Energy-Resolved Real-Space Imaging of Orbital Nematicity in an Fe-Based Superconductor

Electronic nematicity in Fe-based superconductors is manifested by spontaneous rotational symmetry breaking and the formation of nematic domains with mutually orthogonal directions of $d_{xz}$/$d_{yz}$ orbital anisotropy. However, its energy dependence has remained largely unexplored in real space. Using 5.82-eV laser-excited photoemission electron microscopy (laser-PEEM) with an energy-selective slit, we visualize the evolution of linear dichroic (LD) contrast within individual nematic domains of Ba$_{1-x}$Na$_x$Fe$2$As$2$ ($x\approx0.08$). We discover a sign reversal of the LD contrast at an energy $\sim0.4$ eV below the Fermi level, directly revealing an inversion of orbital anisotropy inside each domain. This behavior reflects a different energy-dependent redistribution of spectral weight between the $d{xz}$ and $d{yz}$ states, highlighting the crucial role of orbital-selective coherence in the nematic phase of Fe-based superconductors.

💡 Research Summary

In this work the authors employ an energy‑selective laser‑photoemission electron microscope (laser‑PEEM) to image orbital nematicity in the iron‑based superconductor Ba₁₋ₓNaₓFe₂As₂ (x ≈ 0.08) with unprecedented real‑space and energy resolution. By using a continuous‑wave ultraviolet laser of photon energy hν = 5.82 eV and inserting a narrow energy slit at the end of the PEEM energy analyzer, they can tune the kinetic‑energy window of the emitted electrons and thus probe electronic states from the Fermi level down to about –1.4 eV. Two orthogonal linear polarizations (E₁ and E₂) are alternately incident on the sample; the resulting PEEM intensities I₁ and I₂ are combined into a linear‑dichroism (LD) contrast, LD = (I₁ – I₂)/(I₁ + I₂), which directly reflects the imbalance between dₓz and d_yz orbital occupations because the polarization vectors are aligned with the orthorhombic a‑axis of each nematic domain.

The key observations are: (i) When the LD signal is integrated over the full accessible energy range, the contrast between twin nematic domains is weak. (ii) If only the shallow energy window (E – E_F > –0.4 eV) is selected, a clear stripe‑like domain pattern emerges (denoted S > LD). This indicates that near‑Fermi‑level (coherent) bands carry a strong orbital anisotropy, with the dₓz orbital dominating in domains where the a‑axis is parallel to E₁ and the d_yz orbital dominating where it is parallel to E₂. (iii) Conversely, when only the deep‑energy window (E – E_F < –0.4 eV) is selected, the same spatial pattern appears but with opposite LD sign (S < LD). Thus the orbital anisotropy reverses at an energy of about –0.4 eV: the d_yz spectral weight exceeds that of dₓz in the incoherent, lower‑energy region.

Temperature‑dependent measurements show that the sign reversal persists only in the nematic phase. Both S > LD and S < LD appear below the structural/nematic transition temperature T_s ≈ 135 K and vanish above it. Their amplitudes follow a power‑law (1 – T/T_s)^β with β ≈ 0.16 for the shallow‑energy LD and β ≈ 0.13 for the deep‑energy LD, values close to the critical exponent expected for the magnetic order parameter in this material. This demonstrates that the energy‑dependent redistribution of orbital spectral weight is a genuine order‑parameter‑like feature of the nematic transition, not an artifact of external strain.

The authors interpret the results in terms of orbital‑selective coherence. In the nematic state, Hund’s‑metal physics leads to stronger quasiparticle coherence for the dₓz orbital near the Fermi level, while the d_yz orbital becomes more incoherent at higher binding energies. Consequently, spectral weight is transferred from the coherent dₓz band to the incoherent d_yz background as one moves to lower energies, producing the observed LD sign change. Importantly, the experiment is performed on an unstrained crystal, so the observed domain structure and orbital anisotropy are intrinsic to the material.

This study establishes laser‑PEEM with an energy slit as a powerful tool for mapping orbital‑dependent electronic structure in real space, extending the accessible energy range beyond what conventional ARPES or nano‑ARPES can achieve. It provides direct evidence that orbital selectivity, a hallmark of Hund’s‑metal behavior, plays a central role in the nematic phase of iron‑based superconductors. The findings have implications for understanding anisotropic quasiparticle interference, the symmetry of the superconducting gap, and the microscopic mechanism linking nematicity to high‑temperature superconductivity. Future applications of this technique to other multi‑orbital systems could reveal similar energy‑dependent orbital reconstructions and deepen our grasp of correlation‑driven electronic orders.