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

¹  Goethe-University Frankfurt, Theoretical Physics - 60054 Frankfurt/Main, Germany, EU
²  Goethe-University Frankfurt, Institute of Physics - 60054 Frankfurt/Main, Germany, EU

Corresponding author: kulic@th.physik.uni-frankfurt.de

Received: 2 May 2009
Accepted: 25 June 2009

Abstract

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 ~ d^-4. The EPI_pol coupling is much larger than the one obtained in the LDA band structure calculations, with  (~16 eV/Å) ( < 1 eV/Å) and the bare pairing EPI_pol coupling constant ~ 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 MFe₂As₂, 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 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.