Browsing by Author "Yao, Yugui"

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Engineering symmetry breaking in 2D layered materials
(2021-03) Du, Luojun; Hasan, Tawfique; Castellanos-Gomez, Andres; Liu, Gui Bin; Yao, Yugui; Lau, Chun Ning; Sun, Zhipei
A2 Katsausartikkeli tieteellisessä aikakauslehdessä
Symmetry breaking in 2D layered materials plays a significant role in their macroscopic electrical, optical, magnetic and topological properties, including, but not limited to, spin-polarization effects, valley-contrasting physics, nonlinear Hall effects, nematic order, ferroelectricity, Bose-Einstein condensation and unconventional superconductivity. Engineering symmetry breaking of 2D layered materials not only offers extraordinary opportunities to tune their physical properties but also provides unprecedented possibilities to introduce completely new physics and technological innovations in electronics, photonics and optoelectronics. Indeed, over the past 15 years, a wide variety of physical, structural and chemical approaches have been developed to engineer the symmetry breaking of 2D layered materials. In this Technical Review, we focus on the recent progress on engineering the breaking of inversion, rotational, time-reversal and gauge symmetries in 2D layered materials, and present our perspectives on how these may lead to new physics and applications.
Robust circular polarization of indirect Q-K transitions in bilayer 3R-W S2
(2019-10-23) Du, Luojun; Zhang, Qian; Zhang, Tingting; Jia, Zhiyan; Liang, Jing; Liu, Gui Bin; Yang, Rong; Shi, Dongxia; Xiang, Jianyong; Liu, Kaihui; Sun, Zhipei; Yao, Yugui; Zhang, Qingming; Zhang, Guangyu
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
Valley-contrasting Berry curvature and orbital magnetic moment have led to highly selective circular polarization of direct excitons at the K valleys in transition-metal dichalcogenides. In addition to K valleys, Q valleys, another critical point in the conduction band, also possess well-defined but distinct magnetic moment. Being akin to the direct excitons at K valleys, indirect excitons associated with Q (K) valleys in the conduction (valence) band could allow circular polarization in principle. Here, we report an experimental observation of the circular polarization of indirect Q-K transitions in noncentrosymmetric bilayer 3R-WS2. In stark contrast to the circular polarization of direct excitons which depolarizes with increasing lattice temperature, the circular polarization of indirect Q-K excitons is extremely robust and independent on the temperature. Such robust circular polarization can be understood as follows: The spin-orbit coupling in the Q valley is much stronger than that in the K point of the conduction band, significantly suppressing the temperature induced valley depolarization. Our results open up opportunities for exotic valleytronics and quantum information processing applications.
Strongly distinct electrical response between circular and valley polarization in bilayer transition metal dichalcogenides
(2019-05-10) Du, Luojun; Liao, Mengzhou; Liu, Gui Bin; Wang, Qinqin; Yang, Rong; Shi, Dongxia; Yao, Yugui; Zhang, Guangyu
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
We introduce a physical model to describe the influence of a perpendicular electric field on circular polarization (CP) and valley polarization (VP) in bilayer transition metal dichalcogenides. Our results uncover that electric-field-dependent CP and VP are quite distinct from each other. The dependence of CP on the electric field harbors a W pattern and possesses the minimum when the potential energy difference between the two layers is equal to the strength of spin-orbit coupling. Such dependence of CP stems from the modulation of energy cost for interlayer hopping and spin-dependent layer polarization. In contrast, VP is strictly absent in primitive bilayers and increases monotonically with increasing strength of electric field, resulting from the continuous variation of valley magnetic moments and inversion-symmetry breaking. Our model elaborates well the recent experimental observations for which the origin is under debate. Moreover, we demonstrate that the manipulation of layer and valley pseudospin is fully tunable by perpendicular electric fields, paving the way for prospects in electrical control of exotic layer-valleytronics.