2,992 research outputs found
Demonstration of a beam loaded nanocoulomb-class laser wakefield accelerator.
Laser-plasma wakefield accelerators have seen tremendous progress, now capable of producing quasi-monoenergetic electron beams in the GeV energy range with few-femtoseconds bunch duration. Scaling these accelerators to the nanocoulomb range would yield hundreds of kiloamperes peak current and stimulate the next generation of radiation sources covering high-field THz, high-brightness X-ray and γ-ray sources, compact free-electron lasers and laboratory-size beam-driven plasma accelerators. However, accelerators generating such currents operate in the beam loading regime where the accelerating field is strongly modified by the self-fields of the injected bunch, potentially deteriorating key beam parameters. Here we demonstrate that, if appropriately controlled, the beam loading effect can be employed to improve the accelerator's performance. Self-truncated ionization injection enables loading of unprecedented charges of ∼0.5 nC within a mono-energetic peak. As the energy balance is reached, we show that the accelerator operates at the theoretically predicted optimal loading condition and the final energy spread is minimized.Higher beam quality and stability are desired in laser-plasma accelerators for their applications in compact light sources. Here the authors demonstrate in laser plasma wakefield electron acceleration that the beam loading effect can be employed to improve beam quality by controlling the beam charge
Nature of the spin dynamics and 1/3 magnetization plateau in azurite
We present a specific heat and inelastic neutron scattering study in magnetic
fields up into the 1/3 magnetization plateau phase of the diamond chain
compound azurite Cu(CO)(OH). We establish that the
magnetization plateau is a dimer-monomer state, {\it i.e.}, consisting of a
chain of monomers, which are separated by dimers on the
diamond chain backbone. The effective spin couplings K
and K are derived from the monomer and dimer
dispersions. They are associated to microscopic couplings K,
K and a ferromagnetic K, possibly as
result of orbitals in the Cu-O bonds providing the superexchange
pathways.Comment: 5 pages, 4 figure
Broadband dielectric spectroscopy on benzophenone: alpha relaxation, beta relaxation, and mode coupling theory
We have performed a detailed dielectric investigation of the relaxational
dynamics of glass-forming benzophenone. Our measurements cover a broad
frequency range of 0.1 Hz to 120 GHz and temperatures from far below the glass
temperature well up into the region of the small-viscosity liquid. With respect
to the alpha relaxation this material can be characterized as a typical
molecular glass former with rather high fragility. A good agreement of the
alpha relaxation behavior with the predictions of the mode coupling theory of
the glass transition is stated. In addition, at temperatures below and in the
vicinity of Tg we detect a well-pronounced beta relaxation of Johari-Goldstein
type, which with increasing temperature develops into an excess wing. We
compare our results to literature data from optical Kerr effect and depolarized
light scattering experiments, where an excess-wing like feature was observed in
the 1 - 100 GHz region. We address the question if the Cole-Cole peak, which
was invoked to describe the optical Kerr effect data within the framework of
the mode coupling theory, has any relation to the canonical beta relaxation
detected by dielectric spectroscopy.Comment: 11 pages, 7 figures; revised version with new Fig. 5 and some smaller
changes according to referees' demand
Monte Carlo study of Si(111) homoepitaxy
An attempt is made to simulate the homoepitaxial growth of a Si(111) surface
by the kinetic Monte Carlo method in which the standard Solid-on-Solid model
and the planar model of the (7x7) surface reconstruction are used in
combination.
By taking account of surface reconstructions as well as atomic deposition and
migrations, it is shown that the effect of a coorparative stacking
transformation is necessary for a layer growth.Comment: 4 pages, 5 figures. For Fig.1 of this article, please see Fig.2 of
Phys.Rev. B56, 3583 (1997). To appear in Phys.Rev.B. (June 1998
Vanadium (β-(Dimethylamino)ethyl)cyclopentadienyl Complexes with Diphenylacetylene Ligands
Reduction of the V(III) (β-(dimethylamino)ethyl)cyclopentadienyl dichloride complex [η5:η1-C5H4(CH2)2NMe2]VCl2(PMe3) with 1 equiv of Na/Hg yielded the V(II) dimer {[η5:η1-C5H4(CH2)2NMe2]V(µ-Cl)}2 (2). This compound reacted with diphenylacetylene in THF to give the V(II) alkyne adduct [η5:η1-C5H4(CH2)2NMe2]VCl(η2-PhC≡CPh). Further reduction of 2 with Mg in the presence of diphenylacetylene resulted in oxidative coupling of two diphenylacetylene groups to yield the diamagnetic, formally V(V), bent metallacyclopentatriene complex [η5:η1-C5H4(CH2)2NMe2]V(C4Ph4).
Heterogeneous microstructures tuned in a high throughput architecture
A new method applied to the sensor proposed by Zhang et al. in 2018 is demonstrated in this paper that combines the benefits of this design with the fast heating possible with nanocalorimetry. By applying a PID regulated pulse instead of a constant wattage, we unlock an accessible method to sense morphological changes occurring over short time periods that would be invisible to methods based only on heat capacity. In this study, multilayer Ni/Al thin films were linearly heated at 25, 50, 100, and 200 K/s to over 700°C, showing two distinct peaks in resistance change with activation energies of 554 and 747 kJ/mol, respectively. Through Scanning Transmission Electron Microscopy (STEM) and Energy Dispersive X-ray Analysis (EDX) analysis on cross sections taken ex situ from samples quenched before and after the peaks of interest, we find strong evidence that peak 1 corresponds to Ni diffusing through Al grain boundaries forming intermetallic phases that essentially block the highly conductive Al pathway. This presents the potential to design and calibrate novel heterogeneous structures in a high throughput manner
Monoclonal antibodies against human astrocytomas and their reactivity pattern
The establishment of hybridomas after fusion of X63-Ag8.653 mouse myeloma cells and splenocytes from mice hyperimmunized against human astrocytomas is presented. The animals were primed with 5 × 106 chemically modified uncultured or cultured glioma cells. Six weeks after the last immunization step an intrasplenal booster injection was administrated and 3 days later the spleen cells were prepared for fusion experiments. According to the specificity analysis of the generated antibodies 7 hybridoma products (MUC 7-22, MUC 8-22, MUC 10-22, MUC 11-22, MUC 14-22, MUC 15-22 and MUC 2-63) react with gliomas, neuroblastomas and melanomas as well as with embryonic and fetal cells but do not recognize non-neurogenic tumors. The selected monoclonal antibodies (McAbs) of IgG1 and IgG2a isotypes are not extensively characterized but these antibodies have been demonstrated to be reactive with a panel of glioma cell lines with varying patterns of antigen distribution. Using the McAbs described above and a series of cryosections of glioma biopsies and paraffin sections of the same material as well as glioma cultures established from these, variable antigenic profiles among glioma cell populations could be demonstrated. From these results it is evident that there is not only a distinct degree of antigenic heterogeneity among and within brain tumors, but also that the pattern of antigenic expression can change continuously. Some of the glioma associated antigens recognized by the selected antibodies persist after fixation with methanol/acetone and Karnovsky's fixative and probably are oncoembryonic/oncofetal antigen(s). The data suggest that the use of McAbs recognizing tumor associated oncofetal antigens in immunohistochemistry facilitates objective typing of intracranial malignancies and precise analysis of fine needle brain/tumor biopsies in a sensitive and reproducible manner
Magic Islands and Barriers to Attachment: A Si/Si(111)7x7 Growth Model
Surface reconstructions can drastically modify growth kinetics during initial
stages of epitaxial growth as well as during the process of surface
equilibration after termination of growth. We investigate the effect of
activation barriers hindering attachment of material to existing islands on the
density and size distribution of islands in a model of homoepitaxial growth on
Si(111)7x7 reconstructed surface. An unusual distribution of island sizes
peaked around "magic" sizes and a steep dependence of the island density on the
growth rate are observed. "Magic" islands (of a different shape as compared to
those obtained during growth) are observed also during surface equilibration.Comment: 4 pages including 5 figures, REVTeX, submitted to Physical Review
Near-Threshold eta Meson Production in Proton-Proton Collisions
The production of eta mesons has been measured in the proton-proton
interaction close to the reaction threshold using the COSY-11 internal facility
at the cooler synchrotron COSY. Total cross sections were determined for eight
different excess energies in the range from 0.5 MeV to 5.4 MeV. The energy
dependence of the total cross section is well described by the available
phase-space volume weighted by FSI factors for the proton-proton and proton-eta
pairs.Comment: 9 pages, 1 table, 5 figure
NIRVP: A robust structural proxy for sun-induced chlorophyll fluorescence and photosynthesis across scales
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