Valley Polarization In Two-dimensional Materials And Its Regulation Mechanism

Two-dimensional（2D）valleytronic systems,using the valley index of carriers to perform logic operations,serves as the basis of the next-generation information technologies.Manipulating the different degrees of freedom（charge,spin,valley）of electrons to expand data capacity will greatly improve the performance of next-generation electronic products.In order to effectively utilize the valley degrees of freedom,the main challenge at present is to lift the valley degeneracy to achieve valley splitting.The properties of valleytronics are regulated by means of biaxial stress,external electric field and magnetic doping,which provides theoretical basis for the design of valleytronics devices.In this dissertation,the controllable spin direction in nonmagnetic BX/MX₂（M=Mo or W;X=S,Se and Te）van der Waals heterostructures by switching between the Rashba splitting and valley polarization,conduction band-edge valley splitting in 2D ferroelectric AgBiP₂S₆ by magnetic transition-metal（TM=V,Cr,Mn,Fe,Co,Ni）doping and nonvolatile electrical control of valley splitting by ferroelectric polarization switching in 2D AgBiP₂S₆/CrBr₃ multiferroic heterostructures are investigated systematically based on density functional theory.The electronic properties of the BX/MX₂ heterostructures are calculated.It is found that the Rashba-type spin splitting with the in-plane spin and valley Zeeman splitting with the out-of-plane spin can be formed and coexist at the valence-bands of BSe/MoSe₂ and BSe/WSe₂ by considering the spin-orbit coupling.The dominant carrier spin direction around the valence-band edge can be effectively tuned by exerting a biaxial strain and external electrical field.Using first-principles calculations,it is proposed that valley splitting can be readily achieved in a ferroelectric AgBiP₂S₆ monolayer by transition-metal doping.Especially in a V-doped system,accompanied with a large valley splitting（～26.8 me V）,there is a serious n-type doping in AgBiP₂S₆.The efficient electron-doping moves the Fermi level just located between the conduction band minimum of the K/K’valleys,which is suitable for valley-polarized transport.The valley-polarized index can be flipped by applying a small magnetic field to rotate the magnetocrystalline direction.The magnitude of valley splitting relies on the strength of orbital hybridization between the TM-d and Bi-p states and can be tuned continually by applying biaxial strain.Under an in-plane electric field,such valley degeneracy breaking would give rise to the valley Hall effect,which is crucial to design a valleytronic device.The electronic properties of the AgBiP₂S₆/CrBr₃ multiferroic heterostructure are calculated.It is found that when the ferroelectric AgBiP₂S₆ is combined with the ferromagnetic CrBr₃,the time reversal symmetry is broken due to the magnetic proximity coupling of the ferromagnet,so the degeneracy of the K/K’valley is lifted,resulting in valley splitting（～30.5me V）.The conversion of the polarization direction of the ferroelectric layer in the heterostructure can change the direction of valley splitting and realize the nonvolatile electrical control of valley splitting.