Multiferroic-like Quasiparticles in Ferroelectrics

Abstract

Multiferroics are materials with coexisting electric and magnetic orders that are of central importance for fundamental research and technological applications. Unfortunately, intrinsic multiferroics that operate at room temperature remain rare due to an apparent incompatibility between magnetism and ferroelectricity. Here we predict that a pure ferroelectric support multiferroic-like quasiparticles, termed ``multiferrons", that simultaneously carry \emph{static} magnetic and electric dipoles. The electric dipole moment emerges from the parity-odd anharmonicity of the ferroelectric dynamics, while the magnetic moment has both paramagnetic and diamagnetic origins generated by circularly polarized transverse fluctuations of the ferroelectric polarization. In contrast to the established ``dynamical multiferroicity" of circularly polarized phonons, which involve only \emph{oscillating} electric dipoles, multiferrons cause, apart from Zeeman and Einstein-de Haas effects, a linear dc Stark response, giant electric-field-tunable second-harmonic generation in the THz-frequency regime, and a finite magnetoelectric cross coupling. Multiferrons open a new route toward nonlinear THz optical applications and offer multiferroic functionalities with simple ferroelectrics.

Figures (3)

• Figure 1: Schematics of two multiferron modes corresponding to right- and left-handed circulations of the transverse polarization fluctuations $\delta\mathbf{P}(t)$ around the spontaneous polarization $P_{0}\mathbf{z}$. Each mode carries a quantized angular momentum ($\pm\hbar$) as well as a net electric dipole (red arrows) that reduces the ground state polarization.
• Figure 2: The parity-odd three-body interactions by which the quasiparticles of mode $\mathbf{k}\sigma$ acquire an intrinsic electric dipole moment, i.e., by absorbing (a) and emitting (b) a spatially uniform mode (denoted by wave lines) polarized along the ferroelectric order.
• Figure 3: (a) Electric-field-tunable second-harmonic generation by multiferrons ($\vert\chi_{zxx}^{(2)}\vert$) and ferrons ($\vert\chi_{zzz}^{(2)}\vert$). (b) Multiferron-induced magnetoelectric coupling coefficient multiplied by $\eta$ as a function of the bias magnetic field ($B$). Here $\eta=\gamma_{e}/\gamma_{p}$ denotes the ratio of the gyromagnetic ratio of electrons to that of the electric polarization.