Charge and spin transport through normal lead coupled to $s$-wave superconductor and a Majorana zero mode

TL;DR

This paper investigates how coupling to an s-wave superconductor affects zero-bias conductance peaks and spin transport in systems hosting Majorana zero modes, revealing that spin transport measurements may better indicate MZM presence.

Contribution

It demonstrates that coupling to an s-wave superconductor can transform ZBCCPs into valleys without eliminating MZMs and highlights spin transport as a more reliable detection method.

Findings

Zero-bias conductance peak can turn into a valley due to superconductor coupling.

Spin conductance always shows a zero-bias peak at $e/2 au$ regardless of conditions.

Spin transport measurements are more reliable for detecting MZMs.

Abstract

Zero-bias charge conductance peak (ZBCCP) is a significant symbol of Majorana zero modes (MZMs). The proximity effect of s-wave superconductor is usually demanded in the fabrication of MZMs. So in transport experiments, the system is inevitably coupled to the s-wave superconductor. Here we study how the ZBCCP is affected by coupling of the s-wave superconductor. The results show that the ZBCCP could be changed into a zero-bias valley due to the coupling of s-wave superconductor, although the conductance at the zero bias still keeps a quantized value $2e^2/h$. So it does not mean no MZM exists when no ZBCCP is experimentally observed. In addition, the spin transport is investigated. Four reflection processes (the normal reflection, spin-flip reflection, normal Andreev reflection, and equal-spin Andreev reflection) usually occur. The reflection coefficients are strongly dependent on the spin direction of the incident electron, and they may be symmetrical or Fano resonance shapes. But the spin conductance always shows a zero-bias peak with the height $e/2\pi$ regardless of the direction of the spin bias and the coupling strength of the s-wave superconductor. So measuring spin transport properties could be a more reliable method to judge the existence of MZMs.