Precision microwave dielectric and magnetic susceptibility measurements of correlated electronic materials using superconducting cavities

We analyze microwave cavity perturbation methods, and show that the technique is an excellent, precision method to study the dynamic magnetic and dielectric response in the GHz frequency range. Using superconducting cavities, we obtain exceptionally high precision and sensitivity for measurements of relative changes. A dynamic electromagnetic susceptibility ζ(T)=ζ′+iζ″ is introduced, which is obtained from the measured parameters: the shift of cavity resonant frequency δf and quality factor Q. We focus on the case of a spherical sample placed at the center of a cylindrical cavity resonant in the TE011 mode. Depending on the sample characteristics, the magnetic permeability μ, the dielectric permittivity e=e′+ie″, and the complex conductivity σ=σ′+iσ″ can be extracted from ζH. A full spherical wave analysis of the cavity perturbation indicates that: (i) In highly insulating samples with dielectric constant e′∼1, the measured ζH∼χM, enabling direct measurement of the magnetic susceptibility. The sens...