Optical sum rule in metals with strong electron-phonon interaction

The properties of a superconductor featuring strong electron-phonon and electron-impurity interactions have been studied in a single-band approximation. It is shown that the elastic scattering of electrons on static impurities decreases the electron-phonon interaction constant in proportion to the ratio of the electron-impurity relaxation rate and the seeding band width. The optical spectral weight (integral of the real part of the optical conductivity) in various energy intervals of a metal in the normal and superconducting states has been calculated as a function of the temperature and the superconducting gap. In the region of large (compared to the phonon) energies, the dependence of the spectral weight on the superconducting gap is weak, while the dependence on the temperature is completely determined by the corresponding dependence of the relaxation rate due to the elastic electron-phonon scattering far from the Fermi surface. It is shown that a difference in behavior of the spectral weight between the normal and superconducting states at lower energies is determined by so-called Holstein’s shift of the feature in the optical conductivity spectrum (rather than by the gap width, as it is commonly believed) and sharply decreases upon the introduction of impurities.     

Point-contact spectra and electron-phonon interaction functions of platinum metals

The results of the experimental studies of the electron-phonon interaction (EPI) of all platinum metals by point-contact (PC) spectroscopy are reported. The EPI spectral functions are determined using the microscopic theory of PC background. Averaged phonon frequencies are obtained.     

Anisotropic electron-phonon scattering in metals

A previously published general theory for the effects of electron-phonon scattering in metals involves all relevant anisotropies and permits a virtually exact solution of the Boltzmann equation. The electron-phonon interaction is represented by a few Wannier matrix elements that are adjusted to experimental data. Calculations of the electron lifetime and the mass enhancement in copper show good average agreement with the experiments. For reproducing the exact anisotropies further non-diagonal elements have to be considered. The calculated temperature dependence of the Hall effect agrees qualitatively with the observations and reveals the local maximum found for both pure copper and dilute alloys.     

Anisotropic electron-phonon scattering in metals

A previously published general theory for the effects of electron-phonon scattering in metals involves all relevant anisotropies and permits a virtually exact solution of the Boltzmann equation. The electron-phonon interaction is represented by a few Wannier matrix elements that are adjusted to experimental data. Calculations of the electron lifetime and the mass enhancement in copper show good average agreement with the experiments. For reproducing the exact anisotropies further non-diagonal elements have to be considered. The calculated temperature dependence of the Hall effect agrees qualitatively with the observations and reveals the local maximum found for both pure copper and dilute alloys.     

A comparison of electron-phonon interaction in metallic hydrogen and free electron like metals

The electron-phonon interaction 〈I)²〉 for metallic hydrogen is calculated directly from McMillan's expression and compared with the result recently obtained by Papaconstantopoulos and Klein using the Gaspari-Gyorffy theory. Comparison is also made with 〈I)²〉 for Na.     

Relativistic electron-phonon interaction

On the basis of a new relativistic one-particle equation it is possible to reformulate the problem of the relativistic electron-phonon interaction in terms of “oscillating rigid muffin-tins” such that it can be expressed as in the non-relativistic case. The improvement over the non-relativistic approximation is shown for lanthanum and tungsten.     

Anisotropic electron-phonon interaction in the cuprates

We explore manifestations of electron-phonon coupling on the electron spectral function for two phonon modes in the cuprates exhibiting strong renormalizations with temperature and doping. Applying simple symmetry considerations and kinematic constraints, we find that the out-of-plane, out-of-phase O buckling mode (B(1g)) involves small momentum transfers and couples strongly to electronic states near the antinode while the in-plane Cu-O breathing modes involve large momentum transfers and couples strongly to nodal electronic states. Band renormalization effects are found to be strongest in the superconducting state near the antinode, in full agreement with angle-resolved photoemission spectroscopy data.     

Anisotropy of the electron-phonon scattering rate in metals

A method of measuring the temperature dependent part of the electron scattering ratev(T) for separate groups of electrons has been developed on the basis of the radio-frequency size effect. The temperature dependence of the amplitude of a radio-frequency size effect line yields information on the probability of electron scattering averaged over the vicinity of the corresponding extremal cross section of the Fermi surface (FS). This method was applied to two problems:

1. the determination of the electron-phonon scattering rate on the FS point-by-point in noble metals; and
2. the extraction of the contribution of electron-phonon scattering collisions with electron transfering from one sheet of the FS to another to the total electron-phonon scattering rate.

     

Anisotropy of electron-phonon interactions in alkali metals

Electron-phonon scattering in the solid alkalis is distinguished from that in most other metals by a combination of three circumstances: The phonon spectra and structure factors are very anisotropic, the Fermi surface in the reduced zone is simply connected and virtually spherical and important large momentum transfers (0.7<(q/2k _F)<1.0) fall within the first large peaks of the phonon structure factors. Anisotropy of microscopic contributions to the macroscopic coefficients is controlled by and is quite sensitive to values of electron-ion matrix elements at large momentum transfer, and can be explored by a realistic yet relatively simple theoretical calculation. A brief summary is presented of such calculations, for the all alkalies, of mean free paths, thermoelectric powers, and electron-phonon mass enhancements. The results show marked anisotropy only for lithium, are consonant with experimental low field Hall coefficients and in addition indicate strong anisotropy in the mass enhancement for lithium.     

On the possibility of the direct study of local electron-phonon interaction in semiconductors

Local electron-phonon interaction in deep-level states of defects in semiconductors was studied by induced absorption spectroscopy. Using ZnS:Cu single crystals as an example, it was shown that the laser modulation of two-step impurity absorption is an efficient technique for direct investigations of phonon relaxation effects in deep-level states. It was shown that the localized states in ZnS are prone to extremely strong electron-phonon coupling.     

Electronic-structure dependence of the electron-phonon interaction in Ag

The linewidths of sp- and d-band derived electronic quantum-well states in thin films of Ag on Fe(100) are measured as a function of temperature to yield the electron-phonon coupling parameters. The results vary by a factor of up to 35 among the different states. The origin of these huge differences is traced to the decay path selection for the various initial states of the holes created by the photoemission process. The electron-phonon coupling parameter for the top d-band quantum-well state, 0.015+/-0.006, is the smallest ever reported.     

Resonance enhancement of electron-phonon interaction

A superconducting state near the phase transition curve from paramagnetic phase to superconducting phase for perovskites La_{2 ? x} Sr _x CuO₄ and YBa₂Cu₃O₆ is considered. Expressions for effective parameters of electron-spin-phonon interaction are obtained. It is shown that the critical temperature of the phase transition from paramagnetic phase to superconducting phase T _c is determined by the enhancement of electron-phonon interaction by spin fluctuations of exchange type.     

Suppression of electron-phonon interaction in narrow-band crystals

Taking the inelastic nature of the interaction of electrons with acoustic phonons into account is crucial in crystals having a conduction band width Δɛ comparable to the maximum acoustic phonon energy . In view of this, the laws of conservation of energy and wave vector impose stringent constraints on possible electron scattering processes, and single-phonon scattering becomes impossible for . The phonon contribution to the resistance may be negligible, therefore, in narrow-band conductors. Novosibirsk State University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 8, p. 78–82, August, 1997.     

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.     

Apparent electron-phonon interaction in strongly correlated systems

We study the interaction of electrons with phonons in strongly correlated solids, having high-T(c) cuprates in mind. Using sum rules, we show that the apparent strength of this interaction strongly depends on the property studied. If the solid has a small fraction (doping) delta of charge carriers, the influence of the interaction on the phonon self-energy is reduced by a factor delta, while there is no corresponding reduction of the coupling seen in the electron self-energy. This supports the interpretation of recent photoemission experiments, assuming a strong coupling to phonons.     

Point-contact spectroscopy of electron-phonon interaction in alloys

The point contact spectroscopy is used for the investigation of the electron-phonon interation in alloys. The possibilities offered by the method are studied for the case when the impurity electron mean free path is comparable with or much smaller than the point contact diameter. The changes of the electron-phonon interaction spectra of CuNi and CuFe alloys caused by those in impurity concentrations are studied and compared with the PC spectra of pure copper.     

Band structure and electron-phonon interaction in lead

A self-consistent semi-relativistic APW method has been employed to calculate from first principles the band structure and the electronic density of states of Pb. Our results were used to obtain the electron-phonon interaction and the superconducting transition temperature of lead.