1. Faculty of Physics and Earth Sciences, Felix Bloch Institute for Solid State Physics, Applied Quantum Systems, Leipzig University 1 , Linnéstrasse 5, Leipzig 04103, Germany
2. Accelerator Technology and Applied Physics Division, Lawrence Berkeley National Laboratory 2 , Berkeley, California 94720, USA
3. Materials Sciences Division, Lawrence Berkeley National Laboratory 3 , Berkeley, California 94720, USA
4. Department of Electrical Engineering and Computer Sciences, University of California 4 , Berkeley, California 94720, USA