Spin excitation continuum from degenerate states in the mixed ferro-antiferromagnetic exchange system CeMgAlO

Gao, B.*; Chen, T.*; Liu, C.*; Klemm, M. L.*; Zhang, S.*; Ma, Z.*; Xu, X.*; Won, C.*; McCandless, G. T.*; Rao, K.*; et al.

Science Advances (Internet), 12(10), p.eaed7778_1 - eaed7778_7, 2026/03

https://doi.org/10.1126/sciadv.aed7778

In the search for unconventional magnetism, exotic quantum states are characterized by a lack of order and a broad spin excitation continuum approaching zero temperature. We study the two-dimensional triangular-lattice effective spin-system CeMgAlO, which shows slight disorder but no magnetic ordering down to 100 millikelvin. Spin-wave analysis in the magnetic-field-polarized state determines the spin Hamiltonian featuring a mixed ferromagnetic-antiferromagnetic nearest-neighbor exchange interaction. This places the system near an exactly solvable point of the spin-triangular-lattice XXZ model with extensive ground-state degeneracy. In zero field, neutron spectroscopy reveals a prominent continuum; we show that this arises from an ensemble average of spin-wave spectra across the degenerate ground-state manifold. This demonstrates that the role of weak quenched disorder can be quantitatively constrained: It inhibits unique ground-state selection and stabilizes a local distribution within the degenerate manifold, yielding continuum-like spectra that necessitate a critical reevaluation of the experimental signatures of exotic quantum states.