Sound propagation in density wave conductors and the effect of long-range Coulomb interaction

TL;DR

This paper theoretically investigates sound propagation in charge- and spin-density waves, highlighting the impact of long-range Coulomb interactions and the coupling of transverse sound waves to phasons, with implications for electromechanical effects.

Contribution

It introduces a hydrodynamic framework for quasi-one-dimensional systems and analyzes the influence of Coulomb screening on sound velocities and mode coupling.

Findings

Longitudinal sound velocity increase is much less than transverse below critical temperature.

Only transverse sound waves couple effectively to the density wave phason.

Electromechanical effects are significant only for transverse sound with displacement parallel to the chain.

Abstract

We study theoretically the sound propagation in charge- and spin-density waves in the hydrodynamic regime. First, making use of the method of comoving frame, we construct the stress tensor appropriate for quasi-one dimensional systems within tight-binding approximation. Taking into account the screening effect of the long-range Coulomb interaction, we find that the increase of the sound velocity below the critical temperature is about two orders of magnitude less for longitudinal sound than for transverse one. It is shown that only the transverse sound wave with displacement vector parallel to the chain direction couples to the phason of the density wave, therefore we expect significant electromechanical effect only in this case.