LP12055

Exploiting the Cheerios effect to study heterogeneous matter

Cheerios floating in a bowl of milk are usually seen to aggregate due to
surface tension. We exploit this effect to study the structure of
heterogeneous agglomerates formed by floating spheres by changing the
level of the liquid in a conical container and thus uniformly
compressing the system. Using this trick we can study aggregates formed
over a wide range of density. We find that the structures are not random
as one may expect for objects brought together randomly, but rather have
local hexagonal structure at short scale and pockets of empty space or
voids. We show with statistical measures that this anomalous behavior is
particularly dramatic at low densities and the system becomes
increasingly homogeneous as the density is increased because of
crowding. Agglomeration due to cohesive interactions occurs broadly in
nature including in dust and soot due to electrostatic interactions,
inter‑galactic dust under gravitation, and in powders due to humidity.


***



LP12468
Materials yield to creep stresses... but not uniformly


If a bridge or a piece of paper is stressed, it will start slowly to yield, or
creep, by accumulating irreversible deformation. A simple experiment done with
ubiquitous copy paper demonstrates that a sample does not creep uniformly. As
time goes on, the yielding slows down, but fluctuations ‑ differences between
local creep rates and the total one of the sample ‑ become meanwhile larger
and larger relative to the average yield rate. The experiment and simulations
of simple metal plasticity tell us that all this is a result of hidden
complexity inside the material, and related here to a phase transition: if an
object is loaded enough, it will creep until brought to failure, whereas
stresses smaller than a critical value will bring the deformation eventually
to a stop. The results will have an impact on our understanding of the most
important properties of materials, as such variations in creep have a novel
interpretation via a phase transition. They also show how simple, almost toy
experiments, such as pulling a piece of paper, may contain complex, yet
beautiful physics.

Image caption: A copy paper sample under creep due to a constant stress applied
in the vertical direction. The color code shows how the sample deformed over
100 second period of time: from small creep deformation (blue) over to large
deformation (red). In the background one may discern the fibrous,
heterogeneous structure of paper. The non‑uniform deformation shows a signature
of a phase transition, in the collective dynamics of the material.


***

LT12557

Molecules whittled out by x‑rays

The LCLS x‑ray laser at the SLAC National Accelerator Laboratory
is proving its mettle as a machine for discovery in strong field
molecular physics. The first observations of double core
vacancies (DCH) ‑‑ so‑called “hollow atoms,” which have lost
their core electrons ‑‑ were reported in early July in both
Physical Review Letters [Hoener et al.] and in Nature [Young et
al.]. These exotic states should be particularly sensitive
indicators of the chemical environment inside a molecule.

Now barely a month after these first LCLS results, two new
Physical Review Letters are showing the first detailed pictures of
electrons in hollowed out molecules. The Auger electrons, which
boil away from a core‑excited atom with much of the energy
deposited by the x‑rays, are more energetic when there is a
double vacancy. They also retain information about the shape of
the molecule and the distribution of electrons that form the
chemical bonds.

***

LS11930

How to mix immiscible materials down to the atomic scale

The presence and the conditions for the formation of surface alloys on bulk materials or thin films is of utmost scientific and technological importance in various fields of science, such as metallurgy, catalysis or magnetism. Of particular significance is the formation and the understanding of new type of surface alloys from components that are immiscible in the bulk, because new materials with potential for applications may be found.

On a surface, foreign atoms are often stressed due to the atomic size differences with substrate atoms. They can be too much or not enough packed (like apples and cherries in an egg carton). But if we mix big AND small atoms together, we can obtain a good packing if they are correctly ordered. If we make the good choice of constituents and composition of the alloy, we may force two immiscible metals in the bulk to make an ordered surface alloy.

In a recent experiment reported in Physical Review letters (July 2010), two immiscible metal, iron and gold, have been evaporated on a ruthenium surface (which mimics the egg carton) and by choosing a precise ratio of 1/3 a long range ordered bidimensional alloy, has been created and observed with scanning tunneling microscopy and electron diffraction. It was generally accepted that such mixing might be favored because of the strain imposed by the corrugation of the substrate.

This work goes one step further, since the authors have performed state-of-the-art spin-polarized ab initio density functional theory calculations for their system for various atomic configurations and compositions. These calculations show that the driving force for the stability of the ordered alloy is the development of the high Fe atomic moment, i.e. magnetism, while stress relief plays no major role, which is a significant discovery.

These results open up new directions in surface alloying and magnetism, and may lead to the development of technologically relevant alloys for magnetic storage and catalysis.


***

LP12487BR

Probing energy levels of massless electrons at the surface

We present the first observation of magnetic‑field‑quantized energy
levels of a highly unusual electronic state that is formed at the
surface of a newly discovered kind of insulator. Like conventional
insulators, these new insulators are electronically inert in the bulk;
however, so‑called topological insulators can support electrical
conduction ‑ but only at the surface. This surface state is predicted
theoretically to have a number of unusual properties, a striking one of
which is that, just like photons, the mobile electrons at the surface do
not have a mass. Quantum‑mechanical properties of massless electrons are
distinct from those of conventional electrons and this should be
manifest in their quantized energy levels in a magnetic field. Although
it was experimentally verified that the surface electrons are indeed
massless, there has been a lack of studies exploring their quantum
properties due to the inevitable contribution from the bulk electrons in
a real material. Using the surface‑sensitive scanning tunneling
spectroscopy technique in a magnetic field, we selectively probed the
surface massless electrons and successfully detected the quantized
energy levels, with exactly the indexing and field dependence expected
for topological insulators. Present results may provide basis for
understanding topological insulators as well as open several new avenues
for technological exploitation.

***



LQ12706

Attosecond electron interferometry

Ultrafast motion can be captured using ultrashort pulses, which, when it comes to electrons, means that attosecond pulses (1 as = 10-18 s) are necessary. These extremely short pulses have intrinsically very broad bandwidths, which leads to a fundamental problem: they cannot be used directly to resolve the spectral content of an electron wave packet. In this letter we present experimentally a novel technique to overcome this obstacle by utilizing a basic principle of quantum mechanics that continues to fascinate us all: the fact that a coherent addition of different possible electron paths results in high contrast interference fringes. In our experiment these fringes contain state specific amplitude and phase information. We obtain this interference structure by coherently launching a reference wave packet simultaneously with the excitation of the electron wave packet that we want to study. At a later time we recombine the wave packets using a few cycle infrared probe pulse and the interference fringes are recorded as a function of the delay between the two pulses. The presented technique combines high spectral and temporal resolution simultaneously, thus providing an increased precision when doing attosecond experiments.

Please note that in a theoretical work (LR11894) that will be published back to back with our letter the technique has been used to study wave packets created from doubly excited autoionizing states.


***

BRR1130BR

Modeling an unconventional oscillator

In free space, an electron subjected to a static electric field becomes
uniformly accelerated. In a crystalline solid instead, as predicted by
Bloch in 1928, it performs oscillations with a time frequency
proportional to the electric field strength. This spectacular but very
counterintuitive behavior remained a tantalizing concept till Esaki and
Tsu pointed out that a periodic stacking of semiconductor layers of
nanometric dimension (a superlattice) would allow realizing it in
solids. Its implementation has since then been achieved in various
experiments, which unambiguously demonstrate that a Bloch oscillator
excited in a biased superlattice emits light in the THz range, a
frequency domain where reliable and tunable sources are badly lacking.
In our paper we present a modeling of superlattice’s Bloch oscillations
that combines accuracy and flexibility. It is based on a comprehensive
quantum‑optical like description of the superlattice response to a fast
optical pulse and results in the finding of the stronger THz emission.
Such a theoretical strategy fully responds to the present demand of a
dedicated analysis tool. It paves the way to optimizing the emission
characteristics of such fascinating solid‑state oscillators.

EM10670

Reverse color sequence in the diffraction of white light by the wing of the male butterfly Pierella luna (Nymphalidae: Satyrinae)

The butterfly Pierella luna (Nymphalidae) shows an intriguing rainbow iridescence effect: the forewings of the male, when illuminated along the axis from the body to the wing tip, decompose a white light beam as a diffraction grating would do. Violet light, however, emerges along a grazing angle, near the wing surface, while the other colors, from blue to red, exit respectively at angles progressively closer to the direction perpendicular to the wing plane. This sequence is the reverse of the usual decomposition of light by a grating with a periodicity parallel to the wing surface. It is shown that this effect is produced by a macroscopic deformation of the entire scale, which curls in such a way that it forms a vertical" grating, perpendicular to the wing surface, and functions in transmission instead of
reflection.

LP11880E

Sudden changes of direction of Marching Locust groups

One may find in nature many examples of animals which possess an
undeniable collective behavior, such as ants or bees. In some cases,
this collective behavior manifests itself in patterns of movement in
large groups of animals like fishes, insects or birds. Strikingly,
many of these groups show sudden changes of direction in their
displacements in the absence of any external stimuli. Movement in
groups is nowadays understood as an advantageous strategy in the
search of food or as protection against predation. But the mechanistic
underpinning of these sudden changes of direction has thus far
remained largely unresolved. In this work we analyze experimental data
on the collective movement of a group of marching locusts in an
experimental setup. Our results point to the Poissonian character of
the change of direction stochastic process. This implies that this
process is purely random and as such it may well be the consequence of
the accumulation of errors made by the individuals within the group as
they try to imitate the velocity of their neighbors. These small
errors cancel each other for short times; however there is a small
probability that they may add up to produce a change of direction, and
this small probability becomes indeed significant for long times. We
have also noted the similarity of this effect with the sudden
magnetization changes that appear in magnetic materials, which
apparently share a comparable mechanism.

***

CR10222


Alpha-gas state in heavier nuclear systems

A dilute gas-like state of alpha particles can exist in heavier nuclear systems.
In the last decade, alpha clustering in nuclei has become a topic of great interest,
involving conglomerates of alpha-particles forming inside the atomic nucleus.
Recently, the extension to systems termed nuclear molecules has broadened this interest.
One atomic analogy is Bose-Einstein condensation. Recent advances in nuclear
theory indicate that there are a class of alpha particles (4He nuclei) condense out into
a dilute gas-like state. These states have a considerably extended nuclear radius and
radically different properties to other nuclear states close-by. One of the most well known
examples in light nuclei is the second 0+ state in Carbon-12, the so called Hoyle state,
responsible for our existence, due to its role in nucleosynthesis in the stars.
We have gone a step further by proving from a theoretical point of view that such
a dilute state of alpha particles can exist in heavier systems. We have shown that
a three-alpha cluster around a Calcium-40 nucleus can have a dilute gas-like structure,
which is consistence with experimental results by Kokalova et al.

Picometer‑sized magnets in free‑electron‑laser light

Atomic clusters represent an ideal model system to study size‑driven
effects in solids. Indeed, such clusters provide a self‑contained
`laboratory' where, as a function of the number of constituent atoms,
condensed matter effects gradually appear. Particularly intriguing is
the evolution of the geometric and electronic structure, and their role
in the appearance of magnetic ordering in increasing the size of the
system from a single atom to a macroscopic unit.

Knowing the structure of a cluster is thus the first important step in
understanding more complex properties. This is a challenging task since
none of the usual spectroscopy techniques work on low‑density gas‑phase
clusters. However, we prove it possible using intense tunable infrared
light from a free electron laser. The laser radiation was used to
resonantly shake a small messenger molecule off the cluster surface.
Thus obtained vibrational spectrum was proven to be a unique fingerprint
of not only the geometric, but also the magnetic configuration of the
clusters. For example, a tiny object with only four Fe atoms was shown
to be a ferrimagnet with a large exchange energy.


***

LT12529

How to make water ‘bounce’ at the nanoscale

In our cells, water is stuck between molecules with only a few nanometers to spare. Such ‘nanoconfined’ water has long been suspected of having unique properties. Now a team of physicists at Wayne State University has measured the mechanical properties of water squeezed down to just a few molecules, and found that water can be switched from being a liquid to a bouncy solid by small changes in external conditions. Using a new Atomic Force Microscope technique developed at Wayne State, the team probed the mechanical properties of confined water layers without disturbing them. Oscillating a tiny probe, immersed in the liquid, with amplitudes the size of a hydrogen atom (0.1 nm), they recorded the response as the probe squeezed the water at extremely low speeds. Once squeezed to a layer four molecules thick or less, the water behaved like honey: more viscous than in bulk, but still liquid. However, at squeeze speeds of 0.8 nm/s and above, water became elastic. This speed is so slow, it would take 12 years to move one foot, yet it is enough to change the behavior of water drastically.

LS12549ER

Artificial Runners and Tumblers

A new class of artificial microswimmers with combined translational and
rotational self‑propulsion is fabricated and studied experimentally. The
chemically fueled microswimmers are made of doublets of Janus colloidal
beads with catalytic patches that are positioned at a fixed angle
relative to one another. Our work suggests strategies for designing
microswimmers that could follow prescribed cycloidal trajectories.

***

LR12495

Light‑Sharpened Ultra‑Precise Atomic Compasses

By a subtle manipulation of a beam of light, researchers have been able to improve, for the first time, the measurement of a magnetic field beyond a fundamental quantum limit. As early compasses did for navigation, today’s ultra‑precise magnetometers are opening new, ground‑breaking applications in medical and biological fields. Currently, the best magnetometers use atoms as compass needles and detect their magnetic direction with a laser beam. The sensitivity of these quantum compasses is limited by Heisenberg’s uncertainty principle, which affects the sharpness of the needle, limiting it to a “sphere of uncertainty”. Heisenberg’s principle cannot be violated, but nothing prevents us from squeezing the “sphere of uncertainty” to sharpen the needle in one direction (it will be blunt in the other, but that’s unimportant). The ICFO experiment used rubidium atoms as compasses, and a beam of so‑called squeezed light to read the atoms’ orientation and effectively "sharpen" the magnetic needles. The experiment beat the standard quantum limit, improving the magnetometer’s sensitivity by a factor of two. These results demonstrate the solid promise of quantum‑enhanced measurements, with important applications such as diagnosis of heart and neurological diseases.

LP12104

A light source less ordinary

Fluorescent molecules are efficient light sources that can be found in
many applications, ranging from energy efficient lighting and laptop
screens to medical imaging apparatus and microscopes. In clear
materials, identical molecules will always emit light at the exact same
rate. In many practical situations the molecules are located in opaque
materials such as paint or biological tissue that strongly scatter
light: a "maze for photons". A team of scientists from the Netherlands
and France has discovered that fluorescent molecules inside such a maze
emit photons at a strongly variable rate that strongly deviates from the
average. The emission of light is determined by a molecule's close
surroundings, in particular the closest scatterer. Understanding this
process allows one to design materials for energy‑efficient lamps,
powerful microscopes and efficient solar cells.

***

LP12012

Looking into the molecular origins of viscoelasticity

When a molten polymer liquid is cooled it sets into the shape
of the mould in which it was blown or injected. This is why
e.g. plastic bottles comes in so many useful shapes.
Polymer liquids displays complex flow properties, that
differs vastly from simple liquids such as water. The reason is
to be found on the molecular level, simple liquids are made of
small molecules, that can easily move between each other.
The liquid flows effordlessly. Polymers on the other hand are
long string like molecules, that are highly entangled with each
other like strings of spaghetti, this vastly restricts their freedom
to move. This not only makes polymer liquids highly viscous,
but it even displays elastic properties like a soft solid. Computer
simulations allows us to obtain new and unique insights into
polymer liquids, since we can simultaneously look at the
dynamics of individual polymer molecules (see figure)
and the viscoelastic flow properties of the material.


***

EQJ1049

New Surprises in a Classic Experiment

Inverse Chladni patterns, i.e., grains collecting at the anti-nodes of a resonating horizontal plate, were traditionally believed to occur only for particles light enough to be carried along by the air currents induced by the vibrating plate. Thus it comes as a surprise that there is yet a second mechanism leading to inverse Chladni patterns: Dutch physicists show – in a brief report in Physical Review E – that when the acceleration of the resonating plate remains below g, all grains spontaneously roll towards the anti-nodes, irrespective of their size and even in the absence of air. The authors stress that the new mechanism is a subtle one, and explain why it has escaped detection for more than two centuries, in thousands of demonstrations of this classic experiment.


***

LP12418ER

Bending to Fly: Why Elasticity is The Key To Flapping Flight

Birds, bats and insects flap their wings to fly, and they do it so
skillfully that engineers have gone a long way trying to imitate the
flapping motion to power flying machines. Both biologists and engineers
have since long observed that the amazing propulsive and maneuvering
possibilities offered by flapping flight come at a price: the perpetual
cycle of moving wings back and forth costs a lot of energy. All
flapping flyers in nature have found out a way to minimize this cost and
it always involves the fact that animal wings are flexible. This paper
uses a simplified experimental model of a flapping‑wing flyer to
investigate the problem of the effect of wing flexibility on flight
performance.

***

LM12086

Imaging atoms and molecules with a nanoscale force sensor.

Scanning tunnelling microscopy (STM) changed our perception of the nanoworld because it has made single atoms and molecules visible to us. This “vision” is, unfortunately, quite indirect: A density of valent electronic states, imaged by the STM, may sometimes reflect positions of the atoms, but in the cases when the atoms are bound chemically to each other and share their valence electrons the STM fails to resolve the individual atomic positions without the support from time consuming theoretical simulations. In this work we extended the imaging capabilities of the STM by equipping it with a nanoscale sensor composed of a single hydrogen/deuterium molecule. The sensor probes atomic short-range forces and thus maps the density of the core electronic states, which in contrast to the valence ones are not affected by the by chemical interactions and thus reveal the positions of the individual atoms within the studied chemical compound.

***

LR11957

Close encounters of the third kind between cosmic strings

Magnetic flux tubes, known as Abrikosov vortices, are routinely observed
in superconductors. Yet their relativistic cousins -- known as cosmic strings,
widely predicted in particle physics and superstring theory models-- have
never been observed in the sky. The stakes are high: if found, strings
could provide a "fossil record" for the forces among particles immediately
after the Big Bang. A promising search strategy is to try to detect the
gravitational radiation emitted by the wildly oscillating strings, but this
requires very accurate theoretical predictions of the emitted radiation.
These predictions, in turn, require detailed knowledge of what strings do
when they collide, the object of this study. "Abrikosov" cosmic strings
always reconnect but, occasionally, when the collision happens at
near-luminal speed, they reconnect a second time and effectively pass
through with some deformation. Surprisingly, the present study shows that if
the strings have strongly repulsive cores, double reconnections are much
less uncommon, but also that the strings may even dance around and reconnect
three or four times before parting company. The effect of these close
encounters on the strings' detectability is now under investigation.


***

LG12153E

Consensus is not always driven by the majority

Consensus is a concept which relates to the agreement of a set of
entities. It is very much studied in contemporary physics due to its
widespread presence in interdisciplinary scenarios. Examples of
consensus reaching systems include magnetic materials, animal moving
groups and human systems driven by opinion formation. It is usually
believed that consensus is easily reached when it accommodates the
opinion of the majority. But however things do not necessarily happen
this way. In our work we have considered a simple model inspired in
collective animal motion. Two merging groups of animals, or more
generally entities, travelling with opposite directions form a new
group which selects its direction of motion according to some rule. If
this rule indicates that the direction of motion is chosen randomly,
then a large number of groups initially travelling with different
directions will in short time reach consensus and a unique direction
of travel will be globally selected. If, alternatively, the direction
of motion of the majority is chosen with a higher probability then
consensus is not reached and the initial disordered pattern of
velocities is maintained for all times. This shows that consensus is
not necessarily a consequence of the opinion of the majority and might
contrarily arise out of a multiplicity of random interactions.

LN12179

Spinning Doughnuts in your Coffee, with Video

Everyone knows that after about 10 minutes a hot cup of coffee will cool but if you could spin it about its axis, not only would it cool much less rapidly, but you would not need a doughnut to go with it—they would form right in the cup! Not doughnuts of the usual variety of course (one cannot make dough from water alone) but patterns that underlie the basic physical processes at work in all fluids when their free surfaces are cooled; coffee cups, planetary atmospheres, oceans and any other rotating fluid body. A coffee or tea drinker who takes milk knows that a splash of the cold white stuff sinks to the bottom. If one does not stir with a spoon after a while “clouds” of milk appear at the surface. How? That last bit of water on your back after a shower feels cool because the latent heat of evaporation is taken away from your body. Here, we find that if the cup of coffee is rotated, the cold dense sinking fluid does not go straight down. Rather, for a period of time while the fluid sinks it also rotates with the rest of the liquid, its motion resisted solely by the intrinsic fluid shear viscosity. During this state, using a highly quantitative optical methodology in which we track the motion of all of the fluid parcels in the cup, we observe a concentric pattern of rings forming—doughnuts—from the inside to the outside of the fluid (see b below). Thus, the cup of coffee cools more rapidly in the center than in the outside. Eventually, the relative motion of the rings creates a sufficient shear that they break down into a grid of vortices which characterize not only what is typically assumed to be the usual state of affairs in planetary fluids, but also magnetic vortices in solid materials and other vortex states. All in a cup of coffee…breakfast anyone? See the saga unfold in the movie below and the sequence below taken from it: http://pantheon.yale.edu/~jw378/Convec_rings_vortices.avi


***


AQ10637

The shortest pulse in the world with commercial lasers

The world in the extremely small and fast scale is always mysterious
to human being. One possible "bridge" to this amazing world are
isolated ultrashort pulses. With the shortest pulse as our "eyes" and
"hands", we could watch, touch and even control the inner world.
Researches here may change our view of the nature. However, the
"ticket" to this world is quite expensive: only a few laboratories
have short and strong enough lasers to generate ultrashort pulses. To
debase the high requirement on lasers, some groups have done some
researches about ultrashort pulses generation with commercial lasers,
mainly depend on combining two general lasers instead of one short and
strong laser. Besides their complexity, the final result is not
perfect. The choice of the two lasers and the way to combining them
should be more exact. In this paper, we propose an appropriate combine
of two commercial lasers to generate the shortest pulse in the world
and it's readily available in most laboratories.

***

BR11054

Imaging with Evanescent Waves

Evanescent waves of light can convey information of sub‑wavelength
structures, for example, nanoparticles, polymers, or proteins, but
usually decay exponentially. So, how can we transport the evanescent
waves of light for long distance? This is one of the important questions
for the metamaterial superlens that enables us to realize sub‑wavelength
imaging beyond the diffraction limit. In order to answer the question,
this paper describes experimental and numerical studies of resonant
photon transport through multilayers consisting of silver and insulator.
The authors have clearly demonstrated that the complex, at times,
interplay between various electromagnetic modes including surface
plasmon polaritons in the metal‑insulator‑metal (MIM) system is
important to obtain large tunneling probabilities over relatively large
distances approaching the wavelength. Moreover, this study suggests that
a possible application of plasmonic MIM structures will be a novel type
of hyperlens, which converts evanescent waves to propagating waves of
light.

EFR1051

Relativistic Thermodynamics: an end to 100 years of controversies!

Relativistic thermodynamics has been a controversial subject ever since Einstein first considered the consequences of his new-born theory on quantities like temperature in 1907. Since then the correct relativistic generalization of temperature as well as the famous Maxwell-Boltzmann (MB) velocity distribution, which allows for speeds larger than speed of light, have been hotly debated. Using a simple realistic model of a relativistic gas, we have recently shown (Phys.Rev. E 79, 031124) that Juttner distribution is the correct generalization of MB distribution, while providing strong evidence for the invariance of temperature. In the present paper, we consider the same model under time reparametrization, a possibility that only exists in relativity and has been argued to alter our previous conclusions. Here, again, we show that temperature remains invariant under such reparametrization and that Juttner distribution is simply rescaled by a velocity dependent factor belonging to the (proper) time parameter. Our work provides strong evidence for the invariance of temperature and the legitimacy of Juttner distribution in relativistic generalization of thermodynamics. This, we hope, brings to an end more than

LQ12448

Unlocking the Reversal of Newton’s Cradle

Anyone familiar with a Newton’s cradle – the classic desktop toy consisting of a line of steel balls hanging from pendulums – knows that when one ball is lifted and released, the ball at the opposite end is then ejected up the arc, a motion which then continues back and forth. Researchers at the University of Colorado have recently conducted experiments using a “Stokes’ cradle” – basically a Newton’s cradle except that the balls are coated with a thin layer of viscous liquid – only to find that the well-known outcome is elusive in favor of its reversal (i.e., the striker particle separates from the group after the collision). Further work revealed a surprising source of this behavior: the liquid gap between colliding particles actually solidifies due to extremely high pressures generated as the gap shrinks. This understanding allows for the prediction of the agglomeration/de-agglomeration of “wetted” particles, with applications in processing of biomass for energy production, capturing of floating particles in microgravity to prevent inhalation by astronauts, wastewater treatment, and pharmaceutical processing.


***


BP11313

Frictional duality of metallic nanoparticles: Influence of particle morphology, orientation, and air exposure

While everybody knows from daily experience, that friction is an ubiquitous companion of our daily life, friction might not be that omnipresent on the nanometer scale, where instead a frictionless state called ‘superlubricity’ can be anticipated. In order to track down this phenomenon, researches have used the sharp tip of an atomic force microscope to push antimony nanoparticles over a graphite substrate and measured the force needed to do so. Surprisingly, a dual behavior of the friction forces was found, characterized by the observation that some particles exhibit friction below the detection limit while other similarly sized particles showed finite friction. But what is the difference between the particle types? To elucidate this question, further experiments have now been performed and while proving the general reproducibility of this effect, they also revealed that neither the particles’ morphology nor their alignment relative to the substrate lattice influence the findings. In contrast, it was observed that a temporary exposure to ambient air can lead to a drastic increase in the particle's friction. These findings corroborate the theory that interface contamination, which may consist only of a few molecules, is the most likely culprit for preventing superlubric sliding.

***

LQ12716

NOVEL STATES OF LIGHT FOR QUANTUM INFORMATION PROCESSING

We have demonstrated the generation and characterization of a new class
of optical quantum states, implemented by a modified photon subtraction
method. It has potential applications to quantum information
processing, and for reaching the ultimate capacity of optical
communication channels. A new, recently developed variety of optical
quantum computation relies on information encoded not in single photons
as usual, but rather in states of light fields with contributions from
several photons at the same time. To realize this, it is necessary to
be able to control superpositions (nonclassical combinations) of these
states. While commonplace for single photon states, such control has
always been difficult to achieve for multi-photon states. In this
paper, we report on an experimental realization of a new simple method
for preparing such superpositions of two different multi-photon states.
By a non-deterministic photon detection process, our given input state
is transformed into any desired superposition of itself and the state
which has had one photon removed. We show this by highly efficient
characterization of the outcomes obtained from a range of different
control parameters.


***

Atoms kicked by light provide a means to better measurements of atomic recoil and gravity

We have shown that it is possible to beat a limitation on the precision with which measurements of atomic recoil and gravitational accelerations can be carried out. We took a collection of ultracold atoms and subjected them to “kicks” from short pulses of laser light. For special gravitational accelerations and times between the light pulses, the atomic sample readily absorbs energy from the kicks through a process known as a “resonance”. The width of the energy peak around a resonance usually depends inversely on the square of the number of kicks. This directly impacts the ability to use this technique for measurements since there are always practical limitations on how many kicks can be applied. In our experiments resonances are observed using a method that does not involve determining energies. Instead, after kicking the atoms we use a more intense pulse of light which reverses the effect of the kicks only at a resonance. When observed using this method, the resonant peaks have a width which is inversely proportional to the number of kicks cubed. This seemingly small change in the width dependence has important consequences for precision measurements. For example, with one hundred kicks, the resonances could now be measured with one hundred times greater precision than was previously possible. This may lead to better gravitational sensors and determinations of fundamental physical constants.

BP11087

CO molecules on the wire

Moore’s Law states that the number of processors on a chip doubles roughly every two years due to miniaturization. This miniaturization process is fundamentally limited; the smallest possible processor is expected to be only a single molecule or even atom in size, and the wires connecting them are expected to be a single atom wide. When Nuri Oncel and collaborators observed such thin wires on a germanium surface after platinum deposition, the above image came one step closer to being reality. To discover the nature of the wires a CO adsorption experiment was performed; CO adsorbs on platinum but not on germanium. The adsorption of CO on the wires indicated the wires consisted of platinum atoms. In contrast, theoretical calculations showed the wires actually consist of germanium atoms. So how is it possible for the CO molecules to adsorb on the wires? In this paper we show the CO molecules have binding sites right next to the wires and because the CO molecules bend toward the nearby wire it looks like they are located on the wire. We also found that if one can remove the wire while retaining the rest of the surface undamaged, CO molecules can adsorb on the wire location and form molecular chains … bringing the limit of Moore’s Law back in sight.

***

LM12290

Is the universe fractal?

Quantum gravity is one of the Holy Grails of modern physics. Presently there are many candidate theories, but most of them share a common feature: the universe seems to become lower-dimensional at very small scales. If we were able to observe Nature with an arbitrarily powerful magnifying lens, we would notice that our four-dimensional world acquires a possibily irregular fractal structure at smaller and smaller scales, to the point that we would perceive less than four dimensions. Fractals are objects with bizarre properties (possibly non-integer dimension, self-similarity) which were first introduce by Mandelbrot in the 1960s. They find applications in many branches of physics, mathematics, biology and computer graphics. In a recent paper, Gianluca Calcagni has applied the general but not new idea that spacetime has “fewer dimensions” and a fractal structure at the microscopic level, proposing a model which incorporates this feature in a novel way. The concept of space-time looses its traditional meaning at the scales where gravity and the world become fractal. This could bring new insight in several aspects of quantum gravity models, the cosmology of the early universe, and other scenarios such as non-commutative field theories, with the possibility of building a bridge towards other disciplines.


***

LS12316

Is a "living ocean" the precursor of current living entities?

One of the most important scientific problems concerns the origin of
life. Current theories say that, about 3.5 billion years ago, the
first self‑replicating cell was spontaneously generated from
non‑living matter: in a single step, life began. Now, my researches
allow to propose a different view: life was preceded by a pre‑biotic
entity, constituted by a solution of molecules, without any definite
form or volume, perhaps extending over a whole ocean. Through
numerical modeling, I analyzed some chemical systems, including
biologically relevant molecules. I discovered that it is possible to
create a system that has very unusual chemical properties, namely, it
is marginally stable. This means that it is not unstable (e.g. like
explosives) nor stable: its state is similar to a marble on a
horizontal track. It can smoothly move from a (marginally) stable
point to a neighboring one. Quite surprisingly, I observed that such a
marginally stable system can "evolve", in a sense similar to Darwin
theory, under the effect of thermodynamic fluctuations. While
evolution of cells relies on competition, the kind of evolution I
observed is a collaborative effect of all the molecules, taking place
on macroscopic volumes of water. Since marginally stable chemical
systems can be more simple than cells, I propose that the first
pre‑biotic organism was a marginally stable system, formed by all the
organic molecules dispersed into a volume of water; perhaps, molecules
of a whole ocean contributed to constitute this unusual kind of
organism. That organism was able to evolve;
eventually, evolution led to the fragmentation, compartmentalization
and formation of smaller entities, the cells, that were much more
efficient.


***

LM12390

Magnetic Antenna Transforms Michelson Interferometry

Our cell phones use antennas to receive microwave signals.
Conventional antennas are made of either semiconductors or normal
metals. What happens if an antenna is magnetic? This question, emerged
from the field of spintronics and answered in this paper, has
transformed Michelson interferometry which has been a cornerstone of
wave physics and coherent spectroscopy.

When the antenna is made magnetic, oscillations of the spins of the
electrons are added to the usual oscillations of the charges.
Interestingly, these two oscillations, driven by the microwave
magnetic and electric field, respectively, interfere with each other
and create a measurable voltage. Conventionally, interference refers
to the superposition of two waves of the same dynamic nature, as in
the case of classic Michelson interferometry. Magnetic antenna now
enables coherent coupling of the dynamic electric and magnetic fields.
This led to spintronics Michelson interferometry via which the
relative phase between the dynamic electric and magnetic fields is
directly probed, ‑‑ for the first time ever since James Clerk Maxwell
unveiled the fundamental electromagnetic coherence in 1864.

As a new form of phase detectors, the magnetic antenna adopted in
spintronics Michelson interferometry has broad applications. Two
distinct examples are demonstrated in this paper: (I) Mapping the
phase of spin resonances in magnetic materials via a novel
phase‑resolved spin resonance spectroscopy. (II) Near‑field microwave
phase imaging with sub‑wavelength spatial resolution and on‑site
signal conversion. The second example is particularly exciting since
it paves a new way for developing a digital camera in the microwave
regime, which would be capable for detecting cancer cells via rapid
microwave medical imaging.


***

LN11878

Unraveling information hidden in coherence: from compound eye to cosmic radiation

Our paper elaborates on the challenging idea that wavefront detectors,
used in a variety of fields, can reconstruct the mutual
coherence function when combined with techniques of quantum
tomography. This means, that these systems may be underrated and do
not fully exploit the registered data.

We stress that this is not a technical issue: it can be of interest
in numerous branches. In the manuscript we have considered how this
can influence our understanding of the vision mechanisms in insects
(compound eye), or how the NASA and ESA missions to check the
anisotropy in the cosmic microwave background radiation
may exploit the proposed schemes to improve the resolution.

We also wish to emphasize that our intention is to trigger discussion,
rather than to present a detailed list of possible applications.

LK12461

Ironing out pleats

How does one make a rippled sheet terminate at a straight edge? If the
sheet is sufficiently thin, such as a piece of paper or cloth, then
the obvious solution of stretching it out flat will induce large
stresses near the edge, possibly even tearing the sheet apart. A
solution to this problem is suggested by the series of tiny folds of
fabric generated near a curtain rod. A study of wrinkling patterns on
an ultrathin floating raft of polystyrene led to the discovery of a
new mechanism by which nature resolves such a conflict. Rather than
the hierarchy of pleats seen in curtains, a smooth cascade of
ever‑smaller wrinkles emerges as the edge is approached. This kind of
hierarchical geometry is theoretically explained by the action of the
surface tension of water that tends to ?iron out? any sharp features
in the sheet. Similar types of smooth cascades may appear in other
problems in materials science where a patterned surface comes up
against an edge that is incompatible with the pattern.


***

LR12232

Unexpected arrangement of the first water molecules on a metal

Scientists have answered the long-standing question of the water molecule arrangement on a platinum surface. Water interactions with materials are broadly important. But, at a molecular scale, how water wets a solid is understood in only a few cases, excluding platinum until now, despite this metal’s importance in electrochemistry. In 1997, a Göttingen group showed experimentally that water forms a 2-dimensional crystal on a platinum surface, but, unexpectedly, rotated so that the water molecules are seemingly poorly aligned with the metal’s atoms. High-resolution Scanning Tunneling Microscope images of this very delicate system led researchers to attribute the rotation to clustering of water molecules that lie parallel to the surface plane. Those at cluster centers can then form particularly strong bonds to the metal, and show up in an array of dark features in the images. In the corresponding theoretical model (see figure), the strongly bound “molecular anchor” molecules are connected to the rest of the wetting layer by water molecule pentagons and heptagons, not hexagons as in the conventional understanding of how ice forms. This radically new picture should promote progress in understanding water flow past surfaces, ice-crystal nucleation and growth, and aqueous chemistry at surfaces.

***


EM10525

Financial markets have feelings too.

Financial markets are sensitive to new information, which flow through various technological avenues, keeping the ever-changing world up-to-date. Since stock prices are driven by speculation on future company earnings, they are consequently very sensitive to a variety of news. In this paper, we analyze the reaction of U.S. markets to U.S. Federal Reserve interest rate announcements, which serve as a benchmark and barometer for both American and international economies. Our results are consistent with the “sign effect,” in which “bad news” has a larger impact than “good news.” Furthermore, we observe significant volatility aftershocks immediately before and immediately after the rate change announcement, which can be understood using methods from Earthquake physics. The size of the financial quake is related to the market anticipation before the announcement, and also to the market surprise in response to the Fed’s announcement to increase, decrease, or maintain key target interest rates. We show that the cascade of high volatility lasts for at least one trading day, depending on the magnitude of the news.

BL11352

Fullerene (C60) Nanowire Polymer

Summary

Two important forms of carbon, fullerene (C60) and carbon nanotubes, are
closely related to each other by the structural commonality of their sp2
frameworks. Carbon nanotubes have been widely investigated for the last
decade or so as one‑dimensional (1D) nanomaterials, but fullerene 1D
nanostructures presently only represent laboratory curiosities. In this
paper, we show the formation of a C60‑based nanowire polymer made by
first growing the coresponding crystalline nanowire through a solution
phase of C60 followed by a topochemical polymerization in the solid
state. This new material is scientifically very interesting and may be
potentially important for nanotechnology because of its low
dimensionality, high surface area, large length‑to‑width ratio,
crystalline and molecularly cross‑linked fullerene‑based nanostructure.
In comparison with carbon nanotubes, fullerene 1D nanopolymers could be
even more attractive in specific electronic and photonic applications
especially for bio‑applications as the material would be bio‑compatible
and totally free from any metal, which clearly contrasts the case of
carbon nanotubes, the growth of which is catalyzed by transition metal
nanoparticles, and from which by no means all the metal can be removed
by a post‑purification process.


***

LR12107

Making Ultra-Cold Antimatter

In this article we demonstrate a new technique for obtaining very cold
particles of antimatter. Antimatter is the "mirror image" of the
normal matter that makes up all of the observable universe. The work
took place at CERN in Geneva, Switzerland at the facility that
inspired the popular novel and hit movie "Angels and Demons".
Scientists need cold antimatter atoms in order to perform precision
comparison measurements of matter and antimatter that test the basic
physical laws or symmetries of the universe. The ALPHA collaboration
at CERN adapted a method, called evaporative cooling and commonly used
for obtaining very cold clouds of neutral atoms (Bose‑Einstein
Condensates), to charged antiprotons. The antiproton is the
antimatter equivalent of the proton, which constitutes the nucleus of
Nature's most abundant element, hydrogen. ALPHA scientists study
antihydrogen, which is an atom containing an antiproton and an anti‑
electron, usually called a "positron". To study antimatter,
scientists must store it in high vacuum conditions so that it does not
annihilate with normal matter. ALPHA scientists are hoping to hold on
to their antihydrogen atoms in a device known as an atom trap. For
this to succeed, the antihydrogen atoms must be very close to the
absolute zero point in temperature. The current work demonstrates
that is is possible to cool the antihydrogen nuclei down to about 10
K, by far the lowest temperature ever measured for antiprotons. It is
hoped that the new technique will make it possible for scientists to
trap and study the elusive antihydrogen atoms.

EPJ1042


Maze solving made easy: just follow your nose.

There are many maze solving algorithms. But none as simple as just following your nose. In this paper, it is shown that following your nose will take you through a maze via the shortest possible route to the source of a sweet smelling reward. You will never make a wrong turn. The results neatly explain how nematodes, tiny soil‑dwelling worms, and other creatures can navigate through the labyrinth of air‑filled channels within soils to locate and infest plant roots, causing US$ 70 billion crop losses annually. The predictions are supported by recent experimental studies which have shown that by moving up gradients in acidity, a droplet of organic solvent can find the shortest of multiple possible paths through one particular maze to an acid‑soaked exit. The new results show that nose‑following will work for almost any maze, even fiendishly difficult ones.

***

LC12164

Classic gedankenexperiment comes to life in a granular gas

In 1912 Marian Smoluchowski proposed a device with which he argued that it would be possible to create work from a single heat bath or, in other words, to convert heat directly into work. Richard Feynman explained how the laws of thermodynamics forbid Smoluchowski's ratchet to operate at thermal equilibrium. Now, after almost a century, this classic device has been made to work by immersing it in a non-equilibrium environment of vibrated granular particles. The new Smoluchowski-Feynman ratchet is a prototype for a wide variety of biochemical motors on the molecular scale, which play a key role in living organisms by converting chemical energy into work and directed motion. It is also the first ratchet of its sort capable of delivering work continuously.

***


LM12523

Synthetic mechanochemical molecular swimmer

The first ever blueprint for a simple synthetic molecular scale swimmer
that converts chemical energy directly into mechanical work via a
built‑in mechanochemical coupling is proposed in this Letter. The
swimming mechanism is based on electrostatic actuation, which is coupled
to the catalytic activity of the two enzymes that form the swimmer. As
technological advances allow us to fabricate smaller and smaller
autonomous self‑propelled devices, it is clear that at some point
directed propulsion could not come from pre‑specified deterministic
periodic deformation of the swimmer's body and we need to develop
strategies to extract a net directed motion from a series of random
transitions in the conformation space of the swimmer.
I use a minimal low Reynolds number swimmer model and electrostatic
interactions as actuation mechanism to induce conformational changes
on the device. The swimmer has two enzymes on board that catalyze
chemical reactions that involve ionic products. When the ions are still
attached to the enzymes, electrostatic interaction between them and
other charged elements of the swimmer can lead to conformational
changes. The design proposes how one can take advantage of these
stochastically temporary charged components of the system and achieve
net propulsion at low Reynolds number.

***

LM12230


Poking Holes in Cell Membranes to Make Them Stiffer

During cellular processes, cell membranes undergo various morphological changes that are governed by the interplay between lipid bilayer membranes and proteins. In this paper, we measured for the first time the stiffness of lipid bilayer membranes (mimetic cell membranes) interacting with the pore-forming antimicrobial peptide melittin using neutron spin echo spectroscopy. The measurements revealed three distinct effects on the membrane stiffness as the concentration of melittin was increased. At low melittin concentration, the membrane bending rigidity decreases as individual melittin adsorbs onto the membrane surface, "softening" the membrane. When the melittin concentration is large enough to form pores, the membrane becomes slightly more rigid. At even higher concentrations, additional pores are formed and the inter-pore interactions within the membrane become significant, rapidly “stiffening” the membrane. These findings have improved current understanding of the elastic behavior and morphological changes of cell membranes induced by protein-membrane interactions. In addition, they may guide the development of new theoretical models for membrane fluctuations that include the membrane-mediated interactions between proteins in membrane.

***

AQ10509

HOW THE FIRST ATOMS ARRIVED

After the Big Bang that created our Universe about 14 billion years ago
the structure of the Universe was strongly different from the recent
state: there were no atoms, no molecules, no solids. The first simplest
atoms - hydrogen atoms which consist of one proton and one electron
did arrive about 400 000 years after the Big
Bang. This epoch is called the Recombination Epoch. It was not so easy for
the electrons and protons to recombine: jumping down from one atomic level
to another the hydrogen atom released photons which could excite the
neighbouring atom i.e. turn the process back. This could happen many times
and in this way the radiation was coupled to the matter. The radiation
could escape this coupling mainly due to the two-photon
transitions. After this escape the recombination could finally take
place.The astrophysicists now observe this escaped radiation as Cosmic
Microwave Background ( CMB ). In our paper we apply the methods of the
Quantum Electrodynamics to the more accurate description of the
two-photon transitions in the process of cosmological recombination. The
new corrections found in our paper are noticable at the accuracy level of
the recent CMB observations. They may help to understand better the
history of the Early Universe.

BR11116

The world’s thinnest capacitor


The leakage current between two conducting plates that form a parallel plate capacitor increases as the distance between the plates is decreased. Nevertheless, we show in this paper that two conducting graphene layers separated by a fraction of a nanometer can have incredibly small leakage currents. The degree of isolation depends on the electronic density, on the impurity concentration, and most importantly on the twist angle between the graphene sheets. Orientational disorder in very common in thin graphitic systems, especially in epitaxial graphene samples grown on SiC. Our theory explains why interlayer coupling is sometimes anomalously weak and sometimes more typical in strength. We find that for certain twist angles interlayer tunneling dominates transport despite its minute influence on the band structure. Our theory lays the foundation for studying both linear and non-linear transport in few layer graphene systems and points to some conceptually intriguing aspects of these quasi-3D structures.

AP10418

Ideal Bose Gas revisited: How Many Is Different?

In comparison to classical mechanics, some crucial features of quantum mechanics are that, elementary particles in Nature are identical and also that, physical quantities do not assume continuous values. In this paper, we revisit the ideal Bose gas confined in a cubic box as a simplest quantum system. Powered by a modern supercomputer, without assuming a continuous approximation, we investigate into the thermodynamic instability of the identical particle system. In other words, we examine whether the pressure increases or not if one squeezes the system. We report that if the number of particles is equal to or greater than a certain critical value, which turns out to be 7616, the ideal Bose gas reveals a thermodynamic instability. Accordingly we demonstrate ‑ for the first time ‑ that, a system consisting of finite number of particles can exhibit a discontinuous phase transition featuring a genuine mathematical singularity, provided we keep not volume but pressure constant.

ZQ10042

LHC Protection

The Large Hadron Collider at CERN is a proton‑proton collider and, at
top energy and intensity, will contain 362 MJ of stored energy. This
level of stored energy, equivalent to an aircraft carrier moving at 30
kph, is unprecedented for a particle collider and poses considerable
risk to the elements of the machine and the six delicate experiments
of the LHC. In order to prevent damage, the LHC has a complex machine
protection system to monitor the beam and machine condition, and
safely dump the beams if required. In this paper, the machine
protection of the LHC is described, with a focus on how the various
sub‑systems work together protect the six experiments, and a series of
beam‑based case studies is presented to demonstrate the protection
provided when injecting or storing a proton beam. This includes the
response of the 7 TeV proton beam when magnetic fields change in time,
the detection of particle showers to indicate beam loss and the
triggering of a safe beam dump through the beam interlock system. The
conclusion is the level of protection is adequate and protects the
experiments as much as reasonably possible, giving confidence to the
operation and development of the LHC.

Building an electron dimer with light


Electrons confined in semiconductor quantum dots enable fundamental
studies of Coulomb interaction at the nanoscale; they are also prime
candidates for solid-state quantum information processing.

The case of exactly two confined electrons is especially relevant,
being the benchmark for the exploration of Coulomb correlation. This
setting has been so far difficult to investigate experimentally,
since it requires low-density high-quality quantum dots.

In this Letter the authors realize and investigate a 'dimer molecular
state' formed by two electrons in nanofabricated GaAs quantum dots.
Crucial to this experiment is the novel method to tune the number of
confined electrons, realized via optical illumination. This photo-
depletion technique allows for single electron accuracy, adding
electrons in the quantum dot one by one.

The authors investigated the two-electron system measuring complex
excitation energy spectra. Using sophisticated quantum-mechanical
calculations they were able to link the observed spectra to those of a
surprisingly simple system. The confined electrons in fact behave just
like the ions of a diatomic molecule, forming a dimer electron
molecule. The authors show that the measured excitations are the
vibrational modes of the electron molecule.

In addition to its fundamental interest, this experiment demonstrates
the potential of the new photo-depletion method in controlling few-
electron states in semiconductor quantum dots.

***

LM12403

Catastrophic events governed by a universal law

Surprisingly even minute random forces acting on a flow of particles or waves can conspire to focus the flow along lines. These focal lines form branches of catastrophically increased intensity in the flow. In this letter we calculate how many of these branches emerge as the flow progresses away from its source and through the disorder that is creating the random forces. We can provide a single mathematical expression for this evolution and show that it is universal for a large variety of different types of disorder.
The effect of branching occurs in many physical systems ranging from the electron flow in semiconductors to the sound propagation in the ocean on length scales 1000 billion times larger. Perhaps most intriguingly it was shown to be a source of freak ocean waves of giant proportions endangering seafaring ships and making the catastrophic nature of the intensity enhancement most tangible.

***

LP12186

Wave‑particle‑duality allows observation of classical optical effect
with matter waves

Exploiting the wave‑particle‑duality we have observed a classical
optical effect with atomic matter waves. In 1907 Lord Rayleigh
explained the anomalies that had been observed by R.W. Wood in the
diffraction patterns of visible light from ruled reflection
gratings. Rayleigh‑Wood anomalies occur when conditions (wavelength,
grating period) are such that one of the diffracted beams just
emerges from the grating surface (grazing emergence). As this effect
is general for wave‑like behavior it was, eventually, also observed
with matter waves, i.e., electrons diffracted from crystal surfaces.
In this paper, we demonstrate the first observation of Rayleigh‑Wood
anomalies with atomic matter‑waves. Similar to Wood we have used a
fine ruled reflection grating. Our observations, which are in
agreement with Rayleigh's model, complete the analogy between photon
optics and matter wave optics, and have potential value in improving
the understanding of atom‑surface scattering processes.

"One in a million"‑ atomic resolution electron spectroscopy puts faces to individual atoms

The whereabouts and destiny of a single impurity atom in a material is being unveiled using ultra‑high spatially resolved electron spectroscopy. Following a recent demonstration in which the elemental nature of individual atoms in single‑layer materials was identified using advanced imaging techniques in new‑generation transmission electron microscopes (Krivanek et al., Nature 464, 571, 2010.), we report how the capability of such microscopes can be increased further still, by adding atomic resolution electron energy loss (EEL) spectroscopy, enabling the bonding state of single impurity atoms to be measured. This combination of techniques is a most powerful tool for nano‑technology. Here multi‑walled carbon nanotubes and few‑layer graphene were ion‑implanted with atom species including light elements (e.g., boron), which due to their similarity in atomic number to carbon cannot indubitably be 'seen' by imaging. EEL spectroscopy shows up not only atomic locations of these elements, but also how the element resides electronically in its atomic environment. By monitoring the fate of individual ion‑implanted impurity atoms we show that carbon‑based nano‑materials, which bear enormous promise for nano‑electronics, can be successfully doped to densities larger then one atom nm^‑2 via ion‑implantation, an established routine in device mass‑production in semiconductor industries.

***

LQ12292

Cooling off friction

The atomic scale roots of friction have been the focus of researchers in the
past ever since the invention of the friction force microscope. A
fundamental process, envisioned already back in the early 20th century by
Ludwig Prandtl, is the atomic stick‑slip phenomenon: The atoms of the
sliding contacts first stick together until enough lateral force is applied
to make them jump (or 'slip') suddenly to the neighboring atomic lattice
site. One decade ago Gnecco et al. (Phys.Rev.Lett. 84, 1172(2000)) predicted
that temperature can strongly influence this process: Thermal energy will
tend to induce premature jumps in effect reducing the jump inducing force,
and therefore friction. Our work now provides direct experimental proof that
this notion is indeed true by presenting measurements of atomic friction on
graphite as a function of surface temperature from ambient down to cryogenic
temperatures. While the concept is shown to hold in principle some
corrections are proposed concerning an unexpected temperature dependence of
the attempt frequency of the slip process.

LQ12667

Squeezing the uncertainty out of an atomic clock

A decade ago, researchers at the Paris Observatory first demonstrated
an atomic clock so technically perfect that its precision was limited
only by quantum noise (see the Physical Review Focus story of 8 June
1999, "Fountain Clock Keeps Good Time"). Researchers have now
"squeezed" that quantum noise out of an atomic clock: reducing the
uncertainty in the measurement of time---without violating the
Heisenberg Uncertainty Principle---by redistributing that uncertainty
into another variable which is irrelevant to the measurement.
To conceptualize the difference made by the squeezing, imagine taking
your pulse. The quantum noise in an ordinary clock is like a slight
arrhythmia that makes the number of heartbeats you count in one minute
different from the number you count in the next. In spite of the
arrhythmia, you can determine your average heart rate more and more
precisely simply by keeping count longer and longer. But with a more
rhythmical pulse you could reach the same precision faster. In the
same way, a squeezed clock arrives at a given precision faster than
its unsqueezed counterpart--by almost a factor of three.

***

LL12534

Cosmologists use the Largest Ever Map of Galaxies to Weigh one of Nature's
most Elusive Particles: the neutrino

The elusive nature of neutrinos is particularly startling when one
considers that they are capable of passing through a light‑year (about six
trillion miles) of lead without hitting it. Accurately quantifying their
properties therefore seems like a daunting and formidable task. One such
property is their mass, which is expected to be astonishingly small and is
yet to be determined precisely. However, the neutrinos are also one of the
most abundant particles in the Universe and this has led physicists to
realise they would have a large cumulative effect in the cosmos. As the
neutrinos are so light they move at great speeds and this smooths the
natural clumpiness of matter that forms into groups and clusters of
galaxies. By analysing the distribution of galaxies across the Universe it
has therefore been possible to place upper limits on the tiny neutrino
mass.

Using the idea above cosmologists have placed one of
the tightest limits to date on the particle. They produced and studied the
distribution of galaxies in the largest 3D map available, called MegaZ
from the Sloan Digital Sky Survey. Distances to galaxies were derived from
their colours by innovative methods. This
contained three quarters of a million highly luminous galaxies and allowed
measurements over vast stretches of the observable Universe. By combining
this enormous galaxy map with information from the temperature
fluctuations in the after‑glow of the Big Bang, called the Cosmic
Microwave Background radiation, they found the neutrinos must have a mass
less than a billionth of a single hydrogen atom.

Neutrinos were once thought to be a candidate for the mysterious dark
matter that pervades our Universe but with such minuscule masses it is
clear they constitute around only 1% of all matter and energy at best.

LQ12081

Signature of Superconducting String in Early Universe

Gamma-ray bursts (GRBs) are the most powerful explosions in the Universe. The recent discovery of several very distant GRBs has opened a unique possibility for the study of the physical processes in the early Universe. These GRBs originated when the Universe was around 630 million years old. However, they cannot be classified in the traditional classes considered for nearby GRBs. Moreover, they pose a serious challenge for the conventional star formation theory, since the early Star Formation Rate is too high. In our paper we show that a possibility to solve these observational contradictions is to assume that the distant GRBs have another physical source than the conventional GRBs. We propose that they are observational evidence for the existence of the superconducting cosmic strings, linear topological defects that could be formed in the early Universe, as predicted in unified particle physics theories. However, up to now there is no direct observational evidence for cosmic strings. If the distant GRBs originate from cosmic strings, their strange properties can be immediately explained. We can also constrain the primordial cosmic magnetic field, and its evolution. Therefore the study of distant GRBs can provide some essential information about the fundamental properties of cosmic strings.

AQ10677

Quantum behavior at room temperature

This work reports the first experimental measurement of quantum
correlations and its classical counterpart in a NMR (Nuclear Magnetic
Resonance) system at room temperature. Since the birth of quantum
information theory, entanglement (a strong correlation associated to
the nonseparability between parts of a quantum system) has been
considered as a key resource for the processing of information in a
quantum level. However, the discovery of the existence of
non-classical correlation other than entanglement (as the one revealed
by the quantum discord in separable states) has changed this faith.
The role of such general correlation in quantum information processing
has received a great deal of attention in the recent scientific
literature. Particularly, it has been suggested that this nonclassical
correlation may provide the computational speedup for some quantum
algorithms. Besides the experimental verification of a theoretically
predicted fact, the importance of our work is to give some support to
the conjecture that the existence of quantum correlation other than
entanglement in NMR systems may be the reason for the success of most
bulk NMR implementations of quantum information tasks.

***


LR12633

Positrons Bind to Molecules

The authors report the first measurements of positron (i.e., the antiparticle of the
electron) binding to molecules with large permanent dipole moments. The permanent dipole
moment is due to the molecular electrons shifting from one atom to another and can aid
in forming bound states with an extra electron or positron. The positron binding
energies were found to be surprisingly large ‑ from 10 to 100 times larger than for
similar electron bound states. These results, combined with recent theoretical
calculations, are now beginning to provide details of the positron binding mechanism.
This work is an important step in understanding a key element of the interaction of
antimatter (in this case positrons) with matter.

LJ12204

A new steering technique for orienting gaseous molecules has emerged!

In this Letter, we report the first clear evidence of all‑optical
orientation of polar molecules with an intense nonresonant two‑color laser
field, although an electrostatic field has always been required to determine
the head‑versus‑tail order in the existing molecular orientation techniques.
It has been demonstrated that molecular orientation can be controlled simply
by changing the relative phase between the two wavelength fields. Since
most molecules are anisotropic quantum systems, alignment or orientation
dependence called steric effects is ubiquitous nature and always a matter of
central concern in various phenomena where molecules are involved.
Recently, the importance of molecular alignment and orientation techniques
has been more and more rising. The realization of molecular orientation to
arrange polar molecules in a "head‑versus‑tail" order has been thought to be
much more challenging than that of molecular alignment. The present
technique brings researchers a new steering tool of gaseous molecules. We
hope our new technique will be further developed and contribute to progress
in studies on stereodynamics in chemical reactions, electronic
stereodynamics in molecules, attosecond science, surface science, and the
development of molecular switches.

***

LP12267

How Stars Freeze

In this paper, large-scale computer simulations show how mixtures of carbon and oxygen, in the interior of White Dwarf stars, freeze. These simulations, coupled with Hubble Telescope observations of the globular star cluster Ngc 6397, may allow astronomers to deduce the amounts of carbon and oxygen in the cores of stars. This helps scientists understand how nuclear reactions in stars make the carbon in our bodies and the oxygen that we breathe. When a star is freezing it cools more slowly, just as melting ice cubes cause your drink to warm up more slowly. This slow cooling, which depends on composition as revealed by the new simulations, may be visible in stellar observations. Stars are hot. They are made of not liquids, or gasses, but a plasma of electrons and ions. Even so, White Dwarf and Neutron Stars are so dense that this plasma of ions can actually freeze, producing a solid star. Perhaps the song “twinkle twinkle little star … like a diamond in the sky” is really about solid carbon in White Dwarf stars.


**

LQ11431B

Creating light from sound

In this paper, we show that when a sound wave at gigahertz frequencies pass through a semiconductor nanostructure, it can suddenly stop electrons flowing through the structure and make them orbit around a fixed point at terahertz frequencies a thousand times higher than the sound itself. Such a device has the potential to turn sound into electromagnetic radiation at precisely the frequencies required for medical imaging or naked body scanners at airports. The conversion process is switched on by increasing the loudness of the sound wave. When the wave is quiet it catches electrons in its troughs and carries them through the nanostructure. But when the strength of the sound wave passes a critical threshold, the troughs suddenly spill the electrons, which interact so strongly with the nanostructure’s crystal lattice that they no longer flow. As a result, loud sound kills the current through the nanostructure. In other structures used in high-frequency detectors - speed traps for example - a gradual decrease in current occurs as an applied voltage is increased. But in the structures we consider, the current is killed far more rapidly, suggesting that acoustic control could greatly increase the performance of high-frequency electronics and, remarkably, turn sound into light.

LL12418AR

Novel evidence of matter interference

This is a nanometer-sized version of an experiment imagined by the colorful physicist Richard Feynman half a century ago. Single electrons within a very narrow energy band follow two different paths simultaneously and interfere with themselves. As a result, their distribution would oscillate with the detection angle. The present approach, which will certainly induce a new generation of experiments with matter particles and light, has never been tried or even imagined before. Its novelty is that it seeks and finds oscillations on the energy width that, in principle, were not expected to occur.

LN12132

A new mechanism for atmospheric teleconnections in the laboratory?

‘Teleconnections’ occur in the climate system when variations in the weather at two remotely separated regions follow each other and are noticeably correlated. Conventional wisdom in meteorology explains this phenomenon through waves propagating from one location to another, but is this the only way to make the connection? In a new set of laboratory experiments, using analogs of the mid-latitude atmospheric circulation on Earth, we have demonstrated an alternative mechanism that may represent a new pathway for coupled chaotic behavior in the Earth's climate system. Our experiment consists of two liquid-filled, rotating cylindrical annular tanks, both mounted on the same rotating table. Each tank consists of two thermally conducting cylinders, one inside the other, between which a carefully controlled temperature contrast is applied, recreating the temperature contrast between the warm tropics and the cold polar regions on a rotating Earth-like planet. The combination of differential heating with rapid rotation captures the essential physics of the large-scale atmospheric circulation in mid-latitudes that leads to the formation of cyclone “storms” and anticyclones. When the two tanks are coupled, by allowing variations in the heat transfer in one tank affect the boundary conditions of the other, changes in the amplitude of the “storms” in both systems are found to synchronize, even when the flows are chaotic. Only a tiny perturbation to the boundary conditions is needed to produce detectable synchronization, suggesting that this may be an important mechanism affecting the predictability of the real atmosphere.

EK10620

Common Contagion Patterns for Traders, YouTube users, school kids and insurgents

Despite the flood of published works, network science ‑‑ and in
particular, the study of contagion phenomena on networks ‑‑ has an
Achilles heel. Until now, the transmission of information, viruses or
rumors on a network has always been assumed to be fast enough that the
network could be considered to be quasi‑static, or at most slowly
varying. By contrast, we show that a wide range of important real‑
world phenomena (from YouTube downloads to the transmission of
destabilizing rumors in markets) lie in the network 'twilight zone'
where the network can evolve on the same timescale as the contagion
process. Our accompanying dynamical network model describes social
group dynamics which are quantitatively consistent with empirical
observations from financial markets through to modern insurgent
groups. Despite the visual differences in their activity profiles
(i.e. level of infection vs time), these social and biological systems
can all be explained using this common group‑dynamical contagion
model, simply by varying the relative timescales for groups forming
and breaking as compared to the transmission process. One implication
of our work is that the problems of understanding (i) how flu
pandemics spread around schools, (ii) how destabilizing rumors spread
around financial markets, (iii) how the unexpected popularity of
YouTube downloads (e.g. Susan Boyle) spreads among Internet users, and
(iv) how secret messages should spread among insurgents in Afghanistan
and Iraq, are all the 'same' problem. In particular, their diverse
contagion profiles can all be described using the same dynamical model.

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LN12331

Reprogrammable Control of Spin Wave Flow

A meta-material consisting of magnetic nanowires has been realized where the propagation of wave-like spin excitations is allowed or stopped depending on the magnetic state. This device forms a novel kind of artificial crystal where the dynamic response in the microwave frequency regime is reprogrammable.

Nanotechnology allows one to generate artificial materials with unforeseen physical properties, i.e., meta-materials. Using periodic arrangements of dielectric nanostructures, so-called artificial crystals have been shown to exist for electro-magnetic waves in the optical frequency regime. Such photonic crystals have led to unprecedented control of light propagation on microchips and improved performance of optical devices. We have now shown that magnetic materials periodically patterned on the nanoscale form a novel kind of artificial crystal for collective spin excitations which can be reprogrammed. Due to the non-volatility of magnetic states the fabricated one-dimensional magnonic crystal has provided an enhanced level of control of GHz excitations propagating in magnetic solids. Such spin wave excitations have a wavelength on the nanoscale which is several orders of magnitude shorter than corresponding electromagnetic waves. Nanodevices are thus possible which are operating at microwave frequencies and offer multi-functionality in magnonic applications.