[1] Karlsruhe Inst Technol, Inst Theoret Festkorperphys TFP, D-76131 Karlsruhe, Germany
[2] Beijing Normal Univ, Dept Phys, Beijing 100875, Peoples R China
[3] Karlsruhe Inst Technol, Inst Theorie Kondensierten Mat TKM, D-76131 Karlsruhe, Germany
[4] Saarland Univ, Theoret Phys, D-66123 Saarbrucken, Germany
[5] Chalmers Univ Technol, Dept Microtechnol & Nanosci MC2, SE-41296 Gothenburg, Sweden