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Physics of Neutral-to-Ionic Phase Transition in Organic Charge Transfer Semiconducting Compounds

100%
Cailleau H., Lemée-Cailleau M. H., Le Cointe M., Luty T.
Acta Physica Polonica A
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1997
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vol. 92
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issue 4
597-608
EN
An uncommon excitonic instability takes place in some exotic semiconducting compounds. Indeed, the equilibrium neutral-to-ionic (N-I) phase transition, as well as the non-equilibrium photo-induced phase transformation, observed in some organic charge-transfer complexes, originate from intra- and inter-chain cooperative effects between structurally relaxed charge-transfer excitations. This electronic-structural phase transition manifests itself by a change of the degree of charge-transfer and a dimerization distortion with the formation of donor-acceptor pairs along the stacking axis in the I phase. Thermal charge-transfer excitations associated with the formation of I strings along N chains are at the heart of the mechanism of this phase transition. These relaxed electronic excitations, which are an intrinsic feature of low-dimensional systems with strong electron-phonon coupling, can be described in terms of self-trapping and self-multiplication of charge-transfer excitons. Precise structural studies on the prototype compound, tetrathiafulvalene-p-chloranil allow to highlight the respective role taken by the ionicity and the dimerization. Symmetry and thermodynamics analysis of the N-I transition, based on recent determination of the pressure-temperature phase diagram, make possible to present a consistent picture of this phase transition. Supported by theoretical considerations taking into account the interplay between quantum and thermal effects, the experimental observations show that the N-I transition results from the condensation and the ordering (crystallization) of charge-transfer excitations, following a phase diagram analogous to the solid-liquid-gas one.
2
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White Light Snapshot of Non-Equilibrium High-Spin State in a Molecular Crystal

76%
Kaszub W., Collet E., Cailleau H., Servol M., Boillot M., Tissot A., Lorenc M.
Acta Physica Polonica A
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2012
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vol. 121
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issue 2
324-327
EN
The spin-crossover compound [(TPA)Fe(TCC)]PF_6 was experimentally investigated using time-resolved optical pump and supercontinuum probe technique. Changes of VIS optical density at thermal equilibrium of a crystal with different high-spin/low-spin fractions are compared to changes recorded picoseconds after femtosecond laser excitation. A white light snapshot of the high-spin state at 180 K is extracted from time-resolved spectra. Our results indicate that the temperature of the electronic state affects the absorption spectra of high- and low-spin states.
3
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PIPT from the Beginning to Future

76%
Cailleau H., Luty T., Koshihara S., Servol M., Lorenc M., Buron-Le Cointe M., Collet E.
Acta Physica Polonica A
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2012
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vol. 121
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issue 2
297-306
EN
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.
4
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Time-Resolved Pump-Probe Diffraction Experiment at SOLEIL Synchrotron: Photoinduced Spin Transition in the Molecular Crystal [TPA Fe(III) TCC] PF_6

52%
Laulhé C., Ravy S., Fertey P., Elkaim E., Legrand F., Féret P., Hollander P., Hustache S., Bordessoule M., Ricaud J., Collet E., Lorenc M., Buron-Le Cointe M., Cailleau H., Tissot A., Boillot M.
Acta Physica Polonica A
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2012
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vol. 121
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issue 2
332-335
EN
We present the first time-resolved pump-probe experiment performed at synchrotron SOLEIL at the CRISTAL diffraction beamline. The time-resolved setup will be used in the near future for sub-ns time resolution experiments. We studied spin state switching in a [TPA Fe(III) TCC] PF_6 single crystal as induced by a ≈6 ns laser pulse (1064 nm), by measuring the 3D diffraction volume of the 002 Bragg reflection as a function of time after excitation. The intensity profiles (rocking curves) are found to evolve at two characteristic timescales, namely hundreds of ns and few μs, exhibiting subtle profile changes and peak broadenings. Consistently with previous studies, we interpret these features as due to structural inhomogeneities related to laser-induced deformation wave propagation and heat diffusion, which both start from the absorbing sites.
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