Proximity-induced superconductivity within the InAs/GaSb edge conducting state

We experimentally investigate Andreev transport through the interface between an indium superconductor and the edge of the InAs/GaSb bilayer. To cover all possible regimes of the InAs/GaSb spectrum, we study samples with 10-nm-, 12-nm-, and 14-nm-thick InAs quantum wells. For the trivial case of a direct band insulator in 10 nm samples, differential resistance demonstrates standard Andreev reflection. For InAs/GaSb structures with band inversion (12 and 14 nm samples), we observe distinct low-energy structures, which we regard as direct evidence for the proximity-induced superconductivity within the current-carrying edge state. For 14 nm InAs well samples, we additionally observe mesoscopiclike resistance fluctuations, which are subjected to threshold suppression in low magnetic fields.

Figure 1

(a) Sketch of the sample (not to scale) with electrical connections. 10-$\mu \mathrm{m}$-wide side normal-superconductor In-InAs/GaSb junctions are fabricated by a lift-off technique, after thermal evaporation of a thick In film (gray) over the mesa step. Charge transport is investigated across a single In-InAs/GaSb junction in a standard three-point technique: the superconducting electrode is grounded, while ohmic contacts N1 and N2 (yellow) are employed to feed the current and measure the voltage drop, respectively. (b) Schematic diagrams of the expected energy spectrum for different InAs quantum-well thicknesses; see Refs. [8, 9] for details.

Figure 2

$dV/dI\left(V\right)$ curves for a single In-InAs/GaSb junction for different samples. For every curve, $dV/dI$ is finite within the indium superconducting gap $\left|eV\right|<{\mathrm{\Delta }}_s=0.5$ meV due to Andreev reflection. The top and the bottom panels demonstrate maximum device-to-device fluctuations for a given InAs quantum-well thickness: (a),(b) 10 nm, which is expected to have trivial insulator band structure. There are no any additional $dV/dI$ features. (c),(d) 12 nm, a supposed topological insulator. There is a well-developed $dV/dI$ peak within $\pm 0.07$ mV; the subgap $dV/dI$ resistance is extremely high, about 200–800 kOhm; (e),(f) 14 nm, two-dimensional indirect-band semimetal. A zero-bias resistance dip is accompanied by a number of additional symmetric peaks of different amplitude. The curves are obtained at 30 mK in zero magnetic field.

Figure 3

(a) Suppression of the superconductivity by in-plane magnetic field for the 10 nm sample at 1.2 K and for the 12 nm one at 30 mK. The resistance drop is clearly broadened at high temperature. Monotonous $dV/dI\left(B\right)$ suppression is fully consistent with the classical Andreev reflection picture [26]. (b) Threshold suppression of the mesoscopiclike resistance fluctuations by low in-plane magnetic field for the 14 nm sample at 30 mK. The exact threshold positions depend slightly on the magnetic field direction. The dc bias is fixed at $V=0$ during the field sweep.

Figure 4

$dV/dI\left(V\right)$ behavior with (a) temperature or (b) magnetic field for the 12 nm sample. The superconductivity is gradually suppressed, but the resistance peak within $\pm 0.07$ mV is well visible for temperatures (a) below 0.6 K in zero field and (b) below 40 mT at minimal temperature.

Figure 5

Schematic energy diagram of the edge region for InAs/GaSb structures with band inversion. The proximity-induced superconductivity (blue region ${\mathrm{S}}^{\prime }$) can be expected within the conductive edge state near the indium superconducting lead [22]. The ${\mathrm{NS}}^{\prime }$ interface is responsible for Andreev reflection at biases below the induced gap, $\left|eV\right|<{\mathrm{\Delta }}_{\mathrm{ind}}$, while above this value the NS interface with bulk superconductor (S) governs the reflection process. Because of different single-particle transparency of the interfaces, $dV/dI\left(V\right)$ contains [31] an additional structure at low biases [see Figs. 2 and 2 and Figs. 2 and 2.