Quantized and unquantized zero-bias tunneling conductance peaks in Majorana nanowires: Conductance below and above $ 2e^2/h $

TL;DR

This paper systematically distinguishes between topological Majorana zero modes and trivial states in nanowires by analyzing conductance peak values and robustness, clarifying experimental signatures of Majorana states.

Contribution

It provides a comprehensive analysis of conductance peaks in Majorana nanowires, highlighting differences between topological and trivial states in peak values and stability.

Findings

Majorana and quasi-Majorana peaks are capped at 2e^2/h with stable plateaus.

Trivial Andreev and disorder states can exceed 2e^2/h and lack stable plateaus.

Conductance peak behavior helps differentiate topological Majorana states from trivial states.

Abstract

Majorana zero modes can appear at the wire ends of a 1D topological superconductor and manifest themselves as a quantized zero-bias conductance peak in the tunneling spectroscopy of normal-superconductor junctions. However, in superconductor-semiconductor hybrid nanowires, zero-bias conductance peaks may arise owing to topologically trivial mechanisms as well, mimicking the Majorana-induced topological peak in many aspects. In this work, we systematically investigate the characteristics of zero-bias conductance peaks for topological Majorana bound states, trivial quasi-Majorana bound states and low-energy Andreev bound states arising from smooth potential variations, and disorder-induced subgap bound states. Our focus is on the conductance peak value (i.e., equal to, greater than, or less than $2e^2/h$), as well as the robustness (plateau- or spike-like) against the tuning parameters (e.g., the magnetic field and tunneling gate voltage) for zero-bias peaks arising from the different mechanisms. We find that for Majoranas and quasi-Majoranas, the zero-bias peak values are no more than $2e^2/h$, and a quantized conductance plateau forms generically as a function of parameters. By contrast, for conductance peaks due to low-energy Andreev bound states or disorder-induced bound states, the peak values may exceed $2e^2/h$, and a conductance plateau is rarely observed unless through careful postselection and fine-tuning. Our findings should shed light on the interpretation of experimental measurements on the tunneling spectroscopy of normal-superconductor junctions of hybrid Majorana nanowires.