Topological phase transition to a hidden charge density wave liquid

TL;DR

This paper demonstrates a method to observe a hidden charge density wave liquid state in 1T-TaS2 by using femtosecond laser pulses to bypass structural phase transitions, revealing topological defect dynamics and liquid CDW signatures.

Contribution

It introduces a novel photoexcitation technique to detect and study the elusive CDW liquid state, advancing understanding of correlated electron phases.

Findings

Observation of azimuthal broadening of CDW peaks indicating a hexatic state

Detection of diffuse scattering ring characteristic of a CDW liquid

Evidence of a topological defect-unbinding transition in CDW dynamics

Abstract

Charge density waves (CDWs), electronic crystals that form within a host solid, have long been speculated to melt into a spatially textured electronic liquid. Though they have not been previously detected, liquid CDWs may nonetheless be fundamental to the phase diagrams of many correlated electron systems, including high temperature superconductors and quantum Hall states. In one of the most promising candidate materials capable of hosting a liquid CDW, 1T-TaS2, a structural phase transition impedes its observation. Here, by irradiating the material with a femtosecond light pulse, we circumvent the structural phase transition to reveal how topological defect dynamics govern the otherwise invisible CDW correlations. Upon photoexcitation, the CDW diffraction peaks broaden azimuthally, initially revealing a hexatic state. At higher temperatures, photoexcitation completely destroys translational and orientational order and only a ring of diffuse scattering is observed, a key signature of a liquid CDW. Our work provides compelling evidence for a defect-unbinding transition to a CDW liquid and presents a protocol for uncovering states that are hidden by other transitions in thermal equilibrium.