First-principles Study On Valley Polarization Properties Of Two Dimensional Ferromagnetic FeCl And FeI

Valleytronics is an emerging discipline,which has important potential application in information storage.To achieve valley polarization,the main challenge of valleytronics is to break the degeneracy between different valleys.The storage and computation of two logical states can be realized by manipulating two nequivalent valleys in the Brillouin zone of valleytronics materials.At present,most of the discovered valleytronics materials belong to the hexagonal system.To expand the scope of valleytronics materials,it is necessary to develop more suitable materials for valleytronics devices.Because the breaking of C4 symmetry in square materials will lead to the emergence of inequivalent valleys,it is of great significance to explore the valley polarization properties of square materials and their control methods.Based on the first-principles,the modulation of valley polarization properties for twodimensional square FeCl and FeI is studied.The research contents are listed as follows:（1）The modulation of valley polarization properties for two-dimensional square ferromagnetic FeCl is studied.The calculated results of the cohesive energy,Gibbs free energy and phonon spectrum show that FeCl has energy and kinetic stability.When the spin-orbit coupling（SOC）is not considered,two-dimensional FeCl is a spin-polarized semimetal material,and the crossing point around the Fermi level is protected by rotation and mirror symmetry.When the SOC is considered,the crossing point around the Fermi level opens the local bandgap.A tight binding model based on five d orbitals of Fe atom is constructed,and the Berry curvature distribution and anomalous Hall conductance of FeCl are calculated.It is found that there are four valleys with the same signs in the Berry curvature distribution.Uniaxial strain can effectively modulate the energies of these valleys leading to two inequivalent valleys,so that valley polarization can be realized.（2）Valley polarization anomalous Hall effect of two-dimensional square FeI is studied.The results of the cohesive energy,Gibbs free energy,elastic constants and phonon spectrum show that FeI has energy,mechanics and dynamics stability.The firstprinciples molecular dynamics simulation indicates that FeI has thermodynamic stability.When the SOC is not considered,two-dimensional FeI is a spin-polarized semimetal material,and the crossing point is protected by rotation and mirror symmetry around the Fermi level.When the SOC is considered,the crossing point opens the global bandgap around the Fermi level.A tight binding model is constructed based on five d orbitals of Fe atom,and the Berry curvature distribution and anomalous Hall conductance of FeI are calculated.It is found that the valleys are C₄ symmetry in the Berry curvature distribution,and the anomalous Hall conductance appears a quantum platform.Uniaxial strain can effectively modulate two inequivalent valleys of FeI,so that the valley polarization properties are achieved.