[1] Department of Inorganic Chemistry, Stockholm University, S-10691 Stockholm, Sweden
[2] Department of Applied Physics, Chalmers Univ Technol./Goteborg Univ, SE-41296 Göteborg, Sweden
[3] Department of Physics, Condensed Matter Theory Group, Box 530, S-75121 Uppsala, Sweden
[4] Applied Materials Physics, Dept. of Materials and Engineering, Royal Institute of Technology (KTH), SE-10044 Stockholm, Sweden