On the origin of critical nematic fluctuations in pnictide superconductors

Authors:  S.-F. Wu, W.-L. Zhang, L. Li, H. B. Cao, H.-H. Kung, A. S. Sefat, H. Ding, P. Richard, and G. Blumberg

Fig 1712.06066
We employ polarization-resolved Raman spectroscopy to study critical nematic fluctuations in Ba(Fe1-xAux)2As2 superconductors above and across well separated tetragonal to orthorhombic phase transition at temperature TS(x) and the Neel transition at TN(x). The static Raman susceptibility in XY symmetry channel increases upon cooling from room temperature following the Curie Weiss law, with Weiss temperature Tθ(x) several tens of degrees lower than TS(x). Data reveals a hidden nematic quantum critical point at x = 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 dxz/dyz orbitals.