Dynamical susceptibility of a near-critical nonconserved order parameter and quadrupole Raman response in Fe-based superconductors

Abstract

We analyze the dynamical response of a two-dimensional system of itinerant fermions coupled to a scalar boson φ, which undergoes a continuous transition towards nematic order with a d-wave form factor. We consider two cases: (a) when φ is a soft collective mode of fermions near a Pomeranchuk instability, and (b) when it is an independent critical degree of freedom, such as a composite spin order parameter. In both cases, the order parameter is not a conserved quantity and the d-wave fermionic polarization (q, ) remains finite even at q = 0. The polarization (0, ) has similar behavior in the two cases, but the relations between (0, ) and the bosonic susceptibility χ (0, ) are different, leading to different forms of χ (0, ), as measured by Raman scattering. We compare our results with polarization-resolved Raman data for the Fe-based superconductors FeSe 1−x S x , NaFe 1−x Co x As, and BaFe 2 As 2 .

Quadrupolar charge dynamics in the nonmagnetic FeSe 1− x S x superconductors

“…The behavior arises from degeneracy of the partially filled iron 3dxz and 3dyz orbitals in the tetragonal phase (62)(63)(64). The QEP is related to overdamped dynamical charge oscillations at sub-terahertz frequencies, which give rise to a fluctuating charge quadrupole moment with an amplitude proportional to oscillating dxz /dyz orbital charge imbalance Q ∝ nxz − nyz , where n xz /yz is the orbital occupancy (16,17,60,(65)(66)(67)(68)(69)(70). Such excitations result in Pomeranchuk-like nematic dynamic deformation of the Fermi surface pockets with nodal lines in the X /Y directions (see illustration of a snapshot in Fig.…”

Scattering mechanisms and electrical transport near an Ising nematic quantum critical point

“…The situation is complicated by the fact that despite arXiv:1902.04590v1 [cond-mat.str-el] 12 Feb 2019 the substantial progress made [26][27][28][29][30][31][32][33][34][35][36][37][38], the theory of Ising-nematic quantum criticality at asymptotically low energies is not well understood. However, as we argue below, there is a broad range of energy scales where the theory can be controlled; this "Hertz-Millis-Moriya" regime is described in terms of coherent electrons interacting with strongly renormalized, overdamped collective fluctuations of the order parameter [39][40][41].…”

Raman Response in the Nematic Phase of FeSe

“…In a metal, the Raman response probes densitylike fluctuations at a finite frequency Ω and vanishing momenta q, modulated by a form factor, which depends on relative polarizations of the incoming and outgoing light and transforms according to the point-group representation of the crystal [26]. When the form-factor is constant as, e.g., in the fully symmetric channel in a singleband system, the Raman response is proportional to the density correlator at q ¼ 0 and finite Ω, and it vanishes because fermionic density is a conserved quantity [4,[26][27][28][29][30][31][32][33][34]. The Raman response which probes electronic nematic correlations in FeSe has the nonsymmetric B 1g symmetry [35], so it is finite in the metal because no conservation law applies [4,[29][30][31][32].…”

“…When the form-factor is constant as, e.g., in the fully symmetric channel in a singleband system, the Raman response is proportional to the density correlator at q ¼ 0 and finite Ω, and it vanishes because fermionic density is a conserved quantity [4,[26][27][28][29][30][31][32][33][34]. The Raman response which probes electronic nematic correlations in FeSe has the nonsymmetric B 1g symmetry [35], so it is finite in the metal because no conservation law applies [4,[29][30][31][32].…”