Nanocalorimetric Evidence for Nematic Superconductivity in the Doped Topological Insulator Sr$_{0.1}$Bi$_{2}$Se$_{3}$

Spontaneous rotational-symmetry breaking in the superconducting state of doped $mathrm{Bi}_2mathrm{Se}_3$ has attracted significant attention as an indicator for topological superconductivity. In this paper, high-resolution calorimetry of the single-crystal $mathrm{Sr}_{0.1}mathrm{Bi}_2mathrm{Se}_3$ 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 materials 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.