106 research outputs found
Compact femtosecond electron diffractometer with 100 keV electron bunches approaching the single-electron pulse duration limit
We present the design and implementation of a highly compact femtosecond
electron diffractometer working at electron energies up to 100 keV. We use a
multi-body particle tracing code to simulate electron bunch propagation through
the setup and to calculate pulse durations at the sample position. Our
simulations show that electron bunches containing few thousands of electrons
per bunch are only weakly broadened by space-charge effects and their pulse
duration is thus close to the one of a single-electron wavepacket. With our
compact setup we can create electron bunches containing up to 5000 electrons
with a pulse duration below 100 femtoseconds on the sample. We use the
diffractometer to track the energy transfer from photoexcited electrons to the
lattice in a thin film of titanium. This process takes place on the timescale
of few-hundred femtoseconds and a fully equilibrated state is reached within
one picosecond.Comment: 5 pages, 3 figure
Experimental Investigation of Plastic Deformations Before Granular Avalanche
We present an experimental study of the deformation inside a granular
material that is progressively tilted. We investigate the deformation before
the avalanche with a spatially resolved Diffusive Wave Spectroscopy setup. At
the beginning of the inclination process, we first observe localized and
isolated events in the bulk, with a density which decreases with the depth. As
the angle of inclination increases, series of micro-failures occur periodically
in the bulk, and finally a granular avalanche takes place. The micro-failures
are observed only when the tilt angles are larger than a threshold angle much
smaller than the granular avalanche angle. We have characterized the density of
reorganizations and the localization of micro-failures. We have also explored
the effect of the nature of the grains, the relative humidity conditions and
the packing fraction of the sample. We discuss those observations in the
framework of the plasticity of granular matter. Micro-failures may then be
viewed as the result of the accumulation of numerous plastic events
Momentum-Resolved View of Electron-Phonon Coupling in Multilayer WSe
We investigate the interactions of photoexcited carriers with lattice
vibrations in thin films of the layered transition metal dichalcogenide (TMDC)
WSe. Employing femtosecond electron diffraction with monocrystalline
samples and first principle density functional theory calculations, we obtain a
momentum-resolved picture of the energy-transfer from excited electrons to
phonons. The measured momentum-dependent phonon population dynamics are
compared to first principle calculations of the phonon linewidth and can be
rationalized in terms of electronic phase-space arguments. The relaxation of
excited states in the conduction band is dominated by intervalley scattering
between valleys and the emission of zone-boundary phonons.
Transiently, the momentum-dependent electron-phonon coupling leads to a
non-thermal phonon distribution, which, on longer timescales, relaxes to a
thermal distribution via electron-phonon and phonon-phonon collisions. Our
results constitute a basis for monitoring and predicting out of equilibrium
electrical and thermal transport properties for nanoscale applications of
TMDCs
Femtosecond photoswitching dynamics and microsecond thermal conversion driven by laser heating in FeIII spin-crossover solids.
International audienceIn this paper we review time-resolved studies of ultrafast light-induced spin-state switching, triggered by a femtosecond laser flash,and the following out-of-equilibrium dynamics in FeIII spincrossover crystals. The out-of-equilibrium dynamics involves several steps, resulting fromthe ultrafast molecular photoswitchingof low-spin (LS) to high-spin (HS) states in solids. First, the transient HS state is reached within 200 femtoseconds, and mayrapidly decayinto the stable LS state of the system. A second process at longer delay,associated with volume expansion, drives additional conversion to the HS state during the so-called elastic step occurring at nanosecond time scale. Finally,the laser heating process induces a temperature jump in the crystal that may result in a significant thermal population of the HS state on microsecond time scale. The photoswitching mechanism is of local nature and has linear dependenceon the excitation fluence, whereas the heating effect can macroscopically perturb the LS/HS equilibrium. We discuss similarities and differences between photoswitching dynamics in solution and in different crystals for which the thermal spin conversion is of more or less pronounced cooperative nature
Time-Domain Separation of Optical Properties From Structural Transitions in Resonantly Bonded Materials
The extreme electro-optical contrast between crystalline and amorphous states
in phase change materials is routinely exploited in optical data storage and
future applications include universal memories, flexible displays,
reconfigurable optical circuits, and logic devices. Optical contrast is
believed to arise due to a change in crystallinity. Here we show that the
connection between optical properties and structure can be broken. Using a
unique combination of single-shot femtosecond electron diffraction and optical
spectroscopy, we simultaneously follow the lattice dynamics and dielectric
function in the phase change material Ge2Sb2Te5 during an irreversible state
transformation. The dielectric function changes by 30% within 100 femtoseconds
due to a rapid depletion of electrons from resonantly-bonded states. This
occurs without perturbing the crystallinity of the lattice, which heats with a
2 ps time constant. The optical changes are an order-of-magnitude larger than
those achievable with silicon and present new routes to manipulate light on an
ultrafast timescale without structural changes
A theoretical approach for elastically driven cooperative switching of spin crossover compounds impacted by an ultrashot laser pulse
International audienceIn this paper we use an elastic model in order to study the elastically driven cooperative switching of spin crossover materials after femtosecond laser excitation. In this model, the molecules occupy a triangular lattice in open boundaries systems and are connected by springs. The volume change of a molecule between its two possible spin states, low-spin and high-spin, determines a variation of the spring length and therefore induces elastic interactions between molecules, which propagates throughout the whole sample as elastic distortions. This model is able to reproduce the multi-step out-of-equilibrium response to ultrashort laser excitation and especially the elastically-driven cooperative response. Then this model is developed in order to predict the behaviour of the system as a function of its different physical parameters, such as the magnitude of the elastic constant or the homogeneity of the photoexcitation. The contribution of the reorganisation of the molecular states during elastic steps, leading to clusters of high-spin molecules towards edge or corners is also revealed
Femtosecond optical pump-probe reflectivity studies of spin-state photo-switching in the spin-crossover molecular crystals [Fe(PM-AzA)2(NCS)2]
International audienceWe report here on the ultrafast photo-switching dynamics of a Fe(II) molecular material [Fe(PM-AzA)2(NCS)2]. It undergoes a thermal spin-crossover which can be detected by magnetic measurements or by optical reflectivity. We use here femtosecond optical reflectivity to study the ultrafast photo-switching dynamics. Our results indicate that the HS state is reached from the LS state within less than 100 fs, through an intermediate MLCT state. This ultrafast relaxation from the electronic excited state towards the structurally relaxed HS state is followed by a vibrational cooling of the hot HS molecules within ≈1 ps timescale
Green manure and long-term fertilization effects on available soil zinc and cadmium and their accumulation by wheat (Triticum aestivum L.)
Zinc (Zn) deficiency in humans due to imbalanced diets is a global nutritional problem. It is especially widespread in populations of low-income countries depending on cereals as staple food. Grain Zn concentrations are particularly low in cereals grown on soils with low phytoavailable Zn concentrations. . Plant Zn uptake depends on soil properties such as pH, calcium carbonate, iron and manganese oxides, total Zn and organic matter content (OM). Soil pH, total Zn and OM can be influenced on farms with limited access to mineral fertilizers through organic matter management practises. In this study, we investigated to what extent green manure application could increase soil Zn availability and wheat grain Zn concentrations (biofortification) on soil with different long-term fertilizer management
Time-domain separation of optical properties from structural transitions in resonantly bonded materials
The extreme electro-optical contrast between crystalline and amorphous states in phase-change materials is routinely exploited in optical data storage1 and future applications include universal memories2, flexible displays3, reconfigurable optical circuits4, 5, and logic devices6. Optical contrast is believed to arise owing to a change in crystallinity. Here we show that the connection between optical properties and structure can be broken. Using a combination of single-shot femtosecond electron diffraction and optical spectroscopy, we simultaneously follow the lattice dynamics and dielectric function in the phase-change material Ge2Sb2Te5 during an irreversible state transformation. The dielectric function changes by 30% within 100 fs owing to a rapid depletion of electrons from resonantly bonded states. This occurs without perturbing the crystallinity of the lattice, which heats with a 2-ps time constant. The optical changes are an order of magnitude larger than those achievable with silicon and present new routes to manipulate light on an ultrafast timescale without structural changes.Peer ReviewedPostprint (author's final draft
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