1. Institute
for Solid State Physics, University of Jena, Max-Wien-Platz 1, 07743 Jena, Germany
2. Institute
for Solid State Electronics, Vienna University of Technology, Floragasse
7, 1040 Vienna, Austria
3. Felix
Bloch Institute for Solid State Physics, Universität Leipzig, Linnéstraße 5, 04103 Leipzig, Germany