Theory of electromagnetic wave propagation in ferromagnetic Rashba conductor

TL;DR

This paper investigates electromagnetic wave propagation in ferromagnetic Rashba conductors, revealing how symmetry-breaking and Rashba effects lead to unique optical phenomena like hyperbolic dispersion, birefringence, and nonreciprocal dichroism.

Contribution

It provides a comprehensive quantum mechanical analysis of electromagnetic responses in ferromagnetic Rashba conductors, highlighting novel optical effects induced by symmetry breaking and spin-orbit coupling.

Findings

Enhanced anisotropic properties with large Rashba coupling.

Enlarged frequency range for hyperbolic dispersion and negative refraction.

Prediction of nonreciprocal optical phenomena such as directional dichroism.

Abstract

We present a comprehensive study of various electromagnetic wave propagation phenomena in a ferromagnetic bulk Rashba conductor from the perspective of quantum mechanical transport. In this system, both the space inversion and time reversal symmetries are broken, as characterized by the Rashba field $\alpha$ and magnetization $M$, respectively. First, we present a general phenomenological analysis of electromagnetic wave propagation in media based on the dielectric tensor. Then, we calculate the microscopic electromagnetic response of the current and spin of conduction electrons subjected to $ \alpha$ and $M$, based on linear response theory and the Green's function method. Firstly, it is found that a large $\alpha$ enhances the anisotropic properties of the system and enlarges the frequency range in which the electromagnetic waves have hyperbolic dispersion surfaces and exhibit unusual propagations known as negative refraction and backward waves. Secondly, we consider the electromagnetic cross-correlation effects on the wave propagation. These effects stem from the lack of space inversion symmetry and yield $q$-linear off-diagonal components in the dielectric tensor. This induces a Rashba-induced birefringence. In the presence of $M$, there arises an anomalous Hall effect and the dielectric tensor acquires off-diagonal components linear in $M$. These components yield the Faraday effect and the Cotton-Mouton effect. When $\alpha$ and $M$ are noncollinear, $M$- and $q$-induced optical phenomena, nonreciprocal directional dichroism is possible. In these nonreciprocal optical phenomena, a "toroidal moment," $\alpha\times M$, and a "quadrupole moment," $\alpha_i M_j + \alpha_j M_i$, play central roles. These phenomena are strongly enhanced at the spin-split transition edge in the electron band.