First-principles Study Of Two-dimensional MA₂Z₄ Energy Valley Material

Processing information using valley manipulation is a hot issue in the current research of valley spin informatics and physical electronics.Due to the excellent valley spin properties,multicomponent MA₂Z₄ type two-dimensional energy-valley materials are increasingly becoming the"star"materials in the research of valley spintronics,which are sought after by many experimental and theoretical workers.Based on first principles calculations,this paper investigates the structure and characteristics of two-dimensional MA₂Z₄（M is a transition metal element of the VIB and IVB families,A is the IVA group where the C element is located,Z is the VA group where the N element is located）energy valley materials,providing a candidate material for valley electronic devices in theory.At first,the valley electron properties,optical properties,and biaxial strain regulation of the two-dimensional Mo Ge₂P₄ material are calculated.With seven atomic layers,two dimensional Mo Ge₂P₄has a reverse symmetry breaking in structure.Due to the spin orbit coupling effect,the valley spin splitting value generated by this material is 153 me V,which is conducive to the practical operation of the energy valley.Meanwhile,the two-dimensional Mo Ge₂P₄ material is a direct bandgap semiconductor,which is very suitable for the preparation of optoelectronic devices.By applying biaxial strain,the material undergoes a phase transition between semiconductor and metal.In addition,the two-dimensional Mo Ge₂P₄ has Berry curvature at K and K’and zero at the remaining points.Besides,to systematically study the performance of MA₂Z₄ type valley materials,we calculated the valley electron properties of two-dimensional Janus MSi Ge Z₄（M=Cr,W;Z=N,P,As）with the same host family.The valley spin splitting of WSi Ge Z₄（Z=N,P,As）exceeds 400 me V,which is greater than that of most valley electronic materials.Two-dimensional WSi Ge Z₄（Z=N,P,As）material possess rich electronic properties,with stable dynamic and mechanical properties.Among them,WSi Ge N₄ is an indirect bandgap semiconductor material with a bandgap of 1.654 e V,while WSi Ge P₄ is a direct bandgap semiconductor.The W atoms have more d-orbital electrons and stronger spin orbit coupling effects than Cr atoms,resulting in the valley spin splitting of WSi Ge Z₄ is larger than that of Cr Si Ge Z₄（more than 300 me V）.This work uses the Janus symmetry structure to break the central inverse symmetry and improve the Berry curvature of the material.Finally,two-dimensional MSi₂N₄（M=Ti,Zr,Hf）materials with circular Berry curvature in the IVB family are explored,whose structure are similar to the two-dimensional WSi Ge Z₄,with stable dynamic and mechanical properties.The two-dimensional MSi₂N₄（M=Ti,Zr,Hf）materials are all indirect bandgap semiconductor materials.The bandgaps of the three are 1.64 e V,1.65 e V,and 1.80e V,respectively.In addition,due to the breaking of symmetry and the influence of spin orbit coupling,valley spin splitting occurs in two-dimensional MSi₂N₄（M=Ti,Zr,Hf）.Unlike most valley electronic materials,two-dimensional MSi₂N₄（M=Ti,Zr,Hf）has a special Berry curvature,which is zero at the K and K’valleys forming a ring around it.We believe that this is due to band aggregation at the valence bands K and K’.In summary,both VIB and IVB MA₂Z₄ type materials have good valley properties,and have broad application prospects in valley electronic devices.