Pressure- and temperature-induced structural phase transitions of CaFe$_2$As$_2$ and BaFe$_2$As$_2$ studied in the Hund's rule correlation picture

TL;DR

This study uses the Hund's rule correlation picture to explain phase transitions in CaFe$_2$As$_2$ and predict a phase diagram for BaFe$_2$As$_2$, linking structure, magnetism, and electronic properties in pnictides.

Contribution

It introduces a Hund's rule correlation framework to understand phase transitions and predicts a pressure-temperature phase diagram for BaFe$_2$As$_2$, extending previous understanding.

Findings

Explains collapsed tetragonal phase in CaFe$_2$As$_2$

Predicts phase transitions in BaFe$_2$As$_2$ under pressure

Links structural, magnetic, and electronic properties

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. With the same framework, a pressure-temperature phase diagram is predicted for BaFe$_2$As$_2$ as well, in which a paramagnetic tetragonal and a collinear antiferromagnetic orthorhombic structures 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 better understand the complex correlation among crystal structure, magnetism, and electronic structure in pnictides, a precondition to understand the superconductivity in pnictides.