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

One-unit-cell FeSe films on SrTiO 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 ( 15 meV vs. ~10 meV) and pairing temperatures (T52.0 K vs. T~37.3 K), and correspondingly two-step nonlinear behavior but a concurrent Berezinskii-Kosterlitz-Thouless (BKT)-like transition occurring at ~28.7 K. Moreover, the onset transition temperature ~54 K and zero-resistivity temperature ~31 K are consistent with T and , respectively. Our results indicate the broadened superconducting transition in FeSe/SrTiO is related to intrinsic electronic inhomogeneity due to distinct two-gap features and phase fluctuations of two-dimensional superconductivity.