1. Department of Physics, University of Calabria, 87036 Arcavacata di Rende (CS), Italy
2. CNR-Nanotec, 87036 Cosenza, Italy
3. Department of Physical Chemistry, University of Geneva, 30 Quai Ernest-Ansermet, 1211 Geneva 4, Switzerland
4. Beam Engineering for Advanced Measurements Company, 1300 Lee Road, Orlando, Florida 32789, USA