Propagating S=1/2 particles in S=1 Haldane gap systems

Abstract

Inspired by the recent experiments on Y${}_{2-x}$Ca${}_x$BaNiO${}_5$, (J. F. DiTusa {\it et al.}, Phys. Rev. Lett. 73, 1857 (1994)), we discuss the dispersion relation of the $s$=1/2 particles in the $s$=1 Heisenberg and VBS model in the limit of small hopping amplitudes. The effective $s$=1/2 edge spins mix with the spin of the impurity resulting in one four--fold and two two--fold degenarate bands. We briefly discuss the interaction between the $s$=1/2 particles arising from the background.

4 pages, RevTeX 3.0, 4 figures available on request