On the origin of critical nematic fluctuations in pnictide superconductors

We employ polarization-resolved Raman spectroscopy to study critical nematic fluctuations in Ba(Fe${}_{1-x}$Au${}_x$)${}_2$As${}_2$ superconductors above and across well separated tetragonal to orthorhombic phase transition at temperature $T_S(x)$ and the N'{e}el transition at $T_N(x)$. The static Raman susceptibility in $XY$ symmetry channel increases upon cooling from room temperature following the Curie-Weiss law, with Weiss temperature $T_{\theta }(x)$ several tens of degrees lower than $T_S(x)$. Data reveals a hidden nematic quantum critical point at $x_c=0.031$ when the system becomes superconducting, indicating a direct connection between quantum critical nematic fluctuations and unconventional superconductivity. We attribute the origin of the nematicity to charge quadrupole fluctuations due to electron transfer between the nearly degenerate $d_{xz}\mathrm{/}{d}_{yz}$ orbitals.