Orbital localization and the role of the Fe and As $4p$ orbitals in ${\mathrm{BaFe}}_{2}{\mathrm{As}}_{2}$ probed by XANES

Orbital localization and the role of the Fe and As $4 p$ orbitals in ${BaFe}_{2} {As}_{2}$ probed by XANES

A. G. de Figueiredo, M. R. Cantarino, W. R. da Silva Neto, K. R. Pakuszewski, R. Grossi, D. S. Christovam, J. C. Souza, M. M. Piva, G. S. Freitas, P. G. Pagliuso, C. Adriano, and F. A. Garcia

Phys. Rev. B 105, 045130 – Published 21 January 2022

Abstract

The polarization dependence of the near edge x-ray absorption spectroscopy (XANES) is an element specific probe to the real-space distribution of the density of unoccupied states in solid-state materials. In this paper, we present Fe and As $K$ -edge experiments of $Ba {({Fe}_{1 - x} M_{x})}_{2} {As}_{2}$ ( $M = Mn$ , Co, and $x = 0.0$ and 0.08). The experiments reveal a strong polarization dependence of the probed XANES spectra, which concerns mainly an increase in the intensity of electronic transitions when the beam polarization is set out of the sample's $a b$ crystallographic plane. The results show that states with $p_{z}$ -orbital character dominate the density of unoccupied states close to the Fermi level. Partial substitution of Fe by Co is shown to decrease the intensity anisotropy, suggesting that Co promotes electronic transfer preferentially to states with $p_{z}$ -orbital character. On the other hand, Mn substitution causes the increase in the spectra $p_{z}$ -orbital anisotropy, which is proposed to take place by means of an enhanced local $Fe 3 d 4 p$ mixing, unveiling the role of $Fe 4 p$ states in the localization of the $Fe 3 d$ orbitals. Moreover, by comparing our results to previous experiments, we identify the relative mixing between Fe and pnictide $4 p_{x, y, z}$ orbitals as a clear divide between the electronic properties of iron arsenides and selenides. Our conclusions are supported by multiple-scattering theory calculations of the XANES spectra and by quantum chemistry calculations of the Fe coordination electronic structure.

Received 21 October 2021
Revised 18 December 2021
Accepted 6 January 2022

DOI:https://doi.org/10.1103/PhysRevB.105.045130

Physics Subject Headings (PhySH)

Electronic structure Superconductivity

Iron-based superconductors Strongly correlated systems

Configuration interaction Molecular orbital theory X-ray absorption near-edge spectroscopy

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

A. G. de Figueiredo ¹, M. R. Cantarino ¹, W. R. da Silva Neto ^1,2, K. R. Pakuszewski ³, R. Grossi ³, D. S. Christovam ^3,*, J. C. Souza ³, M. M. Piva ^3,*, G. S. Freitas³, P. G. Pagliuso³, C. Adriano³, and F. A. Garcia ^1,†

¹Instituto de Física, Universidade de São Paulo, São Paulo-São Paulo 05508-090, Brazil
²Instituto de Química, Universidade de São Paulo, São Paulo-São Paulo 05508-090, Brazil
³Inst Fis Gleb Wataghin, Universidade Estadual de Campinas, Campinas-São Paulo 13083-859, Brazil

^*Present address: Max Planck Institute for Chemical Physics of Solids, Nöthnitzer Straße 40, 01187 Dresden, Germany.
^†Corresponding author: fgarcia@if.usp.br

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Vol. 105, Iss. 4 — 15 January 2022

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Figure 1
(a) Composition ( $x$ ) vs $T$ phase diagram for the $Ba {({Fe}_{1 - x} {Mn}_{x})}_{2} {As}_{2}$ and $Ba {({Fe}_{1 - x} {Co}_{x})}_{2} {As}_{2}$ transition-metal substituted iron arsenides (black squares are data from Ref. [35]). (b) Schematic of the experimental geometry, defining the rotation angles $ϕ, θ$ , and $χ$ as rotations around the $a, b$ , and $c$ axes, respectively. (c) Representative ${BaFe}_{2} {As}_{2}$ Fe $K$ -edge normalized XANES spectrum [ $μ (E)$ , left axis, full symbols] and its derivative [ $d μ (E) / d E$ , right axis, open symbols]. Capital letters $A$ – $E$ mark the transitions, and the dashed lines associate the features in the spectrum derivative to the spectrum. (d) The respective As $K$ -edge data of ${BaFe}_{2} {As}_{2}$ with capital letters $F$ and $G$ labeling the absorption features (e)–(j). Polarization dependence of the Fe $K$ -edge and As $K$ -edge XANES spectra of ${BaFe}_{2} {As}_{2}$ . The open red circles in the insets of panels (f) and (i) show the difference spectrum [ $μ {(E)}_{ϕ = 45} - μ {(E)}_{ϕ = 0}$ ] with the dashed lines marking some representative energy positions.
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Figure 2
Fe $K$ -edge XANES spectra of (a)–(c) $Ba {({Fe}_{1.92} {Mn}_{0.08})}_{2} {As}_{2}$ and (d)–(f) $Ba {({Fe}_{1.92} {Co}_{0.08})}_{2} {As}_{2}$ as a function of the polarization (see figures' top). In all cases, the normalized intensities [ $μ (E)$ ] are presented. The open red circles in the insets of panels (b) and (e) show the difference spectrum [ $μ {(E)}_{ϕ = 45} - μ {(E)}_{ϕ = 0}$ ] with the dashed lines marking some representative energy positions.
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Figure 3
(a) Fe $K$ -edge XANES spectra of ${BaFe}_{2} {As}_{2}$ ( $ϕ = 0$ and $ϕ = 45)$ and their respective phenomenological fittings. As shown, two peaks, termed $A_{1}$ and $A_{2}$ , are included to describe the preedge $A$ feature. The shaded regions below the curves are the peak areas which are adopted as the resonance intensities. The intensities are presented in (b) and (c), respectively, for the $A_{1}$ -preedge peak and the $B$ -edge peak as a function of $ϕ$ and composition. In (d), we present polarization-dependent feff calculations of the Fe $K$ -edge spectra (left axis) and its derivative (right axis) compared to the respective experimental data. A shift of $- 1.3 eV$ and a broadening of 1 eV were considered in the calculations. In panel (e), we show the feff calculated site and orbital-projected LDOS of ${BaFe}_{2} {As}_{2}$ . Features $A, B$ , and $C$ are presented for comparison to the results. The inset of the figure compares the feff-calculated spectra ( $ϕ = 45^{\circ}$ ) with and without core-hole effects.
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Figure 4
As $K$ -edge XANES spectra of (a)–(c) $Ba {({Fe}_{1.92} {Mn}_{0.08})}_{2} {As}_{2}$ and (d)–(f) $Ba {({Fe}_{1.92} {Co}_{0.08})}_{2} {As}_{2}$ as a function of the polarization (see figures's top). In all cases, the normalized intensities [ $μ (E)$ ] are presented. The open red circles in the insets of panels (b) and (e) show the difference spectrum [ $μ {(E)}_{ϕ = 45} - μ {(E)}_{ϕ = 0}$ ] with the dashed lines marking some representative energy positions.
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Figure 5
(a) Normalized intensities [ $μ (E)$ ] of the As $K$ -edge XANES spectra of $Ba {({Fe}_{1.92} {Mn}_{0.08})}_{2} {As}_{2}$ for $ϕ = 0$ and $ϕ = 45^{\circ}$ . In each case, the panel shows the spectrum region taken into consideration to perform the numerical integration that represents the $F$ -feature intensity [ $I_{F} (ϕ)$ ]. (b) The polarization dependence of the $F$ -peak intensity as a function of $ϕ$ , for all compositions. In (c), we present polarization-dependent feff calculations of the As $K$ -edge spectra (left axis) and its derivative (right axis) compared to the respective experimental data. A shift of 1 eV and a broadening of 1.2 eV were considered in the calculations. In panel (d), we present the difference spectrum of the ${BaFe}_{2} {As}_{2}$ data (open red circles) compared to feff calculations of $Ba {({Fe}_{1 - x} M_{x})}_{2} {As}_{2}$ with $M = Mn$ or Co and $x = 0.1$ (see the main text). (e) feff-calculated site and orbital projected LDOS of ${BaFe}_{2} {As}_{2}$ focusing on the Fe- and As-derived electronic states.
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Figure 6
Left panel: the energetic ordering of fully occupied, ligand field, and virtual orbitals as obtained from our calculations. Right panel: ligand-field molecular orbitals and their ground-state configurations based on the CAS(6,5)/NEVPT-2/Def2-TZVPP approach [41]. Isosurface values are set to 0.006. Our results indicate a ligand-field splitting of 0.308 eV. Ligand orbitals are plotted via the gabedit program [61].
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Physical Review B

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Orbital localization and the role of the Fe and As $4 p$ orbitals in ${BaFe}_{2} {As}_{2}$ probed by XANES

A. G. de Figueiredo, M. R. Cantarino, W. R. da Silva Neto, K. R. Pakuszewski, R. Grossi, D. S. Christovam, J. C. Souza, M. M. Piva, G. S. Freitas, P. G. Pagliuso, C. Adriano, and F. A. Garcia

Phys. Rev. B 105, 045130 – Published 21 January 2022

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Orbital localization and the role of the Fe and As 4p orbitals in BaFe2As2 probed by XANES

A. G. de Figueiredo, M. R. Cantarino, W. R. da Silva Neto, K. R. Pakuszewski, R. Grossi, D. S. Christovam, J. C. Souza, M. M. Piva, G. S. Freitas, P. G. Pagliuso, C. Adriano, and F. A. Garcia

Phys. Rev. B 105, 045130 – Published 21 January 2022

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Orbital localization and the role of the Fe and As $4 p$ orbitals in ${BaFe}_{2} {As}_{2}$ probed by XANES