1. Non-Equilibrium Dynamics Project, ERATO-JST, c/o KEK, Tsukuba, Ibaraki 305-0801
2. Synchrotron Radiation Research Center, JAEA/SPring-8, Hyogo 679-5148
3. Institute for Material Reseach, Tohoku University, Sendai 980-8577
4. Department of Physics, University of Tokyo, Tokyo 113-0033
5. Departamento de Física Teórica de la Materia Condensada, Universidad Autónoma de Madrid, Madrid 28049, Spain