Vortex viscosity in superconducting granular aluminum resonators

The high kinetic inductance of granular aluminum resonators presents a novel regime for studying vortex dynamics in superconductors. Vortices trapped in regions of large microwave currents typically contribute excess loss for superconducting resonators and qubits. However, in highly disordered films, such as granular aluminum, we observe an anomalously low microwave loss due to vortices. We measure granular aluminum resonators of different sheet resistances cooled in a range of magnetic fields. By analyzing the dependence of the loss and reactance as a function of field and resonator frequency, we extract the vortex viscosity. For the lowest sheet resistance samples, we find the vortex viscosity to be consistent with the conventional Bardeen-Stephen model. However, for more disordered films with higher sheet resistance, we observe a vortex viscosity that is anomalously low. We discuss possible theoretical explanations for this behavior.