Heavy Fermions in 3d Electron Systems

最近，在4f和5f电子系统中发现了一种重费米子，其特征是具有巨大的电子比热系数。当三维电子系统显示出巨大的自旋涨落时，伽马的类似增强可以预期。我们研究了几种合金和Laves相化合物的低温比热。结果如下：1。近反铁磁性Y_<1-X> Sc_XMn_2化合物由于自旋涨落表现出150mJ/K^2mol的巨大γ值。增强因子，定义为gamma/gamma_b，其中gamma_b是波段计算期望的理论伽马值，约为15。这种增强可与典型的5f重费米子UPt_3相媲美，20.2。NbFe_2和Y(Co_<1-X>Al_X)_2等近铁磁性Laves相化合物也表现出γ增强。增强因子最多为5，小于Y_<1-X>Sc_XMn_2.3。betaMn在1.4 K以下没有磁序。这种金属已知具有70mJ/K^2mol的大伽马值，被认为是3d重费米子的候选者。通过用Al代替Mn，我们发现局部矩特征得到了发展。这表明betaMn中的电子态处于局域矩开始的边缘，类似于4f和5f重费米子。从这些结果中，我们得出结论，受挫结构中的反铁磁自旋涨落显著提高了三维电子系统中的伽马值。

Recently, A heavy fermion, characterized by a huge electronic specific heat coefficient, gamma, has been found in 4f and 5f electron systems. Similar enhancement of gamma can be expected for 3d electron systems, when they show giant spin fluctuations. We have investigated the low temperature specific heat of several alloys and Laves phase compounds. The results are as follows.1. The nearly antiferromagnetic Y_<1-X> Sc_XMn_2 compounds show a giant gamma-value of 150mJ/K^2mol due to spin fluctuations. The enhancement factor, defined as gamma/gamma_b, where gamma_b is the theoretical gamma-value expected from the band calculation is about 15. This enhancement is comparable to that of a typical 5f heavy fermion UPt_3, 20.2. The nearly ferromagnetic Laves phase compounds, such as NbFe_2 and Y(Co_<1-X>Al_X)_2 also show an enhancement of gamma. The enhancement factor is at most 5, which is smaller than that of Y_<1-X>Sc_XMn_2.3. betaMn has no magnetic order down to 1.4 K. This metal is known to have a large gamma-value of 70mJ/K^2mol and is considered as a candidate for 3d heavy fermions. By substitution Al for Mn, we have found that the local moment character develops. This suggests that an electronic state in betaMn is on the verge for the onset of local moment, similarly to 4f and 5f heavy fermions. From these results, we conclude that antiferromagnetic spin fluctuations in a frustrative structure significantly enhances gamma-value in 3d electron systems.