arXiv subjects

Matthieu Chollet

Publications and source records attributed to Matthieu Chollet.

23 records · Page 2Linked to original sources

arXiv2017

Non-equilibrium phase precursors to the insulator-metal transition in V2O3

The discovery of novel phases of matter is at the core of modern physics. In quantum materials, subtle variations in atomic-scale interactions can induce dramatic changes in macroscopic properties and drive phase transitions. Despite their importance, the mesoscale processes underpinning phase transitions often remain elusive because of the vast differences in timescales between atomic and electronic changes and thermodynamic transformations. Here, we photoinduce and directly observe with x-ray scattering an ultrafast enhancement of the structural long-range order in the archetypal Mott system V2O3. Despite the ultrafast change in crystal symmetry, the change of unit cell volume occurs an order of magnitude slower and coincides with the insulator-to-metal transition. The decoupling between the two structural responses in the time domain highlights the existence of a transient photoinduced precursor phase, which is distinct from the two structural phases present in equilibrium. X-ray nanoscopy reveals that acoustic phonons trapped in nanoscale blocks govern the dynamics of the ultrafast transition into the precursor phase, while nucleation and growth of metallic domains dictate the duration of the slower transition into the metallic phase. The enhancement of the long-range order before completion of the electronic transition demonstrates the critical role the non-equilibrium structural phases play during electronic phase transitions in correlated electrons systems.

cond-mat.str-el↗

arXiv2016

Femtosecond X-Ray Scattering Study of Ultrafast Photoinduced Structural Dynamics in Solvated [Co(terpy)2]2+

We study the structural dynamics of photoexcited [Co(terpy)2]2+ in an aqueous solution with ultrafast x-ray diffuse scattering experiments conducted at the Linac Coherent Light Source. Through direct comparisons with density functional theory calculations, our analysis shows that the photoexcitation event leads to elongation of the Co-N bonds, followed by coherent Co-N bond length oscillations arising from the impulsive excitation of a vibrational mode dominated by the symmetrical stretch of all six Co-N bonds. This mode has a period of 0.33 ps and decays on a subpicosecond time scale. We find that the equilibrium bond-elongated structure of the high spin state is established on a single-picosecond time scale and that this state has a lifetime of ~ 7 ps.

physics.chem-ph↗

arXiv2015

Watching coherent molecular structural dynamics during photoreaction: beyond kinetic description

A deep understanding of molecular photo-transformations is challenging because of the complex interaction between the configurations of electrons and nuclei. An initial optical excitation dissipates energy into electronic and structural degrees of freedom, often in less than one trillionth (10^-12) of a second. Molecular dynamics induced by photoexcitation have been very successfully studied with femtosecond optical spectroscopies, but electronic and nuclear dynamics are often very difficult to disentangle. X-ray based spectroscopies can reduce the ambiguity between theoretical models and experimental data, but it is only with the recent development of bright ultrafast X-ray sources, that key information during transient molecular processes can be obtained on their intrinsic timescale. In this letter, Free Electron Laser (FEL) radiation is used to measure ultrafast changes in the X-ray Absorption Near Edge Structure (XANES) during the prototypical photoreaction of a spin crossover compound. We reveal its transformation from the ligand-located electronic photoexcitation to the structural trapping of the high spin state. The results require a description beyond a kinetic model and provide a direct observation of a dynamic breathing of the main structural change. The coherent structural oscillations (period of ~265 fs) in the photoproduct potential lose synchrony within ~330 fs, whereas incoherent motions reveal the energy redistribution and vibrational cooling within ~1.6 ps. We foresee that ultrafast X-ray spectroscopies will provide invaluable insight to understand the complex physics of fundamental light induced phenomena, which are of prime interest in a multitude of chemical, physical and biological processes.

physics.chem-ph↗

arXiv2015

Phonon Spectroscopy with Sub-meV Resolution by Femtosecond X-ray Diffuse Scattering

We present a reconstruction of the transverse acoustic phonon dispersion of germanium from femtosecond time-resolved x-ray diffuse scattering measurements at the Linac Coherent Light Source. We demonstrate an energy resolution of 0.3 meV with momentum resolution of 0.01 nm^-1 using 10 keV x-rays with a bandwidth of ~ 1 eV. This high resolution was achieved simultaneously for a large section of reciprocal space including regions closely following three of the principle symmetry directions. The phonon dispersion was reconstructed with less than three hours of measurement time, during which neither the x-ray energy, the sample orientation, nor the detector position were scanned. These results demonstrate how time-domain measurements can complement conventional frequency domain inelastic scattering techniques.

cond-mat.mtrl-sci↗

arXiv2013

Performance of an LPD prototype detector at MHz frame rates under Synchrotron and FEL radiation

A MHz frame rate X-ray area detector (LPD - Large Pixel Detector) is under development by the Rutherford Appleton Laboratory for the European XFEL. The detector will have 1 million pixels and allows analogue storage of 512 images taken at 4.5 MHz in the detector front end. The LPD detector has 500 mm thick silicon sensor tiles that are bump bonded to a readout ASIC. The ASICs preamplifier provides relatively low noise at high speed which results in a high dynamic range of 10^5 photons over an energy range of 5-20 keV. Small scale prototypes of 32x256 pixels (LPD 2-Tile detector) and 256x256 pixels (LPD supermodule detector) are now available for X-ray tests. The performance of prototypes of the detector is reported for first tests under synchrotron radiation (PETRA III at DESY) and Free-Electron-Laser radiation (LCLS at SLAC). The initial performance of the detector in terms of signal range and noise, radiation hardness and spatial and temporal response are reported. The main result is that the 4.5 MHz sampling detection chain is reliably working, including the analogue on-chip memory concept. The detector is at least radiation hard up to 5 MGy at 12 keV. In addition the multiple gain concept has been demonstrated over a dynamic range to 10^4 at 12 keV with a readout noise equivalent to <1 photon rms in its most sensitive mode.

physics.ins-det↗