S=1 2 CHAIN-BOUNDARY EXCITATIONS IN THE HALDANE PHASE OF ONE-DIMENSIONAL S=1 SYSTEMS/

The S= 1/2 chain-boundary excitations occurring in the Haldane phase o f S= 1 antiferromagnetic spin chains are investigated. The bilinear-bi quadratic Hamiltonian is used to study these excitations as a function of the strength of the biquadratic term, beta, between -1 less than o r equal to beta less than or equal to 1. At the Affleck-Kennedy-Lieb-T asaki point, beta= - 1/3, we show explicitly that these excitations ar e localized at the boundaries of the chain on a length scale equal to the correlation length xi=1/ln3, and that the on-site magnetization fo r the first site is [S-1(z)]=2/3. Applying the density matrix renormal ization group we show that the chain-boundary excitations remain local ized at the boundaries for -1<beta<1. As the two critical points beta= +/-1 are approached the size of the S= 1/2 objects diverges and their amplitude appears to vanish.