Coexistence of topological and normal insulating phases in electro-optically tuned InAs/GaSb bilayer quantum wells

We report on the coexistence of both normal and topological insulating phases in InAs/GaSb bilayer quantum well induced by the built-in electric field tuned optically and electrically. The emergence of topological and normal insulating phases is assessed based on the evolution of the charge carrier densities, the resistivity dependence of the gap via in-plane magnetic fields, and the thermal activation of carriers. For the Hall bar device tuned optically, we observe the fingerprints associated with the presence of only the topological insulating phase. For another Hall bar processed identically but with an additional top gate, the coexistence of normal and topological insulating phases is found by electrical tuning. Our finding paves the way for utilizing a different electro-optical tuning scheme to manipulate InAs/GaSb bilayer quantum wells to obtain trivial-topological insulating interfaces in the bulk rather than at the physical edge of the device.

Figure 1

(a) Schematic layout of the devices made from the same InAs/GaSb BQW used for the optical tuning (HB-O, left-hand side) and the electrical tuning (HB-E, right-hand side). Before the deposition of the top-gate electrode, the HB-O device was cleaved from the processing piece. In the middle, an exemplary optical image with a top gate (for HB-E) is displayed. (b) The longitudinal resistivity ${\rho }_{xx}$ in both devices as a function of the illumination time (left panel) and top-gate voltage (right panel). For HB-E, a unique double-peak structure is observed.

Figure 2

(a) Hall resistance traces for different illumination times ranging from $t=9$ s up to 55 s. Starting from $t=25$ s, a pronounced nonlinearity is observed indicating two-carrier transport. (b) Extracted electron (in blue) and hole (in red) densities and the zero-field resistivity ${\rho }_{xx}$ (in black) for different illumination times. After $t=25$ s, electron and hole densities can be extracted indicating electron-hole hybridization. (c) $R_{\mathrm{xy}}$ versus magnetic field for $V_{\mathrm{TG}}-V_{\mathrm{peak}}=+4$ to $–2\mathrm{V}$. In contrast to HB-O, $R_{\mathrm{xy}}$ does not show pronounced nonlinearities. (d) Electron (in blue) and hole (in red) charge carrier densities and ${\rho }_{xx}$ at zero magnetic field (in black) as a function of top-gate voltage normalized to the central peak $V_{\mathrm{peak}}$. Within a small region around the gap, both carrier types are present.

Figure 3

(a),(b) ${\rho }_{xx}$ as a function of illumination time for different in-plane magnetic fields $B_x$ [panel (a)] and $B_y$ [panel (b)] for HB-O. For $B_x=4$ T and $B_y=2$ T, the resistivity peak vanishes indicating the closing of the TI gap. (c),(d) ${\rho }_{xx}$ versus $V_{\mathrm{TG}}-V_{\mathrm{peak}}$ for different in-plane magnetic fields $B_x$ [panel (c)] and $B_y$ [panel (d)] for HB-E. The double-peak feature vanishes at around $B_x=4.5$ T and $B_y=2.5$ T, and the remaining peak prevails until 10 T for both in-plane magnetic field orientations. (e) The relative change in the difference in the longitudinal resistance between the peak and saturated value at the VB as a function of the in-plane magnetic field $\mathrm{\Delta }{\rho }_{xx,\max }$. For the HB-O device, $\mathrm{\Delta }{\rho }_{xx,\max }$ drops to zero, while it remains close to unity in the HB-E device.

Figure 4

(a) ${\rho }_{xx}$ versus illumination time for different temperatures from $T$ = 1.7 K up to 17.5 K for HB-O. The resistance peak vanishes at $T$ = 17.5 K. (b) Arrhenius plot of ${\rho }_{xx,\max }$ (1/$T$). The fitting at high temperatures yields a band gap $E_{\mathrm{gap}}=(1.7\pm 0.6)$ meV. (c) ${\rho }_{xx}$ versus illumination time for different temperatures from $T$ = 1.7–50 K for HB-E. The left-hand peak for the TI phase and the right-hand peak for the NI phase vanishes around $T$ = 15 and 50 K, respectively. (d) Arrhenius plot of the left-hand (in blue) and right-hand (in black) peak values ${\rho }_{xx,\max }$. The fit of the high-temperature regime gives two band-gap values: $E_{\mathrm{gap}}$(TI) = (0.9 ± 0.7) meV and $E_{\mathrm{gap}}$(NI) = (8.7 ± 1.1) meV, attributed to 2D TI and NI phase, respectively.

Figure 5

Band gap of the InAs/GaSb BQW of the HB-O and HB-E devices as a function of the effective electric field along the growth direction at $T$ = 4.2 K. Positive and negative band-gap values correspond to NI and 2D TI phase, respectively. Negative electric field values correspond to the orientation of the electric field strength in the direction from the substrate to the surface (see Fig. 1). The insets show a plot of band dispersion for several electric field values: (a) no electric field, SM phase; (b) $\text{−}13.5$ kV/cm, 2D TI phase with $E_{\mathrm{gap}}$(TI) = 1 meV, and (c) $\text{−}70$ kV/cm, NI phase with $E_{\mathrm{gap}}$(NI) = 8 meV. For the insets, the separation of the main vertical ticks is 10 meV and for the horizontal $0.1\mathrm{n}{\mathrm{m}}^{-1}$.