Orbital-Dependent Modifications of Electronic Structure across the Magnetostructural Transition in ${\mathrm{BaFe}}_2{\mathrm{As}}_2$

Laser angle-resolved photoemission spectroscopy (ARPES) is employed to investigate the temperature ($T$) dependence of the electronic structure in ${\mathrm{BaFe}}_2{\mathrm{As}}_2$ across the magnetostructural transition at $T_N\sim 140\mathrm{K}$. A drastic transformation in Fermi surface (FS) shape across $T_N$ is observed, as expected by first-principles band calculations. Polarization-dependent ARPES and band calculations consistently indicate that the observed FSs at $k_z\sim \pi$ in the low-$T$ antiferromagnetic state are dominated by the $\mathrm{Fe}3d_{zx}$ orbital, leading to the twofold electronic structure. These results indicate that magnetostructural transition in ${\mathrm{BaFe}}_2{\mathrm{As}}_2$ accompanies orbital-dependent modifications in the electronic structure.

Figure 1

(a) Crystal structure of ${\mathrm{BaFe}}_2{\mathrm{As}}_2$ in the tetragonal structure. (b) Crystal and magnetic structure in the stripe-type AF ordered orthorhombic structure. (c),(d) FS of ${\mathrm{BaFe}}_2{\mathrm{As}}_2$ measured by $p$ polarization at 180 K (above $T_N$) and 30 K (below $T_N$), respectively. (e),(f) FS of ${\mathrm{BaFe}}_2{\mathrm{As}}_2$ measured by $s$ polarization at 180 K (above $T_N$) and 30 K (below $T_N$), respectively. (g),(h) Whole FS in the first BZ obtained by LDA calculation considering PM tetragonal and stripe-type AF [6] orthorhombic structure, respectively, using the experimentally obtained structural parameters. PM structure is represented by a space group symmetry of $I4/mmm$, while we use $Cccm$ notation for AF state regarding the Fe sites having opposite spins as nonequivalent.

Figure 2

(a) Schematics of orthorhombic twinned structure at $T<T_N$. Orthorhombic orientation in domain $A$ is 90° rotated with respect to domain $B$. (b),(c) Two permissible cases of the relation between the polarization and orthorhombic orientation, denoted as “case 1” and “case 2,” respectively. Arrows on the square lattice represent the stripe-ordered Fe spins.

Figure 3

(a),(b) ARPES intensity image along $Z\mathrm{\text{−}}{\Gamma }^{\prime }$ ($a^{o*}$) and $Z\mathrm{\text{−}}T$ ($b^{o*}$) directions. (c) Band structure near $E_F$ obtained from LDA calculation in the stripe-type AF ordered state. Blue and red curves correspond to the $V$-shaped band dispersions observed in (a) and (b), respectively.

Figure 4

(a),(b) Calculated Fe $3d$ PDOS in PM and AF state, respectively. The upper panel PDOS in (b) and (d) originates in minority-spin states, while the lower panel is for majority-spin states. Note that $d_{zx}$ and $d_{yz}$ orbitals are degenerate in the tetragonal PM state and their PDOS are equivalent. (c),(d) Calculated As $p$ PDOS in PM state and AF state, respectively. $p_x$ and $p_y$ PDOS are equivalent in PM state.