[1] Karlsruhe Inst Technol, Inst Theoret Condensed Matter Phys, D-76128 Karlsruhe, Germany
[2] Natl Res Nucl Univ MEPhI, Moscow Engn Phys Inst, Moscow 115409, Russia
[3] Karlsruhe Inst Technol, Inst Nanotechnol, D-76021 Karlsruhe, Germany
[4] Ioffe Phys Tech Inst, St Petersburg 194021, Russia
[5] Petersburg Nucl Phys Inst, St Petersburg 188300, Russia
[6] Karlsruhe Inst Technol, Inst Solid State Phys, D-76021 Karlsruhe, Germany