arXiv subjects

C. David

Publications and source records attributed to C. David.

26 records · Page 2Linked to original sources

arXiv2018

Femtosecond phase-transition in hard x-ray excited bismuth

The evolution of the bismuth crystal structure upon excitation of its A$_{1g}$ phonon has been intensely studied with short pulse optical lasers. Here we present the first-time observation of a hard x-ray induced ultrafast phase transition in a bismuth single crystal, at high intensities (~$10^{14}$ W/cm$^2$). The lattice evolution was followed using a recently demonstrated x-ray single-shot probing setup. The time evolution of the (111) Bragg peak intensity showed strong dependence on the excitation fluence. After exposure to a sufficiently intense x-ray pulse, the peak intensity dropped to zero within 300fs, i.e. faster than one oscillation period of the A1g mode at room temperature. Our analysis indicates a nonthermal origin of a lattice disordering process, and excludes interpretations based on electron-ion equilibration process, or on thermodynamic heating process leading to a plasma formation.

cond-mat.mtrl-sci↗

arXiv2018

PICOSEC: Charged particle timing at sub-25 picosecond precision with a Micromegas based detector

The prospect of pileup induced backgrounds at the High Luminosity LHC (HL-LHC) has stimulated intense interest in developing technologies for charged particle detection with accurate timing at high rates. The required accuracy follows directly from the nominal interaction distribution within a bunch crossing ($σ_z\sim5$ cm, $σ_t\sim170$ ps). A time resolution of the order of 20-30 ps would lead to significant reduction of these backgrounds. With this goal, we present a new detection concept called PICOSEC, which is based on a "two-stage" Micromegas detector coupled to a Cherenkov radiator and equipped with a photocathode. First results obtained with this new detector yield a time resolution of 24 ps for 150 GeV muons, and 76 ps for single photoelectrons.

physics.ins-det↗

arXiv2013

Enhanced ULF electromagnetic activity detected by DEMETER above seismogenic regions

In this paper we present results of a comparison between ultra low frequency (ULF) electromagnetic (EM) radiation, recorded by an electric field instrument (ICE) onboard the satellite DEMETER in the topside ionosphere, and the seismicity of regions with high and lower seiismic activity. In particular we evaluated the energy variations of the ULF Ez-electric field component during a period of four years (2006-2009), in order to examine check the possible relation of ULF EM radiation with seismogenic regions located in central America, Indonesia, Eastern Mediterranean Basin and Greece. As a tool of evaluating the ULF Ez energy variations we used Singular Spectrum Analysis (SSA) techniques. The results of our analysis clearly show a significant increase of the ULF EM energy emmited from regions of highest seismic activity at the tectonic plates boundaries. We interpret these results as suggesting that the highest ULF EM energy detected in the topside ionosphere is originated from seismic processes within Earth's crust. We understand the results of the present study as confirming previous evidence that the ULF EM anomalous wave activity in the ionosphere can be considered a useful earthquake precursory signal

physics.geo-ph↗

arXiv2010

Dark-field Tomography: Modeling and Reconstruction

Dark-field images are formed by small-angle scattering of x-ray photons. The small-angle scattering signal is particularly sensitive to structural variations and density fluctuation on a length scale of several ten to hundred nanometers, offering a new contrast mechanism to reveal subtle structural variation of object. In this paper, we derive a novel physical model to describe x-ray absorption and small-angle scattering, and use the proposed model to reconstruct the volumetric small-angle scattering images. The numerical experiments and test experiments demonstrate that the reconstructed scattering images reveal unique features with a high contrast resolution. The proposed approach has great potential in biomedical imaging, nondestructive detections, and other applications.

physics.med-ph↗

arXiv2004

'Spillout' effect in gold nanoclusters embedded in c-Al2O3(0001) matrix

Gold nanoclusters are grown by 1.8 MeV Au^\sup{2+} implantation on c-Al\sub{2}O\sub{3}(0001)substrate and subsequent air annealing at temperatures 1273K. Post-annealed samples show plasmon resonance in the optical (561-579 nm) region for average cluster sizes ~1.72-2.4 nm. A redshift of the plasmon peak with decreasing cluster size in the post-annealed samples is assigned to the 'spillout' effect (reduction of electron density) for clusters with ~157-427 number of Au atoms fully embedded in crystalline dielectric matrix with increased polarizability in the embedded system.

cond-mat.mtrl-sci↗

arXiv2000

Single hole transistor in a p-Si/SiGe quantum well

A single hole transistor is patterned in a p-Si/SiGe quantum well by applying voltages to nanostructured top gate electrodes. Gating is achieved by oxidizing the etched semiconductor surface and the mesa walls before evaporation of the top gates. Pronounced Coulomb blockade effects are observed at small coupling of the transistor island to source and drain.

cond-mat.mes-hall↗

arXiv1998

On the flow of kaons produced in relativistic heavy ion collisions

We investigate the different contributions to the in-plane flow of K+ mesons observed recently by the FOPI collaboration in the reaction Ni(1.93 AGeV)+Ni. Due to the kinematics of the three body phase space decay the flow of kaons produced in baryon-baryon interactions is smaller than that of the baryons in the entrance channel. On the contrary, in pi N interactions the flow of the sources and of the kaons are identical. Therefore the total kaon flow depends on the relative number of Delta N -> K+ and pi N -> K+ reactions and hence on the lifetime of the Delta, in addition to the already known dependence on the potential interaction interaction of the kaons with the nuclear environment.

nucl-th↗

arXiv1996

Influence of rescattering on the spectra of strange particles

Applying a new method of rescattering which is based on the neural network technique we study the influence of rescattering on the spectra of strange particles produced in heavy ion reactions. In contradistinction to former approaches the rescattering is done explicitly and not in a perturbative fashion. We present a comparison of our calculations for the system Ni(1.93AGeV)+Ni with recent data of the FOPI collaboration. We find that even for this small system rescattering changes the observables considerably but does not invalidate the role of the kaons as a messenger from the high density zone. We cannot confirm the conjecture that the kaon flow can be of use for the determination of the optical potential of the kaon.

nucl-th↗