Muon spin rotation study of the topological superconductor Sr_x Bi₂ Se₃

We report transverse-field (TF) muon spin rotation experiments on single crystals of the topological superconductor Sr_x Bi₂ Se₃ with nominal concentrations x = 0.15 and 0.18 (T_c ∼ 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 λ ≥ 2.3 μm. 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 → 0. The data provide microscopic evidence for a superconducting volume fraction of ∼70% in the x = 0.18 crystal and thus bulk superconductivity.