2026/07/29 by Kaiwen Zheng, Nathan J. Johnson, Nathan T. Thobaben +5
Physics and Astronomy · #cond-mat.supr-con #cond-mat.mes-hall #quant-ph
8 pages, 5 figures
arxiv created 2026/07/29 · arxiv updated 2026/07/31
We report the microwave response of coplanar-waveguide (CPW) resonators fabricated from \mathrmYBa2Cu3O7-δ (YBCO) thin films over temperatures from approximately 70~mK to 40~K. The resonators exhibit internal quality factors Qi in the range of 4×103 to 104 at 70 mK, which increase to a maximum of approximately 8×103 to 1.2×104 near 6~K. At low temperatures, both Qi and the fractional shift of the resonance frequency Δfr/fr increases with temperature, qualitatively resembling behavior commonly associated with two-level-system (TLS) defects. However, neither response saturates on the temperature scale set by the resonator frequency, and the loss exhibits no observable microwave-power dependence. We show that low-temperature frequency upturn may be better described by an additional paramagnetic response associated with defect-induced local moments or Andreev bound states, while the low-temperature loss follows an approximately logarithmic temperature dependence whose microscopic origin remains unresolved. These measurements establish the millikelvin performance of patterned YBCO resonators and show that their low-temperature response cannot be understood within the conventional TLS framework alone.