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The electrical resistivity (T) and transverse magnetoresistivity of magnetically ordered CeRu 2 Ge 2 (T N ϭ8.55 K and T C ϭ7.40 K) was measured as a function of pressure up to Pϭ11 GPa and down to Tϭ30 mK. Pressure first increases T N and suppresses T C. Then a second transition, corresponding probably to a modification of the AFM state, appears at T L ϽT N in the pressure range 3.5Ͻ PϽ7.2 GPa. The long-range magnetic order disappears at a critical pressure P c ϭ8.7 GPa like T N ϰ(P c Ϫ P) m , with mϭ0.71(8). The (T) curves at low temperature and low pressure (PϽ7.8 GPa) are well described by a power law (T)ϰT n , with exponents nϾ2. Well above P c , a Fermi liquid behavior (nϭ2) is observed and the A coefficient of the quadratic T dependence decreases with pressure. In the pressure interval P c Ϯ0.8 GPa, exponents close to n ϭ1.5 indicate a deviation from a Fermi liquid description. The magnetoresistivity curves show different anomalies at characteristic magnetic fields B a , B c , and B M. The latter occurs close to P c and is reminiscent of the metamagneticlike field in CeRu 2 Si 2. A quantitative comparison between the (T, P) data of CeRu 2 Ge 2 and the (T,x) data of CeRu 2 (Si 1Ϫx Ge x) 2 on the basis of the unit-cell volume as the crucial parameter shows that the same (T,V) diagram is obtained. ͓S0163-1829͑99͒05705-7͔

A technique for measuring the electrical resistivity and absolute thermopower is presented for pressures up to 30 GPa, temperatures down to 25 mK and magnetic fields up to 10 T. With the examples of CeCu 2 Ge 2 and CeCu 2 Si 2 we focus on the interplay of normal phase and superconducting properties. With increasing pressure, the behaviour of CeCu 2 Ge 2 evolves from that of an antiferromagnetically ordered Kondo system to that characteristic of an intermediate valence compound as the Kondo temperature increases by about two orders of magnitude. In the pressure window 8-10 < P < 20 GPa, a superconducting phase occurs which competes at low pressure with magnetic ordering. For CeCu 2 Si 2 the effective mass of carriers is probed by both the coefficient of the Fermi liquid law and the initial slope of the upper critical field. The magnetic instability is studied notably for CeRu 2 Ge 2 and Yb-based compounds for which pressure-induced magnetic ordering tends to develop. Finally, contrary to conventional wisdom, we argue that in heavy fermions a large part of the residual resistivity is most likely not independent of temperature; tentatively ascribed to Kondo hole, it can be very pressure as well as sample dependent.[electrical resistivity, thermoelectric power, heavy fermion, magnetic order, superconductivity]

The intermetallic compound YbCu 2 Si 2 is a well-known nonmagnetic ͑NM͒ Yb intermediate-valent compound with a Yb valence of 2.9 at ambient pressure and 300 K. In the present work we have investigated the effect of high pressure on the ground state properties of YbCu 2 Si 2 on both microscopic and macroscopic levels by using the 170 Yb Mössbauer effect, electrical resistance, and x-ray diffraction techniques, respectively. High-pressure x-ray diffraction data indicate that the lattice structure of YbCu 2 Si 2 is stable up to 22.2 GPa. The value of the bulk modulus ͓B 0 ϭ168(10) GPa Ϫ1 ͔ is found to be close to the value expected for trivalent RCu 2 Si 2 compounds. The pressure dependence of the electrical resistance reveals evidence for a pressureinduced magnetic order for pу8 GPa. From our Mössbauer data, we conclude a crossover from the NM to a magnetically ordered state of localized Yb moments for pу8 GPa and below 2 K. The pressure-induced change of the electric quadrupole splitting indicates that this transition is accompanied by a valence change towards the Yb 3ϩ state. ͓S0163-1829͑99͒01729-4͔

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