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

Published

March 6, 2026

Author(s)

Bin Gao, Tong Chen, Chunxaio Liu, Mason Klemm, Shu Zhang, Zhen Ma, Xianghan Xu, CHOONGJAE WON, Gregory McCandless, Karthik Rao, Naoki Murai, Seiko Ohira-Kawamura, Stephen Moxim, Jason Ryan, Xiaozhou Huang, Xiaoping Wang, Manh Duc Le, Emilia Morosan, Julia Chan, Sang-Wook Cheong, Oleg Tchernyshyov, Leon Balents, Pengcheng Dai

Abstract

In magnetically ordered insulators, elementary quasiparticles manifest as spin waves - collective motions of localized magnetic moments that propagate through the lattice - observed via inelastic neutron scattering. In effective spin- 1/2 systems where geometric frustrations suppress static magnetic order, spin excitation continua can emerge, either from degenerate classical spin ground states or from entangled quantum spins characterized by emergent gauge fields and deconfined fractionalized excitations. Comparing the spin Hamiltonian with theoretical models can unveil the microscopic origins of these zero-field spin excitation continua. Here, we use neutron scattering to study spin excitations of the two-dimensional (2D) triangular-lattice effective spin- 1/2 antiferromagnet CeMgAl11O19. Analyzing the spin waves in the field-polarized ferromagnetic state, we find that the spin Hamiltonian is close to an exactly solvable 2D triangular-lattice XXZ model, where degenerate 120◦ ordered ground states - assembly of umbrella states - develop in the zero temperature limit. Given that the observed zero-field spin excitation continuum matches the calculated ensemble of classical spin waves from the umbrella state manifolds, we conclude that CeMgAl11O19 is the first example of an exactly solvable spin liquid on a triangular lattice where the spin excitation continuum arises from the classical ground state degeneracy.

Citation

Science Advances

Volume

12

Issue

10

Pub Type

Journals

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DOI Link

Citation

Gao, B. , Chen, T. , Liu, C. , Klemm, M. , Zhang, S. , Ma, Z. , Xu, X. , WON, C. , McCandless, G. , Rao, K. , Murai, N. , Ohira-Kawamura, S. , Moxim, S. , Ryan, J. , Huang, X. , Wang, X. , Le, M. , Morosan, E. , Chan, J. , Cheong, S. , Tchernyshyov, O. , Balents, L. and Dai, P. (2026), Spin excitation continuum from degenerate states in the mixed ferro-antiferromagnetic exchange system CeMgAl11O19, Science Advances, [online], https://doi.org/10.1126/sciadv.aed7778 (Accessed March 25, 2026)