Description
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 doping
concentrations, the tetragonal-to-orthorhombic structural
transition is closely followed by a concomitant spin density
wave transition both being in close proximity to the
superconducting phase. A key question is the microscopic
mechanism of high-Tc superconductivity and its relation to
multi-orbital ordering and magnetism.
Here we study the 111, 122 and 11 families of iron
superconductors using low energy polarization resolved
Raman spectroscopy. The Raman susceptibility shows critical
non-symmetric charge fluctuations across the entire phase
diagram. The charge fluctuations are interpreted in terms of
plasma waves of quadrupole intra-orbital excitations in which
the electron and hole Fermi surfaces breathe in-phase. We
demonstrate that above the structural phase transition these
quadrupolar fluctuations with long correlation times are
precursor to the discrete four-fold symmetry breaking
transition. This is manifested in the critical slowing down of
XY-symmetry collective fluctuations observed in dynamical
Raman susceptibility and strong enhancement of the static
Raman susceptibility. Below superconducting transition, these
collective excitations undergo a metamorphosis into a
coherent in-gap collective mode of extraordinary strength and
at the same time may serve as glue for non-conventional
superconducting pairing.
[1] V. K. Thorsmølle, M. Khodas, Z. P. Yin, C. Zhang, S. V.
Carr, Pengcheng Dai, G. Blumberg. Critical Charge
Fluctuations in Iron Pnictide Superconductors. Phys. Rev. B
93, 054515 (2016).
[2] S.-F. Wu, P. Richard, H. Ding, H.-H. Wen, Guotai Tan,
Meng Wang, Chenglin Zhang, Pengcheng Dai, G. Blumberg.
Superconductivity and electronic fluctuations in Ba1?
xKxFe2As2 studied by Raman scattering. Phys. Rev. B 95,
085125 (2017).
