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

One-unit-cell FeSe films on SrTiO₃ 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 (T_p1~52.0 K vs. T_p2~37.3 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_p1 and $$ , respectively. Our results indicate the broadened superconducting transition in FeSe/SrTiO₃ is related to intrinsic electronic inhomogeneity due to distinct two-gap features and phase fluctuations of two-dimensional superconductivity.