1. Institut de Physique et Chimie des Matériaux, UMR 7504 CNRS, Université de Strasbourg, 23 Rue du Loess, 67034 Strasbourg, France
2. COMP/Department of Applied Physics, Aalto University, P.O. Box 11100, FI-00076 Aalto, Finland
3. Department of Physics and Astronomy, University of Pennsylvania, Philadelphia, Pennsylvania 19104, United States
4. Nanotube Research Center, National Institute of Advanced Industrial Science and Technology (AIST), Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan
5. Department of Physics, University of Helsinki, P.O. Box 43, FI-00014 Helsinki, Finland