1. Joseph Henry Laboratories and Department of Physics, Princeton University, Princeton, NJ 08544, USA.
2. Department of Physics, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA.
3. Condensed Matter Physics and Materials Science, Brookhaven National Laboratory (BNL), Upton, NY 11973, USA.
4. Central Research Institute of Electric Power Industry, Komae, Tokyo 201-8511, Japan.
5. Institute of Scientific and Industrial Research, Osaka University, Ibaraki, Osaka 567-0047, Japan.