Itinerant approach to magnetic neutron scattering of FeSe: Effect of orbital selectivity

Abstract

Recent STM experiments and theoretical considerations have highlighted the role of interactiondriven orbital selectivity in FeSe, and its role in generating the extremely anisotropic superconducting gap structure in this material. We study the magnetic excitation spectrum resulting from the coherent quasiparticles within the same renormalized random phase approximation approach used to explain the STM experiments, and show that it agrees well with the low-energy momentum and energy dependent response measured by inelastic neutron scattering experiments. We find a correlation-induced suppression of (π, π) scattering due to a small quasiparticle weight of states of dxy character. We compare predictions for twinned and untwinned crystals, and predict in particular a strongly (π, 0)-dominated response at low energies in untwinned systems, in contrast to previous itinerant theories.

Anisotropic spin fluctuations in detwinned FeSe

“…In addition to having a weak peak around E ≈ 3.2 meV, we find that the scattering changes from well-defined commensurate peaks centered around Q AF below E = 3.625 ± 0. Although these results on twinned FeSe suggest that spin fluctuations play an important role in the superconductivity of FeSe, they provide no information on the possible orbital selective nature of the fluctuations that may lead to a highly anisotropic electron pairing state [19,31,[35][36][37][38]. From STM quasiparticle interference measurements on a single domain (detwinned) FeSe, where the Fermi surface geometry of electronic bands can be determined in the nematic phase, sign-reversed superconducting gaps are found at the hole [Γ or Q = (0, 0)] and electron [X or Q AF = (1, 0)] Fermi surface states derived from d yz orbitals of the Fe atoms along the orthorhombic a o -axis direction [Figs.…”

“…Finally, the nematic phase and superconductivity in FeSe has also been described by itinerant electrons interacting among quasi-nested hole-electron Fermi surfaces [28,29], as in other iron based superconductors [30]. In this picture, arXiv:1905.08399v1 [cond-mat.str-el] 21 May 2019 the electronic correlation effect is taken into account by orbital-dependent quasiparticle weights [24,31]. Without electron correlation effects, spin fluctuations in the nematic phase below T s exhibit only a minor C 4 asymmetry.…”

“…Without electron correlation effects, spin fluctuations in the nematic phase below T s exhibit only a minor C 4 asymmetry. Including correlations in the theoretical calculations render the spin fluctuations highly C 2 symmetric with negligible weight at (0, ±1), and a neutron spin resonance exhibited only at Q AF = (±1, 0) driven by the d yz orbitals [31]. Approaches based on localized models with magnetic quadrupolar order have also predicted a strong suppression of low-energy (0, ±1) intensity [27].…”

“…In order to achieve a theoretical understanding of the experimental results presented above, we start from an itinerant five-band model that quantitatively matches the low-energy electronic structure of FeSe in its nematic state [9,21,24,36] and compute the magnetic scattering S(Q, E) ∝ χ (Q, E)/(1 − e −E/k B T ) within a standard random phase approximation formulation [28,29,31,36]. The spectral function at the Fermi level is presented in Fig.…”

“…A simple means to incorporate the important effects of such orbital selectivity is through the introduction of orbital-dependent quasiparticle weights Z < 1 [31,36] leading to a modified bare susceptibilityχ 0 1 2 3 4 (Q, E) given byχ…”