Theoretical study of the heavy electron state in Lanthanide and Actinide compounds

大多数镧系元素和榄系元素化合物中的f电子系统都被认为是局域电子系统。然而，在这些基团中的一些化合物如CeCu2Si2和Ube_lt；13&gt；在低温下表现为具有极大有效质量的费米液。这种费米液态被称为重电子态。了解这个态的起源和性质是固体物理中最基本的问题之一。我们这个项目的主要目的是了解重电子系统的低能电子态。为此，我们利用周期Anderson模型计算了低能区的态密度。结果表明，在系统中f电子和S电子的自能修正中，存在f电子自旋涨落引起的大重整化效应时，在低温下，态密度曲线在费米能级附近出现一个非常尖锐的峰。这个窄带对应于定义明确的有效质量非常大的单费米子激发。我们的计算表明，随着温度的升高，由于准粒子寿命的缩短，窄峰变宽，峰高减小。这一结果与在重电子系统中观察到的从重电子态向类Kondo杂质态交叉的实验结果是一致的。我们还研究了高超导氧化物的超导机制，因为我们认为超导氧化物属于重电子超导体。提出了一种基于电荷涨落机制的高温超导机制。成功地解释了YBCO中铜核弛豫时间与温度的反常关系。

The f electron system in most of Lanthanide and Actinide compounds is considered as a localized electron system. However, in some of the compounds such as CeCu_2Si_2 and UBe_<13> in these groups behaves as a Fermiliquid with an extremely large effective mass at low temperatures. This Fermi liquid state is called the heavy electron state. To understand the origin and nature of this state is one of the most fundamental problems in solid state physics.Our major purpose of the present project is to understand the low energy electronic state of the heavy electron system. For this purpose we calculated the density of states in the low energy region using the periodic Anderson model. The obtained result shows that a very sharp peak in the density of states curve appears near the Fermi level at low temperatures when the large renormalization effect due to the f electron spin fluctuations exists in the self-energy correction for the f and s electrons in the system. This narrow band corresponds to the well-defined one-fermion excitation with a very large effective mass. As temperature increases, our calculation shows that the narrow peak broadens and its height decreases due to a shortening of the quasi-particle lifetime. This result is consistent with the experimental result observed in the heavy electon systems which shows the crossover from the heavy electon state to the Kondo-impurity-like state.We also investigated the mechanism of superconductivity in the hight superconducting oxides because we believe that the superconducting oxides are classified into the heavy-electron superconductors. We proposed a possible mechanism for high temperature superconductivity based on the charge-fluctuation-mechanism. The anomalous temperature dependence of the nuclear relaxation time-of cu nucleus in YBCO was successfully explained.