Influence of phonon dimensionality on electron energy relaxation

We studied experimentally the role of phonon dimensionality on electron-phonon (e-p) interaction in thin copper wires evaporated either on suspended silicon nitride membranes or on bulk substrates, at sub-Kelvin temperatures. The power emitted from electrons to phonons was measured using sensitive normal metal-insulator-superconductor tunnel junction thermometers. Membrane thicknesses ranging from 30 to 750 nm were used to clearly see the onset of the effects of two-dimensional (2D) phonon system. We observed for the first time that a 2D phonon spectrum clearly changes the temperature dependence and strength of the e-p scattering rate, with the interaction becoming stronger at the lowest temperatures below approximately 0.5 K for the 30 nm membranes.     

Caustic and anticaustic points in the phonon focusing patterns of cubic crystals

Phonon focusing patterns are dependent on the existence of concave/saddle regions and acoustic axes in the slowness surface. The main feature of the focusing patterns in cubic crystals can be characterized by the caustic and anticaustic points in the symmetry planes. By applying the Stroh formalism, the caustic and anticaustic points in the symmetry planes are investigated in relation to degeneracies in the Stroh eigenvalue equation. A set of analytical expressions for the locations of the caustic and anticaustic points is derived for cubic crystals.     

Dependence of the phonon spectrum of a metal on electron temperature in a nonequilibrium electron-phonon system

The dependence of the phonon spectrum of a crystal and the associated thermodynamic functions on electron temperature in the absence of equilibrium between the electrons and the lattice is investigated. The treatment is performed within the Thomas-Fermi approximation for a body-centered cubic crystal at high pressures. Zh. éksp. Teor. Fiz. 115, 231–242 (January 1999)     

Statistical theory of electron transport in open electron-phonon systems

Within the framework of general statistical mechanics of irreversible processes the electrical resistivity of an open electron-phonon system is calculated. By means of the projection operator technique an evolution equation for coupled subsystems in a heat bath is derived and specialized for electrons and longitudinal phonons, the latter being coupled to a bath of transverse phonons. The influence of heating of the electron-phonon system is investigated and the question of validity of the linear response theory with or without inclusion of a dissipative mechanism is discussed. In the balance equation for the total electron momentum, terms of only third (and higher) order in the electrical field strength and the current density appear; consequently, the transverse phonons act only as a “momentum bath”. A general resistivity formula is derived containing the Bloch-Grüneisen law as a special case and including corrections due to phonon drag up to infinite order without a partial summation.     

The effect of coherent optical phonon on thermal transport

Phonons are quantized lattice vibrations and the major heat carriers in most crystalline materials. We have been utilizing femtosecond phonon spectroscopy to excite and detect optical coherent phonons (CPs) in various materials. However, the impact of CPs to overall thermal transport is still unknown. In this study, we developed a small perturbation model in MD to simulate CPs and investigate the effects of CPs on thermal transport in Bi₂Te₃ at temperatures ranging from 20 to 325 K. The phonon frequency and lifetime predicted by our model agree very well with experimental results. It is found that the effective thermal transport estimated with the heat current autocorrelation function shows a great enhancement upon CP generation and extension, especially at low temperatures. Our results suggest that it is possible to manipulate thermal transport effectively with CPs.     

The tilt effect in the electron-phonon interaction with finite relaxation times of the electrons

A theoretical study of the tilt effect is presented namely the dependence of the longitudinal sound absorption and velocity on the angle between the magnetic field and the propagation vector. As has been shown, the tilt effect is a strong nonadiabatic effect which does not contain the small parameter s/v. The influence of the finite electron relaxation time on the singularities of the absorption and of the sound velocity is investigated. The tilt effect proves to be well-distinguishable if the condition ωτ > 1 is satisfied.     

Transverse electron focusing as a way of studying phonon kinetics. Turbulence of phonon flow

Novel techniques have been created for studying phonon kinetics. A supersonic conduction electron flux is used to produce a phonon flux. Transverse electron focusing (TEF) is applied for checking the electron gas affected by phonons. A turbulent regime of phonon flow-phonon flux step broadening-reveals itself as step broadening of non-equilibrium electron flux.     

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.     

Size effect on phonon transport in two-dimensional silicon film

Phonon transport in two-dimensional silicon film is investigated and frequency dependent Boltzmann transport equation is solved numerically using discrete ordinate method. The transient effects of phonon transport in the film are incorporated in the analysis. The influence of film size on phonon transport is examined through equivalent equilibrium temperature in the film. It is found that increasing film thickness enhances phonon scattering and dispersion in the film while increasing equivalent equilibrium temperature. The rate of equivalent equilibrium temperature increase is high in the early heating period ( \(\hbox {t} \le 50\,\hbox {ps}\) ) and the rate of temperature increase becomes gradual in the film as the heating period progresses.     

The electron-phonon interaction and itinerant electron magnetism

An attempt is made to systematically explore the close relation between the magnetic and the lattice properties in a metal. We ask the following two questions. The first is how the magnetic properties are involved in determining the lattice properties such as volume, structure, elasticity, and phonon frequency in a metal. The second is the opposite to the first: How can the electron-phonon interaction affect the magnetic properties in a metal? Various aspects of these two problems are systematically discussed from a unified point of view based on the jellium model with some extension to simulate real metals. With regard to the first problem, the basis of our discussion is the observation that since the screening of the ion-ion interaction depends on the exchange interaction between electrons, and in the ferromagnetic state on magnetization in addition, the magnetic properties of a metal are quite intimately reflected on its phonon frequency. Thus, we reveal, for instance, how sensitively the magnetic properties are to be involved in the ordinary phonon mechanism of thermal expansion and then, under what condition such phonon mechanism can give rise to the Invar behavior in the ferromagnetic state of a metal. As for the second problem, starting from a comprehensive analysis of the free energy of an interacting metallic electron-phonon system, we show the effect of the electron-phonon interaction on magnetism of a metal is of fundamental importance. We show, for instance, how phonons can be responsible for the Curie-Weiss like temperature dependence of the spin susceptibility in a ferromagnetic metal.     

Singular effect of disorder on electronic transport in strongly coupled electron-phonon systems

We solve the disordered Holstein model in three dimensions considering the phonon variables to be classical. After mapping out the phases of the "clean" strong coupling problem, we focus on the effect of disorder at strong electron-phonon (EP) coupling. The presence of even weak disorder (i) enormously enhances the resistivity (rho) at T=0, simultaneously suppressing the density of states at the Fermi level, (ii) suppresses the temperature dependent increase of rho, and (iii) leads to a regime with drho/dT<0. We locate the origin of these anomalies in the disorder induced tendency towards polaron formation, and the associated suppression in effective carrier density and mobility. These results, explicitly at "metallic" density, are of direct relevance to disordered EP materials such as covalent semiconductors, the manganites, and to anomalous transport in the A-15 compounds.     

Effect of dispersion on the phonon focusing and anisotropy of thermal conductivity of silicon single crystals in the boundary scattering regime

The effect of dispersion on the focusing of thermal phonons and on the thermal conductivity of silicon single crystals in the boundary scattering regime has been investigated. Analysis of the spectra of acoustic modes obtained for silicon single crystals from inelastic neutron scattering data has demonstrated that, upon transition from long-wavelength phonons to short-wavelength phonons, the directions of their focusing change. With an increase in temperature, this leads to a change in the anisotropy of thermal conductivity of phonons with different polarizations and, consequently, to a change in the anisotropy of the total thermal conductivity. Analysis of the temperature dependence of the thermal conductivity has revealed that the presence of extended flattened sections in the spectrum of short-wavelength transverse phonons indicates anomalously low values of the group velocity and, accordingly, a significant decrease in the contribution from these phonons to the thermal conductivity with increasing temperature. The contribution from longitudinal phonons to the thermal conductivity also significantly increases even at temperatures higher than 110 K and becomes dominant.     

Anomalous phonon scattering responsible for the electron relaxation time in the longitudinal magneto-resistance in bismuth

Anomalous longitudinal Shubnikov-de Haas oscillation of bismuth which was found by the authors is confirmed by extending the experimental temperature range up to 40K. With the temperature rise, the oscillation amplitude increases remarkably at lower temperatures, saturates and decreases slowly at higher temperature. The effect is explained by considering two types of relaxation mechanisms of electron orbits, i.e., intra-Landau level and inter-Landau level scatterings due to phonons. Phonons responsible for the inter-level scatterings are less excited at lower temperature and give the observed anomalous effect. A numerical estimate has been presented.