First-principle Study On Magneto-elastic Interactions Of FeAs-based Superconductor

Using first-principle calculation which based on the Density functional theory, the Project-Augmented Wave (PAW) was performed on exchange interaction and magneto-elastic coupling for122FeAs-based parent compounds. Exchange constants for BaFe2As2and CaFe2As2were calculated firstly, and it revealed that exchange interactions in122compounds are anisotropic. Jab’s, exchange interactions lies in in-plane, were very large, and Jbc, Jac and Jcc’s which stand for out-plane exchange interaction terms were very small. This revealed the in-plane exchange interactions play more important role in the magnetism of the system than that out-planes. And then, we calculated the elastic constants for iron-based parent compounds and obtained the Bulk modulus and Shear modulus by Reuss and Vogit approximation. From the view of the Bulk modulus and the Cauchy pressure, we discussed the hardness and tenacity or brittleness and bonds for CaFe2As2and BaFe2As2, and they fit with others very well. At last, we also studied on the magneto-elastic coupling in the122compounds. We found it was the oriented to a-axis of the magnetism that makes the122parent compounds undergoes the structure transitions simultaneously with the magnetism transitions, and the BaMn2As2compounds also has magnetism transition but none structural ones for its magnetic moments were oriented to c-axis. The magneto-elastic coupling constants data revealed122compounds’anisotropy, the magnetism of the system were sensitive to the in-plane structural disordered. And then, in order to research how the magnetic moments and total energies changed by the change of the bond length of Fe-As and the distance h from the As atom to the plane which the Fe atom lies in, we performed calculations for the BaFe2As2and CaFe2As2, by changing the ratios of a/c and a/b for the fixed the As atoms and then changing the site of As fixed the a/c or a/b. The values of the magneto-elastic coupling constants proved that there was very anisotropic coupling between the magnetic and structure in122compounds, it was very large when lies in in-plane and very small when out of the FeAs layer plane. The curves of magnetic moments with the distance of the As to the Fe, shows a very rapid changes when the As deviated from its equilibrium site. This illustrates the interactions of the Fe-As were very localized.