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Proximity-effect-induced superconductivity was studied in epitaxial topological insulator Bi₂Se₃ thin films grown on superconducting NbSe₂ single crystals. A point contact spectroscopy (PCS) method was used at low temperatures down to 40 mK. An induced superconducting gap in Bi₂Se₃ was observed in the spectra, which decreased with increasing Bi₂Se₃ layer thickness, consistent with the proximity effect in the bulk states of Bi₂Se₃ induced by NbSe₂. 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 Bi₂Se₃ on NbSe₂ sample, the bulk state gap value nearmore » the top surface is ~159 μeV, while the second gap value is ~120 μeV at 40 mK. The second gap value decreased with increasing Bi₂Se₃ layer thickness, but the ratio between the second gap and the bulk state gap remained about the same for different Bi₂Se₃ thicknesses. It is plausible that this is due to superconductivity in Bi₂Se₃ topological surface states induced through the bulk states. In conclusion, 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.« less

Microwave resonant activation is a powerful, straightforward technique to study classical and quantum systems, experimentally realized in Josephson junction devices cooled to very low temperatures. These devices typically consist of two single-gap superconductors separated by a weak link. We report the results of the first resonant activation experiments on hybrid thin film Josephson junctions consisting of a multi-gap superconductor (MgB₂) and a single-gap superconductor (Pb or Sn). We can interpret the plasma frequency in terms of theories both for conventional and hybrid junctions. Using these models, we determine the junction parameters including critical current, resistance, and capacitance and find moderatelymore » high quality factors of Q₀~100 for these junctions« less