Observation of a pressure-induced As-As hybridization associated with a change in the electronic state of Fe in the tetragonal phase of EuFe2As2

The electronic and vibrational properties of EuFe2As2 in the tetragonal phase between 0 and 5 GPa have been investigated using 57Fe Mössbauer spectroscopy and 57Fe nuclear resonance inelastic scattering, respectively. We find a discontinuous increase of the center shift around 2.3 GPa, reflecting a change of the electronic state of Fe, and above 2.5 GPa a softening of the optical phonon modes associated with an increase of the relative volume of the FeAs4 tetrahedron in the unit cell. Our findings reveal that an effective As-As hybridization along the c axis appears at approximately 2.3 GPa in the tetragonal phase of EuFe2As2, along with a change in the electronic state of Fe, causing bulk superconductivity to appear at a low temperature. Consequently, the change in the electronic state of the Fe atom and the effective As-As hybridization play key roles in the pressure-induced superconductivity in the tetragonal phase of AFe2As2.