513 research outputs found
Mott transition in Cr-doped V2O3 studied by ultrafast reflectivity: electron correlation effects on the transient response
The ultrafast response of the prototype Mott-Hubbard system (V1-xCrx)2O3 was
systematically studied with fs pump-probe reflectivity, allowing us to clearly
identify the effects of the metal-insulator transition on the transient
response. The isostructural nature of the phase transition in this material
made it possible to follow across the phase diagram the behaviour of the
detected coherent acoustic wave, whose average value and lifetime depend on the
thermodynamic phase and on the correlated electron density of states. It is
also shown how coherent lattice oscillations can play an important role in some
changes affecting the ultrafast electronic peak relaxation at the phase
transition, changes which should not be mistakenly attributed to genuine
electronic effects. These results clearly show that a thorough understanding of
the ultrafast response of the material over several tenths of ps is necessary
to correctly interpret its sub-ps excitation and relaxation regime, and appear
to be of general interest also for other strongly correlated materials.Comment: 6 pages, 3 figures. Europhysics Letters (in press
Femtosecond x rays from laser-plasma accelerators
Relativistic interaction of short-pulse lasers with underdense plasmas has
recently led to the emergence of a novel generation of femtosecond x-ray
sources. Based on radiation from electrons accelerated in plasma, these sources
have the common properties to be compact and to deliver collimated, incoherent
and femtosecond radiation. In this article we review, within a unified
formalism, the betatron radiation of trapped and accelerated electrons in the
so-called bubble regime, the synchrotron radiation of laser-accelerated
electrons in usual meter-scale undulators, the nonlinear Thomson scattering
from relativistic electrons oscillating in an intense laser field, and the
Thomson backscattered radiation of a laser beam by laser-accelerated electrons.
The underlying physics is presented using ideal models, the relevant parameters
are defined, and analytical expressions providing the features of the sources
are given. Numerical simulations and a summary of recent experimental results
on the different mechanisms are also presented. Each section ends with the
foreseen development of each scheme. Finally, one of the most promising
applications of laser-plasma accelerators is discussed: the realization of a
compact free-electron laser in the x-ray range of the spectrum. In the
conclusion, the relevant parameters characterizing each sources are summarized.
Considering typical laser-plasma interaction parameters obtained with currently
available lasers, examples of the source features are given. The sources are
then compared to each other in order to define their field of applications.Comment: 58 pages, 41 figure
Superadiabatic transitions in quantum molecular dynamics
We study the dynamics of a molecule’s nuclear wave function near an avoided crossing of two electronic energy levels for one nuclear degree of freedom. We derive the general form of the Schrödinger equation in the nth superadiabatic representation for all n є N. Using these results, we obtain closed formulas for the time development of the component of the wave function in an initially unoccupied energy subspace when a wave packet travels through the transition region. In the optimal superadiabatic representation, which we define, this component builds up monotonically. Finally, we give an explicit formula for the transition wave function away from the avoided crossing, which is in excellent agreement with high-precision numerical calculations
Observation of longitudinal and transverse self-injections in laser-plasma accelerators
Laser-plasma accelerators can produce high quality electron beams, up to
giga-electronvolts in energy, from a centimeter scale device. The properties of
the electron beams and the accelerator stability are largely determined by the
injection stage of electrons into the accelerator. The simplest mechanism of
injection is self-injection, in which the wakefield is strong enough to trap
cold plasma electrons into the laser wake. The main drawback of this method is
its lack of shot-to-shot stability. Here we present experimental and numerical
results that demonstrate the existence of two different self-injection
mechanisms. Transverse self-injection is shown to lead to low stability and
poor quality electron beams, because of a strong dependence on the intensity
profile of the laser pulse. In contrast, longitudinal injection, which is
unambiguously observed for the first time, is shown to lead to much more stable
acceleration and higher quality electron beams.Comment: 7 pages, 7 figure
Facultative Hyperparasitism: Extreme Survival Behaviour of the Primary Solitary Ectoparasitoid, Dinarmus basalis
This study investigated the egg-laying behaviour of ectoparsitoid, Dinarmus basalis Rondani (Hymenoptera: Pteromalidae), females when faced with a prolonged deprivation of suitable hosts leading to extreme ‘oviposition pressure’. The egg-laying behaviour of virgin D. basalis females was tested with Callosobruchus maculatus (F.) (Coleoptera: Bruchidae) hosts previously parasitized by the conspecific females in which the developing larvae had reached the last larval instar (L5) or pupae. The hyperparasitism did not prevent the occurrence of superparasitism, but only one D. basalis egg from a hyperparasitized D. basalis L5 larvae reached the adult stage due to the solitary behaviour of the D. basalis larvae. Under these experimental conditions, 60.78% of the D. basalis adults emerging from larvae were miniaturized due to the depletion of host resources
Simulations of quantum dynamics with fermionic phase-space representations using numerical matrix factorizations as stochastic gauges
The Gaussian phase-space representation can be used to implement quantum
dynamics for fermionic particles numerically. To improve numerical results, we
explore the use of dynamical diffusion gauges in such implementations. This is
achieved by benchmarking quantum dynamics of few-body systems against
independent exact solutions. A diffusion gauge is implemented here as a
so-called noise-matrix, which satisfies a matrix equation defined by the
corresponding Fokker--Planck equation of the phase-space representation. For
the physical systems with fermionic particles considered here, the numerical
evaluation of the new diffusion gauges allows us to double the practical
simulation time, compared with hitherto known analytic noise-matrices. This
development may have far reaching consequences for future quantum dynamical
simulations of many-body systems
Angular momentum evolution in laser-plasma accelerators
The transverse properties of an electron beam are characterized by two
quantities, the emittance which indicates the electron beam extend in the phase
space and the angular momentum which allows for non-planar electron
trajectories. Whereas the emittance of electron beams produced in laser- plasma
accelerator has been measured in several experiments, their angular momentum
has been scarcely studied. It was demonstrated that electrons in laser-plasma
accelerator carry some angular momentum, but its origin was not established.
Here we identify one source of angular momentum growth and we present
experimental results showing that the angular momentum content evolves during
the acceleration
Betatron emission as a diagnostic for injection and acceleration mechanisms in laser-plasma accelerators
Betatron x-ray emission in laser-plasma accelerators is a promising compact
source that may be an alternative to conventional x-ray sources, based on large
scale machines. In addition to its potential as a source, precise measurements
of betatron emission can reveal crucial information about relativistic
laser-plasma interaction. We show that the emission length and the position of
the x-ray emission can be obtained by placing an aperture mask close to the
source, and by measuring the beam profile of the betatron x-ray radiation far
from the aperture mask. The position of the x-ray emission gives information on
plasma wave breaking and hence on the laser non-linear propagation. Moreover,
the measurement of the longitudinal extension helps one to determine whether
the acceleration is limited by pump depletion or dephasing effects. In the case
of multiple injections, it is used to retrieve unambiguously the position in
the plasma of each injection. This technique is also used to study how, in a
capillary discharge, the variations of the delay between the discharge and the
laser pulse affect the interaction. The study reveals that, for a delay
appropriate for laser guiding, the x-ray emission only occurs in the second
half of the capillary: no electrons are injected and accelerated in the first
half.Comment: 8 pages, 6 figures. arXiv admin note: text overlap with
arXiv:1104.245
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