Muon spin rotation study of the topological superconductor SrxBi2Se3

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 (${T}_c\ensuremath{\sim}3$ 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 $\ensuremath{\lambda}\ensuremath{\ge}2.3\phantom{\rule{4pt}{0ex}}\ensuremath{\mu}\mathrm{m}$. However, when we induce disorder in the vortex lattice by changing the magnetic field below ${T}_c$, a sizable damping rate is obtained for $T\ensuremath{\rightarrow}0$. The data provide microscopic evidence for a superconducting volume fraction of $\ensuremath{\sim}70%$ in the $x=0.18$ crystal and thus bulk superconductivity.