Characterizing microwave loss mechanisms induced by magnetic fields in microstrip superconducting resonators.*

We describe an experimental approach to investigating the magnetic field dependent microwave losses in superconducting niobium microstrip resonators. The key parameters characterizing the losses are obtained by studying the dependence of the resonant frequency and the quality factor on magnetic fields along various directions, and comparing the experimental data with numerically calculated values. We propose a general way to identify contributions from quasiparticle generation and vortex motion to microwave losses.