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Extremely high pressures over a million of atmospheres are required to deform diamonds permanently. Under dynamic high-pressure conditions, even such strong materials lose their strengths so rapidly that the initially pristine lattice transforms into complex dynamics. Here, we report femtosecond x-ray diffraction observations that directly resolve how shock waves deform the crystal lattice in the isotropic nano-polycrystalline form of diamond. The results show that the nano-grain reinforced diamond retains its strength at shock pressures far beyond its elastic limit until it finally approaches zero at 707 GPa, indicating the existence of brittle-ductile transition of nano-polycrystalline diamond under high-strain rate shock compression. The atomic-level deformation process of the diamond observed in this study is a key benchmark for designing high strength materials and simulating their behavior under extreme conditions.

International audience; The x-ray free electron laser (XFEL) enables probing highly compressed material response at the subnanosecond timescale. We exploit the ultrafast XFEL pulse to combine reflection x-ray diffraction and laserdriven shock compression to perform a study of phase transformation and stability in iron and Fe-Si alloys. Our approach enables us to observe that solid-solid phase transformations occur in iron and Fe-Si8.5wt% in ≤130 ps at ∼130 GPa; no transformation is observed in Fe-Si16wt% up to 110 GPa. Density Functional Theory calculations predict similar phase relations.

Polarization control using an X-ray phase retarder in combination with an arrival timing diagnostic on BL3 of SACLA is reported., Control of the polarization of an X-ray free-electron laser (XFEL) has been performed using an X-ray phase retarder (XPR) in combination with an arrival timing diagnostic on BL3 of the SPring-8 Angstrom Compact free-electron LAser (SACLA). To combine with the timing diagnostic, a pink beam was incident on the XPR crystal and then monochromated in the vicinity of samples. A high degree of circular polarization of ∼97% was obtained experimentally at 11.567 keV, which agreed with calculations based on the dynamical theory of X-ray diffraction. This system enables pump–probe experiments to be operated using circular polarization with a time resolution of 40 fs to investigate ultrafast magnetic phenomena.

We present here a new diagnostics based on using LiF crystal detectors that are able to perform measurements an intensity distribution of X-rays beams with diameters ranging from some microns up to some centimeters with high spatial resolution (~1 micron). in situ, 3D visualization of SACLA XFEL focused beam profile along propagation, including propagation inside photoluminescence solid materials, is demonstrated.

Direct metrology of coherent short-wavelength beamlines is important for obtaining operational beam characteristics at the experimental site. However, since beam-time limitation imposes fast metrology procedures, a multi- parametric metrology from as low as a single shot is desirable. Here a two- dimensional (2D) procedure based on high-resolution Fresnel diffraction analysis is discussed and applied, which allowed an efficient and detailed beamline characterization at the SACLA XFEL. So far, the potential of Fresnel diffraction for beamline metrology has not been fully exploited because its high- frequency fringes could be only partly resolved with ordinary pixel-limited detectors. Using the high-spatial-frequency imaging capability of an irradiated LiF crystal, 2D information of the coherence degree, beam divergence and beam quality factor M2 were retrieved from simple diffraction patterns. The developed beam metrology was validated with a laboratory reference laser, and then successfully applied at a beamline facility, in agreement with the source specifications.

We performed single-shot spectrometry to evaluate the pulse duration of the soft X-ray free electron laser (FEL) at BL1 of SPring-8 Angstrom Compact free electron LAser (SACLA). The soft X-ray FEL beam was focused with a Kirkpatrick-Baez (KB) focusing mirror system and single-shot spectra were measured with a grating and a two dimensional detector. The spike widths of the spectra, which are inversely proportional to the pulse duration, varied when we changed the operation conditions of the accelerator. By comparing the spike widths with a simulation, the pulse duration was evaluated to be less than 100 fs.

High energy density physics 32, 63 - 69 (2019). doi:10.1016/j.hedp.2019.05.002, We present results from the SPring-8 Angstrom Compact free electron LAser (SACLA) X-ray free electron laser (XFEL) facility, using an X-ray pump, X-ray probe scheme to observe ultrafast changes in the structure of heated graphite. The 9.8 keV XFEL beam was focused to give an intensity on the order of ∼ 10$^{19}$ W/cm$^2$, and the evolution of the diffraction pattern observed up to delays of 300 fs. The interplanar diffraction peaks weaken significantly within 10s of femtoseconds, but in-plane diffraction orders i.e. those with Miller Index $(hk0)$, persist up to 300 fs, with the observed signal increasing. We interpret this as nonthermal damage through the breaking of interplanar bonds, which at longer timescales leads to ablation by removal of intact graphite sheets. Post-experiment examination of the graphite samples shows damage which is comparable in size to the range of the excited photoelectrons. These results highlight the challenges of accurately modelling X-ray driven heating, as it becomes a routine approach to generating high energy density states., Published by Elsevier, Amsterdam [u.a.]

High intensity X-ray Free Electron Lasers (XFEL) are an ideal tool to heat materials directly and isochorically, which can cause them to be modified and damaged irreversibly. During XFEL-matter interactions, the energy of an XFEL beam will be mainly absorbed by photoionization, creating numerous high-energy photo- and Auger electrons. Modelling this process is quite complex since both atomic physics and plasma physics are involved. Atomic collisional-radiative (CR) codes such as FLYCHK are widely used to simulate such processes. However, the CR codes typically assume local thermodynamic equilibrium (LTE) and are limited to zero dimension. In order to understand the sample damaging mechanisms, we performed two-dimensional kinetic particle-in-cell simulations with radiation transport (PIC-RT) to retrieve the temporal processes of XFEL-matter interactions. The dynamics of XFEL-Matter interactions can roughly divide into three different time scales: 1) electron heating and photoionization by XFEL in ~ 10 fs ; 2) collisional heating and ionization by high-energy photo- and Auger electrons with several keV energy in tens of fs to sub-ps; and 3) collective hydrodynamic explosion driven by ~ TPa thermal pressure from ~100 fs to nanoseconds. The simulation results are compared to our recent experiment that a variety of samples were irradiated by Japanese XFEL SACLA with intensities on the order of 10^20 W/cm^2. The post-analysis of the irradiated samples showed that large holes with radius sizes more than one order of magnitude higher than the XFEL spot were created for metallic samples. The hole size is also much larger than the stopping range of high energy electrons. According to our PIC-RT simulations, we attribute the generation of such large holes to the micro-explosion process. Kinect simulation of the hole generation with multiple time scales is also useful and complementary to understand the change of X-Ray diffraction pattern in the experiment that infers significant material structural change on femtosecond timescales.

The control of materials properties with light is a promising approach towards the realization of faster and smaller electronic devices. With phases that can be controlled via strain, pressure, chemical composition or dimensionality, nickelates are good candidates for the development of a new generation of high performance and low consumption devices. Here we analyze the photoinduced dynamics in a single crystalline NdNiO$_3$ film upon excitation across the electronic gap. Using time-resolved reflectivity and resonant x-ray diffraction, we show that the pump pulse induces an insulator-to-metal transition, accompanied by the melting of the charge order. Finally we compare our results to similar studies in manganites and show that the same model can be used to describe the dynamics in nickelates, hinting towards a unified description of these photoinduced phase transitions., Comment: 17 pages, 6 figures