First-principles Study Of Two-dimensional Ferroic Materials

In the field of condensed matter physics,ferroelectric materials have potential of application in the information storage,sensing devices,field-effect transistors and so on,which are attracting researchers'wide interest.In the past few decades,the research on ferroelectrics has mainly focused on three-dimensional traditional ferroelectric materials such as perovskite oxides.However,traditional ferroic films grown by epitaxy will encounter problems such as quantum tunneling,dangling bonds and depolarization.In order to meet the needs of the rapid development of the information age,the inevitable trend of the future development of electronic devices is low energy consumption,fast response and high storage density.Due to the advantages of the geometric structure,two-dimensional materials have clean surface,which can greatly reduce the thickness of the electronic devices,and have become popular materials for manufacturing integrated and miniaturized electronic devices.Considering the wide application of ferroelectric materials in modern technology,we naturally hope to combine ferroelectric materials with two-dimensional materials to obtain ferroelectricity and miniaturize electronic components at the same time.Of course,due to increasing demand for multi-functional and miniaturized devices,it is obviously difficult for the material having a single ferroic property to compete with multiferroic materials.The cross-coupling between different ferroic order parameters and the corresponding external field cross-control in multiferroic materials can be used to prepare new high-performance electronic devices such as energy harvesting,memory,sensors and actuators.After the survey of two-dimensional ferroelectric material,we pay attention to the two-dimensional multiferroic materials with coexistence of ferroelectricity,ferromagnetism and so on.The charge,spin and other degrees of freedom of electrons in multiferroic materials can interact and couple with each other,showing many novel condensed matter physical phenomena.Based on the first principles,this thesis studies the properties of new two-dimensional ferroelectric materials and multiferroic materials.The thesis is divided into five chapters,and the main contents are as follows:The first chapter is an introduction.It briefly introduces the origin,definition and application of ferromagnetic,ferroelectric and multiferroic,and gives a detailed review of the research status of two-dimensional ferromagnetic,ferroelectric and multiferroic materials.After that,the research purpose and content of this thesis are introduced.The second chapter introduces the theories,calculation methods and software used in this thesis:density functional theory,exchange correlation functional theory,energy band theory,VASP software and modern polarization theory.In chapter 3,based on first-principles calculations,we investigated the ferroelectric properties of the bilayer 1T'-phase WTe₂.By calculating the ferroelectric polarization switching path,we found that the out-of-plane vertical ferroelectric polarization switching of the system is realized by interlayer sliding.The obtained ferroelectric polarization value well reproduced the results of pervious experiments and theoretical calculations.Our present differential charge density results show that the ferroelectricity in the bilayer 1T'-phase WTe₂ is derived from interlayer charge transfer.Furthermore,we calculated the spin textures of the ferroelectric bistable and found that their in-plane spin texture havs opposite chirality.This shows that we can use ferroelectricity and external electric field to regulate the spin texture of the system,which has important applications in spintronics.According to the previous experimental data,we proposed a spin field effect transistor model which can effectively improve the spin polarization injection rate of the device.In addition,we have found that strain has an important effect on the ferroelectric properties of the bilayer 1T'-phase WTe₂,which may realize the transition from the ferroelectric phase to the paraelectric phase.In chapter 4,based on first-principles density functional theory,we studied the magnetoelectric coupling effect in bilayer H-phase VS₂ multiferroic material.We found that ferroelectricity and antiferromagnetism in the system are coupled together by ferrovalley to achieve electrically controlled magnetism.Therefore,we can use bilayer H-phase VS₂ to make a multi-state storage and to realize a two-dimensional new high-performance electronic functional device that uses magnetic methods to read information and electrical methods to write information.Moreover,the interlayer distance has an important effect on the magnetic properties of the system.Further,under the condition of external electric field,we can achieve liner and second-order nonlinear magnetoelectric coupling at different interlayer distance.If the applied electric field is large enough,the transformation from antiferromagnetism to ferromagnetism in the system may also be realized.The new physical microscopic mechanism of the coexistence and coupling between ferroelectric,antiferromagnetic and ferrovalley,is shown not to be a special case only for the bilayer H-phase VS₂,but is generic in the bilayer ferroelectric-antiferromagnetic ferrovalley materials,which is of great significance for understanding and manipulating the interaction of electron charge,spin and valley degrees of freedom.In the fifth chapter,we summarized the research content of this thesis,and outlined the research and development trend of two-dimensional ferroelectric,ferromagnetic and multiferroic materials.