Transport phenomena in the 300 kbar range

Hydrostatic pressure applied to pieces of solids influences in a manifold manner physical properties. This can be easily understood from a reduction of the volume of the solid as a function of increasing pressure. Directly connected with this change is a modification of electronic quantities and parameters like the electronic density of states, which - as central quantity - determines in a unique way physical properties. Besides, other interaction mechanisms are changed upon pressure. Among them are magnetic -, Kondo - or crystal field interactions. The possibility to vary pressure in a smooth manner allows us to change physical properties in a specific way. Of particular interest is the tuning of phase transitions. In order to apply pressure to solids, one needs sophisticated pressure techniques. Two different possibilities were planned and realized in the scope of the research project. i) piston-cylinder cells: there, a number or thick-walled tubes are pressed into each other and filled up by liquids. Samples are centred inside this liquid. When closing these tubes on both ends, pressure can be applied to the liquid and is therefore transmitted to the samples. The maximum pressure value which was realized throughout the project in this manner is about 25 kbar. ii) Bridgman cells: Two anvils, made of high performance materials like sintered diamonds or aluminium-oxide, are pressed together. Depending on the diameter and on the applied force, pressure values one order of magnitude above those of the piston-cylinder cells can be realized. The disadvantage are the small working volumes and the non- hydrostatic conditions inside the pressure cells. The main focus of investigations using these pressure cells was directed to studies on so called Kondo- and heavy fermion systems. Such intermetallics are e.g., characterised by effective masses of the charge carriers which may exceed those of free electrons by up to 3 orders or magnitude. This behaviour, of course, influences most of the physical properties of these systems. In spite of the complexity of such systems, it is possible to account for these intermetallics in very basic pictures (Fermi-liquid model), which are normally used only if the most simplest metals like K or Cu are considered. This can be done, because all interaction mechanisms are projected into the effective mass. In recent years, however, many materials have been found, which dramatically deviate from the Fermi-liquid description. In particular, such discrepancies become evident if phase transitions occur at a temperature T = 0, which can happen accidentally or can be induced by various parameters, best suited is pressure. Such phase transitions at T = 0 are termed quantum phase transitions; they influence the physical behaviour even far above zero temperature due to the occurrence of strong fluctuations. Our recent investigations have shown that such a quantum critical point can exist also in a pressure induced crossover from an insulator to a metallic ground state (SmB6). Interesting effects concerning the pressure response were also observed for superconducting - and magnetic phase transitions.