Disentangling Quasiparticle and Two-Level-System Contributions to Superconducting Cuprate Microwave Resonators

Superconducting on-chip microwave resonators are a powerful tool for probing material properties ranging from superfluid stiffness to dielectric loss. However, while the behavior of resonators made from traditional materials like Nb and NbN is well understood, these systems operate only at limited temperatures and magnetic fields. In this work, we systematically characterize the microwave response of coplanar waveguide resonators fabricated from YBa₂Cu₃O_7-x (YBCO) from 80 mK to 40 K. Our results disentangle the respective roles of quasiparticle dynamics and two level systems in determining the loss and frequency shift, laying the groundwork for using superconducting resonators to explore material properties in previously inaccessible temperature and magnetic field regimes.