Crystal-Field Symmetry Constraints in Layered Honeycomb ErBr${}_3$

Abstract

Crystal-field symmetry restricts the ground-state Kramers doublet of ErBr${}_3$ to one of two classes. We show that the compressed octahedral environment selects the class with $⟨{\psi }_{\pm }|J^{\pm }|{\psi }_{\mp }⟩=0$, suppressing the lowest-order $J^{\pm }$-mediated exchange. Thermodynamic measurements reveal two zero-field anomalies at 0.375 and 0.200~K. Under an in-plane magnetic field, the thermodynamic response separates into a phase boundary and a broader crossover line. Inelastic neutron scattering measurements at 2 K reveal no well-defined low-energy dispersive magnetic modes. These results connect the ground-state symmetry with the field-dependent thermodynamic response of ErBr${}_3$, providing a microscopic starting point for understanding its low-energy magnetic behavior.

9 pages, 4 figures. Accepted by Physical Review B