Electron spin resonance spectroscopy using a Nb superconducting resonator

Recently, micro-resonator structures have demonstrated considerable enhancement of ESR spectroscopy. The high-quality factor and confined mode of these resonators (cavities) lead to an enhanced spin-cavity interaction that both increase sensitivity for conventional measurements and allow access to experiments that can investigate and utilize effects associated with strongly interacting spin ensembles and cavities. Superconducting micro-resonators are particularly interesting due to their natural compatibility with low temperatures, where the reduction of thermal noise permits coherent effects of the spin-cavity interaction to be resolved. In this work, we present X-band CW-ESR measurements of a microcrystalline BDPA sample performed using a niobium (Nb) superconducting micro-resonator. The achieved ultimate sensitivity and power conversion factor for this particular device, interfaced with an X-band Bruker EMX Micro ESR spectrometer, were found to be 5.4 x 10(8) Spins/G and 155 G root W, respectively, at a temperature of 3.8 K. The enhanced spin-cavity interaction in our setup had a profound influence on the measured BDPA spectral line shape, leading to a line-narrowing process not present in measurements performed using a conventional 3D resonator.