Non equilibrium phonon distribution and electron-phonon coupling in semiconductors

A comparison is made for different semiconductors, of the non equilibrium phonon distributions induced by photon absorption. It is found that energetic carriers create preferentially LO-phonons for GaAs where Fröhlich type interaction dominates. For Si and the intermediate case of InSb the energetic carriers still relax through optical phonons. For InSb LO- and TO-phonons participate in the same proportion. Highest phonon temperatures are obtained in the layer semiconductor GaSe.     

Point-contact spectroscopy of electron-phonon interaction in semiconductors

A kinetic theory of current-voltage characteristic of semiconductor-semiconductor and metal-semiconductor point-contact junction is presented showing (a) smooth dependence of differential resistance over the voltage R = R(V), and (b) structure in dR/dV due to phonon emission at energies ?ω < eV.     

Ferromagnetism and electron-phonon coupling in the manganites

The physics of ferromagnetic doped manganites, such as La 1-x Ca x MnO 3 with x ≈ 0.2-0.4, is reviewed. The concept of double exchange is discussed within the general framework of itinerant electron magnetism. The new feature in this context is the coupling of electrons to local phonon modes. Emphasis is placed on the quantum nature of the phonons and the link with polaron physics. However it is stressed that the manganites fall into an intermediate coupling regime where standard small-polaron theory does not apply. The recently-developed many-body coherent potential approximation is able to deal with this situation and Green's recent application to the Holstein double-exchange model is described. Issues addressed include the nature of the basic electronic structure, the metal-insulator transition, a unification of colossal magnetoresistance, pressure effects and the isotope effect, pseudogaps in spectroscopy and the effect of electron-phonon coupling on spin waves.     

Multiphonon hopping in disordered semiconductors

Equations of motion for the nonequilibrium single-frequency density matrix are used to obtain a balance equation describing the kinetics of electron hopping between localized states in disordered semiconductors. Conclusions are drawn about the applicability of the theory of multiphonon processes to systems with a small nonadiabaticity. The decoupling used is a generalization of one used earlier to the multiphonon case. The balance equation found in the Markov approximation for the average number of sites occupied in an electric field contains the field dependence of the multiphonon-transition probability, tending to the well-known weak-field limit.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No.2, pp.35–42, February, 1976.In conclusion the author is grateful to V. L. Bonch-Bruevich and A. G. Mironov for discussing this work.     

Exciton effects in disordered semiconductors

Experimental data on optical and photoelectric properties of molecular (polymeric) disordered semiconductors of inorganic and organic nature are analyzed. It is shown that anomalies in optical properties and in the structure of the internal photoeffect quantum-yield spectrum can be understood by assuming the existence of exciton states in disordered semiconductors.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 2, pp. 86–101, February, 1976.The author is grateful to Prof. S. G. Grenishin for his interest in this work.     

Electron transport in disordered semiconductors

The density matrix of an electron in an amorphous semiconductor, written as a double path integral, is averaged over random disorder. For small disorder the golden rule result is recovered. When the mean disorder energy approaches k_BT, the electronic mobility goes to zero signalling disorder induced localization.     

The electron-phonon coupling strength in TTF-TCNQ

An ab-initio calculation of the electron relaxation rates in TTF-TCNQ for one phonon and two-phonon processes, resulting from the linear and quadratic electron-phonon coupling is performed. This calculation is based on the detailed molecular structure. It is shown that the relaxation rates due to quadratic coupling processes are at least of the same order of magnitude as the relaxation rate for one-phonon processes. The quadratic coupling processes are dominated by molecular librations, while the linear coupling processes are dominated by translations.     

Phase transitions and electron-phonon coupling in platinum hydride

Structural phase transitions and superconducting properties of platinum hydride under pressure are explored through the first-principles calculations based on the density functional theory. Three new low-pressure phases (Pm3m, Cmmm and P4/nmm) are predicted, and all of them are metallic and stable relative to decomposed cases. The superconducting critical temperature of two high-pressure phases correlates with the electron-phonon coupling. The presence of soft modes induced by Kohn anomalies and the hybridization between H and Pt atoms result in the strong electron-phonon coupling. Our results have major implications for other transition metal hydrides under pressure.     

Effects of electron-phonon interaction in tunneling processes in nanostructures

JETP Letters - Tunneling through a system with two discrete electron levels coupled by electron-phonon interaction is considered. The interplay between elastic and inelastic tunneling channels is...     

Trap-assisted recombination in disordered organic semiconductors

The trap-assisted recombination of electrons and holes in organic semiconductors is investigated. The extracted capture coefficients of the trap-assisted recombination process are thermally activated with an identical activation energy as measured for the hole mobility μ(p). We demonstrate that the rate limiting step for this mechanism is the diffusion of free holes towards trapped electrons in their mutual Coulomb field, with the capture coefficient given by (q/ε)μ(p). As a result, both the bimolecular and trap-assisted recombination processes in organic semiconductors are governed by the charge carrier mobilities, allowing predictive modeling of organic light-emitting diodes.     

Quadrupole and octupole electron-phonon coupling in acoustoelectric effects

We propose an experiment to determine the quadrupole and octupole electron-phonon coupling in germanium and silicon. The experiment consists in measuring the electroacoustic absorption and/or the acoustoelectric field in uniaxially stressed n-type crystals. From theoretical values, quadrupole coupling should be well-measurable.     

Similarity renormalization of the electron-phonon coupling

We study the problem of the phonon-induced electron-electron interaction in a solid. Starting with a Hamiltonian that contains an electron-phonon interaction, we perform a similarity renormalization transformation to calculate an effective Hamiltonian. Using this transformation singularities due to degeneracies are avoided explicitly. The effective interactions are calculated to second order in the electronphonon coupling. It is shown that the effective interaction between two electrons forming a Cooper pair is attractive in the whole parameter space. For a simple Einstein model we calculate the renormalization of the electronic energies and the critical temperature of superconductivity.     

Intervalley-scattering-induced electron-phonon energy relaxation in many-valley semiconductors at low temperatures

We report on the effect of elastic intervalley scattering on the energy transport between electrons and phonons in many-valley semiconductors. We derive a general expression for the electron-phonon energy flow rate at the limit where elastic intervalley scattering dominates over diffusion. Electron heating experiments on doped n-type Si samples with electron concentrations (3.5-16.0) x 10(25) m(-3) are performed at sub-Kelvin temperatures. We find a good agreement between the theory and the experiment.     

Anomalous quasiparticle lifetime and strong electron-phonon coupling in graphite

We have performed ultrahigh-resolution angle-resolved photoemission spectroscopy on high-quality single crystals of graphite to elucidate the character of low-energy excitations. We found evidence for a well-defined quasiparticle (QP) peak in the close vicinity of the Fermi level comparable to the nodal QP in high-T(c) cuprates, together with the mass renormalization of the band at an extremely narrow momentum region around the K(H) point. Analysis of the QP lifetime demonstrates the presence of strong electron-phonon coupling and linear energy dependence of the QP scattering rate indicative of a marked deviation from the conventional Fermi-liquid theory.     

Theory of hopping conductivity in disordered semiconductors

The bond percolation method is used to investigate the conditions under which a linear (and not exponential) temperature dependence of hopping conductivity can be observed.Translated from Izvestiya Vysshikh Uchbenykh Zavedenii, Fizika, No.2, pp. 52–62, February, 1976.