Electronic Inhomogeneity and Phase Fluctuation in One-Unit-cell FeSe Films

AbstractOne-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 (${\Delta }_{1}$ Δ 1 ~15 meV vs. ${\Delta }_{2}$ Δ 2 ~10 meV) and pairing temperatures (Tp1~52.0 K vs. Tp2~37.3 K), and correspondingly two-step nonlinear $V \sim {I}^{\alpha }$ V ~ I α behavior but a concurrent Berezinskii–Kosterlitz–Thouless (BKT)-like transition occurring at ${T}_{{{{{{\rm{BKT}}}}}}}$ T BKT ~28.7 K. Moreover, the onset transition temperature ${T}_{{{{{{\rm{c}}}}}}}^{{{{{{\rm{onset}}}}}}}$ T c onset ~54 K and zero-resistivity temperature ${T}_{{{{{{\rm{c}}}}}}}^{{{{{{\rm{zero}}}}}}}$ T c zero ~31 K are consistent with Tp1 and ${T}_{{{{{{\rm{BKT}}}}}}}$ T BKT , 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.