Proximity induced Superconductivity in low carrier density Bi_xSb_2-xTe₃

To investigate superconductivity in topological insulators, we have fabricated and measured arrays of islands of diameter about 160nm on topological insulators grown by molecular beam epitaxy. Typical island spacing is about 40 nm. The superconductivity is induced via the proximity effect which is strongly dependent on the electronic properties of the underlying TI film. Earlier work with n-type Bi₂Se₃ where the carrier density was 3E13 carriers per square cm showed the emergence of superconductivity with a 2D critical current density of more than 1A/m at 0.7 mK. In these samples, both bulk and 2D topological surface states were made superconducting. To study proximity effect coupling into only the topological surface states we used thin films of the alloy Bi_xSb_2-xTe₃. In these materials, the total carrier density can be tuned such that the Fermi surface is completely topological and no bulk states are occupied. In samples with carrier density of 7E12 carriers per square cm, we observed the superconducting proximity effect coupling through the surface states above 0.8 K. Nonlinear transport and magnetoresistance results will be presented and discussed. This study reveals how surface and bulk carriers participate in superconductivity in TIs.