Translational symmetry breaking in the electronic nematic phase of BaFe2As2

TL;DR

This study addresses the origin of electronic nematicity in BaFe2As2 by performing temperature-dependent, detwinned ARPES measurements complemented by DFT calculations. The key finding is that a Dirac-cone feature, arising from SDW folding with wavevector $Q=(1/2,0,1/2)$ in the antiferromagnetic orthorhombic phase, persists into the nematic phase up to $T^* \\sim 170$ K, implying that the nematic state shares the stripe-type periodicity of the SDW. The authors interpret this persistence as evidence for either short-range/dynamical stripe SDW or an antiferro-orbital order involving the shallow $d_{xy}$ orbital that remains active in the nematic regime. They propose that antiferro-orbital correlations coexist with, or accompany, the nematic order, offering a plausible mechanism for $C_4$ symmetry breaking beyond ferro-orbital scenarios and contributing to the broader understanding of nematicity in iron-based superconductors.

Abstract

The microscopic origin of the nematicity, namely, four-fold rotational symmetry breaking in iron-based superconductors has been controversial since its discovery. In particular, its relationship with the stripe-type spin-density-wave order and the orthorhombic lattice distortion in the antiferromagnetic orthorhombic (AFO) phase, which exists at temperatures below the electronic nematic phase, has been highly debated. Here, we report on the temperature evolution of angle-resolved photoemission spectra of the parent compound BaFe2As2, ranging from the AFO to nematic to paramagnetic phases. The Dirac cone feature, which is formed in the AFO phase, is found to persist in the nematic phase, suggesting that an antiferroic order of the same periodicity as the AFO phase persists in the nematic phase. Considering the relatively shallow d_xy orbital in BaFe2As2, we propose that an antiferro-orbital order involving the d_xy and other orbitals takes place in the nematic phase.

Figures (5)

• Figure 1: (a) Crystal and magnetic structures of the 122-type BaFe$_2$As$_2$ in the antiferromagnetic orthorhombic (AFO) phase (adopted from Ref. qhuang). (b) Phase diagram of the P-substituted BaFe$_2$As$_2$kasahara. PM: Paramagnetic phase; SC: Superconducting phase; $T_N$: Néel temperature: $T_s$: Tetragonal-to-orthorhombic transition temperature; $T^\ast$: Nematic transition temperature.
• Figure 2: ARPES spectra of BaFe$_2$As$_2$ in the antiferromagnetic-orthorhombic phase. (a), (b) Fermi surface mapping for the detwinned and twinned BaFe$_2$As$_2$ crystals, respectively. Inset to (a) shows the crystal and magnetic structure on the 2D plane and the direction of the applied strain P. (c) 3D Brillouin zones of the PM (gray) and AFO (green) phases. (d) Corresponding in-plane 2D Brillouin zones. (e) Schematic Fermi surfaces. Orange and blue circles indicate hole pockets and electron pockets, respectively. (f), (h) Energy-momentum plots along cuts 1 and 2 indicated by red arrows in (a). (g), (i) Second-derivative plots of EDCs corresponding to cut 1 and cut 2.
• Figure 3: ARPES spectra of BaFe$_2$As$_2$ in the paramagnetic (PM) tetragonal phase. (a) Fermi-surface mapping. The spectra were taken at $T=$ 200 K. (b), (c) Energy-momentum plots along cut 1 in Fig. 1(a) for $T=$ 150 K and 200 K. (d), (e) Second-derivative plots of (b) and (c) with respect to energy.
• Figure 4: DFT-GGA calculation of the ban structures of BaFe$_2$As$_2$. (a) (d) Band structures of the PM and AFM states, respectively, along the Z-X lines. The Z-X line is perpendicular to cuts 1 and 2. (b), (e) Calculated band structure of the PM and AFM states, respectively, along cut 1. The center of cuts correspond to the Z point and cuts are perpendicular to the Z-X line. (c), (f) Calculated results for cut 2 in the PM and AFM states, respectively. Cut 2 is parallel to cut 1 and crosses the Dirac point indicated by the broken line in (a) and (d).
• Figure 5: Temperature dependence of the Dirac-cone feature in ARPES spectra. (a) Evolution of ARPES spectra of BaFe$_2$As$_2$ with temperature. Spectra from 100 K to 200 K are taken along cut 2 shown in Fig. \ref{['fig2']}(a). The spectra have been divided by the FermiDirac function. (b) Temperature evolution of the momentum distribution curve (MDC) for the binding energies form 10 meV to 20 meV. (c) Temperature dependence of the MDC peak intensity and width.