Proximity effect of Nb/Al superlattice as a superconducting resonator*

Superconducting resonator has evoked growing interest due to the wide spectrum of applications, especially in the booming realm of quantum information and quantum computing. How to achieve high-Q resonators as well as the ones maintaining high Q in a modest magnetic field are the long-standing goals and persistently explored issues. Here we systematically study how the proximity effect influence the performance of Nb/Al superlattice resonators. We fabricate a series of Nb/Al superlattice resonators with total thickness of 50 nm, while the number of repetition ranges from 1 (ie, 25nm Nb/25 nm Al) to 20 (ie, (1.25nm Nb/1.25nm Al)20). The DC transport measurement shows the proximity effect becomes stronger with the increasing number of repetition, while microwave measurement shows non-monotonic dependence of Q value, which may be ascribed to the competition between the enhanced proximity effect and the reduced mean free path of electrons. Based on this work, we are able to find the optimal design for proximity enhancement, which may be beneficial for further exploring high performance resonators.