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

铈化合物中4f电子的性质在磁性和电荷有序、自旋和价态涨落、重费米子和各向异性超导性等方面起着重要作用。de Haas-van Alphen （dHvA）实验是研究4f电子系统费米表面性质的有力工具。例如，CeB_6、CeIn_3和CeRhIn_5中确定了局域化的4f电子特征。另一方面，澄清了CeSn_3、CeRu_2Si_2和CeCoIn_5中4f电子的流动性质。在量子临界点附近，施加压力使反铁磁体的磁性有序态变为顺磁性有序态。我们开发了压力高达3gpa的dHvA实验压力池。研究了反费米磁体CeRh_2Si_2、CeRhIn_5、CeIn_3和CePt_3Si中费米表面性质的变化。我们发现在临界压力P_c下，CeRh_2Si_2、CeRhIn_5和CeIn_3的费米表面发生了突变。费米表面的变化表明，4f电子的性质由局域化转变为流动化。也就是说，P_c以上的4f电子作为传导电子对费米表面的体积做出了贡献。有趣的是，在P_c附近观察到具有大有效质量的压力诱导重费米子态，在那里出现了压力诱导超导性。我们还在没有晶体结构反转对称的CePt_3Si中进行了压力下的dHvA实验。虽然在CePt_3Si中没有观察到完整的费米面，但在P_c以上观察到一个新的分支，有效质量相对较大，表明电子态发生了变化。

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.