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

The possibility for an appreciable many-body contribution to the electron-phonon interaction (EPI) in Fe-pnictides is discussed in the model where EPI is due to the electronic polarization of As-ions. The polarization induced EPI(pol) potential V(ep) is large for vibrations of the As-ions and depends strongly on the As-Fe distance d, i. e. V(ep) similar to d(-4). The EPI(pol) coupling g(pol)(ep) is much larger than the one obtained in the LDA band structure calculations, with g(pol)(ep) (similar to 16 eV/angstrom) >> g(ep)((LDA)) (< 1 eV/angstrom) and the bare pairing EPI(pol) coupling constant lambda(0)(ep),(A1g) similar to 1. It contributes significantly to the intra-band s-wave pairing and an appreciable positive As-isotope effect in the superconducting critical temperature is expected. In the Fe-breathing mode the linear (in the Fe-displacements) EPI(pol) coupling vanishes, while the non-linear (quadratic) one is very strong. The part of the EPI(pol) coupling, which is due to the potential energy (the Hubbard U) changes, is responsible for the giant magneto-elastic effects in MFe(2)As(2), M = Ca, Sr, Ba since it gives much larger contribution to the magnetic pressure than the band structure effects do. This mechanism is contrary to the LDA prediction where the magneto-elastic effects are due to the kinetic energy effects, i. e. the changes in the density of states by the magneto-elastic effects. The proposed EPI(pol) is expected to be operative (and strong) in other Fe-based superconductors with electronically polarizable ions such as Se, Te, S etc., and in high-temperature superconductors due to the polarizability of the O(2-)-ions. Copyright (C) EPLA, 2009