Dresden 2026 – scientific programme

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MA: Fachverband Magnetismus

MA 25: Focus Session: Chiral phonons and crystals coupled to magnetic order I

MA 25.2: Invited Talk

Wednesday, March 11, 2026, 10:00–10:30, HSZ/0002

Towards a modern theory of chiralization (and can chiral phonons help us get there?) — •Nicola Spaldin — Materials Theory, ETH Zürich, Switzerland

The goal of this talk is to answer the question: Given two chiral crystals, which one is the most chiral? In order to compare the amount of chirality, we propose the concept of ``chiralization'', representing the amount of chiral moment per unit volume, as a bulk macroscopic thermodynamic quantity, analogous to magnetization or polarization. Such a quantity will allow us to identify the conjugate field for chirality, classify achiral to ferrochiral phase transitions in terms of an appropriate order parameter, determine the multipole(s) associated with chirality, and predict phenomena in chiral materials. We begin by analyzing ferroaxial materials, which are protochiral, in the sense that they have a symmetry-lowering structural phase transition that breaks mirrors but preserves inversion, and are often characterized by rotations of sub-units around an axis yielding domains of opposite handedness. We show that the electric toroidal dipole, calculated as the cross product of the spin and orbital angular momenta, is a suitable order parameter for ferroaxial phase transitions. We outline the difficulties in extending the physics of ferroaxial materials to ferrochiral materials, and indicate how our emerging understanding of chiral phonons might help.

Keywords: chiral phonon; ferrochiral; ferroaxial; electric toroidal dipole