Transient electronic anisotropy in overdoped NaFe1−xCoxAs superconductors

By combining polarized pump-probe spectroscopic and Laue x-ray diffraction measurements, we have observed nonequivalent transient optical responses with the probe beam polarized along the $x$ and $y$ axes in overdoped $\mathrm{NaF}{\mathrm{e}}_{1\text{--}x}\mathrm{C}{\mathrm{o}}_{x}\mathrm{As}$ superconductors. Such transient anisotropic behavior has been uncovered in the tetragonal phase with the doping level and temperature range far from the borders of static nematic phases. The measured transient anisotropy can be well explained as a result of nematic fluctuation driven by an orbital order with energy splitting of the ${d}_{xz}$- and ${d}_{yz}$-dominant bands. In addition, the doping level dependence and the pressure effect of the crossover temperature show significant differences between the transient nematic fluctuation and static nematic phase, implying spin and orbital orders may play different roles in static and transient nematic behaviors.