  Alloying indium into the topological insulator Sn1-xInxBi2Te4 induces bulk
superconductivity with critical temperatures Tc up to 1.85 K and upper critical
fields up to about 14 kOe. This is confirmed by electrical resistivity, heat
capacity, and magnetic susceptibility measurements. The heat capacity shows a
discontinuity at Tc and temperature dependence below Tc consistent with weak
coupling BCS theory, and suggests a superconducting gap near 0.25 meV. The
superconductivity is type-II and the topological surface states have been
verified by photoemission. A simple picture suggests analogies with the
isostructural magnetic topological insulator MnBi2Te4, in which a natural
heterostructure hosts complementary properties on different sublattices, and
motivates new interest in this large family of compounds. The existence of both
topological surface states and superconductivity in Sn1-xInxBi2Te4 identifies
these materials as promising candidates for the study of topological
superconductivity.
