Photo-induced semimetallic states realised in electron-hole coupled insulators

TL;DR

This study demonstrates how light can induce insulator-to-metal transitions in electron-hole coupled insulators, revealing a non-equilibrium metallic state that offers new possibilities for optical band engineering.

Contribution

It provides direct experimental evidence of photo-induced phase transitions in excitonic insulators using time-resolved photoemission spectroscopy, highlighting the role of excitonic correlation screening.

Findings

Observation of photo-induced insulator-to-metal transition in Ta2Ni(Se1-xSx)5.

Identification of screening of excitonic correlations as key to transition timescale.

Discovery of a non-equilibrium metallic state in an excitonic insulator.

Abstract

Using light to manipulate materials into desired states is one of the goals in condensed matter physics, since light control can provide ultrafast and environmentally-friendly photonics devices. However, it is generally difficult to realise a photo-induced phase which is not merely a higher entropy phase corresponding to a high-temperature phase at equilibrium. Here, we report realization of photo-induced insulator-to-metal transitions in Ta2Ni(Se1-xSx)5 including the excitonic insulator phase using time- and angle-resolved photoemission spectroscopy. From the dynamic properties of the system, we determine that screening of excitonic correlations plays a key role in the timescale of the transition to the metallic phase, which supports the existence of an excitonic-insulator phase at equilibrium. The non-equilibrium metallic state observed unexpectedly in the direct-gap excitonic insulator opens up a new avenue to optical band engineering in electron-hole coupled systems.