In some low-dimensional charge transfer (CT) crystals, for example the ionic (I) and neutral (N) states are energetically nearly degenerated and the changes in temperature and pressure can induce the phase transition between neutral and ionic states (N-I transition). We have revealed that the cooperative 3D Coulomb interaction and charge-lattice coupling play an essential role in the driving mechanism of this unconventional phase transition. In addition, lately we have reported that N-I transition can be controlled by photoexcitation. This finding triggered the extensive studies of the photoinduced phase transition and this project. Because the N- I transition can be characterized as a condensation of relaxed excited state which is an exciting target from the view point of solid state and statistical physics.
The importance of the research of this class of material will be expanded into the field of magneto-optics and solid state chemistry. For systems with strong spin-lattice interaction, magneto-transport and magneto-optical properties can be sensitively changed by photo-excitation. The research on structural multistability will give also physical foundation for reactivity in solid state chemistry (soft organic chemistry): photo-isomerization, -helical structural change, - polymerization, - transfer of molecular groups etc.. From this point of view, the N-I transition can be classified as a cooperative CT reaction in solid. To make clear the physical and chemical foundations of the photoinduced cooperative phenomena, we have started the dynamic study of the phase transition in model materials using time-resolved X-ray measurement techniques. We have achieved the various PIPT in exotic materials such as N-I, spin-Peierls, spin-crossover, photo-ferromagnetism, photo-ferroelectricity and photo control of charge order by virtue of the time-resolved spectroscopy, magneto-optical measurement technique, time-resolved structural analysis and collaboration with synthetic chemists.
The idea proposed in our research will make important and original contributions mainly to the various basic research fields in solid state physics and chemistry, and nonequilibrium statistical physics. The physical concepts which are studied experimentally and theoretically in this project will also supply a new point of view for the studies of soft-materials and complex systems. In addition, we believe that it is also important for giving the impact for the fields of application of this class of materials such as photo-control and /or switching of the magnetic and dielectric memories. We are promoting the research in this direction deeply collaborating with Dr. K.Onda.