Study of the formation of Magnetic Polaron Lattice in Low-Carrier strongly Correlated Systems

本项目的目的是用中子散射方法研究低载流子系统CeP和Yb_4As_3的异常性质。结果表明：1. CePCeP在磁场和高压下均表现出异常的磁相图。它也显示出典型的密集近藤效应的传输异常。我们的中子散射研究建立了磁场低于约6 T和压力低于约2 GPa的磁相图。所得到的磁相图是十分新颖和独特的。为了解释相图，有必要考虑该系统中低载流子密度的事实。2.Yb_4As_3Yb_4As_3在290K左右发生了结构相变，这是一个电荷有序的相变。该化合物在低温下也显示出重电子异常，尽管其载流子密度极低，每个分子式为10^<-3>。我们的极化中子研究直接证明了这种材料中Yb^&lt;2+&gt;和Yb^&lt;3+&gt;在低温下的有序性。非弹性中子散射研究揭示了一维反铁磁体典型的自旋激发的存在。这是由于Yb^&lt;3+&gt;离子的电荷有序性导致的一维结构。除了一维自旋激发外，还存在异常的磁响应。这些事实表明，该系统中的重电子异常与电荷有序化引起的电子态的Mott跃迁型变化密切相关。

The purpose of this project is to study the unusual properties of the low carrier systems CeP and Yb_4As_3 by means of neutron scattering. The results are as follows :1.CePCeP shows unusual magnetic phase diagrams under magnetic field as well as under high pressure. It shows also typical dense-Kondo-effect like transport anomalies. Our neutron scattering studies established the magnetic phase diagrams for the magnetic field below about 6 T and for the pressure below about 2 GPa. The obtained magnetic phase diagrams are quite novel and unique. In order to explain the phase diagrams, it is necessary to take into account the fact of the low carrier density in this system. The magnetic polaron model gives the most plausible picture to understand the system.2.Yb_4As_3Yb_4As_3 undergoes structural phase transition at about 290K,which was claimed to be that of a charge ordering. The compound shows also heavy electron anomalies at the low temperatures in spite of its extremely low carrier density of the order of 10^<-3> per formula. Our polarized neutron study proved directly the ordering of Yb^<2+> and Yb^<3+> in this material at low temperatures. The inelastic neutron scattering studies revealed the existence of spin excitations typical of a one-dimensional antiferromagnet. They are ascribed to the one-dimensional configuration of the Yb^<3+> ions caused by the charge ordering. There is also anomalous magnetic response other than the one-dimensional spin excitations. These facts indicate that the heavy electron anomalies in this system are strongly related with the Mott-transition-type change of the electronic state caused by the charge ordering.