Trends in pressure-induced layer-selective half-collapsed tetragonal phases in the iron-based superconductor family $Ae{}A$Fe$_4$As$_4$

TL;DR

This study uses density functional theory to predict pressure-induced phase transitions in iron-based superconductors, revealing two distinct half-collapsed tetragonal phases and their implications for magnetism and superconductivity.

Contribution

It introduces a detailed analysis of pressure-induced phase transitions in $AeA$Fe$_4$As$_4$ compounds, highlighting the role of magnetic fluctuations and identifying trends across different elements.

Findings

Two consecutive half-collapsed tetragonal phases at different pressures.

Magnetic fluctuations are crucial for accurate structure prediction.

Collapsed phases influence magnetism and superconductivity.

Abstract

By performing pressure simulations within density functional theory for the family of iron-based superconductors $Ae{}A$Fe$_4$As$_4$ with $Ae$ = Ca, Sr, Ba and $A$ = K, Rb, Cs we predict in these systems the appearance of two consecutive half-collapsed tetragonal transitions at pressures $P_{c_1}$ and $P_{c_2}$, which have a different character in terms of their effect on the electronic structure. We find that, similarly to previous studies for CaKFe$_4$As$_4$, spin-vortex magnetic fluctuations on the Fe sublattice play a key role for an accurate structure prediction in these materials at zero pressure. We identify clear trends of critical pressures and discuss the relevance of the collapsed phases in connection to magnetism and superconductivity. Finally, the intriguing cases of EuRbFe$_4$As$_4$ and EuCsFe$_4$As$_4$, where Eu magnetism coexists with superconductivity, are discussed as well in the context of half-collapsed phases.