Abstract
We theoretically consider transport properties of a normal metal (N)-superconducting semiconductor nanowire (S)-normal metal (N) structure (NSN) in the context of the possible existence of Majorana bound states in semiconductor–superconductor hybrid systems with spin–orbit coupling and external magnetic field. We study in detail the transport signatures of the topological quantum phase transition (TQPT) as well as the existence of the Majorana bound states in the electrical transport properties of the NSN structure. Our treatment includes the realistic non-perturbative effects of disorder, which is detrimental to the topological phase (eventually suppressing the superconducting gap completely), and the effects of the tunneling barriers (or the transparency at the tunneling NS contacts), which affect (and suppress) the zero bias conductance peak associated with the zero-energy Majorana bound states. We show that in the presence of generic disorder and barrier transparency the interpretation of the zero bias peak as being associated with the Majorana bound state is problematic since the non-local correlations between the two NS contacts at two ends may not manifest themselves in the tunneling conductance through the whole NSN structure. We establish that a simple modification of the standard transport measurements using conductance differences (rather than the conductance itself as in a single NS junction) as the measured quantity can allow direct observation of the non-local correlations inherent in the Majorana bound states. We also show that our proposed analysis of transport properties of the NSN junction enables the mapping out of the topological phase diagram (even in the presence of considerable disorder) by precisely detecting the TQPT point. We propose direct experimental studies of NSN junctions (rather than just a single NS junction) in order to establish the existence of Majorana bound states and the topological superconducting phase in semiconductor nanowires of current interest. Throughout the work we emphasize that the NSN transport properties are sensitive to both the bulk topological phase and the end Majorana bound states, and thus the NSN junction is well-suited for studying the non-local correlations between the end Majorana modes as well as the bulk TQPT itself.
Highlights
The subject of topological superconductors (SCs) hosting non-Abelian quasiparticles has become one of the most intensively investigated topics in condensed matter physics.[1, 2] In particular, one-dimensional topological superconductors have been predicted to support zero-energy particle-hole symmetric non-Abelian Majorana bound-states (MBS) localized at the ends.[2]
Tunneling transport in nanowire (S)-normal metal (N) structure (NSN) Majorana junctions across the topological quantum phase transition24 precisely at the TQPT and which could be detected in electrical measurements. This is the main idea of our proposal, and the main reason for us to propose experiments in the NSN geometry in order to establish the existence of the TQPT and the MBS in Majorana nanowires
This type of geometry is being explored at present by experimental groups studying Majorana bound states, and our study might be of relevance for the interpretation of these results
Summary
The subject of topological superconductors (SCs) hosting non-Abelian quasiparticles has become one of the most intensively investigated topics in condensed matter physics.[1, 2] In particular, one-dimensional topological superconductors have been predicted to support zero-energy particle-hole symmetric non-Abelian Majorana bound-states (MBS) localized at the ends.[2]. [6,7,8] have shown an intriguing ZBP, in apparent agreement with theoretical predictions for the existence of MBS, which appears upon application of a Zeeman field, providing compelling preliminary evidence of the Majorana scenario.[9,10,11] the interpretation of these experiments seems to be considerably more complex than the ideal models originally proposed and show several deviations from the predicted behavior, among which we mention the most important ones: a) the smallness of the ZBP in comparison to the ideally theoretical value of 2e2/h (i.e., 0.1 − 0.2 e2/h in the low temperature limit), b) the presence of a continuum of fermionic excitations in the subgap region (i.e., the so-called “soft-gap” feature) instead of a well-defined SC gap, and c) the lack of evidence for the closing and reopening of this SC soft-gap upon increasing the Zeeman field across the putative critical field Vc. In principle, the tunneling conductance at the end of the topological SC nanowire should reveal an MBS as a quantized zero-bias peak (ZBP) of magnitude 2e2/h in the conductance at zero temperature, which is a direct manifestation of the perfect Andreev reflection associated with the MBS.[8, 17,18,19,20,21] Recent experiments implementing the proposal in Refs.
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
