Putative excitonic insulating state in narrow-gap semiconductor La$_3$Cd$_2$As$_6$

TL;DR

This paper presents evidence that La$_3$Cd$_2$As$_6$ is a rare bulk excitonic insulator, showing a transition to a highly insulating state driven by electron-hole pairing without structural change.

Contribution

It provides experimental and theoretical evidence for excitonic insulating behavior in La$_3$Cd$_2$As$_6$, a material where density functional theory fails to predict the insulating state.

Findings

Transition at 278 K to a highly insulating state

No structural transition accompanies the electronic transition

Charge fluctuations suggest an excitonic gap opening

Abstract

Excitonic insulators are electronically-driven phases of matter characterized by the spontaneous condensation of electron-hole pairs. Here we show that La$_3$Cd$_2$As$_6$ undergoes a transition at $T_{0}=278$ K to a highly insulating state with no accompanying structural transition. We observe quasi-two-dimensional electrical transport and charge fluctuations consistent with an electronic transition enabled by enhanced Coulomb interactions. Density functional theory calculations are unable to replicate the insulating ground state. Our results support the opening of a gap by excitonic effects at $T_{0}$, placing La$_3$Cd$_2$As$_6$ as a rare example of a bulk excitonic insulator.