Did a Supernova Explode Near Earth in the Recent Past?

LZ12656

- Recent measurements of soil samples brought back from the Moon by NASA astronauts suggest that a supernova may have exploded near our solar system 2 to 3 million years ago. Elevated radioactivity was detected in a sample from the lunar surface. This radioactivity is thought to represent debris from the supernova explosion that was deposited on the lunar surface as the debris passed through the solar system. Similar evidence for this event has previously been found in a rock sample recovered from deep in the Pacific Ocean on Earth. Supernova explosions are a relatively common event in the universe, but finding direct evidence of such an explosion on a planetary body is challenging. Explosion of a nearby supernova could have biological effects on Earth, and it is important to determine if Earth has in fact been subjected to such events in the past.

Scratching as a Fracture Process: From Butter to Steel

LZ12125

- We present results of a hybrid experimental and theoretical investigation of the fracture scaling in scratch tests and show that scratching is a fracture dominated process. Validated for paraffin wax, cement paste, Jurassic limestone and steel, we derive a model that provides a quantitative means to relate quantities measured in scratch tests to fracture properties of materials at multiple scales. The scalability of scratching for different probes and depths opens new venues towards miniaturization of our technique, to extract fracture properties of materials at even smaller length scales.

Baryonic Z' explanation for the CDF Wjj excess

LD13219

- A new force may have been found in a recent CDF result. The CDF
collaboration has announced an exciting observation of a suspicious
bump in W+2 jet production. If this excess holds up, it could be a
discovery of new physics beyond the standard model of particle
physics. We showed that a new force of nature, mediated by a baryonic
Z’ suggested 15 years ago, can explain this excess. This baryonic Z'
boson is very different from the usual Z boson as it only couples to
quarks that carry baryon numbers. With this special property it
survives all other constraints imposed on Z' bosons.

Further tests for Z’ include looking for excess in similar channels
such as photon + 2jet and Z + 2 jet. This baryonic Z' can come from
some specific versions of grand unified theory (GUT) or some entirely
new type of models. If this observation is real and a baryonic Z’ is
confirmed with more data, it will give a significant implication to
the final theory of particle physics.

Randy Moths on the Edge

LX12269

- Male moths can sense and locate females releasing sex pheromones from more than a mile away. This involves the male working on pheromone concentrations across several orders of magnitude - from very small concentration when they are far away to large concentrations on their final approach. It has recently been suggested that this kind of sensory dynamic range can arise in brain networks if their dynamics are close to a critical point analogous with a phase transition in physics (e.g. between solid and liquid). In our paper we describe how such a critical point could arise in, and explain the dynamic range of, the inhibitory neurons that lie at the base of the male moths antennae (the invertebrate equivalent of a nose). Interestingly this large dynamic range in the response to pheromones disappears when the male moth has mated or as a natural part of their diurnal rhythms. Consequently our work really does suggest that the brains of sexually motivated virgin male moths are literally "on the edge" when searching for females.

Superconducting Circuits from Gravity

LA13386

Using Einstein's general relativity, traditionally a theory of gravity, we
construct a model of a Josephson junction and find that it agrees with
results from condensed matter physics.

A Josephson junction is made by sandwiching a non-superconductor between
two superconductors. Superconducting electrons can 'tunnel' through the
non-superconducting barrier and produce a current. These junctions are
well-understood by condensed matter physics and have wide applications in
electronic circuits.

Recently, using tools from string theory, a gravity model of a
superconductor was found. It was created in hopes of understanding one of
the biggest puzzles in condensed matter: high temperature
superconductivity. We test this model by using it to build a Josephson
junction. We then calculate the behavior of the current across the
junction, and find that it matches the expectations from condensed matter
physics.

Surface plasmons can be imprinted on metal nanostructures for subsequent imaging

LA13268

- An unusual observation turned into a scientific breakthrough when researchers investigating the optical properties of nanomaterials discovered a new type of high resolution microscopy for imaging the electric fields of nanostructures.

Optical nanomaterials are mainly based on surface plasmon resonances – the property whereby, in metallic nanostructures, light can collectively excite surface electron waves. With the help of plasmons, light can be captured, modified and even stored in nanostructures. This emerging nanotechnology could find applications in curing cancer, biochemical sensing, solar cells, optical computing, negative refractive index materials, and even invisibility. The imaging of surface plasmons provides a direct way to map and understand the local electric fields that are responsible for the unusual electromagnetic properties of optical nanomaterials; the imaging of surface plasmons, however, is quite challenging. Generally speaking, while there are methods to image plasmons with high resolution, they come at a considerable increase in both cost and complexity.

Now, researchers have demonstrated that upon illuminating nanostructures made of nickel or palladium, the resulting surface plasmon pattern is imprinted on the structures themselves, allowing for subsequent imaging with standard surface probe techniques, such as scanning electron microscopy or atomic force microscopy. The imprinting method is quite unique, combining aspects of both imaging and writing techniques. The combination offers a resolution on plasmons that is, in principle, only limited by that of the atomically-sensitive surface probe techniques.

New wireless devices based on current-induced torques

BA11499

- Current flowing through a magnetic material can alter its magnetization
by spin torque, whereby the spins of the electrons flowing in the
current exert a torque on the magnetization. This mechanism can induce
high-frequency precession of the magnetization. The effect can be used
to make high-frequency wireless devices for future mobile phones,
devices that are significantly smaller and consume less power than the
current state-of-art current technology based, for example, on standard
quartz crystal resonators. In this work, we demonstrate that a very
tiny magnetic tunnel junction device, in which two magnetic layers
(CoFeB) are separated by an oxide barrier (MgO) can emit strong
microwave signals with GHz frequencies. The novelty of our results is
that we are able to demonstrate microwave emission from both the top
magnetic layer, which is called free layer, and the bottom magnetic
layer which is called fixed layer. We show that the precession frequency
of the free layer changes linearly with bias voltage due to the linear
variation of perpendicular component of spin torque. In contrast, the
precession frequency of the fixed layer changes quadratically as a
result of heating effects. By changing the applied field magnitude it is
possible to control which layer is excited and hence to manipulate the
behavior of the frequency with bias voltage. Thus our work provides an
important step towards making next generation wireless devices.

Quantum walking over rough terrain: How obstacles influence the propagation of quantum particles

LA12856

- Quantum physics allows particles to spread quadratically faster than their classical counterparts in a discrete, uniform environment. We have implemented an experimental setup that demonstrates how the dynamics drastically change, if temporal and spatial inhomogeneities are introduced. Fast fluctuations in time lead to a full suppression of the quantum behavior, forcing the particle to act entirely classically. On the other hand, spatial disorders result in a stagnation of the propagation, thus trapping the quantum particle around its initial position, which is in high contrast to
any classical description. Quantum walks serve as underlying theoretical model to explain processes in a variety of different physical systems, as for example, the energy transfer in photosynthesis. The dynamics in such biological systems are hard to measure and highly influenced by disorder and thermal fluctuations. Using controllable photonic quantum networks, we were now able to simulate similar environmental influences and carry out detailed studies of their impact on quantum systems. The experiment not only confirms the theoretical predictions, but opens up new routes for quantum simulations and information processing in mesoscopic structures based on coherent state transfer.

Medley swimming of sleeping sickness parasites.

LW12592


- Though cell locomotion has been examined almost since the discovery of the cell itself, advances in microscopy and biochemical studies have paved the way to a more fundamental understanding of cell motility. More recently, a physical, quantitative approach to understanding the world at the micron scale has gained momentum. This work is a detailed, quantitative characterization of trypanosome motility. Trypanosomes, parasites responsible for deadly disease in humans and cattle, swim with the aid of an appendage called a flagellum. The flagellum, produces rapid undulatory movements that result in cell locomotion. We followed single trypanosomes in a homogeneous environment and found that cells that swim faster also exhibit stronger fluctuations in velocity. Statistical analysis allowed us to develop a mathematical model that could reproduce the diverse trajectories followed by the trypanosomes. Finally, we were able to show that the rapid movements of the body (with time scales on the order of 0.1s) are a result of an active process (requiring energy) and thus cannot be described as simple thermal fluctuations. On the whole, such studies provide insight into basic mechanisms of motility, allow for modeling of cell movement, and may eventually even provide design ideas for artificial microswimmers.

Researchers Study the Interplay between Electric and Magnetic Modes Inside a Material … by Looking Outside

LA12855B

- By examining how garnet crystals reflect and transmit light, we observed the rare occurrence of a hybrid mode where the material displays electric and magnetic characteristics simultaneously. Amazingly, evidence of the hybrid mode vanishes from the reflectivity spectra but remains strong for the transmitted light. We developed an explanation called the Adjusted Oscillator Strength Matching (AOSM) condition to describe this unique electric and magnetic behavior. A possible application of this effect is in antireflection coatings. Using a variety of complementary optical techniques, such as reflectivity, transmittance and ellipsometry, we measured the material’s dielectric permittivity and magnetic permeability in the far infrared frequency range. This work is important in furthering the understanding of when coupling between magnetic excitations such as magnons and electric excitations such as phonons occurs inside a material.

How can you compact a thin sheet of paper?

LB13270

- You can crumple this sheet as a paper ball but the growth of a network of high energy ridges and vertices (d-cones) will hinder the formation of a dense object. It is indeed very difficult to attain 50% of compaction even by applying very large forces !

On the contrary, you can try to make with great care regular folds following a complex origami scheme to get the most compact morphology, i.e., the smallest volume for a given sheet but this is rather time consuming !

Why not using self-organization of folds ?

Here, we show that you can produce very easily optimal self-similar patterns of fold by constraining a sheet at one edge, i.e., a hanging curtain. By exploring these self-organized curtains, we uncover an universal law governing the shape of the sheet whatever the used materials, from graphene to fabrics. In addition, we show that these spontaneous patterns can be manipulated by adding a simple tensile force, regularizing the complex hierarchy of folds.

BOOMERANG BEHAVIOUR OF MASSIVE PARTICLES IN BLACK-HOLE RADIATION

DB10717

- The traditional view on Hawking radiation is that black holes emit (massless)
photons as well as massive particles, provided that the latter have an energy
at least equal to their rest mass. We have demonstrated that this image is
actually not correct for the massive particles: these are radiated from the
black hole horizon independently of their energy. However, the particles with
an energy smaller than their rest mass only arrive at a finite distance from
the black hole before bouncing back in a boomerang-like fashion and being
re-absorbed by the black hole. We have also suggested several examples of
analogue gravity systems (systems where sound waves or other perturbations
behave as if they were moving in a black-hole spacetime) where such an effect
could realistically be detectable in a laboratory setting.

Optically induced crystals of submicron particles

EY10506

- Laser beams sent through microscope lenses are widely used in
physics and biology to trap and manipulate small particles in a solution.
In these so-called optical tweezers, micron-sized particles floating in the
microscope's field of view become trapped at the focal spots of the laser
beams, allowing mechanical control of tiny objects under the microscope.
Using several laser beams, multi-site optical tweezers have been realized
that enable the assembly and control of ensembles of such particles. In the
present work, we use optical tweezers formed by four interfering laser beams
to grow and control large, optically induced crystals of submicron particles
in aqueous solution. In contrast to crystals in the usual sense, these are
non-frozen, periodic particle assemblies held together by the optical forces
generated by the lasers. Several thousands of particles can be arranged into
nearly defect-free, three-dimensional crystals with high packing density.
The crystal structure is controlled by laser beam polarizations and angles,
while the crystal size can be controlled by laser beam diameters and powers.
Future applications of the work may include studies of artificially
crystallized biological matter (bacteria, viruses, proteins) using soft
x-ray Bragg scattering. The research may also lead to the creation of novel
tools for photonics applications, such as materials with a tunable photonic
bandgap.

Break-Up in Granular Jets

LW12658
- Attractive forces between the intimate particles of matter (atoms
or molecules) are at the heart of the conception of cohesion of matter.
This is why matter stick together : gases condense to liquids,and
liquids freeze to solids. Granular jets falling out of a funnel shaped
container, subjected to small vertical vibrations, under the action of
gravity display a strikingly liquid-like appearance. Such jets start out
spatially uniform and break up into clusters farther downstream as may
happen for ordinary liquids under the action of surface tension forces.
This is surprising since attractive forces between grains (at the heart
of surface tension or capillary forces in liquids) are much weaker than
other mechanical forces at play (gravity, friction, inelasticity).
From an analysis of the long wavelength variations of the jet
radius (induced by the vertical vibration), it turns out that these
modes are unstable and produce a long wavelength break up of the jet
which is reminiscent of a "Rayleigh-Plateau" capillary instability
(that leads to drop formation for liquids). This instability and the
break up of the jet can be inhibited when the effect of the surrounding
medium (air) is reduced by enclosing the jet in an evacuated chamber,
showing that the effective surface tension measured is the result of a
strong interaction with the surrounding air.

Two telescope views on drifting constants in the early universe

LC12995

- Since the days of Dirac scientists have been wondering about the possibility that fundamental constants may have varied over the history of the universe. In the past decade this question has been made operational through the comparison of spectral lines observed in objects, known to be old, in comparison with spectral lines observed in the laboratory, i.e. in the present epoch. The mass ratio between a proton and an electron is such an important fundamental constant, which may be tested by looking at hydrogen molecules. Now very detailed observations of the largest set of hydrogen lines so far (over 90) of the brightest know quasar system (J2123-005 at redshift =2.05) with a lookback time of over 10 billion years from the two largest optical telescopes in the world (the Very Large Telescope in Paranal, Chile and the Keck Telescope in Hawaii) show that such observations on "old hydrogen" can be made, and are not overwhelmed by systematic effects. The results from both telescopes perfectly agree with each other and give a slight indication of a change of a fundamental constant. However, the evidence is too small to call proof. More observations will be needed (of other quasar systems), but we know now better about the trustworthyness of such observations.

Quantum “Tricks” in the Biochemical Reactions of the Avian Compass Mechanism

LW12015E

-“Quantum” and “Bio” are two phrases rarely seen together in a scientific context. The idea that quantum physics is underlying biological systems has been entertained for a long time, with evidence, however, being scarce. We here show that a sort of biochemical reactions, namely radical-ion-pair reactions, at the heart of the avian magnetic compass mechanism and central in photosynthesis, are full of the counter-intuitive quantum-mechanical traits usually encountered in experiments dealing with the simplest of quantum objects, atoms or photons. One of the central themes of quantum physics is the infamous Young's double slit experiment, which beautifully manifests the particle-versus-wave duality and the principle of quantum interference, both cornerstones of quantum physics. In this paper we show that radical-ion-pair reactions are governed by the same principles, only now it is the spin of the electrons that is the main actor. Electrons are spinning little magnets, and their magnetic orientation determines the fate of these reactions, as the electrons hop between neighboring molecules. We have unraveled the rich quantum dynamical behavior of these reactions that has been overlooked for more than 40 years, providing further evidence that Nature has invented quantum physics well ahead of quantum physicists and has genuinely applied it in large dangling biomolecules living in the “wet and warm” biological environment.

Undulating Underperformance: Swimming in Elastic Fluids

LB13132

-The main findings from this work is that fluid elasticity, the property that gives materials like silly putty, yogurt, gels, and human mucus their unusual and useful texture, hinders both the swimming speed and efficiency of live micro-organisms. This is a surprising result because many organisms live, move, feed, and reproduce in fluids possessing elasticity, and many biological process of vital importance take place in such media. Examples include the motion of spermatozoa in the female reproductive track (human reproduction), the beating of cilia in the respiratory track (removal of foreign agents), and the motion of worms in wet soil (soil aeration). We find that fluid elasticity decreases swimming speed of the nematode C. elegans up to 35% compared to ordinary fluids. The undulatory motion of C. elegans is typical of many limbless organisms of different sizes including eels, snakes, worms, cilia and flagellated eukaryotes. This is the first study that systematically investigates in experiments the role of fluid elasticity on swimming, which will help in developing and guiding theoretical models in the future.

Hot and Fast

LC13073

-When a drop of water falls onto a hot plate, the vapor layer between the drop and the plate provides levitation and lubrication that allows the drop to skate rapidly over the surface – an effect familiar to every chef. A novel application of this phenomenon, known as the Leidenfrost effect, has shown that a hot solid sphere under free fall in liquid can travel over twice as fast as a cold sphere by maintaining a continuous, robust, thin lubricating vapor layer around the sphere (see Figure). High-speed video imaging showed that the vapor layer reduced the adhesion between the liquid and the sphere surface resulting in a smoother liquid flow pattern and dramatic reduction in hydrodynamic drag (Videos available on EPAPS). These findings complement related lubricating vapor layer technologies such as using superhydrophobic surfaces, microbubbles injection, and supercavitation in the quest for efficient energy usage and reduced carbon emission in high speed under water propulsion applications.

Why chaos is warmer than order?

LB13044

Glasses are remarkably different from crystals at low temperature.
They accumulate more heat and conduct less. This anomaly is related
to a particular ensemble of atomic motions called the "boson peak",
which is universally observed for all glasses. Its nature, however,
remained unknown for more than 50 years. Because of this lengthy
research period, the boson peak has been called the last puzzle of
solid state physics. Most models explain the boson peak by additional
vibrational modes created by the chaos of the atomic positions,
while others attribute it to sound waves. We compared atomic motions
in a glass and a crystal using the nuclear inelastic scattering
technique which determines an exact number of vibrational states.
The results show that around the boson peak, the number of states
in a glass is exactly the same as the number of sound wave states
in the crystal. Furthermore, application of pressure causes a
gradual transformation of the boson peak towards a particular
(van Hove) singularity of the crystal created by sound waves with
a period equal to the crystal periodicity. These observations
unambiguously identify the boson peak with sound waves. Thus,
more heat can be stored in a glass not because chaos allows for
more vibrations, but because it changes sound waves.

Cosmic Dynamos—Coherent Motions, Not Turbulence, Generate Large-Scale Magnetic Fields

LY12726

From whence come the highly-organized, large-scale magnetic fields observed
around planets, stars, galaxies, AGN, and the giant radio lobes emanating from AGN.
According to many theories, dynamo action—the stretching, twisting, and folding of
magnetic flux needed to grow magnetic fields—should arise naturally from turbulent
motions in the molten metal cores of planets or the hot plasmas in the Sun, stars, and AGN. However, in the last few years, laboratory tests of that idea using liquid sodium have yielded negative results. Rather than enhancing the growth of magnetic fields, strong turbulence appears to diffuse magnetic flux and dissipate it away as fast as it’s generated.

Reported here for the first time, a dynamo experiment with liquid sodium has succeeded in shearing a radial magnetic field and wrapping it up in the toroidal direction to create a toroidal field 8 times larger that the original radial field. The secret was to keep the turbulence very low by creating a very rapidly rotating shear flow (Couette flow) stabilized against turbulence by the differential rotation (decrease in angular momentum) between a rapidly rotating inner cylinder and a less rapidly rotating outer cylinder. This type of stabilization occurs in the Keplerian flow of accretion disks around stars and super-massive black holes. Stabilization is achieved in the interior of stars by an entropy gradient at the base of the convective zone, and by viscosity in planets.

The new experiment has demonstrated the omega effect (amplification through stretching) of a classic alpha-omega dynamo. To demonstrate that this field could grow exponentially large from a small seed value, the experiment will add the coherent twisting motion of plumes (the alpha effect) to rotate a small fraction of the toroidal field back into the original radial field. In astrophysics such plumes occur naturally when a small number of early stars plunge back and forth through accretion discs, or convective plumes rise in the convective zone of stars or planets. Thus, the experimental results so far suggest that coherent flows, not turbulence, are the likely origin of the magnetic fields that produce the most dramatic effects of astrophysics—the Earth’s aurora, solar and stellar flares, massive magnetized jets from AGN, and ultra high energy cosmic rays.

Bubble formation in stout beers

EC10816

We show theoretically and experimentally that the same cellulose fibre
nucleation sites responsible for bubble formation in champagne can
also create bubbles in stouts beers, although at a substantially
slower rate. A rough calculation suggests that it may be possible to
replace the widgets of canned stout beers with a coating of cellulose
fibres on the inside of the can. We have extended a model of bubble
formation in champagne, a supersaturated solution of carbon dioxide,
to the case of stout beers, which are supersaturated solutions of
nitrogen and carbon dioxide. This model reveals that the low
solubility of nitrogen retards the rate of bubble formation within a
cellulose fibre, explaining why widgets are necessary to trigger
foaming in canned stout beers. However, the results suggest that a
coating of millions of cellulose fibres, covering an area the size of
a postage stamp might be able to generate the hundred million bubbles
needed to form the head of a pint of stout in the recommended pouring
time of thirty seconds.

Fluid mixing from viscous fingering

LZ11988

In this paper, we explain how the mixing of two fluids can be enhanced
by hydrodynamic instabilities when the mixing fluids have different
viscosities.

For flows at high velocities, turbulence creates chaotic flow
conditions that get a volume of fluid mixed very quickly (which is why
we stir our coffee to get the sugar dissolved quickly). At low
velocities and in small geometries, however, the flow is laminar and,
typically, mixing occurs very slowly. This is important in nature, for
instance, in flows through porous media, because biological activity
and chemical reactions are limited by how fast the fluids come into
contact. It is also important in engineering applications, especially
in the context of microfluidics, where it is difficult to get the
reactants to mix quickly. Many methods have been proposed to achieve
fast mixing in small devices, but all the strategies explored so far
assume that the fluids to be mixed have the same viscosity. In this
paper, we explain how mixing efficiency can be enhanced when the
fluids are of different viscosities. In that case, the flow is
unstable (through a phenomenon called 'viscous fingering'), which
creates disorder in the flow and leads to faster mixing.

LW12496

How Venom Flows

In some snakes the fang delivers deadly venom in much the same way that a hypodermic needle delivers medicine -- by rapid injection of a pressurized mass of fluid. But there is a second means of envenomation that is more common, and, perhaps, also biophysically more elegant. In the majority of venomous snakes and in all other venomous reptiles, the venom is not released under pressure, but rather seeps from the venom gland along an open groove. Nevertheless, to be effective against other organisms, the venom has to penetrate through the superficial layer of skin and infiltrate the deeper tissue. How does venom infiltrate in the absence of pressure? By using a combination of analytical techniques, experiments, and biophysical modeling, a team of researchers has solved the above paradox. Two key findings have emerged. First, the surface tension acting on the venom is the dominant physical force underlying envenomation; it literally shapes the flow of the venom, and ensures that it sticks when necessary while waiting for prey whereas any break in the prey's skin will act as a venom attractant and suck the venom into the deeper tissues. Second, the influence of surface tension is enhanced by the presence of open grooves on the surface of the tooth or fang. A grooved fang is common among living and extinct reptiles. The contours of the groove enable the venom to conform and flow in such a way that, as a consequence of surface tension, it minimizes the surface energy. In this way it can also be understood how during evolution the groove contour might well adapt to the nature of the prey's skin cover, such as feathers.

LY11974

-Small-scale lab experiments reveal new physics possibly responsible
for nebula structure formation-

Interstellar clouds consist mostly of hydrogen, helium, and cosmic
dust. When a massive star explodes as a nova (or supernova), a shock
wave from the explosion produces a complicated structure of giant
swirls in the expanding envelope of gases from the star and in the
surrounding interstellar medium. It has been known for a while that some of
that structure emerges due to Richtmyer-Meshkov instability that
occurs when a shock wave crosses a boundary between two gases of
different densities. However, our recent study shows that swirling
motion will also emerge if the shock travels through a cloud
composed of one gas non-uniformly seeded with dust (like the ubiquitous
dusty hydrogen plasma). We see such swirls form in laboratory experiments
where shock accelerates an air jet seeded with droplets of vapor
made by a theatrical fog machine or with smoke particles. This
observation is important not just for astrophysics, but for many
practical problems, where it changes our understanding of what will
happen – scramjets, where droplets of fuel are injected into a high-speed
gas flow, strong explosions carrying dust, and even inertial
confinement fusion.

LZ11975

New light on microscopic motors

Laser light can be used as delicate and precise fingers to control and
move tiny beads. Many scientific areas have found in optical
micromanipulation a great tool for exploring new horizons. Such is the
case of the study of transport phenomena in the microscopic realm. In
particular, the field of ratchets tries to understand the emergence of
directional motion even though the acting forces add to zero; this was
motivated in part by the study of molecular motors inside cells. Here
we present an experimental realization of an optical ratchet by
creating a periodic and asymmetric pattern of light and an unbiased
external oscillating force. We put our tiny beads on this landscape of
light and we manage to observe a systematic motion. The interplay
between the optical force and the oscillating force gives rise to a
surprising dynamics, even in the simplest case when thermal noise is
negligible. Namely, by properly tuning experimental parameters it is
possible to control the average velocity of the beads and even the
direction of motion in real time. The simplicity and versatility of
our system shine new light, literally, in the fields of ratchets and
transport of microscopic motors.