The Effect Of Uniaxial Strain On Valleytronics Properties Of Molybdenum Disulfide Monolayer

In the year of 2004,the first single atom monolayer material,graphene,was striped successfully from its bulk material,which opens a new gate toward an era that we could study directly the movement and interaction of electrons and pseudo particles in a two-dimensional system.Dimensionality is of great importance for some like electronic,thermal and magnetic properties.And for the main role of this thesis,valleytronics,it also has profound connections with dimensionality.The energy valley can be understand as some region at the lowest conduction band or the highest valance band in where the carriers will have lower energy than others.Valleytronics study energy valley itself and what properties the electron in the valleys have.The concept of energy valley is not only for two-dimensional material but,we will see later,it is the limitation of dimensionality to have proper number of symmetry element of monolayer materials;there are just enough number of independent valley to realize valley degree of freedom by accumulating more carrier in one valley than another,and,in same time,make sure each valley has rotational symmetry to be controlled by some manner of time reversal symmetry breaking which are vital for realizing the valley degree of freedom.This thesis focus on the uniaxial strain effect on vallytronics properties of Mo S₂like two-dimensional monolayers.The most important effect of uniaxial strain is the symmetry lowering.The symmetry lowering will happen in both real and reciprocal space.As a landmark in reciprocal space,the symmetry of energy valley will also be lowered as a result of valley drift.In this thesis,the pattern of valley drift in monolayer Mo S₂ under the uniaxial strain is discussed,and,based on the WSe₂/Cr Se₂heterojunction,how the symmetry lowering and valley drift affect magnetism induced valley splitting is studied.The results show that pattern of valley drift of Mo S₂ is unique and the uniaxial strain can destroy the magnetism induced valley splitting.This work provide more understanding about properties of valleytronics of Mo S₂ like monolayer and give some theoretical suggestion for its application.This thesis is composed by five chapters.In the first chapter,firstly,I will present a brief introduction of two-dimensional material focusing on their classifications and applications.Particularly,it shall focus on Transition metal dichalcogenides（TMDs）which are in the leading role throughout this thesis.And then,an introduction about the basics concept,study purpose and progress of valleytronics will be present.At the last,I shall give some explanation about why I choose this direction and about the overall framework of this thesis to help reader understand my story better.In the second chapter,I will present a brief review about band theory,symmetry properties of band and spin-orbit coupling effect on band structure.This review will focus on the lowest conduction band of Mo S₂ monolayer because this band has unique shape to implement valleytronics applications.In the third chapter,I will systematically discuss the deformation of the first Brillouin zone,the coordination of high symmetry point on the boundary of Brillouin zone and valley drift under the uniaxial strain,which are vital for simulating electronic properties of monolayer Mo S₂ under the uniaxial strain.In the end of this chapter,an explanation for the unique pattern of valley drift will be given.In the fourth chapter,the electronic structure of WSe₂/Cr Se₂ heterojunction under uniaxial strain will be calculated and uniaxial strain effect on vallytronics will be discussed.As the monolayer Cr Se₂ has the ferromagnetic property and is insensitive to tensile strain,the heterojunction could be an ideal platform to study the question about the effect of uniaxial strain on magnetism induced valley splitting in WSe₂.The results show that the uniaxial strain can completely destroy the magnetism induced valley splitting.In the end of this chapter,an explanation for this result from the viewpoint of symmetry and spin orbit coupling effect will be given.In the last chapter,all this thesis will be summarized,and some further question will be expected.