1. Institute of Materials Chemistry, Universität Wien, Währingerstraße 42, Wien A-1090, Austria
2. Erich Schmid Institute of Materials Science, Austrian Academy of Sciences, Jahnstrasse 12, Leoben A-8700, Austria
3. Thermoelectric Materials and Devices Laboratory, Department of Physics, Indian Institute of Science, Bangalore 560012, India
4. Institute of Physics of Materials, Czech Academy of Sciences, Žižkova 22, Brno 61662, Czech Republic
5. Faculty of Physics, Universität Wien, Boltzmanngasse 5, Wien A-1090, Austria
6. Department of Physics and Helsinki Institute of Physics, University of Helsinki, P.O. Box 43, Helsinki FI-00014, Finland
7. Institute of Solid State Physics, TU Wien, Wiedner Hauptstraße, 8-10, Wien A-1040, Austria