Localized optical vibrational modes in GaSb/AlSb superlattices

The vibrational spectrum of short-period GaSb/AlSb superlattices in the frequency range of GaSb optical phonons is investigated in detail. Localization of transverse GaSb optical phonons in GaSb/AlSb superlattices is observed experimentally for the first time. The dispersions of TO and LO phonons in GaSb are measured. Pis’ma Zh. éksp. Teor. Fiz. 64, No. 5, 361–364 (10 September 1996)     

Optical detection of the absorption of acoustical phonons by a two-dimensional electron gas in a magnetic field

The effect of absorption of nonequilibrium acoustical phonons on the intensity of recombination of a two-dimensional electron gas in a magnetic field is investigated. The nonequilibrium acoustical phonons are emitted in the relaxation of electrons in a tunnel junction deposited on the back side of a sample with a two-dimensional electronic channel. It is demonstrated that the optical signal showing the intensity of the recombination of nonequilibrium electrons from a photoexcited size-quantization subband can serve as a sensitive detector of acoustical phonons. Because the general heating of two-dimensional carriers and the intersubband transitions stimulated by the absorption of nonequilibrium acoustical phonons lead to effects of different sign, the useful signal can be discriminated unambiguously. Pis’ma Zh. éksp. Teor. Fiz. 69, No. 1, 30–35 (10 January 1999)     

Theory of Raman light scattering by nanocrystal acoustic vibrations

A theory of Raman scattering of light by acoustic phonons in spherical nanocrystals of zinc-blende and wurtzite semiconductors has been developed with the inclusion of the complex structure of the valence band. The deformation-potential approximation was used to describe the exciton-phonon interaction. It is shown that this approximation allows only Raman scattering processes involving spheroidal acoustic phonons with a total angular momentum F=0 or 2. The effect of phonon quantum confinement on linewidth in Raman scattering spectra and scattered polarization is analyzed. An expression for the shape of the spectral line corresponding to nonresonant scattering from F=0 phonons was obtained. Fiz. Tverd. Tela (St. Petersburg) 41, 1473–1483 (August 1999)     

Magnetoelasticity and effect of thermal phonons on the magnetic properties of ferromagnets

The experimental data for a series of ferromagnets are analyzed to determine quantitatively the magnetoelasticity-reduced effects of thermal phonons on the magnetic properties of ferromagnets. It is shown that thermal phonons have an anomalously strong effect on the magnetic properties of Invar iron-platinum alloys. The effect is governed by shear magnetoelasticity. Pis'ma Zh. éksp. Teor. Fiz. 64, No. 1, 33–37 (10 July 1996)     

Resonant Mandelstam-Brillouin light scattering and Raman scattering in semiconductors with intermediate exciton states belonging to discrete exciton bands and the continuous spectrum

The results of a theoretical and experimental investigation of resonant Mandelstam-Brillouin light scattering by thermal acoustic phonons with k=0 near the direct absorption edge (in the case of ZnSe crystals) are analyzed. The appearance of a new type of resonant increase in the intensity of Raman scattering by optical phonons with k≠0, which corresponds to resonance with the scattered light in the output channel, near the indirect absorption edge (in the case of semi-insulating GaP:N crystals) is also reported. The resonant gain reaches ∼4×10³ at frequencies corresponding to overtone scattering assisted by LO(X) and LO(L) phonons. Exciton states belonging to both discrete exciton bands and to the continuous spectrum are considered as the intermediate states involved in the scattering processes in calculations of the resonant scattering tensors. In addition, all the intraband transitions, as well as the interband transitions between the conduction band, the valence bands, and the spin-orbit split-off band are taken into account, and good agreement with the experimental results is obtained. Fiz. Tverd. Tela (St. Petersburg) 40, 938–940 (May 1998)     

Localization of acoustic phonons in a layered lattice

The possibility of weak localization of acoustic phonons in a nonideal layered-crystal lattice is studied. Analytical expressions are obtained for the thermal conductivity and diffusion coefficient tensors at low temperatures. The role of specific processes of interference scattering of phonons by phonon density fluctuations near defects under conditions of weak interaction between the layers is analyzed. It is shown that because of such processes a substantial renormalization of the diffusion coefficient in the presence of strong diagonal disorder is possible at relatively low frequencies, where the dispersion laws for the acoustic phonons display quasi-two-dimensional properties. A nonstandard low-temperature dependence of the thermal conductivity of the layered systems BSCCO and BSYCO is discussed. Fiz. Tverd. Tela (St. Petersburg) 40, 132–141 (January 1998)     

Melting of the cooperative Jahn-Teller distortion in LaMnO 3 single crystal studied by Raman spectroscopy

We have studied the behavior of the Raman phonons of a stoichiometric LaMnO₃ single crystal as a function of temperature in the range between 77 K and 900 K. We focus on the three main phonon peaks of the Pbnm structure, related to the tilt, antisymmetric stretching (Jahn-Teller mode) and stretching modes of Mn-O octahedra. The phonon frequencies show a strong softening that can be fit taking into account their renormalization because of three phonon anharmonic effects in the pseudoharmonic approximation. Thermal expansion effects, in particular the variation of Mn-O bond lengths with temperature, are not relevant above 300 K. On the contrary, phonon width behavior deviates from the three phonon scattering processes well bellow T _c . The correlation between the magnitude of the cooperative Jahn-Teller distortion, that disappears at 800 K, and the amplitude of the Raman phonons in the orthorhombic phase is shown. Nevertheless, Pbnm phonons are still observable above this temperature. Phonon width and intensity behavior around T _c can be explained by local melting of the orbital order that begins quite below T _c and by fluctuations of the regular Mn-O octahedra that correspond to dynamic Jahn-Teller distortions. Received 25 January 2001 and Received in final form 14 March 2001     

Observation of LO-phonon localization in GaAs quantum wires on faceted (311)A surfaces

The localization of longitudinal optical phonons in GaAs/AlAs lateral superlattices and quantum wires grown on faceted GaAs (311)A surfaces are investigated by means of Raman scattering spectroscopy. The frequencies of the localized phonons are found to decrease as the average thickness of the GaAs layer is decreased from 21 to 15 Å. As the GaAs thickness is decreased further to 11.3 and 8.5 Å, the frequencies of the localized phonons increases sharply in connection with the formation of an array of quantum wires. The frequencies calculated in a two-dimensional chain model agree with the experimental values. This makes it possible to interpret the increase in the frequencies of localized phonon states as being the result of the quantization of phonons in the array of one-dimensional objects. The results obtained support the model of GaAs (311)A surface faceting with a facet height of 10.2 Å. Pis’ma Zh. éksp. Teor. Fiz. 63, No. 12, 942–946 (25 June 1996)     

The peculiarities of the tunneling conductance and phonon induced features in tetrahedrically bonded semiconductors under pressure

Abstract

Summary The 30 kbar high pressure unit for low temperatures and the digital processing have been applied to the tunneling investigations of Ge and GaAs. The obtained results include the values of the mode Gruneisen constants for zone boundary phonons in <100> direction in Ge, the irregular optical phonons behavior in GaAs, the bistability of tunneling voltage-current curve for GaAs at elevated pressure and the observation of optically induced oscillations on the tunneling conductance of GaAs Schottky barrier. The semi-quantitative model involving the conductance band structure change under pressure and DX-centers in GaAs(Te) is used for the tentative explanation.     

Electron-phonon interaction in spherical multilayer nanoheterostructures

The complete hamiltonian of an electron-phonon system in the second quantization representation for a complex spherical nanoheterostructure with an arbitrary number of semiconducting layers is obtained in a polarized continuum model for bounded volume and interface phonons. The energy of the electron ground state is calculated using the experimentally realized CdS/HgS/H₂O system as an example. The cause of the nonmonotonic dependence of the shift in the electron ground state on the HgS thickness is established as due to the interaction with bounded phonons. Fiz. Tverd. Tela (St. Petersburg) 39, 1109–1113 (June 1997)     

Microscopic description of the kinetics of a martensitic transition in real crystals: bcc-hcp transition in Zr

The kinetics of the bcc-hcp transition in Zr in the presence of dislocations is investigated by numerical simulation methods. It is shown that the transition occurs in a nondiffusional (at a velocity of the order of the speed of sound) in two stages: relatively long development of instability of long-wavelength acoustic phonons and a fast stage of instability of short-wavelength phonons. The elastic stresses near a dislocation make it possible for these instabilities to develop at much lower temperatures than in an ideal crystal. Pis’ma Zh. éksp. Teor. Fiz. 70, No. 6, 376–380 (25 September 1999)     

Contribution of nonequilibrium optical phonons to the Peltier and Seebeck effects in polar semiconductors

Additive contributions to the Seebeck and Peltier coefficients made by nonequilibrium longitudinal optical phonons have been calculated. The results obtained are valid for any temperature and applicable to polar nondegenerate semiconductors with low carrier concentrations. The calculated components of the thermoelectric coefficients are exponentially small in the low-temperature domain and reach a maximum at k _BT∼ħω ₀. In materials with a large carrier mass and strong electron-phonon coupling the contribution of optical phonons to the Seebeck coefficient can exceed 1 mV/K. Fiz. Tverd. Tela (St. Petersburg) 40, 1209–1215 (July 1998)     

Anisotropy of optical phonons in semiconductor superlattices: Raman scattering experiments

Experiments on Raman scattering in the “forward” geometry, permitting observation of anisotropy of the optical phonons, are performed on specially prepared short-period GaAs/AlAs superlattice structures with the substrates removed and the surfaces covered with an antireflective layer. The experimental data agree well with the computational results obtained for the angular dispersion of optical phonons in superlattices on the basis of a modified continuum model. Pis’ma Zh. éksp. Teor. Fiz. 68, No. 1, 50–55 (10 July 1998)     

Pressure and temperature-dependent raman study of ZnWO4

Abstract

The Raman spectra of monoclinic ZnWO₄ (c⁴ _2h-space group) are reported over wide ranges of pressure (0-24GPa) and temperature (13-970 K). All 18 Raman active pho-nons are observed throughout these ranges. Combining the pressure and temperature Raman data, an identification scheme is suggested which makes possible to distinguish the internal modes (vibrations in the octahedral WO₆ units) from the external ones (pure lattice modes). All phonons harden with pressure, thus showing normal positive dω/dP slopes, and soften with temperature in the region 250 - 970 K (normal negative dω/dT slopes). However, two phonons show a change of slope sign below 300 K, resulting in small positive slopes at low temperatures. Since the pressure dependence of these two phonons is normal and linear, it is concluded that their anomalous, non-linear dependence with temperature is due to anharmonic (temperature induced) phonon interactions rather that volume expansion. It is found that ZnWO₄ remains stable throughout the pressure and temperature ranges.     

Heating of two-dimensional excitons by nonequilibrium acoustic phonons

The energy distribution functions of two-dimensional excitons in the presence of nonequilibrium acoustic phonons have been calculated for the geometry used in heat-pulse experiments. The results were obtained by solving numerically the kinetic equation for the case where the exciton gas equilibrates with phonons during its lifetime. The cases of the low and high exciton-gas density limits are considered. It is shown that at low exciton-gas densities the distribution does not follow the Boltzmann function and depends on the quantum-well width. A comparison with earlier experimental data is made. Fiz. Tverd. Tela (St. Petersburg) 41, 1707–1711 (September 1999)     

Coherent optical spectroscopy due to lattice vibrations in a single quantum dot

We theoretically investigate a coherent optical spectroscopy of a strongly driven quantum dot without and with the coupling between exciton and phonons. In the absence of this coupling, we achieve the experimental results obtained by Xu et al. [Science 317, 929 (2007)]. However, while taking this coupling into account, we observe some new features in the probe absorption spectrum including two sharp phonon-induced sidebands. Our results also demonstrate that the lifetime of phonons will play an important role in this coherent spectroscopy.     

Exciton-phonon coupling strength in single wall carbon nanotubes

The coupling strength of the exciton and different phonons, including the radial breathing mode (RBM), longitudinal (LO) and transverse (TO) optical phonons, are calculated for different diameter single wall carbon nanotubes (SWNTs) in the framework of tight-binding model. It is found that the exciton-phonon coupling strength with the LO mode or RBM shows a clear (2n+m)-family behavior due to the trigonal warping effect, but it with the TO mode remains to be zero. In the same SWNT, the E₂₂ exciton-phonon coupling strength is found to be slightly smaller than that of E₁₁ exciton. Finally, the exciton-RBM-phonon coupling strengths for several SWNTs are found to be in good agreement with the recent experimental data [Phys. Rev. Lett. 98, 037405 (2007)].     

High-Pressure Vibrational Studies of HCP Metals by Raman Spectroscopy

The pressure response of Raman phonons, determined for several hcp metals, includes positive pressure shifts as well as anomalies like mode softening in connection with phase transitions. It is shown that the phonon frequencies and their pressure dependences are related to macroscopic elastic parameters. More general, these results show that the measurement of Raman-active phonons in metals provides a direct probe of bonding, and agreement with theoretical models gives additional confidence in ab initio techniques.     

Pressure Dependence of Acoustic Modes in AgGaSe 2

The pressure dependence of the transverse acoustic TA[0 0 1] and TA[1 0 0] phonons and the low-frequency optical v 3 phonons of AgGaSe 2 has been measured at pressures up to 4.3 GPa by inelastic neutron scattering. The strong frequency decreases in the whole Brillouin zone of the TA modes show generally a weak nonlinearity. Depending on the wavevector, the softening prior to the phase transition at 2.6 GPa also affects the optical mode to a less extent.     

Determination of the Mode Grüneisen Parameter of AlN using different Fits on Experimental High Pressure Data

To investigate the pressure dependence of the AlN phonon frequencies Raman spectra of single-crystalline bulk AlN under hydrostatic pressure up to 10 GPa were recorded. The Raman peak positions of the A 1 (TO), E 1 (TO), E 2 (high), A 1 (LO) and quasi-longitudinal optical (QLO) phonons were plotted as a function of pressure. The experimental data was fitted using the traditional parabolic fit (M. Kuball et al . (2001) Appl. Phys. Lett., 78, 724 [1]) and fits to physical models, density, volume, etc. The mode Grüneisen parameters of the different phonons were determined for each fit and significant differences are found between the various fits. Results are compared with recent theoretical calculations (J.-M. Wagner et al . (2000) Phys. Rev. B, 62, 4526 [2]).