Proximity-effect-induced Superconducting Gap in Topological Surface States - A Point Contact Spectroscopy Study of NbSe2/Bi2Se3 Superconductor-Topological Insulator Heterostructures

摘要

Proximity-effect-induced superconductivity was studied in epitaxialtopological insulator Bi2Se3 thin films grown on superconducting NbSe2 singlecrystals. A point contact spectroscopy (PCS) method was used at lowtemperatures down to 40 mK. An induced superconducting gap in Bi2Se3 wasobserved in the spectra, which decreased with increasing Bi2Se3 layerthickness, consistent with the proximity effect in the bulk states of Bi2Se3induced by NbSe2. At very low temperatures, an extra point contact featurewhich may correspond to a second energy gap appeared in the spectrum. For a 16quintuple layer Bi2Se3 on NbSe2 sample, the bulk state gap value near the topsurface is ~ 159 {\mu}eV, while the second gap value is ~ 120 {\mu}eV at 40 mK.The second gap value decreased with increasing Bi2Se3 layer thickness, but theratio between the second gap and the bulk state gap remained about the same fordifferent Bi2Se3 thicknesses. It is plausible that this is due tosuperconductivity in Bi2Se3 topological surface states induced through the bulkstates. The two induced gaps in the PCS measurement are consistent with thethree-dimensional bulk state and the two-dimensional surface statesuperconducting gaps observed in the angle-resolved photoemission spectroscopy(ARPES) measurement.