Study of the Instability of the Fermi Surface and the Heavy Fermion State around the Quantum Phase Transition for the Pressure-Induced Superconductor

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The character of the 4f-electron in cerium compounds plays an important role on magnetic and charge orderings, spin and valence fluctuations, heavy fermions and anisotropic superconductivity. The de Haas-van Alphen (dHvA) experiment is a powerful tool to study the Fermi surface properties for the 4f-electron systems. For example, the localized 4f-electron character was confirmed in CeB_6,CeIn_3 and CeRhIn_5. On the other hand, the itinerant 4f-electron character was clarified in CeSn_3,CeRu_2Si_2 and CeCoIn_5.In order to clarify the charter of the 4f-electron around the quantum critical point where the magnetic ordering state of an antiferromagnet changes to the paramagnetic one by applying pressure. We have developed the pressure cell for the dHvA experiment under pressure up to 3 GPa. We have studied the change of the Fermi surface properties in antifermmagnets CeRh_2Si_2,CeRhIn_5,CeIn_3 and CePt_3Si.We found the abrupt change of the Fermi surface at the critical pressure P_c in CeRh_2Si_2,CeRhIn_5 and CeIn_3. The change of the Fermi surface indicates that the 4f-electron character is changed from localized to itinerant. Namely, the 4f electron above P_c contributes to the volume of the Fermi surface as the conduction electron. Interestingly, the pressure-induced heavy fermion state with large effective mass was observed around P_c, where the pressure-induced superconductivity appears. We also carried out the dHvA experiments under pressure in CePt_3Si without inversion symmetry in the crystal structure. Although the overall Fermi surface was not observed in CePt_3Si, a new branch with relatively large effective mass was observed above P_c, indicating the change of the electronic state.