Superconductivity fluctuation in FeSe investigated by magnetic torque measurement using optical interference

We investigated the superconducting fluctuation (SCF) in FeSe by a magnetic torque measurement. This material has a large ratio of the superconducting gap to Fermi Energy, and is proposed to be located in BCS-BEC crossover regime [1]. In our method, the absolute value of the cantilever displacement can be measured with high resolution by detecting the interference intensity of the Fabry-Perot type cavity formed between the cantilever and the optical fiber. We observed SCF signal originating from the surviving vortex liquid state, whose onset temperature increases up to above 1.5 Tc in high magnetic field. Its contribution to the magnetic torque is about 0.5 % of that of the total sample magnetization at 10 K and 10 T. These features are totally different, both qualitatively and quantitatively, from those of the signal observed in the previous torque magnetometry study using a piezoresistive cantilever[1], while similar to what reported in cuprate superconductors [2]. Therefore, we conclude that SCF in FeSe is not anomalous and it can be understood in the same framework as other superconductors using Ginzburg number.
[1]S. Kasahara et al., Nature Comm. 7, 12843 (2016).
[2]L. Li et al., Phys. Rev. B 81, 054510 (2010).