Abstract
We report a high-resolution terahertz spectroscopic study of quantum spin dynamics in the antiferromagnetic Heisenberg-Ising spin-chain compound BaCo₂V₂O₈ as a function of temperature and longitudinal magnetic field. Confined spinon excitations are observed in an antiferromagnetic phase below T_N≃ 5.5 K. In a field-induced gapless phase above B_c=3.8 T, we identify many-body string excitations as well as low-energy fractional psinon/antipsinon excitations by comparing to Bethe-Ansatz calculations. In the vicinity of B_c, the high-energy string excitations are found to be dynamically dominant over the fractional excitations.