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Physical Review Letters reports the Ji Feng group's recent progresses in quantum phases in oxides

    Recently, Prof. Ji Feng’s research group of the International Center for Quantum Materials (ICQM), School of Physics, Peking University, made a series of progresses in computational and theoretical studies of novel quantum states in oxides. These studies include the exploration of magnetic structure of spin-liquid oxides, topological electronic states at the interface of simple oxides, and tunable valley degeneracy splitting. They were recently reported by Physical Review Letters, Nano Letters, and Physical Review B (Rapid Communications), respectively.

    As a 5d metal oxide, Na2IrO3 has attracted much attention in recent years, owing to its potential in realizing the Kitaev model. The Kitaev model describes a spin-1/2 system with a two-dimensional (2D) honeycomb lattice, whose ground state is a spin liquid and can be used to realize topological quantum computation. Therefore, there is a great deal of interest this kind of “Kitaev matter”. Currently Na2IrO3is the most extensively studied candidate in this regard. Na2IrO3 has a layered structure, where Ir atoms form a two-dimensional honeycomb lattice (Figure 1). Due to the crystal-field splitting and strong spin-orbit coupling, the d-electrons of Ir atoms can be described by an effective 1/2 pseudospin.

    However, the experimental studies on the crystal and magnetic structures have undergone several revisions. Kaige Hu, a postdoc in Prof. Feng’s group, carried out first-principles calculations of the magnetic structure of Na2IrO3, and found that the zigzag ordered magnetic moments are not aligned along the crystallographic a axis established by previous experiments, and not even in the Ir honeycomb lattice plane (i.e., the ab plane), but lie in the ac plane and parallel to g=a+c. Such computational results are further explained by a modified Kitaev-Heisenberg model, which is critical to the understanding of the electronic structure of this compound and designing/realizing the Kitaev ground state. This work is a result of a collaboration with Prof Fa Wang, and just appeared in Physical Review Letters on Oct. 16, 2015: PRL 115, 167204 (2015), and Dr. Kaige Hu is the lead author. Remarkably, the theoretical prediction of this work has been confirmed by an independent and very recent experiment (Nature Physics 11, 462 (2015)).

Figure 1Na2IrO3 has a layered structure, where Ir atoms form a 2D honeycomb lattice and the zigzag-ordered magnetic moments are aligned along the g=a+c direction.

    The Feng group is also active in designing new topological states on the oxide interfaces. Quantum anomalous Hall (QAH) effect is a topological quantum state with non-zero Chern number in the absence of external magnetic field.To date theQAH effect was experimentally realized only by magnetic doping of the surface state of topological insulator. Prof. Ji Feng’s research group and collaboratorsproposed a novel theoretical strategy to realize QAH states without the need for topological insulator and magnetic doping, which make possible a Chern insulator proper. By interfacial orbital design and first-principles calculations, they showed that the interface CrO2/TiO2harbors a 2-dimensional electronic phase, dubbedsingle-spin “graphene”. When the spin-orbit coupling is taken into consideration, CrO2/TiO2 superlattice becomes a Chern insulator, one that has perfect crystalline symmetry and requires no doping. This work was published by Nano Letters on Aug. 25, 2015: Nano Letters 15, 6434 (2015). Xiao Li, a Ph. D graduateunder supervision of Profs. Enge Wang and Ji Feng and now a postdoctoral fellow in the University of Texas at Austin, is one of the lead authors.

  Figure 2 Calculated Berry curvature of the CrO2/TiO2 heterostructure, which demonstrates that the interfacial material is a Chern insulator with a Chern number of 2.

    Oxides also play an important role in modifying the electronic properties of 2D layered materials as substrates. 2D transition-metal dichalcogenide, with a pair of degenerate valleys in the band structure, is a class of promising valleytronic materials. Lifting the valley degeneracy of transition-metal dichalcogenidein a controllable way is an attractive route to achieve various optoelectronic manipulations. However, the magnetic field only creates a very small valley splitting (~0.1 meV/T). Using first-principles calculations and taking the MoTe2/EuO heterostructure as an example,Prof. Ji Feng’s research group and its collaborators proposed that a giant valley splitting over 300 meV can be generated by proximity-induced Zeeman effect. The valley splitting in the heterostructure is also continuously tunable by rotating the substrate magnetization. The giant and tunable valley splitting adds an extra dimension to the exploration of unique optoelectronic devices based on magneto-optical coupling and magnetoelectric coupling. This work was published by Physical Review B (Rapid Communications), 2015: PRB 92, 121403(R) (2015). Dr. Xiao Li is again one of the lead authors.

    The above researches are supported by National Basic Research Program of China, National Natural Science Foundation of China, Thousand Talents Program for Young Scientists of China, and Collaborative Innovation Center of Quantum Matter, China. Collaborators include Profs. Fa Wang and Qian Niu from Peking University, Prof. Jingshan Qi from Jiangsu Normal University, and Profs. Sheng Ju and Tianyi Cai from Soochow University.

        最近北京大学物理学院量子材料科学中心的冯济课题组在氧化物新颖量子态的计算和理论研究中取得了一系列进展。这一系列工作包括对氧化物自旋液体材料磁基态结构问题的探索,氧化物界面拓扑电子态和可调能谷简并劈裂的研究。这些工作近期分别被《物理评论快报》(Physical Review Letters)、《纳米快报》(Nano Letters)和《物理评论B-快讯》 (Physical Review B (Rapid Communications))报道。


  然而,在实验上对Na2IrO3结构与磁性结构的研究几经曲折。冯济课题组的博士后胡凯歌运用第一性原理计算详细研究了Na2IrO3的磁性结构,发现其基态zigzag磁序的磁矩方向并不是以往实验认为的晶格a方向,也不在Ir的二维晶格平面(即晶格ab平面)内,而是位于ac平面内、平行于g=a+c(图1),进而通过修改Kitaev-Heisenberg模型解释了计算结果。最后,他们强调理论预言的g方向符合现有的实验结果,并对理解该材料的基本电子结构及设计实现Kitaev基态的方案具有重要意义。该工作于2015年10月16日发表于《物理评论快报》:Physical Review Letters 115, 167204 (2015),胡凯歌博士为第一作者。值得一提的是,此工作的主要理论预言近期已被独立的实验所证实(Nature Physics 11, 462 (2015))。


  图 1  Na2IrO3具有层状结构,其中Ir原子构成二维的蜂窝层状格子,zigzag磁序的磁矩方向平行于g=a+c方向。

  凝聚态物理中近年出现的新的量子拓扑态受到广泛关注。量子反常霍尔(QAH)效应是没有外场作用下的具有非零陈数的拓扑电子态。迄今,唯一实现量子反常霍尔效应的途径是对拓扑绝缘体的表面态进行磁掺杂。冯济课题组与合作者提出了不需要拓扑绝缘体和磁掺杂实现QAH电子态理论方案,实现严格意义上的“陈绝缘体”。通过电子轨道设计和第一性原理计算,在二氧化铬和二氧化钛异质结构的能带结构中发现了四个单自旋的狄拉克点,即这种材料可以看做单自旋版本的“石墨烯”。在自旋轨道耦合作用下, 面外磁化的二氧化铬和二氧化钛超晶格无需外加磁性掺杂,便可以实现真正的“陈绝缘体”。因此,这两个高度产业化的氧化物有望进一步应用于高保真的数据存储和低能耗的电子学器件中。该工作于8月25日发表于《纳米快报》:Nano Letters 15, 6434 (2015)。量子中心王恩哥和冯济共同指导的博士李晓(目前在德州大学从事博士后研究)为文章的共同第一作者。


  图 2  计算得到的CrO2/TiO2异质节贝里曲率,表明界面电子态是陈数为2的“陈绝缘体”。

     氧化物材料也可以作为基底材料,在调控新颖二维层状材料性质方面发挥重要作用。二维过渡金属二硫族化物材料是一类有前途的谷电子学材料,有一对简并的能谷。为实现稳态多途径的光电操纵,需要实现对能谷简并度的有效调节。然而,现有的外加磁场只能引起很小的能谷劈裂(~0.1 meV/T)。冯济课题组与合作者以MoTe2/EuO异质结为例,通过第一性原理计算提出借助磁性氧化物基底产生的近邻塞曼效应,可以使过渡金属二硫族化物产生巨大的能谷劈裂(>300 meV)。通过旋转氧化物基底的磁化方向,能谷劈裂进而可以连续调节。巨大可调的能谷劈裂使基于磁电和磁光耦合的新颖光电子学器件成为可能。该工作于9月发表于《物理评论B –快讯》:Physical Review B (Rapid Communications) 92, 121403 (2015)。李晓是文章的共同第一作者。