Non-ballistic transport characteristics of superconducting point-contacts

TL;DR

This paper explores non-ballistic N/S point-contacts, revealing that their conductance spectra can display diverse features that may be mistaken for signatures of exotic phenomena, even in conventional superconductors.

Contribution

The study provides a theoretical analysis of non-ballistic N/S point-contacts, showing how contact shape and size influence spectral features, potentially mimicking exotic superconducting signatures.

Findings

Spectral features vary with contact geometry in non-ballistic regime.

Features can mimic signatures of topological and unconventional superconductivity.

Non-ballistic effects complicate interpretation of conductance spectra.

Abstract

In the "ballistic" regime, the transport across a normal metal (N)/superconductor (S) point-contact is dominated by a quantum process called Andreev reflection. Andreev reflection causes an enhancement of the conductance below the superconducting energy gap, and the ratio of the low-bias and the high-bias conductance cannot be greater than 2 when the superconductor is conventional in nature. In this regime, the features associated with Andreev reflection also provide energy and momentum-resolved spectroscopic information about the superconducting phase. Here we theoretically consider various types of N/S point contacts, away from the ballistic regime, and show that even when the superconductor under investigation is simple conventional in nature, depending on the shape, size and anatomy of the point contacts, a wide variety of spectral features may appear in the conductance spectra. Such features may misleadingly mimic theoretically expected signatures of exotic physical phenomena like Klein tunneling in topological superconductors, Andreev bound states in unconventional superconductors, multiband superconductivity and Majorana zero modes.