Precise Evaluation of Characteristic of the Multi-layer Thin-film Superconductor Consisting of NbN and Insulator on Pure Nb Substrate
Résumé
In recent years, it has been pointed out that the maximum accelerating gradient of a superconducting RF cavity can be pushed up by coating the inner surface of the cavity with a multilayer thin-film structure that consists of alternating insulating and superconducting layers. In this structure, the principal parameter that limits the performance of the cavity is the critical magnetic field or effective Hc1 at which vortices start penetrating into the superconductor layer, and it is predicted to depend on the combination of the film thickness. We made samples that have NbN/SiO2 thin-film structure on pure Nb substrate with several thicknesses of NbN film deposited using DC magnetron sputtering method. Here, we report the measurement results of effective Hc1 of the NbN sample with a thickness of 200 nm by using the third-harmonic voltage method. In addition, we report the preliminary results to evaluate the dependence of the effective Hc1 on the thickness of the NbN film in the range 50 nm-200 nm.