Darine GHONEIM : Sliding Charge Density Wave Systems Probed by MHz-Repetition-Rate X-ray Free-Electron Lasers

In this thesis, we studied the sliding phenomenon in Charge Density Wave (CDW) systems under a weak electric current. Two systems were studied, NbSe3 and TbTe3. Both systems can slide but differ in the dimensionality of their atomic structures: the former is quasi-1D, and the latter is quasi-2D. To do this, we used an XFEL source that allowed us to observe, by X-ray diffraction, the satellite reflection associated with the CDW, at equilibrium and in the sliding regime, as a function of the ultra-short, MHz-pulsed X-ray pulses generated by the XFEL source. These in situ experiments were conducted at low temperature and under an applied external current at the MID beamline of the European XFEL. There are significant differences between the two systems. In the equilibrium state of NbSe3, we observe a very high sensitivity of the CDW to X-ray pulses. These changes have been observed thanks to the high repetition rate of the source. We show that the evolution of the CDW state with successive pulses cannot be attributed to heating induced by the X-ray beam. It could be related to a reduction in electron-phonon coupling and, consequently, in the CDW gap induced by the X-ray beam. The sliding regime of NbSe3, by contrast, is very well observed through a modification of the satellite peak. No change in the equilibrium state of TbTe3 by the X-ray beam was observed, unlike NbSe3. However, as with NbSe3, the sliding state was clearly observed through a change in the satellite peak associated with the CDW. However, we observe that the host lattice is also involved in the sliding regime of TbTe3, unlike in NbSe3, raising questions about the nature of charge transport in these two systems that underlie this difference.