Microwave impedance study of superconducting (Li_1-xFe_x)OHFeSe single crystal

Microwave (MW) surface impedance Z_s =R_s +iX_s of the recently discovered high-Tc iron-based superconductor (Li_0.84Fe_0.16)OHFe_0.98Se single crystal was measured for the first time. The special X-band "hot finger" resonator allowed carrying out measurements when temperature of the sample under test can be varied from 2K to 90K and in the same time temperature of the resonator remained constant (4.2K). The unusual and important inequality R_s<X_s at T≥T_c =42K was found. The possible reason of the observed feature is considered. The temperature dependence of the quasiparticle σ₁ and superfluid σ₂ conductivities and structure of energy gap are analyzed. Experiments show the temperature dependence of the field penetration depth as Tⁿ with n=2.8 at T<T_c /3. The temperature dependence of the superfluid density confirms fully gapped behavior of a superconductor with a complex gap.