Nanocalorimetric Evidence for Nematic Superconductivity in the Doped Topological Insulator Sr${}_{0.1}$Bi${}_2$Se${}_3$

Abstract

Spontaneous rotational-symmetry breaking in the superconducting state of doped ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ has attracted significant attention as an indicator for topological superconductivity. In this paper, high-resolution calorimetry of the single-crystal ${\mathrm{Sr}}_{0.1}{\mathrm{Bi}}_2{\mathrm{Se}}_3$ provides unequivocal evidence of a two-fold rotational symmetry in the superconducting gap by a \emph{bulk thermodynamic} probe, a fingerprint of nematic superconductivity. The extremely small specific heat anomaly resolved with our high-sensitivity technique is consistent with the material's low carrier concentration proving bulk superconductivity. The large basal-plane anisotropy of $H_{c2}$ is attributed to a nematic phase of a two-component topological gap structure $\vec{\eta }=({\eta }_1,{\eta }_2)$ and caused by a symmetry-breaking energy term $\delta (|{\eta }_1{|}^2-|{\eta }_2{|}^2)T_c$. A quantitative analysis of our data excludes more conventional sources of this two-fold anisotropy and provides the first estimate for the symmetry-breaking strength $\delta \approx 0.1$, a value that points to an onset transition of the second order parameter component below 2K.

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