非共振圆偏振光作用下单层二硫化钼电子结构及其自旋/谷输运性质

基于紧束缚近似下的低能有效哈密顿模型和久保线性响应理论,研究了外部非共振圆偏振光作用下单层二硫化钼(MoS_2)电子结构及其自旋/谷输运性质.研究结果表明:单层MoS_2布里渊区K谷和K′谷附近自旋依赖子带间的能隙随着非共振右旋圆偏振光引起的有效耦合能分别线性增大和先减小后增大,随着非共振左旋圆偏振光引起的有效耦合能分别先减小后增大和线性增大,实现了系统能带结构有趣的半导体-半金属-半导体转变.此外,单层MoS_2在外部非共振圆偏振光作用下,呈现有趣的量子化横向霍尔电导和自旋/谷霍尔电导,自旋极化率在非共振右/左旋圆偏振光有效耦合能±0.79 eV附近达到最大并发生由正到负或由负到正的急剧转变,谷极化率随着非共振圆偏振光有效耦合能先增大后减小并在其绝对值0.79-0.87 eV范围内达到100%.因而,可以利用外部非共振圆偏振光将单层MoS_2调制成自旋/谷以及光电特性优异的新带隙材料.

Electronic structure and spin/valley transport properties of monolayer MoS2 under the irradiation of the off-resonant circularly polarized light

The new-type monolayer semiconductor material molybdenum disulfide (MoS₂) is direct band gap semiconductor with a similar geometrical structure to graphene, and as it owns superior physical features such as spin/valley Hall effect, it should be more excellent than graphene from the viewpoint of device design and applications. The manipulation of the spin and valley transport in MoS₂-based device has been an interesting subject in both experimental and theoretical researches. Experimentally, the photoninduced quantum spin and valley Hall effects may result in high on-off speed spin and/or valley switching based on MoS₂. Theoretically, the off-resonant electromagnetic field induced Floquet effective energy should modulate effectively the electronic structure, spin/valley Hall conductance as well as the spin/valley polarization of the MoS₂, through the virtual photon absorption and/or emission processes. Utilizing the low energy effective Hamilton model from the tight-binding approximation and Kubo linear response theorem, we theoretically investigate the electronic structure and spin/valley transport properties of the monolayer MoS₂ under the irradiation of the off-resonant circularly polarized light in the present work. The band gaps around the K and K' point of the Brillouin region for monolayer MoS₂ proves to increase linearly and decrease firstly and then increase, respectively with the increase of external off-resonant right-circularly polarized light induced effective coupling energy, and decrease firstly and then increase and increase linearly with the increase of left-circularly polarized light induced effective coupling energy, therefore, the interesting transition of semiconducting-semimetallic-semiconducting may be observable in monolayer MoS₂. Furthermore, the spin and valley Hall conductance of the monolayer MoS₂ for the case without off-resonant circularly polarized light are 0 and 2e²/h, respectively, and they will convert into -2e²/h and 0 when the absolute value of the off-resonant circularly polarized light induced effective coupling energy is in a range of 0.79-0.87 eV. Finally, the spin polarization for monolayer MoS₂ increases up to a largest value and changes from positive to negative and/or negative to positive at the vicinity of the effective coupling energy ±0.79 eV of the off-resonant right/left circularly polarized light, while the valley polarization should increase firstly and then decrease with the off-resonant circularly polarized light, and goes up to 100% in the range of 0.79-0.87 eV of the absolute value for effective coupling energy. Therefore, the external off-resonant circularly polarized electromagnetic field should be an effective means in manipulating the electronic structure, spin/valley Hall conductance and spin/valley polarization of the monolayer MoS₂, the two-dimensional MoS₂ may be tuned into a brand bandgap material with excellent spin/valley and optoelectrical properties.