The low-temperature variation of the London penetration depth $\lambda \left(T\right)$ in the candidate topological superconductor ${\mathrm{Nb}}_x{\mathrm{Bi}}_2{\mathrm{Se}}_3 \left(x=0.25\right)$ is reported for several crystals. The measurements were carried out by means of a tunnel-diode oscillator technique in both field orientations ($H_{\mathrm{rf}}\parallel c$ and $H_{\mathrm{rf}}\parallel ab$ planes). All samples exhibited power-law behavior at low temperatures ($\mathrm{\Delta }\lambda \sim T^2$) clearly indicating the presence of point nodes in the superconducting order parameter. The results presented here are consistent with a nematic odd-parity spin-triplet $E_u$ pairing state in ${\mathrm{Nb}}_x{\mathrm{Bi}}_2{\mathrm{Se}}_3$.

• Received 29 August 2016

DOI:https://doi.org/10.1103/PhysRevB.94.180510