Conductance oscillations at the interface between a superconductor and the helical edge channel in a narrow HgTe quantum well

Abstract

We experimentally investigate electron transport through the interface between a superconductor and the edge of a two-dimensional electron system with band inversion. The interface is realized as a tunnel NbN side contact to a narrow 8 nm HgTe quantum well. It demonstrates a typical Andreev behavior with finite conductance within the superconducting gap. Surprisingly, the conductance is modulated by a number of equally-spaced oscillations. The oscillations are present only within the superconducting gap and at lowest, below 1 K, temperatures. The oscillations disappear completely in magnetic fields, normal to the two-dimensional electron system plane. In contrast, the oscillations' period is only weakly affected by the highest, up to 14 T, in-plane oriented magnetic fields. We interpret this behavior as the interference oscillations in a helical one-dimensional edge channel due to a proximity with a superconductor. PACS numbers: 73.40.Qv 71.30.+h Recently, a strong interest appears to the investigations of electron transport through the interface between a superconductor (S) and a normal (N) semiconductorbased low-dimensional structure. Because of the Majorana problem 1 , this interest is mostly connected with different SNS type structures, where the N region is a one-dimensional quantum wire 2-7 or a topological surface state 8,9 .An attractive example of a one-dimensional topological surface state is a current-carrying helical edge channel, realized in a narrow HgTe quantum well 10 . This channel originates if the well thickness exceeds the critical 6.3 nm due to the inverted band structure in the bulk HgTe two-dimensional system [11][12][13][14] . In contrast to the conventional quantum Hall edge states 15 , this channel is helical, i.e. it consists of two spin-resolved counterpropagating states in zero magnetic field. Despite the initial idea of a topological protection 10,12-14 , backscattering appears at macroscopic distances 16,17 , possibly due to the allowed two-particle process 18 and to the electron puddles 19 . The edge current has been directly demonstrated in a visualization experiment 20 in zero magnetic field. Also, the supercurrents have been investigated for a two-terminal SNS Josephson junction, with a rectangular section of quantum well located between two superconducting leads 9 . So, a narrow HgTe quantum well is a promising candidate 21 for the search for a topological superconductivity 22,23 .On the other hand, even a single SN interface is predicted to demonstrate a number of intriguing effects, e.g. conductance oscillations due to a proximity effect 24,25 , giant spin rotation 26 , and localized edge states 27 . Pronounced Fabry-Perot oscillations have been demonstrated for a three-dimensional Bi 2 Se 3 topological insulator sandwiched between a superconducting and normal leads 28 . In this experiment, a proximity with a superconductor doubled the period of the oscillations, although they were present also for normal leads. Thus, it seems to be reasonable to investigate electron trans...

Strong coupling between a permalloy ferromagnetic contact and helical edge channel in a narrow HgTe quantum well

“…The low-current resistive region with two kinks at about ±1.5 mV indicates a significant potential barrier (depletion region 24,25 ) at the 2DEG edge. Similar high-resistive junctions we have obtained for other non-magnetic materials like sputtered Nb and NbN 22 .…”

“…Our principal idea is to use side contact to the 2DEG edge at the mesa step [21][22][23] . Indeed, the usual procedure with annealed In contacts is not selective with respect to the edge state transport.…”

Specular Andreev reflection at the edge of an InAs/GaSb double quantum well with band inversion

“…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].…”

“…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].…”

Interlayer current near the edge of an InAs/GaSb double quantum well in proximity with a superconductor

“…Since the edge effects are of primary importance in InAs/GaSb bilayers 4,6-13 , we fabricate side superconductor-normal (NS) junctions 25,26 by sputtering 50 nm thick Nb or NbN film over the mesa step, see Fig. 1.…”