Microscopic origin of pressure-induced phase transitions in iron-pnictide $AFe_2As_2$ superconductors: an {ab initio} molecular-dynamics study

TL;DR

This study uses ab initio molecular dynamics to explore how pressure induces structural and magnetic phase transitions in $A$Fe$_2$As$_2$ superconductors, revealing the microscopic mechanisms and their impact on superconductivity.

Contribution

It provides a detailed ab initio analysis of pressure-induced phase transitions in $A$Fe$_2$As$_2$, explaining the origin of collapsed tetragonal phases and the role of Fermi surface nesting.

Findings

Structural transition always accompanies magnetic transition.

Collapsed tetragonal phase occurs in CaFe$_2$As$_2$ but not in BaFe$_2$As$_2$.

Fermi surface nesting changes drive the phase transitions.

Abstract

Using {\it ab initio} molecular dynamics we investigate the electronic and lattice structure of $A$Fe$_2$As$_2$ ($A$=Ca, Sr, Ba) under pressure. We find that the structural phase transition (orthorhombic to tetragonal symmetry) is always accompanied by a magnetic phase transition in all the compounds, while the nature of the transitions is different for the three systems. Our calculations explain the origin of the existence of a collapsed tetragonal phase in CaFe$_2$As$_2$ and its absence in BaFe$_2$As$_2$. We argue that changes of the Fermi surface nesting features dominate the phase transition under pressure rather than spin frustration or a Kondo scenario. The consequences for superconductivity are discussed.