Pressure- and temperature-induced structural phase transitions of CaFe2As2and BaFe2As2studied in the Hund’s rule correlation picture

Abstract

With the proposed Hund's rule correlation picture, i.e., the fluctuating Fe local moments with the As-bridged antiferromagnetic superexchange interactions, the exceptional collapsed tetragonal phase and related phase transitions observed in CaFe${}_{2}$As${}_{2}$ are well understood. Within the same framework, a pressure-temperature phase diagram is predicted for BaFe${}_{2}$As${}_{2}$ as well, in which a paramagnetic tetragonal structure and a collinear antiferromagnetic orthorhombic structure to nonmagnetic tetragonal structure transitions take place around 4--8 GPa and 10--15 GPa, respectively, and a nonmagnetic tetragonal to a nonmagnetic collapsed tetragonal structure transition takes place over 26 GPa. Our study helps to better understand the complex correlation among crystal structure, magnetism, and electronic structure in pnictides, a precondition to understanding the superconductivity in pnictides.