Crystal-field symmetry constraints in layered honeycomb ErBr 3

Crystal-field symmetry restricts the ground-state Kramers doublet of ${\mathrm{ErBr}}_{3}$ to one of two classes. We show that the compressed octahedral environment selects the class with $\ensuremath{\langle}{\ensuremath{\psi}}_{\ifmmode\pm\else\textpm\fi{}}|{J}^{\ifmmode\pm\else\textpm\fi{}}|{\ensuremath{\psi}}_{\ensuremath{\mp}}\ensuremath{\rangle}=0$, suppressing the lowest-order ${J}^{\ifmmode\pm\else\textpm\fi{}}$-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 ${\mathrm{ErBr}}_{3}$, providing a microscopic starting point for understanding its low-energy magnetic behavior.