Atomic-scale contrast mechanism in atomic force microscopy

We present numerical results for the ground state and low-lying excited states of theS=1 antiferromagnetic Heisenberg chain with exchange and single-ion anisotropy for free boundary conditions and up to 16 spins. The excitation spectrum in the Néel and Haldane phases is interpreted successfully in terms of domain walls (i.e. solitons) only. We present evidence that the transition between these two phases is characterized by the condensation of the lowest soliton modes. The difference between periodic and open boundary conditions, in particular the relevance of open boundary conditions for numerical calculations on finite chains, is discussed.