1. Department of Quantum Nanoscience, Kavli Institute of Nanoscience, Delft University of Technology, 2628 CJ Delft, The Netherlands
2. Huygens-Kamerlingh Onnes Laboratorium, Leiden University, 2300 RA Leiden, The Netherlands
3. Center for Memory and Recording Research, University of California, San Diego, California 92093-0401, USA