Volume 87, Number 1, July 2009
Article Number 17007
Number of page(s) 6
Section Condensed Matter: Electronic Structure, Electrical, Magnetic and Optical Properties
Published online 24 July 2009
EPL, 87 (2009) 17007
DOI: 10.1209/0295-5075/87/17007

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

M. 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 $g_{\rm ep}^{pol}$ is much larger than the one obtained in the LDA band structure calculations, with geppol (~16 eV/Å) $\gg g_{ep}^{(LDA)}$ ( < 1 eV/Å) and the bare pairing EPIpol coupling constant $\lambda _{ep, A_{1g}}^{0}$ ~ 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