Conductance Spectroscopy of Majorana Zero Modes in Superconductor-Magnetic Insulating Nanowire Hybrid Systems

Abstract

There has been a recent interest in superconductor-magnetic insulator hybrid Rashba nanowire setups for potentially hosting Majorana zero modes at smaller external Zeeman fields. Using the Keldysh non-equilibrium Green's function technique, we develop a detailed quantum transport approach that accounts for the hybrid structure comprising the Rashba nanowire coupled to the bilayer structure which includes the proximity inducing superconductor and the magnetic insulator. We provide a detailed analysis of three terminal setups to probe the local and non-local conductance spectra in both the pristine as well as the disordered nanowire setups. We uncover the conductance quantization scaling with the bilayer coupling and the signatures of the gap closing followed by the emergence of near-zero energy states, which can be attributed to topological zero modes in the clean limit. However, in the presence of a smoothly varying disorder potential, trivial Andreev bound states may form with signatures reminiscent of topological zero modes in the form of a premature gap closure in the non-local conductance spectra. Our results therefore provide transport-based analysis of the operating regimes that support the formation of Majorana modes in these hybrid systems of current interest, while investigating the effect of disorder on experimentally relevant device structures.

Prevalence of trivial zero-energy subgap states in nonuniform helical spin chains on the surface of superconductors

“…Therefore, when the rotation rate of the one-dimensional helical chain is nonuniform, the above mapping is to an effective Hamiltonian that is reminiscent of a one-dimensional semiconductor nanowire with spatially varying parameters. Such nanowires have been considered extensively and it has been shown that trivial zero-energy Andreev bound states (ABSs) are abundant in these systems [51][52][53][54][55][56][57][58][59][60][61][62][63][64][65][66][67].…”

“…In fact, it was shown that trivial Andreev bound states (ABSs) [49,50] are expected to be abundant in nanowires [38]. These trivial states can also result in zero-bias peaks which mimic MBSs and they occur, for instance, due nonuniformities in the nanowire parameters [51][52][53][54][55][56][57][58][59][60][61][62][63][64][65][66][67]. Despite several further signatures of MBSs being proposed, due to the high prevalence of possible zero-energy modes, it remains unclear if a conclusive measurement of MBSs can be performed in nanowire systems.…”