968 research outputs found
Concept of a laser-plasma based electron source for sub-10 fs electron diffraction
We propose a new concept of an electron source for ultrafast electron
diffraction with sub-10~fs temporal resolution. Electrons are generated in a
laser-plasma accelerator, able to deliver femtosecond electron bunches at 5 MeV
energy with kHz repetition rate. The possibility of producing this electron
source is demonstrated using Particle-In-Cell simulations. We then use particle
tracking simulations to show that this electron beam can be transported and
manipulated in a realistic beamline, in order to reach parameters suitable for
electron diffraction. The beamline consists of realistic static magnetic optics
and introduces no temporal jitter. We demonstrate numerically that electron
bunches with 5~fs duration and containing 1.5~fC per bunch can be produced,
with a transverse coherence length exceeding 2~nm, as required for electron
diffraction
Current Induced Fingering Instability in Magnetic Domain Walls
The shape instability of magnetic domain walls under current is investigated
in a ferromagnetic (Ga,Mn)(As,P) film with perpendicular anisotropy. Domain
wall motion is driven by the spin transfer torque mechanism. A current density
gradient is found either to stabilize domains with walls perpendicular to
current lines or to produce finger-like patterns, depending on the domain wall
motion direction. The instability mechanism is shown to result from the
non-adiabatic contribution of the spin transfer torque mechanism.Comment: 5 pages, 3 figures + supplementary material
Magnetic domain structure and dynamics in interacting ferromagnetic stacks with perpendicular anisotropy
The time and field dependence of the magnetic domain structure at
magnetization reversal were investigated by Kerr microscopy in interacting
ferromagnetic Co/Pt multilayers with perpendicular anisotropy. Large local
inhomogeneous magnetostatic fields favor mirroring domain structures and domain
decoration by rings of opposite magnetization. The long range nature of these
magnetostatic interactions gives rise to ultra-slow dynamics even in zero
applied field, i.e. it affects the long time domain stability. Due to this
additionnal interaction field, the magnetization reversal under short magnetic
field pulses differs markedly from the well-known slow dynamic behavior.
Namely, in high field, the magnetization of the coupled harder layer has been
observed to reverse more rapidly by domain wall motion than the softer layer
alone.Comment: 42 pages including 17 figures. submitted to JA
Two-dimensional nanosecond electric field mapping based on cell electropermeabilization
Nanosecond, megavolt-per-meter electric pulses cause permeabilization of cells to small molecules, programmed cell death (apoptosis) in tumor cells, and are under evaluation as a treatment for skin cancer. We use nanoelectroporation and fluorescence imaging to construct two-dimensional maps of the electric field associated with delivery of 15 ns, 10 kV pulses to monolayers of the human prostate cancer cell line PC3 from three different electrode configurations: single-needle, five-needle, and flat-cut coaxial cable. Influx of the normally impermeant fluorescent dye YO-PRO-1 serves as a sensitive indicator of membrane permeabilization. The level of fluorescence emission after pulse exposure is proportional to the applied electric field strength. Spatial electric field distributions were compared in a plane normal to the center axis and 15-20 μm from the tip of the center electrode. Measurement results agree well with models for the three electrode arrangements evaluated in this study. This live-cell method for measuring a nanosecond pulsed electric field distribution provides an operationally meaningful calibration of electrode designs for biological applications and permits visualization of the relative sensitivities of different cell types to nanoelectropulse stimulation. PACS Codes: 87.85.M
Magnetic patterning of (Ga,Mn)As by hydrogen passivation
We present an original method to magnetically pattern thin layers of
(Ga,Mn)As. It relies on local hydrogen passivation to significantly lower the
hole density, and thereby locally suppress the carrier-mediated ferromagnetic
phase. The sample surface is thus maintained continuous, and the minimal
structure size is of about 200 nm. In micron-sized ferromagnetic dots
fabricated by hydrogen passivation on perpendicularly magnetized layers, the
switching fields can be maintained closer to the continuous film coercivity,
compared to dots made by usual dry etch techniques
Life Cycle of an Electropore: A Molecular Dynamics Investigation of the Electroporation of Heterogeneous Lipid Bilayers (PC:PS) In the Presence of Calcium Ions
Cross time-bin photonic entanglement for quantum key distribution
We report a fully fibered source emitting cross time-bin entangled photons at
1540 nm from type-II spontaneous parametric down conversion. Compared to
standard time-bin entanglement realizations, the preparation interferometer
requires no phase stabilization, simplifying its implementation in quantum key
distribution experiments. Franson/Bell-type tests of such a cross time-bin
state are performed and lead to two-photon interference raw visibilities
greater than 95%, which are only limited by the dark-counts in the detectors
and imperfections in the analysis system. Just by trusting the randomness of
the beam-splitters, the correlations generated by the source can be proved of
non-classical origin even in a passive implementation. The obtained results
confirm the suitability of this source for time-bin based quantum key
distribution.Comment: 5 pages, double column, 3 captioned figure
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