Tip-induced Superconductivity

TL;DR

This paper reviews the phenomenon of tip-induced superconductivity (TISC) in topological materials, highlighting experimental signatures, systems where TISC occurs, and its potential for revealing topological superconductivity.

Contribution

It introduces and discusses the concept of tip-induced superconductivity in topological materials, a novel method for inducing and studying superconductivity at mesoscopic interfaces.

Findings

TISC observed in various topological materials since 2014.

Experimental signatures include temperature and magnetic field dependence of superconducting gap.

TISC provides a unique platform for probing topological superconductivity.

Abstract

It is widely believed that topological superconductivity, a hitherto elusive phase of quantum matter, can be achieved by inducing superconductivity in topological materials. In search of such topological superconductors, certain topological insulators (like, Bi$_2$Se$_3$) were successfully turned into superconductors by metal-ion (Cu, Pd, Sr, Nb etc. ) intercalation. Superconductivity could be induced in topological materials through applying pressure as well. for example, a pressure-induced superconducting phase was found in the topological insulator Bi$_2$Se$_3$. However, in all such cases, no conclusive signature of topological superconductivity was found. In this review, we will discuss about another novel way of inducing superconductivity in a non-superconducting topological material -- by creating a mesoscopic interface on the material with a non-superconducting, normal metallic tip where the mesoscopic interface becomes superconducting. Such a phase is now known as a tip-induced superconducting (TISC) phase. This was first seen in 2014 on Cd$_3$As$_2$ at IISER Mohali, India. Following that, a large number of other topological materials were shown to display TISC. Since the TISC phase emerges only at a confined region under a mesoscopic point contact, traditional bulk tools for characterizing superconductivity cannot be employed to detect/confirm such a phase. On the other hand, such a point contact geometry is ideal for probing the possible existence of a temperature and magnetic field dependent superconducting energy gap and a temperature and magnetic field dependent critical current. We will review the details of the experimental signatures that can be used to prove the existence of superconductivity even when the "text-book" tests for detecting superconductivity cannot be performed. Then, we will review different systems where a TISC phase could be realized.