Giant X-ray Circular Dichroism in a Time-Reversal Invariant Antiferromagnet

Jun Okamoto^1*, Ru-Pan Wang², Yen-Yi Chu¹, Hung-Wei Shiu¹, Amol Singh¹, Hsiao-Yu Huang¹, Chung-Yu Mou³, Sukhito Teh⁴, Horng-Tay Jeng⁴, Kai Du⁵, Xianghang Xu⁵, Sang-Wook Cheong⁵, Chao-Hung Du⁶, Chien-Te Cheng¹, Atsushi Fujimori^1,3,7, Di-Jing Huang^1,4,8

¹National Synchrotron Radiation Research Center, Hsinchu, Taiwan
²Department of Physics, University of Hamburg, Hamburg, Germany
³Center for Quantum Science and Technology and Department of Physics, National Tsing Hua University, Hsinchu, Taiwan
⁴Department of Physics, National Tsing Hua University, Hsinchu, Taiwan
⁵Rutgers Center for Emergent Materials and Department of Physics and Astronomy, Rutgers University, New Jersey, USA
⁶Department of Physics, Tamkang University, Tamsui, Taiwan
⁷Department of Physics, University of Tokyo, Tokyo, Japan
⁸Department of Electrophysics, National Yang Ming Chiao Tung University, Hsinchu, Taiwan

* Presenter:Jun Okamoto, email:okamoto.jun@nsrrc.org.tw

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 but exhibit weak X-ray natural circular dichroism. Here, the observation of giant natural circular dichroism in the Ni L₃-edge X-ray absorption of Ni₃TeO₆ is reported, a polar and chiral antiferromagnet with effective time-reversal symmetry. The observed x-ray circular dichroism changes sign between domains of opposite crystal chirality, indicating that the crystal chirality acts as an effective “magnetic field”. To unravel this intriguing phenomenon, a phenomenological model is proposed 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 Ni₃TeO₆. The findings provide evidence for the interplay between magnetism and crystal chirality in natural optical activity. Additionally, the first example of a new class of magnetic materials exhibiting circular dichroism is established with time-reversal symmetry.

Keywords: X-ray circular dichroism, Crystal chirality, Collinear antiferromagnet, Altermagnet