Citation for the 2021 (the 9th) Nishina Asia Award
YAO, Wang(Chair Professor, Department of Physics, The University of Hong Kong,Hong Kong SAR, China)
For his pioneering contributions to valleytronics in two-dimensional
semiconductors and van der Waals heterostructures

Realization of relativistic "Dirac fermions" in two-dimensional materials
such as graphene is not only interesting from cross-disciplinary interest
between condensed matter and high-energy physics, but also as a platform toexhibit various exotic phenomena, and eventually to realize usefulfunctionalities. In condensed matter realization, multiple copies of Diracfermions are often realized as required by the symmetry and topology of thesystem. They usually occur at different points of the momentum space. This
provides an extra degree of freedom called "valley", in addition to the spin
degree of freedom.
As one of his pioneering contributions concerning the valley degree offreedom, Dr. Wang Yao discovered theoretically the "valley Hall effect", inwhich currents associated to two valleys flow to opposite directions
orthogonal to the applied electric field [1]. This effect arises when theinversion symmetry is broken in graphene, for example, by the substrates.
The Dirac fermions then acquire masses (namely the excitation gap opens) sothat the Dirac masses for the two valleys have opposite signs. The resulting
Berry curvature naturally induces the valley Hall effect. This discovery has
generated a significant interest in the field, and become one of thefundamentals of valleytronics: exploitation of the valley degree of freedom torealize devices with useful functionalities. It was still difficult to probe andcontrol the valley degree of freedom with certainty in experiments. Dr. Yao
made another groundbreaking contribution with the prediction of the valleyselective optical transitions by circularly polarized light [2]. This enabledversatile controls and precise measurements of the valley degree of freedom.
Valleytronics has become an experimentally demonstrated reality, largely
thanks to these pioneering contributions by Dr. Yao.

Dr. Yao's works have also vitalized the study of new group of materials,
especially transition metal dichalcogenides, such as monolayer MoS2. They
have a structure similar to graphene, but with the inversion symmetry
broken. Thus they turned out to be an ideal playground for valleytronics,
naturally realizing the valley Hall effect and the valley-selective opticaltransitions as it had been predicted by Dr. Yao. In fact, significant spin-orbit
couplings exist in this class of materials, leading to new physics. Dr. Yao also
elucidated the interplay between the spin and valley degrees of freedom inthese materials [3]. In particular, spin-polarized excitations can be created bychoosing the frequency properly for the circularly polarized light, thanks tothe valley-selective optical transition and the spin-orbit coupling. This further
enriched the emerging field of valleytronics.
Dr. Yao has also provided numerous other interesting ideas, such as longlived interlayer excitons in moiré materials [4], stimulating theoretical andexperimental studies in the field. In addition to his theory papers which havebeen influential for experimental studies, he also often helps experimental
study directly through collaborations with experimentalists.
Valleytronics has evolved into an important field in physics and for futureengineering, with the new class of two-dimensional semiconducting materials
as an ideal platform. There is no doubt that Dr. Yao's contributions have been
essential for this evolution, and that he continues to be a widely recognized
leader in the field. Based on these achievements, Dr. Yao deserves theNishina Asia Award.
References[1] Di Xiao, Wang Yao, and Qian Niu, "Valley-Contrasting Physics inGraphene: Magnetic Moment and Topological Transport", Phys. Rev. Lett.
99, 236809 (2007).
[2] Wang Yao, Di Xiao, and Qian Niu, “Valley-dependent Optoelectronics
from Inversion Symmetry Breaking”, Phys. Rev. B 77, 235406 (2008).
[3] Di Xiao, Gui-Bin Liu, Wanxiang Feng, Xiaodong Xu, and Wang Yao,
“Coupled spin and valley physics in monolayers of MoS2 and other group-VIdichalcogenides”, Phys. Rev. Lett. 108, 196802 (2012).

[4] Hongyi Yu, Guibin Liu, Jianju Tang, Xiaodong Xu, and Wang Yao,
“Moiré excitons: from programmable quantum emitter arrays to spin-orbit
coupled artificial lattices”, Science Advances 3, e1701696 (2017).