Thorsmølle, V.K.Khodas, M.Yin, Z.P.Zhang, ChenglinCarr, S.V.Dai, PengchengBlumberg, G.2017-05-042017-05-042016Thorsmølle, V.K., Khodas, M., Yin, Z.P., et al.. "Critical quadrupole fluctuations and collective modes in iron pnictide superconductors." <i>Physical Review B,</i> 93, no. 5 (2016) American Physical Society: https://doi.org/10.1103/PhysRevB.93.054515.https://hdl.handle.net/1911/94179The 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 nonsymmetric charge fluctuations across the entire phase diagram, which are precursors to a d-wave Pomeranchuk instability at temperature θ(x). The charge fluctuations are interpreted in terms of quadrupole interorbital 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 in-gap mode of extraordinary strength.engArticle is made available in accordance with the publisher's policy and may be subject to US copyright law. Please refer to the publisher's site for terms of use.Journal articlehttps://doi.org/10.1103/PhysRevB.93.054515