1. Department of Physics and Astronomy; University of Nebraska; Lincoln NE 68588 USA
2. Department of Materials Science and Engineering; University of Wisconsin-Madison; Madison WI 53706 USA
3. Crete Center for Quantum Complexity and Nanotechnology; University of Crete; Heraklion 71003 Greece
4. Department of Physics; University of Crete and FORTH; Heraklion 71003 Greece
5. Division of Physics and Applied Physics; School of Physical and Mathematical Sciences; Nanyang Technological University; Singapore 637371 Singapore