63 research outputs found
Photoinduced ultrafast transition of the local correlated structure in chalcogenide phase-change materials
Revealing the bonding and time-evolving atomic dynamics in functional
materials with complex lattice structures can update the fundamental knowledge
on rich physics therein, and also help to manipulate the material properties as
desired. As the most prototypical chalcogenide phase change material, Ge2Sb2Te5
has been widely used in optical data storage and non-volatile electric memory
due to the fast switching speed and the low energy consumption. However, the
basic understanding of the structural dynamics on the atomic scale is still not
clear. Using femtosecond electron diffraction, structure factor calculation and
TDDFT-MD simulation, we reveal the photoinduced ultrafast transition of the
local correlated structure in the averaged rock-salt phase of Ge2Sb2Te5. The
randomly oriented Peierls distortion among unit cells in the averaged rock-salt
phase of Ge2Sb2Te5 is termed as local correlated structures. The ultrafast
suppression of the local Peierls distortions in individual unit cell gives rise
to a local structure change from the rhombohedral to the cubic geometry within
~ 0.3 ps. In addition, the impact of the carrier relaxation and the large
amount of vacancies to the ultrafast structural response is quantified and
discussed. Our work provides new microscopic insights into contributions of the
local correlated structure to the transient structural and optical responses in
phase change materials. Moreover, we stress the significance of femtosecond
electron diffraction in revealing the local correlated structure in the subunit
cell and the link between the local correlated structure and physical
properties in functional materials with complex microstructures
IVIg treatment increases thrombin activation of platelets and thrombin generation in paediatric patients with immune thrombocytopenia
Clinical manifestations and laboratory parameters of haemostasis were investigated in 23 children with newly diagnosed immune thrombocytopenia (ITP) before and after intravenous immunoglobulin (IVIg) treatment. ITP patients with platelet counts of less than 20âĂâ10 /L and mild bleeding symptoms, graded by a standardized bleeding score (BS), were compared with healthy children with normal platelet counts and children with chemotherapy-related thrombocytopenia. Markers of platelet activation and platelet apoptosis in the absence and presence of platelet activators were analysed by flow cytometry; thrombin generation in plasma was determined. ITP patients at diagnosis presented with increased proportions of platelets expressing CD62P and CD63 and activated caspases, and with decreased thrombin generation. Thrombin-induced activation of platelets was reduced in ITP compared with controls, while increased proportions of platelets with activated caspases were observed. Children with a higher BS had lower proportions of CD62P-expressing platelets compared with those with a lower BS. IVIg treatment increased the number of reticulated platelets, the platelet count to more than 20âĂâ10 /L and improved bleeding in all patients. Decreased thrombin-induced platelet activation, as well as thrombin generation, were ameliorated. Our results indicate that IVIg treatment helps to counteract diminished platelet function and coagulation in children with newly diagnosed ITP
Traversing double-well potential energy surfaces: photoinduced concurrent intralayer and interlayer structural transitions in XTe2 (X=Mo, W)
Manipulating crystal structure and the corresponding electronic properties in
quantum materials provides opportunities for the exploration of exotic physics
and practical applications. Here, by ultrafast electron diffraction, structure
factor calculation and TDDFT-MD simulations, we report the photoinduced
concurrent intralayer and interlayer structural transitions in the Td and 1T'
phase of XTe2 (X=Mo, W). Concomitant with the interlayer structural transition
by shear displacement, the ultrafast suppression of the intralayer Peierls
distortion within 0.3 ps is demonstrated and attributed to Mo-Mo (W-W) bond
stretching. We discuss the modification of multiple quantum electronic states
associated with the intralayer and interlayer structural transitions, such as
the topological band inversion and the higher-order topological state. The twin
structure and the stacking fault in XTe2 are identified by the ultrafast
structural response. Our work elucidates the pathway of the photoinduced
intralayer and interlayer structural transitions in atomic and femtosecond
spatiotemporal scale. Moreover, the concurrent intralayer and interlayer
structural transitions reveals the traversal of all double-well potential
energy surfaces (DWPES) by laser excitation in material system, which may be an
intrinsic mechanism in the field of photoexcitation-driven symmetry
engineering, beyond the single DWPES transition model and the order-disorder
transition model
Intrinsic energy flow in laser-excited 3d ferromagnets
Ultrafast magnetization dynamics are governed by energy flow between electronic, magnetic, and lattice degrees of freedom. A quantitative understanding of these dynamics must be based on a model that agrees with experimental results for all three subsystems. However, ultrafast dynamics of the lattice remain largely unexplored experimentally. Here we combine femtosecond electron diffraction experiments of the lattice dynamics with energy-conserving atomistic spin dynamics (ASD) simulations and ab initio calculations to study the intrinsic energy flow in the 3d ferromagnets cobalt (Co) and iron (Fe). The simulations yield a good description of experimental data, in particular an excellent description of our experimental results for the lattice dynamics. We find that the lattice dynamics are influenced significantly by the magnetization dynamics due to the energy cost of demagnetization. Our results highlight the role of the spin system as the dominant heat sink in the first hundreds of femtoseconds. Together with previous findings for nickel [Zahn et al., Phys. Rev. Research 3, 023032 (2021)], our work demonstrates that energy-conserving ASD simulations provide a general and consistent description of the laser-induced dynamics in all three elemental 3d ferromagnets
Observation of Multi-Directional Energy Transfer in a Hybrid PlasmonicâExcitonic Nanostructure
Hybrid plasmonic devices involve a nanostructured metal supporting localized surface plasmons to amplify lightâmatter interaction, and a non-plasmonic material to functionalize charge excitations. Application-relevant epitaxial heterostructures, however, give rise to ballistic ultrafast dynamics that challenge the conventional semiclassical understanding of unidirectional nanometal-to-substrate energy transfer. Epitaxial Au nanoislands are studied on WSe2 with time- and angle-resolved photoemission spectroscopy and femtosecond electron diffraction: this combination of techniques resolves material, energy, and momentum of charge-carriers and phonons excited in the heterostructure. A strong non-linear plasmonâexciton interaction that transfers the energy of sub-bandgap photons very efficiently to the semiconductor is observed, leaving the metal cold until non-radiative exciton recombination heats the nanoparticles on hundreds of femtoseconds timescales. The results resolve a multi-directional energy exchange on timescales shorter than the electronic thermalization of the nanometal. Electronâphonon coupling and diffusive charge-transfer determine the subsequent energy flow. This complex dynamics opens perspectives for optoelectronic and photocatalytic applications, while providing a constraining experimental testbed for state-of-the-art modelling
Lattice dynamics and ultrafast energy flow between electrons, spins, and phonons in a 3d ferromagnet
The ultrafast dynamics of magnetic order in a ferromagnet are governed by the interplay between electronic, magnetic, and lattice degrees of freedom. In order to obtain a microscopic understanding of ultrafast demagnetization, information on the response of all three subsystems is required. A consistent description of demagnetization and microscopic energy flow, however, is still missing. Here, we combine a femtosecond electron diffraction study of the ultrafast lattice response of nickel to laser excitation with ab initio calculations of the electron-phonon interaction and energy-conserving atomistic spin dynamics simulations. Our model is in agreement with the observed lattice dynamics and previously reported electron and magnetization dynamics. Our approach reveals that the spin system is the dominating heat sink in the initial few hundred femtoseconds and implies a transient nonthermal state of the spins. Our results provide a clear picture of the microscopic energy flow between electronic, magnetic, and lattice degrees of freedom on ultrafast timescales and constitute a foundation for theoretical descriptions of demagnetization that are consistent with the dynamics of all three subsystems
Accessing the Anisotropic Nonthermal Phonon Populations in Black Phosphorus
We combine ultrafast electron diffuse scattering experiments and first-principles calculations of the coupled electron-phonon dynamics to provide a detailed momentum-resolved picture of lattice thermalization in black phosphorus. The measurements reveal the emergence of highly anisotropic nonthermal phonon populations persisting for several picoseconds after exciting the electrons with a light pulse. Ultrafast dynamics simulations based on the time-dependent Boltzmann formalism are supplemented by calculations of the structure factor, defining an approach to reproduce the experimental signatures of nonequilibrium structural dynamics. The combination of experiments and theory enables us to identify highly anisotropic electron-phonon scattering processes as the primary driving force of the nonequilibrium lattice dynamics in black phosphorus. Our approach paves the way toward unravelling and controlling microscopic energy flows in two-dimensional materials and van der Waals heterostructures, and may be extended to other nonequilibrium phenomena involving coupled electron-phonon dynamics such as superconductivity, phase transitions, or polaron physics
Gene banks for wild and cultivated sunflower genetic resources
Modern breeding of sunflower (Helianthus annuus L.), which started 100âyears ago, increased the number and the diversity of cultivated forms. In addition, for more than 50âyears, wild sunflower and other Helianthus species have been collected in North America where they all originated. Collections of both cultivated and wild forms are maintained in gene banks in many countries where sunflower is an important crop, with some specificity according to the availability of germplasm and to local research and breeding programmes. Cultivated material includes land races, open pollinated varieties, synthetics and inbred lines. The majority of wild accessions are ecotypes of wild Helianthus annuus, but also 52âother species of Helianthus and a few related genera. The activities of three gene banks, in USA, France and Serbia, are described in detail, supplemented by data from seven other countries. Past and future uses of the genetic resources for environmental adaptation and breeding are discussed in relation to genomic and improved phenotypic knowledge of the cultivated and wild accessions available in the gene banks.LâamĂ©lioration moderne du tournesol (Helianthus annuus L.) a dĂ©butĂ© il y a un siĂšcle, diversifiant et augmentant le nombre des formes cultivĂ©es du tournesol. De plus, des collectes de tournesols sauvages et dâespĂšces du genre Helianthus ont lieu depuis 50âans en AmĂ©rique du Nord dâoĂč ils sont tous originaires. Ainsi, des collections de tournesols cultivĂ©s et sauvages sont conservĂ©es par des centres de ressources gĂ©nĂ©tiques dans de nombreux pays oĂč le tournesol est une culture importante. Chacun dâeux prĂ©sente des spĂ©cificitĂ©s par rapport aux ressources gĂ©nĂ©tiques maintenues, en fonction des programmes de recherche ou de sĂ©lection variĂ©tale locales. Le matĂ©riel gĂ©nĂ©tique cultivĂ© comprend des Ă©cotypes, des populations et des lignĂ©es tandis que les accessions sauvages correspondent eux Ă©cotypes dâHelianthus annuus sauvages et des 52âautres espĂšces apparentĂ©es du genre Helianthus. Les activitĂ©s de trois centres de ressources gĂ©nĂ©tiques des Ătats-Unis, de la France et de la Serbie sont dĂ©crites en dĂ©tail, complĂ©tĂ©es par des donnĂ©es provenant des centres de sept autres pays. Lâhistorique de lâutilisation des ressources gĂ©nĂ©tiques et les perspectives futures pour lâadaptation des variĂ©tĂ©s Ă lâenvironnement sont discutĂ©s ainsi que leur caractĂ©risation au niveau gĂ©nomique et phĂ©notypique.The Supplementary Material is available at [https://www.ocljournal.org/10.1051/ocl/2020004/olm]
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