The First-principle Study Of Fe-based Piezoelectric Quantum Anomalous Hall Insulators

Recently,the research of multi-functional materials has triggered a research upsurge.Because of its ultra-thin thickness,two-dimentional(2D)materials have many special physical properties,which have huge advantage in the realization of multi-functional materials.Magnetism,topology and piezoelectricity in 2D meterials offer more possibilities for the theoretical development of 2D condensed matter physics and the application of new electronic devices.Their coupling is expected to lead more novel physical phenomena and new device applications.Based on density functional theory,by constructing Janus structure,the central symmetry of the center-symmetric quantum anomalous Hall insulator is destroyed,which can induce out-of-plane piezoelectric response,and then realize the coupling of magnetism,topology and piezoelectricity(piezoelectric quantum anomalous Hall insulator).The main research contents and conclusions of this thesis are as follows:(1)2D Janus Fe2IX(X=Cl or Br)materials were constructed based on monolayer Fe2I2 by replacing I atoms with X(X=Cl or Br)atoms.The calculated results show that the monolayer Fe2IX(X=Cl or Br)exhibit the mechanical,dynamical and thermal stabilities.The out-of-plane piezoelectric coefficient d31 of Fe2ICl(Fe2IBr)is 0.467 pm/V(0.384 pm/V).The calculated topological edge states show that there are two chiral edge states along(100)direction,which is consistent with the Chen number C=2.Therefore,the monolayer Fe2IX(X=Cl or Br)are quantum anomalous Hall insulators.Monte Carlo simulation was used to estimate the Curie temperature of monolayer Fe2ICl(Fe2IBr),which is as high as 429 K(403 k).The calculated results show that,within the considered range of strain(-6%～6%)and electron correlation effect(2 eV～4 eV),monolayer Fe2ICl(Fe2IBr)are piezoelectric quantum anomalous Hall insulators.(2)2D Janus Li2Fe2SSe material was constructed by replacing the top Se atoms of monolayer Li2Fe2Se2 with the S atoms.The results show that the monolayer Li2Fe2Se2 is stable,and the piezoelectric coefficient d31 is-0.238 pm/V.Its Curie temperature is 996 K.It has been proved that the monolayer Li2Fe2Se2 has two chiral edge states,and C=2 by Berry curvature integral.The calculated results show that Li2Fe2Se2 is a piezoelectric quantum anomalous Hall insulator in the considered range of strain and electron correlation effect.(3)Based on previous computational studies,in order to make the materials easily synthesize in experiment,we extended the piezoelectric quantum anomalous Hall insulator Fe2IBr monolayer to monolayer FeI1-xBrx(x=0.25 or 0.75)with the α-phase and β-phase structures.It is found that the α and the β phase both are mechanically,dynamically and thermally stable,and they both are piezoelectric quantum anomalous Hall insulators.The βphase of FeI0.75Br0.25 has the largest piezoelectric coefficient value d31 of 0.396 pm/V and d32 of-0.321 pm/V.The α phase of FeI0.25Br0.75 has the highest Curie temperature of 441 K.