Phenomenology of the soft gap, zero-bias peak, and zero-mode splitting in ideal Majorana nanowires

Abstract

We theoretically consider the observed soft gap in the proximity-induced superconducting state of semiconductor nanowires in the presence of spin-orbit coupling, Zeeman splitting, and tunneling leads, but in the absence of any extrinsic disorder. We critically consider the effects of three distinct intrinsic physical mechanisms (tunnel barrier to normal leads, temperature, and dissipation) on the phenomenology of the gap softness in the tunnel spectroscopy of the normal-superconductor junction as a function of spin splitting and chemical potential. We find that all three mechanisms individually can produce a soft gap, leading to calculated conductance spectra qualitatively mimicking experimental results. We also show that the phenomenology of the soft gap is intrinsically tied to the broadening and the height of the Majorana zero mode(MZM)-induced differential conductance peak above the topological quantum phase transition point with both the soft gap and the quality of the MZM being simultaneously affected by tunnel barrier, temperature, and dissipation. We establish that the MZM splitting oscillations can be suppressed by temperature or dissipation, but not by the tunnel barrier. Since all three mechanisms are likely to be present in any realistic nanowires, discerning the effects of various mechanisms is difficult, necessitating detailed experimental data as a function of all the system parameters, some of which (e.g., dissipation, chemical potential, tunnel barrier) may not be known experimentally. While the tunneling-induced soft-gap behavior is benign with no direct adverse effect on the Majorana topological properties with the zero-bias peak remaining quantized at $2e^2/h$, the soft gap induced by finite temperature and/or finite dissipation is detrimental to topological properties and must be avoided as much as possible.