Gate-Defined Josephson Weak-Links in Monolayer WTe2.

Systems combining superconductors with topological insulators offer a platform for the study of Majorana bound states and a possible route to realize fault tolerant topological quantum computation. Among the systems being considered in this field, monolayers of tungsten ditelluride (WTe2 ) have a rare combination of properties. Notably, it has been demonstrated to be a quantum spin Hall insulator (QSHI) and can easily be gated into a superconducting state. Measurements on gate-defined Josephson weak-link devices fabricated using monolayer WTe2 are reported. It is found that consideration of the 2D superconducting leads are critical in the interpretation of magnetic interference in the resulting junctions. The reported fabrication procedures suggest a facile way to produce further devices from this technically challenging material and the results mark the first step toward realizing versatile all-in-one topological Josephson weak-links using monolayer WTe2 .

Fully in situ Nb/InAs-nanowire Josephson junctions by selective-area growth and shadow evaporation.

Josephson junctions based on InAs semiconducting nanowires and Nb superconducting electrodes are fabricated in situ by a special shadow evaporation scheme for the superconductor electrode. Compared to other metallic superconductors such as Al, Nb has the advantage of a larger superconducting gap which allows operation at higher temperatures and magnetic fields. Our junctions are fabricated by shadow evaporation of Nb on pairs of InAs nanowires grown selectively on two adjacent tilted Si (111) facets and crossing each other at a small distance. The upper wire relative to the deposition source acts as a shadow mask determining the gap of the superconducting electrodes on the lower nanowire. Electron microscopy measurements show that the fully in situ fabrication method gives a clean InAs/Nb interface. A clear Josephson supercurrent is observed in the current-voltage characteristics, which can be controlled by a bottom gate. The large excess current indicates a high junction transparency. Under microwave radiation, pronounced integer Shapiro steps are observed suggesting a sinusoidal current-phase relation. Owing to the large critical field of Nb, the Josephson supercurrent can be maintained to magnetic fields exceeding 1 T. Our results show that in situ prepared Nb/InAs nanowire contacts are very interesting candidates for superconducting quantum circuits requiring large magnetic fields.

Strain relaxation and ambipolar electrical transport in GaAs/InSb core-shell nanowires.

The growth, crystal structure, strain relaxation and room temperature transport characteristics of GaAs/InSb core-shell nanowires grown using molecular beam epitaxy are investigated. Due to the large lattice mismatch between GaAs and InSb of 14%, a transition from island-based to layer-like growth occurs during the formation of the shell. High resolution transmission electron microscopy in combination with geometric phase analyses as well as X-ray diffraction with synchrotron radiation are used to investigate the strain relaxation and prove the existence of different dislocations relaxing the strain on zinc blende and wurtzite core-shell nanowire segments. While on the wurtzite phase only Frank partial dislocations are found, the strain on the zinc blende phase is relaxed by dislocations with perfect, Shockley partial and Frank partial dislocations. Even for ultrathin shells of about 2 nm thickness, the strain caused by the high lattice mismatch between GaAs and InSb is relaxed almost completely. Transfer characteristics of the core-shell nanowires show an ambipolar conductance behavior whose strength strongly depends on the dimensions of the nanowires. The interpretation is given based on an electronic band profile which is calculated for completely relaxed core/shell structures. The peculiarities of the band alignment in this situation implies simultaneously occupied electron and hole channels in the InSb shell. The ambipolar behavior is then explained by the change of carrier concentration in both channels by the gate voltage.

MBE growth of Al/InAs and Nb/InAs superconducting hybrid nanowire structures.

We report the in situ growth of crystalline aluminum (Al) and niobium (Nb) shells on indium arsenide (InAs) nanowires. The nanowires are grown on Si(111) substrates by molecular beam epitaxy (MBE) without foreign catalysts in the vapor-solid (VS) mode. The metal shells are deposited by electron-beam evaporation in a metal MBE. High quality superconductor/semiconductor (SC/SM) hybrid structures such as Al/InAs and Nb/InAs are of interest for ongoing research in the fields of gateable Josephson junctions and quantum information related research. Systematic investigations of the deposition parameters suitable for metal shell growth are conducted. In the case of Al, the substrate temperature, the growth rate and the shell thickness are considered. The substrate temperature as well as the angle of the impinging deposition flux are explored for Nb shells. The core-shell hybrid structures are characterized by electron microscopy and X-ray spectroscopy. Our results show that the substrate temperature is a crucial parameter in enabling the deposition of smooth Al layers. Contrarily, Nb films are less dependent on substrate temperature but are strongly affected by the deposition angle. At a temperature of 200 °C Nb reacts with InAs, dissolving the nanowire crystal. Our investigations result in smooth metal shells exhibiting an impurity and defect free, crystalline SC/InAs interface. Additionally, we find that the SC crystal structure is not affected by stacking faults present in the InAs nanowires.

Anisotropic phase coherence in GaAs/InAs core/shell nanowires.

Low-temperature transport in nanowires is accompanied by phase-coherent effects, which are observed as modulation of the conductance in an external magnetic field. In the GaAs/InAs core/shell nanowires investigated here, these are h/e flux periodic oscillations in a magnetic field aligned parallel to the nanowire axis and aperiodic universal conductance fluctuations in a field aligned perpendicularly to the nanowire axis. Both electron interference effects are used to analyse the phase coherence of the system. Temperature-dependent measurements are carried out, in order to derive the phase coherence lengths in the cross-sectional plane as well as along the nanowire sidewalls. It is found that these values show a strong anisotropy, which can be explained by the crystal structure of the GaAs/InAs core/shell nanowire. For nanowires with a radius as low as 45 nm, flux periodic oscillations were observed up to a temperature of 55 K.

Electron Interference in Hall Effect Measurements on GaAs/InAs Core/Shell Nanowires.

We present low-temperature magnetotransport measurements on GaAs/InAs core/shell nanowires contacted by regular source-drain leads as well as laterally attached Hall contacts, which only touch parts of the nanowire sidewalls. Low-temperature measurements between source and drain contacts show typical phase coherent effects, such as universal conductance fluctuations in a magnetic field aligned perpendicularly to the nanowire axis as well as Aharonov-Bohm-type oscillations in a parallel aligned magnetic field. However, the signal between the Hall contacts shows a Hall voltage buildup, when the magnetic field is turned perpendicular to the nanowire axis while current is driven through the wire using the source-drain contacts. At low temperatures, the phase coherent effects measured between source and drain leads are superimposed on the Hall voltage, which can be explained by nonlocal probing of large segments of the nanowire. In addition, the Aharonov-Bohm-type oscillations are also observed in the magnetoconductance at magnetic fields aligned parallel to the nanowire axis, using the laterally contacted leads. This measurement geometry hereby directly corresponds to classical Aharonov-Bohm experiments using planar quantum rings. In addition, the Hall voltage is used to characterize the nanowires in terms of charge carrier concentration and mobility, using temperature- and gate-dependent measurements as well as measurements in tilted magnetic fields. The GaAs/InAs core/shell nanowire used in combination with laterally attached contacts is therefore the ideal system to three-dimensionally combine quantum ring experiments using the cross-sectional plane and Hall experiments using the axial nanowire plane.

Angle-dependent magnetotransport in GaAs/InAs core/shell nanowires.

We study the impact of the direction of magnetic flux on the electron motion in GaAs/InAs core/shell nanowires. At small tilt angles, when the magnetic field is aligned nearly parallel to the nanowire axis, we observe Aharonov-Bohm type h/e flux periodic magnetoconductance oscillations. These are attributed to transport via angular momentum states, formed by electron waves within the InAs shell. With increasing tilt of the nanowire in the magnetic field, the flux periodic magnetoconductance oscillations disappear. Universal conductance fluctuations are observed for all tilt angles, however with increasing amplitudes for large tilt angles. We record this evolution of the electron propagation from a circling motion around the core to a diffusive transport through scattering loops and give explanations for the observed different transport regimes separated by the magnetic field orientation.