Microwave response of vortices in superconducting thin films of Re and Al

TL;DR

This study investigates how vortices in Re and Al superconducting thin films respond to microwave frequencies, affecting the performance of microwave circuits like qubits and detectors, with a focus on vortex trapping, loss, and flux creep effects.

Contribution

It provides a comparative analysis of vortex microwave response in Re and Al thin films, highlighting material-dependent vortex trapping thresholds and flux creep influences.

Findings

Vortices become trapped above specific magnetic field thresholds in Re and Al films.

Al resonators exhibit higher microwave loss due to vortices than Re resonators.

Flux creep effects are more pronounced in Al than in Re films.

Abstract

Vortices in superconductors driven at microwave frequencies exhibit a response related to the interplay between the vortex viscosity, pinning strength, and flux creep effects. At the same time, the trapping of vortices in superconducting microwave resonant circuits contributes excess loss and can result in substantial reductions in the quality factor. Thus, understanding the microwave vortex response in superconducting thin films is important for the design of such circuits, including superconducting qubits and photon detectors, which are typically operated in small, but non-zero, magnetic fields. By cooling in fields of the order of 100 $\mu$T and below, we have characterized the magnetic field and frequency dependence of the microwave response of a small density of vortices in resonators fabricated from thin films of Re and Al, which are common materials used in superconducting microwave circuits. Above a certain threshold cooling field, which is different for the Re and Al films, vortices become trapped in the resonators. Vortices in the Al resonators contribute greater loss and are influenced more strongly by flux creep effects than in the Re resonators. This different behavior can be described in the framework of a general vortex dynamics model.