Microscopic order parameter of the higher-temperature anomaly

Determine the microscopic order parameter associated with the higher-temperature thermodynamic anomaly at T_Q in DyOCl, DyOBr, and DyOI, including whether it corresponds to quadrupolar order or another low-energy degree of freedom.

Background

All three dysprosium oxyhalides exhibit a higher-temperature thermodynamic anomaly near T_Q ≈ 27–30 K in addition to the lower-temperature dipolar antiferromagnetic transition. The anomaly carries substantial entropy, depends only weakly on magnetic field, and is not accompanied by resolved magnetic Bragg peaks or a detectable structural transition.

The authors regard a multipolar interpretation, particularly quadrupolar order involving low-lying crystal-field states, as plausible but not established. They emphasize that thermodynamic and conventional neutron-diffraction measurements do not identify the symmetry or microscopic nature of the order parameter, motivating direct probes such as resonant x-ray scattering, elastic-constant or ultrasound measurements, and Raman spectroscopy.

References

For DyO$X$, the higher-temperature anomaly near 30 K motivates a similar multipolar interpretation, but its microscopic order parameter remains to be determined.

— Tunable inter-bilayer magnetic correlations and candidate multipolar physics in the van der Waals oxyhalides DyOCl, DyOBr, and DyOI  (2608.19421 - Brooks et al., 19 Aug 2026) in Introduction, Section 1; Results, Section 3.3; Discussion, Section 5.1