Origin of Electronic Nematicity in the Iron Pnictide NaFe$_{1-x}$Co$_x$As Superconductor*

Doped iron pnictides present a complex phase diagram with superconductivity in close proximity to antiferromagnetic and structural transitions (ST). In addition to these phases, an electronic nematic phase has been suggested to be associated with the tetragonal-to-orthorhombic transition at $T_S$. Electronic nematicity breaks $C_4$ rotational symmetry and is believed to be the driving force behind the ST. However, at present, the main interaction behind electronic nematicity and nematic fluctuations remain unexplained. Using electronic Raman spectroscopy we show nematic charge fluctuations in the $XY$ symmetry channel to follow a Curie-Weiss-like temperature dependence extending over a $\sim$200 K range above $T_S$ and in the entire phase diagram including the superconducting phase in NaFe$_{1-x}$Co$_{x}$As ($0 < x < 0.08$) single crystals. The nematicity is found to originate from orbital fluctuations, interconnected with local phonons, and are described in the frame of a classical Curie-Weiss law two-level system corresponding to the $d_{xz}$ and $d_{yz}$ Fe-orbitals.