Microscopic origin of pressure-induced phase transitions in iron-pnictide AFe₂As₂ superconductors: an ab initio molecular-dynamics study

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Using ab initio molecular dynamics we investigate the electronic and lattice structure of AFe₂As₂ (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 CaFe₂As₂ and its absence in BaFe₂As₂. 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.