Low-energy excitations and non-BCS superconductivity in Nb$_x$-Bi$_2$Se$_3$

TL;DR

This study uses low-temperature STM spectroscopy to investigate Nb$_x$-Bi$_2$Se$_3$, revealing non-BCS behavior and potential nodal superconductivity, suggesting it may be a topological superconductor unlike other similar compounds.

Contribution

It provides the first evidence of non-BCS, possibly nodal, superconductivity in Nb$_x$-Bi$_2$Se$_3$, indicating a higher likelihood of topological superconductivity.

Findings

Spectra deviate from BCS behavior with large low-bias spectral weight.

Evidence suggests the superconducting order parameter is nodal.

Nb$_x$-Bi$_2$Se$_3$ may be a topological superconductor.

Abstract

When certain elemental metals like Cu, Sr and Nb are intercalated between the layers of Bi$_2$Se$_3$, a topological insulator, the intercalated systems superconduct with critical temperatures around 3 K. Naturally, in all these cases, the possibility of topological superconductivity was suggested and explored. However, in cases of Cu and Sr intercalated systems, the low-temperature scanning tunneling microscopy (STM) experiments revealed fully formed gaps where no signature of low-energy states, a requisite for topological superconductivity, was found. Here, through STM spectroscopy down to 400 mK we show that in Nb$_x$-Bi$_2$Se$_3$ ($x$ = 0.25), the spectra deviate from a BCS-like behavior and the spectral weight at low-bias is large. Our observations are consistent with the idea that the order parameter of Nb$_x$-Bi$_2$Se$_3$ is nodal. Therefore, our results conclude that compared to other members of the family, Nb$_x$-Bi$_2$Se$_3$ has a stronger possibility of being a topological superconductor.