Gate-Tuned Mott Transition in Dilute InAs/GaSb Quantum Wells

We investigate the origin of the bulk gap in inverted InAs/GaSb quantum wells (QWs) that host spatially-separated electrons and holes using charge-neutral point (CNP) density (n\textunderscore o\textasciitilde p\textunderscore o) in gated devices as a tuning parameter. We find two distinct gap regimes: for I), n\textunderscore o \textgreater \textgreater 5×1010/cm2, a soft gap opens predominately by hybridization, which closes under B// \textgreater \textasciitilde 10T; for II), approaching the dilute limit n\textunderscore o\textasciitilde 5×1010/cm2, a hard gap opens leading to a true bulk insulator with quantized helical edges, continuously for B// up to 35T. Our results confirm that hard gap is associated with the Quantum Spin Hall (QSH) effect but cannot be explained by single-particle band theory. Instead it originates from many-body correlations. The data are remarkably consistent with a Mott insulator bulk state in the dilute InAs/GaSb bilayers. Specifically, spontaneous exciton binding is a viable mechanism for driving the Mott transition. Our results point to the importance of charge interactions in properties of QSHE in InAs/GaSb, in addition to single-particle band theories. The work in Rice was supported by DOE (measurements) and NSF (materials).