5-27-08

LP10931
Invisible waves shape continental slopes




Throughout the world, the ocean floor beyond the continental shelf
slopes at an average angle of 2 to 4 degrees, even though marine
sediments can stably pile up with slopes up to about 15 degrees. Zhang
et al. at the University of Texas at Austin have conducted laboratory
experiments that provide an explanation why the continental slopes are
at angles of only a few degrees. The scientists show that tidal flow,
back and forth along a sloping ocean floor, can generate strong internal
gravity waves, which are a type of wave that can form in fluids whose
density increases with depth, as in the oceans. Internal waves are
peculiar in that they travel only at a particular angle, which in the
oceans is set by the frequency of the tides and the variation of density
with depth. The experiments show that the waves near an ocean floor can
be very strong and can prevent deposition of sediments at larger angles,
thereby limiting the continental slopes to be at the angle of the
internal gravity waves.


Caption for VelField.jpg: Rapid flow (the dark red region) along a
sloping ocean floor, observed in a laboratory experiment where internal
gravity waves travel at the same angle as the slope of the ocean floor.

Caption for KH.jpg: The flow along the sloping ocean floor can become so
strong that the fluid rolls up into these patterns, called
Kelvin-Helmholtz billows.

***


LM11241

Images were stored in a gas of atoms for several micro-seconds



Quantum information can be coded in various physical realizations such as light pulses, internal quantum states of matter particles and more. Each realization has its unique benefits and drawbacks, for examples light pulses are ideal for long-distance communication, but are hard to manipulate. Therefore, in the last years several methods to coherently convert quantum information from light pulses to matter excitations and vice versa were explored. In this work we demonstrate a technique to map an image onto a gas of room temperature atoms, utilizing a phenomenon called electromagnetically induced transparency. All the information in the light field, including both its amplitude and phase patterns, is converted into the internal quantum states of the atoms, and transferred back to an image after several micro-seconds. We further explore a technique to immune the images from blurring due to the thermal motion of the atoms, by applying a suitable phase coding to the image. This technique, which is the atomic equivalent of a resolution-enhancement method used in optical lithography, dramatically improves the visibility of the image for long storage durations (see figure). Storing images in a gas of atoms might have future applications in quantum information and all-optical image processing and manipulation.

***

LM11731

From order to disorder when mild impurities hold back

How a system evolves from an ordered to a disordered state (or the opposite), while mild impurities hold back, is a long standing question that has been addressed in different complex systems. We have studied particularly an array of quantized magnetic flux lines in a low Tc superconductor (NbSe2, Tc ~ 7 K) as it transformed from an ordered triangular low temperature lattice to a disordered high temperature configuration in the presence of mild random defects. An advantage of this model system is that by “shaking” the flux lines with an alternating magnetic field at different temperatures, the corresponding stable state is accessed. It is known that the compression modulus of the array varies with the degree of order, so a very sensitive magnetic measuring system was used to determine the elastic response at each temperature, assuring that the array was not modified by the measurement itself. We conclude that in a small temperature interval (of about 0.1 K) while the system goes from order to disorder (or back) , the equilibrium states are a mixture of fully ordered and fully disordered regions, the proportion of which changes across the transition, suggesting that this phenomenology could also occur in colloids, Wigner crystals, charge density waves, etc.

***

LL11135

Save the Notion of Spin for Photons and Gluons

A particle's spin is an important information carrier, and is the
basis for quantum communication and quantum computation. But rather
disturbingly, for many decades physicists are unable to define the
spin properly for photons and gluons. The difficulty is due to a
kind of arbitrariness in physics: the gauge freedom. In the
traditional definition, the spin of photos and gluons depends on the
choice of gauge, and since one is free to change the gauge, such
definition has no concrete physical meaning. In this work, we
succeed in identifying the physical components of photons and gluons
which are unaffected when changing the gauge. With these physical
components, the spin of photons and gluons can be defined
concretely. This saves the notion of spin for photons and gluons,
and allows physicists to properly manipulate the photo spin in
quantum communication, and meaningfully investigate the total amount
of gluon spin inside the nucleon --- a key issue in understanding
where the nucleon obtains its spin.

***


LP10889

New mechanism for a single molecule switch with the negative differential
resistance (NDR) feature is discovered--NDR that constitutes rise and fall
in current with increase in applied voltage is used to build switches for
computers. To continue drive towards miniaturization, ultra-small single
molecule NDR switches are essential. Even after a decade of its first
realization, this device is not yet operational. The reason is that
various groups have proposed contentious mechanisms to explain the origin
of NDR in single molecule device. Unless the origin of NDR is underpinned
resolving one of the greatest nano-debate of this decade, a reliable
device cannot be made.

In this letter, we have unified two distinct dimensions of this debate,
role of junction and the molecule itself, into a single framework.
Specifically, we have found that the ten years long debate was due to
overlooking a well-known phenomenon in non-equilibrium physics. We have
unambiguously shown that the ¿symmetry breaking and appearance of a new
broken symmetry phase in the electronic state¿ of the molecule upon
increasing applied bias is the root cause of NDR. This could be used as a
universal recipe to design single molecule switch for the new generation
molecular computer.


***

LM11414

Temperature can switch the coupling between magnetic layers.

Magnetic thin films coupled through layers of nonmagnetic materials
are at the heart of modern reading devices of magnetic information
in hard disks. The Nobel Price 2007 was awarded to P. Grünberg and
A. Fert for the discovery of the "giant magnetoresistance" in these
layered systems: A strong reduction of the electrical resistance when
an applied external magnetic field changes the relative magnetization
direction of the magnetic layers from antiparallel (antiferromagnetic
coupling) to parallel (ferromagnetic coupling). In the absence of
external field, the coupling strength between the magnetic layers is
known to depend in an oscillatory way on the thickness of the nonmagnetic
spacer layer, changing from ferromagnetic to antiferromagnetic coupling.
In this work, we have grown trilayers of two different magnetic
rare-earth metals (Gd and Tb) separated by a nonmagnetic Y layer.
We found, besides the expected oscillatory dependence of the magnetic
coupling on the Y-spacer layer thickness, a novel temperature-dependent
phase shift of the oscillations. This strong temperature dependence even
leads to sign reversals between ferromagnetic and antiferromagnetic
coupling for constant Y-thicknesses. Results are interpreted in terms of
magnetization-induced changes in the reflectivity of conduction electrons
at the magnetic-nonmagnetic interfaces.

***


LP11610

Increasing thermoelectric efficiency: A dynamical systems approach

POWERFUL HEAT

In our paper we present a mechanism for a drastic increase of efficiency of
thermoelectric power conversion which - if implemented - may lead to
environmentally benign and economically competitive refrigerators and heat
engines. Providing a sustainable supply of energy to the world's population
will become a major problem as fossil fuel supplies decrease and world demand
increases. Also there is an increasing environmental concern about waste heat,
and about chlorofluorcarbons used in most compressor based refrigerators. It
is therefore expected that thermoelectric phenomena, which involve conversion
between thermal and electrical energy, will play an increasingly important
role in meeting the energy challenges of the future. The difficulty is that,
so far, thermoelectric power generators and refrigerators have poor
efficiency. The latter depend on a pure number, the so-called figure-of-merit
ZT of their material components. In spite of the efforts of the last five
decades, the values of ZT remained around one, while a value larger than 3
will make thermoelectric refrigerators economically competitive with the
conventional home refrigerators. In our paper, using an approach from
nonlinear dynamics and chaos, we have discovered a general microscopic
mechanism for an unlimited increase of the thermoelectric figure-of-merit ZT,
thus allowing efficiency to approach the Carnot's limit of an ideal engine.
Our results are demonstrated by a simple numerical calculation on a Lorenz gas
type system.

***


LR10990
Ultrasensitive detection of lowest H2D+ rotational transition

The lowest rotational transition of one of the most important
astrophysical molecules, H2D+, has been observed in the laboratory for
the first time. This result will trigger an astronomical search using
new and upcoming telescopes, like APEX and SOFIA.
In the experiment, only a few hundred of the ionic species are stored in
a low-temperature ion trap and excited by a THz-beam with very high
spectral purity. Using the small energy amount gained by absorbing
THz-photons,
the ions undergo a chemical reaction and the products are detected
with high sensitivity. This is the first example of pure rotational
spectroscopy of molecular ions using this novel technique.
The astronomical observation of this transition based on the present
laboratory work will have far reaching consequences, including the
determination of the coldest temperatures in space and the ortho to para
ratio of molecular hydrogen, the most abundant molecule in the universe.
Unravelling the pivotal role of these two nuclear spin configurations of
hydrogen will finally give a hint to how water came to earth and whether
life was kick-started from space.

***


LM11170

Ultrashort light pulses weld ultracold atoms together

At temperatures close to absolute zero, matter can be controlled in a way that is very different from our every-day perception. Ultrashort pulses of laser radiation, just like those used, for example, as a precision knife in laser eye surgery, have been employed by a collaboration of researchers from the Universities of Freiburg and Berlin to weld atoms in an ultracold gas together into molecules. An advantage of this method of making molecules is that the resulting molecules remain as cool as the atoms from which they are made. The researchers prove the existence of the molecules by firing in a second ultrashort light pulse a short time after the first one. In addition to the laser light creating molecules, it was surprisingly found that the formed molecules interact further with the laser field. The molecules take electromagnetic energy from the laser field and store it internally for a short time before giving it back to the laser field. This coherent process is repeate
d on a time-scale that depends on the laser frequency and the binding strength of the molecules. The observations are perfectly described by full quantum-mechanical calculations. With this work, the combining of the ultracold and the ultrashort worlds has become reality, and new possibilities for the quantum control of atoms and molecules near absolute zero temperature, where quantum mechanics governs the dynamics, have come within reach.

5-19-08

LF11381

Does the size really matter?

It is well known that dynamics of atoms in nanocrystalline materials is remarkably different from their bulk counterparts. The anomalous features are usually attributed to small nanograins with enhanced surface-to-volume ratio. To further reveal the origin of those anomalies we investigated the atomic dynamics of nanocrystalline Fe90Zr7B3 alloy at various phases of crystallization. The atomic vibrations of the nanograins were separated from those of the interfaces for a wide range of grain size and interface thickness. Surprisingly, the results show that the atomic vibrations of the nanograins do not vary with their size even down to 2 nm, and still closely resemble those of the bulk. The known vibrational anomalies originate from the interface atoms and the degree of deviation from bulk dynamics is proportional to the interface fraction.
A practical implication of these results is that in order to optimize particular thermodynamic properties of the nanocrystalline materials one has to control precisely the fraction of the interfaces rather than the size of the nanograins.

***

LM11721

Galactic magnetic fields could originate at the epoch of reheating after inflation


The origin of the microgauss magnetic fields observed in galaxies and clusters
of galaxies is still a mystery. In this paper we propose that they could be produced
at the electroweak transition after a period of low-scale hybrid inflation. We show
how the non-equilibrium processes occurring at the epoch of reheating of the
Universe may give rise to a significant fraction of energy density in the form of
helical magnetic field lines, whose correlation length grows via inverse cascade
in the primordial plasma. These magnetic fields would be seen today in galaxies
and clusters of galaxies, but not in the cosmic microwave background. The
detection of a helical component of cosmological magnetic fields would give yet
another signature, together with the predicted Gravitational Wave Background,
of the violent processes occurring at the Big Bang.

***


LQ11037
Active cooling of massive objects

Several groups worldwide are investigating the possibility of cooling macroscopic mechanical resonators towards the quantum mechanical ground state, using active feedback or laser cooling techniques. A possible outcome of this research could be the observation of quantum behaviour in a macroscopic object. In this paper, we demonstrate that these techniques can be efficiently applied to resonators much more massive than previously analysed systems. In particular, we cooled the resonant modes of the cryogenic gravitational wave detector AURIGA, a 2 ton aluminium bar resonator whose motion is detected by a very sensitive SQUID-based position sensor. Starting from a temperature of 4.2 K, we were able to achieve by active feedback a minimum equivalent temperature of 0.17 mK, which is even lower than the temperature previously reported for much smaller systems. This experiment suggests that, using this technique, it could be possible to cool even massive human-scale resonators towards the quantum ground state.

***

LQ11568
QUANTUM MOTION SOLVED ON CURVED SURFACES WITH ELECTRIC AND MAGNETIC FIELDS

We derive the quantum dynamical equation for a charged particle moving on a curved surface in the presence of an electric and magnetic field.

It is evident that both the geometry of the surface and the applied fields influence the motion of a particle, but, up to now, it was not clear if these factors couple with each other.
We find that it is not the case: the effect of the fields is independent from the surface curvature.
In addition, we demonstrate that, also with fields, the motion of the particle on the surface is not affected by the dynamics along the direction perpendicular to it.

Applying our results, it has been possible to obtain the quantum equation of motion on surfaces of frontier interest for nanosciences, such as spheres, cylinders and tori.

Our new equation will be useful both for the interpretation of experimental results and theoretical predictions involving curved structures immersed in fields, for example in the analysis of new effects in low-dimensional nanostrucures.

The results are the outcome of the collaboration between two young researchers, a solid-state physicist and a field-theorist: this is a proof of how fruitful is the interplay of competences among different areas of physics.

***

LM11290
Probing colored light and ghosts by gentle warming

Sometimes victory of a long-distance racer depends on what is usually
considered to be not a even a physical issue. This is the strength of
the racer's spirit. The colored gluons and quarks -- basic building
blocks of the theory of strong interactions -- emerge naturally with
other objects, ghosts. We demonstrate that the long-distance
propagation of warmed gluons is intimately related to proliferation of
the ghosts. The gluons are massless intensity of the colored "light" of
gluons created by a static quarks falls off as the inverse square of
the distance from the source. The same as for ordinary light. Color is
seen however only at short distances, less than about one femtometer
(one quadrillionth of a meter, 10^{-15}). At long distances only
colorless hadrons, or bound states of quarks and gluons are
observed. This phenomenon is known as color confinement.
In our work we show theoretically that slightly warmed gluons
and ghosts offer a new kind of conspiracy implied by confinement
of color. We demonstrate that the warm colored ghosts propagate
for long distances and the falloff of the intensity of the ghosts
and gluons obeys a certain relation.

***

LN11310
Electronegativity Identification of Novel Superhard Materials

The hardness of materials can be identified via electronegativity. The search for superhard materials is a huge challenge to scientists. Furthermore, people are also facing many new challenges to precisely measure the hardness of synthesized superhard materials. In this letter, the nature of hardness is systematically studied from the viewpoint of electronegativity, one of the most widely used parameters in chemistry, physics and materials science. We find that materials hardness is essentially determined by the electron-holding energy of constituent bonds per unit volume. The hardness of various materials can be satisfactorily predicted solely in terms of electronegativity and crystal structure. A number of bonds which can or cannot form a superhard material are qualitatively distinguished. Our work provides a new approach to design novel superhard materials from the general viewpoint of electronegativity, which will inspire people to further explore new superhard materials.


***

LP11122


"Quantum bit measurement reaches computing regime"

Very precise state measurements necessary to create a working Quantum Computer (QC) have recently been achieved in the lab. The QC uses information encoded in quantum bits ("qubits") which can exist in a quantum "superposition" of the states 0 and 1, in contrast to conventional bits which can only be in one of these states at any time. If built, the QC could exploit the quantum phenomena of superposition and "entanglement" to solve certain problems that are intractable on any conceivable conventional machine. To make an error-free computation, the state of most of the qubits in the computer must be read out with very high accuracy both during the computational process and to determine the final answer. This paper reports the high fidelity measurement of qubits stored in a single trapped Calcium atom. The state of the atom was repeatedly measured with a mistake occurring less than once in every 10000 tests. This 99.99% fidelity is high enough for a working QC and is believed to be currently the best measurement of any physical qubit. This achievement is particularly exciting given recent insights into the power of quantum measurement, which can be the driving force at the very heart of a quantum algorithm.

***

LL11113
ITER operational regime in a simple small experiment

The ITER experiment is now being built in Cadarache, France, with the goal of showing the feasibility of producing controlled thermonuclear fusion energy. Its success will critically depend on reaching an operational regime where matter and heat are highly confined, the so called H-mode regime. Although the discovery of such a high confinement regime dates back to the '80s, the underlying physical mechanisms have not been fully understood yet. In particular, the fact that the H mode is achieved only in big fusion-relevant devices has made its dynamics always very difficult to diagnose and control. In the present paper, a theoretical prediction is made that an H mode-like regime can also be achieved in a simpler and smaller sized device with easy diagnostics access. The relative simplicity of the considered configuration has allowed the theoretical exploration of the properties of this operational regime in great detail.

***


LM11031

Calcium as the Superior Coating Metal in Functionalization of Carbon Fullerenes
for High-Capacity Hydrogen Storage

There has been an expanding effort of the scientific community searching
for the most promising materials for high-capacity hydrogen storage,
with carbon-based nanostructures representing one important class of
systems under intensive study. Because molecular hydrogen and pristine
carbon nanostructures such as fullerenes or nanotubes are both
sufficiently stable and relatively inert, a variety of surface
modification schemes have been proposed in the recent literature to
enhance the binding of molecular hydrogen, such as by doping (Ref. A),
coating (Refs. B and C, D), or charging (Ref. E), but each scheme having
severe limitations or running into disappointments. Our paper shows that
Ca as a coating element satisfies all the requirements to functionalize
fullerenes (and related carbon nanostructures) as potential high storage
media. Even more pleasantly, Ca turns out to be (probably) the only
coating element in the periodic table to deliver the desired
functionality, making this contribution truly refreshing in an otherwise
somewhat frustrated community.

The attached figure shows that the optimized organometallic complex of
Ca32C60 can contains 92 H2, which corresponds to a hydrogen uptake of
8.4wt%.

5-13-08

LG11494

Unraveling the mysteries of how complex liquids flow

Simple fluids, such as water or oil, flow in response to applied forces that are well understood and described using a set of coupled differential equations, known as the Navier-Stokes' equations. An important parameter for these fluids is the viscosity, which describes the friction between the molecules and thus their resistance to flow. However many materials in everyday life that flow are composed of small particles, such as powders or grains, which may be inside a liquid, such as pastes or emulsions. For these materials the friction between particles is very different from that of molecules in simple fluids, and consequently their resistance to flow cannot be described with a hydrodynamic viscosity. Nonetheless researchers are hard at work trying to extend the hydrodynamic descriptions that work well for simple fluids to these complex fluids. Aqueous foams are an appealing model system for a complex fluid, because their composition is elementary: they are merely composed of packed bubbles surrounded fluid. In their recent experiment study, Rheology of steady-state, draining foams, Soller and Koehler find that the friction of sheared bubbles in a foam is inversely proportional to the thickness of the films separating bubbles. Wetter foams with higher liquid content have thicker films, allowing bubbles to slide past each other more easily than drier foams where the films are thinner. It still remains to be understood how friction changes with the shear rate; observations show that foams are shear-thinning and the their composition also affects the friction. Once the shear rate dependence is understood, a hydrodynamic description for foam flow should be possible that may lead the way to hydrodynamic descriptions of other complex fluids such as granular matter.

***

LN11547
A universe in turmoil

A key open question in our understanding of the universe is the
quantum nature of the big bang -- its earliest moment which general
relativity can only describe as an infinitely dense singularity. In
this letter, detailed considerations of quantum cosmology show that
the density of the universe, even when it emerges at the big bang,
must respect a universal upper bound no matter how much it quantum
jumps and fluctuates. The precise form of how the universe starts its
expansion turns out to be dictated by another quantum parameter called
correlation (or squeezing). When we picture the early history of the
universe, we usually think of a steadily expanding tube emerging from
a hot, narrow phase at the big bang. Its initial extension was far
from smooth but quantum and in wildly fluctuating turmoil. The new
results put this phase under high control and restrict the options of
what precisely happened before the big bang: The universe either
sharply bounced back after a long collapse, or it emerged as the
sudden liberation out of a long phase of imprisonment in the quantum
world.

***


LN11566

Millikan's experiment in a liquid

The experiment of Robert Millikan in which the elementary charge is
determined from the motion of charged oil drops in air has been
repeated many times, either to find a more precise value of the
elementary charge, or because it is such a simple and elegant way to
measure a physical constant. In this paper it is revealed for the
first time that the charge of micrometer-sized colloidal particles
suspended in a nonpolar liquid comes in multiples of the elementary
charge. In the experiments, the motion of the particles under
influence of an electric field is measured by microscopic particle
tracking. The particle mobility ?the speed divided by the electric
field strength? changes with a fixed step, if the charge on the
particle is modified by plus or minus one elementary charge. From
detailed mobility measurements on a single particle over many seconds,
the number of elementary charges on the particle can be determined as
a function of time. In addition precise values are found for the
particle size or (if the size is known) for the elementary charge. The
experiment can be seen as the colloid variant of Millikan?s
experiment, but is a bigger challenge due to the higher viscosity of
liquids compared to air. The experiments can be used to characterize
properties and charging mechanisms in all kinds of colloids.

***

LP11079

Wisconsin Researchers Put a New Spin on Decoherence

It is exceedingly difficult to construct bits for a quantum computer (“qubits”). Even minute amounts of noise are sufficient to destroy coherence – the essential quantum nature of the qubit. In the case of qubits based on superconducting circuits, a dominant source of decoherence is a low-frequency magnetic flux noise whose existence has been known for over 20 years, but whose origins were never explained. Now researchers at the University of Wisconsin, Madison have uncovered clear evidence for a high density of unpaired electron spin magnetic moments on the surfaces of the superconducting thin films that are used to make qubits and other sensitive superconducting detectors, including Superconducting QUantum Interference Devices (SQUIDs). Fluctuations of these moments account for the previously unexplained low-frequency flux noise, and for its apparently “universal” character: the noise is only weakly dependent on a wide range of parameters, including superconducting materi
als, the scale of the device, and temperature. This research suggests that optimization of the surfaces of the superconducting thin films could lead to detectors with improved noise performance, and to superconducting qubits with improved coherence times.


***

LN11156

“Mixed solids” of nanoparticles

Most of useful materials are mixed crystalline solids composed from different atoms (eg. stainless, permalloy, etc.). The semiconductor and metal nanoparticles are one of nanoscale building blocks for tailored materials with fascinating and multifunctional properties beyond isolated nanostructured materials. It is expected that the close-packed mixed solids composed of semiconductor and metal nanoparticles make it possible to realize novel optical and electronic properties; semiconductor nanocrystals exhibit size-dependent luminescence with high-quantum yields, while metal nanoparticles exhibit large electric-field enhancement due to the surface-plasmon excitation. In this Letter, Kanemitsu and coworkers successfully fabricated “mixed solids” films composed of semiconductor and metal nanoparticles using a very simple technique and revealed their ultrafast energy flow mechanism from the semiconductor and metal nanoparticles on the nanoscale. This is a
first step toward the future material engineering using “mixed solids” composed of nanoparticles and will contribute to development of novel functionalities in the artificial mixed solids.

5-7-08

LP10816
How fast will materials break as they get tired?


The so-called "fatigue" phenomenon, responsible for the mechanical
failure of structural components when exposed to periodic loading even
if the load amplitude is much below the safety limit, has been largely
recognized in engineering and physics as a very important
technological problem with many involved scientific challenges. For
instance, this subcritical process, which gradually leads to failure
due to macroscopic fractures, typically occurs unexpectedly and has
been responsible for a large number of airplane and railway crashes
with considerable human loss. A crucial dynamical feature of this
fatigue phenomenon is expressed by the empirical "law of Paris", which
states that the growth rate of a crack increases as a power-law of the
instantaneous crack size, with an exponent that is strongly material
dependent. In our study, we have been able to derive analytically and
confirm numerically the connection between the exponent in Paris' law
and the exponent of a damage accumulation mechanism at the microlevel.
To our big surprise, we discovered that there is a "critical" value
for the damage accumulation exponent which separates two very
different regimes of crack growth. In addition, we also studied the
role of disorder (the presence of impurities, for instance) and found
again that this critical point is expected to have the most striking
consequences on crack growth rate, namely that disorder is relevant
below it and irrelevant above it. Our results can have far-reaching
consequences in the understanding and control of subcritical crack
propagation. On one hand the discovered relation between the damage
and the Paris exponents, which in principle could be checked
experimentally, could help to predict lifetimes of samples by studying
the velocity of small cracks. Finally, it is important to mention that
since the microscopic exponent of the damage accumulation law is
material dependent, it could play a central role in the engineering
design to increase the robustness and optimize the mechanical
performance of materials.

***

LN11036


EVEN BLACK HOLES CANNOT HIDE INFORMATION


Black holes form by gobbling up matter. According to Einstein's
general relativity, information contained in the matter is lost
forever. However, in 1974 Hawking discovered that black holes are
like black bodies and they radiate quantum mechanically. As a
result, matter does eventually come out, but it emerges as thermal
radiation which has no memory of how the black hole was formed.
So, it appeared that this information is still lost in spite
of the evaporation process. But this conflicts with a basic tenet of
quantum mechanics. Thus, gravity seemed to make the amazing demand
that the mathematical structure of quantum mechanics itself must
be modified. Since quantum mechanics is the most successful physical
theory we have, the issue of information loss has drawn
much attention over the last thirty years and is widely regarded
as one of the most fundamental problems in theoretical physics.

In recent years, string theorists have appealed to the so-called
AdS/CFT conjecture to argue that information is not lost and
quantum mechanics can remain in tact. However, these arguments
have not been able to explain how the information comes out of
black holes. In the forthcoming PRL, Ashtekar, Tavares and
Varadarajan use quantum gravity to provide a space-time
description of this process for 2-dimensional black holes. These
black holes are technically simpler to work with but are
conceptually very similar to spherical black holes in
4-dimensions. They argue that quantum effects of geometry resolves
the black hole singularity and significantly extend space-time.
Hawking's thermal radiation is an excellent approximation but only
at early times. The apparently lost information emerges at late
times in the extended quantum space-time. Thus, because quantum
space-time does not end at a singularity, it has ample room in the
distant future for the information to reappear.


***

LB11631

Lasing, normally thought to require population inversion is shown to be possible even without population inversion. We show that atomic population undergoes an interesting capitalistic (rich gets richer) behavior, which can be taken as a definitive signature of lasing without inversion.

Einstein's explanation of the stimulated emission has given rise to the devices we now call lasers. To obtain laser light, the medium (atomic or molecular) need to be excited as a condition known as population inversion is to be satisfied. Population inversion means the higher energy states need to be more populated than lower energy states, which makes it very difficult to obtain lasing in the high frequency region of the electromagnetic spectrum. This requirement gets modified if there is an extra transition coupling with the lasing transition. The quantum interference between coupled transitions makes population inversion unnecessary for lasing. However, there are other competing processes and pure LWI contribution is difficult to discern. Dr. Kapale's work with Dr. Kilin and Dr. Scully shows how to extract pure lasing without inversion contribution and show an accompanying capitalistic effect in the population dynamics. An initially more populated ground state gets more populated with time; such behavior is a definitive signature of LWI. The concept of LWI is the only hope to obtain highly efficient X-ray or gamma-ray lasers. Also in the context of quantum information generation of pure states as initial states of the quantum system is a necessity and the capitalistic population dynamics proposed can be used to generate such pure states.

***

EP10430

Listening to singing bubbles

On volcanoes, giant gas bubbles exploding at the top of vents or at the surface of lava lakes generate acoustic signals. Thus, motivated by the potential applications in geophysics, we listen to bubbles bursting at the free surface of a complex fluid. We identify the basic mechanisms at stake in the sound generation. In the laboratory, a diluted hair-gel solution mimics the complex properties of lava, such as its peculiar mechanical behavior: if squeezed gently, it behaves as an elastic solid; if squeezed harder, it flows as a liquid. Due to these complex properties, the bubbles are elongated, and thus, behave as panpipes at bursting. The acoustic signal associated with one event exhibits a well-defined frequency, directly linked to the bubble length. The bubble geometry can be inferred from the spectral content. By contrast, the sound intensity is not only controlled by the bubble geometry but also strongly depends on the bursting dynamics. Consequently, the total amount of energy released by the explosions cannot be deduced from the acoustic energy recorded on the field.

***

ln11264

*Capillary-like Fluctuations at the interface of a granular liquid. *
New experiments with granular jets (made of spherical glass beads)
falling from a funnel shaped container show that capillary-like behavior
known to place at the interface of liquid-gas can be observed on a
macroscopic scale. The experiment was undertaken by Y. Amarouchene and
his colleagues at the CPMOH University of Bordeaux1 (France). At the
scale of a molecule, the free interface between any two fluids is not as
smooth and mirror like as it appears on a macroscopic scale since
thermal motion can easilly roughen the interface (whose interface
roughness is given by a balance between thermal energy and capillarity).
This give rise to waves that propagates at the interface. The same
phenomenolgy seem to hold for granular fluids although thermal energy is
irrelevant in this case. This opens the way to a better understanding of
small scale interfacial phenomena in these widely spread materials.


***

LM11565

Experimental evidence of 2He decay from 18Ne excited states


Two-proton cluster radioactivity in nuclei near the proton drip-line was theoretically predicted about 50 years ago as result of the pairing force. Although few cases were already observed, the reported experiments were either not able to distinguish between the true diproton (2He) emission and the uncorrelated emission of the two protons or suggested the latter as the decay mechanism.

A technique for disentangling the two decay modes was proposed in 1961 and in this paper was successfully applied for the first time to the two-proton decay from 18Ne excited states, demonstrating the existence of di-proton radioactivity.

The 18Ne nucleus, produced as radioactive beam through 20Ne fragmentation and subsequent in flight separation was excited via Coulomb excitation on a Pb target. A complex detection system allowed to identify on an event-by-event basis the charge, mass, energy and angle of each fragment produced in the interaction.

The novelty of this work lies in the analysis of the relative angle and momentum spectra of the two emitted protons which allowed to disentangle the uncorrelated (69%) from diproton (31%) emission and infer the relative branching ratios.

This finding will put new constraints on the description of nuclei near the proton drip-line important to understand the nucleosynthesis rp-process.

***


LP10807
Two-photon nonlinearity with many-atom cavity system
- Photons squabble over only "one chair" in the presence of many chairs -

If the two of you squabble over only one chair in a tiny room, the interaction
between the two should be strong. If many chairs are prepared for you, in
contrast, you do not have to interact each other. Similarly, two
photons--particles of light--interact each other most strongly if they are
confined in a cavity with only one atom. This is a key phenomenon and
currently a crucial subject in order to realize a quantum logic gate for
quantum information technologies. However, if many atoms are inside the cavity,
which is an inevitable situation in the implementation of solid-state devices,
interaction between photons easily vanishes. In this paper, we find a new
scheme to drastically change this standard scenario; the photons' interaction
is strongly enhanced even for a many-atom system. In this mechanism, many atoms
in a cavity are superposed, and this strange collective state behaves as if
there is only one chair under a particular design of many-atom cavity system.
This finding develops a new research field of the multi-particle cavity quantum
electrodynamics, and more importantly, it opens the link between the concept
of the quantum logic gate with an ideal model and the real-world solid-state
device technology.

***

LQ11684
The Potential of Graphene Nanoribbons for Nano-electronics

Graphene, a monolayer of carbon atoms tightly packed into a two-dimensional honeycomb lattice, has been suggested as a promising candidate for nano-electronics due to high carrier mobility in large sheets of graphene. Recently it has been demonstrated that very narrow (<10nm) graphene nanoribbons (GNRs) were semiconductors with adequate bandgap for transistor operation (Science 319, 1229 (2008)). The present work reported the highest-performance sub-10nm GNR transistors, with on state current density similar to small diameter carbon nanotubes (CNTs). However, a key advantage of sub-10nm GNRs over CNTs is that GNRs are all semiconductors without metallic species. The study suggests that the intrinsic properties such as electron mobility in narrow GNRs are limited by the scattering by the edges and may still have room to improve. Thus, all-semiconducting, high current sub-10nm GNRs are possible for future electronics beyond silicon.

***

LL11068

Non-locality fundamental for nano-scale fluids

What is it about the world of the very small that makes it so different to
the everyday world that we can directly experience? The standard answer to
this is quantum mechanics, where non-local phenomena dominate. However,
researchers from Swinburne University and RMIT University in Melbourne
Australia have shown that non-local phenomena are also of critical
importance for nano-scale systems that behave purely classically. They
simulated a system of inert atoms in the liquid state that are affected by
an external spatially dependent oscillatory force. The force generates a
shear stress in the fluid that can be computed exactly. This computed stress
was then compared to the stress predicted by Newton¹s law of viscosity,
which states that it should be directly proportional to the local velocity
gradient of the fluid, the constant of proportionality being the standard
shear viscosity. However, when the variation in the velocity gradient varied
significantly over a few atomic diameters Newton¹s law was shown to break
down dramatically. Using an expression that relates the stress to a
convolution of a non-local viscosity kernel and the velocity gradient over
all space, the team was able to predict the stress exactly. It turns out
that exact predictions of the flux of any fluid property under such extreme
conditions requires the use of non-local constitutive equations that relate
this flux to its conjugate thermodynamic driving force. This in fact is a
central feature of generalised hydrodynamics, which until now had not been
demonstrated so directly. Conditions where rapid variations in velocity
gradient occur on the order of atomic dimensions are to be expected in a
range of phenomena such as shock waves, shear banding and nano-fluidics,
which is where this work could be expected to have greatest significance.


***


Possible explanation of the breakdown of a fundamental symmetry in the
cuprate superconductors


In Phys. Rev. Lett. 100, 127002 (2008),
Kapitulnik's group at Stanford University experimentally
demonstrated that time reversal invariance, one of the fundamental
symmetries of nature, spontaneously breaks down in a class of high
transition temperature cuprate superconductors. In LL11262, a group of
theoretical physicists in the University of Maryland, Collge Park, gave
convincing arguments that the breakdown of this fundamental symmetry
could be due to local bond currents, also proposed to be spontaneously
generated, in these materials. The breakdown of time reversal
invariance and the state with spontaneous local currents may
ultimately provide important clues to the origin of the
anomalously high transition temperature of the cuprate
superconductors.

***


LM11356

Multi-Energy Anomalous Diffuse Scattering

Although characteristic properties of
many substances are related to the short-range
arrangement of atoms, a general-purpose method of
providing reliable information on local atomic
deviations from the average structure is still
missing. In this article, a novel x-ray method of
multi-energy anomalous diffuse scattering (MADS)
for imaging substances with atomic resolution is introduced.
MADS makes possible to reconstruct
numerically local atomic structures using the
measured intensities of x-ray diffuse scattering
from the sample. The method was tested using a
strontium titanate single crystal with
outstanding results. A sample volume with a
dimension of tens of ångstrøms was successfully
reconstructed. Images of several thousands of
atoms, including the light oxygen atoms, were
obtained (see Figure). Application of this method
to the imaging of local structures in both
crystalline and non-crystalline solids seems to be very promising.

***

LK10874
Manipulation of a cloud of a large number of antiprotons

We have succeeded in controlling the radial distribution of a single
component antiproton cloud (consisting of about one million
antiprotons) in a strong magnetic field by applying a rotating
electric field.
The rotating field was believed to be effective when some cooling
mechanisms were available, but the compression here was realized
under ultra high vacuum conditions.

The success of the active control on a large number of antiprotons is
an important step for a controlled synthesis of a point source of
antihydrogen atoms.

It also enables to prepare intense monoenergetic beams of ultraslow
antiprotons in eV ranges for the first time, which is practically
three orders of magnitude lower in energy than ever realized.

One can now study various collision dynamics involving "heavy" electrons
(i.e., antiprotons) such as antiprotonic atom formation and ionization
processes under single collision conditions.

***

LM10936
A fresh view on the classical problem of Poincare recurrences: how an
invariant fractal set governs the long-time recurrences in chaotic
systems.

While studying the stability of the solar system, Poincare noted a
remarkable property: unless planets collide or escape, they will
return to their initial configuration some time in the future. Later,
his famous recurrence theorem, published in 1892, played a central
role in the debate over the foundation of non-equilibrium statistical
mechanics: while Zermelo emphasized the formal contradiction between
recurrences and the irreversibility of macroscopic processes,
Boltzmann argued that the average recurrence time in macroscopic
systems is huge and therefore irrelevant in practice. This is no
longer true for low-dimensional systems, and recurrence is currently a
fundamental tool in the theory of dynamical systems. In this paper
we bring a new insight to this problem by considering an analogy to a
problem of escape in an open system. We find that the recurrence time
distribution is governed by an invariant fractal set, called chaotic
saddle, and the exponent governing its decay coincides with a
characteristic number of the chaotic saddle, its escape rate. In
typical Hamiltonian systems, as those considered by Poincare, we find
that this saddle can be effectively split in a hyperbolic and a
non-hyperbolic component that are responsible, respectively, for an
intermediate time exponential and asymptotic power-law decay of the
recurrence time distribution.

4-21-08

LL11292

Silicon lens breaks the diffraction limit in the near-infrared wavelength range

With increase in demand for fabricating as well as visualizing structures much smaller than the wavelengths of light, the resolving capabilities of lenses used in conventional microscopes need to be improved. Today, commercial lenses are limited in their resolution to the Abbe diffraction limit because of their inability to focus both the propagating and the evanescent waves. The evanescent waves, which are well known to contain intricate details of objects (much smaller than the wavelengths of light), decay with distance and hence, almost never restored at the focus. This limits the resolution of the lenses to the details provided only from the propagating waves. A certain class of metamaterials, known as left-handed metamaterials (LHMs) however, has been extensively researched and demonstrated to amplify evanescent waves, and therefore information contained in both propagating and evanescent fields can now be reconstructed into a ‘perfect’ image using LHM based lenses. This property of “superlensing” or, more rigorously, imaging beyond the diffraction limit, has recently been demonstrated in a series of experiments involving different classes of LHMs. In most of these experiments the LHMs consists of metallo-dielectric periodic microstructures that operate at microwave frequencies. Metal-based LHMs, however, show large absorption losses especially when scaled to visible or near-infrared wavelengths. Although recent remarkable efforts have examined important pathways for improvements to these losses, the current loss values can possibly act as practical limit on the resolution of sub-diffraction limit imaging at optical frequencies. A promising alternative is to use dielectric-based photonic crystal (PhC) LHMs, whose optical losses could be considerably smaller. In addition, these CMOS-compatible materials platform promises seamless integration with existing technologies and scalability to optical and near-infrared wavelengths, critical for nanolithography applications, imaging, and detection nanodevices. Thus, this first experimental observation of subdiffraction imaging in negative refraction photonic crystals at the near-infrared suggests interesting opportunities towards achieving these applications, where the commonly perceived diffraction limit is no longer a barrier.

***

LH11494
The not-so-unsual stripe formation on surfaces

The simplest possible approach to making nanoscale patterns is for the
material to spontaneously order into the pattern. Such self-assembling
systems are known to occur if a material is deposited on a substrate. A
common physical origin of self-assemble is balancing the cost of breaking
bonds on the material with the deformations produced on the substrate.
Standard theories indicate that the periodicities of such patterns should
depend very strongly on the properties of deposit and substrate, so strongly,
in fact, that observing such patterns at all should be rare. In contrast, we
experimentally observe and study in real time by electron microscopy the
formation of self-assembling patterns in a very simple system: gold deposited
on tungsten. By modeling the formation of the patterns, we conclude that such
patterns should be common on metal surfaces.

***

LN11483
Controlling colloidal valence

Chemistry as we know it is realized binding atoms together to form
molecules, whose natural scale is in the nanometer range. However,
in 2002, David Nelson showed that a chemistry on the micrometer scale
should also be possible, using liquid crystal-covered colloidal particles
as ``atoms'' and DNA or polymer linkers as ``bonds''. In our study, we
propose to control the number of bonds per colloidal particle --- the
``colloidal valence'' --- by means of an electric field of appropriate
symmetry. Colloidal valence will greatly affect the actual architecture
of colloidal assemblies, which is crucial for their interaction with
light and hence for their application as photonic materials. As the
bonding linkers can only be attached to the so-called liquid crystal
defect points, we have thoroughly explored molecular ordering in the
liquid crystal layer covering each colloid. We show, via large-scale
computer simulations, that the number, position, and type of defect
points, and hence the colloidal valence, can be changed from the
value 4 in the absence of the electric field, to 2, 4, 8, or higher,
depending on field strength and symmetry.

***

LN11300
A single molecule as an electro-mechanical system

When passing electric current through a single-molecule bridging between
two metallic needles, some of the electrons can activate molecular
motion (vibrations). We have found that the conductance of the molecule
can either increase or decrease by the activation of such vibration. By
measuring the molecule conductance and the current noise across the
molecule we discovered that a crossover between conductance enhancement
to conductance suppression takes place when the main probability of
electrons to cross the molecule exceeds 1/2. The effect of molecular
vibrations on the conductance of a single-molecule is a central issue in
the field of molecular electronics and has been recently much debated in
theory. Our findings provide the first experimental support for several
theoretical models that predict such a crossover.

***LN11503

Exploding Molecules with Light!

Physicists often like to blow up microscopic objects and study what comes out. This approach has certainly proved effective in high energy particle physics and, in the less energetic world of atomic collisions, new experimental techniques are now enabling physicists to view molecular interactions in unprecedented detail. A case in point probes what happens when polarized light, of sufficient energy, is absorbed by the simplest molecule H2 resulting in both electrons being ejected simultaneously. Once the glue that held the molecule together has gone, the positively charged nuclei rapidly fly apart in opposite directions creating a so-called “Coulomb Explosion”. Understanding the details of this fundamental process has been a hot topic for both theory and experiment in recent years. The new collaborative work of Reddish et al (2008) Phys Rev Letts (LN11503), using state-of-the-art theoretical methods and 3-dimensional momentum imaging techniques, effectively views this double ionisation process as the H2 molecule - fixed in space at specific orientations - vibrates, i.e. as a function of internuclear separation. They have been able to unambiguously analyse and interpret the complex 4-particle dynamics during the Coulomb explosion by comparing their results with the simpler problem of photoionization of H2+.

***

LL11508

"Quantum Carpentry": The Spirit Level goes Quantum


We propose a quantum version of the commonly known engineering
instrument, the ``Spirit Level''. This instrument is typically
used to determine if a given surface is horizontal or plumb. While the
spirit level uses an air bubble within a spirit (usually ethanol)
filled glass tube, the proposed version on the other hand uses
a bubble of quantum fluid within another quantum fluid. We have shown
that by using a quantum fluid that is created by Bose condensing
trapped gases, the quantum version of the spirit level, so called
``Quantum Level'', will allow experimentalists to achieve sensitivity
20,000 times better than the most sensitive engineering spirit
level. This implies, in simple terms, measuring an incline approximately
of the size of an human hair in 100 Kilometers.

***

LQ11331

Quantum jumps with memory

Quantum mechanical system can be in a superposition of two or more states whose coexistence is forbidden by classical physics. When a quantum system interacts with its environment, superpositions get destroyed due to decoherence. This process can be described by quantum jumps which change the system state suddenly and indicate the flow of information from the system to its environment. However, quantum systems which have memory are able to regain the information which they leaked earlier. We show that this leads to a concept of quantum jumps that counterintuitively restore superpositions instead of destroying them. Earlier, quantum jumps have been widely used to describe quantum dynamics without memory. However, the use of the same theoretical description for systems with memory leads to a serious problem: negative probability for a quantum jump to occur. Our results solve this problem with an interesting consequence. Namely, a negative probability for a usual jump, which
destroys superpositions, actually corresponds to positive probability for a novel quantum jump which restores quantum superpositions. The solution allows to design efficient simulation algorithm for open quantum systems and demonstrates what happens when the direction of the information flow between the system and the environment gets reversed due to memory and the system regains the information it leaked earlier.

***

LN10907
Visualizing Motion of Non-Resolvable Objects in a Microscope

It is well known that objects smaller than the wavelength of light (about 1/2000 of mm) can not be resolved individually in a microscope. In this paper we show that the motion of sub-wavelength particles can still be measured very precisely with an ordinary microscope without resorting to track individual particles. This is realized by an advanced image processing technique that isolates efficiently the signal due to the particle motion in a time series of microscopy images.
Our technique, named Differential Dynamic Microscopy (DDM), is a complement to techniques which probe the dynamics of particles, such as dynamic light scattering or particle video tracking. DDM can be used to determine the object size and shape or to measure the mechanical properties of the host fluid, such as for example its viscosity or elasticity. We envision its use in a wide range of systems, such as cells, macromolecules, emulsions, foams, colloidal glasses and gels.

***

LN11239
How fast can one tunnel into chaos?

Tunneling through a barrier is a striking and well studied consequence of
quantum mechanics. In contrast, tunneling from regular to chaotic dynamics
is barely understood. In this paper, we provide a theory predicting tunneling
rates in billiards. Microwave experiments with a mushroom-shaped billiard
confirm these predictions. Playing billiards with photons and electrons is
relevant for applications in microlasers and semiconductor nanostructures.

***


LK11063

The soft strength of Surface Plasmons

Using optical forces to manipulate with laser light small objects – such as living cells- at the surface of a chip is a fascinating aspiration of photonics. In this paper (LK11063), we show how surface plasmon fields engineered at a surface patterned with gold disks enable trapping micro-objects with ultra gentle forces down to a few tens of femto-Newtons. Surface plasmon tweezers not only are the softest optical tweezers ever reported but have the peculiarity of being tunable. In particular, we have discovered that a suitable adjustment of the laser illumination parameters (such as polarization and incident angle) enables controlling their selectivity and achieving trapping of a specific object out of a mix. This novel generation of integrated optical tweezers opens new perspectives for the implementation of future optically driven lab-on-a-chip devices with inestimable applications to biomedicine.

***

LN11227

Monsieur Poincaré, how long do I have to wait?

In 1890 the French mathematician Henri Poincaré proved
that conservative dynamical systems eventually return
to their initial neighborhood. But he could not tell how
long one has to wait for this to happen.
In the present work it is shown that the distribution of
these return times is universal for all Hamiltonian systems
with two degrees of freedom, like e.g. a double pendulum.
These are among the simplest systems that show chaotic dynamics
and the search for their universal behavior attracted the
interest of physicist and mathematicians for decades.
With the help of a newly proposed model and on the basis of
numerical studies this work elucidates the universal
character of the distribution of Poincaré return times.

***


LM11311

Scientists at NIST have created a new kind of "optical lattice" in which to
study ultracold atoms. Instead of each 100nm-scale site of the lattice having
a bucket-like well to hold a few atoms, Lundblad et al. created a lattice with a
ringlike "Mexican hat" geometry at each site. This means that the probability
distribution of the atom in the well is no longer the normal bell curve, but as
dictated by quantum mechanics, approaches a doughnut-like shape. Actual atomic
"doughnuts" were not observed, but the regime of squashed or dimpled
wavefunctions was approached, using a Bose-Einstein condensate as the source of
cold atoms to load the lattice. The mechanisms by which these new lattice
states decay was also studied. A lattice of sites with ringlike symmetry is of
interest to researchers using the tailored and customizable milieu of cold atoms
to explore condensed-matter physics analogues.

***


LP10959
Using Time Reversal to Understand Single-Molecule Mechanical Properties

NIH scientists have discovered a new way to understand the mechanical properties of individual molecules. Using technology such as optical tweezers, single molecules can be held at their ends and stretched, much like rubber bands. Beyond the technical difficulty of performing these precise measurements, there remains the theoretical challenge of recovering what their mechanical properties would be without such manipulation. For example, how likely is it that the molecule will have a certain end-to-end distance versus another? It has been previously understood how to calculate this property from repeatedly pulling the molecule in a single direction. Now, a new theory has been developed, based on optimally using the time reversal of paths that the molecule takes after being pulled in the opposite direction, to efficiently calculate the relative probabilities of molecular lengths. This theory should aid in the understanding of biology at the single-molecule level.

***

LF10925
Strange properties of cryogenic liquids:
From fundamental laws of fluids to technological applications.

Drop formation from the breakup of liquid jets is a universal feature of daily
life, as when water drops form at a dripping tap. It has been scrutinized for
centuries by scientists like Bohr, Eötvös, Laplace, Lenard, Rayleigh, Savart,
and Young. We have developed generators for periodic fluxes of mono-disperse
drops with diameters down to 10 mikrometers. When operated with liquid
Hydrogen or Nitrogen, we observe jets that are significantly more resistant
to breakup than predicted by well established theories. Moreover, at high
evaporation rates, axial symmetry of the dynamics is lost and spontaneous jet
bending is observed. Our findings illustrate how incomplete our understanding
remains of phenomena involving fluids with free boundaries. Our patentet
cooling method may also pave the way to novel technological applications,
like EUV sources for producting highly-integrated micro chips and laser-based
particle accelerators.

***

LL11771
The M1 resonance: a nuclear ingredient that might have been
overlooked in the synthesis of heavy elements in stars

The M1 resonance near the neutron threshold may be a nuclear
ingredient that has been overlooked in the past in the description of
the nuclear properties entering the complicated stellar
nucleosynthesis of the elements heavier than Fe. A systematic
measurement of zirconium isotopes (Zr-91,92,94) with monochromatic
photon beams has revealed the presence of a strong M1 resonance close
to the neutron threshold. This resonance is located in the energy
region in which stellar neutron capture and photodisintegration take
place most effectively. The E1 gamma-ray strength in the low-energy
tail of the giant E1 resonance has so far been considered as the main
excitation or de-excitation electromagnetic mode. Recently,
the emergence of a possible E1 pigmy resonance near the neutron
threshold was theoretically predicted in neutron-rich nuclei and
its clear signature was experimentally confirmed in a tin isotope
(Sn-132). In additon to the tail of the giant E1 resonance and the
pigmy E1 resonance, this research has shown that the M1 contribution
needs to be better understood and determined. Investigating resonance
with higher multipolarities of direct relevance to the synthesis of
heavy elements near neutron threshold may follow in the future.

4-8-08

LN11253

Subdiffusion in peptides originates from fractal-like structure of
configuration space

The origin of 'anomalous' subdiffusion in internal motions in proteins
has attracted considerable attention recently. Here, the analysis of
molecular dynamics simulations of oligopeptide chains reveals that even
molecules of about ten amino acids show configurational subdiffusion at
equilibrium extending from 1ps to 10ns. Trap models, involving a random
walk with a distribution of waiting times, cannot account for the
subdiffusion, which is found rather to arise from the fractal-like
structure of the accessible configuration space.

***

LH11158
Magnetically driven microscopic swimmer

Small organisms such as bacteria are able to ¿swim¿ very efficiently through
what is to them a very viscous environment. Scientists have been trying for
many years to replicate this behaviour in microscopic devices that could be
used in micromachines or for processes such as drug delivery, but with
little success. Scientists at the University of Exeter have now shown (Ogrin
et al., PRL) that a device consisting of two magnetic particles coupled by a
spring can swim under the influence of an external magnetic field. Key to
this behaviour is the different magnetic properties of the two particles.
One is ¿soft¿ and able faithfully to follow changes in the direction of the
external field, but the other is ¿hard¿ and unable to do so. The different
time-varying forces experienced by the two particles, coupled to the elastic
forces in the spring and hydrodynamic interactions with the surrounding
fluid generate a rich variety of motions, including ¿swimming¿ with
efficiency comparable to that of biological organisms. Efforts are now
underway to understand this behaviour, both theoretically and
experimentally, and to produce ¿swimmers¿ ranging in size from a few
millimeters to microscopic dimensions that can be used in engineering and
medicine.

***

LM11759

Many large molecules have similarities to dolls - their limbs are
flexible and move around. Physicists from the Fritz-Haber-Institute of
the Max-Planck-Society in Berlin, Germany can now sort molecules by
the directions of their arms and legs. These conformers, molecules
with distinct poses, are generally hardly distinguishable, yet their
limbs are even flouncing heavily. However, for biomolecules the pose
is important: they can only fulfill their biological purpose, if they
orient their limbs correctly.

We have now for the first time separated the conformers of a
prototypical neutral biomolecule. Our separation technique utilizes
the gradients of strong inhomogeneous switched electric fields and
exploits the different electric dipole moments of the individual
structures. Similar to the separation of charged particles based on
their mass-to-charge ratios in a quadrupole massfilter for ions, we
select neutral molecules based on their mass-to-dipole-moment ratios.
With our new separation method it is possible to prepare clean samples
containing practically only molecules of a single structure. This will
allow a variety of new experiments, such as tomographic
reconstructions of their orbitals and X-ray- or electron-diffraction
imaging of the individual structures, removing the blurring by
different structures from such images.

***

LL11757

A new class of wave function statistics : random models vs
semiclassical analysis.



For a wide variety of systems, the statistical properties of
single particle eigenfunctions play a critical role in the
determination of a host of physical properties. One example is
conductance fluctuations for fairly isolated quantum dots that are
coupled to conduction leads, whether by tunneling or through open
channels. For the past thirty years or so, eigenfunction fluctuations
have usually been described in terms of local correlation functions.
On the other hand, new statistical measures that require both spatial
integration and energy summation enter into expressions for calculating
residual interaction effects on many-body ground state energies. We
introduce a semiclassical framework for deriving analytical expressions
for these ``integrated'' statistical measures. In just the context of
fully and strongly chaotic systems, it turns out interestingly that the
oft-used random plane wave model is found to fail for not imposing
normalization properly and also for not incorporating longer range
correlations correctly, both of which are properly incorporated in the
semiclassical framework. For the particular class of statistical
measures treated in our work and for all dynamical systems, independent
of considerations of integrable or chaotic behavior, the dominant role
will be played by the interplay of Friedel oscillations near the
boundary and eigenstate fluctuations.

***

LL11771
The M1 resonance: a nuclear ingredient that might have been
overlooked in the stellar synthesis of heavy elements

The M1 resonance near the neutron threshold may be a nuclear
ingredient that has been overlooked in the past in the description of
the nuclear properties entering the complicated stellar
nucleosynthesis of the elements heavier than Fe. A systematic
measurement of zirconium isotopes (Zr-91,92,94) with monochromatic
photon beams has revealed the presence of a strong M1 resonance close
to the neutron threshold. This resonance is located in the energy
region in which stellar neutron capture and photodisintegration takes
place most effectively. The E1 gamma-ray strength in the low-energy
tail of the giant E1 resonance has so far been considered as the main
excitation or de-excitation electromagnetic mode. Recently,
the emergence of a possible E1 pigmy resonance near the neutron
threshold was theoretically predicted in neutron-rich nuclei and
its clear signature was experimentally confirmed in a tin isotope
(Sn-132). In additon to the tail of the giant E1 resonance and the
pigmy E1 resonance, this research has shown that the M1 contribution
needs to be better understood and determined. Investigating resonance
with higher multipolarities of direct relevance to the synthesis of
heavy elements near neutron threshold may follow in the future.

***

LP11313
Meyer-Neldel Compensation Effect Explained

Since Meyer-Neldel¿s (1937) report, it has been repeatedly observed that the temperature-independent factor of a thermally activated diffusion constant increases exponentially with its activation energy. In a forthcoming issue of Physical Review Letters, David Emin (University of New Mexico) explains the origin of the Meyer-Neldel compensation effect for thermally activated polaron (multi-phonon electronic) hopping. He notes that while a hopping electron follows atoms¿ motion to negotiate a hop, the electron¿s inter-site motion also lowers the frequencies of the associated atomic vibrations. Meyer-Neldel compensation occurs because the concomitant increase of vibrations¿ entropy is proportional to the hopping activation energy. Emin also shows that Poole-Frenkel behavior (1938), the activation energy falling in proportion to the square-root of the applied field, can result simply from the long-range electron-phonon interaction arising from a carrier¿s Coulomb interactions with displaceable ions of ionic and polar media. Both phenomena significantly affect the performance of electronic devices that utilize molecularly-doped polymers and organic semiconductors.

***

LK11490

Surprising simplicity in the merger of two spinning black holes.

Spinning black holes are tornado-like whirls of spacetime predicted by
Einstein's celebrated theory of General Relativity. These black holes
often come in pairs. As the two black holes orbit around each other, they
emit gravitational waves which cause them to spiral toward one another and
eventually merge into a single black hole. Just within the past few
years, physicists have solved the long-standing problem of simulating this
complicated merger process in detail on a supercomputer. In our letter,
we have shown that the results of these simulations obey suprisingly
simple patterns which may be understood on the basis of elementary
symmetry arguments. Again and again over the past few decades, single
(isolated) black holes have turned out to be simpler and more beautiful
than physicists had any right to expect. Our findings suggest the same
may turn out to be true for binary black hole pairs.

***

LK11778

What was there before the Big Bang?

Einstein's general relativity told us that there was a beginning of time,
the Big Bang. Asking what happened before does not even make sense. But
physicist know that general relativity
can not be valid all the way to the beginning of time since the theory is
full of infinities.
Everybody's best bet is that a new theory, a theory of quantum gravity, will
come to the rescue and be free from infinities.
A simple cosmological model based on Loop Quantum Gravity, proposed by Dr.
Alejandro Corichi at Universidad Nacional Autonoma de Mexico and Dr.
Parampreet Singh at the Perimeter Institute for Theoretical Physics, has
provided new insights on this issue. Their model predicts that the Big Bang
singularity is not there and replaces it with a quantum bounce to a pre big
bang branch. So there is a before the Big Bang. But what?
Their model did also give an answer to that. For a universe like ours,
before
the Big Bang there was also a universe just like it. Not exactly the same
but with
most of its properties. Like its identical twin.

***

LN11581

------------------------------------------------------------------
Black holes define a new universality class for critical phenomena
------------------------------------------------------------------

There are many examples of physical systems which differ microscopically, but share a number of universal macroscopic properties when heated up or cooled down to the point of a so-called second order phase transition. In this paper we propose that such phase transitions (the most common example being the transformation of water into vapor) admit a novel notion of universality which is based on the way in which the system stores information. The amount of information stored can be counted by looking at either short-distance or long-distance properties of the system. It turns out that the two ways of counting information give exactly the same answer in a variety of models, an unexpected result which had only been known to hold in one spatial dimension. Our argument uses a recently discovered connection between second order phase transitions and black hole physics, and is closely related to the universality of shear viscosity. The latter has attracted much attention recently in the context of heavy-ion experiments; in turn, our work explores an alternative application of the intriguing universalities of black hole physics to (quantum) critical phenomena.

***


028813PRB

Novel magnetic gradient material

The enormous progress in hard disk data storage densities is mainly based on the ability to tailor the properties of magnetic systems by structuring them in all three spatial directions. Following this route, we produced and studied a novel magnetic gradient material, which changes its properties locally on a nanometer length scale. By alternately growing thin layers of Cobalt and Platinum, vertically stacked layers and thus structuring along a first direction is obtained. For the remaining lateral directions we do not follow the slow and expensive “top-down” approach of nanostructuring. Instead, we let nature do the job by using the self-organization process that occurs if a solution with small polymer particles is slowly evaporated on a flat substrate. This fast and cheap “bottom-up” method produces well ordered single layers of densely packed nanospheres, which are used as a patterned substrate for the deposition of the Co/Pt system. The curved surface generates a varying film thickness on each spherule, which in turn alters the properties of the magnetic system in a dramatic way. A local change in the orientation of the magnetic moments on a length scale of less than 100 nanometres could be detected via high resolution x-ray microscopy.



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LN11519
A new vision of the old picture of graphene

The nature of charge carriers in graphene (a one atom thick material having
a honeycomb lattice) is unique because on one hand, they appear as
quasi-particles in condensed-matter physics, where the Schrödinger equation
directs their performance, but on the other hand, they behave as
relativistic particles which are subject to the Dirac equation. Both
scenarios originate from carbon atoms arranged in a plane honeycomb lattice.
In the language of Solid State Physics, this can be translated into the
so-called band structure shown to the left in the figure. It was predicted
by P.R. Wallace more than 60 years ago, using the standard periodic boundary
conditions. We have found this band structure taking into account the
armchair and zigzag shaped boundaries of graphene. A lot about the interplay
between non-relativistic and relativistic-like properties of electrons and
holes in graphene has been learned from the Wallace band structure. A lot
more can be more easily and more naturally understood on the basis of the
right-hand-side band structure reported in our paper.

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LJ11331

A giant piezoresistance switch

The ease with which electricity passes through a solid is determined by its resistance. Metals, such as copper, have a very low resistance whose value is fixed. In contrast, semiconductors such as silicon have a high resistance which makes electrical current flow difficult. However, by physically stretching a semiconductor, its resistance can be slightly modified. New experiments show that in specially designed artificial structures combining both metals and semiconductors, electricity can be switched between the metal and semiconductor upon* *stretching, resulting in surprisingly large resistance changes; large enough to be useful for sensing nanoscale movements in nano-mechanical devices.

3-31-08

LL11352

Multiple interactions of molecules in biology and chemistry and pharmacy

Biology uses membrane bound receptor molecules for controlling and signal transduction. Molecular ligands binding to receptors activate or block their function. Receptors can act as dimers or tetramers possessing several equivalent ligand binding sites (multivalency). For instance tetravalent ion–channels are fully activated only, if saturated with ligands. Receptor control increases with ligand concentration. The local ligand concentration at a receptor binding site can be enhanced by connecting two ligands with a polymeric linker. The resulting enhanced binding affinity works for the second ligand, if the first ligand is already bound to the multivalent receptor. Although a matter of debate this concentration effect was known in the scientific community. Recently, Diestler and Knapp (PRL) established a simple model, which explains experiments on enhanced ion-channel activation and puts the concentration effect of multivalency on a firm basis. Multivalency is ubiquitous in nature, used for processes like cell adhesion, cell-cell recognition in immune response, but unfortunately, also used by bacteria and viruses to adhere to and to penetrate in host cells. Pharmacy can make use of multivalency to create new drugs working at much lower concentration that reducee side effects. Chemistry uses this mechanism to foster self-organization of molecular complexes and chemical reactions leading to new molecular architectures. These promising perspectives lead to foundation of the collaborative research center “Multivalency as chemical organization and action principle: new architectures, functions and applications” speaker Prof. Haag, Freie Universität Berlin.


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LL10946

Visible yet invisible: The blue band gap of indium oxide

Researchers from the United States and Europe have combined to dispel to
long-standing popular belief that the direct band gap of indium oxide is
on the order of 3.75 eV. Through the combination of high-resolution
spectroscopic measurements and ab initio quantum mechanical electronic
structure calculations, A. Walsh et al. have demonstrated that the
fundamental band gap is in fact up to 1 eV lower than previously
considered. Direct optical absorption is symmetry forbidden until well
below the top of the valence band, which results in the maintenance of
optical transparency in the visible wavelength range, despite having a
fundamental band gap in the blue range. Indium oxide belongs to a
special class of transparent conducting oxides, which makes it a
ubiquitous component in solar cells and thin film displays. These new
findings have important consequences for understanding and improving
device performances and charge transport in next generation
optoelectronic devices.

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LG11102
Spreading of information across a system of mobile agents: unveiling a
hidden dynamics.


The understanding of information spreading across a system of moving
agents is crucial for many applications ranging from chemical reactions to
epidemic spreading. Though propagation of information has been extensively
studied on lattices and networks, the spatial dynamics of agents plays, in
many applications, an essential role in the evolution of information
spreading.

In this letter we study the spreading of a particular sort of information:
excitations, whose most simple and classical example is a contagious
disease. We analyze 2D systems of mobile agents where the excitation is
transmitted when a quiescent agent keeps contact with an excited one
during a non-vanishing time [*]. The study reveals that the coupling
between the exposition time (to the excitation) and the agent-agent
contact rate becomes crucial to understand the excitation dynamics.
Moreover, we unveil a third regime associated to the contact rate which
has remained unnoticed in the classical epidemiological literature.

From a methodological point of view, this letter seeks to stimulate
further theoretical modeling of the epidemiology of mobile agents, and
convince the scientific community that the study of this kind of systems
opens a new powerful research avenue towards a better understanding of
information spreading.

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LM11313

Stuffed Boron Buckyball is Better than a Hollow One

Fullerenes, and a recently predicted boron analog B80, have spherical shape with nothing inside. Based on quantum mechanical calculations, in this paper, we predict that boron fullerenes become even more stable if covalently bound boron atoms are placed inside. In arriving at the structures such as B84 we followed the leads from the chemistry of boron. B84 is a building block of beta-rhombohedral boron with a C60-like surface built around a B12 icosahedron. The trick is to make B84 electron sufficient so that it is stable and does not associate to form extended solids. Our earlier work on the structure of elemental boron and condensed boranes helped us to estimate the charge requirement of B84 as 50. If 50 electrons are provided by additional boron atoms, where each boron atom provides three valence electrons, 16.66 additional boron atoms are needed. We still cannot compute fractional atoms in a molecule easily! The closest approach towards the 50 electrons is by adding 16,17, and 18 atoms, making the highly stable clusters B100, B101, and B102. These results emphasize the importance of chemical valence rules and encourage exploration of novel boron based nano materials experimentally.

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LM11699
ELECTRON DENSITY OSCILLATIONS HELP TO VISUALIZE INTERACTIONS

It is usually not possible to see any ripples
on the water surface in vicinity of a bathtub wall. However, making the
bathtub small enough and filling it with quantum particles such as
electrons changes the situation - the ripples can become
as deep as the depth of the "liquid" in the bathtub. This is exactly
what happens with free electrons in metals. The surfaces of any
metallic sample are hard walls for the electrons. Within the sample the
electrons move freely just as water molecules in the bathtub analogy.
Due to their quantum mechanical nature there is a maximal momentum for
the electrons, called Fermi momentum, which determines the position of
the "water surface". As the
electrons are waves, their density profile follows the
very complex interference pattern of electronic wave functions. The
scattering of these waves on the hard walls leads to a buildup of
oscillations in the electron density known as Friedel oscillations.
Their periodicity depends on the Fermi momentum. Friedel oscillations
are generated not only at hard walls but also around impurities.
In our Letter we consider a metal with
an impurity and assume that the Fermi momenta on both sides
of the impurity are different, which is often the case for metals
carrying electric current. Naively one would then expect the density
profile to be a superposition of two waves with different periods
generating a beating pattern. In our Letter we show that this is the
case only in _interacting_ systems, where the electrons are supposed to
be correlated by genuine interaction such as electrostatic Coulomb
repulsion. As the imaging of the Friedel oscillations recently became a
standard technique we can literally "see" the interactions in the
beating pattern of the electron density profile.

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LG11361
Tunable source of spin current

By sandwiching a short section of a carbon nanotube between a metal and a
ferromagnet, we have created a tunable spin diode. Devices based on an
electron's spin, rather than its charge, have the potential to
revolutionize the electronics industry. The spin currents are typically
manipulated using optics or magnetic fields, which is inconvenient for
integration with conventional electronics. In contrast, our spin diode
provides a method to generate spin current using only electric fields. As
the device is fully tunable, it could be used to inject controllable spin
currents into non-magnetic materials.

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LJ11257
WHAT IS THE MAXIMAL CHARGE OF A SPHERE ?

THIS LETTER: A universal law has been found that predicts the maximal electric
charge possible for any spherical system larger than 400 fermi, such as
superheavy nuclei, Van de Graaf generators, neutron stars, black holes, or more
exotic systems such as quark nuggets, Q-balls or quark stars. The maximal
charge is proportional to radius for spheres smaller than 10,000 fermi, and
proportional to the surface area for larger objects. This result complements
and in several cases improves various charge relations known for specific
systems.

BACKGROUND: The law is a consequence of the formation of pairs of electrons
and positrons in very strong electric fields. If the total charge of the
sphere is positive the positrons will be expelled and the electrons
remain to reduce the net charge. For spheres larger than 10,000 fermi the
formation is not "spontaneous" but requires a finite time related to quantum
tunneling; therefore the law changes its radius dependence at that radius.

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LP10809
Forbidden coexistence?
It is a well-established fact that ferromagnetic materials such as iron
become nonmagnetic above a "critical" temperature, the so-called Curie
temperature. In this paper we show that this is not true in an iron monolayer
on a single crystal substrate, but that ferromagnetic and nonmagnetic regions
can coexist below the critical temperature as seen in the magnetization-
sensitive image of such a layer. Black and white regions are ferromagnetic
with the magnetization pointing in opposite directions and the gray regions
are nonmagnetic. This phenomenon is a consequence of the microstructure of
the substrate surface which consists of many terraces of various widths and
differing in height by one monolayer. Across the terrace edges the
ferromagnetic interactions are weakened resulting in a terrace width-
dependent critical temperature. Measurements of the temperature dependence of
the magnetization on terraces with different widths confirm theoretical
predictions, made several decades ago that the critical temperature depends
on the width and length of the terraces. This confirmation became possible
only recently with modern imaging techniques.

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LM11256
Stabilization of Solitons Generated by a Supersonic Flow
of Bose-Einstein Condensate Past an Obstacle

In recent numerical simulations, surprising stabilization of the flow
picture with formation of dark solitons behind the obstacle has
been observed, on the contrary to well-known instability of
such solitons generated by other known methods. In this paper,
we show that in the reference frame attached to the obstacle a
transition occurs at some critical value of the flow velocity
from absolute instability of dark solitons to their convective
instability, which means that soliton disturbances are "gone
with the wind" created by the flow. This leads to decay of
disturbances of solitons at fixed distance from the obstacle
and formation of effectively stable dark solitons. This
phenomenon explains surprising stability of the flow picture
and can be used for experimental realization of dark solitons
in Bose-Einstein condensates.

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LP10830
Classical physics goes quantum

Long-held beliefs concerning quantum systems
have now been proved wrong in a Swedish-French
study to appear in Physical Review Letters.
Conventional wisdom states that the quantum world
appears at low temperatures (e.g. Bose-Einstein
condensates), high densities (e.g. metals), or at
nanoscales. However, by combining Wolfgang Pauli's
work on the electron's properties (awarded the Nobel
prize in 1945) with the seemingly disparate work
on so called plasma waves by Hannes Alfvén
(awarded the Nobel prize in 1970), the research team have shown
that plasmas, i.e. electrically conducting gases, can
display quantum behaviour under circumstances
normally considered as classical. By analyzing the
self-interaction of waves in a plasma, the team has been
able to show that the collective quantum spin of the
electrons may give rise to strong modifications of
classical wave propagation. Plasmas are common in
our Universe, and occur on all scales, from fluorescent lights
and fusion schemes to accretion discs around black
holes and neutron stars. Therefore, this discovery can
lead to new ways to probe quantum properties of
matter, and could also help to increase our
understanding of, for example, laser-plasma interactions
and astrophysical environments.