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.
		What can be and should be done by photo-induced phase transition (PIPT) : from opto-spintronics to ultrafast nonequilibrium statistics Shin-ya Koshihara Japan Topics It is well known fact that various phase transitions can be triggered in condensed matter by the changes in external conditions such as temperature, pressure, electric field and magnetic field. It is an attractive target for materials science to find a system which shows the phase transition triggered by external stimulation of light. Of course, in the research history of the fields of photochemistry and solid-state physics, we can easily find that extensive studies have been made on the structural changes by optical pumping for a long interval. However, those studies have been mainly concerned with microscopic reactions confined in small molecules or local structure changes in crystals. Photo-induced cis-trans isomerization of the stilbene molecule, optically induced defect state in amorphous materials (fatigued photo-luminescence center) [1] and self-trapped exciton in alkaline-halide [2] are typical examples of such a local structure change by photo-excitation. By contrast, the purpose of this work is to review experimental evidences indicating that the photo-injected local excitation can really trigger the macroscopic phase transition by virtue of cooperative interactions such as electric correlation, spin-lattice interaction and electron-lattice coupling. This unique effect induced by photo-excitation which is analogous to domino effect has been named as photo-induced phase transition (from now on, abbreviated as PIPT). Over sixteen years, extensive studies on PIPT have been accumulated and realistic examples have been reported for various materials such as charge transfer (CT) crystals [3], p-conjugated polymers [4], transition metal-oxides [5], spin crossover complexes [6], prussian-blue analogs [7], quantum structure of diluted magnetic semiconductors [8], low-dimensional transition metal organo complexes [9] and A2B molecular crystals [10]. In all examples, it is essential to utilize the intrinsic instability in electronic, magnetic and structural properties due to cooperative interactions for enhancement of photo-response. Especially, in case of the p-conjugated polymers which was the first example of the reversible PIPT, the bistability in free energy plays an essential role for promoting the growth process from localized excitations into macroscopic phase change. In the following part, we review the works on the dynamical behavior of PIPT in p-conjugated polymer as a typical example [4]. We also review our recent work on ultra fast dynamics of PIPT in 1/4 filled molecular crystals relating with the importance of femto-second (fs) time-resolved structural study. [11] References [1] T.Tada, Ph. D. Thesis, The University of Tokyo (1990). [2] N.Itho, in 'Optical Properties of Solids and Electron- lattice Interaction', Solid State Physics, Special Issue, ed. K.Nasu, (Agne Gijutsu Center, Tokyo, 1987). [3] R.S.Potember, T.O.Poehler and R.C.Benson, Appl. Phys. Lett. 41, 548 (1982). S.Koshihara, Y.Tokura, T.Mitani, G.Saito and T.Koda, Phys. Rev. B42, 6853 (1990). S.Koshihara, Y.Tokura, Y.Iwasa and T.Koda, Phys. Rev.B44, 431 (1991). S.Koshihara, Y.Takahashi, H.Sakai, Y.Tokura and T.Luty, J. Phys. Chem.B103, 2592 (1999). S. Iwai et.al., Phys. Rev. Lett. 88, 57402, (2002). E.Collet et.al., Science 300, 612 (2003). H. Okamoto et.al., Phys. Rev. B70, 165202, (2004). [4] S.Koshihara, Y.Tokura, K.Takeda and T.Koda, Phys. Rev. Lett. 68, 1149 (1992). S.Koshihara, Y.Tokura, K.Takeda and T.Koda, Phys. Rev. B52, 6265 (1995). [5] M.Fiebig, K.Miyano, Y.Tomioka and Y.Tokura, Science 280, 1925 (1998). N.Ogawa et.al., Phys. Rev. Lett. 87, 256401 (2001). A.Cavalleri et al., Phys. Rev. Lett. 87, 237401 (2001). [6] P.Gutlich, A.Hauser and H.Spoering, Angew. Chem. 33, 2024 (1994). Y.Ogawa et.al., Phys. Rev. Lett. 84, 3181 (2000). C. Enachescu, U. Oetliker, and A. Hauser, J. Phys. Chem. B 106, 9540 (2002). N.Huby et.al., Phys. Rev. B69, 20101 (2004) [7] O.Sato, T.Iyoda, Afijishima and K.Hashimoto, Science 272, 704 (1996). [8] S. Koshihara et al., Phys. Rev. Lett. 78, 4617 (1997). [9] S.Iwai et.al, Phys. Rev. Lett. 91, 57401 (2003). [10] F.O.Karutz, J.U.von Schutz, H.Wachtel and H.C.Wolf, Phys.Rev.Lett. 81, 140 (1998). N.Uchida et.al., J.Phys. IV France 114, 143 (2004) (Proceedings of ISCOM2003). N.Tajima et.al., J.Phys. Soc. Jpn. 74, 511 (2005). [11] M.Chollet et.al., Science 307, 86 (2005).