Suspended InAs nanowire Josephson junctions assembled via dielectrophoresis

Abstract

We present a novel technique for the realization of suspended Josephson junctions based on InAs semiconductor nanowires. The devices are assembled using a technique of drop-casting guided by dielectrophoresis that allows to finely align the nanostructures on top of the electrodes. The proposed architecture removes the interaction between the nanowire and the substrate which is known to influence disorder and the orientation of the Rashba vector. The relevance of this approach in view of the implementation of Josephson junctions based on HighTemperature Superconductors is discussed. INTRODUCTION.Hybrid devices based on low---dimensional materials coupled to superconducting (SC) electrodes emerged in the last years as an ideal platform for the investigation unconventional quantum effects. In these devices the macroscopic quantum correlations typical of SCs can be induced via proximity effect in a wide class of low---dimensional materials such as graphene 1,2 and semiconducting nanowires (NWs) 3---7 that offer a unique degree of freedom in the design and control of quantum states 8---11 . The most intriguing opportunity offered by these material combinations is the possibility to drive novel excitations and properties that none of the individual device components originally possessed. Recently, this possibility was at the base of the proposal to obtain Majorana---like excitations in semiconducting NWs 11---15 , where pairing correlations can be combined with Rashba spin---orbit coupling 15---18 . Hybrid NWs---SC Josephson junctions are typically obtained by transferring NWs to a Si/SiO2 substrate, on top of which device electrodes are nanofabricated by aligned e---beam lithography. Future advancements in the field and a better understanding of the exotic excitations induced in NW---SC hybrid structures will also depend on the possibility to realize devices having complex architectures, such as systems comprising several NWs connected in a controlled way 19 . Moreover, in these hybrid devices the NW---to---substrate interaction is known to play an important role in terms of induced disorder and by the breaking of inversion symmetry in the electrostatic environment in which the nanostructure is embedded. This is believed to lead to a pinning of the Rashba vector within the plane of the host Si substrate 20 . In this letter we demonstrate a rapid and effective method to realize NW---based suspended Josephson junctions where nanostructure positioning is obtained by dielectrophoresis (DEP) on previously patterned electrodes 21---23 . In our devices, single NWs are aligned on electrode pairs separated by nanogaps approx. , corresponding to a dimension which is over one order of magnitude smaller than the NW length. This is an unusual and challenging DEP configuration and we discuss a novel geometry for achieving a good deposition in this regime. We also show that the properties of the hybrid devices realized using this method are comparable with those of conventional non---suspended devices. We believe that ou...

Helical gaps in interacting Rashba wires at low electron densities

“…Disorder is always an important concern in one dimension, both from the point of view of Luttinger liquids where it is renormalization-group relevant for repulsive interactions, and in the Wigner crystal where it can drive a Peierls instability. In this respect, it is encouraging to note that recent experiments have managed to realize good contacts, 10 suspended wires, 9 and ballistic transport with a mean free path of several µm in InSb nanowires. 11 We therefore expect our predictions to be observable in these state-of-the-art wires.…”

Vectorial Control of the Spin–Orbit Interaction in Suspended InAs Nanowires

“…5,[28][29][30][31][32][33][34] Such an asymmetry causes the pinning of the spin-orbit field in the plane of the substrate, as demonstrated for nanowire quantum dots and for Majorana nanowires. 24,29,35 Additionally, the lack of spatial inversion symmetry in the crystal structure can introduce another spin-orbit contribution, namely the Dresselhaus SOI . 25,[31][32][33][34]36 Furthermore, local internal electric fields, caused by the Fermi level pinning at the nanowire surface, can also give extra Rashba contributions.…”

What happens in Josephson junctions at high critical current densities

“…1(a) I-V curves of a Al-BiSe (flake) (TI)-Al junction as a function of the temperature clearly indicate an overdamped regime [9], which is retrieved in Fig. 1(c) for a Al-InAs (nanowire)-Al device [14]. In these devices, the value of the critical current density c J is strongly affected by the properties of the interface [17].…”

“…In Fig. 1, panels (c) [14] and (d) [15], I-V characteristics refer to junctions where the barrier is a nanowire. In all these cases we present curves with and without hysteresis to encompass nominally the relevant limits in the phase dynamics.…”