1. Max Planck Institute for Chemical Physics of Solids Nöthnitzer Straße 40 01187 Dresden Germany
2. Department of Physics National Tsing Hua University 101, Section 2, Kuang Fu Road Hsinchu 30013 Taiwan
3. Department of Physics and Astronomy University of British Columbia Vancouver BC V6T 1Z1 Canada
4. Graduate Institute of Applied Physics and Department of Physics National Taiwan University Taipei 10617 Taiwan