Welcome to Yao group

We are a theoretical group working in an interdisciplinary field across condensed matter physics, quantum physics, and optical physics.

Research Highlights

Valley physics in 2D transition metal dichalcogenides

A trend in future electronics is to utilize internal degrees of freedom of electron, in addition to its charge, for nonvolatile 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.  

In monolayer group-VI transition metal dichalcogenides, a class of direct bandgap multi-valley 2D semiconductor, we find the low energy electrons and holes are described by massive Dirac electrons with strong spin-valley coupling. We find 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. Our prediction of valley optical selection rule and optical generation and detection of valley polarization are independently confirmed by several experimental groups. Optical generation of valley coherence is also first demonstrated.  

In dichalcogenide bilayers, with the AB-stacking order, we find that the spin-valley coupling in monolayer manifests as an effective coupling between the valley and spin with the layer pseudospin degree of freedom. The latter is associated with an electric polarization. The effective coupling between spin, valley and pseudospin makes possible a variety of magnetoelectric effects and prototype interplay between the spin and valley as information carriers.

Review

Spins and pseudospins in layered transition metal dichalcogenides,

X. Xu*, W. Yao*, D. Xiao and T. F. Heinz,

Nature Physics 10, 343 (2014). — (Link).

Our Main Theory Works

Nonlinear valley and spin currents from Fermi pocket anisotropy in 2D crystals,

H. Yu, Y. Wu, G. Liu, X. Xu and W. Yao,

http://arxiv.org/abs/1406.2931

Dirac cones and Dirac saddle points of bright excitons in monolayer transition metal dichalcogenides,

H. Yu, G. Liu, P. Gong, X. Xu and W. Yao,

Nature Communications 5, 3876 (2014). — (Link)

Z. Gong, G. Liu, H. Yu, D. Xiao, X. Cui, X. Xu and W. Yao,

Nature Communications 4, 2053 (2013). — (Link)

A three-band tight-binding model for monolayers of group-VIB transition metal dichalcogenides,

G. Liu, W. Shan, Y. Yao, W. Yao* and D. Xiao*

Phys. Rev. B 88, 085433 (2013). — (Link)

See also our earlier papers:
Phys. Rev. B 77, 235406 (2008). — (Link)

Phys. Rev. Lett. 99, 236809 (2007). — (Link)

Collaboration with experimental groups

Electrically tunable excitonic light-emitting diodes based on monolayer WSe2 p-n junctions,

J. Ross, P. Klement, A. Jones, N. Ghimire, J. Yan, D. Mandrus, T. Taniguchi, K. Watanabe, K. Kitamura, W. Yao, D. Cobden, X. Xu

Nature Nanotechnology 9, 268 (2014). — (Link)

A. Jones, H. Yu, J. Ross, P. Klement, N. Ghimire, J. Yan, D. Mandrus, W. Yao, X. Xu

Nature Physics 10, 130 (2014). — (Link)

A. Jones, H. Yu, N. Ghimire, S. Wu, G. Aivazian, J. Ross, B. Zhao, J. Yan, D. Mandrus, D. Xiao, W. Yao*, X. Xu*

Nature Nanotechnology 8, 634 (2013). — (Link)

Electrical Tuning of Valley Magnetic Moment via Symmetry Control in bilayer MoS2,

S. Wu, J. S. Ross, G. Liu, G. Aivazian, A. Jones, Z. Fei, W. Zhu, D. Xiao, W. Yao, D. Cobden, X. Xu,

Nature Physics 9, 149 (2013). — (Link)

Electrical Control of Neutral and Charged Excitons in a Monolayer Semiconductor,

J. S. Ross, S. Wu, H. Yu, N. Ghimire, A. Jones, G. Aivazian, J. Yan, D. Mandrus, D. Xiao, W. Yao, X. Xu,

Nature Communications 4, 1474 (2013). — (Link)

Optical signature of symmetry variantions and spin-valley coupling in atomically thin tungsten dichalcogenides,

H. Zeng, G. Liu, J. Dai, Y. Yan, B. Zhu, R. He, L. Xie, S. Xu, X. Chen, W. Yao*, X. Cui*,

Scientific Reports 3, 1608 (2013). — (Link)

Valley polarization in MoS2 monolayers by optical pumping,

H. Zeng, J. Dai, W. Yao, D. Xiao, X. Cui,

Nature Nanotechnology 7, 490 (2012). — (Link).

Media coverage: Ars Technica, Nature Nanotech. News and Views