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We studied the pressure effect by measuring the electrical resistivity for a high-quality single crystal of an antiferromagnet CeRh 2 Si 2 with the tetragonal structure. The Néel temperature becomes zero around 1 GPa. An onset of superconductivity appears below 0.5 K in a relatively wide pressure region from 0.97 to 1.20 GPa, while the resistivity zero, which appears below 0.4 K, is found to be located in an extremely narrow region from 1.03 to 1.08 GPa. Correspondingly the A value of the Fermi liquid relation in the electrical resistivity (ρ = ρ 0 +AT 2 ) becomes maximum around 1 GPa. Superconductivity is realized in a heavy fermion state.The f electrons of cerium and uranium compounds exhibit a variety of characteristics including spin and valence fluctuations, heavy fermions, Kondo insulators and anisotropic superconductivity [1]. Recently, a new aspect of these compounds with magnetic ordering has been discovered. When pressure P is applied to the cerium compounds with antiferromagnetic ordering such as CeIn 3 , CePd 2 Si 2 [2], CeCu 2 Ge 2 [3] and CeRhIn 5 [4], the Néel temperature T N → 0 is reached at the quantum critical pressure P c . Superconductivity and/or the non-Fermi liquid nature are observed around P c . Very recently superconductivity was also found in a ferromagnetic state in UGe 2 [5] and URhGe [6]. The crossover from the magnetic ordered state to the non-magnetic state under pressure, crossing the quantum critical point, is the most interesting issue in strongly correlated f-electron systems.CeRh 2 Si 2 crystallizes in the tetragonal ThCr 2 Si 2 -type structure and orders antiferromagnetically below the Néel temperature T N1 = 36 K at ambient pressure. Its magnetic structure was investigated by the neutron diffraction measurements [7]. Below T N1 , the propagation

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We carried out the de Haas-van Alphen (dHvA) experiment under pressure for a ferromagnet UGe2. The dHvA frequency of a main dHvA branch named α, that corresponds to a majority up-spin band 40-hole Fermi surface, decreases monotonously with increasing pressure, but in the pressure range from pc* (≃1.2 GPa) to pc (≃1.5 GPa) the dHvA signal disappears completely, where pc and pc* correspond to critical pressures for a Curie temperature TC and the second phase transition temperature T* (pc we observed new dHvA branches with large cyclotron masses of 19-64m0, which correspond to main Fermi surfaces in the paramagnetic state.

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