Analysis of spin density wave conductivity spectra of iron pnictides in the framework of density functional theory

Abstract

The optical conductivity of LaFeAsO, BaFe${}_2$As${}_2$, SrFe${}_2$As${}_2$, and EuFe${}_2$As${}_2$ in the spin-density wave (SDW) state is investigated within density functional theory (DFT) in the framework of spin-polarized generalized gradient approximation (GGA) and GGA+U. We find a strong dependence of the optical features on the Fe magnetic moments. In order to recover the small Fe magnetic moments observed experimentally, GGA+$U_{\mathrm{eff}}$ with a suitable choice of negative on-site interaction $U_{\mathrm{eff}}=U-J$ was considered. Such an approach may be justified in terms of an overscreening which induces a relatively small U compared to the Hund's rule coupling J, as well as a strong Holstein-like electron-phonon interaction. Moreover, reminiscent of the fact that GGA+$U_{\mathrm{eff}}$ with a positive $U_{\mathrm{eff}}$ is a simple approximation for reproducing a gap with correct amplitude in correlated insulators, a negative $U_{\mathrm{eff}}$ can also be understood as a way to suppress magnetism and mimic the effects of quantum fluctuations ignored in DFT calculations. With these considerations, the resulting optical spectra reproduce the SDW gap and a number of experimentally observed features related to the antiferromagnetic order. We find electronic contributions to excitations that so far have been attributed to purely phononic modes. Also, an orbital resolved analysis of the optical conductivity reveals significant contributions from all Fe 3d orbitals. Finally, we observe that there is an important renormalization of kinetic energy in these SDW metals, implying that the effects of correlations cannot be neglected.

arXiv comment8 pages, 4 figures; recalculated spectra for U_eff=-1.9 eV for better comparison to experimental results, added discussion of the role of U and J in LDA+U

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• Magnetism and Charge Dynamics in Iron Pnictides
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• Accounting for spin fluctuations beyond LSDA in the density functional theory
• Electronic correlations in iron-pnictide superconductors and beyond; what can we learn from optics
• Optical conductivity and Raman scattering of iron superconductors
• Extended Drude model and role of interband transitions in the mid-infrared spectra of pnictides
• Analysis of the optical conductivity for A2IrO3 (A = Na, Li) from first principles
• Optical Conductivity Anisotropy in the Undoped Three-Orbital Hubbard Model for the Pnictides
• Theoretical Investigation of Optical Conductivity in Ba [Fe(1-x)Co(x)]2 As2
• Electronic excitations in $\gamma$-Li${}_2$IrO${}_3$
• Orbital selective phase transition induced by different magnetic states: A dynamical cluster approximation study
• Importance of Itinerancy and Quantum Fluctuations for the Magnetism in Iron Pnictides
• Possible Way to Make MgFeGe a New Fe-based Superconductor
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