We show that the local crystal-field symmetry of ErBr3 enforces ⟨ψ±∣J±∣ψ∓⟩=0 within the ground-state Kramers doublet, thereby removing the lowest-order transverse channel from the low-energy sector. Thermodynamic measurements reveal two zero-field anomalies. Under an in-plane magnetic field, the thermodynamic response separates into a phase boundary and a broader crossover line. Consistently, inelastic neutron scattering measurements above the ordering temperature reveal no well-defined low-energy dispersive magnetic modes. These results show that the crystal-field ground-state symmetry strongly constrains the low-energy dynamics and provides a natural framework for understanding the field-dependent thermodynamic response of ErBr3.
@article{arxiv.2604.24951,
title = {Crystal-Field Symmetry Constraints in Layered Honeycomb ErBr$_3$},
author = {Biaoyan Hu and Mingyuan Hu and Franz Demmel and Andrey A. Podlesnyak and Jiaqing He and Liusuo Wu},
journal= {arXiv preprint arXiv:2604.24951},
year = {2026}
}