1. Institut für Theoretische Physik, Technische Universität Berlin, PN 7-1, Hardenbergstr. 36, D-10623 Berlin, Germany; Fritz Haber Research Center and Department of Physical Chemistry, The Hebrew University, IL-91904 Jerusalem, Israel; and Institute of Polymers, Polymer Physics, ML H 18, Sonneggstr. 3, ETH Zentrum, CH-8092 Zürich, Switzerland