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

We analyze microwave cavity perturbation methods, and show that the techniq ue is an excellent, precision method to study the dynamic magnetic and diel ectric 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 <(zeta)over til de>(T)=zeta'+i zeta' is introduced, which is obtained from the measured par ameters: the shift of cavity resonant frequency delta f and quality factor Q. We focus on the case of a spherical sample placed at the center of a cyl indrical cavity resonant in the TE011 mode. Depending on the sample charact eristics, the magnetic permeability <(mu)over tilde>, the dielectric permit tivity <(epsilon)over tilde>=epsilon'+i epsilon', and the complex conductiv ity sigma=sigma'+i sigma" can be extracted from <(zeta)over tilde>(H). A fu ll spherical wave analysis of the cavity perturbation indicates that: (i) I n highly insulating samples with dielectric constant epsilon'similar to 1, the measured <(zeta)over tilde>(H)similar to<(chi)over tilde>(M), enabling direct measurement of the magnetic susceptibility. The sensitivity of the m ethod equals or surpasses that of dc superconducting quantum interference d evice measurements for the relative changes in magnetic susceptibility. (ii ) For moderate epsilon and conductivity <(sigma)over tilde>, <(zeta)over ti lde>(H)proportional to<(epsilon)over tilde>+i omega<(sigma)over tilde>/epsi lon(0)-1, thus enabling direct measurement of the sample dielectric constan t epsilon, even though the sample is placed in a microwave magnetic field. (iii) For large sigma we recover the surface impedance limit. (iv) Expressi ons are provided for the general case of a lossy dielectric represented by <(epsilon)over tilde>+i omega<(sigma)over tilde>/epsilon(0). We show that a n inversion procedure can be used to obtain <(epsilon)over tilde>+i omega<( sigma)over tilde>/epsilon(0) in a wide range of parameter values. This anal ysis has led to the observation of new phenomena in novel low-dimensional m aterials. We discuss results on magneto dynamics of the three-dimensional ( 3D) antiferromagnetic state of the spin chain compound Sr2CuO3. In dielectr ic susceptibility measurements in Sr14Cu24O41, we directly observe a dielec tric loss peak. Dimensional resonances in the paraelectric material SrTiO3 are shown to occur due to the rapid increase of dielectric constant with de creasing temperature. The cavity perturbation methods are thus an extremely sensitive probe of charge and spin dynamics in electronic materials. (C) 2 000 American Institute of Physics. [S0034-6748(00)04008-9].