The multiband nature of iron pnictides gives rise to a rich temperature-doping phase diagram of competing orders and a plethora of collective phenomena. At low dopings, the tetragonal-to-orthorhombic structural transition is closely followed by a spin density wave transition both being in close proximity to the superconducting phase. A key question is the nature of high-Tc superconductivity and its relation to orbital ordering and magnetism. Here we study the NaFe1−xCoxAs superconductor using polarization-resolved Raman spectroscopy. The Raman susceptibility displays critical enhancement of non-symmetric charge fluctuations across the entire phase diagram which are precursors to a d-wave Pomeranchuk instability at temperature θ(\mboxx). The charge fluctuations are interpreted in terms of quadrupole inter-orbital excitations in which the electron and hole Fermi surfaces breathe in-phase. Below Tc, the critical fluctuations acquire coherence and undergo a metamorphosis into a coherent ingap mode of extraordinary strength.
@article{arxiv.1410.6456,
title = {Critical Quadrupole Fluctuations and Collective Modes in Iron Pnictide Superconductors},
author = {V. K. Thorsmølle and M. Khodas and Z. P. Yin and Chenglin Zhang and S. V. Carr and Pengcheng Dai and G. Blumberg},
journal= {arXiv preprint arXiv:1410.6456},
year = {2016}
}