Proximity-effect-induced Superconducting Gap in Topological Surface States – A Point Contact Spectroscopy Study of NbSe(2)/Bi(2)Se(3) Superconductor-Topological Insulator Heterostructures

Proximity-effect-induced superconductivity was studied in epitaxial topological insulator Bi(2)Se(3) thin films grown on superconducting NbSe(2) single crystals. A point contact spectroscopy (PCS) method was used at low temperatures down to 40 mK. An induced superconducting gap in Bi(2)Se(3) was observed in the spectra, which decreased with increasing Bi(2)Se(3) layer thickness, consistent with the proximity effect in the bulk states of Bi(2)Se(3) induced by NbSe(2). At very low temperatures, an extra point contact feature which may correspond to a second energy gap appeared in the spectrum. For a 16 quintuple layer Bi(2)Se(3) on NbSe(2) sample, the bulk state gap value near the top surface is ~159 μeV, while the second gap value is ~120 μeV at 40 mK. The second gap value decreased with increasing Bi(2)Se(3) layer thickness, but the ratio between the second gap and the bulk state gap remained about the same for different Bi(2)Se(3) thicknesses. It is plausible that this is due to superconductivity in Bi(2)Se(3) topological surface states induced through the bulk states. The two induced gaps in the PCS measurement are consistent with the three-dimensional bulk state and the two-dimensional surface state superconducting gaps observed in the angle-resolved photoemission spectroscopy (ARPES) measurement.