Theoretical Design Of Spin And Valley Device Controlled By External Field In Monolayer WSe₂

Two-dimensional（2D）materials,that is,thin film materials with only one or several atomic layers thick,such as graphene,silicene,and transition metal dichalcogenides（TMDs）,etc.Among them,monolayer graphene is a zero-band-gap material and can not be directly used to make electronic switches.Monolayer silicene is an indirect band gap material,and its luminous efficiency is much lower than that of direct band gap semiconductor materials.However,the monolayer TMDs is a direct band gap semiconductor material,so it has broad application prospects in the field of photoelectricity such as solar cells.In addition,the monolayer TMDs has two significant characteristics compared with graphene,one is the strong spin-orbit coupling（SOC）derived from the d orbit of the heavy metal atom,and the other is the inversion symmetry breaking gives rise to different Berry curvatures at two inequivalent valleys,which results in the different valleys responding to circularly polarized light.This provides a prerequisite for designing of logic devices and encoding information by using spin and valley indexes.In this paper,we mainly focus on the spin and valley transport properties controlled by external field（such as optical,electrical,magnetic and velocity fields,etc.）in the monolayer WSe₂,and to design high-efficiency optoelectronic devices in theory.The main contents of this dissertation are shown follows（1）The study on the ballistic conductance controlled by optical,electric and magnetic field in monolayer WSe₂ junction.It has found that an electric switch is available by increasing potential barrier beyond one critical value in WSe₂.Due to the larger SOC in WSe₂,nearly 100%perfect valley and spin polarizations are gained by using different valley responding to non-resonant circularly polarized light.Additionally,the increasing FM exchange field can assist significantly the spin polarization in a wider region of light intensity and barrier height plane due to the large shift of spin-up and down bands.This work provides an important theoretical basis for designing the optical,electric and magnetic controlled spin valley electronic devices.（2）The study on spin and valley polarized transport controlled by velocity barrier in monolayer WSe₂ junction.It has found that both the spin-valley resolved transmission probabilities and conductance are strong dependent on the velocity barrier.As the velocity barrier decreases to 0.06,a spin-valley polarization of exceeding 90%is observed,which is distinct from the monolayer Mo S₂ owing to incommensurable SOC.In addition,the transport properties under the control of multiple velocity barrier are further studied and many extraordinary velocity barrier-dependent transport gaps are observed due to evanescent tunneling.This work provides an important theoretical basis for the manufacture of electronic switching devices controlled by velocity barrier.（3）The study on the spin and valley beam splitter through the Goos–H?nchen shift in monolayer WSe₂ tunnelling junction.It has found that a two-fold degenerate lateral shift induced by the spin-valley locking occurs at Fabry–Perot resonance width in single barrier structure.By introducing the FM exchange field,one spin and valley dependent lateral shift can be further obtained.For double barriers structure,the lateral shifts exceeding thousands of Fermi wavelengths are observed at Fabry–Perot resonance width due to the effect of localized states between these barriers.This work provides an important theoretical basis for designing the spin and valley beam splitter based monolayer WSe₂.