Valley dependent physics for potential information processing, from graphene to atomically thin transition metal dichalcogenides
Speaker: Prof. Wang Yao, Assistant Professor, The University of Hong Kong

Abstract: A trend in future electronics is to utilize internal degrees of freedom of electron, in addition to its charge, for information processing. A paradigmatic example is spintronics based on spin of electrons. Degenerate valleys of energy bands well separated in momentum space constitute another discrete degrees of freedom for low energy carriers with long relaxation time. This has led to the emergence of valleytronics, a conceptual electronics based on the valley index. Our previous work showed that when the inversion symmetry is broken in graphene, the two inequivalent valleys develop contrasted topological physical properties [1-3]. The valley index is then associated with the phenomena which the spin has in a conventional semiconductor, including the magnetization, Hall transport, optical transition selection rules, and chiral edge modes. These properties make possible the control of valley dynamics by magnetic, electric and optical means, forming the basis for potential valley-based device applications. We recently generalize the study to monolayers of group VI transition metal dichalcogenides, which are multi-valley direct bandgap semiconductors with strong spin-orbit coupling [4]. We find the low energy electrons and holes are described by massive Dirac electrons with strong spin-valley coupling. We discover valley and spin dependent optical transition selection rules, and coexistence of valley Hall effect and spin Hall effect for both electrons and holes. These make possible valley and spin controls for potential integrated spintronics and valleytronics applications on this platform. I will also present experiments performed on monolayer MoS2 which confirm our prediction of valley optical selection rule and demonstrate optical generation and detection of valley polarization [5].
[1] Di Xiao, Wang Yao and Qian Niu, Phys. Rev. Lett. 99, 236809 (2007).
[2] Wang Yao, Di Xiao and Qian Niu, Phys. Rev. B 77, 235406 (2008).
[3] Wang Yao, Shengyuan Yang and Qian Niu, Phys. Rev. Lett. 102, 096801 (2009).
[4] Di Xiao, Gui-Bin Liu, Wanxiang Feng, Xiaodong Xu and Wang Yao, Phys. Rev. Lett. 108, 196802 (2012).
[5] Hualing Zeng, Junfeng Dai, Wang Yao, Di Xiao and Xiaodong Cui, http://arxiv.org/abs/1202.1592, to appear in Nature Nanotechnology.

Biography: Prof. Yao is an Assistant Professor in Department of Physics in the University of Hong Kong. He obtained his Bachelor of Science in Physics from Beijing Univesity and his PhD in Physic from University of California, San Diego.