Helium nanodroplets ignited from inside
Clusters can be tailored to contain from a few atoms to some
million atoms. When exposed to strong short laser pulses,
electrons are released from these atoms and trapped by the
cluster forming tiny nano-scale plasmas. Thereby, they absorb
extremely efficiently energy from the laser, outperforming
single atoms and bulk material under similar conditions. We have
discovered a dramatic ignition effect. It turns a naturally
transparent helium cluster, so-called nanodroplet, into a fast
and strong absorber of laser light when doped by only a handful
of xenon atoms. These few seed atoms in the center of the
droplet spark the plasma which grows to an unusual cigar shape.
Such a shape allows for a tremendously strong resonant
absorption within a few femtoseconds only. The energy absorption
is so strong, that a "hole" forms in the middle of the laser
pulse. This leads to a startling possibility, which awaits
exploration as a promising technological application of our
work: The creation of short and intense dark laser pulses
***
LY10902BR
Perfect lens and compensated media unified by transformation optics
Transformation optics has provided us a convenient guideline for designing
exotic electromagnetic devices, such as invisibility cloaks. In this rapid
communication, it is shown that transformation optics also opens new horizon
for interpreting in a unified manner several optical meta-phenomena, namely
Pendry's perfect lens, indefinite media lens, and compensated bilayer media.
The proposal of these devices has created a lot of excitement in recent
years and motivates current metamaterial technology. Here we reveal that
these devices share the same physical root: they are all bilayer media
obtained with the coordinate transformation technique. As an extension of
our finding, we predict and numerically confirmed that, by incorporating the
obstacle in the electromagnetic space, perfect imaging beyond passive
objects or active sources is possible. Such transformed bilayer system can
be naturally extended to arbitrary geometries, such as cylindrical and
spherical ones.
***
LZ10950
RESEARCHERS FIND A NEW VIBRATION
Researchers discovered a new atomic oscillation that could affect a
range of phenomena in solids.
When hydrogen atoms diffuse into the crystal aluminum antimonide, they
form bonds with the aluminum atoms. The bond stretching and bending
vibrations of the hydrogen showed a bizarre isotope effect. While
aluminum-deuterium pairs have one stretch-mode frequency, the
aluminum-hydrogen pairs have two frequencies.
To solve this puzzle, the scientists performed calculations on massively
parallel supercomputers. The computations revealed that there is a
transverse mode where the aluminum and hydrogen atoms oscillate
together, as a single unit. The transverse mode plus two bending modes
just happens to equal the hydrogen stretch-mode frequency. This
accidental resonance causes the stretch mode to split in two.
Normally, a stretch mode is localized. Only the hydrogen atom
oscillates. The neighboring atoms barely move. The accidental resonance,
however, causes its spatial extent to increase dramatically. Instead of
only one atom moving, hundreds do.
The researchers discovered a new quasi-particle that exists somewhere
between a sound wave and a localized vibration. In the future, it is
possible that these strange quasi-particles will be found in many
condensed-matter systems.
***
EZ10345
Mathematical modeling of fluid-particle behavior in ureteral peristalsis
Transport of body fluids in humans, animals and plants generally occur by
peristalsis. This refers to successive waves of contraction along the walls
of a hollow muscular structure that push their contents forward. In the
urinary system, urine flows from the kidney to the bladder by peristaltic
action of the ureteral wall. Sometimes this is accompanied by bacteria or
calcium oxalates. Bacterial attachment to the ureteral wall can produce
inflammation, and calcium oxalates can precipitate and form ureteral stones.
A mathematical model of peristaltic flow with particles is developed in this
work. An analytical solution of the fluid
velocity field is first obtained. This is then used in conjunction with an
equation of motion for a small rigid sphere in nonuniform flow under the
action of several forces to calculate particle motion. Retrograde motion of
particles, such as bacteria or stones, can occur in
the upper urinary tract when there is a partial occlusion of the peristaltic
wave. Some of the particles participate in the formation of a recirculating
bolus, and some are delayed in transit and eventually reach the walls.
***
LT11994
Absence of fundamental length scale explains dynamical dark energy.
Fundamental theories without an explicit length scale are invariant
under a change of scale. In a world with more than three space
dimensions scale invariance can have profound consequences for the fate
of the dark energy in the universe. We discuss the presence of two
phases for possible stable cosmological solutions, somewhat analogous to
the phases in many body physics as vapor and water. Within a given
phase, certain physical properties do not depend on the details of the
unknown fundamental theory. For one of the phases we find that
Einstein's cosmological constant vanishes by a mechanism of
self-adjustment. Due to quantum fluctuations, this phase is approached
only as the cosmological time goes to infinity. Therefore the dark
energy vanishes only in this limit. In the present very old universe a
tiny dynamical dark energy density remains, which is typically of the
magnitude required to explain the cosmological observations.
***
LA12142
When superconductivity meets ferromagnetism in ferropnictides
Summary Text: Superconductivity and ferromagnetism are antagonistic. On one hand, a superconducting state tends to expel magnetic fields. On the other hand, ferromagnetism, which produces strong internal magnetic field, generally kills superconductivity. It is fundamentally interesting to find when and how the two phenomena live together. In this paper we report the coexistence of superconductivity and ferromagnetism induced by isovalent phosphorus doping in a ferropnictide system of EuFe2(As1-xPx)2. On cooling, superconductivity associated with the d-electrons in iron atoms appears first at 26 K, and ferromagnetism due to the f-electrons in europium atoms then comes below 20 K. Strikingly, the zero-resistance superconducting state is robust against the ferromagnetism at low temperatures, making the material as a true ferromagnetic superconductor. Besides, the isovalent phosphorus doping, which generates chemical pressure, provides an alternative route to realize superconductivity in ferropnictides.
***
LX10931
How does the Earth's magnetic field reverse?
The magnetic field of the Earth is roughly a dipole aligned with its
axis of rotation. Paleomagnetic measurements have shown that the direction
of the field is not constant: from time to time the magnetic field reverses and
the poles shift in an apparently random way. In this paper we present a model
that explains how these reversals occur.
We propose that the reversals result from the competition between the dipolar
mode and a second unstable dynamo mode. This explains many features of the
Earth's magnetic field. Not only the existence of reversals that thus can
be triggered by a small amount of fluctuations but also their shape: the
dipolar field first slowly decays to zero and then grows with the
opposite sign on a much faster time scale. Aborted reversals, also called
excursions, are predicted. The statistical properties of the duration
between reversals are calculated and this allows to understand the
existence of long durations without reversals, also named superchrons.
If the second mode is a quadrupole, which is likely from numerical
simulations, we show how reversals of the magnetic field are correlated
with the flow in the Earth's inner core: they require breaking of its
equatorial symmetry. Thus, paleomagnetic records can provide informations
about the history of the internal structure of the flow in the inner core
of the Earth.
***
LX11825
Rise and fall of black hole entropy
We have shown analytically that the rise of the entropy of
a black hole with the area is strongly modulated.
Black holes, which show intriguing thermodynamic features,
have the area in Planck units behaving like an entropy.
The theory of loop quantum gravity has a
way of counting states corresponding to discrete eigenvalues of
the area operator, involving square roots of the
familiar eigenvalues of the squared spin operator.
By counting states with the area approximately fixed,
it had been seen earlier that the spins are distributed
in a Boltzmann fashion, involving an analogue temperature and
the area instead of the energy, while the entropy increases
linearly with the area. In the present work, by recognizing the
irrational nature of the exact area eigenvalues, we have found that the
spins have several segregated classes, each with a Boltzmann distribution
governed by its own analogue temperature,
while the linear rise of the entropy is modulated so that it
rises to peaks lying on the linear curve and falls
drastically between the peaks.
This structure is prominent for small black holes and
recent numerical calculations were puzzled by it.
***
BA11272
Relating superconductivity to structure.
This work indicates that strontium nickel phosphide (SrNi2P2) is a
conventional superconductor with a transition temperature of 1.4 K
(similar to more than 10,000 other compounds which have been
discovered). However, the surprising aspect of this work is that we
establish a trend among this and other related nickel based
superconductors, which mimics that of the structurally similar
iron-arsenide-based cousins. The transition temperatures of the
iron-based systems, which are widely believed to be unconventional
superconductors, reach a remarkable 55 K, second only to the
copper-oxide based superconductors. It is completely unexpected that
structural trends would be identical between a class of
unconventional and conventional superconductors. Whether our work
indicates that the nickel-based systems have a similar pairing
mechanism to the iron-arsenide systems and are simply not as well
optimized, or a deeper relationship between crystal structure and the
general phenomenon of superconductivity is an open question.
***
LA11995
Dancing Algae
Scientists at the University of Cambridge have discovered that freshwater
algae can swim about each other in intricate dances, held together only by
the fluid flows they create. The researchers studied the organism Volvox,
which has thousands of cells arranged on the surface of a spherical matrix
about half a millimeter in diameter. Each of those cells has two hair-like
appendages known as flagella, whose beating propels the organism through the
fluid and simultaneously makes it spin about an axis. When two nearby Volvox
swim close to a surface, the deflection of the flow by the boundary leads to
an attractive interaction that pulls them together. Once the individuals are
close, they can orbit around each other like waltzing dancers, or oscillate
back and forth like a minuet. This behaviour has been explained by
mathematical models, which also suggest that the fluid flows set up by the
individuals could assist with fertilization during the sexual phase of their
life cycle.
***
LY11325
A Chemical Quorum
A quorum, in legal terms, refers to the minimum number of members required at a meeting before
business can be done. In biology, quorum sensing (QS) refers to the ability of cells to "switch on"
behavior in response to an increase in group size or density. QS can be thought of as an example of
emergent or collective behavior, the macroscopic properties of a system that arise as a result of
the interactions of its components. We investigated the collective behavior of catalytic
micro-particles that individually display nonoscillatory steady state behavior when immersed in a
catalyst-free Belousov-Zhabotinsky solution. When the particles are gathered into groups larger
than a critical group size, however, spatiotemporal oscillations are exhibited. The activity of the
particles is regulated by the exchange of species with the surrounding solution. The transition from
steady state behavior in small groups of particles to spatiotemporal oscillations in groups larger
than a critical size has the features of a dynamical quorum sensing transition.
***
LZ11313
Magic-Sized Diamond Nanocrystals
Diamond is known to be the hardest of materials and the strongest of
electrical insulators. Making diamond metallic is possible by doping
this material: the process of inserting into diamond lattice large
amounts of atomic impurities. When the number of such dopants exceeds
the critical limit diamond becomes a metal. Such insulator-to-metal
transitions are well known in physics. The researchers from Air Force
Research Laboratory and North Carolina State University found that
metallic diamond possesses very unique structural properties. Using
scanning tunneling microscopy they found magic-sized nanocrystals of
metallic diamond. These tiny nanocrystals are parallelogram shaped and
are closely-packed into a continuous solid film. The AFRL/NCSU
researchers have noticed that the height-ratio of magic nanocrystals is
extremely close to 2/3, the observation which brought them to the
conclusion that quantized electrons play crucial role in the growth
process of these nanocrystals. Indeed, since the discovery of quantum
physics it was known that quantum effects manifest through quantized
sizes: quantized electron states inside atoms and quantized electron
orbits in magnetic field. Thus, the recent discovery reveals the new
phenomenon: the quantized sizes of "artificial" diamond atoms. Each of
these "artificial atoms" contains 7 electrons.
***
LA12237
Understanding the ashes of supernovae and nuclear reactors
Whether one considers supernovae events or the operation of nuclear fission
reactors, extremely exotic neutron-rich nuclei that beta decay back to the
stable isotopes around us will be created. As these nuclei decay, some can
proceed by the process of beta-delayed neutron emission which changes their
mass thus altering the path to stability and the elemental composition of
the ashes of the supernovae event or nuclear fission reactor. Precise
measurements carried out at the Oak Ridge National Laboratory's Holifield
Radioactive Ion Beam Facility, a DOE national user facility, have revealed
yields for beta-delayed neutron emission in the copper and gallium isotopes
which are two to four times higher than those previously reported. Our
measurements, which have increased accuracy and precision in comparison to
the previous measurements, were made possible by new and improved techniques
in beam purification. Revised theoretical calculations are able to
reproduce our results. Additional measurements are needed to determine if
the lack of accuracy in the previous experiments is systematic or isolated
only to the copper and gallium isotopes.
***
LZ11375
Neutron spins visualize magnetic fields
With the presented novel neutron imaging technique it is possible to visualize quantitatively magnetic fields in- and outside of magnetic samples with a sub-millimeter resolution.
In conventional neutron radiography a neutron beam passing through a sample gets attenuated according to the material properties and one obtains an image similar to an x-ray picture. However, due to the spin with its associated magnetic moment, the neutron also senses the strength of magnetic fields. This interaction results in a subtle change in its Larmor precession frequency, which can be detected with a spin echo method similar to the ones used in neutron scattering or NMR. From this measurement a two dimensional magnetic field map can be reconstructed.
The technique offers a wealth of new possibilities in real space condensed matter research, which could help to shed more light on various macroscopic magnetic phenomena.
The attached image shows the two dimensional projection of the characteristic shape of a dipolar magnetic field in the vicinity of a ferromagnetic steel rod of 9 mm length, obtained with this new technique.
***
Two-way traffic: Coexistence of Melting and Crystallization in Polymers.
Everyday experiences show that substances either melt or crystallize
with an increase or decrease in temperature. In contrast, polymers can
undergo both transitions simultaneously at the same time during a
heating process. Using Molecular Dynamics simulations of long chain
molecules, we have shown that melting and crystallization processes
coexist. Furthermore, we were able to reveal some of the molecular
details which lead to this unusual behavior. In particular, we have
shown that at intermediate stages of heating some micro-crystalline
domains disappear while others continue growing (see picture). Because
polymer crystals are far from thermodynamic equilibrium, heating can
increase their thermal stability. Thus, during a heating process,
micro-crystalline domains "decide" whether they are going to melt or
to become more stable. Based on this effect, treatments with
particular temperature-time characteristics might be developed to grow
polymer crystals with designed structural properties.
***
LZ10969
Spin-wave goes further
Anyone who ever threw a stone into a lake could observe generation of wave whose amplitude rapidly attenuates as it propagates, because of energy dissipation. Spin-wave is the magnetic analogy of wave and thus also rapidly attenuates in general. In this paper, we theoretically show that spin-wave attenuation can be suppressed by injecting an electric current and thus spin-wave can go further. It is caused by nonadiabatic interaction between conduction electron spins and spin-wave. We also find that spin-wave can be amplified at a sufficiently high current (see figure). Our finding is important from the viewpoint of fundamental understanding of spin transport mechanism since it provides a new way to experimentally estimate the magnitude of nonadiabatic interaction, which is highly controversial at this moment. Furthermore, our finding will be potentially useful for spin-wave-active devices such as spin-wave logic devices and spin-wave interconnect buses which require as high signal output as possible.
***
LU11904
Ants hate traffic jams
Have you seen ants marching on a trail in a platoon? The ants are
almost as disciplined as soldiers marching in a line. Sometimes,
analogies have been drawn between the traffic of vehicles on busy
highways and ants on a crowded trail. But, which one is a better
analogy? In their forthcoming letter in PRL, John et al. report
empirical results which demonstrate striking differences between
ant-traffic and vehicular traffic. John et al. have analyzed their
empirical data by computing quantities which are known to characterize
vehicular traffic on highways. Surprisingly, unlike vehicular traffic,
free flow of ants on trails can take place even at high number
densities at which vehicles would be stuck in a jam! In other words,
phenomenon of jamming is practically non-existent in the world of ants!
