Superconducting phase boundaries and gap symmetry of the candidate topological superconductor In_0.8Pb_0.2Te

The family of (Sn,In,Pb)Te compounds is expected to contain members which are topological superconductors. Recently, it was observed that the solubility limit of Pb in In_1-xPb_xTe can be substantially raised via high-pressure synthesis, forming a cubic NaCl-type structure with an enhancement of T_c from ~3 K to ~5 K. We present the superconducting phase diagram of the x = 0.2 member (T_c ~4.7 K), finding lower and upper critical fields of 3.2 mT and 2.12 T, respectively. We report on a full superconducting gap as deduced from penetration depth measurements using the TDO technique at temperatures as low as ~450 mK, eliminating a nodal, odd-parity topological superconducting state as a possibility.