Non-reciprocal circular dichroism of ferro-rotational phonons in MnTiO$_{3}$

TL;DR

This paper demonstrates non-reciprocal circular dichroism in MnTiO3 due to ferro-rotational phonons, revealing a new mechanism where phonon excitations break symmetry conditions typically forbidding such effects.

Contribution

It introduces ferro-rotational phonons as a new class of excitations causing non-reciprocal XCD in a symmetric system, linking phonon condensates to ferro-rotational order.

Findings

Identification of circularly polarized phonons in MnTiO3

Observation of non-reciprocal XCD in a symmetric system

Proposed link between phonon condensates and ferro-rotational order

Abstract

X-ray circular dichroism (XCD), defined as the difference in absorption or scattering intensity between X-rays of opposite polarizations, arises from the breaking of spatial inversion or time-reversal symmetry and is thus sensitive to chirality, magnetism, and their interplay. Non-reciprocal XCD, in which the dichroic response changes upon reversing the propagation direction of the probe, is generally forbidden in systems with both symmetries. Using resonant inelastic X-ray scattering, we identify circularly polarized phonons in ferro-rotational MnTiO$_3$, which we term ferro-rotational phonons. Their excitations provide a direct demonstration of non-reciprocal XCD in a system that globally preserves 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 interplay among photon helicity, phonon polarization, and the axial ferro-rotational order gives rise to the observed non-reciprocal circular dichroism.