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2D Magnetic Systems
Published in Ram K. Gupta, Sanjay R. Mishra, Tuan Anh Nguyen, Fundamentals of Low Dimensional Magnets, 2023
Valleytronics is based on the manipulation of the valley degree of freedom of electrons in semiconductors to process and store information. An ideal valleytronic material is a semiconductor whose electronic band structure has more than one degenerate but inequivalent valley state (a valley state is formed by a local minimum in a conduction band and a local maximum in a valence band at the same or different crystal momentum, k) located at or near the X symmetry points in the Brillouin zone. The energy of a valley state can be tuned by strain or magnetic field and is termed valley polarization. 2D valleytronic materials can be polarized by magnetic proximity with 2D materials. This concept was realized recently when a monolayer of WSe2 was magnetically coupled to 2D CrI3 below its Curie temperature, which led to the emergence of spontaneous circularly polarized PL at zero applied magnetic field. The spontaneous Zeeman splitting of ~2 meV between right-handed and left-handed PL has been observed due to valley polarization in monolayer WSe2 by magnetic exchange interaction. The magnetic field changes the magnetization of the CrI3 layer, which further affects the Zeeman splitting and PL handedness [50].
Spintronics in 2D Materials
Published in Evgeny Y. Tsymbal, Igor Žutić, Spintronics Handbook: Spin Transport and Magnetism, Second Edition, 2019
With the emergence of graphene, the concept of valleytronics based on manipulating the valley index has attracted great interest [103, 104]. There are two degenerate and inequivalent valleys at the corners of the Brillouin zone in the band structure of graphene. The K and –K valleys are related to each other by time reversal. To manipulate this binary degree of freedom, we need to have measurable physical quantities that distinguish the ±K valleys. In the presence of inversion symmetry, the even parity of pseudo vectors under the inversion operation requires such quantities to take the same value for states related by inversion. Thus, inversion symmetry breaking is a necessary condition for the ±K valleys to exhibit valley contrast properties.
Recent advances in two-dimensional ferromagnetism: strain-, doping-, structural- and electric field-engineering toward spintronic applications
Published in Science and Technology of Advanced Materials, 2022
Sheng Yu, Junyu Tang, Yu Wang, Feixiang Xu, Xiaoguang Li, Xinzhong Wang
The 2D heterostucture based on 2D non-magnetic material/magnets can have promising valleytronic applications. Valleytronics is a novel practical approach for information processing and storage by utilizing the spin and valley degrees of electron freedom. The 2D heterostructure can lift the valley degeneracy and produce the valley splitting by interfacial effect and ultimately induce a robust and nonvolatile valley polarization, which is of great importance to the creation, conduction and storage of magnetic informations. Sabirianov et al. [82] fabricated the heterostructure of WS2 monolayer and ferromagnet EuS. They found that the giant valley splitting of 16 meV can be induced by magnetic proximity effect, which is two orders of magnitude larger than that obtained by applying external magnetic field. Their first-principles calculations revealed that the sign reversal between WSe2/EuS and WS2/EuS is attributed to the different atomic termination at EuS surface sites. Kang et al. [83] performed first-principles calculations and observed a considerable valley splitting energy of 376 meV at the valance band of 2D WS2/h-VN heterostructure, which corresponds to an effective Zeeman magnetic field of 2703 T. Feng et al. [84] also used first-principles calculations to demonstrate a large valley spitting of 214 meV at the valence band of monolayer WS2 on the MnO(111) surface, which is equivalent to a Zeeman magnetic field of 1516 T. Gong et al. [85] observed a large valley splitting over 30 meV at the conduction band of WSe2 monolayer in 2D MnPSe3/WSe2 heterostructure.