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Electrical resistivity of a crystal is the measure of how easily electrons flow in it under the influence of an external electrical field. Although it is a familiar physical quantity, important insights can still be gathered by investigating it. Resistivity originates from collisions of electrons with other scatters, such as electrons and phonons. Its temperature dependence is indicative of the most common scattering event in the given temperature range. For example, according to Landau’s Fermi liquid theory, the scattering between two electrons produces resistivity, which depends quadratically on temperature. This is a consequence of the fact that electrons are fermions, and it also demonstrates that electrons carrying a current can be treated as “quasiparticles”.
However, a variety of strongly correlated metals exhibit a puzzling linear dependence of resistivity on temperature, referred to as “strange metal” behavior, inside the temperature regime in which electron-electron scattering is dominant. This challenges the “quasiparticle” explanation of charge transport. So far, in many cases, such T-linear resistivity has been observed when an antiferromagnetic transition is suppressed to zero temperature. This indicates that the intensification of spin fluctuations might be the source of the observed strange metal behavior. Indeed, several theories have been developed that corroborate this hypothesis.
This study presents systematic measurements of the electrical resistivity of single crystals of the iron-based superconductor, Ba_1-xRb_xFe₂As₂. The parent compound BaFe₂As₂ undergoes an antiferromagnetic transition accompanied by tetragonal-orthorhombic structural distortion, which breaks the four-fold rotational symmetry. By substituting Ba with Rb ions, antiferromagnetic structural transition is rapidly suppressed. Surprisingly, resistivity of the crystal exhibits T-linear behavior around the compositions farther from the endpoint of the antiferromagnetic order, where the spin fluctuations are thought to become strongest. However, it has been recently pointed out that although structural transition already disappears at these compositions, a critical instability remains in the electronic systems towards the four-fold rotational symmetry breaking, which may be potentially associated to the observed strange metalbehavior.
Recently, several quantum materials have been observed to exhibit an intrinsic instability of electrons towards the spontaneous rotational breaking of the underlying lattice. Based on an analogy with nematic liquid crystals, this state is called the electronic nematic state. Our conclusions suggest that fluctuations of an electronic nematic order can lead to strange metal behavior as well as antiferromagnetic fluctuations. However, further investigation of the precise form of the resistivity down to zero temperature masked by high superconducting transition temperature is required to derive more decisive conclusions. The current results serve as adequate motivation to perform such a challenging experiment.

(Written by Kousuke Ishida on behalf of all authors)