Organized by the Institute of Experimental Physics, University of Bialystok and the Institute of Atomic Energy, Swierk under the auspices of the European Crystallographic Association; Committee of Crystallography, the Polish Academy of Sciences; and the Polish Neutron Scattering Society The School is financially supported in part by the International Union of Crystallography; the European Crystallographic Association; the European Office of Aerospace Research & Development; the Ministry of Science and Information Society Technologies; the Warsaw University of Technology; and the University of Bialystok.
		Theory of photoinduced phase transitions: from semiclassical to quantum aspects Tetsuo Ogawa Osaka University, Japan Topics I review recent progress of theoretical studies for the photoinduced phase transitions (PIPTs) to clarify what the PIPTs are. The PIPTs are classified into two types: (a) global phase change via optically excited states and (b) new phase creation in optically excited states. First, concerning with (a), photoinduced structural phase transitions via excited electronic states are discussed theoretically using a one-dimensional model composed of localized electrons and lattices under the adiabatic or diabatic approximation. I show that the global structural change by photoexcitation only at a site is possible, and I clarify conditions for the occurrence of such phenomena. Spatiotemporal dynamics of nonequilibrium first-order phase transitions is also investigated in detail in terms of photoinduced nucleations and domino processes of the domain boundaries (domain walls), which are in striking contrast to the mean-field dynamics. In the adiabatic regime, after the spontaneous emission of a photon, an initial local structural change (i) remains locally, (ii) induces cooperatively a global structural change, or (iii) disappears and returns to the initial phase. Dynamical features of the case (ii) are characterized by the deterministic domino process; domain walls between the two phases move deterministically at a constant velocity (with changing speed) without further spontaneous emissions in the case of strong (weak) dissipation. In the diabatic regime, similar three types of structural change exist. The domain-wall dynamics is described as the stochastic domino process, which is accompanied by the successive radiative transitions. Second, concerning with (b), I discuss quantum states of electron-hole systems, which are optically excited states consisting of many electrons and holes in two bands. In particular, the exciton Mott transition, the "from-insulator-to-metal transition" of the electron-hole systems as the particle density increases is introduced in detail. In the one-dimensional case, bozonization technique and the renormalization group method are employed. The one-dimensional systems are found to be insulating even at the high density limit and that the exciton Mott transition never occurs at absolute zero temperature. The insulating ground state exhibits a strong instability towards the crystallization of biexcitons. In the case of higher dimensions, we analyze a two-band Hubbard model with interactions of electron-electron (hole-hole) repulsion U and electron-hole attraction -U'. With the use of the dynamical mean-field theory, the phase diagram in the U-U' plane is obtained (which is exact in infinite dimensions) assuming that electron-hole pairs do not condense. When both electron and hole bands are half-filled, two types of insulating states appear: the Mott-Hubbard insulator for U>U' and the biexciton-like insulator for U