Coplanar microwave resonators made of 330 nm-thick superconducting YBCO have been realized and characterized in a wide temperature (T, 2-100 K) and magnetic field (B, 0-7 T) range. The quality factor QL exceeds 104 below 55 K and it slightly decreases for increasing fields, remaining 90% of QL(B=0) for B=7 T and T=2 K. These features allow the coherent coupling of resonant photons with a spin ensemble at finite temperature and magnetic field. To demonstrate this, collective strong coupling was achieved by using DPPH organic radical placed at the magnetic antinode of the fundamental mode: the in-plane magnetic field is used to tune the spin frequency gap splitting across the single-mode cavity resonance at 7.75 GHz, where clear anticrossings are observed with a splitting as large as ∼82 MHz at T=2 K. The spin-cavity collective coupling rate is shown to scale as the square root of the number of active spins in the ensemble.
@article{arxiv.1503.06145,
title = {YBCO microwave resonators for strong collective coupling with spin ensembles},
author = {Alberto Ghirri and Claudio Bonizzoni and Dario Gerace and Samuele Sanna and Antonio Cassinese and Marco Affronte},
journal= {arXiv preprint arXiv:1503.06145},
year = {2015}
}