Transport in superconductor–normal metal–superconductor tunneling structures: Spinful p -wave and spin-orbit-coupled topological wires

Abstract

We theoretically study transport properties of voltage-biased one-dimensional superconductornormal metal-superconductor tunnel junctions with arbitrary junction transparency where the superconductors can have trivial or nontrivial topology. Motivated by recent experimental efforts on Majorana properties of superconductor-semiconductor hybrid systems, we consider two explicit models for topological superconductors: (i) spinful p-wave, and (ii) spin-split spin-orbit-coupled s-wave. We provide a comprehensive analysis of the zero-temperature dc current I and differential conductance dI/dV of voltage-biased junctions with or without Majorana zero modes (MZMs). The presence of an MZM necessarily gives rise to two tunneling conductance peaks at voltages eV = ±∆ lead , i.e., the voltage at which the superconducting gap edge of the lead aligns with the MZM. We find that the MZM conductance peak probed by a superconducting lead without a BCS singularity has a non-universal value which decreases with decreasing junction transparency. This is in contrast to the MZM tunneling conductance measured by a superconducting lead with a BCS singularity, where the conductance peak in the tunneling limit takes the quantized value GM = (4 − π)2e 2 /h independent of the junction transparency. We also discuss the "subharmonic gap structure", a consequence of multiple Andreev reflections, in the presence and absence of MZMs. Finally, we show that for finite-energy Andreev bound states (ABSs), the conductance peaks shift away from the gap bias voltage eV = ±∆ lead to a larger value set by the ABSs energy. Our work should have important implications for the extensive current experimental efforts toward creating topological superconductivity and MZMs in semiconductor nanowires proximity coupled to ordinary s-wave superconductors.

Evolution of pair correlation symmetries and supercurrent reversal in tilted Weyl semimetals

“…20,21 Since Majorana fermions follow non-Abelian statistics and are their own antiparticles, superconducting Weyl systems are anticipated to play a promising role in topological quantum computing. [22][23][24][25] Experimentally, superconductivity in Weyl materials has been discovered in a variety of settings, including in MoTe 2 under pressure, and in TaAs by photoemission spectroscopy. 26,27 It was reported that surface superconductivity can be induced in NbAs through ion irradiation.…”

Influence of the Majorana nonlocality on the supercurrent

“…Further, information on the spin canting angle of the MBS can be extracted using this Majorana non-locality [29,31]. Junctions consisting of a conventional s-wave (BCS) lead and a TSC have also been in the focus of some investigations where most of the works address on non-equilibrium transport [33][34][35][36][37]. For a pure s-wave-p-wave junction it was concluded that the supercurrent is blocked [38].…”

Detection of a pair density wave state in UTe2

“…Surface Andreev bound states must occur in p -wave topological superconductors 41 . Moreover, based on the phase-changing quasiparticle reflections at the p -wave surface, finite-energy Andreev resonances should also occur in the junction between a p -wave and an s -wave superconductor 42 and are observed in UTe 2 . Inside the SIS gap, we measure the map in the energy range from −500 µeV to 500 µeV.…”