[1] Univ Gottingen, Inst Theoret Phys, Friedrich Hund Pl 1, D-37077 Gottingen, Germany
[2] Max Planck Inst Phys Komplexer Syst, Nothnitzer Str 38, D-01187 Dresden, Germany
[3] Wurzburg Dresden Cluster Excellence Ct Qmat, Wurzburg-Dresden Cluster Excellence ct, D-01187 Dresden, Germany
[4] Wroclaw Univ Sci & Technol, Inst Theoret Phys, PL-50370 Wroclaw, Poland