Observation of the massive Lee-Fukuyama phason in a charge density wave insulator

TL;DR

This study provides the first direct evidence of a massive Lee-Fukuyama phason in a charge density wave insulator, demonstrating how Coulomb interactions can gap the phason mode, with implications for THz radiation sources.

Contribution

The paper experimentally confirms the existence of a massive phason in a CDW insulator, resolving a long-standing theoretical debate about its nature.

Findings

Observation of coherent THz emission indicating a massive phason

Temperature and polarization dependence consistent with Coulomb-coupled phason

First direct experimental evidence of the Lee-Fukuyama massive phason

Abstract

The lowest-lying fundamental excitation of an incommensurate charge density wave (CDW) material is widely believed to be a massless phason -- a collective modulation of the phase of the CDW order parameter. However, as first pointed out by Lee and Fukuyama, long-range Coulomb interactions should push the phason energy up to the plasma energy of the CDW condensate, resulting in a massive phason and a fully gapped spectrum. Whether such behavior occurs in a CDW system has been unresolved for more than four decades. Using time-domain THz emission spectroscopy, we investigate this issue in the material (TaSe$_4$)$_2$I, a classical example of a quasi-one-dimensional CDW insulator. Upon transient photoexcitation at low temperatures, we find the material strikingly emits coherent, narrow-band THz radiation. The frequency, polarization and temperature-dependence of the emitted radiation imply the existence of a phason that acquires mass by coupling to long-range Coulomb interaction. Our observations constitute the first direct evidence of the massive "Lee-Fukuyama" phason and highlight the potential applicability of fundamental collective modes of correlated materials as compact and robust sources of THz radiation.