1. Departament d'Electrònica and IN2UB, Universitat de Barcelona, 08028 Barcelona, Spain
2. Technische Universität Kaiserslautern, 67663 Kaiserslautern, Germany
3. Graduate School of Excellence, Materials Science in Mainz, 55128 Mainz, Germany
4. IBM Research Division, Almaden Research Center, San Jose, California 95120, USA