1. Data Storage Institute, A*STAR (Agency for Science, Technology and Research), 138634 Singapore.
2. Nonlinear Physics Centre, Research School of Physics and Engineering, Australian National University, Canberra, ACT 2601, Australia.
3. Geballe Laboratory for Advanced Materials, Stanford University, Stanford CA 94305, USA.
4. Division of Physics and Applied Physics, School of Physical and Mathematical Sciences, Nanyang Technological University, 637371 Singapore.