Theoretical Studies Of Ferroic And Optical Properties In Multiferroics

Charge,spin and valley of electrons correspond to traditional electronics,spintronics and the emerging valleytronics,respectively.Such degrees of freedom,consitituting the binary states,can be manipulated by external fields.Therefore,as units for data storage,they are widely used in information processing industry.Individually,there are various and peculiar optical properties related to these order parameters of electrons.Due to the coexistence of two or more primary ferroic orders,multiferroics are generally regarded as the promising candidate to explore the coupling among charge,spin and valley.More importantly,these multifunctional materials provide possibilities for studies of the fundamental physics in areas crossing electronics,spintronics,valleytronics and optics.In this regard,using multiferroics as objects,theoretical research has been systmetically carried out on electorons' order parameters and their interactions.Particular attention has been concentrated in their influence on optical properties,including linear and nonlinear ones.Combining first-principles calculations based on density functional theory,tight-binding models and group theory analysis,the study covers multiferroic magnetoelectrics and the newfashioned one rooted in the valley degree of freedom.Its implement would offer novel physics in multiferroics,and promote their practical application as next-generation electro-optical and information functional devices,as well as shed new light for the multiferroic field.As the beginning of the dissertation,the concept of multiferroics,its history,classification and potential applications are introduced in Chapter 1.The chapter puts emphasis on novel optical properties individually related to charge,spin and valley of electrons.Research objects and main contents are pointed out here as well.Chapter 2 briefly reviews the development of density functional theory.In addition,the functions and characteristics of our own developed optical code OPTICPACK and symmetry analysis code IRREP are introduced.EuO,belonging to the small group of ferromagnetic semiconductors,attracts interest as a spintronics material.The astonishing finding of epitaxial strained EuO to become ferroelectric offers additional opportunies to research the multiferroicity in EuO.In Chapter 3,using first-principles calculations and especially the optical code OPTICPACK,spin-dependent optical properties of multiferroic EuO are investigated.Since the linear and nonlinear optical spectra are separated into different spin states,the information of spin-related electron transitions,and subsequentely the spin-dependent band structures can be presented.The optical spectra from individual spin channel is necessary and of great importance to the study of ferromagnetic and even multiferroic materials.Our findings would be helpful to understand the regular spectrum of such systems and more importantly spread the vanguard of the experimental characterization methods.Electrons in different spin channels possess distinct optical response.The influence of magnetic ordering on optical properties is definitely more significant.Taking the "superstar" among multiferroic materials BiFeO₃ as an example and combining density functional theory calculations with tight-binding models,Chapter 4 reveals that when magnetic ordering changes,ferromagnetic interactions between magnetic atoms is totally different.As a result,band dispersion and then the optical properties dramatically change.The energy shift of the optical band gap in tetragonal BiFeO₃ between Cl-and G-type antiferromagnetic ordering is as large as 0.4 eV,demonstrating the possibility of using the noncontact and nondestructive optical method,rather than the inconvenient and costly neutron scattering experiments,to determine the magnetic ordering.Utilizing the magnetic ordering induced giant optical properties change,an electrically writing and optically reading memory device is proposed,which is of great interest in next-generation information storage technologies.Above chapters focus on traditional multiferroics with magnetic and ferroelectric orders.The birth of valley degree of freedom offers the opportunity to explore a new ferroic-family member and new concept of multiferroic materials based on valleytronics.In Chapter 5,the ferrovalley material with spontaneous valley prlarization is unveiled.Among transition metal dichalcogenides?TMDs?,the coexistence of the spin-orbit coupling effect and intrinsic exchange interaction is the key to hunt for such new ferroic materials.Interestingly,in TMDs monolayers,the ferrovalley couples to ferromagnetism,signifying new types of multiferroics with valley degree of freedom.Combining a two-band k·p model with first-principles calculations,chirality-dependent optical band gap is revealed in such multiferroics,which makes them the promising platform for studies of the coupling among magnetic,optical and valley-related properties.Besides,anomalous valley Hall effect?AVHE?is discovered.The new form of Hall effect indicates the possibility of electrical detection for valley polarization and may promote technological application of valleytronics in promising electonic products with nonvolatility.However,in the proposed valley-polarized monolayers,the valley polarization as well as the sign of the Hall voltage in the AVHE should be controlled by energy-intensive magnetic fields.Through elaborate structural design,Chapter 6 proposes the antiferrovalley bilayer as an ideal candidate for tuning the AVHE through advantageous electric method.The realization of the effect hinges on optionally manipulation of valley degeneracy via an electric field perpendicular to the plane of the system.Taking the concrete bilayer VSe₂ as an example,all-electric tuning and detecting of the AVHE is confirmed by both the k-p model and first-principles calculations,demonstrating the feasibility towards all-electric valleytronic devices in such bilayers where the valley information can be reversed by the vertical electric field and easily read out through the sign of the transverse Hall voltate.In this work,charge degree of freedom is additional imported.in valleytronic-related multiferroics.Its coexistence and coupling with magnetic and valley ones would serve as a bridge among traditional electronics,spintronics and valleytronics.The main conclusions are summarized in Chapter 7.Prospect of the dissertation is given as well.More efforts will be devoted into type-II multiferroic magnetoelectrics whose ferroelectricity is induced by noncollinear magnetic ordering and newfashioned valley-related multiferroics beyond hexagonal honeycomb lattice.Based on deep understanding of the interaction among electric,magnetic,valley-related and optical properties,crossover studies combining traditional electronics,spintronics,valleytronics and optics would further advance.The multiferroic field itself and its potential application would be widen.