Electric and Magnetic Tuning Between the Trivial and Topological Phases in InAs/GaSb Double Quantum Wells

Abstract

Among the theoretically predicted two-dimensional topological insulators, InAs=GaSb double quantum wells (DQWs) have a unique double-layered structure with electron and hole gases separated in two layers, which enables tuning of the band alignment via electric and magnetic fields. However, the rich trivialtopological phase diagram has yet to be experimentally explored. We present an in situ and continuous tuning between the trivial and topological insulating phases in InAs=GaSb DQWs through electrical dual gating. Furthermore, we show that an in-plane magnetic field shifts the electron and hole bands relatively to each other in momentum space, functioning as a powerful tool to discriminate between the topologically distinct states. DOI: 10.1103/PhysRevLett.115.036803 PACS numbers: 73.21.Fg, 71.30.+h, 72.80.Ey Two-dimensional topological insulators (2DTIs), known also as quantum spin Hall insulators, are a novel class of materials characterized by an insulating bulk and gapless helical edges [1][2][3][4]. Double quantum wells (DQWs) of indium arsenide and gallium antimonide (InAs=GaSb) have a unique type-II broken gap band alignment and are especially interesting since the electron and hole gases that form a topological band structure are spatially separated [5][6][7][8]. For the appropriate layer thicknesses, the top of the hole band in GaSb lies above the bottom of the electron band in InAs; hence, for small momentum (around k ¼ 0) the band structure is inverted. At the crossing point (k cross ) of the two bands, coupling of the electrons and holes opens up a bulk hybridization gap [9][10][11][12][13][14][15][16] with gapless helical edge modes [5]. The size of the gap is determined by both k cross and the overlap of the electron and hole wave functions [17]. Because of the spatial separation of the two gases, electric and magnetic fields can induce relative shifts of the bands in energy and momentum [10,18,19], respectively. By controlling such shifts, it is possible to in situ tune between the trivial and topological insulating phases, which is the key advantage of InAs=GaSb compared to the other known 2DTIs [5,[20][21][22].Here, for the first time, we map out the full phase diagram of the InAs=GaSb DQWs by independent control of the Fermi level and the band alignment through electric dual gating. In particular, we observe the phase transition between the trivial insulator (normal gap) and topological insulator (hybridization gap). Moreover, the evolution of the resistance for in-plane magnetic fields is different in the two distinct phases, consistent with the fact that one is trivial, and the other topological.In InAs=GaSb DQWs, the band alignment can be controlled by top and back gate electrodes [5,18] [see the structure shown in Fig. 1(a) ]. The two gates control the perpendicular electric field E z , which shifts the electron and the hole bands relatively to each other in energy by ΔE ¼ eE z hzi (hzi is the average separation of the electron and hole gases), and the position of the Fermi level E F . ...

Decoupling Edge Versus Bulk Conductance in the Trivial Regime of anInAs/GaSbDouble Quantum Well Using Corbino Ring Geometry

“…1(a) is the phase diagram of the InAs=GaSb device as a function of back gate bias (V BG ) and top gate bias (V TG ), calculated using the capacitor model that we have introduced in Ref. [9] (see, specifically, its Supplemental Material, Sec. III for details).…”

“…Without a complete 2D phase diagram, the inverted regime can only be indirectly inferred, e.g., based on the resistance behavior under in-plane [3,10] or out of plane [10,11] magnetic field. It was not until recently that the complete 2D phase diagram was experimentally demonstrated [9] with the use of high mobility materials and efficient back gate coupling [12].…”

Engineering quantum spin Hall insulators by strained-layer heterostructures

“…Despite theoretical predictions for various materials [4][5][6][11][12][13][14][15][16][17][18][19] , however, experimental demonstrations of QSHIs have so far been limited to two systems-HgTe/CdTe [6][7][8] and InAs/GaSb 12,[20][21][22][23] -both of which are lattice-matched semiconductor heterostructures. InAs/GaSb quantum wells (QWs), characterized by a broken-gap type-II band alignment, have recently been attracting increasing interest fueled mainly by their favorable properties, including their good interface with superconductors 24,25 and in-situ electric tunability 26,27 . However, the residual bulk conductivity associated with the small energy gap in this system has been an obstacle to unambiguous identification of edge properties even at low temperatures 28 .…”

Edge transport in InAs and InAs/GaSb quantum wells

“…While a non-local measurement proved the existence of edge transport [4], the confirmation of spin-polarized transport demanded more complex transport experiments [5]. For the coupled quantum well system InAs/GaSb, double gating was predicted to tune density and band alignment independently [6,7] resulting in a tunable two-dimensional topological insulator. According to theory, conductance in helical edge states is switched on or off when crossing the boundary between the topological and trivial insulator by a proper change of front-and back-gate voltage.…”