Chirality manipulation of ultrafast phase switchings in a correlated CDW-Weyl semimetal

TL;DR

This paper demonstrates ultrafast, light-induced control of phase transitions in a topological semimetal, enabling manipulation of correlated states and revealing potential for quantum computing applications.

Contribution

It introduces a novel method for ultrafast, non-thermal switching between polaron, CDW, and Weyl phases using terahertz light in a topological material.

Findings

Ultrafast, non-thermal melting of polarons and CDW gap observed.

Continuous switching from polaron to CDW to Weyl phase with increasing fluence.

Emergence of non-thermal chiral photocurrent indicating phase transitions.

Abstract

A recently emerging concept for quantum phase discovery is the controlled gapping of linear band crossings in topological semimetals. For example, achieving topological superconducting and charge-density-wave (CDW) gapping could introduce Majorana zero modes and axion electrodynamics, respectively. Light engineering of correlation gaps in topological materials provides a new avenue of achieving exotic topological phases inaccessible by conventional tuning methods such as doping and straining. Here we demonstrate a light control of correlation gaps and ultrafast phase switchings in a model CDW and polaron insulator (TaSe$_4$)$_2$I recently predicted to be an axion insulator. Our ultrafast terahertz photocurrent spectroscopy reveals a two-step, non-thermal melting of polarons and electronic CDW gap via studying the fluence dependence of a {\em longitudinal} circular photogalvanic current. The helicity-dependent photocurrent observed along the propagation of light reveals continuous ultrafast switchings from the polaronic state, to the CDW (axion) phase, and finally to a hidden Weyl phase as the pump fluence increases. Other distinguishing features corroborating with the light-induced switchings include: mode-selective coupling of coherent phonons to polaron and CDW modulation, and the emergence of a {\em non-thermal} chiral photocurrent above pump threshold of CDW-related phonons. The ultrafast chirality control of correlated topological states revealed here is important to realize axion electrodynamics and quantum computing.