h/e Superconducting Quantum Interference through Trivial Edge States in InAs

Abstract

Josephson junctions defined in strong spin orbit semiconductors are highly interesting for the search for topological systems. However, next to topological edge states that emerge in a sufficient magnetic field, trivial edge states can also occur. We study the trivial edge states with superconducting quantum interference measurements on non-topological InAs Josephson junctions. We observe a SQUID pattern, an indication of superconducting edge transport. Also, a remarkable h/e SQUID signal is observed that, as we find, stems from crossed Andreev states.Topological systems are a hot topic in condensed matter physics [1]. This is largely motivated by the existence of states at the interface between two topologically distinct phases, for example helical edge states in a quantum spin Hall insulator (QSHI) [2, 3]. Inducing superconductivity in these edge states would form a topological superconductor [1]. Superconducting edge transport has already been found in materials that are predicted to be QSHI [4, 5]. However, edge states can also have a non-topological origin. Trivial edge conduction is found in InAs alongside the chiral edge states in the QH regime [6] and recently in the proposed QSHI InAs/GaSb as well [7,8]. To be able to discriminate between topological and trivial states it is crucial to study transport through trivial edges also and clarify differences and similarities between them. In this work we study the superconducting transport through trivial edge states in non-topological InAs Josephson junctions using superconducting quantum interference (SQI) measurements. We find supercurrent carried by these edge states and an intriguing h/e periodic signal in a superconducting quantum interference device (SQUID) geometry.Trivial edge states arise when the Fermi level resides in the band gap in the bulk, while being pinned in the conduction band at the surface.Then, band bending leads to electron accumulation at that surface as schematically drawn in Fig. 1(a). The Fermi level pinning can have several origins: truncating the Bloch functions in space [9,10], a work function difference [11], the built-in electric field in a heterostack [12] and the surface termination [13]. In our 2D InAs Josephson junctions the accumulation surface is located at the edge of the mesa that is defined by wet etching. The quantum well is MBE grown on a GaSb substrate serving as a global bottom gate [14]. The superconducting electrodes are made of sputtered NbTiN with a spacing of 500 nm and a width of 4 µm. A SiN x dielectric separates the top gate from the heterostructure. Electrical quasi-four terminal measurements [see Fig.1(b)] are performed in a dilution refrigerator with an electron temperature of 60 mK unless stated otherwise.The electron density in the InAs quantum well is altered by using the electrostatic gates, V tg and V bg , located above and below the 2DEG. Decreasing the density subsequently increases the normal state resistance R n and reduces the switching current I s as shown in Fig. 2(a). A full resistance map ...

Imaging work and dissipation in the quantum Hall state in graphene

“…Interestingly, in proximity-induced superconductivity in graphene and InAs 2DEGs, the supercurrent was observed to flow preferentially along the edges [25][26][27][28]. Our results may shed light on the underlying mechanism, which is in turn important for studies of topological superconductivity and Majorana physics [29][30][31]. Note that the upstream edge channels can undermine the apparent topological protection only if the channels are not well equilibrated [6].…”

Gate-tunable electronic transport in p -type GaSb quantum wells

“…resistivity ρ xy , represents the total charge carrier density and is depicted in Fig. 1(d [43][44][45][46] . Next, we investigate ρ xx as function of B at fixed V tg .…”

Counterflowing edge current and its equilibration in quantum Hall devices with sharp edge potential: Roles of incompressible strips and contact configuration

“…Recent theories [6][7][8][9][10] predict that, by coupling their edge states to superconductors, QH as well as quantum spin Hall systems can be exploited to engineer exotic quasiparticles with non-Abelian statistics, a building block for robust quantum computation [11,12]. Semiconductor heterostructures comprising InAs, which can form transparent junctions with superconductors [13][14][15], are promising for such purposes. Theory further predicts that certain fractional QH edge states coupled through a superconductor may harbor even more exotic quasiparticles that would allow for universal topological quantum computation [6][7][8][9][10].…”

“…In contrast, in InAs the surface pinning occurs in the conduction band [21,22], which implies that in heterostructures the edge potential is bent downward so the electron density increases near the edge. While this is advantageous for superconducting junctions, it gives rise to trivial edge conduction with no topological origin at zero magnetic field [15,[23][24][25][26]. In a quantizing magnetic field, this suggests that the Fermi level can cross Landau levels extra times (see the inset to Fig.…”