Superconducting coplanar waveguide resonators for low temperature pulsed electron spin resonance spectroscopy

We discuss the design and implementation of thin film superconducting coplanar waveguide micro-resonators for pulsed electron spin resonance experiments. The performance of the resonators with P doped Si epilayer samples is compared to waveguide resonators under equivalent conditions. The high achievable filling factor even for small sized samples and the relatively high Q-factor result in a sensitivity of 4.5 × 10⁸ spins per shot, which is superior to that of conventional waveguide resonators, in particular to spins close to the sample surface. The peak microwave power is on the order of a few milliwatts, which is compatible with measurements at ultra-low temperatures. We also discuss the effect of the nonuniform microwave magnetic field on the Hahn echo power dependence.