Mutual entrainment of hot electrons and optical phonons in polar semiconductors

Kinetic equations for nonequilibrium electrons and optical phonons are constructed and solved for the case in which the interaction between these particles is the primary mechanism for the relaxation of the electron energy and quasimomentum. The calculations reflect the circumstance that for the optical phonons the equivalent primary relaxation mechanism is the interaction with acoustic phonons (which are at equilibrium in this case). Constitutive equations are derived for polar semiconductors which reflect the mutual entrainment of electrons and optical phonons. Energy balance equations, which determine the temperatures of these particles, are also derived. These temperatures are generally different from each other and from the reservoir temperature.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 2, pp. 32–36, February, 1984.We wish to thank É. A. Kaner and I. B. Levinson for useful discussions.     

Anharmonic renormalization and non-Markovian decay of coherent optical phonons in polar semiconductors

The influence of anharmonic renormalization effects on the decay dynamics of coherent longitudinal optical phonons is investigated from a microscopic point of view. Time-resolved coherent anti-Stokes Raman signals are calculated for GaP on the basis of a full phonon dispersion calculation, and the relevant decay channels are identified and compared. Anharmonic renormalization effects are found to induce non-Markovian behaviour of the decay dynamics and lead to a decrease of the decay time. The renormalization effects only depend on the special properties of the phonon dispersion of the given material. This underlines the intrinsic nature of the non-Markovian decay dynamics of phonons for any material. Non-Markovian dynamics of the decay of coherent LO-phonons is calculated for GaP and result in a 30% faster decay signal than the corresponding Markovian dynamics.     

Interaction of electrons with polar optical phonons in semiconductor superlattices

On the basis of the microscopic theory of lattice dynamics, simulation of the electric potentials created by optical phonons in semiconductor superlattices is performed. It is shown that the spatial distribution of the amplitudes of electric potentials differs from that in a dielectric continuum predicted by the conventional macroscopic model without dispersion. A modified macroscopic continuum theory is proposed that takes into account the dispersion of short-range interatomic forces and allows one to obtain analytical expressions for the potentials of electron-phonon interaction.     

A note on surface magnetoplasmons in semiconductors

The results of Brion et al. [Phys. Rev. Lett.28, 1455 (1972)] are reinterpreted to predict, in facile manner, the type of inequalities that need to be satisfied by the surface parameters for the existence of gaps and multiple branches in the dispersion curves for surface waves along the interface of a semiconductor-semiconductor structure.     

Interaction between an electron and optical phonons in polar semiconductor heterostructures

Interface phonons and bulk-like longitudinaloptical (LO) phonons and their interaction with an electron are studied for a finite four-layer heterostructure (FFLHS). An analysis of the field eigenvectors shows that, in the vicinity of the Brillouin-zone center, an interface transverse-optical (TO) mode oscillates at the bulk LO frequency, and an interface LO mode oscillates at the bulk TO frequency. Analytic expressions and numerical illustrations for dispersion relations of interface modes and for electron-phonon coupling functions and scattering rates are obtained for finite, semi-infinite and infinite quantum well (QW) structures which are important special cases of an FFLHS. It is shown that the scattering rates depend strongly on the well width of a QW structure, and that interface modes are much more important than bulk LO modes when the well width is small. The calculated results also show that the usual selection rules for intersubband and intrasubband transitions break down in asymmetric heterostructures. Moreover, we have found an interesting result. That is, in comparison with the negligibly small interaction between an electron and the lowest-frequency interface-mode in symmetric single QWs and commonly used step QWs, this interaction may be very large in asymmetric single QWs and general step QWs.     

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)     

Damping of optical phonons in metals and strongly doped semiconductors

The electron-phonon-induced damping of optical phonons arising in metals and strongly doped semiconductors under laser irradiation is investigated. The damping of both short-wave KV_F > ω₀ and long-wave KV_F < ω₀ optical phonons is calculated; K is the wavevector, ω₀ is the frequency of the optical phonon; V_F is the Fermi velocity. The electron- phonon-induced damping is important if the frequency of the optical phonon is larger than two frequencies of acoustic phonons of all branches in the range of the whole Brillouin zone. The damping of a soft transverse optical phonon in narrow-gap ferroelectric-semiconductors is also defined by the electron-phonon interaction. In other cases the main relaxation process for optical phonons in metals is the decay into two acoustic phonons due to lattice anharmonicity.     

Amplification of optical phonons in the multiphoton absorption in semiconductors

The optical phonon scattering by electrons in the presence of a strong radiation field is discussed. It is shown that in the strong-field limit the optical phonon population may become unstable.     

Light scattering from surface acoustic phonons in metals and semiconductors

Several examples of Brillouin scattering from thermally excited surface acoustic phonons in metals and opaque semiconductors are presented. The measured surface velocities are consistently lower by 1–5% than the values calculated from the bulk elastic constants. The spectra also show the continuum of Lamb excitations having some surface-type character with velocities ranging between the bulk transverse velocity and the bulk longitudinal velocity.     

Optic phonon-magnetoplasmon surface oscillations in polar semiconductors

An interface between two polar semiconductors in parallel magnetic field geometry can support at most four types of surface oscillations; the actual number (?4), however, depends on the strength of the magnetic field. The interface effects on these relevant ranges of magnetic field are analysed in detail.     

Interaction of optical phonons with anisotropic imperfections

Inelastic (Raman) light scattering by phonons interacting with anisotropic imperfections is investigated. Three different kind of disorder-induced defects (point, linear and planar) have been considered. The optical phonon width and line shape are found to depend importantly on the dimension of the imperfections. There is a close correspondence between the scale of the imperfection and the phonon line shape observed in the Raman scattering experiments. The dependence of the phonon frequency shift and width on the defect concentrations is calculated, and the critical concentrations at which the optical phonon can no longer be observed are determined. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 12, 817–822 (25 December 1997) Published in English in the original Russian journal. Edited by Steve Torstveit.     

The interaction of optical phonons with magnetoplasma waves in ionic semiconductors

The interaction of magnetoplasma waves with optical phonons is investigated for propagation both parallel and perpendicular to the external de magnetic field. The low frequency region is considered where helicons and Alfvén waves propagate for uncompensated and compensated materials, respectively. The results are applied to degenerate, ionic semiconductors where the plasma frequency is much larger than the infrared dispersion frequency and the cyclotron frequency. The theory is applied to heavily-doped InSb for the case of helicon-optical phonon interactions.     

Interaction between optical phonons and Josephson vortices in layered superconductors

The effect of optical phonons on the stability of the in-phase motion regime of Josephson vortices in a layered superconductor has been studied by using an expression for the dielectric constant of a layered high-temperature superconductor. The dispersion characteristics of linear waves have been analyzed. It has been shown that the in-phase motion regime of the Josephson vortices can be spontaneously established in the layered superconductor.     

Proton-exchange influence on the polar phonons of LiTaO3 optical waveguides

Refractive index change and Raman scattering of Z-cut proton-exchanged LiTaO₃ optical waveguides with different composition have been studied. Probing of the waveguide depth was carried out by using micro-Raman spectroscopy in order to distinguish layers with different phase states. Varying the proton-exchange and post-exchange annealing regimes, two phases –α and β– in addition to the pure LiTaO₃, were clearly observed. The behaviour of the A ₁ vibrations of the TaO₆ groups was found to be very sensitive to the phase changes. Using data from waveguide Raman spectra, normal and oblique E(TO, LO) phonons were sorted and their properties were followed in the α and β phases. The OH-stretchings are considered to correspond to dipoles turned out of the oxygen planes. Received: 9 July 1999 / Accepted: 11 October 1999 / Published online: 24 March 2000     

Theory of magnetoresistive and thermomagnetic effects with electron scattering by optical phonons in semiconductors

The magnetoresistance and transverse Nernst-Ettingshausen effect in classical magnetic fields are calculated for the case of low-temperature scattering of electrons by optical phonons. It is shown that even though the electron relaxation time does not depend on the energy, not only are the indicated effects still possible, they can have a magnitude much greater than for other scattering mechanisms.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No.2, pp. 116–120, February, 1976.The author is indebted to V. L. Bonch-Bruevich and A. G. Mironov for a useful discussion.     

Interaction of interface plasmons and longitudinal optical phonons in a multilayer structure

The interaction between two-dimensional interface plasmons and longitudinal optical phonons in multilayer structures is analysed in this communication. The dispersion relations for the mixed modes are obtained for periodic boundary conditions in the direction normal to the layer planes. The energy gap between the two bands of mixed modes can be seen in the density of states ?_k(ω) at fixed k parallel to the layers. ?_k(ω) also exhibits singularities at the band edges characteristics of the one-dimensional periodic array of parallel layers.     

Amplification of optical phonons by a radiation field in semiconductors under strong magnetic fields

The effect of a strong magnetic field on the optical phonon damping in an electron-phonon system is discussed. It is found that as the laser frequency approaches the electron cyclotron frequency the optical phonon population may become unstable.