YBa2Cu3O7 microwave resonators for strong collective coupling with spin ensembles

Coplanar microwave resonators made of 330 nm-thick superconducting YBa2Cu3O7 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 55K and it slightly decreases for increasing fields, remaining 90% of QLðB ¼ 0Þ for B¼7 T and T¼2K. 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 di(phenyl)-(2,4,6-trinitrophenyl)iminoazanium 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 T¼2K. The spin-cavity collective coupling rate is shown to scale as the square root of the number of active spins in the ensemble.