Doniach’s phase diagram) has been constructed as a function ofJN(EF) or pressure: the magnitude of J increases approximately with increasing pressure. The presented results are also discussed in connection with this phase diagram. We have reported the experimental results that have been obtained by using diffraction techniques, thermal expansion and heat capacity measurements, electrical resistance, and magnetic measurements under multiple extreme conditions, such as high pressures, high magnetic fields, and low temperatures.
InSection 2, the high-pressure studies of rare earth compounds using X-ray and ND techniques have been described. Structural transforma- tions, volumes, and magnetic structures were examined under high pres- sure. The valence transition of Ce atoms was explored at high pressure, and the pressure–volume relationships are analyzed using equation of state. The large changes in the temperature dependence of electrical resis- tance are observed at high pressure, indicating a pressure-induced cross- over in the electronic states. These results are combined with structural studies and then a pressure-induced valence transition is suggested to exist at room temperature and high pressure. No discontinuous change in volume has been observed around the valence transition. The anomalous elastic properties and a possible TPT were also pointed out for CeAl2. The recent progress in the high-pressure ND techniques was briefly reported in this section, and the results obtained by using this new high-pressure apparatus are introduced to search for the new magnetic phases at high pressure. Close relationships between SC and magnetic order were also clarified at high pressure for UGe2and CeRhIn5compounds.
InSection 3, the thermal properties of rare earth compounds at high pressure have been reviewed. InSections 3.1 and 3.2, the general survey and the recent progress in the measuring techniques are briefly discussed.
Until now, there have been only a few publications on the thermal expan- sion measurements under pressure, which is mainly due to technical difficulties with such measurements. The heat capacity measurements under pressure have been reported inSection 3.3for HF materials, CeIrSi3, CeRhSi3 and CeRhIn5 compounds, and UGe2, in which the interplay between magnetism and SC was investigated in details. The thermal expansion and specific heat of nonmagnetic narrow-gap semiconductor SmS and the phase transition and the magnetic properties under pressure were discussed. The thermal expansion measurements have been shown inSection 3.4for a wide range of materials, including HF materials with or without magnetic order, and valence fluctuating materials. The overall behavior of these materials is largely different from one other. The relation between magnetic order and lattice properties is discussed on the basis of Doniach’s phase diagram. The presented results indicate that the HF and valence fluctuating states are generally suppressed by the application of pressure. In other words, the electron correlation effects exhibit a tendency
to disappear at high pressure. The effect of pressure on the magnetic ordering temperature seems to be complicated, but the results for antifer- romagnetic order are explained qualitatively by Doniach’s phase diagram.
The anomaly in the thermal expansion coefficients observed in CeRh2Si2
and CeAu2Si2 may be related to the occurrence of SC. The MS of HF materials under pressure was also reviewed. The magnitude of MS decreases with pressure due to an increase in Kondo temperature.
Novel electronic phases at high pressure are discussed inSection 4. The pressure-induced crossovers in the electronic states in some HF materials are Summarized in Section 4.1 on the basis of electrical resistance measure- ments. Drastic changes in the temperature dependence of electrical resistiv- ity are observed by applying pressure, and the temperature dependence was also analyzed in detail. Extremely large coefficients ofT2dependence were observed at ambient pressure, and their magnitude is found to decrease significantly at high pressure. This fact implies that the effect of strong correlation is suppressed at high pressure. The HF states are collapsed and then show a crossover to a valence fluctuating state. The most recent topics in this research fields is to explore the pressure-induced SC and NFL behavior on the border of magnetic instability in highly correlated electron systems.
Some typical examples with magnetic order are presented inSection 4.2. All materials mentioned in this section show pressure-induced SC- and NFL- like behavior near the QCP, where the ordering temperature becomes 0 at high pressure. The Gru¨neisen parameters were estimated from these data, and we conclude that pressure suppresses their magnitude indicating that the valence fluctuating states are stabilized at high pressure, and then the electronic state may become normal by further increases of pressure.
InSection 5, we mentioned some interesting materials including rare earth elements, which are expected to show novel pressure-induced electronic states. Generally speaking, the pressure suppresses the mag- netic order as expected from the negative pressure coefficients of TCor TN. Some rare earth compounds show magnetic order under uniaxial pressure or hydrostatic pressure. For CeNiSn, the relation between the magnetic order and lattice properties under uniaxial pressure is dis- cussed. For YbInCu4, the occurrence of magnetic order and SC at high pressure was discussed in connection with QCPs. InSection 5.2, amor- phous materials including rare earth elements are briefly reviewed. It was found that the pressure dependence of the Kondo coupling JN(EF) is larger than in crystalline materials, and the Kondo temperature estimated from the data shows a stronger pressure dependence when compared with crystalline materials. But there may be some discrepancies between the available data. So at present, it may be safe to say that HF properties are sensitive to the arrangement of rare earth atoms. InSection 5.3, we showed high-pressure studies of magnetic multilayers including rare earth elements. In the nanoscale magnet, we expect novel electronic
properties under pressure. MR on the order of 8% was observed, but the enhancement of MR ratio by applying pressure was not found.
Finally, we want to emphasize that high pressure is an excellent tool to investigate the electronic structure particularly for materials that have unstable electronic states on the border of magnetic instability.
The volume of interesting data on rare earth compounds under high pressure which have been accumulated over the decades is large. Obvi- ously, we could not mention all of them in this chapter. Interested readers are directed to a recent review describing both the progress and new trends in high-pressure research of highly correlated electron systems that are summarized in the following Proceedings of an International Conference, ‘‘Novel Pressure-induced Phenomena in Condensed Matter Systems,’’ J. Phys. Soc. Jpn. 76, 2007 (edited by T. Kagayama, M. Ohashi, and Y. Uwatoko).