GROUND-STATES OF A SPIN BOSON MODEL

Phase transitions with respect to ground states of a spin-boson Hamiltonian are investigated. The spin-boson model under discussion of one spin and infinitely many bosons with a dipole-type coupling. It is shown that the order parameter of the model vanishes with respect to arbitrary ground states if it vanishes with respect to ground states obtained as (biased) temperature to zero limits of thermic equilibrium states. The ground states of the latter special type have been investigated by H. Spohn. Spohn'ss respective phase diagrams are therefore valid for arbitrary ground states. Furthermore, disjointness of ground states in the broken symmetry regime is examined. © 1991.