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Study of the structural dynamics of phase transitions using time resolved electron and X-ray diffraction

Abstract : The application of an external perturbation in certain materials (such as temperature, pressure or light) often gives rise to the emergence of new macroscopic properties with their origin at the atomic level. Therefore, a detailed study of the atomic dynamics becomes essential to the understanding of processes such as chemical transformations or phase transitions. In the particular case of structural phase transitions, the symmetry of the crystal undergoes a transformation between two different states at a given critical value. The typical time scales of the structural dynamics occur on the order of few hundreds of femtoseconds to several picoseconds. The development of femtosecond laser pulses has enabled scientists to access the required time scales to explore the ultrafast dynamics of the lattice structure in the relevant time scales. More precisely, time-resolved diffraction has proven to be an ideal technique to track and unveil the out-of-equilibrium pathways followed by the lattice after a short laser pulse.This thesis presents experimental studies performed by time-resolved electron and X-ray diffraction techniques on two different structural phase transitions. In a first part, we demonstrate the capabilities of the ultrafast electron diffraction (UED) experimental setup developed at Laboratoire d’Optique Apliquée. Our UED experimental results performed on high quality single crystal samples are presented along with a quantitative study of the implications of dynamical diffraction effects in UED experiments. In a second part, we present a set of pump-probe electron diffraction experiments performed on GdTe₃, a compound belonging to the rare-earth Tritellurides family which presents a charge density wave state. The arrival of an optical excitation triggers the phase transition non-thermally with the out-of-equilibrium relaxation dynamics of the charge density wave state characterized by slowing down that increases with incident fluence as well as with initial sample temperature. These results shed more light on current controversial interpretations involving the emergence of photoinduced topological defects. In a third part, we present time resolved X-ray diffraction experiments performed at CRISTAL beamline at SOLEIL synchrotron. In this case we have studied the A15 compound Nb₃Sn, which displays a displacive phase transition evolving from cubic to tetragonal symmetry at thermal equilibrium. Our pump-probe results present evidence of a different lattice response with respect to the thermal equilibrium transition, encouragingf uture investigations on the dynamics of this material.
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Submitted on : Saturday, May 2, 2020 - 1:06:57 AM
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Isabel Gonzalez Vallejo. Study of the structural dynamics of phase transitions using time resolved electron and X-ray diffraction. Other [cond-mat.other]. Université Paris Saclay (COmUE), 2019. English. ⟨NNT : 2019SACLS496⟩. ⟨tel-02560554⟩



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