DOMAIN-WALLS IN THE QUANTUM TRANSVERSE ISING-MODEL

We discuss several problems concerning domain walls in the spin-S Isin g model at zero temperature in a magnetic field, H/(2S), applied in th e x direction. Some results are also given for the planar (y-z) model in a transverse field. We treat the quantum problem in one dimension b y perturbation theory at small H and numerically over a large range of H. We obtain the spin-density profile by fixing the spins at opposite ends of the chain to have opposite signs of S-2. One dimension is spe cial in that there the quantum width of the wall is proportional to th e size L of the system. We also study the quantitative features of the ''particle'' band which extends up to energies of order H above the g round state. Except for the planar limit, this particle band is well s eparated from excitations having energy J/S involving creation of more walls. At large S this particle band develops energy gaps and the low est subband has tunnel splittings of order H2(1-2S). This scale of ene rgy gives rise to anomalous scaling with respect to (a) finite size, ( b) temperature, or (c) random potentials. The intrinsic width of the d omain wall and the pinning energy are also defined and calculated in c ertain limiting cases. The general conclusion is that quantum effects prevent the wall from being sharp and in higher dimension would preven t sudden excursions in the configuration of the wall.