1. Department of Physics, Stockholm University, SE-10691 Stockholm, Sweden
2. Department of Molecular Sciences and Nanosystems, Ca' Foscari University of Venice, 30172 Venice, Italy
3. Linear Coherent Light Source, SLAC National Accelerator Laboratory, Menlo Park, California 94025, USA
4. CIC nanoGUNE BRTA, San Sebastian, Spain and IKERBASQUE, Basque Foundation for Science, Bilbao, Spain
5. Department of Physics, Emory University, Atlanta, Georgia 30322, USA