Microwave complex conductivity measurement of superconducting very thin FeSe_1-xTe_x films (x = 0 - 0.5)

Studies on FeSe_1-xTe_x films have attracted significant attention. Although the nematic transition temperature of the FeSe_1-xTe_x films decreased after Te substition, the superconducting transition temperature, T_c, was largely enhanced following the disappearance of nematic order, which is contrary to the FeSe_1-xS_x films and bulk FeSe_1-xTe_x. To investigate the origin of this increase in T_c, we studied complex conductivity using microwaves. In particular, complex conductivity measurements spanning the entire temperature range, including the vicinity of T_c, were conducted on systematically varied FeSe_1-xTe_x (x = 0 - 0.5) thin films. To accomplish this, we developed a novel technique for measuring complex conductivity in very thin films with a thickness significantly smaller than the penetration depth using a microwave electric field.

By applying this novel technique to FeSe_1-xTe_x films, we observed a definite change in the temperature dependences of superfluid density and quasi-particle scattering rate at the nematic boundary. These changes at the nematic boundary suggest different superconducting gap structures between samples in the nematic and non-nematic phases. Furthermore, superconducting fluctuation is visible up to 1.2 T_c, irrespective with or without the nematic order. Our experimental results offer insights into the electronic states of FeSe_1-xTe_x thin films.