Dynamics Of A Bond-Disordered S=1 Quantum Magnet Near Z=1 Criticality

Neutron scattering is used to study NiCl2-2x Br-2x center dot 4SC(NH2)(2), x = 0.06, a bond-disordered modification of the well-known gapped S = 1 antiferromagnetic quantum spin system NiCl2 center dot 4SC(NH2)(2). The magnetic excitation spectrum throughout the Brillouin zone is mapped out at T = 60 mK using high-resolution time-of-flight spectroscopy. It is found that the dispersion of spin excitation is renormalized, as compared to that in the parent compound. The lifetime of excitations near the bottom of the band is substantially decreased. No localized states are found below the gap energy Delta similar or equal to 0.2 meV. At the same time, localized zero wave vector states are detected above the top of the band. The results are consistent with a more or less continuous random distribution of bond strengths, and a discrete, possibly bimodal, distribution of single-ion anisotropies in the disordered material.