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A heterostructure of a topological insulator and an s-wave superconductor is a promising platform for topological superconductivity (TSC). Among the heterostructures developed so far, the surface of superconducting Pb grown on topological insulator TlBiSe₂ is expected as an ideal system to detect a signature of TSC, since the TSC is exposed at the surface of Pb due to topological proximity effect [1]. However, it has been still unclear how the carrier doping and the bulk band affect its superconducting gap structure in such a system.
Here, we performed tunneling spectroscopy on Pb nanoislands grown on Bi₂Se₃ substrates with several doping levels, using a dilution-refrigerator STM [2]. Tunneling spectra indicate a superconducting gap along with finite density-of-states within the gap. The weights of the in-gap states decrease with increasing the island thickness and with increasing the Dirac point energy of Bi₂Se₃ substrate. These results suggest that the observed in-gap states are related to the metallic bulk states of the topological insulator substrate.

[1] C. X. Trang et al., Nat. Commun. 11, 159 (2020)
[2] T. Machida et al., Rev. Sci. Instrum. 89, 093707 (2018)