Evidence for Berezinskii–Kosterlitz–Thouless transition in atomically flat two-dimensional Pb superconducting films

• The superconductivity of crystalline lead films was studied by STM and transport measurements.
• Cooper pairing and superconductivity disappear at two distinct temperatures.
• The I–V and R–T features are consistent with BKT physics.

We report results from scanning tunneling microscopy and transport measurements on a series of crystalline lead films containing an integer number of atomic layers, and find that the observed features in sufficiently thin films are consistent with Berezinskii–Kosterlitz–Thouless (BKT) physics. Specifically, Cooper pairing and superconductivity disappear at two distinct temperatures; the current–voltage characteristics in the intermediate phase are non-Ohmic; and the temperature and current dependences of resistance agree with the expectation from the BKT theory.