1. Centre de Nanosciences et de Nanotechnologies, CNRS UMR 9001, Université Paris-Sud, Université Paris-Saclay, C2N-Orsay, 91405 Orsay cedex, France
2. Laboratorio TASC, CNR-IOM, Area Science Park, S.S. 14 km 163.5, Basovizza, I-34149 Trieste, Italy
3. Department of Physics and Astronomy, University of Southampton, Highfield Campus, Southampton SO17 1BJ, United Kingdom
4. INO-CNR BEC Center and Dipartimento di Fisica, Universita di Trento, I-38123 Povo, Italy