First-principles Study On Electric Control Of Ferrovalley In Low-dimensional Materials

Charge and spin,as the two degrees of freedom of electrons,determine the intrin-sic properties of materials.Based on this,spintronics and traditional electronics have become the foundation of modern information technology.Ferroic materials,a class of materials with spontaneous polarization,have a wide scope of applications in infor-mation storages,mainly including ferromagnetics and ferroelectrics.With the advent of the information age,people are increasingly pursuing low-power,high-density and miniaturized functional devices.However,traditional three-dimensional ferroic materi-als are not suitable for the new generation of micro-nano information devices due to the limitation of critical size effect.With the successful preparation of two-dimensional graphene,low-dimensional materials have gradually attracted more attention.Low-dimensional ferroic materials,in particular,are regarded as the perfect platform for next-generation electronic devices with excellent performance.In recent years,with the emergence of two-dimensional materials,the valley degree of freedom,as an emerging electronic degree of freedom,has led the development of valleytronics.In this regard,the dissertation combined with first-principles calculations and a variety of analytical methods,taking low-dimensional materials as the research object,aims to study the exotic physical properties related to the valley degrees of freedom and their regulation.As the introduction of this dissertation,the first part mainly introduces the research background.Firstly,the rise and progress of two-dimensional polar materials are briefly recalled,which mainly include two-dimensional ferromagnetic and two-dimensional ferroelectric materials.At the same time,the development of valley degree of freedom and valleytronics is reviewed,including the discovery of valley degree of freedom,its rise and the usefulness of external fields to regulate valley degree of freedom to realize the application of information devices.Then,a brief overview of the well-known Hall family is given.The second part mainly introduces the research methods and tools involved in this dissertation.The development history of density functional theory and the computational softwares used in subsequent research are outlined.In addition,the research objectives and content of the dissertation are summarized.The third part focuses on valleytronics,mainly through theoretical calculation suc-cessfully predicting the ferrovalley materials with Janus structure,broadening the mem-bers of ferrvalley materials,and providing new ideas for the cross research of valleytron-ics and piezoelectric electronics.We confirmed the system has a spontaneous valley polarization that can be reversed by a magnetic field by analysing the band structures.Using the simplified k_·p model,the importance of ferromagnetic exchange field for spontaneous valley polarization is analyzed.Through group theory analysis combined with first-principles calculations,the valley-dependent selection rule for circularly po-larized light in the system is expounded.At the same time,the opposite Berry curvature at the two valleys also makes the system have anomalous valley Hall effect,so as to experimentally detect the polar state in the ferrovalley materials.More interestingly,the Janus structure breaks the mirror symmetry along the vertical direction,resulting in the simultaneous in-plane and out-of-plane piezoelectric effects of the system under the strain.Besides,the second harmonic generation properties of the system are also studied,which is an important mean to experimentally detect the out-of-plane electron dipole.Such a ferrovalley material,coupling valley and charge degree of freedom,is a ideal material for studying the cross feild of valleytronics and piezoelectric electronics.The fourth part realizes the electric control of valley degree of freedom and mainly reflects the cross research between valleytronics and topology.Through first-principles calculations,we predict that the monolayer Te Se is a rare ferrovalley material with ferroelectricity-induced spontaneous valley polarization in the tetragonal structure.Un-like circularly polarized light absorption in the hexagonal lattice,the system has a valley-dependent linearly polarized light selection rule at valleys.The opposite signs of Berry curvature at the valleys also make sure the valley Hall effect could be induced in this system,meaning that the valley electrons can be separated by the electric field in the real space.More interestingly,the band structures of two valleys connected by the time-reversal symmetry are similar to Dirac cone dispersion relation,which provides a strong guarantee for the quantum spin Hall effect.By means of external fields,we have also achieved a multi-field controlled topological phase transition,enabling the system to transition from a topologically trivial state to a topologically non-trivial state.The physical mechanism behind it is that the spin-resolved Berry curvatures flip at the two energy valleys,which ensures the existence of a non-zero spin Chern number in the system,thereby inducing a quantum spin Hall state.Based on this,we also designed a field effect transistor with multi-field regulation,and used multi-means to control the topological phase transition of the system to achieve the switching state.It is hoped that this material with cross-integration of valleytronics and topology can provide a new platform and ideas for the investigation of valleytronics.The fifth part mainly extends new research ideas on the basis of the effective coop-eration between theory and experiment.By collaborating with experiments,the struc-tural phase transition from antiferroelectric to ferroelectric is firstly realized by using electric field in ferrovalley material Ge Se,which effectively broadens the research di-rection of layered antiferroelectric materials.By comparing the calculated Raman fre-quencies with experimental data,the initial state of the material was determined to be antiferroelectric.Then through theoretical calculation on the energy barrier of the tran-sition states from antiferroelectric to ferroelectric,it provides a reliable theoretical basis for the experimental realization of phase transition.As an extension of our work,we systematically studied the Raman spectra of Ge Se in different structures by computa-tional means,and found that the Raman spectra have great differences in stress and different layers,which can provide guidance for experiments to identify the number of layers and the stress state of van der Waals materials.More interestingly,combined with the phenomenon of slide ferroelectricity,we found that the ferroelectric phase can also be induced by interlayer slide in layered Ge Se,and the Raman frequencies of the ferroelectric and antiferroelectric phases are distinctive different.Therefore,our results demonstrate Raman spectroscopy can also be used as an effective mean under non-contact state to probe the polarity of polar materials.The sixth part summarizes the dissertation,and looks forward to the in-depth re-search direction in valleytronics.We also hope to lay a certain foundation for future research.