Photoinduced phase transitions in strongly correlated electron systems

The search for materials that show a phase transition triggered by weak external stimulation of light is an important and attractive target for photonic and materials science today. My talk reviews experimental evidence indicating that photo-injected local excitation can really trigger an electron-lattice coupled cooperative phenomenon named as photo-induced phase transition (PIPT: Photo-domino effect) in wide class of materials based on the dynamical x-ray technique combined with the ultrafast spectroscopy. Obtained results also demonstrate that 'hidden phase' in strongly correlated systems which never appears in the phase diagram under thermo-equilibrium condition plays an essential role for the gigantic photo-response of the strongly correlated electron systems.