In recent years, a novel electronic state called the “electronic nematic phase” has attracted much attention. The electronic nematic phase is characterized by rotational symmetry breaking driven by electronic degrees of freedom, such as spins and orbitals, and behaves like a liquid crystal. Iron-based superconductors (IBSs), which are unconventional high-temperature superconductors, are prominent candidates in which an electronic nematic phase appears at low temperatures.

The electronic nematic phase in IBSs breaks the four-fold rotational symmetry of the lattice, and the electronic state has two-fold rotational symmetry. In this case, two energetically equivalent states with different preferred orientations can coexist in real space. This real-space structure, called the domain structure, often provides essential information for understanding the electronic states. In some IBSs, novel domain structures called mesoscopic nematicity waves (MNW) have been observed in which the domains behave like sinusoidal waves with mesoscopic wavelengths of approximately 500 nm. This phenomenon can be interpreted as the boundary between two neighboring domains becoming very long. The length of the domain boundary implies how hard it is to change from one domain to another, indicating the degree of the “stiffness” of the electronic nematicity. From this perspective, the MNW with a thick domain wall possesses very high stiffness. However, the origin of the high stiffness has not yet been elucidated.

We used a laser-PEEM to observe the domains in BaFe₂As₂ and FeSe_0.9S_0.1 and compared their stiffness with those of FeSe and BaFe₂(As_0.87P_0.13)₂ studied previously. We found that the length of the domain boundary varies in different materials and is related to the “strange metal” behavior of the transport properties.

The electrical resistivity of normal metals is proportional to the square of the temperature, T. In strange metals, however, the temperature dependence of the resistivity deviates from this standard T² dependence and exhibits T-linear behavior owing to spin–orbital fluctuations. Thus, we fitted the electrical resistivity above the nematic phase transition temperature with r = r₀ + ATⁿ, and the deviation of the temperature exponent n from 2 was used as the degree of unusual metallicity. By comparing the value of n with the thickness of the domain boundaries, we found that the stiffness increased as the material approached a strange metal. This suggests that the stiffness of the nematicity can be enhanced by the spin–orbital fluctuations responsible for strange-metal behavior. Our results provide important information for understanding and controlling electronic nematic phases.

(Written by Y. Kageyama and T. Shibauchi on behalf of all authors)

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