Strong Coupling of Microwave Photons to Antiferromagnetic Fluctuations in an Organic Magnet*

We present experiments on the coupling between a crystal of di(phenyl)-(2,4,6-trinitrophenyl)iminoazanium radicals and a superconducting microwave resonator in a circuit quantum electrodynamics (circuit QED) architecture [1]. The crystal exhibits paramagnetic behavior above 4 K, with antiferromagnetic correlations appearing below this temperature, and we demonstrate strong coupling at base temperature. The magnetic resonance acquires a field angle dependence as the crystal is cooled down, indicating anisotropy of the exchange interactions. These results show that multispin modes in organic crystals are suitable for circuit QED, offering a platform for their coherent manipulation. They also utilize the circuit QED architecture as a way to probe spin correlations at low temperature.

[1] Mergenthaler, M. et al., Phys. Rev. Lett. 119, 147701 (2017).