Elektrischer Widerstand, Supraleitung und Phasenumwandlungen von Rubidium und Cäsium unter hohem Druck

Four lead electrical resistance measurements were performed on Rb under pressures up to 210 kbar for temperatures in the range 0.05 K to 300 K. Pressure was applied using a Bridgman-anvil-configuration with dense sintered diamond in the highly stressed tip regions of the Carboloy pistons. The sampIe cell was pressurized at room temperature by a mechanical press connected to the mixing chamber of a $^{3}$He-$^{4}$He-cryostat. The pressure remained essentially constant during cooling. Discontinuous changes in resistance at pressures of 70 and 140 kbar indicate two phase transitions and confirm the results of other authors. The resistance of Rb increases after a minimum at 20 kbar by about two orders of magnitude. Large jumps in the resistance at 180 kbar and 207 kbar indicate two additional transitions. In the investigated pressure-range Rb possesses a rich phase diagram similar to Cesium which exhibits at least four high-pressure modifications. Althoughthe pressure dependence of the electrical resistance shows a similarity to the alkaline metal Cesium, we conclude, that the series of high pressure phases is different in Rb and Cs. None ofthe phases of Rb shows superconductivity down to 0.05 K in striking contrast to the heaviest alkaline metal Cs. In a single measurement-series on Cs superconductivity in the phase Cs IV was discovered. There is a significant rise of T$_{c}$ from 0.05 K until 0.5 K with increasing pressure as we approach thephase transition Cs IV/V. The wellknown superconductivity of Cs IV was confirmed in some experiments. When consideration is given to the systematical variation ofsuperconducting Tc's in the periodic system it's our opinion, that our experiments show, that Cs in an annomalously good superconductor. We think, that this experiments confirm the hypothesis, that Cs under pressure is a nonmagnetic rare earth metal with a significant 4f-hybridisation at the fermi-level. This hypothesis is in contrast to present bandstructure calculations, which don't show substantial 4f-contributions