Evidence for a topological excitonic insulator in InAs/GaSb bilayers

Lingjie Du,Wenkai Lou,Gerard Sullivan,Rui-Rui Du,Junichiro Kono,Kai Chang,Xinwei Li

doi:10.1038/s41467-017-01988-1

Abstract

Electron–hole pairing can occur in a dilute semimetal, transforming the system into an excitonic insulator state in which a gap spontaneously appears at the Fermi surface, analogous to a Bardeen–Cooper–Schrieffer (BCS) superconductor. Here, we report optical spectroscopic and electronic transport evidence for the formation of an excitonic insulator gap in an inverted InAs/GaSb quantum-well system at low temperatures and low electron–hole densities. Terahertz transmission spectra exhibit two absorption lines that are quantitatively consistent with predictions from the pair-breaking excitation dispersion calculated based on the BCS gap equation. Low-temperature electronic transport measurements reveal a gap of ~2 meV (or ~25 K) with a critical temperature of ~10 K in the bulk, together with quantized edge conductance, suggesting the occurrence of a topological excitonic insulator phase.

Highlights

Electron–hole pairing can occur in a dilute semimetal, transforming the system into an excitonic insulator state in which a gap spontaneously appears at the Fermi surface, analogous to a Bardeen–Cooper–Schrieffer (BCS) superconductor
The excitonic insulator (EI) phase emerges below a density-dependent critical temperature (Fig. 1a), where a gap opens at the energy of the original Fermi surface of the semimetal, in a manner analogous to the BCS gap in a superconductor[3,4,5,6,7,8,9]
The originally proposed EI was based on a low density, equilibrium e–h gas that exists in certain semimetals, and the possibility of the EI phase was systematically studied in the vicinity of a pressure-tuned semimetal-semiconductor transition[5]

Summary

Introduction

Electron–hole pairing can occur in a dilute semimetal, transforming the system into an excitonic insulator state in which a gap spontaneously appears at the Fermi surface, analogous to a Bardeen–Cooper–Schrieffer (BCS) superconductor. We report optical spectroscopic and electronic transport evidence for the appearance of a BCS-like excitonic insulator gap in gated InAs/GaSb QW devices (Fig. 1f) at low temperatures with a low intrinsic e–h pair density, n0 ~ p0 ~ 5.5 × 1010 cm−2 (we use n and p to denote, respectively, the band electron and hole densities, and in particular, n0 and p0 denote the electron and hole densities at the charge-neutrality point).

Results

Conclusion