Goldstone modes and electromagnon fluctuations in the conical cycloid state of a multiferroic

TL;DR

This paper analyzes the low-energy fluctuation spectrum of the conical cycloid state in a multiferroic, identifying Goldstone modes and electromagnons, and predicts experimental signatures for neutron and optical measurements.

Contribution

It introduces a phenomenological Ginzburg-Landau model for the conical cycloid state in multiferroics, detailing its fluctuation spectrum and experimental signatures.

Findings

Identification of Goldstone modes and their anisotropic dispersion

Prediction of electromagnon polarization fluctuations

Experimental signatures for neutron and optical experiments

Abstract

Using a phenomenological Ginzburg-Landau theory for the magnetic conical cycloid state of a multiferroic, which has been recently reported in the cubic spinel CoCr$_{2}$O$_{4}$, we discuss its low-energy fluctuation spectrum. We identify the Goldstone modes of the conical cycloidal order, and deduce their dispersion relations whose signature anisotropy in momentum space reflects the symmetries broken by the ordered state. We discuss the soft polarization fluctuations, the `electromagnons', associated with these magnetic modes and make several experimental predictions which can be tested in neutron scattering and optical experiments.