[1] IFW Dresden, Inst Festkorpertheorie, D-01171 Dresden, Germany
[2] Acad Sci Czech Republ, Inst Phys, Prague, Czech Republic
[3] Univ Magdeburg, Inst Theoret Phys, D-39106 Magdeburg, Germany
[4] Max Planck Inst Chem Phys Fester Stoffe, Dresden, Germany