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Recently, superconductivity in the parent compounds AFe2As2 (A =Ca, Sr, Ba) was reported under application pressure. In BaFe2As2 superconductivity is found to appear gradually with increasing pressure while in SrFe2As2 the onset of superconductivity occurs abruptly. In CaFe2As2, detailed neutron and X-ray diffraction analysis indicates that the phase transition to the superconducting state is probably of first order. Using ab initio molecular dynamics we investigate the electronic and lattice structure of these systems 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. The microscopic origin of this diversity is attributed to the different Fermi surface behavior under pressure. We propose that a three-band model should be the minimal model for describing the superconductivity under pressure. Finally we discuss the role of correlation in these materials. |
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