Structure of end states for a haldane spin chain
Inelastic neutron scattering experiment on Mg-doped Y2BaNiO5 was performed. An upward renormalization of the Haldane gap with doping, which is consistent with numerical predictions for a distribution of finite chain segments, was observed. In addition, well-defined excitations were found below the Haldane gap whose field dependence demonstrates that they originate from S=1/2 chain-end degrees of freedom. Their wave-vector dependence showed chain-end states that carry an AF "Friedel" oscillation that extends into the chain segment with an exponential decay length ξ=8(1) in a field H=11 T. The experiment also demonstrated a new technique, neutron ESR, by which the structure of composite spin degrees of freedom liberated from a macroscopic singlet can be probed.