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The vortex state in FeSe superconducting thin film

We have investigated the low temperature isothermal current-voltage characteristics of FeSe epitaxial films with thickness of ∼ 360 nm at different magnetic fields parallel to the c axis. At zero magnetic field, with decreasing temperature a Berezinskii–Kosterlitz–Thouless transition, which often appears in two-dimensional (2D) superconductors, is observed. The current dependence of the voltage obeys the scaling relation V ∼ I 3 at the transition temperature TBKT≈4.7  K, and the temperature dependence of sheet resistance follows the prediction of the Halperin-Nelson formula in the vicinity of TBKT . When a magnetic field with magnitude 0.1 T ≲μ0H≲6  T is applied, the low temperature isothermal current-voltage data in each field can be scaled onto two different branches by using the scaling relation predicted by the vortex-glass theory. When comparing with the scaling relation, the dimensionality D can only be taken as 2, indicating a vortex-liquid phase to quasi-2D vortex-glass phase transition occurs in the FeSe film. The presence of Berezinskii–Kosterlitz–Thouless transition and the quasi-2D vortex-glass phase are related to the weak coupling between Fe-Se layers in the FeSe film.