1. Institut Charles Gerhardt Montpellier, UMR CNRS 5253 UM2, ENSCM, UM1, Place E. Bataillon, 34095 Montpellier cedex 05, France
2. CEA, IRAMIS, SIS2M, UMR CEA‐CNRS 3299, 91191 Gif‐sur‐Yvette cedex, France, Fax: +33‐1‐69089806, http://iramis.cea.fr/Pisp/112/thibault.charpentier.html
3. Current address: Equipe Nanostructure, Laboratoire Charles Coulomb, UMR CNRS‐UM2 5221, Place Eugène Bataillon, 34095 Montpellier cedex 05, France