We report transverse-field (TF) muon spin rotation experiments on single crystals of the topological superconductor ${\mathrm{Sr}}_x{\mathrm{Bi}}_2{\mathrm{Se}}_3$ with nominal concentrations $x=0.15$ and 0.18 ($T_c\sim 3$ K). The TF spectra ($B=10$ mT), measured after cooling to below $T_c$ in field, did not show any additional damping of the muon precession signal due to the flux line lattice within the experimental uncertainty. This puts a lower bound on the magnetic penetration depth $\lambda \ge 2.3\mu \mathrm{m}$. However, when we induce disorder in the vortex lattice by changing the magnetic field below $T_c$, a sizable damping rate is obtained for $T\to 0$. The data provide microscopic evidence for a superconducting volume fraction of $\sim 70\%$ in the $x=0.18$ crystal and thus bulk superconductivity.

• Received 20 September 2017

DOI:https://doi.org/10.1103/PhysRevB.97.054503