Structure of end states for a haldane spin chain

M. Kenzelmann, Johns Hopkins University
G. Xu, Brookhaven National Laboratory
I. A. Zaliznyak, Brookhaven National Laboratory
C. Broholm, Johns Hopkins University
J. F. DiTusa, Louisiana State University
G. Aeppli, The London Centre for Nanotechnology
T. Ito, National Institute of Advanced Industrial Science and Technology
K. Oka, National Institute of Advanced Industrial Science and Technology
H. Takagi, The University of Tokyo


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.