Electron Raman scattering in metals with strong electron-phonon coupling

Inelastic light scattering by the carriers interacting with phonons in the anisotropic metals with large penetration depth is theoretically studied. It is shown that the strong temperature dependence of the Raman scattering intensity in the region of phonon frequencies is the main characteristic feature of these processes. The effects of anisotropy, impurities and the strength of electron-phonon interaction on the frequency and temperature dependences of the polarization operator are analysed. Taking into account the anisotropy vertex corrections which obey a system of the Boltzman-type integral equations should leads to the considerable changes of the frequency behavior of scattering cross section for low frequencies. However, the changes of the temperature dependence are not so drastic. Increasing the electron-phonon coupling constant affects the particle-hole polarization operator in two possible ways to weaken temperature dependence and to make flatter frequency curves. The same effects are also from impurities. Some theoretical consequences which concern the role of electron-phonon interaction for electron Raman scattering in high-T _c superconductors aboveT _c are proposed.     

Mode-selected electron-phonon coupling in superconducting Pb nanofilms determined from He atom scattering

The electron-phonon coupling (EPC) strength for each phonon mode in superconducting Pb films is measured by inelastic helium atom scattering (IHAS). This surprising ability of IHAS relies on two facts: (a) In ultrathin metal films, the EPC range exceeds the film thickness, thus enabling IHAS to detect most film phonons, even 1 nm below the surface; (b) IHAS scattering amplitudes from single phonons are shown, by first-principle arguments, to be proportional to the respective EPC strengths. Thus IHAS is the first experiment providing mode-selected EPC strengths (mode-lambda spectroscopy).     

Variable electron-phonon coupling in isolated metallic carbon nanotubes observed by Raman scattering

We report the existence of broad and weakly asymmetric features in the high-energy (G) Raman modes of freely suspended metallic carbon nanotubes of defined chiral index. A significant variation in peak width (from 12 cm(-1) to 110 cm(-1)) is observed as a function of the nanotube's chiral structure. When the nanotubes are electrostatically gated, the peak widths decrease. The broadness of the Raman features is understood as the consequence of coupling of the phonon to electron-hole pairs, the strength of which varies with the nanotube chiral index and the position of the Fermi energy.     

The superconducting transition temperature for very large electron-phonon coupling

A rigorous derivation of the range of validity of the asymptotic relation $\text{k}{}_{\mathrm{B}}\text{T}{}_{\mathrm{c}}\text{?0.1827[λ<ω}{}^2\text{>]}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ is presented.     

Excitation of nonlinear impurity electron-phonon oscillations. Raman scattering

The impurity is shown to transit into the state of non-linear electron-phonon oscillation upon the passage of a resonance electromagnetic wave pulse. In this case some lines appear in the Raman spectrum at frequencies characteristic for the oscillations.     

Normal and superconducting state in the presence of forward electron-phonon and impurity scattering

By assuming that the superconducting pairing is due to the forward E-P scattering (FEP pairing) it is shown that the critical temperature of clean systems depends linearly on the E-P coupling constant and the isotope effect is very small. Impurities with the pronounced forward scattering (FS impurities) change analytical properties of the quasiparticle Green's function substantially compared to the case of the isotropic scattering. The FS impurities are pair-breaking and affect in the same way s- and d-wave FEP pairing making in the dirty limit.The usual isotropic impurity scattering is pair-weakening for s-wave and pair-breaking for the d-wave FEP pairing. Received 1st July 1998     

Influence of the inelastic electron-phonon scattering on the superconducting surface resistance

Experimental investigations of the rf losses in superconductors show deviations from the predictions of the weak-coupling theory. In order to explain the observations one has to take into account a number of mechanisms neglected in the weak coupling theory. One of these is the finite lifetime of the electrons due to their interaction with thermally excited phonons described by the strong-coupling theory. We have developed a computer program which calculates the surface impedance of strong-coupling superconductors at finite temperatures. In this paper the main features of the program and numerical calculations of the surface resistance of Sn, Pb, Nb and amorphous Ga are presented.     

Raman scattering in graphite intercalation compounds

Raman scattering results are reported on graphite intercalated with Br₂, ICℓ and IBr. In all of these acceptor compounds, the single E_2g2 Raman peak for pure graphite is replaced by a doublet structure identified with in-plane carbon atom vibrations. In addition, Raman peaks specific to the intercalate species are found at frequencies down-shifted from the stretching modes of the free intercalate molecules.     

On the electrical resistivity in intermediate valence compounds arising from electron-phonon scattering

The static and frequency dependent electrical resistivity in intermediate valence compounds arising from electron-phonon scattering is calculated by applying the memory function method to the periodic Anderson model. It is shown that in the static limit the resistivity may be considerably enhanced due to a finite fraction of 4f electrons being present at the Fermi level. This partial filling of the 4f shell is also responsible for the fact that the real part of the conductivity, which by Drude's formula is a Lorentzian of width 1/τ₀, is drastically reduced to an effective width 1/τ.     

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.     

Resonant Raman scattering in a superconducting transition metal compoundTiN

Resonant Raman scattering has been observed in TiN. The resonance energy at 2.8 eV corresponds to the low-energy edge of interband transitions. The measured one-phonon Raman spectra which show a gap between the acoustic and optic modes are discussed.