Microscopic origin of pressure-induced phase transitions in iron-pnictide $AF{e}_{2}A{s}_2$ superconductors: an {ab initio} molecular-dynamics study

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.

some figures and text have been improved and comparison with experiment has been added

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• Possible strong electron-lattice interaction and giant magneto-elastic effects in Fe-pnictides