1. Departamento de Física Aplicada; Universidad Autónoma de Madrid; C/Francisco Tomás y Valiente 7 28049 Madrid Spain
2. Department of Physics and Materials Science; City University of Hong Kong; Kowloon Hong Kong
3. Department of Electrical and Electronic Engineering; Saga University; Saga 840-8502 Japan
4. Materials Sciences Division; Lawrence Berkeley National Laboratory; 1 Cyclotron Road Berkeley CA 94720 USA