Pulsed electron spin resonance of an organic microcrystal by dispersive readout.

We establish a testbed system for the development of high-sensitivity Electron Spin Resonance (ESR) techniques for small samples at cryogenic temperatures. Our system consists of a NbN thin-film planar superconducting microresonator designed to have a concentrated mode volume to couple to a small amount of paramagnetic material, and to be resilient to magnetic fields of up to 400mT. At 65mK we measure high-cooperativity coupling (C≈19) to an organic radical microcrystal containing 10 spins in a pico-litre volume.