Spin dynamics of the spin-1 triangular lattice Heisenberg antiferromagnet: Magnons or Spinons?
Chaebin Kim

School of Physics, Georgia Institute of Technology, Atlanta, Georgia 30332, USA

Strong quantum fluctuations and unconventional spin dynamics are well established in the spin-1/2 triangular lattice Heisenberg antiferromagnet. However, their survival in the spin-1 case remains an open question. We investigate the spin dynamics of K₂Ni(SeO₃)₂, a nearly ideal spin-1 triangular lattice Heisenberg antiferromagnet, using inelastic neutron scattering. Below the ordering temperature T_N, we observe coherent one-magnon excitations coexisting with a broad high-energy continuum. Two complementary approaches, a spectrally consistent 1/S-corrected spin wave theory and a beyond-mean-field Schwinger boson theory, reproduce different facets of the continuum. Neither alone is complete, demonstrating substantial quantum fluctuations survive for S = 1 and are reflected primarily in the spectral distribution of the continuum. Above TN, the continuum bandwidth is conserved while spectral weight is redistributed as magnons lose spatial coherence. Our results establish K₂Ni(SeO₃)₂ as a model triangular antiferromagnet, identifying bandwidth conservation and the distribution of spectral weight within the continuum as organizing principles to understand the spin dynamics of ordered quantum magnets beyond spin-1/2. Our results highlight the need for controlled calculations of the interacting multi-magnon sector of 2D antiferromagnets.