# Superconducting phase diagram and nontrivial band topology of structurally modulated Sn$_{1-x}$Sb$_{x}$

## TL;DR

This study discovers superconductivity in Sn$_{1-x}$Sb$_{x}$ alloys with a modulated structure, revealing a complex relationship between structural modulation, band topology, and superconducting properties, including nontrivial topological states.

## Contribution

It reports the first observation of superconductivity in structurally modulated Sn$_{1-x}$Sb$_{x}$ alloys and explores their nontrivial band topology, linking structural features to topological and superconducting behaviors.

## Key findings

- Superconductivity observed in Sn$_{1-x}$Sb$_{x}$ with $x$ from 0.43 to 0.6.
- Bulk $T_c$ peaks at 1.58 K near $x$=0.46.
- Zero-resistivity transition occurs above bulk $T_c$, likely due to grain boundary strain.

## Abstract

We report the discovery of superconductivity in binary alloy Sn$_{1-x}$Sb$_{x}$ with $x$ in the range of 0.43 to 0.6, which possesses a modulated rhombohedral structure due to the incommensurate ordering of Sn and Sb layers along the $c$-axis. The specific heat measurements indicate a weakly coupled, fully gapped superconducting state in this homogeneity range with a maximum bulk $T_{\rm c}$ of 1.58 K at $x$ = 0.46, though the electronic specific heat and Hall coefficients remain nearly $x$-independent. The nonmonotonic dependence of the bulk $T_{\rm c}$ is discussed in relation to the effects of Sb-layer intercalation between the [Sn$_{4}$Sb$_{3}$] seven-layer lamellae that are the essential building block for superconductivity. On the other hand, a zero-resistivity transition is found to take place well above the bulk superconducting transition, and the corresponding $T_{\rm c}$ increases monotonically with $x$ from 2.06 K to 3.29 K. This contrast, together with the uniform elements distribution revealed by energy dispersive x-ray mapping, implies that the resistive transition is due to the strain effect at the grain boundary rather than the compositional inhomogeneity. The first-principles calculations on the representative composition Sn$_{4}$Sb$_{3}$ ($x$ = 0.43) indicate that it is topologically nontrivial similar to Sb, but with different Z$_{2}$ invariants (0;111). Our results not only identify a new superconducting region in the Sn-Sb phase diagram, but also provide a viable platform to study the interplay between structural modulation, nontrivial band topology and superconductivity.