Microwave Investigations on Ferroelectrics and Superconductors

In order to understand the possible processes of charge transport common to ferroelectric ceramics and high Tc superconductors a variety of investigations aie underway. Microwave studies on these materials have attracted the attention of scientists in recent years. Some potential experimental techniques for studying ferroelectrics and superconductors will be discussed. Some common intrinsic features of these materials will be highlighted. Results on dielectric behavior, microwave absorption, and surface impedance of some of these materials will be discussed. Introduction Discovery of high Tc superconductors has brought a kind of scientific and technological revolution. Expectations are high and a long journey has just become. Recently a possible relationship between ferroelectricity and high Tc superconductivity has been pointed. ' Miiller initiated these efforts that have been extended to a larger extent." The presence of ferroelectric like polarizabilities in superconductors may exist in the phase preceeding the metallic or the superconducting state. To further strengthen these possibilities, experiments with a variety of probing tools are going on. Why microwave studies? There are several favorable situations to support microwave investigations. It is being strongly felt that first useful applications of high Tc superconductors will emerge in the area of microwave devices and circuits. In fact, cavities, filter circuits, and interconnects using these materials have been successfully fabricated. If materials are going to be handled at microwave frequencies their properties need to be investigated in that frequency region since low frequency extrapolations will sometimes be wrong and often inaccurate. Further, in many situations (often encountered with ferroelectrics and superconductors) microwave measurements offer advantage over their low frequency counterpart. Some of these advantages are: 1. Higher accuracy of measurement arises due to large Q's (~ 10,000) of cavities often used in microwave measurements and due to very high signal detection efficiency (~ KHZ in GHZ up to 80/90 dB dawn). Considerations of measurement accuracy become important, for example, in the detection of intermediate phases. Phases even in dilute concentrations are detectable. Figure (1) shows a result on 0.7 PMN-0.3 PT ferroelectric relaxor. A mild hump (region A) in the low frequency dielectric response turns to a well formed peak in the microwave measurements. This peak indicates the presence of morphotropic boundary in the 0.7 PMN-0.3 PT system. X-ray and pyroelectric studies also support this observation. 2. Microwave studies are capable of revealing information on the behavior of high Tc superconductors below transition temperature also. As an example, results are shown in Fig. (2) on the effect of magnetic field on temperature dependence of the transmitted power for YBaCuO sample. 3. Microwave measurements are basically electrodeless and theres no need for fixing electrodes on the specimen. Microwave results, therefore, represent true material characteristics free from possible modifications arising due to space charge/electrode effects. 4. The microwave measurements arc less susceptible to pathological current geometries as compared to dc methods and thus represent intrinsic nature of the material. 5. Many microwave techniques require extremely small quantities of the specimen. For example, the perturbation resonance technique uses a superconductor specimen of volume 0.004 cm. This may be a useful consideration for studying small crystal, for example. F.xperimental Techniques In the study of high Tc superconductions two types of microwave measurement techniques have been normally used. 1. Resonance cavity perturbation techniques. 2. Reflection/transmission techniques. Cavity Perturbation Techniques In these techniques, the presence of small amounts of a material changes the resonance frequency and the quality factor Q of the microwave cavity. Cylindrical and rectangular shaped cavities are often used in practice. Also cavities made from purely superconductor materials as well as from conventional metals have been used and both yield identical results.