Fluctuating Cooper pairs in FeSe at temperatures exceeding double T c

Abstract

Abstract Temperature dependencies of excess conductivity, σ ′ , have been studied in detail for three FeSe0.94 textured polycrystalline samples prepared by partial melting and solid state reaction. It was revealed that both σ ′ and its temperature dependence are extremely sensitive to the method of sample preparation. Then, it was shown that in the range from the superconducting transition temperature T c ~9 K up to the characteristic temperature T 01 ~19 K, σ ′ ( T ) obeys the classical fluctuation theories of Aslamazov–Larkin (AL) and Hikami-Larkin (Maki-Thompson (MT) term) pointing to the existence of fluctuating Cooper pairs in FeSe at temperatures exceeding double T c . Like in cuprates, AL–MT crossover at T 0 < T 01 is observed, which means the appearance of 3D–2D dimensional transition at this temperature. This allows us to determine the coherence length along the c-axis, ξ c (0) ~3 Å, and a set of additional samples’ parameters, including the phase relaxation time, τ ϕ , of fluctuating Cooper pairs (FCPs), within a simple two-dimensional free-carrier picture. It was shown that τ ϕ in FeSe coincides with that found for YBa2Cu3O 7 − δ suggesting that the nature of superconducting fluctuations is very similar for these high-temperature superconductors of different types.