Spin excitation continuum from degenerate states in the mixed ferro-antiferromagnetic exchange system CeMgAl 11 O 19
- Bin Gao
- Tong Chen
- Chunxiao Liu
- Mason L. Klemm
- Shu Zhang
- Zhen Ma
- Xianghan Xu
- Choongjae Won
- Gregory T. McCandless
- Karthik Rao
- Naoki Murai
- Seiko Ohira-Kawamura
- Stephen J. Moxim
- Jason T. Ryan
- Xiaozhou Huang
- Xiaoping Wang
- Manh Duc Le
- Emilia Morosan
- Julia Y. Chan
- Sang-Wook Cheong
- Oleg Tchernyshyov
- Leon Balents
- Pengcheng Dai
2026-03-06
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- 1 2 system CeMgAl 11 O 19 , 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 [ J z = −0.024(5) milli–electron volts, J ⊥ = 0.056(3) milli–electron volts]. This places the system near an exactly solvable point of the spin- 1 2 triangular-lattice XXZ model ( J z = − 1 2 J ⊥ ) 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.