1. Institute of Physics, Academy of Sciences of the Czech Republic, Cukrovarnická 10, 162 53 Prague, Czech Republic
2. Paul-Drude-Institut für Festkörperelektronik, Hausvogteiplatz 5–7, D–10117 Berlin, Germany
3. Department of Electrical and Computer Engineering, North Carolina State University, Raleigh, North Carolina 27606, USA