The connection between the effects observed and the magnitude of the multipartial exchange couplings is discussed. (author)

323

Tightly bound excitons in monolayer WSe(2).  

Science.gov (United States)

Exciton binding energy and excited states in monolayers of tungsten diselenide (WSe(2)) are investigated using the combined linear absorption and two-photon photoluminescence excitation spectroscopy. The exciton binding energy is determined to be 0.37 eV, which is about an order of magnitude larger than that in III-V semiconductor quantum wells and renders the exciton excited states observable even at room temperature. The exciton excitation spectrum with both experimentally determined one- and two-photon active states is distinct from the simple two-dimensional (2D) hydrogenic model. This result reveals significantly reduced and nonlocal dielectric screening of Coulomb interactions in 2D semiconductors. The observed large exciton binding energy will also have a significant impact on next-generation photonics and optoelectronics applications based on 2D atomic crystals. PMID:25062219

He, Keliang; Kumar, Nardeep; Zhao, Liang; Wang, Zefang; Mak, Kin Fai; Zhao, Hui; Shan, Jie

2014-07-11

324

Relaxation time in the exciton model  

International Nuclear Information System (INIS)

An equation for the time-dependent mean number of excitons is derived from the system of kinetic equations of the exciton model. The relaxation time is obtained as a function of the excitation energy and the number of nucleons, both with and without the effect of the Pauli exclusion principle in the densities of states with a fixed number of excitons. An equation for the mean square fluctuation of the number of excitons is also obtained. We discuss the connection between the equations of the exciton model and the stochastic differential equation for the number of collisions in a medium with a random distribution of scatterers having a time-dependent average density

325

Tightly Bound Excitons in Monolayer WSe2  

Science.gov (United States)

Exciton binding energy and excited states in monolayers of tungsten diselenide (WSe2) are investigated using the combined linear absorption and two-photon photoluminescence excitation spectroscopy. The exciton binding energy is determined to be 0.37 eV, which is about an order of magnitude larger than that in III-V semiconductor quantum wells and renders the exciton excited states observable even at room temperature. The exciton excitation spectrum with both experimentally determined one- and two-photon active states is distinct from the simple two-dimensional (2D) hydrogenic model. This result reveals significantly reduced and nonlocal dielectric screening of Coulomb interactions in 2D semiconductors. The observed large exciton binding energy will also have a significant impact on next-generation photonics and optoelectronics applications based on 2D atomic crystals.

He, Keliang; Kumar, Nardeep; Zhao, Liang; Wang, Zefang; Mak, Kin Fai; Zhao, Hui; Shan, Jie

2014-07-01

326

Iridium-catalyzed intramolecular [4 + 2] cycloadditions of alkynyl halides  

Directory of Open Access Journals (Sweden)

Full Text Available Iridium-catalyzed intramolecular [4 + 2] cycloadditions of diene-tethered alkynyl halides were investigated by using [IrCl(cod]2 as catalyst, and dppe was found to be the most suitable phosphine ligand for the reaction. No oxidative insertion of the iridium into the carbon–halide bond was observed, and the reactions proceeded to provide the halogenated cycloadducts in good yield (75–94%. These results are the first examples of cycloadditions of alkynyl halides using an iridium catalyst.

Andrew Tigchelaar

2012-10-01

327

Photoacoustic measurements of excitons in CdSe nanorods  

Energy Technology Data Exchange (ETDEWEB)

Photoacoustic (PA) spectra were obtained for CdSe nanorods (NRs) of different aspect ratios, prepared via the organometallic synthesis. The second derivative spectra were used to have an accurate determination of the different excitonic transitions. Using the lowest transition energy (band gap) 1S(e)-1S1/2(h) and applying the effective mass model, the NRs diameters were determined. The obtained diameters were then compared to direct measurements of scanning tunneling microscopy (STM) and XRD. It is observed that the band gap depends on the diameter of the rods due to quantum confinement effect, since diameters are of the order of the bulk CdSe Bohr radius. The second derivative of the PA spectra for CdSe NRs also shows clearly a second excitonic transition1P(e)-1P1/2(h) in contrast to UV-Vis absorption spectra carried out for colloidal samples. The thermal parameters for samples were also measured and compared to the bulk values.

Awad, H; Abdallah, T; Talaat, H [Physics Department, Faculty of Science, Ain Shams University, Abbassia, Cairo (Egypt); Mohammed, M B [NILES, Cairo University, Giza (Egypt); Easawi, K; Negm, S, E-mail: hassantalaat@hotmail.co [Department of Mathematical and Physical Engineering, Faculty of Engineering (Shoubra), Banha University, Cairo (Egypt)

2010-03-01

328

Photodissociation spectroscopy of cyanogen halides in the extreme vacuum ultraviolet region  

International Nuclear Information System (INIS)

Full text: Photodissociation of cyanogen halides, XCN (X=Cl, Br and I), was investigated using synchrotron radiation. The spectral assignments of the absorption bands beyond 105 nm were reported in the previous works. In the present study, the photoabsorption cross section and the absolute and relative cross sections for the CN(B2?+ X2?+ X2?+) emission and CN(A?i-X2?+) emissions, respectively, were determined in the wavelength range 50-120 nm with the spectral resolution of 0.1 nm fwhm using Ar gas filter. The observed photoabsorption spectra display congested structures, indicating a number of Rydberg and high-lying valence states in the energy region under the first ionization potentials. On the other hand, no structure was observed beyond the second ionization potentials. The quantum yields for the CN(B2?+) and CN(A2?) productions were observed to increase with excitation energy and decrease suddenly at the excitation wavelengths corresponding to the opening of the ionization channels of XCN. The spectral assignments of the photoabsorption bands observed in the extreme ultraviolet region and the photodissociation dynamics of cyanogen halides were discussed

329

Two-component exciton model  

International Nuclear Information System (INIS)

The exciton model of nuclear reactions with the distinguishability of protons and neutrons explicitly included is discussed. The Pauli master equation and transition rates in this two-component formulation are given. Both the simple considerations and actual calculations show that the two-component model exhibits new features and modifies conclusions of the one-component formulation. The ratio of the proton to neutron pre-equilibrium emissions in proton induced reactions decreases twice to three times when compared with the usual one-component model. (orig.)

330

Lattice-mismatch-strain effects on excitons in GaAs{sub 1-x}N{sub x}/GaAs heterostructures  

Energy Technology Data Exchange (ETDEWEB)

We have investigated the lattice-mismatch-strain effect on excitons in GaAs{sub 1-x}N{sub x}(500 nm)/GaAs heterostructures with x=0.4, 0.8, and 1.5% grown on a GaAs substrate using photoreflectance spectroscopy that is sensitive to excitonic transitions. It is found that the heavy-hole (HH) and light-hole (LH) excitons of the GaAs{sub 1-x}N{sub x} layer, which are degenerate in a bulk crystal, are split and that the energy of the LH exciton is lower than that of the HH exciton. In this case, a quantum-size effect is negligible because the GaAs{sub 1-x}N{sub x} layer thickness is sufficiently thick. The HH-LH splitting energy increases with an increase of nitrogen concentration (x) in addition to a drastic decrease of the energies of the HH and LH excitons. We have quantitatively analyzed the nitrogen-concentration dependence of the energies of the HH and LH excitons, taking account of the lattice-mismatch-strain effect on the band structure and nitrogen-induced giant bowing of the band-gap energy. (copyright 2009 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)

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

2009-01-15

331

Ion clusters sputtered from alkali halide crystals  

International Nuclear Information System (INIS)

Sputtered ion clusters of alkali halides possess magic numbers of N = 13,22, etc, which can be well explained by classical electrostatic interactions and quantum 'short-range' interactions. When lithium fluoride crystals of high purity were bombarede by 8 keV argon ions, two groups of cluster ions were observed. Both were positively charged, but one of them, [Li(LiF)N]+, with microcrystal structure, and the other, (LiF)N+, complex ions. And, as the natural lithium has two isotopes, both groups contain great amount of hetero-isotopic clusters. The isotopic effect suggests another important property, in addition to the magic numbers, of mass spectra of the sputtered ion cluster

332

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)

333

Thermoluminescence of alkali halides and its implications  

International Nuclear Information System (INIS)

Trapping levels present in some alkali halides namely NaCl, KCl, KBr, and KI are determined by deconvolution of the thermoluminescence (TL) curves. Unlike most of the studies undertaken over the last few decades, we have presented a comprehensive picture of the phenomenon of TL as an analytical technique capable of revealing the position of the trapping levels present in the materials. We show that for all practical purposes, TL can be described involving only the three key trapping parameters, namely, the activation energy (E), the frequency factor (s), and the order of kinetics (b) even for complex glow curves having a number of TL peaks. Finally, based on these, we logically infer the importance of TL in development and characterization of materials used in dosimetry, dating and scintillation.

334

Thermoluminescence of alkali halides and its implications  

Energy Technology Data Exchange (ETDEWEB)

Trapping levels present in some alkali halides namely NaCl, KCl, KBr, and KI are determined by deconvolution of the thermoluminescence (TL) curves. Unlike most of the studies undertaken over the last few decades, we have presented a comprehensive picture of the phenomenon of TL as an analytical technique capable of revealing the position of the trapping levels present in the materials. We show that for all practical purposes, TL can be described involving only the three key trapping parameters, namely, the activation energy (E), the frequency factor (s), and the order of kinetics (b) even for complex glow curves having a number of TL peaks. Finally, based on these, we logically infer the importance of TL in development and characterization of materials used in dosimetry, dating and scintillation.

Gartia, R.K., E-mail: rkgartia02@yahoo.in [Physics Department, Manipur University, Imphal 795003 (India); Rey, L. [Aerial-CRT-parc d' Innovation, B.P. 40443, F-67412 Illkirch Cedex (France); Tejkumar Singh, Th. [Physics Department, Manipur University, Imphal 795003 (India); Basanta Singh, Th. [Luminescence Dating Laboratory, Manipur University, Imphal 795003 (India)

2012-03-01

335

Ground state properties of heavy alkali halides  

CERN Document Server

We extend previous work on alkali halides by calculations for the heavy-atom species RbF, RbCl, LiBr, NaBr, KBr, RbBr, LiI, NaI, KI, and RbI. Relativistic effects are included by means of energy-consistent pseudopotentials, correlations are treated at the coupled-cluster level. A striking deficiency of the Hartree-Fock approach are lattice constants deviating by up to 7.5 % from experimental values which is reduced to a maximum error of 2.4 % by taking into account electron correlation. Besides, we provide ab-initio data for in-crystal polarizabilities and van der Waals coefficients.

Doll, K; Doll, Klaus; Stoll, Hermann

1998-01-01

336

Application of VUV harmonics light source for investigation of energy transfer mechanisms in the wide band gap solids  

International Nuclear Information System (INIS)

Complete test of publication follows. The experimental aims were the study of interaction mechanism of excitons generated in the crystals with a wide forbidden gap. For this investigation, we studied the dependence of luminescence from excitation density. As the source of VUV was used high harmonic source based on 1 kHz Ti:sapphire 800 nm 40 fs laser. For our experiments we used harmonics from 15 to 27 generated in Ar (23-42 eV). 1. Interaction of molecular excitons in CdWO4. CdWO4 is the good modeling system with one type of excitations - excitons. It has only one luminescence band, corresponding to Self-Trapped Exciton luminescence. Observation of these experiments is the strong quenching of luminescence at high density of excitation. The mechanism of this quenching is interaction between excitons. The basic interaction mode of electronic excitations is the dipole-dipole interaction. A. Vasiliev has made mathematical model, witch describes this interaction. This model gives the equation to describe the decay kinetics. We can find the parameters of exciton-exciton interaction via fitting measured decay curve data by this equation with known parameters of the crystal and excitation beam. Result: radius of dipole-dipole interaction of excitons in CdWO4 estimated from the fitting: Rd-d = 4.3 nm. 2. Interaction of core and valence band excitations in BaF2. BaF2 has two types of localized electronic excitations: Self Trapped Excitons and Auger Free Luminescence. In contrast to the CdWO4 in BaF2 it also presents interaction between excitons and core holes. Experimental evidence: strong quenching of luminescence at the expense of interaction between core holes and excitons. This interaction is much stronger and faster then exciton-exciton interaction.

337

Preformed excitons, orbital selectivity, and charge density wave order in 1T-TiSe2  

Science.gov (United States)

Traditional routes to charge density wave (CDW) in transition-metal dichalcogenides, relying on Fermi surface nesting or Jahn-Teller instabilities, have recently been brought into question. While this calls for exploration of alternative views, a paucity of theoretical guidance sustains lively controversy on the origin of, and interplay between, CDW and superconductive orders in transition-metal dichalcogenides. Here, we explore a preformed excitonic liquid route, heavily supplemented by modern correlated electronic-structure calculations, to an exci-tonic CDW order in 1T-TiSe2. We show that orbital-selective dynamical localization arising from preformed excitonic liquid correlations is somewhat reminiscent of states proposed for d and f band quantum criticality at the border of magnetism. Excellent quantitative explication of a wide range of spectral and transport responses in both normal and CDW phases provides strong support for our scenario, and suggests that soft excitonic liquid fluctuations mediate superconductivity in a broad class of transition-metal dichalcogenides on the border of CDW. This brings the transition-metal dichalcogenides closer to the bad actors (where the metallic state is clearly not a Fermi liquid) in d and f band systems, where anomalously soft fluctuations of electronic origin are believed to mediate unconventional superconductivity on the border of magnetism.

Koley, S.; Laad, M. S.; Vidhyadhiraja, N. S.; Taraphder, A.

2014-09-01

338

Efficient exciton funneling in cascaded PbS quantum dot superstructures.  

Science.gov (United States)

Benzenedithiol (BDT) and ethanedithiol (EDT) ligand-exchange treatments can be used to cross-link colloidal PbS quantum dots into nanocrystalline film structures with distinct optoelectronic properties. Such structures can provide a unique platform to study the energy transfer between layers of quantum dots with different sizes. In this report, efficient exciton funneling and recycling of surface state-bound excitons is observed in cascaded PbS quantum dot-based multilayered superstructures, where the excitons transfer from the larger band gap or donor layers to the smallest band gap or acceptor layers. In this system, both the BDT- and EDT-treated cascaded structures exhibit dramatically enhanced photoluminescence from the acceptor layers. As we show, the energy transfer mechanisms involved and their efficiencies are significantly different depending on the ligand-exchange treatment. In the future, we believe these efficient exciton recycling and funneling mechanisms could be used to improve significantly the photocurrent, charge-transport, and conversion efficiencies in low-cost nanocrystalline and hybrid solar cells and the emission efficiencies in hybrid light-emitting devices. PMID:22085035

Xu, Fan; Ma, Xin; Haughn, Chelsea R; Benavides, Jamie; Doty, Matthew F; Cloutier, Sylvain G

2011-12-27

339

Femtosecond spectroscopy study of the exciton relaxation dynamics in silicon quantum dots  

Energy Technology Data Exchange (ETDEWEB)

This contribution is targeted to the development of surface-modified silicon quantum dots (Siqdots) with tailored luminescence properties. The surface modification of Siqdots with sizes between 1 and 5 nm has been successfully achieved via two different synthesis routes, first, by controlled oxidation followed from silanization and second, by thermal hydrosilylation with chromophores. The luminescence properties of ethanolic Siqdots dispersions were characterized using stationary and time-resolved luminescence spectroscopy techniques, whereas the ultrashort exciton relaxation dynamics were examined using femtosecond transient absorption spectroscopy. Silanized Siqdots were observed to exhibit two species of photoluminescence (PL): the blue emission at 380 nm is assigned to localized surface states, whereas radiative recombination of quantum confined excitons gives rise to a broad PL band around 800 nm. Whereas the latter is ascribed to Siqdots with sizes larger than 3 nm, for Siqdots smaller than 1.5 nm exciton relaxation dynamics is understood to occur predominantly by trapping due to lower-lying surface states which may radiatively decay. Siqdots terminated with suited chromophores were observed to exhibit only one PL band in the visible that is ascribed to exciton states involving resonant couplings to the conjugated electron system of the chromophores.

Kryschi, Carola; Kuntermann, Volker; Cimpean, Carla [Institut fuer Physikalische Chemie I, FAU, Erlangen (Germany); Haarer, Dietrich [BIMF, Universitaet Bayreuth (Germany)

2008-07-01

340

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

International Nuclear Information System (INIS)

Emission related to rare earth ions in solids takes place usually due to 4fn ? 4fn and 4fn?15d1 ? 4fn 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 Eu2+ the impurity trapped exciton states become stable and contribute to the radiative processes though anomalous luminescence that appears apart of the 4f7 ? 4f7 and 4f75d1 ? 5f7 emission. In this contribution pressure effect on energies of the 4fn?15d1?5fn 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 Pr3+, and Eu2+ in dif, and Eu2+ in different lattices are reviewed.

341

Tuning the near-gap electronic structure of tin-halide and lead-halide perovskites via changes in atomic layering  

Science.gov (United States)

Using density functional theory (DFT)-based calculations, we explore the extent to which achievable modes of atomic layering can tune the near-gap electronic structure of tin- and lead-halide perovskites with applications in dye-sensitized solar cells. We show that regardless of how atomic layering is achieved—whether by the growth of layered inorganic phases such as the Ruddlesden-Popper series, hybrid perovskites connected by organic linker molecules, or layered perovskite heterostructures—their band gaps can similarly be widened by several tenths of an eV or more. Because these classes of compounds are known to have band gaps spanning much of the visible region of the solar spectrum, the ability to controllably tune their near-gap electronic structure could further optimize their performance in solar energy conversion applications. Throughout this work, trends in band gap are explained based on the effects of atomic layering and quantum confinement on the character and energy of band-edge crystal orbitals.

Grote, Christopher; Ehrlich, Bradley; Berger, Robert F.

2014-11-01

342

Cluster ion formation of alkali halides by electrospray droplet impact  

Science.gov (United States)

Positive and negative cluster ions formed by electrospray droplet impact on the alkali halides (MX) were measured for all alkali halides. Small amounts of powder samples deposited and crushed into thin films on the metal substrate were served for the measurement. The size distribution of cluster ions is very limited for LiF, CsF, and LiI. Little or no fluoride cluster ions F-(MF)n were detected. In general, distribution of n for negative cluster ions X-(MX)n is narrower than that for positive cluster ions M+(MX)n, probably due to the instability caused by the mutual Coulomb repulsion between the larger-size halide ions than alkali ions. While the cubic-like structures are preferred for alkali halides with smaller ionic radii, the stacked hexagonal structures with n = 3, 6, and 9 are preferred for those with larger ionic radii.

Mori, Kunihiko; Hiraoka, Kenzo

2008-01-01

343

Excitation Energy Dependence of the Exciton Inner Ring  

OpenAIRE

We report on the excitation energy dependence of the inner ring in the exciton emission pattern. The contrast of the inner ring is found to decrease with lowering excitation energy. Excitation by light tuned to the direct exciton resonance is found to effectively suppress excitation-induced heating of indirect excitons and facilitate the realization of a cold and dense exciton gas. The excitation energy dependence of the inner ring is explained in terms of exciton transport ...

Kuznetsova, Y. Y.; Leonard, J. R.; Butov, L. V.; Wilkes, J.; Muljarov, E. A.; Campman, K. L.; Gossard, A. C.

2012-01-01

344

Exciton localization and drift in tailored-potential quantum nanowires  

International Nuclear Information System (INIS)

Exciton recombination dynamics in tailored-potential, site-controlled AlGaAs quantum wires (QWRs) are studied. Time-resolved photoluminescence spectra evidence exciton localization in weakly disordered “uniform” QWRs, whereas deterministic bandgap grading is shown to suppress localization and promote exciton drift along the potential gradient. Measured exciton transit times between two quantum dot probes placed at opposite ends of the potential gradient yield the effective 1D exciton mobility as >1300 cm2/(eVs).

345

Exciton localization and drift in tailored-potential quantum nanowires  

Energy Technology Data Exchange (ETDEWEB)

Exciton recombination dynamics in tailored-potential, site-controlled AlGaAs quantum wires (QWRs) are studied. Time-resolved photoluminescence spectra evidence exciton localization in weakly disordered “uniform” QWRs, whereas deterministic bandgap grading is shown to suppress localization and promote exciton drift along the potential gradient. Measured exciton transit times between two quantum dot probes placed at opposite ends of the potential gradient yield the effective 1D exciton mobility as >1300 cm{sup 2}/(eVs).

Szeszko, J., E-mail: justyna.szeszko@epfl.ch; Rudra, A.; Kapon, E. [Laboratory of Physics of Nanostructures, École Polytechnique Fédérale de Lausanne, 1015 Lausanne (Switzerland); Belykh, V. V.; Sibeldin, N. N. [P. N. Lebedev Physical Institute, Russian Academy of Sciences, Moscow 119991 (Russian Federation)

2014-06-30

346

Two Exciton States in Semiconductor Quantum Dots  

Science.gov (United States)

In a semiconductor quantum dot (QD), the conventional biexciton is formed by the coulomb attraction between two excitons. The interaction of two excitons can also create two-exciton states in a QD that in higher dimensional systems are unbound and part of the continuum. However, the confinement potential of the QD binds these excitons into an interaction that creates a biexciton whose energy is greater than the non-interacting two-exciton energy. We report here a nonlinear spectral signature consistent with the creation of an interdot ``anti-bound'' biexciton in GaAs interface fluctuation QDs obtained by high resolution nonlinear spectroscopy. These measurements show evidence of a coupled excitonic system distinct from both the conventional and the intradot ``anti-bound'' biexcitons [1,2] but similar to two interacting separately localized excitons. This work was supported by the ARO, AFOSR and ONR. [1] Y.Z. Hu, et al. Phys. Rev. Lett. 64, 1805 (1990). [2] H. Kamada, et al. Phys. Rev. B 58, 16243 (1998).

Batteh, E. T.; Chen, Gang; Cheng, Jun; Steel, D. G.; Gammon, D.; Katzer, D. S.; Park, D.; Chen, Pochung; Piermarocchi, C.; Sham, L. J.

2002-03-01

347

Characterization of Catalytically Active Octahedral Metal Halide Cluster Complexes  

OpenAIRE

Halide clusters have not been used as catalysts. Hexanuclear molecular halide clusters of niobium, tantalum, molybdenum, and tungsten possessing an octahedral metal framework are chosen as catalyst precursors. The prepared clusters have no metal–metal multiple bonds or coordinatively unsaturated sites and therefore required activation. In a hydrogen or helium stream, the clusters are treated at increasingly higher temperatures. Above 150–250 °C, catalytically active sites develop, and th...

Satoshi Kamiguchi; Sayoko Nagashima; Teiji Chihara

2014-01-01

348

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

349

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

350

Cavity polaritons of heavy-hole and light-hole excitons in a CuI microcavity  

Science.gov (United States)

We have investigated the exciton-photon coupling phenomena in a CuI bulk microcavity consisting of a CuI active layer with an effective thickness of ?/2 and HfO2/SiO2 distributed Bragg reflectors. In the CuI active layer, a thermal strain removes the degeneracy of the heavy-hole (HH) and light-hole (LH) excitons at the ? point. Angle-resolved reflectance spectra measured at 10 K demonstrate strong coupling between the HH and LH excitons and cavity photon, resulting in the formation of three cavity-polariton modes: lower, middle, and upper polariton branches. The photoluminescence band from the lower polariton branch was clearly observed. The energies of the three cavity-polariton modes as a function of incidence angle are reasonably explained by analysis using a phenomenological Hamiltonian for the strong coupling. The vacuum Rabi splitting energies are evaluated as 50 and 29 meV for the HH and LH excitons, respectively. The magnitude of the Rabi splitting energy results from the large oscillator strength of the relevant exciton. Furthermore, it is confirmed that the strong coupling regime is fully maintained at room temperature because of the high stability of the excitonic system.

Nakayama, M.; Kameda, M.; Kawase, T.; Kim, D.

2011-06-01

351

Switching Exciton Pulses Through Conical Intersections  

Science.gov (United States)

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 nonadiabatic 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. The fundamental ideas discussed here have general implications for excitons on a dynamic network.

Leonhardt, K.; Wüster, S.; Rost, J. M.

2014-11-01

352

Excitons in boron nitride nanotubes: Dimensionality effects  

OpenAIRE

We show that the optical absorption spectra of boron nitride (BN) nanotubes are dominated by strongly bound excitons. Our first-principles calculations indicate that the binding energy for the first and dominant excitonic peak depends sensitively on the dimensionality of the system, varying from 0.7 eV in bulk hexagonal BN via 2.1 eV in the single sheet of BN to more than 3 eV in the hypothetical (2,2) tube. The strongly localized nature of this exciton dictates the fast con...

Wirtz, Ludger; Marini, Andrea; Rubio Secades, A?ngel

2006-01-01

353

Spontaneous exciton dissociation in carbon nanotubes.  

Science.gov (United States)

Simultaneous photoluminescence and photocurrent measurements on individual single-walled carbon nanotubes reveal spontaneous dissociation of excitons into free electron-hole pairs. The correlation of luminescence intensity and photocurrent shows that a significant fraction of excitons are dissociating before recombination. Furthermore, the combination of optical and electrical signals also allows for extraction of the absorption cross section and the oscillator strength. Our observations explain the reasons why photoconductivity measurements in single-walled carbon nanotubes are straightforward despite the large exciton binding energies. PMID:24702413

Kumamoto, Y; Yoshida, M; Ishii, A; Yokoyama, A; Shimada, T; Kato, Y K

2014-03-21

354

Harvesting Excitons Through Plasmonic Strong Coupling  

CERN Document Server

Exciton harvesting is demonstrated in an ensemble of quantum emitters coupled to localized surface plasmons. When the interaction between emitters and the dipole mode of a metallic nanosphere reaches the strong coupling regime, the exciton conductance is greatly increased. The spatial map of the conductance matches the plasmon field intensity profile, which indicates that transport properties can be tuned by adequately tailoring the field of the plasmonic resonance. We exploit this effect in a plasmonic nanostructure composed of three metal nanospheres, showing efficient transport of excitons into a very deep sub- wavelength region.

Gonzalez-Ballestero, Carlos; Moreno, Esteban; Garcia-Vidal, Francisco J

2015-01-01

355

Quantum-dot excitons in nanostructured environments  

DEFF Research Database (Denmark)

The interaction between light and quantum-dot (QD) excitons is strongly influenced by the environment in which the QD is placed. We have investigated the interaction by measuring the time-resolved spontaneous-emission rate of QD excitons in different nanostructured environments. Thereby, we have determined the oscillator strength, quantum efficiency and spin-flip rates of QD excitons as well as their dependencies on emission wavelength and QD size. Enhancement and inhibition of QD spontaneous emission in photonic crystal membranes (PCMs) is observed. Efficient coupling to PCM waveguides is demonstrated and the influence of disorder is discussed. The findings have a strong bearing on future nanophotonic devices.

Hvam, JØrn Märcher; Stobbe, SØren

2011-01-01

356

Spontaneous Exciton Dissociation in Carbon Nanotubes  

Science.gov (United States)

Simultaneous photoluminescence and photocurrent measurements on individual single-walled carbon nanotubes reveal spontaneous dissociation of excitons into free electron-hole pairs. The correlation of luminescence intensity and photocurrent shows that a significant fraction of excitons are dissociating before recombination. Furthermore, the combination of optical and electrical signals also allows for extraction of the absorption cross section and the oscillator strength. Our observations explain the reasons why photoconductivity measurements in single-walled carbon nanotubes are straightforward despite the large exciton binding energies.

Kumamoto, Y.; Yoshida, M.; Ishii, A.; Yokoyama, A.; Shimada, T.; Kato, Y. K.

2014-03-01

357

Nonlinear photoluminescence in atomically thin layered WSe2 arising from diffusion-assisted exciton-exciton annihilation  

Science.gov (United States)

We studied multiexciton dynamics in monolayer WSe2 using nonlinear photoluminescence (PL) spectroscopy and Monte Carlo simulations. We observed strong nonlinear saturation behavior of exciton PL with increasing excitation power density and long-distance exciton diffusion, reaching several micrometers. We demonstrated that the diffusion-assisted exciton-exciton annihilation (EEA) model accounts for the observed nonlinear PL behavior. The long-distance exciton diffusion and subsequent efficient EEA process determined the unusual multiexciton dynamics in atomically thin layered transition metal dichalcogenides.

Mouri, Shinichiro; Miyauchi, Yuhei; Toh, Minglin; Zhao, Weijie; Eda, Goki; Matsuda, Kazunari

2014-10-01

358

Low absorption state of phycocyanin from Acaryochloris marina antenna system: On the interplay between ionic strength and excitonic coupling  

Science.gov (United States)

This paper studies the excitonic factor in the excited state energy transfer of phycobilisome (PBS) by using a polarized time-resolved pump-probe and by changing the ionic strength of the cofactors' medium in the PBS of Acaryochloris marina (A. marina). As a result, the interplay between the surrounding medium and the closely excited adjacent cofactors is shown to be a negligible factor of the excitonic decay kinetics at 618 nm of the phycocyanin (PC), while it appears as a driving factor of an increase in excitonic delocalization at 630 nm. The obtained anisotropy values are consistent with the contribution of ionic strength in the excitonic mechanism in PBS. These values were 0.38 in high ionic strength and 0.4 in low ionic strength at 618 nm, and 0.52 in high ionic strength and 0.4 in low ionic strength at 630-635 nm. The anisotropy value of 0.52 in high phosphate is similar at 630 nm and 635 nm, which is consistent with an excitonic delocalization band at 635 nm. The 635 nm band is suggested to show the true low energy level of PC in A. marina PBS. The anisotropy decay kinetic at 630 nm suggests that the excited state population of PC is not all equilibrated in 3 ps because of the existence of the 10 ps decay kinetic component. The presence of the slow kinetic decay component in high, and low ionic strength, is consistent with a 10 and 14 ps energy transfer pathway, while the 450 fs kinetic decay component is consistent with the presence of an additional excitation energy transfer pathway between adjacent ?84 and ?84. Furthermore, the 450 fs decay kinetic is suggested to be trapped in the trimer, while the 400 fs decay kinetic rules out an excitonic flow from low energy level PC to allophycoyanin. This excitonic flow may occur between ?84 in adjacent trimers, towards the low energy state of the PBS rod.

Nganou, Collins

2013-07-01

359

Enhancement Of Free Exciton Peak Intensity In Reactively Sputtered ZnO Thin Films On (0001) Al2O3  

International Nuclear Information System (INIS)

Wide bandgap materials such as GaN with its direct bandgap structure have been developed rapidly for applications in short wavelength light emission. ZnO, II-VI oxide semiconductor, is also promising for various technological applications, especially for optoelectronic light emitting devices in the visible and ultraviolet (UV) range of the electromagnetic spectrum. Above-band-edge absorption spectra of reactively sputtered Zn- and O-rich samples exhibit free exciton (FX) and neutral acceptor bound exciton (A deg. X) features. It is shown that the residual acceptors which bind excitons with an energy of 75 meV reside about 312 meV above the valence band, according to effective mass theory. An intra-bandgap absorption feature peaking at 2.5 eV shows correlation with the characteristically narrow A-free exciton peak intensity. Relevant annealing processes are presented as a function of time and temperature dependently for both Zn- and O- rich thin films. Enhancement of the free exciton peak intensity is observed without disturbing the residual shallow acceptor profile which is necessary for at least background p-type conductivity

360

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

361

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

Science.gov (United States)

We study the linear optical absorption of bulk semiconductors in the presence of a uniform and 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 k · 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 k · p model; the enhancement depends on the strength of the dc field and the polarization of the optical field. With respect to the polarization dependence, we find that the anisotropy described by the independent particle approximation can be modified significantly by the Coulomb interaction.

Duque-Gomez, Federico; Sipe, J. E.

2015-01-01

362

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

363

Temperature dependence of the exciton gap in monocrystalline CuGaSe{sub 2}  

Energy Technology Data Exchange (ETDEWEB)

Near-band-edge photoluminescence properties of as-grown undoped CuGaSe{sub 2} single crystals have been investigated in the range between 10 K and room temperature. The temperature dependence of the exciton gap energy E{sub g} was studied by means of a three-parameter thermodynamic model, the Einstein model, Varshni's model and the Paessler model. The values of the band gap energy at T = 0 K, the effective phonon energy and the Einstein temperature, the cut-off phonon energy of the phonon spectrum as well as a dimensionless constant related to the electron-phonon coupling were estimated. Comparing all applied models points out that a minimum set of four parameters is required for a complete analytical description of the experimentally determined temperature dependence of the exciton gap in CuGaSe{sub 2}.

Meeder, A [Hahn-Meitner Institut GmbH, Glienicker Strasse 100, D-14109 Berlin (Germany); Jaeger-Waldau, A [EC, DG-JRC, IES-Renewable Energies Unit, Ispra (Italy); Tezlevan, V [Institute of Applied Physics, Academy of Sciences of Moldova, Kishinev, MD 2028 (Moldova, Republic of); Arushanov, E [Institute of Applied Physics, Academy of Sciences of Moldova, Kishinev, MD 2028 (Moldova, Republic of); Schedel-Niedrig, T [Hahn-Meitner Institut GmbH, Glienicker Strasse 100, D-14109 Berlin (Germany); Lux-Steiner, M Ch [Hahn-Meitner Institut GmbH, Glienicker Strasse 100, D-14109 Berlin (Germany)

2003-09-17

364

Electron-hole liquid and exciton and biexciton gas in elastically deformed silicon crystals  

International Nuclear Information System (INIS)

Exciton condensation into an electron-hole liquid (EHL) in silicon deformed elastically along the 100, 110 and 111 crystallographic directions is investigated. The binding energy PHI and equilibrium EHL, density, N0 for various deformation directions are determined by analyzing the shape of the recombination emission spectrum. It is shown that on decrease of the multiplicity of degeneracy of the valency band and of the number of valleys in the conductivity band, PHI and N0 decrease. It is found that in Si crystals deformed along 100 the partial pressures of the exciton and biexciton gases along the gas-liquid equilibrium line are commensurable. The biexciton recombination emission spectrum is measured. The probability for biexciton annihilation is calculated and the shape of the biexciton emission spectrum is analyzed and the biexciton binding energy has been estimated with value 0.55+-0.15 mev

365

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

366

Gravitational Excitons as Dark Matter  

CERN Document Server

In earlier work it was pointed out that for warped product spacetimes the conformal (geometrical moduli) excitations of the internal compactified factor spaces should be observable as massive scalar fields in the external spacetime. Here we show that these scalar fields (gravitational excitons) describe weakly interacting particles and can be considered as dark matter component. Masses of the gravexcitons are defined by the form of the effective potential of the theory and the stabilization scales of the internal space. This implies that different stabilization scales result in different types of DM. An essential role is played by the effective potential. On the one hand, its minima fix possible stabilization scales of the internal spaces; on the other hand, they provide possible values for the effective cosmological constant.

Günther, U

2000-01-01

367

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

368

Triplet excitons: Bringing dark states to light  

Science.gov (United States)

Semiconducting quantum dots have been used to harvest triplet excitons produced through singlet fission in organic semiconductors. These hybrid organic-inorganic materials may boost the efficiency of solar cells.

Bardeen, Christopher J.

2014-11-01

369

Excitons in boron nitride nanotubes: dimensionality effects  

CERN Document Server

We show that the optical absorption spectra of boron nitride (BN) nanotubes are dominated by strongly bound excitons. Our first-principles calculations indicate that the binding energy for the first and dominant excitonic peak depends sensitively on the dimensionality of the system, varying from 0.7 eV in bulk hexagonal BN via 2.1 eV in the single sheet of BN to more than 3 eV in the hypothetical (2,2) tube. The strongly localized nature of this exciton dictates the fast convergence of its binding energy with increasing tube diameter towards the sheet value. The absolute position of the first excitonic peak is almost independent of the tube radius and system dimensionality. This provides an explanation for the observed ``optical gap'' constancy for different tubes and bulk hBN [R. Arenal et al., to appear in Phys. Rev. Lett. (2005)].

Wirtz, L; Rubio, A; Wirtz, Ludger; Marini, Andrea; Rubio, Angel

2005-01-01

370

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

371

Cascade-exciton model of nuclear reactions  

International Nuclear Information System (INIS)

An approach combining essential features of the exciton and intranuclear cascade models is developed. The cascade-exciton model predictions for the energy spectra, angular distributions and double differential cross sections of nucleons and complex particles as well as for the excitation functions are analyzed at incident nucleon energies T0 < or approx. 100 MeV and in a large range of nuclear target masses. We discuss the relative role of different interaction mechanisms and their possible experimental identification. (orig.)

372

Exciton in type-II quantum dot  

International Nuclear Information System (INIS)

We study the quantum-size effect and the influence of the external magnetic field on the exciton ground state energy in the type-II InP quantum disk, lens and pyramid deposited on a wetting layer and embedded in a GaInP matrix. We show that the charge distribution over and below quantum dot and wetting layer induced by trapped exciton strongly depends on the quantum dot morphology and the strength of the magnetic field.

373

Exciton in type-II quantum dot  

Energy Technology Data Exchange (ETDEWEB)

We study the quantum-size effect and the influence of the external magnetic field on the exciton ground state energy in the type-II InP quantum disk, lens and pyramid deposited on a wetting layer and embedded in a GaInP matrix. We show that the charge distribution over and below quantum dot and wetting layer induced by trapped exciton strongly depends on the quantum dot morphology and the strength of the magnetic field.

Sierra-Ortega, J; Escorcia, R A [Universidad del Magdalena, A. A. 731, Santa Marta (Colombia); Mikhailov, I D, E-mail: jsierraortega@gmail.co [Universidad Industrial de Santander, A. A. 678, Bucaramanga (Colombia)

2009-05-01

374

Direct observation of dark excitons in micelle-wrapped single-wall carbon nanotubes.  

Science.gov (United States)

We have performed electroabsorption spectroscopy on micelle-wrapped single-wall carbon nanotubes. In semiconducting nanotubes, many oscillating structures composed of the increase and decrease of absorption are observed in the spectra in the region of the first and second absorption bands, E11 and E22. The spectral shape is reproduced mainly by the second-derivative curve of the absorption spectrum, which indicates the presence of nearly degenerate bright and dark excitonic states. PMID:18352747

Kishida, Hideo; Nagasawa, Yoshiaki; Imamura, Sadanobu; Nakamura, Arao

2008-03-01

375

Temperature Effect on Exciton Absorption of CuBr Nanocrystals in Potassium-Alumina-Borate Glass  

Directory of Open Access Journals (Sweden)

Full Text Available The paper describes the research of temperature effect of potassium-alumina-borate (PAB glass with CuBr nanocrystals, which was obtained in optical wavelength region for the first time. Temperature dependence of optical density at different wavelengths was examined. Melting and crystallization temperatures were evaluated for different nanocrystals sizes. Great changes of exciton absorption band influenced by temperatures below 100°C were recorded. Propositions for studied glass application as tunable filters in electro-optic circuit were suggested.

Babkina A.N.

2013-09-01

376

Temperature Effect on Exciton Absorption of CuBr Nanocrystals in Potassium-Alumina-Borate Glass  

OpenAIRE

The paper describes the research of temperature effect of potassium-alumina-borate (PAB) glass with CuBr nanocrystals, which was obtained in optical wavelength region for the first time. Temperature dependence of optical density at different wavelengths was examined. Melting and crystallization temperatures were evaluated for different nanocrystals sizes. Great changes of exciton absorption band influenced by temperatures below 100°C were recorded. Propositions for studied glass ...

Babkina A.N.; Shirshnev P.S.; Tsekhomsky V.A.; Nikonorov N.V.

2013-01-01

377

Exciton transition and electronic structure of PbMoO{sub 4} crystals studied by polarized light  

Energy Technology Data Exchange (ETDEWEB)

Polarized reflectivity spectra of PbMoO{sub 4} crystals have been measured using synchrotron radiation up to 20 eV. The optical constants for the crystallographic axes are derived by using a Kramers-Kronig analysis. It is found that the exciton band at 3.6 eV shows a doublet structure with distinct dichroism. X-ray photoemission spectroscopy (XPS) and the calculation of the electronic structure by a discrete variational X{alpha} method are also carried out. The calculation shows that the valence band and the conduction band are mainly composed of the O 2p and Mo 4d states, respectively, and the Pb state contributes appreciably to the top of the valence band and the bottom of the conduction band. The valence-band XPS spectrum of PbMoO{sub 4} is compared with that of PbWO{sub 4}, which reveals a remarkable difference between them. This difference reflects different magnitudes of hybridization of Mo 4d or W 5d state to the valence band. The exciton transition is explained in terms of the cationic Pb 6s{yields}6p excitation model taking into account the crystal-field splitting and the spin-orbit interaction of Pb 6p state. From a comparison of the doublet structure of the exciton band of PbMoO{sub 4} and PbWO{sub 4}, it is suggested that the electron-hole exchange interaction plays an important role for the exciton transitions in both materials. (Abstract Copyright [2010], Wiley Periodicals, Inc.)

Fujita, Masami [Japan Coast Guard Academy, Wakaba, Kure 737-8512 (Japan); Itoh, Minoru; Mitani, Hiroyuki [Department of Electrical and Electronic Engineering, Shinshu University, Nagano 380-8553 (Japan); Sangeeta; Tyagi, Mohit [Crystal Technology Laboratory, TPPED, Bhabha Atomic Research Center, Trombay, Munbai 400 085 (India)

2010-02-15

378

Femtosecond time-resolved photodissociation dynamics of methyl halide molecules on ultrathin gold films  

Directory of Open Access Journals (Sweden)

Full Text Available The photodissociation of small organic molecules, namely methyl iodide, methyl bromide, and methyl chloride, adsorbed on a metal surface was investigated in real time by means of femtosecond-laser pump–probe mass spectrometry. A weakly interacting gold surface was employed as substrate because the intact adsorption of the methyl halide molecules was desired prior to photoexcitation. The gold surface was prepared as an ultrathin film on Mo(100. The molecular adsorption behavior was characterized by coverage dependent temperature programmed desorption spectroscopy. Submonolayer preparations were irradiated with UV light of 266 nm wavelength and the subsequently emerging methyl fragments were probed by photoionization and mass spectrometric detection. A strong dependence of the excitation mechanism and the light-induced dynamics on the type of molecule was observed. Possible photoexcitation mechanisms included direct photoexcitation to the dissociative A-band of the methyl halide molecules as well as the attachment of surface-emitted electrons with transient negative ion formation and subsequent molecular fragmentation. Both reaction pathways were energetically possible in the case of methyl iodide, yet, no methyl fragments were observed. As a likely explanation, the rapid quenching of the excited states prior to fragmentation is proposed. This quenching mechanism could be prevented by modification of the gold surface through pre-adsorption of iodine atoms. In contrast, the A-band of methyl bromide was not energetically directly accessible through 266 nm excitation. Nevertheless, the one-photon-induced dissociation was observed in the case of methyl bromide. This was interpreted as being due to a considerable energetic down-shift of the electronic A-band states of methyl bromide by about 1.5 eV through interaction with the gold substrate. Finally, for methyl chloride no photofragmentation could be detected at all.

Mihai E. Vaida

2011-09-01

379

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

380

Color silver halide hologram production and mastering  

Science.gov (United States)

Color reflection holograms recorded with the Denisyuk geometry have been demonstrated by the recently formed HOLOS Corporation in New Hampshire. The Slavich red-green-blue (RGB) sensitized ultra-high resolution silver halide emulsion was used for the hologram recording. The employed laser wavelengths were 647 nm, 532 nm, and 476 nm, generated by an argon ion, a frequency doubled Nd:YAG, and a krypton ion laser, respectively. A beam combination mechanism with dichroic filters enabled a simultaneous RGB exposure, which made the color balance and overall exposure energy easy to control as well as simplifying the recording procedure. HOLOS has been producing limited edition color holograms in various sizes from 4' X 5' to 12' X 16'. A 30 foot long optical table and high power lasers will enable HOLOS to record color holograms up to the size of one meter square in the near future. Various approaches have been investigated in generating color hologram masters which have sufficiently high diffraction efficiency to contact copy the color images onto photopolymer materials. A specially designed test object including the 1931 CIE chromaticity diagram, a rainbow ribbon cable, pure yellow dots, and a cloisonne elephant was used for color recording experiments. In addition, the Macbeth Color Checker chart was used. Both colorimetric evaluation and scattering noise measurements were performed using the PR-650 Photo Research SpectraScan SpectraCalorimeter.

Bjelkhagen, Hans I.; Huang, Qiang

1997-04-01

381

Melt synthesis of inorganic nitrides and halides  

Science.gov (United States)

Novel halide chlorides Sr7Cl2H12, Sr 7Br2H12, and Ba2ClH3 were formed from sodium chloride and strontium metal in sodium metal melts at 900°C. Sr7Br2H12 crystallizes in the anti-Fe 12Zr2P7 structure type, and Sr7Cl 2H12 crystallizes in a slightly distorted variant of this structure. Ba2ClH3 crystallizes in a structure containing infinite two-dimensional sheets of edge-sharing Ba6Cl chlorine-centered octahedra. Single crystals of gallium nitride can be obtained by reaction of gallium metal and dinitrogen gas at 750°C. Small amounts of alkaline earth metals added to the reaction mixture promote the formation of large crystals. Polycrystalline GaN nucleates rapidly under these conditions on the surface of a stainless steel autoclave; nucleation is far less pronounced on tungsten surfaces. Trace concentrations of hydrogen greatly enhance GaN nucleation. Melts of magnesium metal with cerium, gadolinium, and lanthanum were exposed to ammonia and nitrogen gas at temperatures from 700°C to 900°C. Binary rare earth and magnesium nitrides and hydrides are formed, but no ternary compounds are seen. The absence of ternary compounds is discussed in the light of a thermodynamic model predicting ternary formation in these systems.

Molstad, Jay Clark

382

Structured alkali halides for medical applications  

Science.gov (United States)

Image plates based on storage phosphors are a major application of radiation defects in insulators. Storage phosphors absorb X-ray quanta creating trapped electron-hole pairs in the material. Optical stimulation of the electron causes recombination leading to light emission. Application of image plates requires an optimal compromise between resolution (represented by the modulation transfer function (MTF)) and sensitivity. In our paper we present a new solution of the problem of combining a high MTF with a high sensitivity by structuring the image plates in form of thin needles acting as light guides. This suppresses the lateral spread of light which is detrimental to resolution. As doped CsBr, e.g. CsBr:Ga [Physica Medica XV (1999) 301], can pose a good storage phosphor evaporated layers are of interest in computed radiography. Needle structured CsI:Tl is used as scintillator in direct radiography [IEEE Trans. Nucl. Sci. 45 (3) (1998)]. CsBr layers have been produced by evaporation in vacuum and in inert gas atmosphere varying pressure and temperature. The resulting structures are of fibrous or columnar nature being in good agreement with the zone model of Thornton [Ann. Rev. Mater. Sci. 7 (1977) 239]. A zone model for CsBr has been developed. Measurements on doped alkali halide image plates having needle structure show good MTF at high sensitivity making a significant progress in image plate technology.

Schmitt, B.; Fuchs, M.; Hell, E.; Knüpfer, W.; Hackenschmied, P.; Winnacker, A.

2002-05-01

383

Structured alkali halides for medical applications  

International Nuclear Information System (INIS)

Image plates based on storage phosphors are a major application of radiation defects in insulators. Storage phosphors absorb X-ray quanta creating trapped electron-hole pairs in the material. Optical stimulation of the electron causes recombination leading to light emission. Application of image plates requires an optimal compromise between resolution (represented by the modulation transfer function (MTF)) and sensitivity. In our paper we present a new solution of the problem of combining a high MTF with a high sensitivity by structuring the image plates in form of thin needles acting as light guides. This suppresses the lateral spread of light which is detrimental to resolution. As doped CsBr, e.g. CsBr:Ga [Physica Medica XV (1999) 301], can pose a good storage phosphor evaporated layers are of interest in computed radiography. Needle structured CsI:Tl is used as scintillator in direct radiography [IEEE Trans. Nucl. Sci. 45 (3) (1998)]. CsBr layers have been produced by evaporation in vacuum and in inert gas atmosphere varying pressure and temperature. The resulting structures are of fibrous or columnar nature being in good agreement with the zone model of Thornton [Ann. Rev. Mater. Sci. 7 (1977) 239]. A zone model for CsBr has been developed. Measurements on doped alkali halide image plates having needle structure show good MTF at high sensitivity making a significant progress in image plate technology

384

GaN nanotubes grown by halide vapor phase epitaxy  

International Nuclear Information System (INIS)

Full text: Wide-band gap GaN nanostructures such as quantum dots, nanorods, nanowires, nano columns and nanotubes have a strong potential within areas of biochemical sensing, nanofluidics, and optoelectronics. GaN nanotubes play a role of the building blocks for several applications such as solution-based transistors and highly sensitive nanotube molecular sensors. We have studied non-catalytic and Au-assisted growth of GaN nanotubes using halide vapor phase epitaxy (HVPE) technique. The growth was performed in the temperature range 480 degrees Celsius to 520 degrees Celsius using pure N2 as a carrier gas at atmospheric pressure. The nanotubes size, shape, density and the selectivity of growth have been studied depending on V/III ratio, growth temperature and substrate material. By increasing the GaCl partial pressure, the structure changed from dot-like to nanotubes. The nanotubes were about 1 ?m long with a diameter of typically 200 nm. In addition, it was observed that the nanostructures were spontaneously nucleated at droplets of Ga or, when using Au-coated Al2O3, on droplets of Au/Ga alloy. By varying the growth temperature, the inner diameter of the nanotubes could be controlled. A growth model is suggested, where the nanotubes are nucleated at droplets of Ga or an Au/Ga alloy. Our experimental results suggest that the approach with pre-patterned Au-coated Al2O3 substrates has the potential for fabrication of well-organized nanotubes with a high density. Nanostructures were characterized using electron microscopy methods and by low temperature time-resolved photoluminescence (TRPL). Studies were performed on samples with different wall thickness in the range of 35-75 nm. Two recombination processes with different dynamics contribute to the emission spectra of the GaN nanotubes. The photoluminescence peak shifts rapidly to the higher energy from 3.47 eV to 3.75 eV within a very short time of 30 ps. The origin of the emission having a short lifetime is related to the near band gap luminescence in GaN, while the higher energy luminescence with a relatively slow decay of 2 ns is associated with the interface underlayer formed on the substrate during the growth. (authors)

385

Electronic fine tuning of the structures of reduced rare-earth metal halides.  

Science.gov (United States)

Electronic band-structure calculations on Pr3I3Ru and Y3I3Ru have been performed in order to analyze the large structural differences found in these isoelectronic compounds. These constitute two structural extremes within the family of monoclinic Re3I3Ru phases (RE = rare-earth metal) that exhibit distortions ranging from one-dimensional double chains of trans-edge-sharing octahedra (bioctahedral chains, BOH) to one-dimensional chains of trans-edge-sharing square pyramidal units bonded base to base (bisquare pyramidal chains, BSP). The structure of La3I3Ru was established by single-crystal X-ray diffraction (monoclinic, P2(1)/m, Z = 4, a = 9.343(1) A, b = 4.3469(8) A, c = 12.496(3) A, beta = 93.42(2) degrees) and found to be isomorphous with the BOH Pr3I3Ru. It is determined that the structural variation in this RE3I3Z family of materials depends largely on the differences in orbital energies between the corresponding rare-earth metal and the interstitial. These bonding considerations can be generalized to account for structural variations in a variety of other rare-earth halides as well as several group 4 or 5 reduced metal halide cluster phases. PMID:11170548

Köckerling, M; Martin, J D

2001-01-15

386

Reference spectroscopic data for hydrogen halides. Part I: Construction and validation of the ro-vibrational dipole moment functions  

International Nuclear Information System (INIS)

Knowledge of the infrared transition moments of hydrogen halides, namely HF, HCl, HBr, and HI, is essential for atmospheric, astrophysical, and laser applications. Recently, a new polynomial empirical dipole moment function (DMF) for HCl has been constructed using an efficient approach that involves a direct fit of experimental ro-vibrational intensities [Li et al. J Quant Spectrosc Radiat Transfer 2011;112:1543–50]. In the present study, this method was extended to the use of Padé approximation representations of the DMF and applied to all four hydrogen halides. To carry out the fits, the best available experimental data were collected and critically evaluated. Combining dipole moment functions with the wavefunctions obtained from highly-accurate empirical potential energy curves, line intensities were computed numerically for numerous ro-vibrational bands, and compared with the experimental values and with intensities calculated using the most recent ab initio dipole moment functions. Results obtained in this work form basis for calculating intensities of spectral lines of hydrogen halides and their isotopologues in the HITRAN 2012 database. -- Highlights: ? New semi-empirical dipole moment functions (DMF) of HF, HCl, HBr and HI were derived. ? Fit employed experimental ro-vibrational intensities with their uncertainties. ? New DMFs can predict intensities over extensive range of internuclear distances

387

Exciton-Exciton Interactions in Tensile-Strained GaN  

Science.gov (United States)

We report on the four-wave mixing (FWM) spectroscopy of excitons in tensile-strained GaN films. Three excitonic resonances (A-, B-, and C-excitons) were clearly observed with energy separations of ?AB˜4.7 meV and ?BC˜14.5 meV. Owing to tensile strain, these values are much smaller than those of free-standing GaN, allowing the simultaneous excitations of A-B and B-C excitons pairs. The time evolutions of the FWM in different polarization configurations resolved the differences in the simultaneous excitonic transitions; the excitation of A-B excitons showed an oscillation with a critical ?-phase shift in the orthogonal polarization configuration as expected in the quantum beat, and the oscillation of B-C excitons pair is independent of the polarization configuration, indicating polarization interference. The polarization-dependent FWM spectra of A-B excitons revealed peaks associated with biexciton transitions with binding energies of EAA = 4.6 meV, EAB = 2.1 meV, and EBB = 1.7 meV.

Toda, Yasunori; Shigematsu, Kyohhei; Hasegawa, Akira; Adachi, Satoru

2013-08-01

388

Exciton quantum confinement in nanocones formed on a surface of CdZnTe solid solution by laser radiation  

OpenAIRE

The investigation of surface morphology using atomic force microscope has shown self-organizing of the nanocones on the surface of CdZnTe crystal after irradiation by strongly absorbed Nd:YAG laser irradiation at an intensity of 12.0?MW/cm2. The formation of nanocones is explained by the presence of a thermogradient effect in the semiconductor. The appearance of a new exciton band has been observed after irradiation by the laser which is explained by the exciton quantum confinement effect i...

Medvid, Artur; Litovchenko, Natalia; Mychko, Aleksandr; Naseka, Yuriy

2012-01-01

389

Exciton quantum confinement in nanocones formed on a surface of CdZnTe solid solution by laser radiation.  

Science.gov (United States)

The investigation of surface morphology using atomic force microscope has shown self-organizing of the nanocones on the surface of CdZnTe crystal after irradiation by strongly absorbed Nd:YAG laser irradiation at an intensity of 12.0?MW/cm2. The formation of nanocones is explained by the presence of a thermogradient effect in the semiconductor. The appearance of a new exciton band has been observed after irradiation by the laser which is explained by the exciton quantum confinement effect in nanocones. PMID:22992253

Medvid', Artur; Litovchenko, Natalia; Mychko, Aleksandr; Naseka, Yuriy

2012-01-01

390

Vitrification of IFR and MSBR halide salt reprocessing wastes  

International Nuclear Information System (INIS)

Both of the genuinely sustainable (breeder) nuclear fuel cycles (IFR - Integral Fast Reactor - and MSBR - Molten Salt Breeder Reactor -) studied by the USA's national laboratories would generate high level reprocessing waste (HLRW) streams consisting of a relatively small amount ( about 4 mole %) of fission product halide (chloride or fluoride) salts in a matrix comprised primarily (about 95 mole %) of non radioactive alkali metal halide salts. Because leach resistant glasses cannot accommodate much of any of the halides, most of the treatment scenarios previously envisioned for such HLRW have assumed a monolithic waste form comprised of a synthetic analog of an insoluble crystalline halide mineral. In practice, this translates to making a 'substituted' sodalite ('Ceramic Waste Form') of the IFR's chloride salt-based wastes and fluoroapatite of the MSBR's fluoride salt-based wastes. This paper discusses my experimental studies of an alternative waste management scenario for both fuel cycles that would separate/recycle the waste's halide and immobilize everything else in iron phosphate (Fe-P) glass. It will describe both how the work was done and what its results indicate about how a treatment process for both of those wastes should be implemented (fluoride and chloride behave differently). In either case, this scenario's primary advantages include much higher waste loadings, much lower overall cost, and the generation of a product (glass) that is more consistent with current waste management practices. (author)

391

Process for oxidation of hydrogen halides to elemental halogens  

Science.gov (United States)

An improved process for generating an elemental halogen selected from chlorine, bromine or iodine, from a corresponding hydrogen halide by absorbing a molten salt mixture, which includes sulfur, alkali metals and oxygen with a sulfur to metal molar ratio between 0.9 and 1.1 and includes a dissolved oxygen compound capable of reacting with hydrogen halide to produce elemental halogen, into a porous, relatively inert substrate to produce a substrate-supported salt mixture. Thereafter, the substrate-supported salt mixture is contacted (stage 1) with a hydrogen halide while maintaining the substrate-supported salt mixture during the contacting at an elevated temperature sufficient to sustain a reaction between the oxygen compound and the hydrogen halide to produce a gaseous elemental halogen product. This is followed by purging the substrate-supported salt mixture with steam (stage 2) thereby recovering any unreacted hydrogen halide and additional elemental halogen for recycle to stage 1. The dissolved oxygen compound is regenerated in a high temperature (stage 3) and an optical intermediate temperature stage (stage 4) by contacting the substrate-supported salt mixture with a gas containing oxygen whereby the dissolved oxygen compound in the substrate-supported salt mixture is regenerated by being oxidized to a higher valence state.

Lyke, Stephen E. (Middleton, WI)

1992-01-01

392

Design and construction of a copper-halide laser  

International Nuclear Information System (INIS)

The considerations necessary for the design and construction of an inexpensive, reliable copper halide laser are presented. A broad range of parameters which govern the output of the double-pulse copper-halide laser was studied. The laser has an optimum operating temperature which is different for each halide uCl, CuBr, CuI), but corresponds to a halide vapor pressure of about 0.1 torr for all three halides. At low repetition rates the highest laser output is achieved with helium as buffer gas. The laser output increases linearly with laser tube cross-section. As the tube diameter is increased, the dissociation energy must be increased, but not the excitation energy. A thyratron bias circuit has been developed which permits both dissociation and excitation pulses to be obtained from the same capacitor (with a small delay) without thyratron latch. Use of the single thyratron reduces the laser cost considerably. A laser tube with an operating life of many hundreds of hours has been designed. The description of a laser with an output of 0.5 mJ per pulse at repetition rates of up to 120 pulses per second is given. The design and construction of various components of this laser are given in detail. (authors)

393

Vitrification of IFR and MSBR halide salt reprocessing wastes  

Energy Technology Data Exchange (ETDEWEB)

Both of the genuinely sustainable (breeder) nuclear fuel cycles (IFR - Integral Fast Reactor - and MSBR - Molten Salt Breeder Reactor -) studied by the USA's national laboratories would generate high level reprocessing waste (HLRW) streams consisting of a relatively small amount ( about 4 mole %) of fission product halide (chloride or fluoride) salts in a matrix comprised primarily (about 95 mole %) of non radioactive alkali metal halide salts. Because leach resistant glasses cannot accommodate much of any of the halides, most of the treatment scenarios previously envisioned for such HLRW have assumed a monolithic waste form comprised of a synthetic analog of an insoluble crystalline halide mineral. In practice, this translates to making a 'substituted' sodalite ('Ceramic Waste Form') of the IFR's chloride salt-based wastes and fluoroapatite of the MSBR's fluoride salt-based wastes. This paper discusses my experimental studies of an alternative waste management scenario for both fuel cycles that would separate/recycle the waste's halide and immobilize everything else in iron phosphate (Fe-P) glass. It will describe both how the work was done and what its results indicate about how a treatment process for both of those wastes should be implemented (fluoride and chloride behave differently). In either case, this scenario's primary advantages include much higher waste loadings, much lower overall cost, and the generation of a product (glass) that is more consistent with current waste management practices. (author)

Siemer, D.D. [Idaho National Laboratory, 12N 3167E, Idaho Falls, ID 83402 (United States)

2013-07-01

394

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

395

Double strong exciton-plasmon coupling in gold nanoshells infiltrated with fluorophores  

Energy Technology Data Exchange (ETDEWEB)

We report on the broadband resonant energy transfer processes observed in dye doped gold nanoshells, consisting of spherical particles with a dielectric core (SiO{sub 2}) covered by a thin gold shell. The silica core has been doped with rhodamine B molecules in order to harness a coherent plasmon-exciton coupling between chromophores and plasmonic shell. This plasmon-exciton interplay depends on the relative spectral position of their bands. Here, we present a simultaneous double strong coupling plasmon-exciton and exciton-plasmon. Indeed, experimental observations reveal of a transmittance enhancement as function of the gain in a wide range of optical wavelengths (about 100?nm), while scattering cross sections remains almost unmodified. These results are accompanied by an overall reduction of chromophore fluorescence lifetimes that are a clear evidence of nonradiative energy transfer processes. The increasing of transmission in the range of 630–750?nm is associated with a striking enhancement of the extinction cross-section in the 510–630?nm spectral region. In this range, the system assumes super-absorbing features. This double behavior, as well as the broadband response of the presented system, represents a promising step to enable a wide range of electromagnetic properties and fascinating applications of plasmonic nanoshells as building blocks for advanced optical materials.

De Luca, A., E-mail: antonio.deluca@fis.unical.it; Dhama, R.; Rashed, A. R. [CNR-IPCF UOS Cosenza, Licryl Laboratory, Department of Physics, University of Calabria, Via P. Bucci, 87036 Rende (Italy); Coutant, C.; Ravaine, S.; Barois, P. [CRPP, Centre de Recherche Paul Pascal, CNRS and University of Bordeaux, 115 Avenue Schweitzer, 33600 Pessac (France); Infusino, M. [Department of Physics, Case Western Reserve University, 10600 Euclid Avenue, Cleveland, Ohio 44106-7079 (United States); CNR-IPCF UOS Cosenza, Licryl Laboratory, Department of Physics, University of Calabria, Via P. Bucci, 87036 Rende (Italy); Strangi, G., E-mail: gxs284@case.edu [Department of Physics, Case Western Reserve University, 10600 Euclid Avenue, Cleveland, Ohio 44106-7079 (United States)

2014-03-10

396

Radiative lifetime of barbell excitons in semiparabolic double quantum wells under intense laser fields  

International Nuclear Information System (INIS)

The binding energy and optical properties of barbell excitons in GaAs–Ga1?xAlxAs semiparabolic double quantum wells under intense laser fields are investigated. Calculations are performed within the effective mass and envelope-function approximations, including the conduction band nonparabolicity. The dependence of the binding energy, oscillator strength and exciton absorption spectrum on the laser field in symmetric and asymmetric quantum wells is studied by using a finite difference method. It is shown that the exciton radiative lifetime can be tuned to a large extent by a proper choice of the structure design (double well size, middle barrier position and its thickness) as well as by varying the laser field intensity. -- Highlights: • Laser and heterostructure asymmetry are useful tools to tune the excitonic binding energy. • Radiative lifetime has an oscillatory behavior as the laser intensity grows. • Interwell coupling can be changed via the central barrier width and laser intensity. • Peak positions of the absorption coefficient are blue shifted by the laser field

397

Excitonic states in GaAs quantum dots fabricated by local droplet etching  

Science.gov (United States)

The influence of dot size and shape on the excitonic structure of GaAs/AlGaAs quantum dots (QDs) is studied experimentally and theoretically. Almost strain-free GaAs QDs are fabricated by epitaxially filling of nanoholes in an AlGaAs surface. The nanoholes are formed in a self-assembling fashion by local droplet etching. As an important point, the size of the QDs can be adjusted by the hole filling level. As a consequence, the exciton recombination energy can be controllably varied over a range of 130 meV by the dot size. We present micro- and macro-photoluminescence measurements of the s-shell recombinations. With the hole shape determined using atomic force microscopy and the amount of GaAs filled into the holes, we have quite precise input parameters for a simulation of the excitonic states in the QDs. The measured exciton and biexciton recombination energies are well reproduced by calculations based on eight band k .p theory and configuration interaction scheme with a deviation of less than 7 meV.

Graf, A.; Sonnenberg, D.; Paulava, V.; Schliwa, A.; Heyn, Ch.; Hansen, W.

2014-03-01

398

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

399

Chemistry of gaseous lower valent actinide halides. Final technical report, August 1979-August 1985  

International Nuclear Information System (INIS)

The objective of this program was to provide accurate thermochemical information for key actinide-halide and oxyhalide systems, starting with uranium halides, so that the basic factors underlying the chemical bonding and chemical reactivity in these systems can be elucidated in a systematic way. Our principal focus was on the gaseous halides, mainly the lower valent halides, because these molecular species largely will define the high temperature chemistry in the nuclear applications of interest. 6 refs., 2 tabs

400

Different temperature dependence of excitonic and defect-related photoluminescence spectra in ZnS nanobelts and nanowires  

International Nuclear Information System (INIS)

In this paper, both excitonic and defect-related information of ZnS nanobelts and nanowires have been investigated by a temperature-dependent photoluminescence (PL) spectrum. PL spectra of ZnS nanobelts and nanowires differ significantly in the ultraviolet (UV) and visible emission regions. In UV emission regions, due to high-quality crystals, free exciton B (FXB), free exciton A (FXA), FXA-one longitudinal optical (LO) phonon replica are observed in ZnS nanobelts, as well as free-to-bound (e, A) with its one LO phonon replica, while neutral-donor bound exciton (Do, X) and free-to-bound (e, A) are observed in ZnS nanowires at 10 K. The peak and relative intensity of the FX and (Do, X) versus temperature follow well with conventional empirical relations. In the visible emission regions, weak donor-acceptor pair (DAP) and self-activated (SA) emission from ZnS nanowires are commonly observed, but the Y band emission is only observed at 10 K in ZnS nanobelts. The Y band emission disappears at some temperature lower than 50 K. The peak position and full width at half maximum of DAP and SA emission bands display different temperature dependences. Detailed study on temperature-dependent PL spectra of ZnS nanobelts and nanowires provides crucial information on the nature of the electronic states and recombination mechanisms in these nanostructures. (paper)

401

Measurement of high exciton binding energy in the monolayer transition-metal dichalcogenides WS2 and WSe2  

Science.gov (United States)

Monolayer transition-metal dichalcogenides are direct gap semiconductors with great promise for optoelectronic devices. Although spatial correlation of electrons and holes plays a key role, there is little experimental information on such fundamental properties as exciton binding energies and band gaps. We report here an experimental determination of exciton excited states and binding energies for monolayer WS2 and WSe2. We observe peaks in the optical reflectivity/absorption spectra corresponding to the ground- and excited-state excitons (1s and 2s states). From these features, we determine lower bounds free of any model assumptions for the exciton binding energies as E2sA-E1sA of 0.83 eV and 0.79 eV for WS2 and WSe2, respectively, and for the corresponding band gaps Eg?E2sA of 2.90 and 2.53 eV at 4 K. Because the binding energies are large, the true band gap is substantially higher than the dominant spectral feature commonly observed with photoluminescence. This information is critical for emerging applications, and provides new insight into these novel monolayer semiconductors.

Hanbicki, A. T.; Currie, M.; Kioseoglou, G.; Friedman, A. L.; Jonker, B. T.

2015-02-01

402

Stacking in colloidal nanoplatelets: tuning excitonic properties.  

Science.gov (United States)

Colloidal semiconductor quantum wells, also commonly known as nanoplatelets (NPLs), have arisen among the most promising materials for light generation and harvesting applications. Recently, NPLs have been found to assemble in stacks. However, their emerging characteristics essential to these applications have not been previously controlled or understood. In this report, we systematically investigate and present excitonic properties of controlled column-like NPL assemblies. Here, by a controlled gradual process, we show that stacking in colloidal quantum wells substantially increases exciton transfer and trapping. As NPLs form into stacks, surprisingly we find an order of magnitude decrease in their photoluminescence quantum yield, while the transient fluorescence decay is considerably accelerated. These observations are corroborated by ultraefficient Förster resonance energy transfer (FRET) in the stacked NPLs, in which exciton migration is estimated to be in the ultralong range (>100 nm). Homo-FRET (i.e., FRET among the same emitters) is found to be ultraefficient, reaching levels as high as 99.9% at room temperature owing to the close-packed collinear orientation of the NPLs along with their large extinction coefficient and small Stokes shift, resulting in a large Förster radius of ?13.5 nm. Consequently, the strong and long-range homo-FRET boosts exciton trapping in nonemissive NPLs, acting as exciton sink centers, quenching photoluminescence from the stacked NPLs due to rapid nonradiative recombination of the trapped excitons. The rate-equation-based model, which considers the exciton transfer and the radiative and nonradiative recombination within the stacks, shows an excellent match with the experimental data. These results show the critical significance of stacking control in NPL solids, which exhibit completely different signatures of homo-FRET as compared to that in colloidal nanocrystals due to the absence of inhomogeneous broadening. PMID:25469555

Guzelturk, Burak; Erdem, Onur; Olutas, Murat; Kelestemur, Yusuf; Demir, Hilmi Volkan

2014-12-23

403

High Biomass Specific Methyl Halide Production Rates of Selected Coastal Marsh Plants and its Relationship to Halide Content  

Science.gov (United States)

Salt tolerant coastal marsh plants (halophytes) have previously been shown to be globally significant producers of methyl chloride (MeCl) and methyl bromide (MeBr). While halophytes are known for their high salt content, there are few reports of their halide content. Our studies have attempted to quantify biomass specific methyl halide (MeX) production from these plants and relate it to tissue halide levels. MeCl, MeBr and MeI production rates and tissue chloride, bromide and iodide concentrations from selected coastal marsh plants were measured for nearly a year. Certain halophyte species (i.e. Batis and Frankenia) have very high summer biomass specific production rates for MeX (e.g. Frankenia: 1 ug MeCl /gfwt/hr; 80 ng MeBr/gfwt/hr; 8 ng MeI/gfwt/hr). These rates of MeCl and MeBr production are much higher than those from other coastal marsh plants or seaweeds. Plant halide levels remain high throughout the year, while MeX production peaks at a high level in mid summer falling to low winter rates. This implies a linkage to plant growth. Higher levels of chloride and bromide were seen in the fleshy marsh plants such as Batis (saltwort, approximately 20 percent dry wt chloride, 0.4 percent dry wt bromide) and Salicornia (pickleweed) than in the others such as Frankenia (alkali heath) approx 7 percent dry wt chloride, 0.1 percent dry wt bromide) or Spartina (cordgrass). No such trend was seen for iodide, which ranged from 4 - 10 ppm. Calculations show the daily halide losses from MeX production are far less than the variability in tissue halide content. MeX production removes a small fraction of the total tissue halide from these plants suggesting that MeX production is not a mechanism used by these species to control internal halide levels. Saltwort cell-free extracts incubated with bromide or iodide in the presence of S-adenosyl-L-methionine (SAM) produced the corresponding MeX. MeBr production was inhibited by caffeic acid the substrate of lignin-specific O-methyl transferase. MeX production may be a result of non-specific methylation of halides by SAM utilizing methyl transferases used in other biosynthetic pathways.

Manley, S. L.; Wang, N.; Cicerone, R. J.

2002-12-01

404

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

405

Hybridized exciton-plasmon polaritons in nanostructured silver films  

Science.gov (United States)

We report on the resonant coupling between localized surface plasmon resonances (LSPRs) in nanostructured Ag films, and an adsorbed monolayer of Rhodamine 6G dye. Hybridization of the plasmons and molecular excitons creates new coupled polaritonic modes, which have been tuned by varying the LSPR wavelength. The resulting polariton dispersion curve shows an anticrossing behavior which is very well fit by a simple coupled-oscillator Hamiltonian, giving a giant Rabi-splitting energy of ~400 meV. The strength of this coupling is shown to be proportional to the square root of the molecular density. The Raman spectra of R6G on these films show an enhancement of many orders of magnitude due to surface enhanced scattering mechanisms; we find a maximum signal when a polariton mode lies in the middle of the Stokes shifted emission band.

Cade, Nic; Ritman-Meer, Tom; Richards, David

2010-05-01

406

Dynamics of indirect exciton transport by moving acoustic fields  

Science.gov (United States)

We report on the modulation of indirect excitons (IXs) as well as their transport by moving periodic potentials produced by surface acoustic waves (SAWs). The potential modulation induced by the SAW strain modifies both the band gap and the electrostatic field in the quantum wells confining the IXs, leading to changes in their energy. In addition, this potential captures and transports IXs over several hundreds of ?m. While the IX packets keep to a great extent their spatial shape during transport by the moving potential, the effective transport velocity is lower than the SAW group velocity and increases with the SAW amplitude. This behavior is attributed to the capture of IXs by traps along the transport path, thereby increasing the IX transit time. The experimental results are well-reproduced by an analytical model for the interaction between trapping centers and IXs during transport.

Violante, A.; Cohen, K.; Lazi?, S.; Hey, R.; Rapaport, R.; Santos, P. V.

2014-03-01

407

Cu halide nanoparticle formation by diffusion of copper in alkali halide crystals  

Directory of Open Access Journals (Sweden)

Full Text Available Atomos de cobre han sido introducidos por difusión en cristales de NaCl, KCl y KBr a 500±C. Los cristales han sido analizados óptimamente con medidas de fotoluminiscencia y por microscopía electrónica de barrido. Los espectros de emisión y excitación, medidos a baja temperatura muestran el efecto de confinamiento de exciton, indicando la formación de nanopartículas de CuX (X=Cl, Br, lo cual ha sido confirmado por imágenes de microscopía electrónica. Este método es propuesto como un método alternativo para obtener nanopartículas de CuX en cristales halogenuros alcalinos.

A. P\\u00E9rez-Rodr\\u00EDguez

2006-01-01

408

Photoluminescence and exciton resonances over the scattered light in multiphonon spectra of resonant scattering in the CdTe/ZnTe superlattices with narrow quantum wells  

Science.gov (United States)

Photoluminescence and Raman scattering spectra in CdTe/ZnTe heterostructures and superlattices with narrow quantum wells (4.8-9.2 Å) in a temperature range of 5-300 K have been measured. The temperature dependences of the intensity of exciton luminescence in isolated quantum wells have been studied, and the thermal activation energies associated with the effective barriers for electrons and holes have been determined. In CdTe/ZnTe heterostructures, the binding energies of an exciton with a heavy hole have been determined as functions of the quantum well width. The multiphonon Raman spectra that exhibit distinctive features, such as the weak intensity of nLO phonon lines of ZnTe ( n 2), and the multiple participation in scattering of acoustic LA phonons with large wave vector, have been investigated. The results have been explained based on the concept of the relaxation of hot excitons over the exciton band.

Zavaritskaya, T. N.; Kucherenko, I. V.; Mel'nik, N. N.; Karczewski, G.; Skorikov, M. L.

2013-11-01

409

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

410

Chemistry of gaseous lower-valent actinide halides  

International Nuclear Information System (INIS)

The objective of this program is to provide accurate thermochemical information for the uranium and thorium halide and oxyhalide systems, two of the key actinide systems, so that the basic factors underlying the chemical bonding and chemical reactivity in these systems can be elucidated in a systematic way. A complete characterization of the uranium and thorium systems will serve as a reliable base for modeling the behavior of the heavier actinides, which will be much more difficult to study because of sample size and safety restrictions. Our principal focus is on the the gaseous halides, mainly the lower-valent halides, because these molecular species largely will define the high-temperature chemistry in the nuclear applications of interest. 2 refs., 1 tab

411

Ultrafast Exciton Formation at the ZnO(101¯0) Surface  

Science.gov (United States)

We study the ultrafast quasiparticle dynamics in and below the ZnO conduction band using femtosecond time-resolved two-photon photoelectron spectroscopy. Above band gap excitation causes hot electron relaxation by electron-phonon scattering down to the Fermi level EF followed by ultrafast (200 fs) formation of a surface exciton (SX). Transient screening of the Coulomb interaction reduces the SX formation probability at high excitation densities near the Mott limit. Located just below the surface, the SX are stable with regard to hydrogen-induced work function modifications and thus the ideal prerequisite for resonant energy transfer applications.

Deinert, J.-C.; Wegkamp, D.; Meyer, M.; Richter, C.; Wolf, M.; Stähler, J.

2014-08-01

412

Stimulated emission and excitonic bleaching in GaN epilayers under high-density excitation  

OpenAIRE

Measurements of edge emission on a GaN epilayer under N-2 laser excitation demonstrate stimulated emission and gain. The mechanisms involved are investigated by using laser pulses of 250 fs duration to measure simultaneously the photoluminescence and the time-resolved reflectance near the band edge, over a range of excitation densities. The excitons are bleached at high densities due to free carriers. A broad luminescence band is seen, extending similar to 100 meV below the low-density emissi...

Taylor, Ra; Hess, S.; Kyhm, K.; Smith, J.; Ryan, Jf; Yablonskii, Gp; Lutsenko, Ev; Pavlovskii, Vn; Heuken, M.

1999-01-01

413

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

414

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

415

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

416

Assignment of the optical absorption bands in CsI:Tl and CsBr:Tl by application of high pressure  

International Nuclear Information System (INIS)

The assignment of the absorption bands in cesium halide thallous phosphors (CsI:Tl and CsBr:Tl) is performed by making use of the phase transition from NaCl-Type to CsCl-type under high pressures. It is concluded that the longest wavelength band corresponds to the A band in alkali halide thallous system and the origin of the next wavelength one is the A-like transition, that is, the transition to the excited state similar to that in the case of the A band which has two relaxed excited states. (author)

417

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)

418

Far IR spectra of Th(IV) halide complexes of some heterocyclic bases  

International Nuclear Information System (INIS)

The synthesis and IR spectra of Th(IV) perchlorato, nitrato and thiocyanato complexes of some heterocyclic bases have been reported. Halogens are common ligands in coordination chemistry forming coordinate bonds with metals readily. Metal halogen (M-X) stretching bands show a strong absorption in the far-IR region. Very little information is available on Th-X stretching frequencies. In the present communication, adducts of Th(IV) halide with certain nitrogen heterocyclic bases such as pyridine, ?-picoline, 2-amino pyridine, 2:4-lutidine, 2:6-lutidine, quinoline, 2,2'-bipyridine and 1,10-phenanthroline were synthesised and characterised. Experimental details are given. Results are presented and discussed. (author)

419

An optical criterion to obtain miscible mixed crystals in alkali halides  

Scientific Electronic Library Online (English)

Full Text Available SciELO Mexico | Language: English Abstract in spanish Este trabajo ofrece un nuevo criterio para predecir la formación de soluciones sólidas en halogenuros alcalinos cristalinos y discute algunos resultados obtenidos en el desarrollo de mezclas dieléctricas cristalinas miscibles de halogenuros alcalinos ternarias y cuaternarias. Estas mezclas son misci [...] bles en cualquier concentración de sus componentes. Tener el resultado de estas mezclas cristalinas está relacionado al centro F a través del comportamiento observado en la banda F de absorción en función de la constante de red de los halogenuros alcalinos donde el defecto fue formado (centro F). Dando un vistazo a la gráfica de Energía de banda F versus constante de red (ley de Mollwo-Ivey), se observa un conjunto de puntos, que dan la pauta (tal como KCl, KBr, RbCl), de posibles mezclas de materiales correspondientes a puntos adyacentes y una solución sólida podría formarse, significando un cristal de una sola fase, que dan por resultado cristales ternarios y cuaternarios. Así, la banda F de absorción nos permite tener un criterio numérico, basado en el porcentaje de cambio de la energía de la banda F que permite obtener soluciones sólidas. Encontramos información experimental, usando difractogramas de las mezclas cristalinas, se obtienen las constantes de red y se comparan con la obtenida teóricamente a través de la generalización de la Regla de Vegard, finalmente se discute la posibilidad de crecer cristales partiendo de cinco componentes, tomando cinco puntos consecutivos en la grafica de la Ley de Mollwo-Ivey. Abstract in english This work gives a novel criterion to predict the formation of alkali halide solid solutions and discusses some results obtained in the development of ternary and quaternary miscible crystalline dielectric mixtures of alkali halides. These mixtures are miscible in any concentration of their component [...] s. The miscibility of these mixed crystals is quite related to the F center through the behavior observed in the spectral position of the optical absorption F band as a function of the lattice constant of the alkali halide where the F center was formed. By inspection of an energy graph of the F band energy versus lattice constant (Mollwo-Ivey law), a set of points is observed corresponding to several pure alkali halides (such as KCl, KBr, RbCl), which gives a notion of possible mixed materials that would correspond to adjacent points and a solid solution could be obtained, meaning a single phase crystal, which result in ternary and quaternary mixed crystals. Thus, the optical absorption F band allows have a numerical criterion, based on the percentage respective of the F band energy, in order to predict possible solid solutions. We obtained experimental information using diffractograms of the mixed crystals, from which the lattice constant was obtained and compared with a theoretical calculus using a generalization of Vegard's law, finally it is discussed the case of a crystal growing, starting from five components, picking up five consecutive dots from the graph of Mollwo-Ivey's law.

R, Rodríguez-Mijángos; G, Vázquez-Polo; J.J., Palafox; R, Pérez-Salas.

2008-11-01

420

Ground-state properties and optical excitations of a solvated electron in molten alkali halides  

International Nuclear Information System (INIS)

Properties of solvated electrons at high dilution in four molten alkali halides are investigated theoretically. A self-consistent evaluation of the bound ground state of the electron and of the surrounding liquid structure, already developed in a previous paper, is compared with the results of NMR hyperfine shifts and magnetic susceptibility measurements. The absorption bands associated with Franck-Condon 1s?2p and 1s?3p transitions are calculated and compared with the available data of optical absorption, with special emphasis on the detailed analysis by Yuh and Nachtrieb for molten CsCl. An instability of the excited states against ionic relaxation and the origin of a finite lifetime for the ground state are also discussed. Finally, the perturbation induced by the solvated electron in the fundamental absorption of the molten salt is estimated. (author)

421

Exciton decay dynamics controlled by impurity occupation in strongly Mn-doped and partially compensated bulk GaAs  

Science.gov (United States)

We report on a pronounced prolongation of the exciton decay in strongly p-doped and partially compensated direct band-gap semiconductor GaAs:Mn with increasing optical excitation power. Using time-resolved photoluminescence we show that the intricate interplay of excitons, shallow and deep impurity centers in GaAs:Mn results in a complex recombination behavior that cannot be characterized in terms of simple rates. The decay can be precisely described by a model based on Shockley-Read-Hall recombination, which shows that the observed dynamics arise from a varying neutralization of shallow and deep recombination centers. This enables the investigation of the carrier dynamics in the impurity system by measuring only the exciton decay time.

Münzhuber, F.; Henn, T.; Kiessling, T.; Ossau, W.; Molenkamp, L. W.; Gieseking, B.; Astakhov, G. V.; Dyakonov, V.

2014-09-01

422

Influence of excitonic oscillator strengths on the optical properties of GaN and ZnO  

Science.gov (United States)

We report on an extensive study of the excitonic properties of GaN and ZnO bulk samples with an accurate determination of excitonic parameters by linear and nonlinear spectroscopies. The in-depth comparative study is carried out between these two competitive wide band gap semiconductors for a better understanding of damping processes. In GaN, it is shown that due to microscopic disorder, such as lattice strain fluctuations, inhomogeneous broadening prevails over homogeneous broadening at low temperature. The opposite situation occurs in ZnO, where the homogeneous broadening dominates due to resonant Rayleigh scattering of exciton polaritons and their interaction with acoustic phonons. This comparative study also allows us to highlight the influence of oscillator strengths on spectrally resolved four-wave mixing and time-integrated four-wave mixing.

Mallet, E.; Réveret, F.; Disseix, P.; Shubina, T. V.; Leymarie, J.

2014-07-01

423

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.

424

Magnetic field tuning of exciton polaritons in a semiconductor microcavity  

OpenAIRE

We detail the influence of a magnetic field on exciton-polaritons inside a semiconductor microcavity. Magnetic field can be used as a tuning parameter for exciton and photon resonances. We discuss the change of the exciton energy, the oscillator strength and redistribution of the polariton density along the dispersion curves due to the magnetically-induced detuning. We have observed that field-induced shrinkage of the exciton wave function has a direct influence not only on ...

Pietka, B.; Zygmunt, D.; Krol, M.; Szczytko, J.; Lusakowski, J.; Molas, M. R.; Nicolet, A. A. L.; Stepnicki, P.; Zieba, P.; Tralle, I.; Morier-genoud, F.; Matuszewski, M.; Potemski, M.; Deveaud, B.

2014-01-01

425

Exciton-Phonon Dynamics with Long-Range Interaction  

OpenAIRE

Exciton-phonon dynamics on a 1D lattice with long-range exciton-exciton interaction have been introduced and elaborated. Long-range interaction leads to a nonlocal integral term in the motion equation of the exciton subsystem if we go from discrete to continuous space. In some particular cases for power-law interaction, the integral term can be expressed through a fractional order spatial derivative. A system of two coupled equations has been obtained, one is a fractional di...

Laskin, Nick

2011-01-01

426

Dynamics of dissipative multiple exciton generation in nanocrystals  

OpenAIRE

The population dynamics of single- and bi-exciton states in semiconductor nanocrystals is modeled numerically in the presence of Coulomb coupling between single- and two-exciton states and a dissipation channel in order to study the transient bi-exciton population that occurs in an optically excited semiconductor nanocrystal. The results show that the system evolution strongly changes if the dissipation is included. In a certain range of parameters, the growth of the exciton...

Azizi, Maryam; Machnikowski, Pawe?

2013-01-01

427

Thermodynamic reactivity, growth and characterization of mercurous halide crystals  

Science.gov (United States)

Thermodynamic calculations were carried out for the Hg-X-O system (X = Cl, Br, I) to identify the potential sources of contamination and relative stability of oxides and oxy-halide phases. The effect of excess mercury vapor pressure on the optical quality of mercurous halide crystal was studied by growing several mercurous chloride crystals from mercury-rich composition. The optical quality of crystals was examined by birefringence interferometry and laser scattering studies. Crystals grown in slightly mercury-rich composition showed improved optical quality relative to stoichiometric crystals.

Singh, N. B.; Gottlieb, M.; Henningsen, T.; Hopkins, R. H.; Mazelsky, R.; Singh, M.; Glicksman, M. E.; Paradies, C.

1992-01-01

428

Oxoacidity scales of ionic solvents based on alkali metal halides  

International Nuclear Information System (INIS)

The relative acidity scales of the ionic melts on the basis of the halides of the alkali metals MIX (MI = Li-Cs; X = Cl-I) and alkaline earth metals MIIX2 (MII = Ca-Ba; X = Cl) are plotted with application of the method of the oxides solubility in the halides melts at the temperature of 600, 700 and 800 deg C. The regularities of the effect of the solvent composition on its acidic-basic properties and also on the acidic-basic reactions on its background are identified

429

Axial cataphoresis effects in continuously pulsed copper halide lasers  

Energy Technology Data Exchange (ETDEWEB)

It has been observed in a continuously pulsed copper halide laser that laser performance deteriorates rapidly due to axial cataphoresis effects. Dissociated copper ions are pumped cataphoretically toward the cathode by the applied electric field, causing nonuniform laser discharges and preferential copper condensation near the cathode. These effects can be eliminated experimentally by alternating the polarity of the applied voltage either mechanically or electronically. This improvement permits long-lived copper halide laser operating at average power levels in the 1--10-W range. (AIP)

Liu, C.S.; Feldman, D.W.; Pack, J.L.; Weaver, L.A.

1977-01-01

430

Reactivity of halide and pseudohalide ligands in transition metal complexes  

International Nuclear Information System (INIS)

The review generalizes experimental material on reactions of coordinated halide ligands, as well as cyanide, azide, thiocyanate and cyanate ligands in transition metal (Mo, W, Ru, etc.) complexes. It is shown that transformation of the intrasphere pseudohalide ligands is a very important method of directed synthesis of coordination compounds. The tendency of halide and pseudohalide ions to the formation of low-soluble salts or strong complexes can be widely used in preparation coordination chemistry for the ''forced'' introduction of solvent molecules into the complexes

431

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

International Nuclear Information System (INIS)

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

432

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

Energy Technology Data Exchange (ETDEWEB)

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; Rusydi, Andrivo, E-mail: phyandri@nus.edu.sg [Singapore Synchrotron Light Source, National University of Singapore, 5 Research Link, Singapore 117603 (Singapore); Department of Physics, National University of Singapore, 2 Science Drive 3, Singapore 117542 (Singapore); NUSNNI-NanoCore, National University of Singapore, 2 Science Drive 3, Singapore 117542 (Singapore); Department of Physics, Institut Teknologi Bandung, Ganesa 10, Bandung 40132 (Indonesia); Seng Herng, Tun [Singapore Synchrotron Light Source, National University of Singapore, 5 Research Link, Singapore 117603 (Singapore); Department of Material Science and Engineering, National University of Singapore, 7 Engineering Drive 1, Singapore 117574 (Singapore); Marlina, Resti; Fauziah, Resti [Department of Physics, Institut Teknologi Bandung, Ganesa 10, Bandung 40132 (Indonesia); Ding, Jun [Department of Material Science and Engineering, National University of Singapore, 7 Engineering Drive 1, Singapore 117574 (Singapore)

2014-02-24

433

Resonance effects of excitons and electrons. Basics and applications  

Energy Technology Data Exchange (ETDEWEB)

Represents the first book on non-traditional resonance effects of excitons in semiconductors. Explains resonance phenomena of excitons and electrons in solids. Presents the Knight shift at the Bose-Einstein condensation of excitons. This book presents the various types of resonance effects on excitons, biexcitons and the local electronic centers (LEC) in solids, such as paramagnetic and paraelectric resonances on excitons, exciton acoustic resonance at intra- and interband transitions, radio-optical double resonance on excitons, hole-nuclear double resonance on localized biexcitons, ENDOR and acoustic ENDOR on LEC. The criteria for the generation of coherent photons, phonons and magnons by excitons are explained. The interactions of excitons and biexcitons with paramagnetic centers and nuclear spins, the indirect interaction between the PC through a field of excitons as well as the quasienergy spectrum of excitons and spin systems are discussed. It is proved that the interaction of paramagnetic centers with excitons increases the spin relaxation rate of paramagnetic centers in comparison with the case of their interaction with free carriers. The giant magneto-optical effects in semi-magnetic semiconductors are theoretically interpreted. In recent years, a new perspective has been added to these systems and their interactions: they can be used for storing and processing information in the form of quantum bits (qubits), the building blocks of quantum computers. The basics of this emerging technology are explained and examples of demonstration-type quantum computers based on localized spins in solids are discussed.

Geru, Ion [Moldovan Academy of Sciences, Chisinau (Moldova, Republic of). Inst. of Chemistry; Suter, Dieter [Technische Univ. Dortmund (Germany). Fakultaet Physik

2013-08-01

434

Thermodynamic model of the macroscopically ordered exciton state  

OpenAIRE

We explain the experimentally observed instability of cold exciton gases and formation of a macroscopically ordered exciton state (MOES) in terms of a thermodynamic model accounting for the phase fluctuations of the condensate. We show that the temperature dependence of the exciton energy exhibits fundamental scaling behavior with the signature of the second order phase transition.

Andreev, Sergey

2012-01-01

435

Collective state in the Bose gas of interacting interwell excitons  

International Nuclear Information System (INIS)

Experiments associated with direct observations of a collective state in a gas of interacting interwell excitons in GaAs/AlGaAs double quantum wells are discussed. The structures constitute Schottky photodiodes. It is found that the photoluminescence of interwell excitons under certain conditions exhibits an axisymmetric spatial structure. The effect is a result of the collective behavior of interacting interwell excitons

436

Massive creation of entangled exciton states in semiconductor quantum dots  

CERN Document Server

An intense laser pulse propagating in a medium of inhomogeneously broadened quantum dots massively creates entangled exciton states. After passage of the pulse all single-exciton states remain unpopulated (self-induced transparency) whereas biexciton coherence (exciton entanglement) is generated through two-photon transitions. We propose several experimental techniques for the observation of such unexpected behavior.

Hohenester, U

2002-01-01

437

Excitation energy transfer between closely spaced multichromophoric systems: Effects of band mixing and intraband relaxation  

CERN Document Server

We theoretically analyze the excitation energy transfer between two closely spaced linear molecular J-aggregates, whose excited states are Frenkel excitons. The aggregate with the higher (lower) exciton band edge energy is considered as the donor (acceptor). The celebrated theory of F\\"orster resonance energy transfer (FRET), which relates the transfer rate to the overlap integral of optical spectra, fails in this situation. We point out that in addition to the well-known fact that the point-dipole approximation breaks down (enabling energy transfer between optically forbidden states), also the perturbative treatment of the electronic interactions between donor and acceptor system, which underlies the F\\"orster approach, in general loses its validity due to overlap of the exciton bands. We therefore propose a nonperturbative method, in which donor and acceptor bands are mixed and the energy transfer is described in terms of a phonon-assisted energy relaxation process between the two new (renormalized) bands. ...

Didraga, C; Knöster, J

2006-01-01

438

Nonradiative decay of core excitons in the Auger-free luminescence materials CsCl and BaF2  

Science.gov (United States)

The present work has been conducted to understand the decay processes of the Cs-5p and Ba-5p core excitons in CsCl and BaF2, respectively, where the Auger decay process of the Cs-5p and Ba-5p core holes is energetically forbidden. The core-exciton peaks are missing in the excitation spectra for the Auger-free luminescence, which arises from a radiative decay of the valence electrons into these core holes. Resonant enhancement effects of the valence-band photoelectron spectra are observed around 13 eV for CsCl and 17 eV for BaF2. The effects are attributed to the nonradiative decay through the direct-recombination process of an excited electron and the Cs-5p or Ba-5p core hole forming the core exciton. It is also found that the core exciton deforms its surrounding lattice with relaxation energies of about 0.3 eV in CsCl and 0.9 eV in BaF2. The nonradiative direct-recombination probabilities of the core excitons are estimated to be about 32% and 75% for CsCl and BaF2, respectively, by taking account of the lattice relaxation effects.

Kamada, Masao; Itoh, Minoru

2002-05-01

439

Charge carrier recombination channels in the low-temperature phase of organic-inorganic lead halide perovskite thin films  

Directory of Open Access Journals (Sweden)

Full Text Available The optoelectronic properties of the mixed hybrid lead halide perovskite CH3NH3PbI3?xClx have been subject to numerous recent studies related to its extraordinary capabilities as an absorber material in thin film solar cells. While the greatest part of the current research concentrates on the behavior of the perovskite at room temperature, the observed influence of phonon-coupling and excitonic effects on charge carrier dynamics suggests that low-temperature phenomena can give valuable additional insights into the underlying physics. Here, we present a temperature-dependent study of optical absorption and photoluminescence (PL emission of vapor-deposited CH3NH3PbI3?xClx exploring the nature of recombination channels in the room- and the low-temperature phase of the material. On cooling, we identify an up-shift of the absorption onset by about 0.1 eV at about 100 K, which is likely to correspond to the known tetragonal-to-orthorhombic transition of the pure halide CH3NH3PbI3. With further decreasing temperature, a second PL emission peak emerges in addition to the peak from the room-temperature phase. The transition on heating is found to occur at about 140 K, i.e., revealing significant hysteresis in the system. While PL decay lifetimes are found to be independent of temperature above the transition, significantly accelerated recombination is observed in the low-temperature phase. Our data suggest that small inclusions of domains adopting the room-temperature phase are responsible for this behavior rather than a spontaneous increase in the intrinsic rate constants. These observations show that even sparse lower-energy sites can have a strong impact on material performance, acting as charge recombination centres that may detrimentally affect photovoltaic performance but that may also prove useful for optoelectronic applications such as lasing by enhancing population inversion.

Christian Wehrenfennig

2014-08-01

440

Excitation energy transfer between closely spaced multichromophoric systems: Effects of band mixing and intraband relaxation  

OpenAIRE

We theoretically analyze the excitation energy transfer between two closely spaced linear molecular J-aggregates, whose excited states are Frenkel excitons. The aggregate with the higher (lower) exciton band edge energy is considered as the donor (acceptor). The celebrated theory of F\\"orster resonance energy transfer (FRET), which relates the transfer rate to the overlap integral of optical spectra, fails in this situation. We point out that in addition to the well-known fa...

Didraga, C.; Malyshev, V. A.; Knoester, J.

2006-01-01

441

Resonant Transfer of Excitons and Quantum Computation  

CERN Document Server

The excitation-energy transfer--the so-called Forster resonant energy transfer--plays a key role in light harvesting processes in photosynthetic organisms in nature. Here we give two methods for performing quantum logic operations by tailoring this interaction. The first implementation uses a coupled quantum dot molecule where the exciton-exciton interaction and the Forster coupling are controlled by means of the dot size, interdot separation, material composition, confinement potential and applied electric field to obtain high fidelity logic. The second proposes the use of biological systems for embodying qubits where, as a result of a stronger Forster interaction, extended exciton states are expected. These states are likely to be more immune to decoherence.

Lovett, B; Nazir, A; Kothari, B; Briggs, A; Lovett, Brendon; Reina, John H.; Nazir, Ahsan; Kothari, Beeneet; Briggs, Andrew

2003-01-01

442

Optical model calculation of exciton decay rates  

International Nuclear Information System (INIS)

The decay rate of an exciton is calculated using the imaginary part of both the phenomenological optical potential of Becchetti and Greenlees and the microscopic potential of Jeukenne et al. The decay rate thus calculated is found to be about the same for proton- and neutron-induced reactions and weakly sensitive to mass number. Kalbach's two body matrix element, obtained from the analysis of precompound reactions using the Williams exciton model, also leads to a decay rate in agreement with the phenomenological potential estimate. However, the exciton model of Gadioli et al. is inconsistent with optical model estimates. Further, remarks are made on the concept of a mean free path in finite nuclear matter

443

Topological excitonic superfluids in three dimensions  

International Nuclear Information System (INIS)

We study the equilibrium and non-equilibrium properties of topological dipolar intersurface exciton condensates within time-reversal invariant topological insulators in three spatial dimensions without a magnetic field. We elucidate that, in order to correctly identify the proper pairing symmetry within the condensate order parameter, the full three-dimensional Hamiltonian must be considered. As a corollary, we demonstrate that only particles with similar chirality play a significant role in condensate formation. Furthermore, we find that the intersurface exciton condensation is not suppressed by the interconnection of surfaces in three-dimensional topological insulators as the intersurface polarizability vanishes in the condensed phase. This eliminates the surface current flow leaving only intersurface current flow through the bulk. We conclude by illustrating how the excitonic superfluidity may be identified through an examination of the terminal currents above and below the condensate critical current.

444

Förster coupling in nanoparticle excitonic circuits.  

Science.gov (United States)

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 Forster 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, spatially varying dielectric environments, and externally applied electric fields. Depending on the orientation of the transition dipole, the Forster coupling is shown to be either increased or decreased as a function of the nanoparticle shape and of the properties of the dielectric environment. In the presence of an electric field, we investigate the relation between excitonic binding and confinement effects. We also study a type II core-shell quantum dot where electron and hole are spatially separated due to a particular configuration of the bandstructure. PMID:20698598

Rebentrost, Patrick; Stopa, Michael; Aspuru-Guzik, Alán

2010-08-11

445

Properties of Excitons Bound to Ionized Donors  

DEFF Research Database (Denmark)

Binding energies, interparticle distances, oscillator strengths, and exchange corrections are calculated for the three-particle complex corresponding to an exciton bound to an ionized donor. The results are given as functions of the mass ratio of the electron and hole. Binding of the complex is obtained for mass ratios up to 0.426. The interparticle distances are up to 50 times larger than the corresponding exciton radius. The oscillator strengths are about 104 times greater than those of free excitons, while the exchange corrections for the complex are comparable to those of free excitions. The results are applied to CdS and ZnO and compared with experimental results on these.

Skettrup, Torben; Suffczynski, M.

1971-01-01

446

Exciton annihilation and diffusion in semiconducting polymers  

Science.gov (United States)

We show that time-resolved luminescence measurements at high excitation densities can be used to study exciton annihilation and diffusion, and report the results of such measurements on films of P3HT and MEH-PPV. The results fit to an exciton-exciton annihilation model with a time independent annihilation rate ?, which was measured to be ? = (2.8+/-0.5)×10 -8 cm 3s -1 in MEH-PPV and ? = (5.2+/-1)×10 -10 cm 3s -1 in P3HT. This implies much faster diffusion in MEHPPV. Assuming a value of 1 nm for the annihilation radius we evaluated the diffusion length for pristine P3HT in one direction to be 3.2 nm. Annealing of P3HT was found to increase the annihilation rate to (1.1+/-0.2)×10 -9 cm 3s -1 and the diffusion length to 4.7 nm.

Shaw, P. E.; Lewis, A. J.; Ruseckas, A.; Samuel, I. D. W.

2006-08-01

447

Coordination polymers of copper(I) halides.  

Science.gov (United States)

A total of 21 complexes of CuX (X = Cl, Br, I) with bridging ligand (B = 4,4'-dipyridyl (Bpy), pyrazine (Pyz), quinoxaline (Quin), phenazine (Phz), 1,4-diazabicyclo[2.2.2]octane (DABCO), and hexamethylenetetramine (HMTA)) have been synthesized. The products show two stoichiometries: [CuXB] (type 1) and [(CuX)2B] (type 2). Both types can be obtained for B = Bpy, depending on the conditions of preparation. In these cases, the type 2 stoichiometry is the kinetic product. Type 2 complexes only are found for B = Pyz (X = I), Quin, Phz, DABCO, and HMTA. Type 1 complexes form for Pyz (X = Cl, Br). Thermogravimetic analyses of the complexes reveal the general decomposition trend: 1 --> 2 --> [(CuX)2B(1/2)] --> CuX. The X-ray crystal structure of [CuBr(Pyz)] (type 1) features copper atoms bridged by Br and Pyz, forming 2D sheets of fused rectangular Cu4Br2(Pyz)2 units. The X-ray structure of [(CuI)2(Quin)] (type 2) shows 2D layers composed of [Cu2I2]infinity "stair step" chains which are cross-linked by Quin ligands. A total of 16 complexes of CuXL (L = P(OPh)3) with bridging ligand (B = those above and 1,4-dimethylpiperazine (DMP)) have also been prepared. All of these products, except those of HMTA, are of type 3 formulation, [(CuXL)2B]. The HMTA products have the formula [CuX(HMTA)], type 4. Thermal decomposition of the type 3 and 4 complexes occurs with initial loss of B, L, or both. The X-ray structures of [(CuBrL)2(Bpy)] and [(CuBrL)2(Pyz)] (type 3) reveal 1D chains formed from rhomboidal (LCu)2Br2 units linked by the B ligand. The type 4 structure of [CuBrL(HMTA)] is shown by X-ray to be a simple halide-bridged dimer. PMID:11233211

Graham, P M; Pike, R D; Sabat, M; Bailey, R D; Pennington, W T

2000-10-30

448

Multiband theory of multi-exciton complexes in self-assembled quantum dots  

CERN Document Server

We report on a multiband microscopic theory of many-exciton complexes in self-assembled quantum dots. The single particle states are obtained by three methods: single-band effective-mass approximation, the multiband $k\\cdot p$ method, and the tight-binding method. The electronic structure calculations are coupled with strain calculations via Bir-Pikus Hamiltonian. The many-body wave functions of $N$ electrons and $N$ valence holes are expanded in the basis of Slater determinants. The Coulomb matrix elements are evaluated using statically screened interaction for the three different sets of single particle states and the correlated $N$-exciton states are obtained by the configuration interaction method. The theory is applied to the excitonic recombination spectrum in InAs/GaAs self-assembled quantum dots. The results of the single-band effective-mass approximation are successfully compared with those obtained by using the of $k\\cdot p$ and tight-binding methods.

Sheng, W; Hawrylak, P; Sheng, Weidong; Cheng, Shun-Jen; Hawrylak, Pawel

2004-01-01

449

Binding energy and radiative lifetime of an exciton in a type-II quantum well  

International Nuclear Information System (INIS)

Donor binding energies of ground- and excited state excitons have been computed with two sets of effective masses linked to X valleys of a GaSb/InAs/GaSb quantum well. The calculations have been performed using the variational technique within a single-band effective mass approximation. The valence-band anisotropy is included in our theoretical model by using different hole masses in different spatial directions with the strain-induced confinement potential. We observe that the values of variational parameters are consistent when a two-parametric trial wave function is used. The oscillator strength and the lifetime for radiative recombination of the exciton have been computed as a function of well width. It is inferred that these binding energies are quite sensitive to the effective masses that are used in the calculation and the spatial separation of electrons and holes, allowing for the formation of very stable excitons. The observed radiative lifetime is in good agreement with the results of other investigators.

450

Intraband exciton relaxation in a biased lattice with long-range correlated disorder  

Science.gov (United States)

We numerically study the intraband exciton relaxation in a one-dimensional lattice with a scale-free disorder in the presence of a linear bias. Exciton transport is the incoherent hopping over the eigenstates of the static lattice. The site potential of the unbiased lattice is long-range-correlated with a power-law spectral density S(k)˜1/k? , ?>0 . The lattice supports a phase of extended states at the center of the band, provided ? is larger than a critical value ?c [F. A. B. F. de Moura and M. L. Lyra, Phys. Rev. Lett. 81, 3735 (1998)]. When the bias is applied, the absorption spectrum displays clear signatures of the Wannier-Stark ladder [E. Díaz, F. Domínguez-Adame, Yu. A. Kosevich, and V. A. Malyshev, Phys. Rev. B 73, 174210 (2006)]. We demonstrate that in unbiased lattices and in weakly correlated potentials, the decay law is nonexponential. However, the decay is purely exponential when the bias increases and ? is large. We relate this exponential decay to the occurrence of the Wannier-Stark ladder in the exciton band.

Díaz, E.; Domínguez-Adame, F.

2008-04-01

451

Inner-shell excitations and secondary excitons in rare-gas solids  

International Nuclear Information System (INIS)

Luminescence of the rare gas solids Xenon, Krypton, and Argon was investigated following excitation of the inner-shell levels using time-resolved photoluminescence techniques. As a probe for luminescence, emission from the free and self-trapped exciton states was used. The measurements were performed in the energy range of 35-750 eV, covering excitations from the 4d, 4p, and 3d shells of Xe, the 3d shells in Kr, and the 2p shells in Ar. For fast emissions, the time structure on the nanosecond scale shows two distinct regions, the region of prompt luminescence from excitons created simultaneously with the excitation, and the delayed luminescence, which stems from slow recombinational excitons. For the prompt luminescence, a resonance is observed for photon energies above a threshold which equals the ionization energy of the respective core state plus the energy of the valence exciton. This threshold is explained in two independent models, the multiple parabolic branch band model, and the electronic polaron complex model. A comparison with related measurements from the literature demonstrates that the occurrence of the resonance in prompt exciton creation is a fundamental property of the rare gas solids. The structure of the delayed luminescence shows a marked dependency on the excitation density. The shift of the decay structure to shorter times with increasing density is explained in a recombinational model for electrons and holes. Numerical simulations within the fraoles. Numerical simulations within the frame of this model are presented, which reproduce the measured date within good accuracy. (orig.)

452

Theory of lifetime of exciton incoherently created below its resonance frequency by inelastic scattering  

OpenAIRE

When an exciton in semiconductor is scattered and its energy is decreased far below the resonance energy of the bare exciton state, it has been considered that an exciton-polariton is created immediately by the scattering process, because there is no exciton level at that energy. However, according to recent time-resolved measurements of P emission originating from inelastic exciton-exciton scattering, it looks rather natural to consider that the exciton-polariton is created...

Bamba, Motoaki; Wakaiki, Shuji; Ichida, Hideki; Mizoguchi, Kohji; Kim, Daegwi; Nakayama, Masaaki; Kanematsu, Yasuo

2014-01-01

453

Quantum Dot Lattice Embedded in An Organic Medium: Hybrid Exciton State and Optical Response  

OpenAIRE

We propose a new model to implement organic exciton-semiconductor exciton hybridization by embedding a semiconductor quantum dot array into an organic medium. A Wannier-Mott transfer exciton is formed when the exciton in each semiconductor dot interacts via the multipole-multipole coupling with other excitons in the different dtos of the array. A new hybrid exciton appears in the system owing to strong dipole-dipole interaction of the Frenkel exciton of the organic molecules...

Huong, Nguyen Que; Birman, Joseph L.

1998-01-01

454

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

455

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)

456

Probing of molecular aggregates with exciton annihilation  

International Nuclear Information System (INIS)

It is characteristic for the kinetics of singlet-singlet exciton annihilation in a finite-size molecular aggregate to approach with time the steady state corresponding to the very last single excitation. The elimination of corresponding signal from the kinetic trace exposes the dominance of the exponential decay. The characteristic time constant representing the slowest annihilation mode is related to the number of exciton hopping sites and the pairwise hopping time by solving the single excitation trapping problem. The relevance of proposed approach for the analysis of annihilation kinetics is demonstrated for the bacterial light-harvesting complex LH1, the reaction center complex D1-D2 and C60 structures

457

Device design for optimal exciton harvesting  

Science.gov (United States)

Organic light emitting diodes (OLEDs) show potential as the next generation solid state lighting technology. A major barrier to widespread adoption at this point is the efficiency droop that occurs for OLEDs at practical brightness (~ 5000 cd/m2) levels necessary for general lighting. We highlight recent progress in highly efficient OLEDs at high brightness, where improvements are made by managing excitons in these devices through rational device design. General design principles for monochrome OLEDs are discussed based on recent device architectures that have been successfully implemented. We expect that an improved understanding of exciton dynamics in OLEDs in combination with innovative device design will drive future development.

Ingram, G. L.; Lu, Z. H.

2014-10-01

458