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 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 nm 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 mesoscopic-like resistance fluctuations, which are subjected to threshold suppression in low magnetic fields.

We experimentally investigate Andreev transport through a single junction between an s-wave indium superconductor and a thick film of a three-dimensional Bi2T e3 topological insulator. We study Bi2T e3 samples with different bulk and surface characteristics, where the presence of a topological surface state is confirmed by direct ARPES measurements. All the junctions demonstrate Andreev transport within the superconducting gap. For junctions with transparent In − Bi2T e3 interfaces we find a number of nearly periodic conductance oscillations, which are accompanied by zero-bias conductance anomaly. Both effects disappear above the superconducting transition or for resistive junctions. We propose a consistent interpretation of both effects as originating from proximity-induced superconducting correlations within the Bi2T e3 topological surface state.

We experimentally investigate transport through the side junction between a niobium superconductor and the mesa edge of a two-dimensional system, realized in an InAs/GaSb double quantum well with band inversion. We demonstrate, that different transport regimes can be achieved by variation of the mesa step. We observe anomalous behavior of Andreev reflection within a finite low-bias interval, which is invariant for both transport regimes. We connect this behavior with the transition from retro-(at low biases) to specular (at high ones) Andreev reflection channels in an InAs/GaSb double quantum well with band inversion. Recent interest to an InAs/GaSb double quantum well is mostly connected with the problem of twodimensional (2D) topological insulator [1][2][3]. Similarly to the CdTe/HgCdTe quantum well [4,5], an inverted band structure can be realized in an InAs/GaSb double quantum well at some growth parameters [6][7][8][9][10][11]. For the 10 nm GaSb well, a spectrum with an inversion gap δ is realized for the 12 nm InAs quantum well [6][7][8][9][10][11], see Fig. 1. If the position of the Fermi level is properly tuned by external gates, the hybridization gap (minigap) can appear at the bands' crossings. The thinner (10 nm) or thicker (14 nm) InAs well produces a direct band 2D semiconductor or an indirect band 2D semimetal, respectively [8]. In comparison with the wellknown CdTe/HgCdTe system, the InAs/GaSb double quantum well provides the stability of a III-V material and well-developed preparation technology.Different correlated systems with band inversion are expected to demonstrate non-trivial physics in proximity with a superconductor. For the topological insulators [1][2][3], it allows topological superconductivity regime [12,13], which stimulates a search for Majorana fermions [14]. In the case of a Weyl semimetal [15], the proximity is predicted [16] to produce specular Andreev reflection [17,18].Andreev reflection [19] allows charge transport from normal metal (N) to superconductor (S) at energies below the superconducting gap. An electron is injected through the NS interface by creating a Cooper pair, so a hole is reflected back to the N side of the junction [19,20]. Usually, the reflected hole remains in the conduction band of the normal metal (so called retro-, or intraband, Andreev reflection -RAR) [19]. However, for some specific situations, a hole can appear in the valence band, which is known as specular (or interband) Andreev reflection (SAR) [17,18]. The latter has been recently reported for graphene [21].For some superconducting or ferromagnetic metals, a junction with 2D systems can be conveniently realized as a side junction at the mesa step [22][23][24]. A side superconducting contact is primary connected to the 2D edge, which transport properties are defined by the edge potential [25,26]. Since the Andreev reflection is strongly affected by the scattering at the NS interface [27], different transport regimes can be achieved by variation of the edge potential strength, e.g. by variation o...