Non-reciprocal circular dichroism of axial phonons coupled to ferro-rotational order

H. Y. Huang; G. Channagowdra; D. Banerjee; E. V. Komleva; J. Okamoto; C. T. Chen; M. Guennou; S. Johnston; S. V. Streltsov; C. Y. Mou; A. Fujimori; S-W. Cheong; D. J. Huang

arXiv · 2512.22050

Non-reciprocal circular dichroism of axial phonons coupled to ferro-rotational order

H. Y. Huang·G. Channagowdra·D. Banerjee·E. V. Komleva·J. Okamoto·C. T. Chen·M. Guennou·S. Johnston·S. V. Streltsov·C. Y. Mou·A. Fujimori·S-W. Cheong·D. J. Huang

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Abstract

Circular dichroism (CD) in X-ray absorption, defined as the difference in absorption between opposite circular polarizations, is fundamentally enabled by the breaking of time-reversal symmetry or inversion symmetry. It is therefore sensitive to magnetism, chirality, and their interplay. In contrast, the sample symmetry alone is insufficient to determine whether CD in resonant inelastic X-ray scattering (RIXS) is allowed. Rather, RIXS-CD is governed by both the sample symmetry and the scattering geometry. Here, using RIXS, we identify circularly polarized phonons coupled to ferro-rotational order in MnTiO$_3$, which we refer to as ferro-axial phonons. Their excitations provide a direct demonstration of non-reciprocal RIXS-CD, in which the dichroic response changes upon reversing the propagation direction of the incident X-rays, although the system globally preserves both inversion and time-reversal symmetries. We propose that a condensate of these phonons, manifested as standing waves, underlies the ferro-rotational order in MnTiO$_3$. The observed non-reciprocal CD reflects the interplay among photon helicity, phonon polarization, and ferro-rotational order.

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H. Y. Huang, G. Channagowdra, D. Banerjee, E. V. Komleva, J. Okamoto, C. T. Chen, M. Guennou, S. Johnston, S. V. Streltsov, C. Y. Mou, A. Fujimori, S-W. Cheong, D. J. Huang. 2025-12-26. Non-reciprocal circular dichroism of axial phonons coupled to ferro-rotational order. https://arxiv.org/abs/2512.22050

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