Laser driven electron acceleration approaches the stage of
suitability for medical uses
A joint team from Italian, French and
German laboratories proved with an experiment at the CEA laser
facility in Saclay (France) that a table-top accelerator based on an
ultra-short pulsed laser and a gas-jet can deliver high energy
electrons (see right hand side of Figure) at a rate suitable for
either a) efficient generation of gamma-rays (i.e. hard X-rays) able
to produce radioactive elements via photonuclear reactions or b)
Intra-Operative Radiation Therapy (IORT) of tumors. The numerical
simulation reveals (see left hand side of figure) that the
unprecedented efficiency of this accelerator was due to the
achievement of a physical regime in which multiple electron bunches
are accelerated in the gas-jet plasma during the action of each laser
shot. Technical features of the accelerator output include
1-nanoCoulomb of electrons per each Joule of laser energy, while the
electron energy ranges between 10 and 45 Mega electron-Volt.
Comparison of the main parameters of electron bunches produced by a
commercial RF Hospital accelerator for IORT treatment and those of
the present laser driven accelerator is highly promising.
***
ER10334
Exotic orbits of two interacting wave sources
As shown recently, it is possible to create, on a vibrating fluid
interface, mobile emitters of Faraday waves [Y. Couder, S. Protière,
E. Fort and A. Boudaoud, Nature 437, 208 (2005)]. They are formed of
droplets bouncing at a sub-harmonic frequency which couple to the
surface waves they emit. The droplet and its wave form a spontaneously
propagative structure called a "walker". In the present article we
investigate the large variety of orbital motions exhibited by two
interacting walkers having different sizes and velocities. The various
resulting orbits which can be circular, oscillating, epicycloidal or
"paired walkers" are defined and characterized. They are shown to
result from the wave-mediated interaction of walkers. Their relation
to the orbits of other localized dissipative structures is discussed.
***
LP11094ER
A more complete theory for dissipative solitons
As it arises in highly diverse physical contexts, the cubic-quintic complex Ginzburg-Landau (CGL) equation plays an important role in modern science. Mathematically, it can be viewed as a dissipative extension to the nonlinear Schrödinger (NLS) equation and thus describes a more broad spectrum of spatial and/or temporal complexity appearing in nature, e.g., solitons and fronts. Unfortunately, even for stationary solitons, an accurate prediction of their dissipative dynamics at an analytical level is still a challenge. In treating such a problem, the variational approach and moment method are two oft-used theories but they approximate well only for a small portion of parameter regime of solitons. In this paper, a more complete theory for stationary solitons in CGL systems is proposed, with its suitability for parameter range significantly wider than existing theories. By using this theory, one can predict accurately the regime of existence of dissipative solitons in the five-dimensional parameter space, and for an arbitrary choice of a fixed point in space, determine all of its soliton characteristics with small errors. As expected, this work may be well adapted to both the interpretation of experimental results and the future optimization of related experimental systems.
***
AT10285
The Taming of the Shrew Molecules: Laser-Assisted Symmetry-Breaking in
Isotropic Molecular Ensembles
Breaking isotropic angular distribution of molecules in gas phase is a
crucial point for many stereochemistry studies and for controlling the
properties of media. We propose how to orient small linear molecules
in "head vs. tail" manner at high temperatures, up to room ones. For
this, short optical or UV laser pulse is employed. Such pulses had
already shown its ability to align small molecules, but without
preference between forward-backward directions. Additionally, the
higher the temperature in alignment scenarios, the stronger pulse is
necessary, which often causes collapse of molecules prior to their
ordering at room temperatures. To solve these problems we propose a
new approach based on indirect affecting the molecular rotations via
slight changes in molecular geometry (bond lengths) produced by
internal excitation of molecule by laser pulse. To achieve necessary
orientation-dependent character of an excitation we propose to use the
multifrequency femtosecond laser pulse with phase matching between its
components. Well-oriented state of the molecules occurs well after the
laser impact (i.e. in “field-free” regime), and quality of orientation
is almost temperature-independent at that. Modeling the photoinduced
dynamics of BF molecules shows high efficacy of the method.
***
ES10433
Correlations in a Nonequilibrium Assembly of Particles
Study of correlations of interacting subunits in equilibrium systems be they particles, spins or other degrees of freedom,
has been a time-honoured way of investigating them. Correlations are the key to understand their responses as well as thermodynamic
behaviour. The common wisdom about correlations is that they decay with distance. However, this expectation is belied for the so
called 'Single file diffusion', which is an assembly of particles with hard cores, diffusing on a line such that they cannot cross
each other. Such diffusion is relevent in a number of situations like ion channels in cell membranes, sliding of proteins along DNA
and diffusion of molecules in zeolites etc. This paper studies diffusion of N particles on an infinite line starting from
arbitrary initial positions. This is not an equilibrium situation, as the assembly expands with time. The correlations in the system
are found to be unusually strong. If one looks at the correlations between the displacements of the central particle with others,
they fall exponentially with distance in particle labels, but with a correlation length that is greater than N/2.
If one considers similar correlation between a particle on one edge with the others, the correlation decreases upto
the central particle linearly, for the most part, but as one goes further it changes sign and begins increasing till the other edge.
As N increases, the magnitude of the correlator decreases, but correlations extend over the entire assembly as described.
This implies that the assembly expands in a highly symmetric and correlated manner with most of the movement occuring at the edges.
***
LQ11630ER
SEGREGATION IN GRANULAR MEDIA
Segregation and mixing of dissimilar grains is one of the most fundamental
issues in granular matter both from a fundamental and a practical point of
view. In some cases it is a desired and useful effect to separate
particles of different types, but in other industrial processes it is
undesired and can be difficult to control. Due to the great relevance of
this phenomenon, many studies have been performed in the past few years to
understand the physics behind this problem. However, although there is an
extensive observational evidence of these phenomena, much less is known
form a more fundamental point of view and so, a deeper understanding of
the physical mechanisms involved in the segregation process is still
lacking. In this paper, a complete theoretical description based on
kinetic theory of granular gases is provided which covers some of the
aspects not accounted for in previous studies: (i) it goes beyond the
nearly elastic description, (ii) considers the combined effect of thermal
gradients and gravity on segregation and (iii) applies for dense systems.
The results show that the form of the phases diagrams delineating the
different segregation states depend significantly on the value of gravity
relative to the thermal gradient, so that it is possible to switch between
the different states for given values of the mechanical parameters. In
addition, the theory is in qualitative agreement with some computer
simulation results and also with previous experimental works.
***
LR10941
Helical (nano)ribbons: pull tight or give slack?
See animations
We study what happens when one stretches a coiled ribbon made of
inextensible material.
For coils of small pitch, such a ribbon turns out to behave differently
from usual springs.
Under some extension it suddenly gives slack and reshapes itself to lose
a coil.
This can happen several times depending on the number of initial coils.
Such a highly nonlinear response can help explain behaviour and shapes
of real tiny ribbons in nanomechanical experiments,
e.g. cholesterol or zink oxide crystalline ribbons. Their unusual
mechanical properties could be exploited in future nanodevices.
***
CQ10112
The Pathway from Order to Chaos in Nuclei is Explored
Most nuclei are reasonably well ordered near their ground states; i.e.,
the nucleons are in well defined shell-model-type orbitals (like the
electrons in atoms) and these orbitals, or configurations, can usually
be identified experimentally. However, as one adds thermal excitation
energy to the nucleus this order is gradually lost through mixing of the
shell-model-type levels and the nucleonic motion becomes chaotic. In
this paper the full range of this transition is studied for the first
time using the special properties of rotational nuclei. Such nuclei
generate a sequence of nearly equally spaced gamma rays, emitting sharp
gamma-ray lines of regularly decreasing energy (or frequency) as they
rotate slower and slower while remaining in a given shell-model-type
configuration. This is a signal that one can follow as the thermal
energy is increased. At first the sharp rotational lines broaden since
the mixed state can decay to the band member of any of its components.
This is a process called rotational damping. At still higher thermal
energies, this paper provides the first evidence that the width of these
rotational lines narrows again due to a curious process called motional
narrowing, well known in other branches of physics. In this case the
motional narrowing arises from the interplay of the time it takes the
nucleus to change components and the time it takes to change rotational
frequency. Throughout these processes the nucleus is becoming more
chaotic and a quantitative measure of this change was developed in this
paper by relating the probability that the configuration remains
unchanged after a transition to the probability of ordered behavior.
This probability ranges from about 100% near the ground state to less
then 1% at the "high" thermal excitation energy of about 4 MeV in the
ytterbium nuclei studied.
***
LR11485
When a material is stretched along a particular direction, the material is
expected to shrink in the lateral direction - like a rubber band. Conversely,
the sides of a material usually bulge when compressed in a vice. Most materials
change their lateral dimensions in this fashion when subjected to tension or
compression. Figure 1 (a) shows an unloaded material, while Figure 1 (b) shows
how a normal material responds to tension. However, materials can respond in
the completely opposite way - that is, they expand laterally when stretched or,
equivalently, shrink laterally when compressed. Figure 3 (c) shows this unusual
response. Such materials are said to have negative Poisson's ratio and are
referred to as auxetic materials. Auxetic materials have technological
importance. For example, they can used to improve the performance transducers,
components in microelectromechanical systems, strain amplifiers, shock
absorbers and fasteners, to mention a few examples.
We report in Physical Review Letters that under tension, standard isotropic
interactions of two- and three-dimensional many-particle systems (such as
colloids) can result in elastically isotropic (nondirectional) auxetic behavior
provided that the pressure of the system is negative. Matter characterized by
negative pressure is unusual. An air-filled spherical balloon will shrink in
size if placed deep in the ocean consisting of ordinary water. On other hand,
such a spherical balloon will expand in size if placed in liquid ocean
possessing negative pressure. Matter under negative pressure exists. A mundane
example is tempered glass. A more exotic example comes from cosmology, where
present thinking links the expanding Universe to a negative pressure.
The result reported in Physical Review Letters is an unexpected, since an
inherently anisotropic behavior (auxetic behavior) arises from isotropic
interactions. Indeed, most previously discovered auxetic materials exhibit
complex, carefully designed anisotropic interactions. We have shown the
existence of elastically isotropic auxetic behavior
at zero temperature for common crystal structures in two and three
dimensions, namely,the triangular lattice in two dimensions, and the
face-centered cubic lattice in three dimensions. These lattices provide the
densest arrangements
of spheres in these dimensions.
***
BUR1118
Making nanoparticles stick
Understanding the collision process between projectile nanoparticles and
target surfaces is of key importance because identifying conditions that can
lead to efficient sticking, i.e., deposition with preservation of
nanoparticle integrity, will open the possibility to produce novel
nano-materials. The challenge is that under low energies, sticking is
prevented by the low reactivity of surfaces. Under energetic impact
conditions, the deposited nanoparticle is significantly damaged.
Here we show via molecular dynamics simulations that sticking of
hydrogen-passivated Si nanospheres is possible even under the conditions of
poor reactivity of surfaces, as long as the impinging speed is in the
hypersonic range. The underlying microscopic mechanism is unexpected and
nanoscale specific: It involves a phase transition occurring in the particle
core, from diamond to beta-tin structure, the latter eventually evolving to
amorphous. Inducing phase transitions under low-energy impacting conditions
is a surprising finding, since in bulk they are achieved under extreme
pressures. We show that although the impacting energies are low and the
nanosphere-substrate contact forces are not large, the extremely small size
of the contact zone renders pressures high enough to induce the beta-tin
change. Besides the evident fundamental interest, this mechanism brings new
understanding to current nanotechnologies, such as hypersonic plasma
particle deposition.
