The dynamics of charge-density waves

TLDR

Charge‑density waves arise in highly anisotropic materials where electron‑phonon coupling creates a pinned condensate that can still carry current under small electric fields. The review aims to examine the theories and experimental observations that underpin collective transport in charge‑density‑wave systems. It surveys theoretical models and experimental measurements of conductivity, current oscillations, transport coefficients, elastic properties, and NMR signals in these materials. The evidence from these studies shows translational motion of the condensate, with classical and quantum‑mechanical theories successfully accounting for the observed phenomena.

Abstract

In many materials with a highly anisotropic band structure, electron-phonon interactions lead to a novel type of ground state called the charge-density wave. The condensate is pinned to the underlying lattice by impurities and by boundary effects, but can, even for small electric fields, carry current in a fashion originally envisioned by Fr\"ohlich. This review discusses some of the underlying theories and the main experimental observations on this new collective transport phenomenon. The frequency- and electric-field-dependent conductivity, current oscillations, electric-field-dependent transport coefficients and elastic properties, together with nuclear-magnetic-resonance experiments, provide clear evidence for a translational motion of the condensate. Various theories, involving classical and quantum-mechanical concepts, are able to account for a broad variety of experimental findings, which were also made in the presence of combined dc and ac fields.