Theoretical Studies Of Novel Two-dimensional Ferroic Materials

Traditional electronics,spintronics,and valleytronics with electronic charge,spin,and energy valley all use the binary state of electron degrees of freedom to perform data transmission behavior that can be manipulated by exteral field,which is the core content of new information industry.Ferroic materials with spontaneous polarization properties play an important role in data storage devices and the information industry.With the rise of graphene,two-dimensional materials have developed vigorously.In order to realize non-volatile functional devices with low power consumption,intrinsic ferroic materials with high density and faster speed at low dimensional scale have great research value.Under the above background,our dissertation will use two-dimensional ferromagnetic materials as the initial research platform.Through first-principles and other calculation methods,combined with a variety of theoretical analysis methods,the research of the new physical properties and devices in Valleytronics and Spintronics are applied.The first chapter,as an introduction to this thesis,briefly reviews the basic ferroelectricity including ferroelectricity and ferromagneticity in two-dimensional materials.At the same time,focusing on the new ferroic family member,the ferrovalley materials,detailed introduction is made from the development of valleytronics and the regulation of spontaneous valley polarization.The development of the quantum anomalous Hall effect in two-dimensional materials and the characteristic nonlinear optical properties,second harmonic generation,are also summarized accordingly.Finally,at the end of the chapter,the research objectives and main contents of this paper are summarized.The second chapter mainly introduces the theoretical research calculation tools in this thesis.First,we make a brief introduction from the solution of the multi-body problem in the early approximation to the evolution of the single-electron approximation in the density functional theory,and at the same time we explain the Wannier function and its applications.Finally,the calculation softwares involved in this article are briefly introduced.The emergence of two-dimensional materials has promoted the research of valleytronics based on valley degrees of freedom.The research of valley degrees of freedom has formed an emerging field of condensed matter physics.Similar to the important concept—half-metal in spintronics,where one spin channel is conducting whereas the other is insulating.In Chapter 3,we propose the concept of half-valley-metal,in which conduction electrons are intrinsically 100%valley polarized,as well as100%spin-polarized even when spin-orbit interactions are considered.Combining first-principle calculations with two-band k·p model,the physical mechanism to form the half-valley-metal is illuminated.Taking the ferrovalley H-FeCl₂ monolayer with strong exchange interaction as an example,we find that the strong electron correlation effect can induce the ferrovalley to half-valley-metal transition.Due to the valley-dependent optical selection rules,such system could be transparent to,e.g.,left-circularly polarized light,yet the right-circularly polarized light will be reflected,which can in turn be used as a crucial method to detect half-valley-metal state.In addition,we find that in the so obtained half-valley-metal state,the conduction valley demonstrates Dirac cone-like linear energy dispersion.Interestingly,with the increase of the correlation effect,the system becomes insulating again with all valleys follow same optical selection rule.We confirm that in this specific case,the valence bands,which consist of single spin,possess non-zero number and consequently intrinsic quantum anomalous valley Hall effect emerges.Therefore,the concepts of half-valley metal and quantum anomalous valley Hall effect provide novel physical images for the cross-research of valleytronics,optics,spintronics and topology materials,and also open up an appealing route toward functional 2D materials design of valleytronics.In addition to proposing new concepts,in Chapter 4 we focuse on the use of low energy consumption methods to control valley polarization.Studies have shown that in the transition metal dichalcogenide compound with hexagonal lattice,ferromagnetism will induce spontaneous valley polarization,and the reversal of valley polarization requires relatively energy-consuming magnetic field means.The potential application of valleytronics based on energy valley degrees of freedom in information storage,the realization of valley polarization reversal and binary logic regulation in an electrically controllable and non-volatile manner is still the most concerned issue for researchers.In this chapter,we give the idea of using the two-dimensional CuInP₂S₆/MnPS₃ system to implement valley polarization control using electric field means.First of all,we learn through group theory analysis that the single layer of room temperature ferroelectric material CuInP₂S₆ is a natural paravalley material.In these ferroelectric/antiferromagnetic heterostructures,the ferroelectric CuInP₂S₆ layer,which is originally in the paravalley state,turns into ferrovalley status due to the magnetic proximity effect of the antiferromagnetic MnPS₃ substrate.Interestingly,when the ferroelectric state of the CuInP₂S₆ layer is changed,the valley polarization of the system is significantly reversed,providing the possibilities to manipulate the valley degree of freedom by electrical means.On account of above findings,all-electrically reading and writing memory devices based on such heterostructures is proposed utilizing the valley degree of freedom.At the same time,it also broadens the research scope of Valleytronics.In the research process of the previous chapter,we notice that the spin-splitting in the ferroelectric CuInP₂S₆ monolayer with paravalley state depends on the strong spin-orbit coupling effect.Spin-orbit interaction,as the basic mechanism of spin-polarized electron generation,has potential application prospects in spin-electronic devices.The electrical controllability and non-volatility of the spin structure in semiconductors provide a theoretical basis for the design of new electronic devices.In Chapter 5,through first-principle calculations,the electronic properties of the ferroelectric monolayer CuInP₂S₆ are studied.The inversion symmetry broken due to ferroelectric polarization makes it a potential member of the ferroelectric Rashba semiconductor.The non-equivalent valleys K₊and K_-also confirm that monolayer CuInP₂S₆ is a potential paravalley material.A typical Rashba spin texture is observed in the ferroelectric phase of CuInP₂S₆ monolayer.When the ferroelectric polarization is reversed,the spin texture chirality corresponding to the Rashba type spin-orbit coupling changes immediately.In addition,whether in the ferroelectric or paraelectric states,the Zeeman-type spin-orbit coupling occurs at the two valleys,forming a large out-of-plane spin splitting.At the same time,we find that Zeeman-type spin splitting is related to the ferroelectric polarization,and the direction of spin splitting is different between the ferroelectric state and the paraelectric state.This bizarre spin polarization change comes from the contribution of in-plane dipoles during the ferroelectric phase transition.The ferroelectric CuInP₂S₆ monolayer with Rashba spin-orbit coupling and Zeeman spin-orbit splitting provides a suitable platform for the realization of electrically controlled spin polarization.The special SHG polarizability and polarization anisotropy in the CuInP₂S₆ monolayer also greatly expand the research direction of 2D ferroelectrics and nonlinear optical electronics and their interdisciplinary subjects.In the last chapter,we briefly summarize the research results obtained in this dissertation,and based on that,we look forward to the current problems and potential research directions.