Visualizing the electronic nematic state by laser-photoemission electron microscopy*

Nematicity is ubiquitous in the electronic phases of iron-based superconductors [1,2]. Previous angle-resolved photoemission spectroscopy revealed the orbital polarization of Fe 3dxz and 3dyz electrons as an order parameter that characterizes the nematic phase [3], but its real-space arrangement remains largely unexplored. We use linear dichroism in a low-temperature laser-photoemission electron microscope [4] to map out the orbital polarization of nonmagentic FeSe and antiferromagnetic BaFe₂(As_0.87P_0.13)₂ [5]. In contrast to structural domains, which have atomic-scale domain walls [6], the linear dichroism patterns in both materials show peculiar sinusoidal waves of electronic nematicity with wavelengths more than 1000 times longer than the unit cell. According to the Ginzburg-Landau theory, the sinusoidal waves can be understood by a train of the nematic domain walls with mesoscopic coherence length. These observations suggest the nematic order parameter with high stiffness against real-space modulation and its unusual decoupling from lattice. In this talk, I will also discuss the temperature dependence of the linear dichroism signals.

 

[1] E. Fradkin et al., Annu. Rev. Condens. Matter Phys. 1, 153 (2010).

[2] H.-H. Kuo et al., Science 352, 958 (2016).

[3] T. Shimojima et al., Phys. Rev. B 90, 121111 (2014).

[4] T. Taniuchi et al., Rev. Sci. Instrum. 86, 023701 (2015).

[5] T. Shimojima et al., Science 337, 1122 (2021).

[6] T. Watashige et al., Phys. Rev. X 5, 031022 (2015).