1. Department of Physics and Astronomy, QTF Centre of Excellence, Turku Centre for Quantum Physics, University of Turku, Turku 20014, Finland
2. Department of Mathematics, Politehnica University of Timişoara, Victoriei Square 2, 300006 Timişoara, Romania
3. Laboratoire de Physique Théorique, Université de Toulouse, CNRS, UPS, France