1. Departamento
de Fı́sica Teórica de la Materia Condensada C5
and Condensed Matter Physics Center (IFIMAC), Universidad Autónoma de Madrid, E-28049 Madrid, Spain
2. Centre of New
Technologies, University of Warsaw, Banacha 2c, 02-097 Warsaw, Poland
3. Department
of Applied Physics and Gothenburg Physics Centre, Chalmers University of Technology, SE-412 96 Göteborg, Sweden