1. Dpt. Física Aplicada
III, Universidad Complutense de Madrid,
Madrid, 28040 Spain
2. Materials
Science and Technology
Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 32831, United States
3. Dpt. Fisica Fonamental and Institut
de Nanociència i Nanotecnologia (IN2UB), Universitat de Barcelona, 08028 Barcelona, Spain
4. Department of Physics and Astronomy, Vanderbilt University, Nashville, Tennessee 37235,
United States