Frustrated Magnetic Interactions, Giant Magneto-Elastic Coupling, and Magnetic Phonons in Iron-Pnictides

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 $J_{i,j}(R)$ are calculated up to $\approx 12$Å$.Wefindthat$J_{i,j}(R)$hasanoscillatorycharacterwithanenvelopdecayingas$1/R^3$alongthestripe-directionwhileitisveryshortrangealongthediagonaldirectionandantiferromagnetic.Abriefdiscussionoftheneutronscatteringdeterminationoftheseexchangeconstantsfromasinglecrystalsamplewithorthorhombictwinningisgiven.Thelatticeparameterdependenceoftheexchangeconstants,$dJ_{i,j}/da$arecalculatedforasimplespin-Peierlslikemodeltoexplainthefinedetailsofthetetragonal-orthorhombicphasetransition.Wethendiscussgiantmagneto-elasticeffectsinthesesystems.WeshowthatwhentheFe-spinisturnedofftheoptimizedc-valuesareshorterthanexperimetnalvaluesby1.4\mathring{A}$ $forCaFe$_2$As$_2$,by0.4\mathring{A}$ $forBaFe$_2$As$_2$,andby0.13\mathring{A}$ $ 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.

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)

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