In-situ electron paramagnetic resonance studies of paramagnetic point defects in superconducting microwave resonators

The physical nature and concentration of paramagnetic point defects in the dielectrics of superconducting planar microwave resonators have been determined using in-situ electron paramagnetic resonance spectroscopy. To perform this work, the quality factor of parallel plate and stripline resonators was measured as a function of the magnitude of a magnetic-field applied parallel to the electrode surfaces. YBa₂Cu₃O_7-δ thin film electrodes proved to be a preferred choice over Nb and MgB₂ because they are readily available and have a small surface resistance (R_s) up to high temperatures (∼77 K) and magnetic fields (i.e., <1 T). Stripline resonators with a widely used high performance microwave dielectric, Co²⁺-doped Ba(Zn_1/3Nb_2/3)O₃, are shown to have losses dominated by d-electron spin-excitations in exchange-coupled Co²⁺ point-defect clusters, even in the absence of an applied magnetic field. A significant enhanced microwave loss in stripline and parallel plate resonators is found to correlate with the presence of paramagnetic Mn²⁺ dopants in Ba(Zn_1/3Ta_2/3)O₃ ceramics and dangling bond states in amorphous Si thin films, although the identification of the dominant loss mechanism(s) in these dielectrics requires further investigation.