Spin Ferroquadrupolar Order in the Nematic Phase of FeSe

TL;DR

This paper presents evidence that spin ferroquadrupolar order is the ground state in the nematic phase of FeSe, supported by theoretical models and consistent with neutron scattering experiments, suggesting a new understanding of FeSe's magnetic properties.

Contribution

The study identifies spin ferroquadrupolar order as the likely ground state in FeSe's nematic phase using variational mean-field, DMRG, and flavor-wave theory, connecting it to experimental observations.

Findings

FQ phase is close to the columnar antiferromagnet in the phase diagram.

FQ state’s dynamical spin structure factor matches neutron scattering data.

FQ can coexist with C4 symmetry breaking environments.

Abstract

We provide evidence that spin ferroquadrupolar (FQ) order is the likely ground state in the nonmagnetic nematic phase of stoichiometric FeSe. By studying the variational mean-field phase diagram of a bilinear-biquadratic Heisenberg model up to the 2nd nearest neighbor, we find the FQ phase in close proximity to the columnar antiferromagnet commonly realized in iron-based superconductors; the stability of FQ phase is further verified by the density matrix renormalization group. The dynamical spin structure factor in the FQ state is calculated with flavor-wave theory, which yields a qualitatively consistent result with inelastic neutron scattering experiments on FeSe at both low and high energies. We verify that FQ can coexist with $C_4$ breaking environments in the mean-field calculation, and further discuss the possibility that quantum fluctuations in FQ act as a source of nematicity.