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

Abstract

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. YBa2Cu3O7−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 (Rs) 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(Zn1/3Nb2/3)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(Zn1/3Ta2/3)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.

Superconducting micro-resonators for electron spin resonance - the good, the bad, and the future

“…This fact has motivated works on miniature resonators that would manage to maintain a high Q. One obvious way to improve the quality factor is to develop resonators made of superconducting materials [8][9][10][11][12][13]. Most relevant to this paper are works on surface micro-resonators that employ superconducting patches deposited on a dielectric substrate.…”

Low microwave loss in deposited Si and Ge thin-film dielectrics at single-photon power and low temperatures

“…Carrier concentrations determined using room-temperature Hall Effect measurements find the deposited Ge film is n-type with 6×10 12 cm -3 net carriers, essentially identical to the original wafer. Electron Paramagnetic Resonance (EPR) measurements on these a-Ge films (performed using the in-situ parallel plate EPR technique described in our earlier work 17 ) find the paramagnetic defect concentration is below the detection limit of ≈10 17 cm -3 . X-ray diffraction data on the a-Ge films do not exhibit any sharp Bragg diffraction peaks, characteristic of amorphous material.…”