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 Na2IrO3 is 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)).

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 the QAH effect was experimentally realized only by magnetic doping of the surface state of topological insulator. Prof. Ji Feng’s research group and collaborators proposed 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/TiO2 harbors a 2-dimensional electronic phase, dubbed single-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 graduate under 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.

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 dichalcogenide in 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.