Many-body interactions can produce novel ground states in a condensed-matter system. For example, interacting electrons and holes can spontaneously form excitons, a neutral bound state, provided that the exciton binding energy exceeds the energy separation between the single particle states. Here we report on electrical transport measurements on spatially separated two-dimensional electron and hole gases with nominally degenerate energy subbands, realized in an InAs(10 nm)/GaSb(5 nm) coupled quantum well. We observe a narrow and intense maximum (~500 k\Omega) in the four-terminal resistivity in the charge neutrality region, separating the electron-like and hole-like regimes, with a strong activated temperature-dependence above T = 7 K and perfect stability against quantizing magnetic fields. By quantitatively comparing our data with early theoretical predictions, we show that such unexpectedly large resistance in our nominally zero-gap semi-metal system is probably due to the formation of an excitonic insulator state.
@article{arxiv.1701.07417,
title = {Evidence for an excitonic insulator phase in a zero-gap InAs/GaSb bilayer},
author = {W. Yu and V. Clericò and C. Hernández Fuentevilla and X. Shi and Y. Jiang and D. Saha and W. K. Lou and K. Chang and D. H. Huang and G. Gumbs and D. Smirnov and C. J. Stanton and Z. Jiang and V. Bellani and Y. Meziani and E. Diez and W. Pan and S. D. Hawkins and J. F. Klem},
journal= {arXiv preprint arXiv:1701.07417},
year = {2017}
}