Spin excitation continuum from degenerate states in the mixed ferro-antiferromagnetic exchange system CeMgAl₁₁O₁₉

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-[Formula: see text] system CeMgAl₁₁O₁₉, 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 [[Formula: see text] = -0.024(5) milli-electron volts, [Formula: see text] = 0.056(3) milli-electron volts]. This places the system near an exactly solvable point of the spin-[Formula: see text] triangular-lattice XXZ model ([Formula: see text]) 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.