Broadband microwave spectroscopy of FeSe

The large enhancements of Tc that occur when FeSe is pressurized or grown as a single layer on an insulating substrate have generated immense current interest in this superconductor. Measurements of the electrodynamic response of FeSe at microwave frequencies provide some of the key means of probing its gap structure and superconducting charge dynamics. Recently, precision measurements of FeSe's microwave surface impedance were reported in which a quasiparticle mean free path in excess of 50 µm was found at 0.1 K, a record for a compound superconductor [1]. Additionally, strong evidence for a non-zero energy-gap minimum was presented, in contention with previous reports of line nodes in FeSe [2]. These measurements were made using cavity perturbation of a self-resonant coil and were limited to two discrete frequencies. Here we present measurements of surface resistance continuously from 1 to 20 GHz using a home-built broadband bolometric microwave spectrometer, and discuss their bearing on the superconducting gap structure of FeSe.
[1] Li, M. et al. New J. Phys. 18(2016)082001
[2] Kashara, S. et al. Proc. Natl. Acad. Sci. 111 16309-13