A Theoretical Study On The Topological Phase Transition And Photoregulation In Dirac Semimetals

Topological materials,including topological insulators,Dirac and Weyl semimetals have aroused great interest in condensed matter physics,because of the intriguing physical properties such as negative magnetoresistance,quantum spin/anomalous Hall effect,Fermi arcs and chiral anomaly.Conductive edge states which are topologically protected exist in the topological insulators.In Dirac semimetals,the valence bands and conduction bands touch at fourfold degenerate points(Dirac points)accompanied by linear energy-momentum relations near the Dirac points.Typical examples include graphene,Na3Bi,Cd3As2 and so on.While for Weyl semimetals,the degenerate points(Weyl points)become twofold,as evidenced in TaAs family,Y2Ir2O7,WTe2.Weyl points have the chiralities of+1/-1 corresponding to the source/sink of Berry curvature in momentum space,which can be treated as the monopoles of Berry magnetic field.The Weyl points with opposite chirality always appear or annihilate in pairs.The unusual surface states featured by Fermi arcs connecting Weyl nodes of opposite chirality are closely related to the nontrivial topology of Weyl semimetals.Topological materials have wide application prospects in electronic devices,energy and other fields due to their excellent properties.The electronic properties of materials cannot always meet the demand in practical applications,which makes the regulation of electronic structures necessary.In order to improve and change the properties of materials,it is necessary to regulate the electronic properties.Especially for topological materials which depend on the symmetry of materials,adjusting the symmetry can greatly change their electronic properties.Recently,there are a variety of regulatory means,such as stress,doping or adsorption,heterojunction,etc.,to regulate the topological states.The study of topological phase transition associated with the electronic structure regulation has attracted wide attention.In this paper,the effects of strain,doping and periodic light field on the properties of topological materials are studied by using first-principles calculations,maximally localized Wannier function and Floquet theory.The main research contents and achievements of this paper are as follows.We reveal the topological phase transitions of the experimentally-synthesized RbAg5Se3 induced by tensile strain and dopants using first-principles calculations in combination with the maximally localized Wannier function method.The Dirac points of RbAg5Se3 are protected by the inversion and time-inversion(PT)symmetry in addition to the C4z rotation symmetry.As a tensile strain along the[001]direction is applied to RbAg5Se3,the type-II Dirac points will convert into type-III and type-I Dirac points,which are promising for black-hole-horizon analog in high-energy physics.Moreover,the Dirac points of RbAg5Se3 can be regulated to hybrid Weyl points by intercalating a Pt atom to the center of the unit cell,which lifts the inversion symmetry.Eight pairs of Weyl points corresponding to the space group P422(No.89)emerge in the resulted PtRbAg5Se3.The topological properties of the Weyl points are confirmed by the chirality,Berry curvature,surface states and Fermi arcs.Our work provides a platform for experimental realization of topological phase transition and the coexistence of different types of Weyl points may leads to fascinating transport phenomena as the Fermi level is regulated to the Weyl points.Based on Floquet theory,the topological phase transition of the experimentally synthesized two-dimensional Dirac semi-metallic material BeN4 under periodic circulally polarized light is studied by using the first principles calculation and maximally localized Wannier function method.It is found that the weak spin-orbit coupling effect of BeN4 not affect the band structure.When the time inversion symmetry in the system is broken by irradiating the material with circularly polarized light,original Dirac point open a gap,along with a clear edge state crosses the band gap and connects the two valleys.It is found that quantum anomalous Hall effect exists in this band gap.When the chirality of the circularly polarized light irradiated is changed,the material conducts electricity in the opposite direction.It is possible to realize the photoinduced topological phase transition of BeN4 experimentally.These excellent properties also make BeN4 have great potential in the field of electronic devices.