Orbital-Selective Band Structure Evolution in BaFe$_{2-x}$M$_x$As$_2$ (M = Cr, Co, Cu, Ru and Mn) Probed by Polarization-Dependent ARPES

TL;DR

This work uses polarization-dependent ARPES to map how the Fe-derived electronic structure in BaFe$_{2-x}$M$_x$As$_2$ (M = Cr, Co, Cu, Ru, Mn) evolves with light substitution. The central finding is that the central hole pockets $\alpha$ and $\beta$ are largely insensitive to substitution, while the outer $\gamma$ hole pocket and the electron pockets $\delta$, $\eta$ show substitution-dependent changes linked to As height and Fe–As bond length, with planar orbitals ($d_{xy}$, $d_{x^2-y^2}$) particularly susceptible. The data support an orbital-selective mechanism in which the suppression of the SDW temperature $T_ ext{SDW}$ correlates with structural changes affecting planar orbitals, while Cr acts as a hole dopant and Mn mainly enhances electronic correlations and disorder; a linear $\Gamma(E)$ across the series signals marginal Fermi-liquid behavior. Together, the results emphasize the role of orbital character in the band-structure evolution and its connection to magnetism and potential superconductivity in lightly substituted BaFe$_{2}$As$_{2}$ systems.

Abstract

We present a systematic study of the evolution of the band structure in the Fe-based superconductor family BaFe$_{2-x}$M$_x$As$_2$ (M = Cr, Co, Cu, Ru and Mn) using polarization-dependent angle-resolved photoemission spectroscopy (ARPES). Low-substituted samples, with comparable spin-density wave transition temperatures ($T_\text{SDW}$), were chosen to facilitate controlled comparisons. The sizes of the central hole pockets ($α$, $β$, and $γ$) remain largely unchanged across different substitutions, showing no clear correlation with either $T_\text{SDW}$ or the As height relative to the Fe planes. However, subtle trends are observed: a modest increase in the size of the $η_\text{X}$ electron pocket correlates with the suppression of $T_\text{SDW}$. Furthermore, the contraction of the $η_\text{X}$ pocket appears to be linked to an increase in the As height relative to the Fe planes. Our results suggest that the suppression of $T_\text{SDW}$ is primarily driven by changes in the Fe-As bond length, with the effect being more pronounced in electronic states with planar character. These findings provide insight into the electronic structure of BaFe$_{2-x}$M$_x$As$_2$.

Figures (10)

• Figure 1: Overview of the macroscopic measurements and ARPES experimental configuration for the BaFe2-xMxAs2 family. (a) Heat capacity as a function of temperature, highlighting the magnetic transition temperature $T_\textrm{SDW}$. The X plus a number means that the y scale was multiplied by this quantity. (b) Tetragonal Brillouin zone (BZ) of the BFA parent compound (space group I4/mmm). (c) Schematic of the polarization-dependent ARPES experimental setup. (d) FS of the parent compound measured along the $\Gamma\text{X}$ direction with $\sigma$ polarization, indicating three high-symmetry cuts (cut 1, cut 2 and cut 3).
• Figure 2: ARPES spectral function analysis. MDC waterfall plots of the BFA compound along the $\Gamma\text{M}$ direction measured with (a) $\pi$ and (b) $\sigma$ polarization. Red circles represent the MDC data, while blue dashed lines indicate the corresponding fits. Panels (c) and (d) display the MDC fits highlighted by green solid lines in panels (a) and (b), respectively. Here, black circles represent the experimental MDC data, green dashed lines show the overall fit, and light blue lines correspond to the background. In panel (c), individual Lorentzian peaks used in the fit are also shown, with blue and red curves corresponding to the $\gamma$ and $\alpha$ bands, respectively.
• Figure 3: Summary of the ARPES measurements for the BFA parent compound. The upper panels (a1 and a2) correspond to measurements performed with $\pi$ polarization, while the lower panels (b1 and b2) correspond to those taken with $\sigma$ polarization. Left panels (a1 and b1) show the raw BMs, and right panels (a2 and b2) display the corresponding curvature BMs. Dashed lines serve as guide-to-eye to indicate the band dispersions. The colors red, green, and blue denote the $\alpha$, $\beta$, and $\gamma$ bands, respectively. Furthermore, at the X corner, the two electron bands $\delta$ (pink) and $\eta$ (orange) are indicated with dashed lines. The open squares denotes the points extracted via fitting or second derivative/curvature analysis.
• Figure 4: $k_z$-dispersion measurements of Cr-BFA taken along the $\Gamma\text{X}$ direction using $\pi$ polarization. (a) Fermi surface map as a function of excitation energy ($h\nu$) and in-plane momentum ($k_x$). (b) MDC waterfall plot corresponding to the data in panel (a). Green and blue circles/triangles indicate the extracted band positions, obtained from Lorentzian fits to the MDCs.
• Figure 5: Summary of the hole band dispersion analysis for the BaFe2-xMxAs2 family. Upper panels (a1 to f1) show data taken along the $\Gamma\text{X}$ direction, while lower panels (a2 to f2) show data taken along the $\Gamma\text{M}$ direction, both using $\pi$ and $\sigma$ polarizations. Circles represent data obtained with $\pi$ polarization, and squares with $\sigma$ polarization. Open markers (circles or squares) correspond to points extracted from curvature or second-derivative BMs, while filled markers indicate values obtained from MDC fitting of the raw BMs. Dashed lines are guide-to-eye of the band dispersions for each compound.