Tuning from ferroelectric topological insulator to superconductor in (Pb,Sn,In)Te*

At low temperature, SnTe is both a ferroelectric and a topological crystalline insulator.  One can tune both the bulk insulating (and conducting surface) character and achieve a superconducting state, with a transition as high as 4.7 K, by partial substitution of Pb and In for Sn.  Our neutron scattering measurements confirm the soft zone-center transverse-optical phonon in superconducting (Pb,Sn,In)Te, making it similar to the case of doped SrTiO₃.  Intriguingly, in the case of Pb_0.5Sn_0.5Te, superconductivity is only observed when sufficient In is substituted to push the system into an electron-doped regime, where the normal-state resistivity is non-metallic and larger than that for SrTiO₃ when doped into its regime of superconductivity.  This unexpected combination of features is in need of an explanation.