Spinon confinement and deconfinement in spin-1 chains

Motivated by the rich phase diagrams recently unveiled in frustrated spin-1 chains with competing next-nearest-neighbor and four-spin interactions, we investigate the nature of the elementary excitations of spin-1 chains in the vicinity of phase transitions using the matrix-product state (MPS) representation of elementary excitations. First, we show that both spinons and magnons naturally arise in SU(2) invariant spin chains when describing ground states and elementary excitations using MPSs. Then we investigate the nature of the elementary excitations across the first-order transition between the Haldane phase and the topologically trivial next-nearest-neighbor Haldane phase. We show explicitly that spinons deconfine at the transition, and we calculate the dispersion of these deconfined spinons with MPSs. We also show that, immediately away from the transition line, spinons confine, forming dispersive spinon/antispinon bound states on both sides of the transition. Finally, we show that deconfined spinons also appear at the transition between the Haldane phase and the spontaneously dimerized phase when this transition is first order.