Dual frequency resonator for the correct determination of the in-field surface impedance frequency dependence of superconductors

In high frequency applications of superconductors, a relevant parameter used to synthetically describe their performances in terms of surface resistance (proportional to power losses) dependence on the operating frequency is the so-called pinning frequency νp. Customarily, high sensitivity resonant techniques, intrinsically operating at discrete frequencies, are used to estimate νp. By exploiting a dual frequency resonator, we show here that the inaccurate evaluation of νp in the common approach based on single-frequency measurements leads to heavy underestimations of the superconductor surface resistance.