Possible strong electron-lattice interaction and giant magneto-elastic effects in Fe-pnictidesM. L. Kulić1 and A. A. Haghighirad2
1 Goethe-University Frankfurt, Theoretical Physics - 60054 Frankfurt/Main, Germany, EU
2 Goethe-University Frankfurt, Institute of Physics - 60054 Frankfurt/Main, Germany, EU
received 2 May 2009; accepted in final form 25 June 2009; published July 2009
published online 24 July 2009
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 EPIpol potential Vep is large for vibrations of the As-ions and depends strongly on the As-Fe distance d, i.e. Vep ~ d-4. The EPIpol coupling is much larger than the one obtained in the LDA band structure calculations, with geppol (~16 eV/Å) ( < 1 eV/Å) and the bare pairing EPIpol 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) EPIpol coupling vanishes, while the non-linear (quadratic) one is very strong. The part of the EPIpol 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 EPIpol 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 O2--ions.
74.25.Jb - Electronic structure.
74.25.Kc - Phonons.
74.70.Dd - Ternary, quaternary, and multinary compounds (including Chevrel phases, borocarbides, etc.).
© EPLA 2009