1. Institute for Theoretical Solid State Physics, RWTH Aachen University, DE-52074 Aachen, Germany
2. Department of Physics, Indian Institute of Technology (ISM) Dhanbad, IN-826004 Dhanbad, India
3. Department of Theoretical Solid State Physics, Institute of Solid State Physics and Optics, HUN-REN Wigner Research Centre for Physics, H-1525 Budapest, Hungary
4. Department of Theoretical Physics, Budapest University of Technology and Economics, H-1111 Budapest, Hungary