Electronic inhomogeneity and phase fluctuation in one-unit-cell FeSe films.

One-unit-cell FeSe films on SrTiO 3 substrates are of great interest owing to significantly enlarged pairing gaps characterized by two coherence peaks at ±10 meV and ±20 meV. In-situ transport measurement is desired to reveal novel properties. Here, we performed in-situ microscale electrical transport and combined scanning tunneling microscopy measurements on continuous one-unit-cell FeSe films with twin boundaries. We observed two spatially coexisting superconducting phases in domains and on boundaries, characterized by distinct superconducting gaps ( Δ 1 ~15 meV vs. Δ 2 ~10 meV) and pairing temperatures (T p1 ~52.0 K vs. T p2 ~37.3 K), and correspondingly two-step nonlinear V ~ I α behavior but a concurrent Berezinskii-Kosterlitz-Thouless (BKT)-like transition occurring at T BKT ~28.7 K. Moreover, the onset transition temperature T c onset ~54 K and zero-resistivity temperature T c zero ~31 K are consistent with T p1 and T BKT , respectively. Our results indicate the broadened superconducting transition in FeSe/SrTiO 3 is related to intrinsic electronic inhomogeneity due to distinct two-gap features and phase fluctuations of two-dimensional superconductivity.