1. Fraunhofer Institute for Mechanics of Materials IWM, MicroTribology Center μTC 1 , Woehlerstraße 11, Freiburg 79108, Germany
2. Department of Physics, University of Freiburg 2 , Hermann-Herder-Straße 3, Freiburg 79104, Germany
3. Theoretical Physics Division, Department of Physics, Chemistry, and Biology, Linköping University 3 , Linköping SE-58183, Sweden
4. Department of Physics, University of Helsinki 4 , P.O. Box 43, Helsinki FI-00014, Finland
5. Department of Physics, KTH Royal Institute of Technology 5 , Roslagstullsbacken 21, Stockholm 114 21, Sweden