1. H.H. Wills Physics Laboratory, University of Bristol, Tyndall Avenue, Bristol BS8 1TL, UK; ITA Institut für Innovative Technologien GmbH Köthen, Aussenstelle Halle, Edvard-Grieg-Weg 8, D-06124 Halle, Germany; and Department of Physics, University of Wales Swansea, Swansea, UK