Giant X-Ray Circular Dichroism in a Time-Reversal Invariant Antiferromagnet.

Abstract

X-ray circular dichroism, arising from the contrast in X-ray absorption between opposite photon helicities, serves as a spectroscopic tool to measure the magnetization of ferromagnetic materials and identify the handedness of chiral crystals. Antiferromagnets with crystallographic chirality typically lack X-ray magnetic circular dichroism because of time-reversal symmetry, yet exhibit weak X-ray natural circular dichroism. Here, wereport the observation of giant natural circular dichroism in the Ni L3 -edge X-ray absorption of Ni3 TeO6 , a polar and chiral antiferromagnet with effective time-reversal symmetry. To unravel this intriguing phenomenon, wepropose a phenomenological model that classifies the movement of photons in a chiral crystal within the same symmetry class as that of a magnetic field. The coupling of X-ray polarization with the induced magnetization yields giant X-ray natural circular dichroism, revealing typical ferromagnetic behaviors allowed by the symmetry in an antiferromagnet, i.e., the altermagnetism of Ni3 TeO6 . Ourfindings provide evidence for the interplay between magnetism and crystal chirality in natural optical activity. Additionally, weestablish the first example of a new class of magnetic materials exhibiting circular dichroism with time-reversalsymmetry. This article is protected by copyright. All rights reserved.