Orbital nature of the hole-like Fermi surface in superconducting Ba(Fe${}_{1-x}$Co${}_x$)${}_2$As${}_2$

We report a high-momentum and energy resolution angle-resolved photoemission study of superconducting Ba(Fe${}_{1-x}$Co${}_x$)${}_2$As${}_2$. Polarization-dependent measurements performed at low photon energy along high-symmetry directions in the Brillouin zone made it possible to get new insight into the role of the five Fe 3$d$ orbitals in the complex electronic structure of this compound close to the Fermi level, in particular, into the nature of the holelike pockets. Two distinct inner $\alpha$ pockets could be disentangled, suggesting that their origin is probably due to hybridized $d_{\mathit{xz}}$ and $d_{\mathit{yz}}$. We also show that the complex outer $\beta$ pocket is mainly of $d_{z^2}$ nature, with contributions of other orbitals of different symmetry only in specific points of the Brillouin zone, in the proximity of the $k_z=$1 $[4\pi /c]$ plane. The identification of a $d_{z^2}$ nature for $\beta$ contributes to our understanding of the role of electron correlations in 122 pnictide superconductors and of the enhancement of the three-dimensional character of the Fermi surface in Co-doped compounds.

Figure 1

(a) The crystallographic structure of BaFe${}_2$As${}_2$ is shown, along with (b) Fe 3$d$ orbitals and ARPES geometry in the (c) $\Gamma M$ and (d) $\Gamma X$ orientation.

Figure 2

The angle-resolved photoemission matrix element calculations for the three even orbitals in the $\Gamma X$ sample orientation.

Figure 3

ARPES measurements in different sample orientations and photon polarizations for $x=0.07$ and $\mathrm{h}\nu =31$ eV. Along $\Gamma M$, (a) $p$-pol, (b) $s$-pol, and (c) MDCs at $E-E_F=-30$ meV. Along $\Gamma X$, (d) $p$-pol, (e) $s$-pol, and (f) MDCs at $E-E_F=-30$ meV. The markers in images indicate the band positions extracted from MDC fits, performed as shown in (c) and (f). All images have the same color scale, as shown in (a).

Figure 4

MDCs at $E-E_F=-50$ meV for Ba(Fe${}_{1-x}$Co${}_x$)${}_2$As${}_2$ ($x=0.08$) along $\Gamma X$ for different photon energies in (a) $p$-pol and (b) $s$-pol. The markers indicate the band positions for $k_{\parallel }>0$; the positions are identical for $k_{\parallel }<0$.

Figure 5

The $k_F$ values (extracted from MDCs at $E-E_F=-10$ meV) for (a) $p$-pol and (b) $s$-pol (lines are guides for the eye). The marker weights for $k_F>0$ (right-hand side) are an indication of the intensity of the $\beta$-band signal normalized with respect to the $\alpha$-band signal. Square markers correspond to the spectra at $h\nu$ $=$ 31 eV. The $k_z$ values are given modulo $4\pi /c$.

Figure 6

ARPES images and MDCs at $E-E_F=-50$ meV showing the symmetry reversal of $\beta$ in $x=0.08$ doped sample along $\Gamma X$. (a)–(c) $h\nu =9$ eV; (d)–(f) $h\nu =10$ eV; and (g)–(i) $h\nu =11$ eV. The color scales are similar to Fig. 3.