[1] Univ Hamburg, Inst Angew Phys, D-20355 Hamburg, Germany
[2] Univ Hamburg, Zentrum Mikrostrukturforsch, D-20355 Hamburg, Germany
[3] Univ Iceland, Inst Sci, IS-107 Reykjavik, Iceland
[4] Inst Natl Fiz Mat, Bucharest, Romania
[5] Tech Univ Munich, Walter Schottky Inst, D-85748 Garching, Germany
[6] Univ Regensburg, D-93053 Regensburg, Germany