5d过渡金属氧化物—Weyl半金属奇异量子物性研究  

Novel properties of 5d transition metal oxides

作　　者：万贤纲[1] 

机构地区：[1]南京大学物理学院南京大学固体微结构国家实验室,南京210093

出　　处：《物理学进展》2014年第1期28-46,共19页Progress In Physics

基　　金：国家973项目(No.2011CB922101;No.2010CB923404);国家自然科学基金项目(No.91122035;No.11174124;No.11374137);江苏高校优势学科建设工程资助项目

摘　　要：绝缘体系统的拓扑量子行为是当前物理学领域的焦点。那么没有能隙的金属体系是否也有拓扑非平庸行为呢?答案是肯定的,5d过渡金属元素既有着大的自旋轨道耦合,又有着不可忽略的电子关联作用,由其构成的材料体系因而具有奇异的量子物性。本文先简介人们最近关注的几类5d材料,着重讨论烧绿石结构铱(Ir)氧化物(A2Ir2O7,其中A=Y或稀土元素)。我们通过第一性原理计算结合有效模型成功地预言了这一大类磁阻挫材料的基态磁构型,并预言A2Ir2O7是Weyl半金属。我们发现和拓扑绝缘体一样,作为一种新型拓扑量子态,Weyl半金属态也具有其拓扑性质:Weyl点是受拓扑保护稳定的;Weyl半金属有着受拓扑保护的表面态,即非闭合的费米面(Fermi arc);它对外场的响应也由其拓扑性质决定(只与Weyl点的位置有关,和能带的细节无关)。我们进一步预言尖晶石结构锇(Os)氧化物(AOs2O4,其中A=Ca或Sr)是有着特别磁电响应的Axion绝缘体。最后我们讨论了电子关联、自旋轨道耦合、磁序结构对钙态矿结构锇氧化物的影响,进一步确定NaOsO3是由磁序导致金属–绝缘体相变的Slater绝缘体。In 5d transition metal oxides, novel properties arise from the interplay of electron correlations and spin-orbit interactions. In this article, we introduce several our recent works. We investigate the interplay of electron correlations and spin-orbit interactions in the pyrochlore iri- dates, （such as Y2Ir2O7） using the LDA＋U method and effective low energy models. A remarkably rich phase diagram emerges on tuning the correlation strength U. The Ir magnetic moments are always found to be nonocollinearly ordered. However, the electronic properties vary; the ground state evolves from a magnetic metal at weak U to a Mott insulator at large U. Most interestingly, the intermediate U regime is found to be a Weyl semimetal, with vanishing density of states at the Fermi energy. This phase also exhibits topological properties--manifested by special surface states in the form of Fermi arcs that connect the bulk Weyl points. This Weyl phase, a three dimensional analog of graphene, is proposed as the ground state of Y2Ir2O7 and related com- pounds. We propose that hypothetical Osmium compounds such as CaOs2O4 and SrOs2O4 can be stabilized in the geometrically frustrated spinel crystal structure. They also show some exotic electronic and magnetic properties in a reasonable range of U such as ferromagnetism and orbital magnetoelectric effect characteristic to Axion electrodynamics. A comprehensive investigation of the electronic and magnetic properties of NaOsO3 has also been made using the first principle calculations. Its ground state is of a G-type antiferromagnet, and it is the combined effect of U and magnetic configuration that results in the insulating behavior of NaOsO3.

关 键 词：Weyl半金属 Axion绝缘体 5d过渡金属氧化物 Fermi Arc Slater绝缘体 能带计算 

分 类 号：O469[理学—凝聚态物理]