Excitonic transverse and amplitude fluctuations in the noncollinear and charge-ordered RbFe$^{2+}$Fe$^{3+}$F$_{6}$

TL;DR

This paper explores excitonic fluctuations in the noncollinear, charge-ordered antiferromagnet RbFe$^{2+}$Fe$^{3+}$F$_{6}$, revealing how orbital degeneracy and complex spin structures lead to unusual excitations like amplitude modes.

Contribution

It extends spin-orbit exciton models to noncollinear magnetic structures and analyzes neutron spectroscopy data to identify amplitude fluctuations related to orbital degrees of freedom.

Findings

Identification of momentum and energy broadened continuum scattering.

Evidence for amplitude fluctuations due to orbital degeneracy.

Extension of exciton models to noncollinear spin arrangements.

Abstract

RbFe$^{2+}$Fe$^{3+}$F$_{6}$ is an example of an antiferromagnet with charge ordering of the octahedrally coordinated Fe$^{2+}$ and Fe$^{3+}$ ions. As well as different spin values, Fe$^{2+}$ ($S=2$) and Fe$^{3+}$ ($S={5\over2}$) possess differing orbital ground states with Fe$^{2+}$ having an orbital degeneracy with an effective orbital angular momentum of $l=1$. The resulting low temperature magnetic structure is non collinear with the spins aligned perpendicular to nearest neighbors (S. W. Kim \textit{et al.} Chem. Sci. {\bf{3}}, 741 (2012)). The combination of an orbital degeneracy and non collinear spin arrangements introduces the possibility for unusual types of excitations such as amplitude modes of the order parameter. In this paper we investigate this by applying a multi-level analysis to model neutron spectroscopy data (M. Songvilay \textit{et al.} Phys. Rev. Lett. {\bf{121}}, 087201 (2018)). In particular, we discuss the possible origins of the momentum and energy broadened continuum scattering observed in terms of amplitude fluctuations allowed through the presence of an orbital degree of freedom on the Fe$^{2+}$ site. We extend previous spin-orbit exciton models based on a collinear spin structure to understand the measured low-energy excitations and also to predict and discuss possible amplitude mode scattering in RbFe$^{2+}$Fe$^{3+}$F$_{6}$.