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 $\tilde{\zeta}(T)=\zeta ^{\prime}+i\zeta ^{\prime \prime}$ is introduced, which is obtained from the measured parameters: 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 cylindrical cavity resonant in the $TE_{011}$ mode. Depending on the sample characteristics, the magnetic permeability $\tilde{\mu}$, the dielectric permittivity $\tilde{\epsilon}$ and the complex conductivity $\tilde{\sigma}$ can be extracted from $\tilde{\zeta}_{H}$. A full spherical wave analysis of the cavity perturbation is given. This analysis has led to the observation of new phenomena in novel low dimensional materials.Comment: 16 pages, 5 figure

Thermal conductivity and specific heat of the linear chain cuprate Sr2_{2}CuO3_{3}: Evidence for thermal transport via spinons

We report measurements of the specific heat and the thermal conductivity of the model Heisenberg spin-1/2 chain cuprate Sr$_{2}$CuO$_{3}$ at low temperatures. In addition to a nearly isotropic phonon heat transport, we find a quasi one-dimensional excess thermal conductivity along the chain direction, most likely associated with spin excitations (spinons). The spinon energy current is limited mainly by scattering on defects and phonons. Analyzing the specific heat data, the intrachain magnetic exchange $J/k_{B}$ is estimated to be 2650 K.Comment: 4 RevTeX pages, 3 figures, to appear in Phys. Rev.

Heat transport by lattice and spin excitations in the spin chain compounds SrCuO_2 and Sr_2CuO_3

We present the results of measurements of the thermal conductivity of the quasi one-dimensional spin S=1/2 chain compound SrCuO_2 in the temperature range between 0.4 and 300 K along the directions parallel and perpendicular to the chains. An anomalously enhanced thermal conductivity is observed along the chains. The analysis of the present data and a comparison with analogous recent results for Sr_2CuO_3 and other similar materials demonstrates that this behavior is generic for cuprates with copper-oxygen chains and strong intrachain interactions. The observed anomalies are attributed to the one-dimensional energy transport by spin excitations (spinons), limited by the interaction between spin and lattice excitations. The energy transport along the spin chains has a non-diffusive character, in agreement with theoretical predictions for integrable models.Comment: 12 pages (RevTeX), 8 figure