1. CNISM, Dipartimento di Fisica and LENS, Universita' di Firenze, via Sansone 1, 50019 Sesto Fiorentino, Italy
2. Microelectronics Research Group, FORTH/IESL, P.O. Box 1527, 71110 Heraklion, Crete, Greece
3. Materials Science and Technology Dept., Univ. of Crete, P.O Box 2208, 71003 Heraklion Crete, Greece
4. Physics Dept., Univ. of Crete, P.O. Box 2208, 71003 Heraklion Crete, Greece