PIPT from the Beginning to Future

Abstract

The birth of the field of photoinduced phase transitions was strongly influenced by the conceptual viewpoint expressed by Professor Y. Toyozawa on the condensation of relaxed excitons. Since this first period, twenty years ago, this young field has been expanding rapidly along a diversity of directions. Nowadays, it goes hand in hand with the challenges of today's science: emergence, nonlinearity, coherence, far away from equilibrium, for example. The control of the functionality of a material via photoexcited states poses many new fundamental questions. Some of them will be overviewed: (i) the nature of the control parameters and the nature of the relevant collective variables, especially the order parameters, which characterize the evolution of the system, (ii) the difference between photoinduced transformations under continuous light irradiation and those resulting from an ultrashort laser pulse, (iii) the physical mechanisms of ultrafast photoinduced phase transitions from the formation and proliferation of phototransformed entities to the softening of a collective mode. 78.47.D- Flash-backThis paper is dedicated in memoriam of Professor Yutaka Toyozawa, a founding father of the field of photoinduced phase transitions (PIPT). After the pioneering experimental work of Koshihara on the observation of the photoinduced neutral-to-ionic instability [1], the paper of Prof. Y. Toyozawa, "Condensation of relaxed excitons in static and dynamic phase transitions" [2], is the fingerprint of his inspiring contribution. He brought out some conceptual keys which strongly influenced our research and that of many of our colleagues. In the conclusion of this paper he emphasizes "the usefulness of the global view-point . . . considering the crossover of the excited and the ground electronic states and correlating static and dynamic phase transitions through the motion of (spontaneously or optically) condensed elementary excitations".A second step was the Taniguchi symposium in 1996 "Relaxations of excited states and photoinduced phase transitions" chaired by Prof. Nasu [3]. The aim of this symposium was to discuss the new problem of unconventionally photoactive materials. In these materials the relaxation of optically excited states result in collective motions or drastic structural changes, involving a large number of atoms and electrons. Actually, in a way this symposium was the precursor of the PIPT conferences. The extension of the physics of self-trapped excitons, which Toyozawa has largely contributed to [4], to systems with electron-spin-lattice cooperative interactions was discussed from the viewpoint of both material science and solid state spectroscopy. * corresponding author; e-mail:herve.cailleau@univ-rennes1.frThroughout this first period different theoretical papers were published, developing this approach to establish some backgrounds in the understanding of photoinduced phase transitions [3,5,6]. At the same period, different approaches of photoinduced phase transitions were introduced [7], in particular to t...

Local response to light excitation in the charge-ordered phase of(EDO−TTF)2SbF6

“…The chemical substitution by larger anions compared to the case of the PF 6 derivative can be assimilated to a negative pressure effect and underlines the role of elastic interaction and volume change in the vicinity of phase transition. As a consequence, the transition toward the metallic phase occurs at a lower temperature (T I−M = 242 K) and shows a larger thermal hysteresis (22 K) for (EDO-TTF) 2 SbF 6 compared to (EDO-TTF) 2 PF 6 (T I−M = 278 K, 2.5 K width hysteresis) [10,25]. It can be noted that for all derivatives of the isostructural (EDO-TTF) 2 XF 6 family, the phase transition occurs for almost the same critical value of the D chain cell parameter b and the magnitude of their discontinuous changes at T I−M is nearly the same [10].…”

“…The phase transition at thermal equilibrium of (EDO-TTF) 2 PF 6 between M and I phases reveals different facets such as Peierls distortion and charge ordering, as well as a significant reduction of the orientational disorder of XF 6 counteranions. The main feature is the charge order along donor stack ( [11][12][13].…”

“…During the last decade, the field of ultrafast photoinduced transformation has been expanding rapidly along diverse directions: correlated electrons, charge density waves, magnetism, molecular switching, etc. [1][2][3][4]. A typical example is provided by a quasi-one-dimensional quarter filled organic conductor (EDO-TTF) 2 PF 6 where EDO-TTF is the abbreviation of the electron donor (D) molecule ethylenedioxytetrathiafulvalene.…”

“…[1][2][3][4]. A typical example is provided by a quasi-one-dimensional quarter filled organic conductor (EDO-TTF) 2 PF 6 where EDO-TTF is the abbreviation of the electron donor (D) molecule ethylenedioxytetrathiafulvalene. It exhibits first-order insulator-to-metal (I-M) phase transition at thermal equilibrium and a gigantic ultrafast change of reflectivity under irradiation by a laser pulse in the I phase [5].…”

“…The initial electronic excited state is trapped at the molecular level within 40 fs following the electron-phonon coupled precursor state [7], while the crystal transformation is driven by slower propagation at the 100 ps time scale. Actually, different isostructural compounds of the same (EDO-TTF) 2 XF 6 family, where X stands for P, As, or Sb, have already been synthesized. They undergo similar I-M phase transition at thermal equilibrium.…”