[1] Lund Univ, LTH, Div Math Phys, S-22100 Lund, Sweden
[2] Peking Univ, Inst Microelect, Beijing 100871, Peoples R China
[3] Peking Univ, Ctr Comp, Beijing 100871, Peoples R China
[4] Univ Duisburg Essen, Fachbereich Phys, D-47048 Duisburg, Germany
[5] Lund Univ, Div Solid State Phys, S-2100 Lund, Sweden