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

TL;DR

This study demonstrates how the crystal-field symmetry in ErBr$_3$ constrains its low-energy magnetic dynamics, explaining the absence of dispersive modes and the thermodynamic anomalies observed.

Contribution

It reveals the impact of local crystal-field symmetry on the magnetic properties and low-energy excitations in layered honeycomb ErBr$_3$.

Findings

No well-defined low-energy dispersive magnetic modes above the ordering temperature.

Thermodynamic measurements show two zero-field anomalies and distinct responses under in-plane magnetic fields.

Crystal-field symmetry constrains the transverse channel in the ground-state doublet.

Abstract

We show that the local crystal-field symmetry of ErBr$_3$ enforces $\langle \psi_\pm | J^{\pm} | \psi_\mp \rangle = 0$ 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 ErBr$_3$.