Equilibrium correlation functions of the spin-boson model with sub-Ohmic bath

The spin-boson model is studied by means of flow equations for Hamiltonians. Our truncation scheme includes all coupling terms which are linear in the bosonic operators. Starting with the canonical generator eta(c) = [H-0; H] with H-0 resembling the non-interacting bosonic bath, the flow equations exhibit a universal attractor for the Hamiltonian flow. This allows to calculate equilibrium correlation functions for super-Ohmic, Ohmic and sub-Ohmic baths within a uniform framework including finite bias. (C) 2002 Elsevier Science B.V. All rights reserved.