Gate-Tuned Spontaneous Exciton Insulator in Double-Quantum Wells

TL;DR

This study provides experimental evidence for a nontrivial exciton insulator phase in dilute InAs/GaSb quantum wells, revealing two distinct gap formation regimes and their magnetic field responses.

Contribution

It demonstrates the existence of a nontrivial exciton insulator in dilute quantum wells, distinguishing it from hybridization-induced gaps and analyzing magnetic field effects.

Findings

Two regimes of gap formation identified: hybridization-dominated and exciton insulator.

Hard gap persists under high in-plane magnetic fields in the dilute regime.

Data supports the formation of a nontrivial exciton insulator in very dilute quantum wells.

Abstract

It was proposed that a dilute semimetal is unstable against the formation of an exciton insulator, however experimental confirmations have remained elusive. We investigate the origin of bulk energy gap in inverted InAs/GaSb quantum wells (QWs) which naturally host spatially-separated electrons and holes, using charge-neutral point density (no~po) in gated-device as a tuning parameter. We find two distinct regimes of gap formation, that for I), no >> 5x1010/cm2, a soft gap opens predominately by electron-hole hybridization; and for II), approaching the dilute limit no~ 5x1010/cm2, a hard gap opens leading to a true bulk insulator with quantized edge states. Moreover, the gap is dramatically reduced as the QWs are tuned to less dilute. We further examine the response of gaps to in-plane magnetic fields, and find that for I) the gap closes at B// > ~ 10T, consistent with hybridization while for II) the gap opens continuously for B// as high as 35T. Our analyses show that the hard gap in II) cannot be explained by single-particle hybridization. The data are remarkably consistent with the formation of a nontrivial exciton insulator in very dilute InAs/GaSb QWs.