Superconducting properties of Cu intercalated Bi${}_2$Se${}_3$ studied by Muon Spin Spectroscopy

We present muon spin rotation measurements on superconducting Cu intercalated Bi${}_2$Se${}_3$, which was suggested as a realization of a topological superconductor. We observe a clear evidence of the superconducting transition below 4 K, where the width of magnetic field distribution increases as the temperature is decreased. The measured broadening at mK temperatures suggests a large London penetration depth in the $ab$ plane ($\lambda_{$\mathrm{e}\mathrm{f}\mathrm{f}$}\sim 1.6$ $\mu$m). We show that the temperature dependence of this broadening follows the BCS prediction, but could be consistent with several gap symmetries.