Spin Excitation Continuum in the Exactly Solvable Triangular-Lattice Spin Liquid CeMgAl11O19

TL;DR

This study uses neutron scattering to reveal that CeMgAl11O19 exhibits a spin excitation continuum arising from ground state degeneracy, providing a rare example of an exactly solvable spin liquid on a triangular lattice.

Contribution

It demonstrates that CeMgAl11O19's spin excitation continuum originates from ground state degeneracy in an exactly solvable 2D triangular-lattice XXZ model.

Findings

Spin excitation continuum matches theoretical spin wave calculations.

CeMgAl11O19 is the first known exactly solvable spin liquid on a triangular lattice.

Ground state degeneracy explains the observed spin excitation continuum.

Abstract

In magnetically ordered insulators, elementary quasiparticles manifest as spin waves - collective motions of localized magnetic moments propagating 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$^\circ$ ordered ground states - umbrella states - develop in the zero temperature limit. We then find that the observed zero-field spin excitation continuum matches the calculated ensemble of spin waves from the umbrella state manifold, and thus 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 ground state degeneracy.