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The crossover from the Bardeen-Cooper-Schrieffer (BCS) state for weakly paired fermions to the Bose Einstein condensation (BEC) state of tightly-bound pairs has attracted much attention as a new paradigm of superconductivity. In the BCS-BEC crossover regime, exotic phenomena, such as pseudogap formation owing to preformed pairs above superconducting transition temperature T_c, have been predicted.
It has been reported that multiband iron-based superconductor FeSe with the small Fermi energies is located at the BCS-BEC crossover regime. Moreover, very recent heat capacity and ARPES measurements have suggested a possibility that the tetragonal phase of Fe(Se_1-xS_x) (x > 0.17) is located at the state closer to the BEC limit. Pairing fluctuation and pseudogap formation above T_c, however, have not been reported so far.
In this study, we perform STM and Nernst measurements on FeSe_0.87S_0.13 and FeSe_0.75S_0.25, which are in the orthogonal and tetragonal phases, respectively. In our Nernst measurements, well exceeding Gaussian fluctuations are observed above T_c in FeSe_0.75S_0.25, while pseudogap formation is not observed by the STM measurements. We will discuss the present results in terms of the BCS-BEC crossover in multiband superconducting systems.