Quantum Critical Phenomena in Heavy-Fermion System under Pressures

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We studied the quantum critical phenomena of heavy fermion systems under pressures by means of high-quality single crystals. New findings are as followings.1. Hexagonal heavy fermion system Ce_7Ni_3 : Ce_7Ni_3 undergoes tow antiferromagnetic transitions at 1.8 K and 0.7 K. The antiferromagnetic order vanishes at B//c = 0.3 T. For B//c > 0.6 T, a new magnetic ordered phase appears below 0.5 K. For 0.3 T < B//c < 0.6 T, the specific heat C follows the form of C/T∝-lnT, so called non-Fermi-liquid behavior. The origin of the field-induced magnetic phase may be attributed to a geometrical frustration in the magnetic moment of Ce sites formed a quasi-regular tetrahedron2 Uniaxial-pressure induced magnetic order in CeNiSn : CeNiSn is a paramagnetic semimetal with a pseudo-gap. By applying the uniaxial pressure of 0.13 GPa along the c axis, an antiferromagnetic order appears at around 3 K. For P//a, however, the density of carriers increases. The Kondo temperature for the crystal field ground state estimated by the resistivity for P//c is suppressed with pressure. Therefore, we may deduce that the formation of magnetic order is attributed to the weakened hybridization in a-b plane between the 4f electrons for the crystal field ground state and conduction electrons.3 Non Fermi liquid behavior in Ce_2Rh_3Ge_5 under pressures : The critical pressure Pc where the antiferromagnetic order of orthorhombic Ce_2Rh_3Ge_5 with T_N = 4.9 vanishes was estimated to be 0.45 GPa. Even for P>3Pc, non-Fermi-liquid behavior is observed in both the resistivity ρ and the susceptibility χ : ρ ∝T^<1.45> and χ ∝1-T^<0.27>. Because the magnetization hardly decreases for P = 3Pc, the origin of the non-Fermi-liquid behavior may be due to a short-rang magnetic order of the remaining local moments.