Study On The Control Of Spin And Valley Degrees Of Freedom In Layered Transition Metal Dichalcogenides

The establishment and development of conventional semiconductor microelectronics led to the third scientific and technological revolution in human history,which rapidly brought human society to the present information age.Nowadays,all aspects of people’s learning,life and work are closely linked to the rapid development of information technology.The traditional semiconductor technology was established and developed on the basis of electronic charge degree of freedom,and in the decades of development since its establishment,the integration of integrated circuit chips has been increasing according to the regularity of Moore’s Law.However,as the size of the chip technology continues to decrease,the wave of electrons becomes more and more pronounced,so the performance of the chip circuit devices is greatly affected and limited.In order to keep the rapid development of information technology,in addition to the continuous research on more advanced chip fabrication methods,people are also exploring other degrees of freedom of electrons in addition to charge degrees of freedom,which are used to realize the logical functions of encoding,storage and processing of information.In addition to the charge degree of freedom,the electrons also have spin as an intrinsic degree of freedom.Based on the spin degrees of freedom of electrons,the possibility of using the spin properties of electrons for information technology has been continuously studied and explored,and the mature field of spintronics has been formed.Additionally,in two-dimensional crystalline materials with special symmetry,such as graphene and transition metal dichalcogenides,electrons will have an extra energy valley degree of freedom,which also has great potential for encoding,transmission and processing of information.The primary challenge in the study of the application of spin and energy valley degrees of freedom is to achieve significant and controllable spin polarization and valley polarization.In this thesis,we investigate,mainly from the theoretical aspect,the topological property changes of monolayer transition metal dichalcogenides materials and the transport properties associated with the energy valley degrees of freedom under the electrical tuning.Firstly,we briefly introduce the development and research status of spintronics and valleytronics,especially the research achievements and some difficulties encountered in graphene and transition metal dichalcogenides.A brief introduction is also given for the topological theory of low-dimensional systems due to the relevance of the research work.Then,we introduce and give a brief derivation analysis of the main theoretical and computational methods used in this thesis,which mainly include the energy band theory in lattice space,the Green’s function method,and the channel-resolved transport calculation method.Next,we studied a heterojunction system composed of a monolayer transition metal dichalcogenides with a ferromagnetic substrate.Taking a system consisting of monolayer molybdenum ditelluride and europium oxide as an example,using a three-orbital tight-binding model approach,we have investigated the effects of magnetic exchange interaction and Rashba field on the topological properties of monolayer molybdenum ditelluride materials.Through tuning the strength of the Rashba field and magnetic exchange interaction,we have discovered a new quantum state of matter,the valley-resolved quantum anomalous Hall state,in monolayer molybdenum ditelluride.Different from the conventional quantum anomalous Hall effect,the valley-resolved quantum anomalous Hall effect here has a completely valley-resolved nature,i.e.,the system in this quantum state will have its edge states all localized in a single energy valley-K(or(10)K).We calculated the Berry curvature and Chern number as well as the wave-function distributions in the valley-resolved quantum anomalous Hall state.The results show that two pairs of valley-resolved chiral edge modes exist at the boundary of the monolayer molybdenum ditelluride nanoribbons.In addition,further calculations and analysis of the valley-Chern number show that the different simultaneity of the energy band inversions in two unequal energy valleys leads to the emergence of the valley-resolved quantum anomalous Hall effect.By calculating the energy gaps in two unequal energy valleys,we give the phase diagram of the system,which contains three regions: the valley Hall insulator,the valley-resolved quantum anomalous Hall effect,and the band insulator.The transport properties of systems with different nanoribbon sizes and different concentrations of atomic vacancy defects in the valley-resolved quantum anomalous Hall state were analyzed by the nonequilibrium Green’s function method,and the computational results showed that the edge states of the valley-resolved quantum anomalous Hall effect are robust to atomic vacancy defects in nanoribbon materials.And then,we investigated an electrical tuning mechanism in monolayer transition metal dichalcogenides for generating valley polarization.The modulation mechanism originates from the backscattering caused by the Rashba spin-orbit coupling effect in the transition metal dichalcogenides nanoribbons.Since the Rashba spin-orbit coupling effect in monolayer transition metal dichalcogenides can be induced by an externally applied perpendicular electric field,the valley polarization of carriers generated through this mechanism can then be dynamically controlled by means of an external gating.We have studied the valley-dependent electron transmission and reflection phenomena in the presence of the Rashba spin-orbit coupling effect in the central scattering region,with a representative molybdenum disulfide nanoribbon.The results explicitly show that a very significant valley polarization is observed in the carrier flux when the Rashba spin-orbit coupling effect is present in the central scattering region.The maximum efficiency of valley polarization can even exceed 80% by tuning the Fermi energy and the strength of the Rashba spin-orbit coupling.We investigate the dependence of the valley polarization efficiency on the Rashba spin-orbit coupling strength,and the results show that the stronger the Rashba spin-orbit coupling effect does not generate a more pronounced valley polarization efficiency,and a relatively moderate Rashba spin-orbit coupling strength is more favorable for achieving a pronounced valley polarization.Through analyzing the effect of vacancy defects in the monolayer transition metal dichalcogenides nanoribbons on the valley polarization efficiency,we found that the valley polarization efficiency remains significant even when there is a relatively high concentration of vacancy defects in the nanoribbons.In order to confirm the generality of this electrical tuning mechanism,we also calculated the valley polarization in the(4,1)nanoribbon.Furthermore,in addition to molybdenum disulfide,we also calculated the valley polarization efficiency in five other transition metal dichalcogenides crystals,and the results showed that monolayers of molybdenum-based transition metal dichalcogenides are more appropriate for generating a significant valley polarization current.Finally,we conclude with a summary and outlook of the research content of this thesis.