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.