Evidence of nodes in the order parameter of the superconducting doped topological insulator Nb$_{\mathrm{x}}$Bi$_{\mathrm{2}}$Se$_{\mathrm{3}}$ via penetration depth measurements

The low-temperature variation of the London penetration depth $\lambda $(T) in the candidate topological superconductor Nb$_{\mathrm{x}}$Bi$_{\mathrm{2}}$Se$_{\mathrm{3}}$ (x$=$0.25) is reported for several crystals. The measurements were carried out by means of a tunnel-diode oscillator technique in both field orientations (H$_{\mathrm{rf}}$ // c and H$_{\mathrm{rf}}$ // ab planes). All samples exhibited quadratic temperature dependence at low temperatures clearly indicating the presence of point nodes in the superconducting order parameter. The results presented here are not consistent with a complete superconducting gap. We interpret our data on Nb$_{\mathrm{x}}$Bi$_{\mathrm{2}}$Se$_{\mathrm{3\thinspace }}$in terms of a nematic odd-parity spin-triplet pairing state with E$_{\mathrm{u}}$ symmetry. This work was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division, Contract No. DE-AC02-06CH11357. MPS thanks ND Energy for supporting his research and professional development through the ND Energy Postdoctoral Fellowship Program. YSH acknowledges support from National Science Foundation grant number DMR-1255607.