February 23, 2010

LM11860

Bacterial spreading with randomly oriented motors

How can randomly oriented molecular motors generate cellular movement? We have addressed this question in the human pathogen Neisseria gonorrhoeae which employs grappling hooks called type IV pili for pulling itself over surfaces. We found that up to twenty randomly oriented pili generate persistent cellular movement and we suggest a tug-of-war mechanism for movement.
Many bacterial pathogens live at surfaces. On the one hand they must attach firmly to avoid clearance but on the other hand they must be motile to spread. The type IV pilus is a polymeric cell appendage that has solved this problem; it elongates through polymerization, attaches to the surface and generates force on the cell body by retraction.
In this paper we showed that pilus-mediated movement is persistent even though the orientation of pilus motors is random. Persistence increased with the number of pili per cell. To resolve this discrepancy, we demonstrated that the force at which a single pilus detached from the surface was 10 times smaller than the motor force. We suggest a tug-of-war mechanism, in which the winning motors pull the opposing motors off the surface thereby generating directed movement.

***



LM12636

First Experimental Demonstration of Selective and Efficient Quantum
Evolution Characterization


The characterization of the temporal evolution of quantum systems is
not only one of the most important tasks in physics, it is also one of
the most difficult ones. In fact, the implementation of 'Quantum
Process Tomography' (the technical name under which the above task is
known) requires resources that scale exponentially with the size of
the system, making it practically impossible for large systems.
Moreover even extracting partial useful information about the
evolution also turns out to be exponentially hard. Quantum Process
Tomography is essential to achieve quantum information processing and
quantum computation (as it is required to device appropriate quantum
error correcting strategies to protect quantum information from
natural degradation inducing decoherence). Only recently methods have
been developed which can extract useful partial information about
quantum processes efficiently for systems of any size. In this paper
the authors present the first experimental implementation of a quantum
algorithm of this kind. The experiment, performed at a quantum optics
laboratory recently set up in Buenos Aires (Argentina) involves the
manipulation and detection of single photons where quantum information
is encoded both in the polarization and momentum degree of freedom.

***


LL11804

Bacteria stampede

Who gets trapped in the burning building: the stampeding crowd or the calm individual? The answer might be obvious if you're dealing with human beings, but we show that in the microscopic world of bacteria, it pays to stampede.

We create a microchannel no wider than a few human hairs, but more than one centimeter long, and fill it with about one hundred fish-trap like barriers that are difficult for incoming E. coli bacteria to cross. Although E. coli bacteria can migrate great distances, it is virtually impossible for a single cell to cross all 100 fish traps in sequence. If we add food on the other side of each barrier, however, something interesting happens: several hundred cells team up and collectively swim against all the barriers.

We show that the bacteria achieve collective escape because they are attracted by the food: if we allow the cells to deplete the nutrients, they no longer defeat the fish traps. This behavior also depends on the number of cells present: too few bacteria in the initial injection means they lack the critical density they need to escape the burning building and reach the banquet next door.

***

February 22, 2010

LM11942

Quantum Hindsight

Researchers have shown that by changing the way they process data, quantum-limited measurement devices can be twice as sensitive as they were previously thought to be. This means that scientists will be able to produce more precise measurements of the tiny objects that make up the quantum world.

Until recently, it was thought that the best strategy was to use a data processing technique known as quantum filtering which is based only on past observations. This paper presents the findings of recent experiments that demonstrate an idea introduced by M. Tsang only 6 months ago called quantum smoothing, which uses past and future observations.

Remarkably, the new technique can produce more precise results than filtering, without requiring any changes to the actual measurement device. Such a result is very significant for quantum sensing applications like gravitational wave detectors where it is more important to have precise rather than real-time measurement.

***

BK11152

HOW CARBON NANOTUBES CAN HELP IMPROVE MATERIALS?

Aircraft fatigue is a term generally used for failure of aircraft components under stress. Researchers now have found a possible solution for aircraft fatigue by designing improved materials. They used large scale computer simulations to fabricate composite materials by embedding carbon nanotubes (CNTs), one of the strongest materials known, into the nickel matrix, primary ingredients for air-craft engine. This work will be helpful for experimentalists to fabricate materials with stronger properties, which may eventually be applied to commercial airlines. The results predict that single-walled nanotubes do not help to improve the mechanical properties that determine strength of the materials. However, multi-walled carbon nanotubes with higher concentration are the material of choice for fabrication of composite materials and this improves materials properties. The research also predicts that there is strong interaction between nickel-matrix and carbon nanotubes.

February 19, 2010

BMR1106

Strong optomechanical interaction is achieved in novel bi-layer photonic crystals

Bi-layer systems have been studied in various fields in physics, such as quantum Hall
systems, quantum wells, graphenes, superconductors, and strongly-correlated electron
systems. Here, the authors realized a novel bi-layer photonic crystal * system (schematically
shown in the figure), for the first time, which can retain most of high performance in conventional
single-layer photonic crystal slabs but of which new degree of freedom in the
vertical direction leads to enormously large optomechanical interaction. Usually, optical
force (that is , radiation force) is very weak because a photon has very small momentum,
but it can be greatly enhanced by this structure. The authors experimentally demonstrated
large displacement of the slab by optical pumping via optically-generatged force. The energy
conversion efficiency is as large as 0.4microN/pJ, which is much larger than conventional
optical tweezers. This bi-layer configuration of photonic crystals may have strong impact
on emerging optomechanics engineering, and has potential to be applied to ultra-energy-
efficient optical force generators.

***

LJ12295




How the motion of micro-organisms can change the viscosity of liquids?

Take a droplet of liquid and estimate its viscosity by shearing it
between your fingers. This is the common perception of viscosity. If
some particles (like pollens for example) are present in the droplet, we
call it a suspension. The viscosity of a suspension increases in a
non-linear way with the number of particles. Now, instead of pollen
seeds, imagine that the particles are micro-organisms such as
spermatozoa, micro-algae or bacteria which can swim in fluids and are
classified as micro-swimmers or motile cells. In this letter, we show
that a given number of motile micro algae (Chlamydomonas Reinhardtii)
suspended in a droplet of water drastically changes the viscosity of the
resulting suspension (when compared to the same suspension made out of
dead cells). It is therefore possible to quantify the averaged motility
of a large population of a billion of cells. We believe that this study
will pave the way towards reliable quantitative study on other
suspension of swimming cells such as spermatozoa for example, for which
motility has a direct impact on fertility.

February 17, 2010

LL11905

Weighing "Invisible" Particles at the LHC through Singularities

We proposed a novel and powerful method for determining the masses of the
so-called "invisible" particles at the Large Hadron Collider (LHC).
These "invisible" particles, which escape direct detection because they
are electrically neutral and interact weakly, are expected in many ideas
for new physics at the Tera-electron volt (TeV) scale, and are in fact of
particular importance because they may make up the dark matter of the
universe. To test this hypothesis, it is crucial to measure their masses.

Mass measurement techniques based on studying specific kinematic
variables have been proposed, but previous approaches are not systematic
and are highly dependent on the process in question. In this paper, we made the key
observation that invisible particle mass measurements rely on mathematical
singularities in the distribution of the momenta of the observable
particles; these singularities arise because this distribution is a
projected image of the full (nonsingular) distribution of the particle
momenta. Based on this observation, we proposed the "algebraic singularity
method," a systematic method for finding all singularities and
constructing optimized kinematic variables for determining the invisible
particle masses. The algebraic singularity method generalizes and improves
upon the previously known techniques by providing a more mathematically
rigorous approach that can be applied in general situations. The method will be an
invaluable tool in the search for new physics and the tests of the TeV
scale dark matter hypothesis at the LHC.

February 14, 2010

LM12448

How do plastic balls break?

Fragmentation phenomena are ubiquitous in nature and play a crucial
role in numerous industrial processes related to mining and ore
processing. The most interesting feature of
fragmenting systems is that the size/mass distribution of pieces
exhibits a surprising universality, i.e. power law distributions are
obtained independent of materials' details and of the way of energy
input. Detailed laboratory experiments on the breakup of disordered
solids have revealed that mainly the effective dimensionality of the
system determines the value of the exponent, according to which
universality classes of fragmentation phenomena can be distinguished.

The breakup of heterogeneous brittle materials (rocks, concrete,
ceramics, glass,...) is very well understood by today, however, hardly anything
is known about the fragmentation of materials with more complicated
rheological behaviors such as plastic. In order to
understand how plastic materials fragment, we accelerated
polypropylene balls of a few millimeter diameter and impacted them
against a hard wall. In the experiments we found
again a power law distribution of fragment masses; however, the value
of the exponent 1.2 is astonishingly small compared to any known
exponents 1.9-2.4 of bulk materials. To understand the physical origin
of this novel behavior we worked out a discrete model
of plastic and performed computer simulations of the impact process of
balls. The simulations reproduce both the large permanent deformation
of plastic during impact, and the novel value of the mass distribution
exponent. We demonstrated that the dominance of shear in the crack
formation and the plastic material response are the key
features which give rise to the emergence of the novel universality
class of fragmentation phenomena.

Attached figure:
Final states of impact at low impact velocities in the experiment
(a) and in the simulation (b). In the contact area with the hard
wall large permanent deformations occur due to compression, while
above it vertical cracks are formed due to tensile stresses.
The simulations are in very good agreement with the measurements.



***

EL10508
Order and Chaos in the Slashdot Body Problem

Click image for animation

A novel analysis highlights the fascinating mix of simplicity and complexity in the orbits of a slash and a dot. A massive line segment or slash (/) and a massive point or dot (.) interact gravitationally to form a slashdot (/.) system. The resulting dynamics is especially beautiful, balancing order with chaos. Online movies and three dimensional strobed animations communicate the graceful pirouettes of this pas de deux. [http://www3.wooster.edu/Physics/Lindner/Research/SlashdotTrailsLarge.mov] The extension of the slash provides an extra degree of freedom that enables the interplay between rotation and revolution, which characterize actual planets, natural and artificial satellites, but not the interaction of idealized points. The International Space Station and a docking space shuttle form an extended million-pound system that is definitely not well-approximated by two points. The asteroid Ida and its moonlet Dactyl form a natural slashdot body system. The slashdot body problem is an instructive semi-solvable model problem in the vicinity of the famous two and three body problems that anchor celestial mechanics.


***

LC12642B

Thermal rectifiers; from nano-electronic devices to energy-saving buildings

Generally, rectification is a transport process that is faster in one direction than in the opposite. This phenomenon is well-known for the current of charges; diodes are electric rectifiers. Thermal rectification, i.e., the non-equivalence of the heat transport in two opposite directions, however, has been detected only recently. Thermal rectification phenomena can be used to create thermal diodes and hence lead to a revolution in the nano/micro devices. Moreover, based on this phenomenon, in macroscopic world, special walls and windows for the energy efficiency of buildings can be designed. Using molecular dynamics simulations, we have demonstrated that thermal rectification can be forced in mass-graded systems (i.e., via isotope substitution), via a gradient in the force-constant (i.e., via impurity doping), or in special designed topologies.


***

AH10518


Classical structures smaller than Planck constant do influence Quantum Mechanics.

It is a commonly held belief that dynamical classical structures smaller than Planck constant can not be "see" by quantum wave-functions and therefore have no effect on their evolution in time. This paper shows that this is not true in general: a correspondence is found between classical resonances (the ratio of two frequencies of the system being equal to the ratio of two integer numbers) and a number of quantum resonances (interactions between energy levels). In the system studied, the resonant quantum states reproduce the underlying resonant classical structures in phase space, even when these structures are smaller than Planck constant. Classical related quantum resonances are present only when the distance between two levels allows them to interact through the classical resonance itself. Varying one of the system parameters, several quantum levels can get close enough to enter resonances related to small classical structures, leading to significant population transfer. In the case studied in this paper, this results in to a remarkable agreement between classical, quantum, and experimental results. Studies of small classical structures could be used to improve methods for the transfer of population among quantum states.

February 4, 2010

LR11713

Collider Reaches Hottest Temperature Ever Achieved - 4 trillion degrees Celcius

The PHENIX Collaboration at Brookhaven National Laboratory's
Relativistic Heavy Ion Collider (RHIC) has found that collisions of
gold ions traveling at nearly the speed of light create matter at a
temperature of about 370 MeV (about 4 trillion degrees Celsius).
This is the hottest temperature ever reached in a laboratory, and
about one hundred thousand times hotter than the center of the Sun.
This temperature is higher than the temperature needed to melt
protons and neutrons into a plasma of quarks and gluons. Such a
plasma filled the universe a few microseconds after it came into
existence. At RHIC, the plasma is a freely flowing liquid with almost
no frictional resistance, or viscosity. The matter is formed, and
the quoted temperature reached, in less time than it takes light
to travel across a single proton. The temperature was determined
by looking at the color, or energy distribution, of the light emitted -
similar to the way one can tell that an iron rod is hot by looking at
its glow. Light interacts very little with the hot liquid produced at
RHIC, so bears accurate witness to the early conditions. We deduce
the yield of photons from the production of electron pairs beyond those
from particle decays. By matching theoretical models of the expanding
plasma to the data, we infer the initial temperature of the liquid.

LJ12535

Wind moves Martian sand with surprisingly little effort

Blowing sand plays an important role on Mars: the bombardment of the
Martian soil by bouncing sand provides the Martian atmosphere with
fine dust particles, which dominate the climate and help give the red
planet its distinctive color. Moreover, blowing sand is the driving
force behind sand dunes, which are abundant on Mars. In this paper, we
use sophisticated computer simulations to show that once the wind
becomes strong enough to start blowing sand on Mars, the sand will
keep blowing across the surface, even when the wind speed drops by up
to a factor of 10. This phenomenon, known in physics as “hysteresis,”
thus lets blowing sand occur on Mars at much lower wind speeds than
were previously thought necessary. This discovery helps explains a
long-standing contradiction: that sand moves in many areas of Mars,
even though wind speeds are only rarely sufficient to set the sand
grains in motion. It also could explain why dunes on Mars are much
smaller than expected from previous calculations and it could play an
important role in Mars’ abundant dust storms.

***

LB11991

CHOREOGRAPHED QUANTUM COIN TOSSING

If you flip a hundred coins, you are unlikely to get exactly fifty
heads and fifty tails; there is a statistical uncertainty in the
outcome. Researchers at MIT have reduced the statistical uncertainty
in the quantum mechanical equivalent of a coin toss. This quantum
mechanical coin toss is more than a game: its uncertainty limits the
precision of one of the world’s most sensitive measurement devices,
the atomic clock. An atomic clock consists of tens of thousands of
atoms, each of which can be in either of two states, much like a coin
that can show either of two faces. Each atom is placed in a quantum
superposition of the two states—each coin, as it were, suspended in
mid-air with the potential to land on either face. The researchers at
MIT use light to probe an ensemble of such atoms in a way that allows
them to count how many atoms are “heads” without revealing the state
of any individual atom—without disturbing the superposition.
Thereafter, the laws of quantum mechanics demand that the count remain
the same on any subsequent measurement. Thus, while each individual
coin continues to tumble at random, the tumbling of the different
coins is now choreographed: as one twists towards heads, another must
turn towards tails. In the jargon of quantum mechanics, the states of
the different atoms are now entangled. When one ultimately measures
the states of the individual atoms—letting the coins land—the
statistical uncertainty in the outcome is reduced. Just such a
measurement is used to read out an atomic clock; if the clock is
operated in an entangled state, its precision is no longer at the
mercy of an ordinary coin toss.

LL11945

Reducing quantum noise in atomic clocks with light

Researchers have demonstrated a new method to improve the sensitivity
of atomic clocks, by using a light field to shuttle information
between distant atoms so that their quantum errors cancel out.
Ensembles of atoms in vacuum are exquisitely sensitive detectors for
measuring time, magnetic fields, gravity and other fundamental
physical effects. Normally, each atom acts as an independent
detector, with some unavoidable measurement noise due to quantum
uncertainty. Adding up the signal from these independent atoms also
adds up their noise. However, if the atoms can communicate, they can
be coaxed into an entangled quantum state where fluctuations of
different atoms are correlated in such a way that the total quantum
noise is reduced, while the full signal is maintained. This is
achieved by placing the atoms in an optical resonator, configured so
that the intensity of the light circulating within it depends on the
internal state of all atoms in the ensemble. Distant atoms, which
never interact directly with one another, nevertheless acquire quantum
correlations as they experience a light field that depends on the
state of all their peers. The new method results in the greatest
entanglement-enabled improvement in signal-to-noise-ratio to date.

January 29, 2010

LE12795

Loops, leafs, and optimal transport networks

In this paper we show that the optimal architecture of a transport network that is subject to random damage or fluctuations in the load is not the simple tree-like architecture of networks optimized for efficiency. Instead, it contains intricate, hierarchical loops that obviate the ordered branching from top to bottom level. The intricate architecture of the loops allows the flow to be optimally redirected in response to disruptions, and might confer a strong evolutionary advantage to the organisms that adopt it.

Leaf venation is a pervasive example of an organism endowed with a complex biological transport network. Dicotyledon leaf venation in particular has a large number of functional, nested, closed loops.


Damage, as would be imposed on a leaf by an insect of herbivore, or fluctuations in the load, as observed in the leaf stomatal openings when the leaf is under conditions of stress, might be the driving force behind
the reticulate, elaborate venation patterns that we are so familiar with.


***

LH12229DR

The Holographic Universe

In the paper "Holography for Cosmology" we develop a holographic theory for the very early Universe,
the epoch when the seeds for the structure we see in today's Universe (the stars and galaxies)
were first created. The holographic principle, suggested by 't Hooft and Susskind in the
early nineties, states that our Universe resembles a hologram:
everything in the Universe, even the laws of physics themselves, may be reconstructed from
a theory living in only two space and one time dimensions. This theory contains forces
such as the electromagnetic and nuclear forces, but not gravity. Examples
of how this might work have been developed in string theory over the
last ten years. In this paper, we show how the very early Universe may be described holographically.
The holographic framework we develop automatically incorporates the conventional theory - inflation -
which proposes that the early Universe underwent a brief period of accelerated expansion.
However, our holographic framework also reveals completely new theories that only
have a holographic description and were thus invisible to previous approaches.
We show that these new theories are consistent with current observations, yet their predictions
are nonetheless different from standard inflation allowing for a 'smoking gun'-type detection.
Over the next few years, new observations from the Planck satellite and other experiments are expected to
pinpoint the precise value of many cosmological parameters: the results might well provide the first direct
observational evidence for the holographic nature of our Universe.


***


LM12444

All correlations, one theory

Entanglement, a quantum correlation, is called by some to be the
resource that allows faster computing, better measurements, and secure
communication. Others have argued in favor of other quantum
correlations for such enhancements, like quantum discord, an
information theoretic measure of 'quantumness' of correlations. In
this paper, we develop a method to quantify different correlations as
distances. Our method allows us encompass entanglement, quantum
discord, and classical correlations under a single umbrella theory and
is applicable for arbitrary number of particles. We further go on to
construct an new quantity, called quantum dissonance, which similar to
quantum discord but excludes entanglement. We found in our studies,
that many of these correlations added to give other correlations,
called additivity properties. All of these facts put together, our
theory will allow physicists to study and compare different
correlations for a variety of experiments and applications.


***

AJ10498

Atmospheric turbulence can be useful for global quantum
communication

The reliability of modern secure communication, including electronic
payment or secure network protocols, is based on the ambiguous
supposition that some mathematical problems cannot be resolved in a
reasonable time. The situation can be successfully improved by the
methods of quantum cryptography, which provides a physical defence
for sending confidential information. A problem is that such
methods, which are usually based on quantum light, appear to be
highly fragile. With increasing losses in the transmission channels
the chance for a successful realization of quantum protocols quickly
diminishes.

In a recent experiment the group of A. Zeilinger has demonstrated
that light may preserve its quantum properties even after
propagation through extremely lossy atmospheric channels [Nature
Physics 5, 389 (2009)]. We have theoretically analyzed this
interesting situation and obtained a clear interpretation of the
experiments. Due to the turbulence phenomena, the atmosphere
temporarily appears to be very transparent. The used measurement
techniques just choose such events and discard the data recorded
with really high losses. This knowledge may play an important role
for designing systems for purposes of global quantum communication
based on satellites.

***

LH12061

Neutral but not indifferent: When net neutral bodies behave like charged
ones

That likes repel and opposites attract is a statement that does not seem
to hold only for
electrostatics but has found its way from physics into broader aspects
of life as well.
In the past several years it has become clear though, that in the case
of strongly charged
bodies likes can attract and opposites repel! This constitutes a major
shift in paradigm
in colloidal and soft matter science in general and has allowed us to
get a deeper
understanding of complicated phenomena like DNA collapse. We now show
that there
can be long-range interactions even between net neutral objects if they
carry small
amounts of positive and negative charges, randomly frozen within their
body or on their
surface. The physical basis of this effect is that even though a frozen
charge in the system
on the average feels no net charge from other charges, it feels its
image charges and is thus
either attracted to or repelled from its image charges. This long-range
interaction stands
at odds with the commonly accepted view that neutral objects can
interact only via
multipolar or van der Waals forces and could have major implications in
other fields
such as colloidal science. Our paper shows that a disorder induced
long-range
attraction between net neutral bodies may even swamp the ubiquitous
Casimir force
under certain circumstances. Indeed, such an additional attractive
force has recently
been inferred from experimental observations and our work may help
disentangle
disorder effects from the real Casimir effect.

January 27, 2010

BKR1161

Multiple Exciton Generation in InP Nanoparticles Promises Safe, Cheap Solar Power

The efficiency of solar cells is currently limited by the fact that each photon of sunlight absorbed produces, at best, only one free electron. Multiple exciton generation (MEG) in nanoparticles has been exciting many researchers in the field because it is process by which a single solar photon can generate mulitple electrons and thereby boost solar cell efficiency. MEG has been observed in a number of different nanoparticles before but these have either contained toxic materials such as lead, cadmium or arsenic, or were made by expensive, energy-intensive processes. In this paper, researchers have now shown that MEG can be efficient in indium phosphide nanoparticles, which are non-toxic and can be made by a cheap, solvent chemistry based method. MEG-boosted solar cells based on cheap, non-toxic nanoparticles have the potential to compete economically with fossil fuels whilst being less damaging to the environment.


***


LJ11735E


Maxwell’s Demon controls heat transport without thermal bias


As our everyday experiences, heat transfers spontaneously from high temperature to low one, thus so far the control of heat flow is based on the thermal bias. However, is there any hiding place for the Maxwell’s Demon to generate and direct heat flow without temperature difference? In this work, researchers have proposed a novel way for the spontaneous emergence and control of heat flow from ZERO thermal bias on molecular level, which somewhat is in contrast to common sense. Their work reveals three necessary conditions for Maxwell’s Demon, i.e., generating and controlling heat flow without thermal bias at nano-scale: (1) non-equilibrium source; (2) symmetry breaking; (3) nonlinearity. In addition, it is shown that when the two heat baths are correlated, only (2) symmetry breaking is sufficient already. These intriguing results will advance our understanding of energy transfer and control in nano-scale from the viewpoint of statistical mechanics and nonlinear dynamics. Also, these results will shed light on the novel design of nano-scale heat engine (pump) and molecular machines in the field of nano-technology.


***


LK11907


Noise encodes network structure in time series

Can the internal structure of the Internet be probed from fluctuations in the data flow? Can the topology of the complex brain network be unmasked by observing neuron's activities? Can the structures of gene regulatory networks be revealed by their expressions? Can the relations among companies be inferred from the time series of stock price? An international team from Singapore and the United States has provided an affirmative answer to these questions.

In a recent article to be published in PRL, described a general noise-induced relationship between the topology of and dynamics on complex networks. Noise, usually regarded as annoying and harmful for obtaining and analyzing information, actually encodes the underlying network structure. The team then uses this relation to precisely identify links based on the measured time series from nodes. They demonstrated that prediction of the full network topology becomes possible even when the underlying network structure and dynamics are totally unknown, which makes the methodology extremely appealing to practical applications. The approach appears to be the first that is capable of predicting the full network structure by taking advantage of noise.

The results not only are important to fundamental science, but also can have significant applications in diverse areas such as computer networks, biomedical systems, neuroscience, socio-economics and even homeland defense. In the future, making a microscope of complex networks may not be Arabian nights. Just listen to the noise-induced fluctuations!

January 23, 2010

LB11875

Backwards Sound

We fabricated one-dimensional acoustic medium in which sounds propagated backwards. Sound characteristics is determined by the mass density and elastic stiffness (bulk modulus) of the acoustic medium. If the density and the modulus are simultaneously negative (double negative), the velocity of sound is negative. No naturally occurring material, however, has negative density, nor negative modulus. In our medium the density and the modulus were made effectively negative with an inter-spaced regular array of very thin tight membranes and transverse air-columns. Our double negative material exhibited negative phase velocity, which means that the wave crests moved towards the source. Backward sound propagation is a new wave characteristics that has not been observed until the present work, and is expected to bring powerful applications on carrying information and energy.

January 22, 2010

LE12390BR

Atoms rattle in Boron due to frustration

Could an elemental crystal prefer to be defective rather than
perfectly ordered? A recent study shows that this is indeed the case
for Boron, and that its defects arrange in a very peculiar geometry.
This geometry and the way defects interact are related to a very well
know concept in the physics of disordered systems: frustration. So in
addition to being a defective solid, boron is also a frustrated element

In general chemistry classes we learn that all elements, with the
exception of helium, solidify into an ordered crystal structure at
low temperature and that defects destabilize ordered solids, making
their energy higher. Therefore it has been a mystery for decades why
the fifth element, boron, has an extraordinarily complicated crystal
structure with 4 percent atomic defects.

Research now shows that it is precisely the presence of these
defects that stabilizes the complex phase of boron against all the
other allotropes, by providing a geometry where the number of bonds
is commensurate to the number of electrons. Most surprisingly, the
geometrical arrangement of these defects forms a very specific two-
dimensional geometrical lattice known as a double layer expanded
kagome lattice. The hopping or “rattling” of boron defects between
nearly degenerate configurations appears to be responsible for the
peculiar transport properties of boron that have puzzled scientists
for the past four decades.

***

BK10995

The strength of diamond at extreme conditions.

New measurements using intense laser pulses show that diamond - the strongest solid- becomes even stronger during rapid compression, supporting almost a million times atmospheric pressure before being crushed (dynamic strength exceeding 60-80 billion Pascals, depending on crystal orientation). Diamond has been characterized at high pressures and temperatures using shock waves generated by the intense laser pulses. Diamond is found to exhibit considerable strength right up to the point that it melts, at around 6 million atmospheres pressure and 14,000 degrees Fahrenheit (8000 Kelvin). These findings underscore the remarkable properties and technological utility of diamond: for example, as a capsule material for fusion-energy experiments at the new National Ignition Facility, the world's largest laser. They may also provide insights into the ancient history of natural diamonds found on Earth and in meteorites, where shock waves caused by impact are common.

January 19, 2010

BMR1099

Taking snapshots of a magnetic multilayer within 30 femtoseconds at the
X-ray laser FLASH

X-ray free electron lasers (XFEL) are unique machines providing
ultra-short flashes of x-ray photons of unmatched intensity. The number
of photons delivered in a single 20-30 femtosecond long XFEL pulse
equals the number of photons delivered within one second at a
synchrotron source. Spectacular new possibilities open up when using the
photon energy of today's FELs at X-ray energies where inner shell
electrons carrying magnetic moments can be excited. The very short X-ray
pulses will then allow to record magnetic diffraction pattern within
femtoseconds - the timescale of spin-flip processes in magnetic
materials. Our paper demonstrates this possibility of performing
resonant magnetic scattering experiments with a single pulse of the
(linearly polarized) UV/soft x-ray free-electron laser FLASH at DESY,
Hamburg. The magnetic domains This means that a new probe of magnetic
properties is now available, that allows to perform magnetic diffraction
experiments within a sub- 100 fs time-scale.

***

LY11228B


SILICON HARDWARE MAY OUTLIVE MOORE'S LAW

The end of the road for Moore's law, predicted to occur within a decade,
is associated to the fall of the computer-chip silicon supremacy. But this
may not be the case with the advent of new proposals for future silicon
computers, based on quantum behavior at a microscopic level. This paper
proposes a new strategy for quantum computation, which takes advantage of
the available silicon-based microelectronics technology and of optical
techniques borrowed from atomic physics. The estimated errors in the
operation of the device should be sufficiently small to allow for useful
applications in numerical simulations and quantum optics. The basic
processing units (quantum bits or qubits) are arsenic atoms implanted in
photonic-crystal silicon cavities, which are known to have very small
losses. Operations on single qubits and pairs of qubits, which are the
basic blocks for universal quantum computation, are implemented by a
combination of laser pulses and static electromagnetic fields. Reducing
the errors per operation in a silicon-based architecture is an important
step in the quest for viable quantum computers, and an indication that
silicon hardware technology may outlive Moore's law.

***

AK10597

Critical temperature of the interacting Bose gas - Case closed?

The dependence of the critical temperature for Bose-Einstein
condensation (BEC) on particle interactions has been a topic of
debate for many decades. It was not until 2003 that a consensus
emerged in the physics community, that is supported by numerical
and field theoretic considerations: Weak interactions enhance BEC,
and the dimensionless constant that controls the linear correction
of the critical temperature is c = +1.3. The flow of articles devoted
to this topic then came to an end.

Motivated by probability theory and statistical mechanics, we have
revisited this question using the model of "spatial permutations",
where quantum particles are represented by Brownian trajectories,
and where the onset of BEC is signaled by the occurrence of infinitely
long permutation cycles (see the illustration). Much to our surprise,
our calculations did not confirm the consensus, but led instead to
the constant c = -2.33. This means that particle interactions
discourage BEC! Our method involves several independent steps,
each of which was carefully justified. At the very least, our article
suggests that the case of the effects of interactions on BEC should
be reopened.

***

LL11906


Mind the Twist

Nanotubes can vary by diameter and length, but also by the angle at which
they are twisted. This later property is known as chirality and a useful
analogy is a roll of gift-wrap paper. If the roll is rewound carefully,
there is no overhang on either end. However, if the roll wound at an odd
angle, technically known as the Eshelby¹s twist, excess paper hangs off at
one end. Recent computational investigations of inorganic metal sulfide
nanotubes, published in Physical Review Letters, revealed the interesting
connection between chirality and the macroscopic phenomena of Eshelby¹s
twist. Most remarkably, the study demonstrates that varying chirality can
have a major role in engineering the mechanical and electronic properties of
these nanostructures. Responsible for this effect is the Eshelby¹s twist
present in chiral tubes.

***

LD12443

Chemistry produces fluid motion

Hydrodynamics is known to impact reactive processes, as stirring is
often used to enhance reaction rates. In this work, we
show that the reverse is also true: chemical reactions as simple as
A+B->C can strongly affect or even trigger fluid flows. By modifying
the density of a given solution, chemical reactions are indeed
strongly impacting buoyancy-driven flows. Not only are reactions
breaking the symmetry of classical hydrodynamic instabilities and
patterns, they can even induce flows in systems that would remain
quiescent in the absence of reaction.
Using a reaction as simple as the HCl-NaOH neutralization known to
every scientist, the authors demonstrate experimentally that
chemically-driven convection can develop. The beautiful patterns
appearing are asymmetric with regard to the initial contact line
between reactants. Numerical simulations of a
reaction-diffusion-convection model quantitatively explain the
instability scenarios and highlight conditions for which
chemically-driven flows can be observed.

***

LK12198

Freezing Friction – Microscopic bonds determine rubbing at cryogenic temperatures

In winter everyday experience teaches us that friction may dramatically increase if sliding surfaces are freezing together, an effect related to freezing water trapped at the sliding interface. But how does friction behave at cryogenic temperatures under perfectly dry conditions? In our research we investigated the friction of a nanoscale contact at temperatures down to -200°C in perfectly clean vacuum conditions. On different surface materials we consistently find an almost tenfold friction enhancement below temperatures of -120°C even in the absence of any water molecules. Our model simulations show that this effect is related to the dynamic behaviour of multiple atomic bonds at the sliding interface. Temperature leads to an enhanced attachment as well as rupturing rate of molecular scale bonds between the sliding surfaces, resulting in a peak-like friction increase at cryogenic temperatures, in agreement with the experiments. These results provide a new conceptual framework to describe the dynamics of dry friction.

***


LK12520

Can one see through paint?

Light propagates through a disordered scattering medium (such as a layer of paint or a biological tissue) in such a complex way that all spatial information seems lost, which shortly prevents imaging or focusing. In this paper, we present a method to measure the transmission matrix of a scattering medium, i.e. the relationship between what enters and what goes through the medium, independently of how long and complex the propagation has been. On top of demonstrating our ability to focus light through an opaque layer of paint, we show that we are able to reconstruct or “view” a simple object through it. Beyond the obvious imaging applications, this technique provides a new tool to understand and study the intriguing domain of wave propagation in complex media.

***

LJ11739


Large increase of the Curie temperature by orbital ordering control

We were able to first theoretically predict and then synthetize
superlattices
with Curie temperature (up to which the magnetic order is stable) far
above the bulk one of the building magnetic material and far above room
temperature (up to 1000K).

The stability of magnetic orders is directly related to the effective
magnetic exchange integrals between neighboring magnetic atoms.
Simultaneously, these exchange integrals are strongly dependant of weak
structural distortions due to the quasi-degenerate and highly
directional character of the magnetic d or f orbitals.
Synthesis in thin films allows a certain control over the material
structure through the in-plane coercion applied by the substrate.
Using fist principle calculations we showed that the Curie
temperature of manganites thin films can be increased by more than an
order of magnitude by applying appropriate strains. We
showed how the constraints, first applied by the substrate of the film,
need to be
maintained over the growth direction by the alternation of a manganite
layers with another appropriate material. Finally we synthesized such
super-lattices and verified the theoretical predictions. Indeed,
super-lattices with Tc up to 1000K were obtained.

The present results provide a new set of concepts for the control of
magnetic order and Curie temperature and thus open new perspectives to
design artificial materials with desired magnetic properties.

***

LF12190

Material failure classification breaks up!

Dynamic fracture drives catastrophic materials failures. Engineers and researchers generally categorize materials into three classes: (i) Ductile materials, like metals, which develop irreversible plastic deformations before breakdown (ii) Quasi-brittle materials, like rocks or mortar, which damage through the nucleation of microcracks before breaking through their coalescence (iii) Brittle materials, like glassy polymers and oxide glasses, which behave elastically up to failure, which occurs through the stretching and breaking of the atomic bonds at the tip of the propagating crack. The experiments presented in this paper challenge this classification and show that a given material can belong to two of these categories depending on the crack velocity. Beyond a critical velocity which is a well-defined fraction of the upper limit for crack speed, Plexiglas (that was considered up to now as an archetype of nominally brittle materials) damages through microcracking ahead of the main crack tip, as in quasi-brittle materials.

January 8, 2010

LG12689

Turning Lord Kelvin's 1875 argument into a general approach to determine stability of fluid flow


Over a century ago, Lord Kelvin proposed an argument for stability of flows with negligible dissipation. The practical application of Kelvin’s ideas has been the subject of extensive debate among researchers from Caltech, Princeton, and Cambridge University, and elsewhere. As a result, it is presently believed that Kelvin’s argument cannot be used to determine stability, for most flows of practical interest. In our Letter, we overturn what has been, so far, the accepted wisdom on the use of Kelvin’s argument.

Flows with negligible dissipation are important in a wide variety of fields; examples include vortices and waves in the atmosphere and oceans, plasmas in nuclear fusion reactors, liquid helium at temperatures near absolute zero, and the dynamics of quantum condensates. Determining whether such a fluid flow is stable usually requires laborious calculations, if standard techniques are used. On the other hand, the approach that we introduce can be used with little effort, since no additional stability calculations are needed, beyond those required for finding the steady flows. Instead, we rely on the construction of a special bifurcation diagram involving the velocity and impulse of the equilibrium flows. In addition, we discover new families of solutions, originating at otherwise hidden bifurcations, by effectively promoting symmetry-breaking behavior in the fluid.

We have applied the resulting approach to a wide range of fluid flows. For all cases considered, we find precise agreement with more involved stability approaches. In addition, for each flow examined, we discover new, symmetry-breaking, fluid flows.

January 4, 2010

LK12193

Terahertz sound laser

This paper presents new experimental evidence for sound laser, or saser (for: sound amplification by stimulated emission of radiation), action in a semiconductor nanostructure under electrical excitation.

Four years ago we demonstrated a semiconductor device which showed some of the characteristics expected of a saser, emitting sound in the terahertz (1012 Hz) range when excited by an electrical current [Phys. Rev. Lett. 96, 215504 (2006)]. However, at the time it was not possible to measure the spectral properties of the sound generated in the device.

It is well known that optical lasers emit a beam of spectrally pure light, i.e. spectral line is narrow and the radiation is coherent, and the same should be true of a saser. In this latest paper we demonstrate that acoustic spectral line narrowing occurs in the saser device. We show the acoustic spectrum sharpens up and the sound becomes more coherent when a voltage is applied to the device.

As a source of spectrally pure sound having nanometre wavelengths, a Terahertz sound laser could find many applications including in probing and imaging of nanostructures and high-speed “clocking” of electronic nanodevices.

***

LK11905

Fluid Metamaterials

Metamaterials, artificially structured composite materials with extraordinary properties, have attracted considerable interest around the world. In one recent paper published in Physical Review Letters, a new class of metamaterials in a fluid configuration is proposed. Such metamaterials exhibit unique characteristic “softness”, in sharp contrast to previous metamaterials made of solid materials. Moreover, the tunability of the proposed metamaterial controlled by an external magnetic field promises a number of reconfigurable optical devices. The new metamaterial is an aqueous fluid system which contains spherical Fe3O4 nanoparticles, each coated with a thin Ag shell. In the presence of a DC magnetic field, nanoparticle chains can be formed as the magnetic dipolar interaction overwhelms the thermal energy. It is theoretically demonstrated that light can be bent negatively for all incident angles and over a wide frequency band in the optical region, due to the strong anisotropy of the nanoparticle chains. With respect to future applications, the self-assembly system of nanoparticles regulated by magnetic fields may realize three-dimensional bulk metamaterials, which are difficult to fabricate using top-down techniques. More importantly, it is possible to engineer the local density of nanoparticles, thus the spatial profile of the dielectric constant, to implement tunable optical devices, such as invisible cloaks, lenses and waveguides in an aqueous environment.

***

LE11953A

Entangling isolated elementary particles

The non-classical, non-local quantum correlations so called entanglement are one of the most remarkable features of quantum mechanics. A variety of systems have been entangled in real experiments, including photons, ions, atoms, or superconducting qubits. However, no isolated elementary particles other than photons have been yet entangled.

In this work, we give a proposal for entangling two electrons -elementary particles- trapped in a Penning trap. The proposal is based on building blocks already proven with single electrons, in Harvard experiments. In those experiments, the aim was to measure the electron´s magnetic moment and the fine structure constant, one of the most important fundamental constants of Nature, to an unprecedented degree of accuracy.

We analyze possible applications of this study, including investigations of decoherence with isolated elementary particles different from photons -not realized so far-, and universal two-qubit gates for quantum computing with electrons. Additionally, we propose a protocol for measuring the so-called anomaly frequency, necessary for measuring the electron´s magnetic moment, by using two entangled electrons, based on quantum metrology theory. This would allow in principle to reach the Heisenberg limit of precision, the maximum allowed by the laws of quantum mechanics, beating the classical shot-noise limit, the ideal limit with classical physics.

***

LF12024BR

How atoms make sound

We have measured the tiny atomic movements responsible for the
macroscopically observed giant magnetostriction effect.
The sound produced by our loudspeakers derives from vibrations induced
when magnetic energy is transformed into mechanical energy.
At an atomic level this occurs because some specific atoms (the
so-called rare earths) strongly deform following the direction
of the magnetic field and “move” the neighboring atoms around.
We have been able for the first time to simultaneously observe the
distortion of the electronic cloud in the Tb atoms
and the tiny changes in neighboring bond lengths (of the order of 60
femtometers) in a film of amorphous TbFe2.
Being able to quantify the amplitude and sign of atomic displacements is
important because in general these manifest the competition
existing in matter between various energy terms.
In magnetic systems, as is the case here, they provide a unique tool for
analyzing the coupling that exists between magnetic
and elastic properties at the atomic scale.

December 31, 2009

LL12277

Nonlinear diffusion model for Rayleigh-Taylor mixing

An heavy fluid placed over a light one is a classical hydrodynamic
instability studied by Lord Rayleigh more than one century ago.
The development of the instability at the interface of the
two fluids generate a mixing layer in which heavy and light fluids
are well mixed by turbulence generated by gravitational forces.

In this Letter we develop a simple closed model for the spatial-temporal
evolution of the mixing layer based on a non-linear diffusion equation.
A variance with usual constant diffusion model, here the diffusivity
is allowed to depend both on time and space, reflecting the complex
structure of the evolving mixing layer.

The nonlinear model reproduces with high accuracy the density
profile of the mixing layer measured in high resolution Direct
Numerical Simulation of the complete Navier-Stokes equations and
predicts the temporal evolution of global quantities such as the
vertical density flux and the width of the mixing layer.

***

LE12465

Understanding the plant cell's self-assembling corset

Growing plant cells show a peculiar structure, the cortical array, made
up of parallel stiff protein tubes called microtubules. It is thought
that the cortical array plays an important role in the growth of cells,
and that it emerges spontaneously from the interactions of the microtubules.

Unlike humans, plants cannot rely on a skeleton for their rigidity;
instead, every cell must contribute by being enclosed in a rigid cell
wall. This presents a challenge, because for the plant to grow in length
these cell walls need to be stretched in a particular direction. Plant
cells have solved this by placing long fibers in the wall, wrapping the
cell like a corset and preventing it from expanding sideways.

These fibers reflect the structure of the microtubules that are inside
the cell, attached to the cell membrane. These microtubules effectively
‘crawl’ across the cell membrane, they grow or shrink, and they can
collide with one another, resulting in shrinkage of a growing
microtubule or its reorientation. It is believed that the collisions
allow the microtubules to ‘negotiate’ a common orientation.

In this paper, we present a model of interacting microtubules, based on
biological observations. The model allows us to make predictions about
the circumstances in which the microtubules can successfully align. We
compare the model predictions with the results from computer
simulations. Our results suggest that the collision-induced shrinkage of
growing microtubules plays a dominant role in their alignment.



***

LK12170BR

SHEDDING LIGHT ON ELECTRONS IN LUMINESCENT ORGANIC SEMICONDUCTOR NANOSTRUCTURES


In organic semiconductors, molecular motifs are the building blocks of periodic architectures, and delocalized electrons within individual molecules interact weakly between molecules. The nature of intermolecular electronic interactions is what defines the semiconductor properties of these advanced materials, including electronic transport and optical properties. These materials are now used in a variety of applications such as organic light-emitting diodes and plastic solar cells. In this letter, an international research team explore the nature of electronic interactions in an organic semiconductor nanostructure by studying the dynamics of electron relaxation to produce light. By exciting the nanostructures with short laser pulses and analysing the time dependence of their subsequent light emission, the research team unravel the spatial extent of electronic intermolecular interactions. Electrons in neighboring molecules interact strongly compared to many organic semiconductor used in applications, such as polymers, allowing electrons to transfer readily between molecules on very fast timescales. However, the spatial extent of excitations resulting from such transfer processes, generated by the laser pulse, is only delocalized over two to three molecules due to strong modulation of the electronic energy by reorganization of the molecular geometry. This work opens the door to the understanding and design of novel organic semiconductor materials for applications in optoelectronics.


***

LK12568


Discovery of the Heaviest Halo Atomic Nucleus

Physicists have measured the radius of Carbon-22, an extremely neutron-rich
carbon isotope with a nucleus comprised of 6 protons and 16 neutrons. In
this paper the radius of Carbon-22 is reported to be twice the
size of the Carbon-12 that predominates in nature. This is a remarkable
finding since the size of a normal nucleus is known to be proportional to
the cube root of its mass number and (22/12)^(1/3) is only 1.2. The radius
of Carbon-22 exceeds that of Lead-208. Nuclei with abnormal sizes are
known as "halo nuclei". The only other halo nuclei observed to date are
Helium-6, Lithium-11, Beryllium-14 and Boron-17. Carbon-22 is the heaviest
halo nucleus found thus far.

December 23, 2009

LH11964

What crumpled aluminum in common with liquid crystals?

Take a sheet of aluminum foil from your kitchen and crumple it
as hard as you can. Give it a few whams with a hammer before
throwing the ball into the water. You will be surprised to find
this metal ball floating, which suggests that it still contains more
than 70% of air. An architect will be dying to know the secret to
this amazingly strong structure out of such magically little material.
In this paper, we employed a special version of microtomography,
similar to the CT scan in medical hospitals, to study the interior of
a crumpled aluminum ball. Patches of ordered domain were found to
appear near the crust, which is reminiscent of the lamellae phase in
lyotropic liquid crystals. We believe the strong resistance offered by
a crumpled ball is intimately related to these brick-like domains whose
lateral size shortens while thickness grows with further compression.

***

LH12192

Whispering Gallery Tube Resonator for Spin Waves

We present a novel type of spin wave resonator which resembles the universal concept of whispering-gallery modes as discovered by Lord Rayleigh in 1910, and well known from acoustics and optics, for spin waves.

We employ strain driven ‘rolled-up nanotech’ to curl up permalloy/semiconductor nanolayers with multiple rotations into a rolled-up-carpet shaped micro tube (cf. teaser image). The homogeneous magnetization in our rolled-up structures resembles the refractive index in optical resonators or the stiffness in acoustic materials and can be tuned simply by changing the external magnetic field.

In contrast to spin-wave resonators presented so far in literature, which are often dominated by complicated inhomogeneous magnetization patterns arising from the edges of the structures, our rolled up structures exhibit well separated sharp spin-wave resonances. The frequencies of the resonances can be understood considering the mode interference in a closed geometry very similar to acoustic or optical whispering gallery modes. The modes exist over a broad magnetic field range and can be tuned over several GHz. These findings are not only appealing from a general standpoint of physical aesthetics but also of high importance for future logic spin-wave devices which require high quality tuneable spin-wave filters.


***

EK10568

Not so spherical bubbles

Small bubbles in liquids are round because surface tension forces them into a shape with minimum surface. Yet bubbles in nature happen in all shapes, in particular when external forces act on them, such as gravity or an external flow. Researchers from the Nanyang Technological University now devised a new technique to create bubbles on demand with arbitrary shapes. They are using a laser and a digital hologram to create, ellipsoidal, toroidal, and square shaped bubbles. All these bubbles are short lived and therefore have to be recorded with a high-speed camera. Some of the bubbles change shape during their lifetime. Their dynamics is compared to simulations and excellent agreement is found. Initially their research was curiosity driven, yet interesting applications open up from these not so round bubbles. These bubbles create unique flow patterns which have not been available so far, e.g. a compression only flow is occurring at the center of a donut shaped bubble. This and other flow patterns are promising to assess mechanical properties of elastic objects in liquids, such as the rigidity of biological cells or the stability of emulsions.

December 15, 2009

LF12555E


If the leaf can do it, can we do it even better?

Photosynthesis is the process by which light energy is converted to chemical energy. It is one of the most important biological processes as it provides energy, food, and oxygen for all organisms living on earth. Artificial photosynthesis is a novel strategy with the aim to improve the underlying mechanism for producing sustainable fuels. The question is: Can we beat nature at her own game?
Plants and many species of bacteria are photosynthetic. The light energy is trapped by the chlorophyll molecules which are arranged as regular arrays within membranes. When a chlorophyll molecule absorbs light, it is electronically excited. It may either realize a photochemical energy transfer or return to its ground state through relaxation mechanisms. A high efficiency of photosynthesis requires a minimization of the relaxation mechanisms.
In this paper, we introduced neutron spectroscopy to measure the light-induced relaxation effects on a photosynthetic membrane system. We detected the existence of photo-excited lattice modes which tells us where, and how much, energy is lost in the initial steps of photosynthesis. In addition, we found evidence for „freezing“ of lattice modes under light irradiation which results in an optimum orientation of the chlorophylls. These exploratory results are essential towards the engineering of new photosynthetic materials to improve the efficiency of the photosynthetic process beyond the limits given by nature.

***

LH12180

Cloudy skies above a metal surface

According to quantum mechanics electrons that carry electrical currents in metals do not appear as particles with a well defined position, but rather as fuzzy moving objects, i.e. electron clouds. While it has been known for a long time that electron clouds can escape from the metal surface and move freely above it, we demonstrate in our paper that the clouds can get trapped above a small nanometer size metal island.

The electrons escaping from the surface still interact with the surface because they create positively charged holes that attract the negatively charged electrons. This way a series of “image-potential” states with quantized separation emerges that can be occupied by the electrons. The occupied states are detected with the atomically sharp needle of a tunneling microscope that probes the shape of electron clouds by measuring the electrical current flowing between the needle and the clouds. Our results reveal the triangular symmetry of the clouds that are trapped by the edges of a triangular cobalt island on a flat gold surface. The fuzzy edges of the clouds reflect the “uncertainty” that is imposed by quantum mechanics, implying that the clouds can leak out from the triangular confinement.

***

LL11820
Atomic-scale Distribution of Water Molecules at Mica/Water Interface Visualized by Three-dimensional Scanning Force Microscopy

We have developed a method referred to as three-dimensional scanning force microscopy (3D-SFM), which enables to visualize water distribution at solid/water interface with atomic-scale resolution in less than 1 min. Interfacial phenomena at solid/water interface play important roles in the industrial technologies as well as in the biological processes. However, the lack of a method able to visualize 3D distribution of water molecules has impeded progresses in molecular-scale understanding of such interfacial phenomena. 3D-SFM sheds light on this issue. As the first application of 3D-SFM, we investigated mica/water interface. This interface has been intensively studied due to the importance in ecological and geological sciences as well as in engineering and physics. The atomically-resolved 3D-SFM image visualizes 3D distributions of adsorbed water molecules as well as a hydration layer. In addition, the image also reveals the detailed atomic-scale structure of a cleaved mica surface next to an aqueous environment, indicating the existence of surface relaxation. So far, such real-space information has been discussed based on indirect spectroscopic data obtained by beam technologies. The direct real-space 3D images obtained by 3D-SFM should provide important information that has not been accessible by conventional technologies.

December 3, 2009

LG13021

Dense Magmas Deep within the Earth

The nature of silicate glasses and liquids under high-pressure
conditions are of key importance for deducing the formation and
differentiation of the terrestrial planets, especially through massive
primordial melting events of the protoplanets. Extensive melting of
the primordial terrestrial planets and the formation of a deep “magma
ocean”. Because silicate melts are so compressible, melts become
denser than their crystalline counterparts under high-pressure. This
characteristic is crucially important to consider in the evolution of
terrestrial planets. Murakami and Bass (PRL in press.) experimentally
examined the densification mechanism of SiO2 glass, the simplest
analogue for silicate melt, up to pressures equivalent to ~3500 km
depth in the Earth’s interior by the acoustic wave velocity
measurements using Brillouin spectroscopy. They discovered that the
SiO2 glass becomes super-dense above 140 GPa owing to a
pressure-induced structural change. This finding strongly suggests the
possible presence of gravitationally stable “super-dense magmas” deep
within the giant terrestrial planets through solidification from a
primordial deep magma ocean.

December 1, 2009

LJ12113

Stopping light for more than one second

Over the past decade, studies of ultra-slow and stopped light have
represented an extremely active area of research. Here we demonstrate that
a light pulse can be stopped and stored for 1.5 seconds in a Bose-Einstein
condensate of laser cooled atoms. During this time, light could – under
normal circumstances - travel roughly 10 times around the earth. In our
experiments, a light pulse, injected into the condensate, is dramatically
slowed and compressed, then extinguished, and in the process converted to
a matter imprint that is a perfect copy of the extinguished light pulse.
We manipulate atom interactions with use of magnetic fields, and the
system enters a phase separating regime: Much like oil and water separate,
the created matter imprint digs a hole for itself in the host condensate.
The imprint is nestled in this void and avoids collisions with condensate
atoms thereby minimizing losses. As a result, the matter imprint can be
stored for extended time scales. The imprint can further be moved to the
outer tip of the condensate where it can be converted back to light, and
from this location, the light pulse can exit the condensate with minimal
loss. The long storage times and the achieved control over the matter
imprint have important applications for creation of entangled states of
light and matter and for creation of long-distance quantum networks for
quantum computing, teleportation, and cryptography for secure data
transfer.

***

LJ12151

A stone hitting water creates supersonic air jet

A stone which lands on quiescent water triggers an impressive scene.
After the appearance of the familiar “crown” splash, a cavity forms
below the surface, whose collapse ends with the ejection of a thin
liquid jet. Hidden to the unarmed eye of a human observer is the equally
rich behavior of the surrounding air which culminates, as we show in
this work, in an air jet issuing from the cavity at /supersonic/ speeds.

Combining our visualization experiments with computer simulations we
show that the mechanism which produces this fast air jet is very similar
to that used in air plane jet engines where pressurized air is squeezed
through a narrow constriction. The unique feature of our situation is
that the confining geometry is a /liquid/ cavity which is rapidly
changing in time.

To study the impact process in a very controlled fashion, we use a
circular disc which is pulled through the liquid surface by a linear
motor. We visualize the air flow with small smoke particles produced by
a commercially available smoke machine commonly used for light effects
in theatres and discotheques. The motion of the smoke particles is
illuminated with a strong laser sheet and recorded with a high-speed
camera at up to 30,000 frames per second.

Figure caption:
The impact of a disc (a) creates a liquid cavity (b) which collapses (c)
and forms a very fast air jet being squeezed through the neck of the
shrinking cavity (d). The addition of smoke particles into the air (here
artificially colored in orange) allows us to visualize this air flow.

***

LF12383

Entangle global, act local: enhancing network connectivity with quantum operations

Complex networks describe many natural and socio-economical systems
such as protein-protein interactions, the brain, the internet, the
power-grid, and friendship networks. Structural properties of complex
networks have a crucial effect on their functionality, determining for
example the spread of information or deseases, the resilence of
internet, or the small world effect in human relations.

In this letter we show that new phenomena appear if networks are
governed by the laws of quantum mechanics. The transmission of quantum
information has already been studied in detail in 1-D chains and some
regular lattices. Here we study how to establish entanglement between
two arbitrary parties in a quantum complex network by means of local
quantum operations that change drastically the network's topology. We
thereby shift the attention from regular lattices to the feature-full
complex networks, opening the door to new ways of distributing,
protecting and manipulating quantum information as well as to the
study of new network models and dynamics generated by quantum
mechanisms.

With the promise of quantum computation and quantum cryptography, the
field of Quantum Information has had its role in the information
age, phenomena like entanglement percolation indicate that Quantum
information has also something to say in the forthcoming "connected
age".



***

LJ12114

Splitting Cooper pairs on demand

Superconductors are a potential source to produce entangled electron
pairs in a solid state device. The implementation of a superconducting
Y-junction in which the Cooper pairs injected from a superconductor
split into two electrons at different normal electrodes provides an
important step in this direction. Although there have been many
theoretical proposals in the last ten years it is the first time that a
tunable Cooper pair splitter is experimentally implemented. Two teams
have reached this goal independently. In the work, the Y-junction is implemented using a single wall carbon nanotube with a central superconducting electrode deposited on top and contacted at each side by two normal electrodes. The system thus fabricated behaves as two coupled quantum dots whose resonant levels can be
controlled by two independent side gates. The Cooper pair splitting
action of the device is demonstrated by comparing the conductance of
each arm in the superconducting and in the normal state, which is
reached by applying a magnetic field. The theoretical estimates
indicate that up to 50% of the injected Cooper pairs can be splitted at
the junction when the two quantum dots are at resonance. The findings
presented in this paper bring on very exciting perspectives and are
very likely to generate a renewed and intense experimental and
theoretical activity.

***

EG10639EJ

Ubiquitous chaos in the modern technological world

We found that a kind of chaos, i.e., chaotic magnetic-field lines
(CMFLs) exist everywhere and are ubiquitous in the modern
technological world. Although it is commonly believed that magnetic-
field lines generated by a wire carrying electric current always have
simple closed-loop structures, this belief is wrong, because CMFLs
can be generated by simple three-dimensionally crossed-wires. We
investigated the existence of CMFLs in realistic systems, and
revealed that this chaos exists widely in conventional electrical and
electronic products in our daily life, for example, in mobile phones,
personal computers, power wires in house etc., although we are
unaware of this fact.


***

LK12343

Understanding Explosive Percolation via Powder Kegs

Networks of various types (e.g., social networks, economic networks, computer networks, etc.)play a central role in modern society and have numerous applications to many fields of study, including sociology, epidemiology, and physics, to name just a few.

Recently, computer simulations have revealed that networks can sometimes undergo a very sudden change in character, dubbed an "explosive transition" (see in particular Achlioptas et al., Science 323, 5920, (2009)). A new paper in PRL describes the mechanism responsible for explosive behavior in some of these networks. It shows that, somewhat surprisingly, the key to explosive transitions is not the details of the edge-addition rules at work during the actual "explosion," but rather lies in the period preceding the explosion when a "powder keg" develops. In effect, the importance of the rules is to create an explosive situation, which can be detonated with almost any rule. The paper discusses the origin and nature of the powder keg, along with a heuristic criterion for determining whether a given network will be explosion or not. It illustrates these ideas in the context of social networks.

November 23, 2009

LH12027

Cloaking Device for Electromagnetic Surface Waves

A 'cloaking device' renders an object undetectable for electromagnetic radiation so that an outside observer cannot register any scattering or reflection effects from that object. Recent approaches commonly consider electromagnetic waves in three dimensions, i.e. light, as a probe. In this paper we demonstrate how it is possible to hide an object located on a metal surface from interaction with surface plasmon polaritons, a collective excitation of light and free electrons in only two dimensions. Our cloaking device consists of point scatterers distributed on two concentric rings around the object to be cloaked. By computer simulations we determine the physical properties of these point scatterers required to acheive the cloaking effect. As it turns out, the cloaking geometry efficently reduces scattering from the enclosed object to a minimum. We conclude that such point scatterers can be formed by surface defects that seem to be easy to create by the photon scanning tunneling microscope/direct-write lithography technique.