Critical quadrupole fluctuations and collective modes in iron pnictide superconductors

Abstract

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(x). 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.

Superconductivity and non-Fermi liquid behavior near a nematic quantum critical point

“…Recent Monte Carlo studies have examined the scaling structure of nematic and related QCPs (31,32) as well as the role of fluctuations in promoting superconductivity (33,34). Moreover, the model is particularly topical, as there is good evidence that a nematic QCP underlies the superconducting dome in many (possibly all) of the Fe-based superconductors (35)(36)(37)(38)(39)(40)(41) and possibly even the cuprate high-temperature superconductors (42)(43)(44).…”

Raman scattering as a probe of nematic correlations

“…First, the buildup of the lowenergy B 2g Raman intensity upon cooling is the dominant feature observed experimentally above T s . [31][32][33] Second, both orbital and the nematic fluctuations have B 2g symmetry. Indeed, Eq.…”

“…30 In this paper we do not attempt to resolve the above controversy, but rather explore the consequences of the nematic fluctuations as observed in recent Raman experiments. [31][32][33] Even though the region of the phase diagram contained between T s and T SDW is either absent or quite tiny, the dynamical nematic fluctuations revealed by Raman spectroscopy kick in far into the paramagnetic phase up to room temperatures. The Raman spectroscopy is essentially a dynamic probe of electronic correlations of prescribed symmetry.…”

Nematic Quantum Critical Fluctuations inBaFe2−xNixAs2Liu1, Gu2, Zhang3 et al. 2016Phys. Rev. Lett.Self Cite496751View full textAdd to dashboardCite

“…Xa, The normal-state electronic states in many iron-based superconductors show strong in-plane anisotropic properties that break the fourfold rotational symmetry of the lattice due to the presence of nematic order [1][2][3][4][5][6][7][8][9]. The nematic order is typically accompanied by a structural transition at T s following an antiferromagnetic (AF) transition at T N ≤ T s , except for FeSe where no AF order is found [10,11].…”

“…It is shown that the nematic order may go through a zero-temperature order-to-disorder quantum phase transition as shown by elastoresistance [3,9], elastic constants [4,5], and Raman scattering [7,8]. The nematic susceptibility shows divergent behavior with the form of (1/T ) γ around optimal doping [3,5,[7][8][9]20]. However, the value of γ is found to be 1, which suggests that the nematic QCP may result in a mean-field scaling.…”