СВЧ-импеданс тонкопленочных гибридных структур сверхпроводник--нормальный металл с большим отношением проводимостей

Temperature dependence of linear electrodynamic response of thin-film hybrid structures superconductor (MoN) — normal metal (Al) with large ratio of normal-state conductivities was studied theoretically and experimentally. Low-frequency measurements of the mutual inductance of two coils with a sample placed between them indicated an increase in magnetic screening ability of the superconductor – normal metal (SN) hybrid structures at liquid helium temperatures as $d_Al$ increases, where $d_Al$ is the thickness of the Al layer. Measurements of the frequency shift $\delta f$ of the microwave dielectric resonator, which was in contact with the SN samples as a function of temperature and $d_Al$ demonstrated that (i) type of the $\delta f(T)$ dependence depends significantly on $d_Al$ and (ii) the shift of resonant frequency of the SN structures at temperatures close to the critical temperature Tc cannot be approximated by a functional dependence $const/(1–T/T_c)$, which is typical for thin superconducting films. Numerical calculations performed within the Usadel model describe the observed effects quite well. Thus, the mentioned anomalies of the electrodynamic properties of the SN hybrid structures can be explained by an appearance of a mini-gap in the spectrum of quasi-particle excitation caused by the proximity effect in the normal metal layer, which depends on $d_Al$ as well as by the high conductivity of the Al layer.