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

Abstract

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 ($${\Delta }_{1}$$ 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.

Atomic-Site-Dependent Pairing Gap in Monolayer FeSe/SrTiO3(001)–(√13 × √13)

“…FeSe resides in the crossover regime between weak coupling Bardeen–Cooper–Schrieffer (BCS) and strong coupling Bose–Einstein condensation (BEC), − for the large pairing gap magnitude and the relatively small Fermi energy. − Given the increasing experimental supports for the presence of fluctuated Cooper pairs in the monolayer FeSe, − the lattice-scale varied pairing gap with the particle–hole asymmetry observed here signals the preformed pairing, which interpreted the wide resistivity transition. − In contrast to the missing pseudogap feature in bulk FeSe, which has been attributed to the multiorbital screening effect, , the pseudogap-like state in the monolayer FeSe emerges upon exclusive M-centered electron pockets crossing the Fermi surface (the Γ-centered hole bands shift downward to approximately −80 meV below E F ) . − Momentum resolution is requisite to disclose the BCS-BEC crossover mechanism in multiband systems, e.g., quasi-particle interference spectrum and ARPES.…”