1. Center for Micro‐ and Nanotechnologies, Technical University Ilmenau, 98688 Ilmenau, Germany
2. Walter Schottky Institute, TU‐Munich, Am Coulombwall, 85748 Garching, Germany
3. Instituto Nazionale per la Fisica della Materia, Dipartimento di Fisica, Università degli Studi di Cagliari, Italy
4. Electrical and Computer Engineering Department and Materials Department, College of Engineering, University of California, Santa Barbara, California 93106, USA
5. Electrical and Computer Engineering, Cornell University, Ithaca, NY 14853‐5401, USA