Hyperfine spectroscopy in a quantum-limited spectrometer
We report measurements of electron spin echo envelope modulation (ESEEM) performed at millikelvin temperatures in a custom-built high-sensitivity spectrometer based on superconducting micro-resonators. The high quality factor and small mode volume (down to 0.2pL) of the resonator allow to probe a small number of spins, down to 5· 10^2. We measure 2-pulse ESEEM on two systems: erbium ions coupled to ^183W nuclei in a natural-abundance CaWO_4 crystal, and bismuth donors coupled to residual ^29Si
... uclei in a silicon substrate that was isotopically enriched in the ^28Si isotope. We also measure 3- and 5-pulse ESEEM for the bismuth donors in silicon. Quantitative agreement is obtained for both the hyperfine coupling strength of proximal nuclei, and the nuclear spin concentration.