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 Asions. The polarization induced EPI potential (EPIpol) is large for vibrations of the As-ions and depends strongly on the As-Fe distance dAs−F e, i.e. Vep ∼ d −4 As−F e. The EPIpol coupling gep(= ∂Vep/∂dAs−F e) is much larger than the one obtained in the LDA band structure calculations, with gep(∼ 20 eV/Å) ≫ g (LDA) ep (< 1 eV/Å) and the bare pairing EPIpol coupling constant λ0ep,A1g ∼ 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. The proposed 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. While the linear coupling vanishes the non-linear EPIpol coupling is very strong in the Fe-breathing mode. 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 O ions. Introduction. – Recently, superconductivity (SC) with high critical temperature Tc was discovered in several families of Fe-pnictides. In the electron (e) doped (1111) system LaFeAsO1−xFx one has Tc ≈ 26 K (and 43 K at high pressure) [1]. The record values are Tc ≈ 55 K in SmFeAsO1−xFx [2] and Tc ≈ 56 K in Sr1−xSmxFeAsF [3], etc. In the hole (h) doped (122) system Ba0.6K0.4Fe2As2 one has Tc = 38 K [4]. Other families are reported, such as MFeAs with M = Li,Na and Tc = 18 K, and the binary systems Fe(Te, Se) with Tc < 12 K. A common phase diagram has emerged: (i) the structural transition appears around Tstr =(140−200); (ii) the SDW-type magnetic ordering occurs at Tsdw ≤ Tstr, while superconductivity appears when the SDW ordering vanishes. The latter can be done either by eor h-doping or by applying high pressure. The important question is what is the pairing mechanism in Fe-pnictides? The vicinity of these systems to the antiferromagnetic phase was inspiration for the spin fluctuation (SF ) pairing models with the repulsive interaction in the s-wave channel. This line of thinking was encouraged by the small electron-phonon coupling λep obtained in the LDA band structure calculations for LaO1−xFxFeAs [5], with λ LDA) ep ∼ 0.2 and T (LDA) c,ep ∼ 1 K. However, this repulsive interaction in the singlet channel might be effective only if the coupling constant for the scattering of pairs from the hole to the electron-band the hh ⇆ ee scattering, is much larger than the intraband (repulsive) couplings, i.e. ∣