Microscopic phase-transition theory of charge density waves: revealing hidden crossovers of phason and amplitudon

TL;DR

This paper presents a microscopic theory of charge density waves that explains thermal phase fluctuations, depinning crossover, and amplitudon behavior, aligning well with experimental data and resolving previous uncertainties.

Contribution

It introduces a self-consistent microscopic model incorporating thermal fluctuations, revealing hidden crossovers and providing quantitative agreement with experiments on CDW materials.

Findings

Identification of a thermal depinning crossover at temperature T_d.

Prediction of a first-order phase transition at T_c.

Quantitative explanation of amplitudon damping and ultrafast spectroscopy signals.

Abstract

We develop a self-consistent phase-transition theory of charge density waves (CDWs), starting from a purely microscopic model. Specifically, we derive a microscopic CDW gap equation $|\Delta_0(T)|$, taking into account of thermal phase fluctuations (i.e., thermal excitation of phason) and their influence on CDW pinning (i.e., the phason mass) and CDW gap. We demonstrate that as temperature increases from zero, the phason gradually softens, leading to a thermal depinning crossover (where the phason becomes gapless) at $T_d$ and a subsequent first-order CDW phase transition at $T_c>T_d$. The predicted values of $T_d$, $T_c$ as well as the large ratio of $|\Delta_0(T=0)|/(k_BT_c)$ for the quasi-1D CDW material (TaSe$_4$)$_2$I show quantitative agreements with experimental measurements and explain many of the previously observed key thermodynamic features and unresolved issues in literature. To further validate the theory, we calculate the energy gap of CDW amplitudon and its lifetime, and reveal a crossover of amplitudon from a lightly damped to a heavily damped excitation during pinning-depinning crossover while its energy gap is nearly unchanged throughout the entire CDW phase. This finding quantitatively captures and explains the recently observed coherent signal in ultrafast THz emission spectroscopy on (TaSe$_4$)$_2$I.