Fluctuating Cooper pairs in FeSe at temperatures exceeding double Tc

TL;DR

This study investigates fluctuating Cooper pairs in FeSe at temperatures above twice its critical temperature, revealing their presence and properties through detailed conductivity analysis and fluctuation theories.

Contribution

It demonstrates the existence of fluctuating Cooper pairs in FeSe above 2Tc and determines key parameters like coherence length and phase relaxation time using fluctuation theories.

Findings

Fluctuating Cooper pairs exist in FeSe above 2Tc.

The AL-MT crossover indicates a 3D-2D transition at T_0.

Phase relaxation time in FeSe is similar to that in YBa2Cu3O7.

Abstract

Temperature dependencies of excess conductivity, Sigma', have been studied in detail for three FeSe_{0.94} textured polycrystalline samples prepared by partial melting and solid state reaction. It was revealed that both Sigma' 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 Tc ~ 9 K up to the characteristic temperature T_01 ~ 19 K, Sigma'(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 Tc. 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, Xi_c(0) ~ 3 A, and a set of additional samples' parameters, including the phase relaxation time, Tau_{Phi}, of fluctuating Cooper pairs, within a simple two-dimensional free-carrier picture. It was shown that Tau_{Phi} in FeSe coincides with that found for YBa2Cu3O7 suggesting that the nature of superconducting fluctuations is very similar for these high-temperature superconductors of different types.