1. Department of Electronic and Electrical Engineering UCL Torrington Place London WC1E 7JE UK
2. Department of Physics and Astronomy UCL Gower Street London WC1E 6BT UK
3. Institut für Werkstoffe der Elektrotechnik II RWTH Aachen University 52074 Aachen Germany
4. Departament d'Enginyeria Electronica Universitat Autonoma de Barcelona 08193 Bellaterra Spain
5. Dipartimento di Elettronica e Informazione Politecnico di Milano Milan 20133 Italy
6. Department of Electrical and Computer Engineering University of Massachusetts Amherst MA 01003 USA