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. In this talk I will 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 dilute 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. We will also mention the transport properties of the edge states, which suggest the occurrence of a novel helical Luttinger liquid. The unique applications of the hosting materials, InAs/GaAs and InAs/InGaSb, to topological Qubits via proximity-induced Majorana Fermions and Parafermions will be discussed.
1. Ivan Knez et al, Evidence for Helical Edge Modes in Inverted InAs/GaSb Quantum Well, Phys. Rev. Lett. 107, 136603 (2011)
2. Lingjie Du at al, Evidence for a topological excitonic insulator in InAs/GaSb bilayers, Nature Commun. 8, 1971 (2017)
3. J. Stajic, Probing an excitonic insulator, Science 358, pp. 1552-1553 (2017)