LOW-TEMPERATURE EQUILIBRIUM CORRELATION-FUNCTIONS IN DISSIPATIVE QUANTUM-SYSTEMS

We introduce a new theoretical approach to dissipative quantum systems . By means of a continuous sequence of infinitesimal unitary transform ations, we decouple the small quantum system that one is interested in from its thermodynamically large environment. This yields a trivial f inal transformed Hamiltonian. Dissipation enters through the observati on that generically observables ''decay'' completely under these unita ry transformations, i.e. are completely transformed into other terms. As a nontrivial example the spin-boson model is discussed in some deta il. For the super-Ohmic bath we obtain a very satisfactory description of short, intermediate and long time scales at small temperatures. Th is can be tested from the generalized Shiba-relation that is fulfilled within numerical errors.