1. Theoretical Sciences Unit, Jawaharlal Nehru Centre for Advanced Scientific Research, Jakkur P.O., Bangalore 560064, India
2. Institut für Physik, Johannes-Gutenberg Universität Mainz, Staudinger Weg 7-9, 55128 Mainz, Germany
3. School of Advanced Materials, Jawaharlal Nehru Centre for Advanced Scientific Research, Jakkur P.O., Bangalore 560064, India