1. Department of Physics, Budapest University of Technology and Economics and Condensed Matter Research Group of the Hungarian Academy of Sciences, 1111 Budapest, Budafoki ut 8., Hungary
2. Institute of Molecular Physics, Polish Academy of Sciences, 60-179 Poznań, Poland
3. Faculty of Electronics and Telecommunications, Poznań University of Technology, Poznań, Poland
4. Department of Applied Physics and Applied Mathematics, Columbia University, New York, New York 10027, United States