Excess conductivity and Berezinskii–Kosterlitz–Thouless transition in superconducting FeSe thin films

Abstract

Temperature-dependent electronic transport in the vicinity of the superconducting transition is reported for quasi-two-dimensional textured FeSe thin films. The conspicuous rounding of the resistive transitions and large transition widths are indications of excess conductivity due to thermal Cooper-pair fluctuations, which can be well-described by the two-dimensional Aslamazov–Larkin theory. The Halperin–Nelson form of the sheet resistance between the phase-ordering temperature TBKT and the mean-field temperature TMF, and the power-law behaviour of the voltage–current characteristics, with a distinctive jump of the exponent at TBKT, are indicative of a Berezinskii–Kosterlitz–Thouless transition. The complementary results suggest a two-dimensional character of superconductivity in the FeSe films and allow a quantitative estimate of the Ginzburg number Gi.