# Sign change of polarization rotation under either time or space inversion in magnetoelectric YbAl3(BO3)4

## TL;DR

This paper reports the discovery of a novel polarization rotation effect in magnetoelectric YbAl3(BO3)4, which changes sign under both time and space inversion, indicating a new form of gyrotropic birefringence linked to magnetoelectric susceptibility.

## Contribution

It introduces a new polarization rotation phenomenon in magnetoelectric materials that changes sign under both time and space inversion, expanding understanding of optical activity.

## Key findings

- Sign change of polarization rotation under time and space reversal.
- Identification of gyrotropic birefringence in YbAl3(BO3)4.
- Contribution of magnetoelectric susceptibility to optical activity.

## Abstract

Materials with optical activity can rotate the polarization plane of transmitted light. The most typical example is the natural optical activity, which has the symmetry property of changing sign after space inversion but being invariant to time inversion. Faraday rotation exhibits the opposite: it is invariant to space inversion but changes sign after time reversal. Here, we demonstrate that in a magnetoelectric material, another type of polarization rotation is possible. This effect is investigated in magnetoelectric YbAl3(BO3)4 under the viewpoint of time and space inversion symmetry arguments. We observe the sign change of the rotation sense under either time or space reversal. This investigation proves that the polarization rotation in YbAl3(BO3)4 must be classified as gyrotropic birefringence, which has been discussed within the idea of time-reversal breaking in underdoped cuprates. The diagonal terms in the magnetoelectric susceptibility are responsible for the observed signal of gyrotropic birefringence. Further analysis of the experimental spectra reveals a substantial contribution of the natural optical activity to the polarization rotation. We also demonstrate that the observed activity originates from the magnetoelectric susceptibility.