Frustrated Magnetic Interactions, Giant Magneto-Elastic Coupling, and Magnetic Phonons in Iron-Pnictides
Superconductivity
2010-06-23 v1 Materials Science
Abstract
We present a detailed first principles study of Fe-pnictides with particular emphasis on competing magnetic interactions, structural phase transition, giant magneto-elastic coupling and its effect on phonons. The exchange interactions Ji,j(R) are calculated up to ≈12\AA .WefindthatJ_{i,j}(R)hasanoscillatorycharacterwithanenvelopdecayingas1/R^3alongthestripe−directionwhileitisveryshortrangealongthediagonaldirectionandantiferromagnetic.Abriefdiscussionoftheneutronscatteringdeterminationoftheseexchangeconstantsfromasinglecrystalsamplewithorthorhombictwinningisgiven.Thelatticeparameterdependenceoftheexchangeconstants,dJ_{i,j}/daarecalculatedforasimplespin−Peierlslikemodeltoexplainthefinedetailsofthetetragonal−orthorhombicphasetransition.Wethendiscussgiantmagneto−elasticeffectsinthesesystems.WeshowthatwhentheFe−spinisturnedofftheoptimizedc−valuesareshorterthanexperimetnalvaluesby1.4A˚ forCaFe_2As_2,by0.4A˚ forBaFe_2As_2,andby0.13A˚ $ for LaOFeAs. Finally, we show that Fe-spin is also required to obtain the right phonon energies, in particular As c-polarized and Fe-Fe in-plane modes. Since treating iron as magnetic ion always gives much better results than non-magnetic ones and since there is no large c-axis reduction during the normal to superconducting phase transition, the iron magnetic moment should be present in Fe-pnictides at all times. We discuss the implications of our results on the mechanism of superconductivity in these fascinating Fe-pnictide systems.
Cite
@article{arxiv.0902.3462,
title = {Frustrated Magnetic Interactions, Giant Magneto-Elastic Coupling, and Magnetic Phonons in Iron-Pnictides},
author = {Taner Yildirim},
journal= {arXiv preprint arXiv:0902.3462},
year = {2010}
}
Comments
New Results: Fe-mangetism is required to explain the ROOM TEMPERATURE phonon DOS (Fig. 21); Jij(R) up to 12 Ang, indicating that J2 is short range while J1 decays as 1/R^3 (Fig. 7) ; A simple spin-Peierles model (i.e. dJ/da) (see Fig. 11-12); Spin-wave spectrum to determine the sign of exchange interaction (see Fig. 8-9)