1. Centro S3, Istituto di Nanoscienze-CNR, 41125 Modena, Italy
2. Institute of Physics, Laboratory for Ultrafast Microscopy and Electron Scattering (LUMES), École Polytechnique Fédérale de Lausanne, Station 6, Lausanne 1015, Switzerland
3. Department of Materials Science, University of Milano-Bicocca, Via Cozzi 55, Milano 20121, Italy
4. Department of Electrical Engineering, Technion-Israel Institute of Technology, Haifa 32000, Israel
5. Ernst Ruska-Centre for Microscopy and Spectroscopy with Electrons and Peter Grünberg Institute, Forschungszentrum Jülich, Jülich 52425, Germany
6. Dipartimento FIM, Universitá di Modena e Reggio Emilia, Modena 41125, Italy
7. Department of Physics, University of Ottawa, 25 Templeton Street, Ottawa, Ontario K1N 6N5, Canada