Fermi Surfaces of the Heavy-Ferimion Superconductor near a Quantum Critical Point

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The purpose of this study is to examine the heavy-fermion superconductivity state in the vicinity of quantum critical point(QCP) from the standpoint of the control of the hybridization between an almost localized 4f-electrons and band electrons by a pressure. We have established the conditions of the single crystal growth by a flux method for a heavy-fermion superconductor Ce_2CoIn_8 and antiferromagnet Ce_2RhIn_8. We have measured a pressure dependence of magnetization, electrical resistivity, Hall effect and transverse magnetoresistance for Ce2CoIn_8 and Ce2RhIn_8 over the temperature range from 0.4 to 300 K. The non-Fermi liquid behavior in Ce_2CoIn_8 is enhanced by a applied pressure. The electronic characters of Ce_2RhIn_8 above 2 GPa is very similar to that for heavy-fermion superconductor Ce_2CoIn_8 at ambient pressure. At low temperature region, T<2 K, the sample dependencies for electrical properties were observed for the both materials. These behaviors may be due to microscopic structural imperfections. Higher quality single crystal is required to study the Fermi surface features of Ce_2MIn_8 system. A study was also made of compound formation in the ternary systems Ce-Rh-Ga with the objective of finding a new heavy-fermion superconductor. We succeeded in growing the single crystals of CeRh_2Ga and YbRh_2Ga by the flux methods. CeRh_2Ga exhibits non-Fermi liquid behavior at low temperatures. We speculated that the NFL behavior observed in CeRh_2Ga may be attributed to the existence of a Griffiths phase in the vicinity of a QCP. YbRh_2Ga is a typical mixed-valent system with a magnetically non-ordered ground state.