Confinement and deconfinement of spinons in two dimensions.

We use Monte Carlo methods to study spinons in two-dimensional quantum spin systems, characterizing their intrinsic size λ and confinement length Λ. We confirm that spinons are deconfined, Λ→∞ and λ finite, in a resonating valence-bond spin-liquid state. In a valence-bond solid, we find finite λ and Λ, with λ of a single spinon significantly larger than the bound state-the spinon is soft and shrinks as the bound state is formed. Both λ and Λ diverge upon approaching the critical point separating valence-bond solid and Néel ground states. We conclude that the spinon deconfinement is marginal in the lowest-energy state in the spin-1 sector, due to weak attractive spinon interactions. Deconfinement in the vicinity of the critical point should occur at higher energies.