Quantum Critical Phenomena in Heavy-Fermion System under Pressures

利用高质量单晶研究了压力下重费米子系统的量子临界现象。新发现如下：1.在1.8 K和0.7 K下，重费米子系统Ce_7Ni_3：Ce_7Ni_3发生了两次反铁磁转变。反铁磁序在B//c = 0.3 T时消失。当B//c > 0.6 T时，在0.5 K以下出现了一个新的磁有序相。当0.3 T < B//c < 0.6 T时，比热C符合C/T ε-lnT的形式，即非费米液体行为。场致磁相的起源可以归因于Ce位磁矩的几何挫折，形成准规则四面体2单轴压力诱导的CeNiSn中的磁序：CeNiSn是具有赝能隙的顺磁半金属。沿c轴施加沿着0. 13 GPa的单轴压力，在3 K附近出现反铁磁有序。然而，对于P//a，载流子的密度增加。由P//c的电阻率估计的晶体场基态的近藤温度随压力而被抑制。因此，我们可以推断磁有序的形成是由于晶场基态的4f电子与传导电子在a-b平面上的杂化减弱。3压力下Ce_2Rh_3Ge_5的非费米液体行为：反铁磁序消失的临界压力Pc为0.45GPa。即使对于P&gt; 3 Pc，在电阻率ρ和磁化率χ中观察到非费米液体行为：ρ π τ ^<1.45>和χ π 1-τ ^<0.27>。由于P = 3 Pc时磁化强度几乎没有降低，因此非费米液体行为的起源可能是由于剩余局部磁矩的短程磁序。

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.