Superconducting fluctuation in FeSe investigated by precise torque magnetometry

We investigated the superconducting fluctuation in FeSe, which is assumed to be located in the BCS--BEC crossover region, via magnetic torque measurements. In our method, the absolute cantilever displacement is measured by detecting the interference intensity of the Fabry--Perot cavity formed between the cantilever and optical fiber. Our findings are totally different from the results of the previous torque magnetometry using a piezoresistive cantilever; the "giant" fluctuation diamagnetism related to the BCS--BEC crossover does not exist. Instead, a considerably smaller fluctuation signal originating from the vortex liquid was observed that showed a qualitatively similar behavior to those in cuprate superconductors. We also discuss the inconsistency between our torque data and the existence of a pseudogap proposed by an NMR experiment.