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

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 $T_{\mathrm{c}}$ 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 $T_{\mathrm{c}}$ 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 $T_{\mathrm{c}}$ 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.

8 pages, 8 figures