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.



***


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.

***

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.


***

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.

***


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.

***

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.

***

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.

***


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.

***

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.

***


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.

***

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.

***

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.

3-24-08

LL11090
Smashing without splashing : Bouncing mechanisms of a droplet onto a bath

How to dribble with an oil droplet ? Droplets may experience a periodic bouncing motion on a vertically vibrated viscous oil bath, exactly as a
basketball on the ground. Nevertheless, the challenge is much greater since the droplet may disappear by merging with the bath. To avoid that
coalescence, a thin air layer must exist between the droplet and the bath, and needs to be replenished at each bounce. This letter introduces
the mechanisms used by the droplet to refill its vital air cushion : the droplet deformation and its internal motions are required ingredients
to ensure the bouncing. The bouncing ability also depends on the forcing frequency of the oscillated bath. We discovered that there is an
optimum frequency for which the forcing required to bounce is minimum, and a cut-off frequency above which the droplet needs to change of
bouncing mode.

***LN11458
A Peak in the Melting Line of Hydrogen

A peak has been found in the melting line of hydrogen at T=1055 K and a pressure of 0.65 megabar. Over 70 years ago it was predicted that at high pressure the molecules of solid hydrogen would dissociate to form an atomic metallic lattice, later predicted to be a possible room temperature superconductor. For most materials the melting temperature increases with pressure or densification, but recent theories have predicted the melting temperature of hydrogen would rise, then fall with increasing pressure, as a possible precursor of metallization. Theorists speculate that at multi-megabar pressures the melting temperature might decrease to zero Kelvin so that hydrogen remains an atomic liquid with both the electrons and protons being superconducting. Earlier experiments designed to observe this peak were foiled by the diffusion of hydrogen at high temperature into the elements of high-pressure cells, the hydrogen sample either disappearing into confining metals or embrittling the cell. An innovative technique of melting for nanoseconds using pulsed laser heating was used to overcome this problem, as in the short time of melting there is insufficient time for hydrogen diffusion.


***

LM11601
Stunt Doubles: Ultracold Atoms Could Replicate the Electron ‘Jitterbug’

Ultracold atoms moving through a carefully designed arrangement of
laser beams will jiggle slightly as they go. If observed, this never-
before-seen “jitterbug” motion would shed light on a little-known
oddity of quantum mechanics arising from Paul Dirac’s 80-year-old
theory of the electron.

Dirac’s theory, which successfully married Einstein’s theory of
relativity to quantum mechanics, famously predicted antimatter,
electron spin, and perhaps most enigmatically, that an isolated
electron moving through empty space would vibrate back and forth at
the speed of light. This shaking—named Zitterbewegung from the
German for ‘trembling motion’—is so rapid and so tiny in amplitude
that the relativistic uncertainty principle prohibits its direct
observation in free electrons.

In this paper, we devise an experimental arrangement of laser beams
in which atoms, such as rubidium-87, mimic the behavior of the
electrons in Dirac’s theory. The atoms will show ZB—but with
vibrations slow enough and large enough to be detected. Our proposal
may offer access to an aspect of electron behavior that would
otherwise remain beyond observational scrutiny.

Figure Caption: A) Optical lattice of laser beams trapping an atomic
cloud. Color scale indicates cloud density: black is low,
white high. B) Jittering motion of the atomic cloud in the optical
lattice. The horizontal axis represents the spatial distribution of
the cloud along one direction, while the vertical axis shows the
variation of cloud density with time.


***


LE11279
High-magnetic-field atom and plasma trap

Developments in cooling and trapping of atoms and ions have given birth to
the emerging area of cold, ionized, strongly magnetized matter (magnetized
plasmas). At the University of Michigan, such plasmas have recently been
created in a particle trap that has the unique capability to simultaneously
laser-cool and trap neutral atoms as well as to confine electrically neutral
plasmas in extremely strong magnetic fields. The plasma dynamics observed in
this work is characterized by a breathing-mode oscillation of the positive
(ionic) plasma component, which feeds back on the behavior of the negative
(electron) component of the plasma. The electron component has also been
found to undergo significant cooling. These results could help researchers
produce electrically neutral, strongly coupled plasmas, an unusual state of
matter that is also found in neutron-star crusts and in gas-planet cores. It
is further possible to study recombination of magnetized plasmas into
Rydberg atoms, which are highly excited, neutral atoms. The demonstrated
capability of the apparatus used in this work to trap Rydberg atoms may
become relevant in efforts elsewhere to trap anti-matter, in quantum
computing applications, and in research on correlation and diffusion in
many-body quantum-mechanical systems.

***

LM11364

The mechanism of B diffusion in amorphous Si revealed.

The migration of B atoms within an amorphous matrix of Si has been investigated through proper experimental characterizations and advanced modeling. In this paper, the diffusion of B is shown to be a mediated process via the interaction of point defect of the amorphous matrix (i.e., unsaturated Si bonds). With respect to the case of migration in crystalline Si, B diffusivity in amorphous Si is measured to be orders of magnitude higher, also depending on the thermal history of the surrounding host and on the B concentration itself. In fact, since boron is incorporated in 3-fold coordinated matrix sites, the presence of B enhances the density of dangling bonds, thus self-improving its migration ability. This effect is of high impact, since B migration results to be far cry from a standard diffusion regulated by the Fick’s law, having also significant implications on the dopant properties in Si crystals generated from the amorphous phase. Boron atoms are modelled to diffuse after the interaction with dangling bonds promoting the formation of a temporary and metastable, 4-fold coordinated B. Finally, in case of density higher than 0.5% at., B precipitation is observed, through quick formation of B complexes, quite stable against dissolution.

***

LK11711
Discovery of a New Theory of Stressed Solid-Solid Phase
Transformations

Using new data, researchers have advanced a new theory for the
rate at which one solid phase of a material transforms into
another solid phase in the presence of applied stress. Studying
the amorphous (unordered) to crystalline (ordered) phase
transformation in impurity-free silicon under applied stress as a
model system, researchers were able to isolate the individual
atomic-level processes responsible for the phase transformation
and gain greater insight into the atomistic nature of the
phenomenon. The study of stressed solid-solid phase
transformations is of vast fundamental significance due to the
ubiquitous nature of stresses during phase transformation
processes and the technological importance of such
transformations. For example, the amorphous to crystalline phase
transformation in many semiconductors is critical to the
manufacturing of integrated circuits. The new results and theory
have called into question the previously accepted and highly
recognized model of stressed solid-solid phase transformations
advanced more than seventeen years ago.

***

LJ11046

Mathematics for mental blocks from spiny dendrites

Santiago Raman y Cajal's hand tracings of individual neurons in the
1890s recorded dendritic trees bristling with small micrometre sized
protrusions, or spines. Many studies have since shown that changes in
the morphology and distribution of spines are correlated with disease
processes, substance abuse and ageing; but a mechanistic understanding
of this correlation has been elusive. Recently, using fluorescence
microscopy experiments, Santamaria and colleagues (Neuron, 2006) found
that spines behave as transient molecular
traps, that cause anomalous diffusion along spiny dendrites. The shapes
and distributions of spines are critical determinants of the efficacy of the
anomalous diffusion. Electrical signalling in nerve cells is dependent on
electro-diffusion of ions along dendrites. Applying insights from the
mathematics of fractional calculus we have introduced new fractional
cable equations for electro-diffusion along spiny dendrites, that
incorporate anomalous diffusion due to transient trapping by spines.
The fractional cable equations for spiny dendrites extend Rall's cable
theory for smooth dendrites, which has dominated mathematical
descriptions of electrical signalling in nerve cells for the past half
century. Importantly, the new fractional cable equations reveal how
changes in the morphology and distributions of spines along dendrites
will alter electrical signalling in nerve cells, in ways that could not
be anticipated from standard cable theory. The new theory provides novel
insights into likely mechanisms for the cognitive decline that
accompanies disease processes, substance abuse, and normal ageing.

***

LG11054


A Moving Laboratory Atomic Collision Experiment: Observation of
Strong Correlation Between Two Low-Energy Electrons

The correlation between two low-energy electrons emerging from the
near-threshold single ionization of a stationary target atom by electron
impact (or double ionization by photon irradiation) has been subject of
numerous experimental and theoretical studies in the past decades. It was
Gregory H. Wannier who showed for the first time in 1953 that at the
threshold the two-electron break-up is a highly correlated process, in which
the electrons move symmetrically in opposite directions. According to
Wannier, the angular correlation of 180 degree is expected to increase with
decreasing electron energies.

In the field of energetic atomic collisions a unique possibility of
observing extremely low-energy (~ meV) electron emission is provided by the
so-called cusp phenomenon. The cusp is a peak appearing in the energy
spectrum of the electrons emitted from the collision in forward direction.
The electrons contributing to the cusp move with velocities approximately
equal to that of the bombarding ion (atom), i.e., they fly with very small
velocities relative to the projectile. Therefore, an electron spectroscopic
experiment performed for the cusp can be regarded as a moving laboratory
experiment by which one can obtain information about the properties of the
low-energy electron emission in the projectile-centered reference system.

In this Letter we report about a moving laboratory experiment by which we
showed that the Wannier-type correlated two-electron state can also be
formed in atomic collisions. In the experiment we collided neutral helium
atomic projectiles of 100 keV energy with helium atoms. We measured the
energies of two electrons ejected simultaneously following the mutual
target and projectile ionization. We observed a strong correlation between
the electron energies in the vicinity of the cusp, corresponding to angular
correlation of 180 degree in the projectile frame.

***

LK11773
The rich behavior of electronic polarizability of crystalline solids in a strong magnetic field

When an electric field is applied to an insulator, the electric charges are displaced, and the coefficient of proportionality between the field and the displacement is called the polarizability. Polarizability is an important property of insulators. In this letter, we show that the polarizability of the electrons in a crystalline solid, in the presence of a strong magnetic field, has a rich and complex behavior – the sign of the polarizability can for example be changed when the electron concentration is changed ! We explain that this behavior is strongly related to the presence of current carrying edge states in the system: electrons which propagate near the border of the sample, because of the crossed electric and magnetic fields. Our work also sheds light on the fascinating properties of the energy spectrum of a crystalline solid in a strong magnetic field. It has been known for more than thirty years that this spectrum is described by the so-called "Hofstadter butterfly": an intriguing self-similar structure, whose shape is similar to a butterfly. In particular, the gaps of the spectrum are described by an equation where the two unknown variables are integers, called topological gap numbers. The physical interpretation of one of these integers was already known: it is directly related to the number of current carrying edge states. In this work, we provide the first interpretation of the other topological gap number: it is directly related to the electronic polarizability!

***

LK11292
*Melting Temperature Controversy is Resolved*

A long-standing dispute over the high-pressure melting temperature of
the metal molybdenum has now been resolved. Remarkably, two
well-established experimental techniques for melting metals at extremely
high pressures, namely, shock wave compression and laser-heated diamond
anvil cells (DAC), yield more than a twofold difference in the melting
temperature of molybdenum at a pressure comparable to that at the center
of the earth. In view of these contradictory experimental results,
physicists turned to large-scale computer calculations based on a sound
theoretical framework, density functional theory (DFT), for a
resolution. Calculated high-pressure melting temperatures for molybdenum
with an assumed body-centered cubic (bcc) crystal structure were all
well above the DAC-measured values, but the controversy remained because
there was no explanation for the results of the carefully conducted DAC
experiments. In this paper, we perform DFT-based computations from which
we draw three important conclusions. First, the face-centered cubic
(fcc) structure is more stable than the bcc at high pressures and
temperatures; hence, molybdenum does /not/ melt from the bcc structure
at high pressures, as is commonly assumed. Second, our calculated fcc
melting temperatures are in good agreement with the shock wave results.
Third, our calculated bcc-fcc phase boundary is consistent with the DAC
data. We conclude that the high temperature phase transition detected in
the shock wave experiments was melting, as claimed, and the DAC
experiments are mapping out a solid-solid phase boundary.

***

LK11561

Singlet state through a tiny multiplet of mediators

The singlet state of two particles with spin (or ¿spins¿) is a well-known
entity to physicists. This is due to its very special properties such as
invariance under various processes where the total spin is conserved, and
entanglement (i.e. correlations between the spins which cannot be attained
through classical mechanisms), together with many other potential
applications. If the particles interact properly, the singlet state may arise
naturally as a ground state. But, how do we prepare the singlet of two
non-interacting particles which are far apart? In this paper, we demonstrate
how to achieve the singlet of two remote spins (magnetic impurities or atoms)
on a wire. Mobile mediators (electrons or photons) in a given internal state
are sent in succession. Once it has interacted with the (initially
non-correlated) spins, each mediator is detected, reflected or transmitted, in
the same internal state it was initially prepared. After a small number of
iterations, a singlet state is achieved. The advantages of this scheme are
manifolds. For example, no precise control over time and strength of the
mediator-particles interaction is required. Due to all this, our scheme holds
the promise to generate singlet states in the lab in the near future.


***

LG11405

"Don’t disturb – it’s a classical correlation!"

Correlations are ubiquitous – both in classical and quantum worlds. We address the fundamental question of distinguishing classical and quantum correlations. Consider Alice and Bob sharing a quantum state. When Alice performs measurement on her part of the system, the overall state gets modified, in general. But if the correlation happens to be classical, Alice must be able to find an optimal measurement scheme leaving the entire state intact! In contrast, quantum correlated states are sensitive to partial measurements. Quantum discord proposed by Ollivier and Zurek (and an independently similar suggestion by Henderson and Vedral), as a measure of quantumness, aims towards quantifying the minimum disturbance upon an optimized partial measurement by Alice. However, such a scheme leads to a conflicting result: quantum discord (and another variant, quantum deficit, proposed by Rajagopal and Rendell) yield non-zero values for a large class of separable states - traditionally considered to be classically correlated. This raises a question on the division of composite states into separable and non-separable (quantum entangled) and even on whether quantumness of correlation is more general than quantum entanglement. Since quantum entanglement is considered to be an important resource in quantum communication and computation, this question needs a clear answer. We approach this issue by considering extended three party states shared by Alice, Bob and Charlie - but leaving the state of Alice-Bob unaltered. Our new measure “quantumness” quantifies the least disturbance suffered by the state of Alice-Bob, when an optimal partial measurement is performed at Alice-Charlie end. Strikingly, quantumness vanishes if Alice-Bob system is separable as Alice and Charlie together can always end up with an optimal measurement scheme, under which the Alice-Bob separable state remains insensitive! On the other hand, an optimal measurement by Alice-Charlie projects an entangled Alice-Bob state to its nearest separable state – with the same marginal state at the Bob's side. So, our measurement-based quantification serves as an upper bound to relative entropy of entanglement - a well-known measure of quantum entanglement. Our proposal thus leads to a conflict free union of quantumness of correlation with quantum entanglement itself.

***

LL11576
Streams of change

The boundary of an electronic droplet in a strong magnetic field may
be viewed as a collection of streams of charge flowing along the edge.
Traditionally, the interaction effects between the electrons are
treated by taking into account only scattering processes that keep the
amount of charge in each stream fixed. We show that when the
interaction is sufficiently strong, other scattering processes, which
do transfer electrons between the channels, lead to generation of
additional streams and rearrangement of the flow pattern. This
reconstruction of channels affects the non-Ohmic current-voltage
relation for electrons injected into the edge of the droplet. As such,
it may hold a clue to the solution of the outstanding discrepancy
between the experimental data and the predictions of the traditional
model for the edges of electronic systems in strong magnetic fields.
The instability generated by the aforementioned scattering processes
calls for a re-examination of their role in other strongly interacting
systems, and may help in designing new switching devices of nanometer
size



***


LX10824

Lifelike self-organizations of freely moving objects in a fluid

Thermal convection is responsible for generating wind in our atmosphere
and motion in the mantle inside the earth that leads to continental drift.
We demonstrate in this work, that a thermal convection system that
includes freely moving objects can turn into an oscillating and
self-organizing, lifelike machine. These free bodies assemble and
disassemble in a cyclic fashion, constantly counteracting the flow that
tends to force them together. This collection of spheres can be viewed as
a deformable mass that is entrained and reshaped by the convective flow.
Geophysicists might regard this system as a prototype, mimicking the
interaction between many small continents and the convective mantle. One
might also view it as a lifelike machine that conducts semi-regular
assemblies, simply powered by a heat flux through a fluid containing
mobile objects.

***

LM11517
In this paper we developed a method to assemble paramagnetic colloidal
particles into regular two dimensional micro-periodic structures. We
control the particle motion by using a magnetically structured film as
substrate (e.g. a Garnet film) and following application of an oscillating
magnetic field normal to the film. Tiny defects in the film's magnetic
pattern act as nucleation agents for growth of particle clusters. The film
morphology (a series of ferromagnetic striped domains) reflects into the
assembled clusters in such way that, adjusting the frequency and/or strength
of the applied field one obtains different order in the colloidal aggregate,
like hexagonal, square, chain-like etc… The strength of the external field
controls also the stability of the formed structures. The latter can be easily
melted by increasing the filed higher than a critical value. All these results
will appear in the issue of Physical Review Letters.
The discovered method is useful towards the controlled design of novel micro
and nano-structured materials and devices. Also, our colloidal assembly method
can be used as a model system in two-dimensions where to study phenomena of heterogeneous
nucleation driven by substrate defects, crystal formation or phase transitions
between morphological orders.


***


LK11276

Hopping dislocation on an atomic wire

The dislocation or crystallographic misfit defect is ubiquitous in materials and greatly affects various properties of materials from growth and fracture to optical and electrical properties. The effects of dislocation can become more and more important as the size of the material shrinks down to, like, nano particles and nano wires. In this paper, we have succeeded in visualizing the existence and the motion of dislocations on the extremely small materials system, that is, one-nanometer wide atomic wires assembled on a silicon substrate. A pair of extrinsic impurity atoms is favored to adsorbed commensurately with the underlying wire lattice (top panel in the figure). However, if the distance between the adsorbates is incommensurate occasionally, the wire lattice in between has a misfit dislocation (middle panel) of atomic scale. At room temperature, this dislocation hops between the two adsorbates making a characteristic modulation pattern in the scanning tunneling microscopy image (bottom).

***

LK11399
Interactions between three Hadrons from Quantum Chromodynamics

For the first time, the three-body interaction in a multi-hadron
system has been
determined from Quantum Chromodynamics (QCD), the underlying theory
of the strong
interactions that binds quarks and gluons into protons, pions and all
hadrons.
Since the 1970s, it has been known that the interactions between hadrons
are governed by the equations of QCD, and since that time an enormous
effort has been
dedicated to solving these equations. Recent advances in lattice QCD,
a numerical technique for solving QCD,
and in computational power are finally allowing for quantitative
calculations. The NPLQCD collaboration
have succeeded in extracting the properties of simple multi-hadron
systems (systems composed of two, three,
four and five pions) directly from lattice QCD. Using cutting-edge
supercomputers at
The Thomas Jefferson National Accelerator facility, FermiLab, the
National Center
for Supercomputing Applications, Lawrence Livermore National
Laboratory, and
the Mare-Nostrum Supercomputer in Barcolona, Spain, the team has
demonstrated
the presence of an irreducible three-pion interaction.
This pioneering research, points the way to QCD studies of nuclear
structure and interactions which,
in turn, can address fundamental questions about the nature of our
Universe.
Their results appear in the current issue of Physical Review Letters.

***

LL11060
Ever since Feynman first proposed the idea of a quantum Turing machine
in the 1980s, the field of quantum information theory has blossomed,
promising super-powerful machines solving classically-intractable
problems, perfectly secure communication, and shedding new light on
fundamental issues such as the interpretation of quantum mechanics.

The power of quantum mechanics to promise so much relies heavily on
the resource of entanglement. This is a strong correlation between
particles, stronger than allowed under the laws of classical
mechanics. Entangled particles, in some sense, lose their individual
identities and only truly exist as part of a pair or group.

Entanglement occurs naturally in groups of physical systems; for
example, spin chains, which are often used as a model of magnets. The
amount of entanglement occurring is often given as an indication of
how powerful such a system would be as a quantum computer.

The standard measures of entanglement used at present are a little
artificial, in as much as they measure the amount of entanglement that
can be extracted from a system if one has an infinite number of copies
of the system! This is called the 'asymptotic limit'. A far more
realistic measure is given by a quantity called the 'single copy
entanglement', which is how much entanglement exists in one copy of
the system.

In this paper, I give the first indication that for a particular class
of systems (those with a finite gap between the energies of the ground
state and the first-excited state), the single copy entanglement is
equal to the asymptotic amount! This means that all the entanglement
present can be extracted from a single copy of the system, and that
the presence of many copies of the system does not convey any
advantage. Previously, it was known that for systems that have no
energy gap, half the entanglement present may be extracted from a
single copy.

I hope that this result will go some way in to understanding the
process of entanglement extraction from naturally-occurring many-body
systems, and the structure of this particular class of system.

***


LM11759
Picking biomolecules by structure

The molecular building blocks of life, so called biomolecules, are an
important class of molecules. Their intrinsic properties are nowadays
often examined using the molecular beam technique. It was realized in
the 1980s, that even at the very low temperatures of only a few Kelvin
in these molecular beams, multiple structures of the molecules are
present. While all individual structures have the same mass, they
possess different properties, i. e., electric dipole moments. We have
now for the first time separated the individual structures of a
prototypical neutral biomolecule, namely 3-aminophenol, that exhibits
two distinct structures. 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, 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, what will allow a variety of new
experiments, such as X-ray- or electron-diffraction imaging of the
individual structures or tomographic reconstructions of their charge
distributions.

***


LH11343


We observed spontaneous structural change of single proteins at a
temperature of 1.5 K by visible fluorescence spectroscopy. The
structural information obtained from this type of the experiments will
disclose the details of the structure-function relationship of
proteins. During the last decade, fluorescence spectroscopy of single
proteins at a liquid helium temperature has been successfully applied
in the near-infrared region to bacterial photosynthetic antenna
complexes. In the present paper, we developed a new design of a
low-temperature fluorescence microscope in order to generalize the
method to study proteins fluorescing in visible. The technical
difficulties in working in the visible region were overcome by using
reflective optics. The fluorescence spectrum of single
green-fluorescent proteins at 1.5 K makes a distinction between
different metastable conformations that last for tens of seconds.

***

LD11800
*How do foams and concentrated emulsions flow?*

In this paper we explain and describe theoretically the basic process
governing viscous friction in flowing foams and concentrated emulsions.
Our model is based on detailed consideration of the dynamics of the
transient planar films, formed between two neighboring bubbles (drops),
which slide along each other in flowing foams (emulsions). By comparing
the shear rate of the system, which determines how long is the time of
contact between the neighboring bubbles (drops), with the rate of
thinning of these transient films, we explain the experimentally
observed square-root dependence of the shear stress vs. shear rate,
which is known for more than 20 years, without being explained till now.
The model predictions are in good agreement with various experimental
results, obtained with both foams and emulsions. Further development of
the model, accounting for the energy dissipation on the surface of the
bubbles and drops, is in progress.

***

LG11883

Hurricanes in a Soap Bubble

Can hurricanes be produced in the lab? Well, that's what our recent
paper to be published in PRL shows. Motivated by the physics of
atmospheric phenomena, our experiment uses a soap bubble (a half
bubble is used actually) heated at the equator and cooled at the top.
The heat difference between the equator and the 'pole' creates
turbulent convection in the soap bubble. We show that once in awhile,
a large single vortex emerges from the turbulent background. The shape
and dimensions of this vortex resemble those seen for natural
hurricanes or cyclones, or Jupiter's Red Spot for that matter. By
tracking this vortex, we show that its movement is random, and when
the statistics of this random motion is compared to that of natural
hurricanes, they turn out to be very much alike. In simple terms, the
chaotic motion of our 'hurricanes' resembles that of the real ones in
the atmosphere making our system attractive for the understanding of
the randomness of hurricane movements and opening further
possibilities for predictive models of such large scale natural
phenomena.

***

prl 100, 090402

Thermal quantum physics in Lineland observed

A team of researchers from the University of Amsterdam, The Netherlands
and the University of Queensland, Australia, has succeeded in comparing
temperature and density of a one-dimensional quantum gas to an exact
theory that was developed back in 1969 by Nobel Laureate C. N. Yang and
his brother, C. P. Yang. The results are published in the 7 March issue of
Physical Review Letters [Phys. Rev. Lett. 100, 090402 (2008)] and were
selected as Editors' Suggestion as a means of promoting reading across
fields.

The experiments in Amsterdam were performed using an 'atom chip' a
lithographically produced pattern of gold wires on a silicon substrate.
By sending currents through the wires, a gas of rubidium atoms is
magnetically trapped and cooled to the point where the atoms can only
move in one dimension. The atoms thus live in 'Lineland'. The
needle-shaped atomic cloud, with a width less than a percent of a human
hair, is cooled to temperatures down to 100 nanokelvin.
In this way, the first direct comparison was made possible between
experiment and the nearly 40 years old Yang-Yang theory. The Yang-Yang
exact solutions to the one-dimensional Bose gas model at finite
temperature were once only a tour-de-force of mathematical and quantum
many-body physics. The experimental techniques of laser cooling and
magnetic trapping are now allowing physicists to precisely engineer these
model systems in the laboratory.
In addition, the experiment allowed access to the momentum distribution
of the gas. This distribution cannot be directly obtained using the
Yang-Yang method and the measurement thus poses a new challenge to theory.

***

LM10983
Gravity Wave “Smoking Gun” fizzles, but nevertheless provides more sensitive probe of processes the physics of the early universe than previously envisaged.

Inflation is currently the best theory we have to explain the observed isotropy and flatness of the Universe. Moreover, it also predicts a spectrum of primordial density fluctuations arising from quantum mechanics that agrees strikingly well with observations of large scale structure. However, the many different model manifestations of Inflation can also accommodate the results of almost any observation, from the curvature of the observable universe, to the nature of large-scale structure. But if the scale of inflation is close to the Grand Unified Scale, where the three non-gravitational forces appear to come together in strength, a primordial spectrum of gravitational waves will result that would leave a measurable imprint on the cosmic microwave background (CMB) radiation via a certain type of polarization in the radiation, called a B mode polarization. This has motivated an active effort to search for such a signal, the apparent ‘smoking gun’ that would finally directly confirm the fact that inflation indeed occurred. Our work, however, demonstrates that a strikingly large (almost 10,000 times larger than naïve dimensional estimates had suggested) and very similar primordial gravitational wave signal can result from non-inflationary phase transitions that might happen later in the early universe. This unfortunately implies that observing a gravitational wave imprint in the CMB may not provide unambiguous proof that inflation actually occurred. Nevertheless the large enhancement of a non-inflationary signal also implies that the gravitational wave imprint in the CMB is a far more sensitive probe of new physics near the GUT scale than previously envisaged and further motivates the search for such a signal.

***


LM11151
Evidence for a New Behavior of Anti-Matter is Confirmed at Fermilab

In 1955 physicists realized that a fundamental particle can have the
"ghost-like" behavior of changing back and forth between its "normal"
state and its "anti-matter" state. The back and forth oscillation
occurs at a fixed frequency, as if the particle was "ringing" like a
bell or tuning fork. Last May, two separate teams of physicists, from
the Stanford Linear Accelerator Laboratory in California and the KEK
Laboratory in Japan, published evidence for a new type of matter
anti-matter oscillation involving particles with a "charm" quark. A
race was on to see if either of the team's results could be confirmed.
The Collider Detector at Fermilab Collaboration has now confirmed the
type of evidence seen at Stanford. The Figure shows the data for a
key ratio of decay rates that changes as a function of time (dashed
curve) and is not consistent with a flat behavior (dotted line). The
implications of the newly observed behavior are still to be determined
- whether it can be explained by the "standard model" or
whether new physical laws have been unveiled. For now, physicists
want to make more precise measurements of the "charm oscillations" and
study them using different techniques.

***

LK11787

A new recipe to cook superheavy elements

Producing superheavy elements is a delicate procedure. While light ions
fuse easily with heavy nuclei, resulting compound nuclei are hot.
Generally, prompt fission destroys them and only very rarely a nucleus
survives after evaporation of four or five neutrons. In contrast, more
symmetric target-projectile combinations, leading to colder compound
nuclei that are more prone to survive, do not fuse so easily due to
increased Coulomb repulsion. We took the best of both worlds: a light
projectile guaranteeing fusion and a low projectile energy to keep
things cold. By doing so we risked falling below the Coulomb barrier.
However, the deformation of the used actinide target nuclei causes the
barrier height to be no longer at a fixed value but to depend on the
orientation of the colliding nuclei. Using a highly efficient chemical
technique, we proved that compound nuclei are formed at energies well
below the classical fusion barrier. Fusion of ^26 Mg with ^248 Cm
produced the new nuclide ^271 Hs after evaporation of only 3 neutrons
with surprisingly high yield, comparable to those of ^270 Hs and ^269 Hs
produced at higher energies. The new reaction type holds the promise to
synthesize other new neutron-rich superheavy nuclei.

***

LK11083

Efficiency of multiple-exciton generation in quantum dots

Efficient multiple-exciton generation (MEG) has been recently reported in semiconductor quantum dots. By producing multiple carriers per absorbed photon (“several for the price of one”), MEG has the potential to significantly increase the output current, and therefore the efficiency, of solar cells. The difficult question is: Exactly how many carriers are generated by a single photon? Up to now, the number of carriers produced by MEG has been estimated by considering that the absorption spectrum is “bleached” by the presence of electron-hole pairs generated by MEG, and assuming that this bleaching is linear with the number of electron-hole pairs N. In this paper we critically examine this assumption using atomistic calculations for colloidal CdSe quantum dots. We find that the bleaching of the absorption spectrum depends non-linearly on N. As a result, MEG efficiencies obtained from the linear scaling assumption are significantly underestimated, if the bleaching is entirely due to MEG. We also find that there is a maximum value of the bleaching that can be attributed to MEG. Thus, any measured value in excess of this maximum must originate from other bleaching mechanisms.

***

LN11138

Non-local Cooper pairs in a graphene flake

Graphene, a single-layer hexagonal lattice of carbon atoms, is a promising candidate for future nanoelectronics. Combining its unique properties with superconductivity, we predict a very special scenario for the Cooper pair injection within a graphene sheet: the outgoing electrons are emitted in opposite directions rather than in the same direction as it is the case in usual conductors. This enabling property allows the spin-entangled unbound electrons to be detected far apart from each other and may help to confirm the non local character of quantum mechanics. Such fundamental tests, performed successfully in quantum optics with polarization-entangled photons, remain a challenge in electronic systems. Previous attempts to collect electrons with nearby metallic tips failed due the electrons tunnelling directly from one tip to another. Such spurious processes are eliminated owing to the unique band structure of this novel bipolar graphene device.

***

LG11298


COMPLEX NETWORKS: IS THERE A WAY BACK?

The complex network enterprise has been largely based on models that
ignore link directionality, for the excellent reason that undirected
networks are much simpler to study. But, given that most real networks are
directed [1], how consequential is the asymmetry of network connections
for widespread network phenomena such as the small-world effect? In
undirected networks, Watts and Strogatz [2] have shown that the
small-world effect results from a combination of small node-to-node
distance AND very large number of short loops (or clustering coefficient).
In fact, it can be shown that these two ingredients are essential for the
success of the celebrated Milgram message-passing small-world experiment
[3], or any search algorithm based on
the same principle [4]. Our study shows, however, that directed networks are
fundamentally different: they tend to have very few loops when compared to
random versions of the same network. The directed neural network of C.
elegans, for example, has less than 50% of the short loops expected from
the random counterpart despite the well-known fact that, when regarded as
an undirected network [2], it has a very large clustering coefficient
compared to randomly rewired versions of the network. Similar behavior was
confirmed for most networks in our database. We suggest that this
distinctive property of directed networks will have widespread
implications in various domains. For example, the reduced number of loops
implies enhanced stability in foodwebs, improved optimization in
transportation networks, reduced reinforcement in the adoption of norms in
social networks, and enhanced
synchronizability in oscillator networks, just to name a few.


***

LG12027
Dynamical clustering of counterions on flexible polyelectrolytes


In contrast to previous studies, which considered equilibrium properties of polyelectrolyte alone, we use molecular dynamics simulations to study the spatio-temporal dynamics of charge fluctuations around a polyelectrolyte molecule at charge densities above and below the classic counterion condensation threshold. Surprisingly, the counterions are found to form weakly interacting clusters which exhibit slowly decaying short range orientational order. Local charge fluctuations are shown to create energy fluctuations comparable to activation barriers for transitions between chemical states and sufficient to affect the polyelectrolyte interaction with an approaching ligand molecule in biological systems. Our results provide new insight into the behavior of counterion-charged polymer association at nanometer and nanosecond scales, information essential for understanding a wide variety of phenomena, ranging from inter-cellular transport in biological systems to self-assembly of macromolecules and to stability of polymer gels and colloids.

***

LK11486

The strength limit of hydrogen bonds in protein materials

Hydrogen bonds in proteins are abundant weak chemical interactions that govern the mechanical behavior of biological protein materials such as spider silk, muscle tissue and amyloid fibers, yet, their mechanical strength remains unknown. Many biological processes involving cell mechanics and cell adhesion depend on the unique properties of these weak bonds, in particular for enhanced robustness as well as their ability to self-heal and to self-assemble. The lack of understanding of mechanical strength has prevented us from developing accurate models for genetic diseases, diagnosis and treatment options for molecular medicine, and the ability to utilize proteins as a platform to develop novel bioinspired materials.

In the upcoming Physical Review Letters article, we report a simple theoretical framework, based on Griffith’s model of fracture mechanics introduced in 1921, which predicts the maximum strength of hydrogen bond assemblies in proteins, providing a universally valid model that unifies previously contradicting experimental observations. The model captures the key physical mechanisms that govern the strength of protein materials, and thereby provides a unifying explanation to some of the most widely discussed and unresolved aspects of protein unfolding experiments. Our findings may have implications for nanoscience, biological sciences and molecular medicine, and may help in the interpretation of AFM and optical tweezers experiments.

***

LK11748
Snipping the Light Fantastic: The World's Shortest Single Photons

Single photons (discrete wavepackets of light) are not only interesting in terms of fundamental physics, but also from the point of view of applications in the emerging field of quantum information processing - a field that has the potential to revolutionize computing by harnessing the data processing power inherent in quantum mechanics. In this paper we present the results of a new technique, based on photon pair generation, that for the first time allows the preparation of single photons of exceptionally high quality, conditioned on the detection of their twin. Furthermore, our photons have a temporal duration of as little as 65 femtoseconds (65 millionths of a billionth of a second), to our knowledge the shortest single photons ever generated. The precise timing and consistent attributes of these photons make them ideal for implementing photonic quantum logic gates and conducting experiments requiring large numbers of single photons, as required by quantum computing algorithms.

***

LL11701
Existence of a spin-gap in the normal state of the superconductor Mo3Sb7

Though the existence of a pseudo-gap (PG) in the quasi-particle density of states in the normal state of high-Tc (HTc) superconductors has been well documented, there are no full answers to the question concerning the origin of the PG, and therefore to the feedback effect between the PG and superconductivity. In this paper, we want to point the existence of a spin-gap in the normal state of the superconductor Mo3Sb7, thus resembling the situation found in HTc superconductors. The experimental data, including magnetization, electrical resistivity and specific heat exhibit that before the superconductivity sets in at 2.3 K, a spin-gap at a relatively high temperature of 50 K is formed due to a dimerization of the nearest-neighbors Mo-Mo pairs. The electronic structure calculations using the LSDA approximation show nesting property in the Fermi surface, favoring the superconductivity.



***


LP10906

We present Lagrangian studies of the local temperature mixing and heat transport in turbulent convection, based on three-dimensional direct numerical simulations. A detailed analysis of the acceleration and dispersion properties of Lagrangian tracers is provided. Caused by the interplay of buoyancy and gravity, convective turbulence is inhomogeneous and anisotropic. Consequently, significant differences between the vertical and lateral transport properties are found. Contrary to vertical tracer pair distances, the temporal growth of lateral tracer pair distances agrees with the Richardson law, but yields a smaller Richardson constant due to correlated pair motion in plumes. Our results thus imply that Richardson dispersion is also found in anisotropic turbulence. Furthermore, we take a "Lagrangian fingerprint'' of local heat transfer events, so-called thermal plumes which carry blobs of hotter or colder fluid from the boundaries into the bulk. The height-dependent joint Lagrangian statistics of vertical plume acceleration and local heat transfer allows us to identify a zone which is dominated by thermal plume mixing. Such studies are important for a better understanding of the transport of phytoplankton in the ocean or aerosols in the atmosphere.

***


LL11737

Visualization of nanoscale nucleation dynamics at femtosecond resolution

Nucleation processes are all around us, familiar to anyone who has observed the appearance of bubbles in a pot of boiling water. In this work we investigate the same process but, for the first time, with atomic-scale resolution and femtosecond time resolution. We use short pulses of x-rays to visualize the nanoscale fluctuation dynamics that underlie superheated liquids, first order phase transitions in materials, and ablation physics. Our measurements capture a glimpse of never-before-seen transient states, and direct evidence for the spontaneous formation of fluctuating nanoscale voids. Because of the high spatial and temporal resolution, these experiments provide the first direct validation of theoretical simulations of the laser ablation process in semiconducting materials. The techniques developed in this work form the basis for a range of multidisciplinary experiments at future x-ray free electron laser facilities. More generally, they describe a new way of looking at materials in motion, on length-scales and time-scales far from human experience.

***

LN11428

Smaller is Stronger: Plasticity of Crystals at Nano-scale

The strength of materials at nano-scale is important for the fabrication and reliable functioning of devices. Plasticity in crystals generally occurs by the motion of dislocations, or line defects. While it is extremely difficult to directly observe the motion of individual dislocations, correlations can be made between the stress-strain behavior and dislocation activity. In bulk metals, dislocations multiply in the course of plastic deformation causing work-hardening. Although this fundamental concept is often assumed to be applicable to crystals of any dimensions, numerous recent studies have shown that conventional plasticity breaks down at the sub-micron scale and have demonstrated a pronounced size effect, whose main premise is "smaller is stronger." In this work we investigate for the first time the stress as a function of diameter in two fundamental types of crystal: gold (face-centered cubic) and molybdenum (body-centered cubic) single crystalline nano-pillars subjected to uniaxial micro-compression. Our results suggest that these crystals have fundamentally different plasticity mechanisms when reduced to nano-scale with significant work-hardening present in the latter type and virtually none in the former. The attainment of nearly 50% of the theoretical strength in gold suggests that plasticity is controlled by the nucleation of new dislocations rather than by interactions of the pre-existing ones. On the contrary, the smallest molybdenum nano-pillar achieves only ~7% of its theoretical strength, implying that plasticity is most likely driven by the intricate motion and interactions of dislocations inside the pillar rather than by nucleation events.

3-17-08

LL11022

Nanofridge doesn’t mind a jolt

The design of minuscule devices that operate for example within the
cells of living organisms could be severely hampered by a tremendous
obstacle: the thermal motion of molecules at this scale. In the last
10 years, research has intensified to find ways to convert this
thermal nuisance into useful purposes. Now a tiny refrigerator has
been uncovered that would be very happy to work in such a hostile
environment. Although the device has not yet been built, thousands of
them could one day be integrated onto a chip to cool computers and
other electronic devices.

The nanodevice, reported recently in Physical Review Letters by
physicists Martijn van den Broek and Christian Van den Broeck of
Hasselt University, Belgium, consists of two snail-shaped spiral
rotors separated by a thin membrane. If an external power source is
used to turn the device, it can operate as a heat pump, transferring
heat from a cold to a warm environment via the rotors. The report
also unveils how a theoretical principle can turn the better-known
motor functionality of the device into a fridge.

The rotors could be very useful in creating bio-engineered systems in
which the careful control of temperature would be needed to ensure
that certain biochemical reactions occur at the desired rates.

See also ‘Tiny fridge thinks it’s a motor’, IOP physicsworld.com
headline news, http://physicsworld.com/cws/article/news/31997.


***

LH10869

Carbon-Based Semiconductors Show Unique Sensitivity to Magnetic Fields

Usually small magnetic fields of a few mT not much stronger then the earth's magnetic field are not expected to effect electric current in semiconductors. However, the amount of electrical current flowing through organic (carbon-based) semiconductors can be strongly modified by such a magnetic field, in an effect called magnetoresistance (MR). In this paper, we report the unique property of organic semiconductors to show an inversion of MR, i.e., in one set of conditions, the current through an organic semiconductor may increase when a small magnetic field is applied, but small changes to the temperature, voltage or thickness of the thin-film organic semiconductor can cause the current to, instead, decrease. Additionally, we apply the MIST model (MR by the interconversion of singlets and triplets) we developed to describe the quantum mechanical phenomena responsible for the increase or decrease of current in the presence of magnetic field. We proposed that MR and the inversion of MR are due to the role of the very weak and usually ignored hyperfine interaction in a magnetic field. In contrast, inorganic semiconductors used in common microchips, such as silicon or gallium arsenide, are largely insensitive to such magnetic fields.

***


LC10969

Archimedes found in the coldest place.

Archimedean lattices, named by J. Kepler, are found to exist in the Bose-Einstein condensate, the coldest state of matter in the universe. For the first time, Authors have demonstrated that when the atom-molecule mixture of Bose-Einstein condensate rotates, vortices appear in a form of lattice made of triangles and rectangles. Surprisingly, this lattice agrees exactly with one of the eleven Archimedean lattices first elucidated by Kepler [1].

Authors explain how the two-to-one ratio of molecule and atom mass leads to the formation of Archimedean lattice and also the onset of lattice formation due to rotation.



***

LM11370

TRANSPORTING MATERIAL UPSTAIRS DURING NANOSTRUCTURE FORMATION

Bilayer and multilayer islands often develop during the initial stages
of epitaxy. Their formation requires upward transport of material, even
for bilayer islands where atoms must climb ¿upstairs¿ from the ¿first to
the second floor¿. How do atoms overcome binding to the edge of lower
layers in order to climb up? Often edge-bonded atoms in growing islands
become less strongly bonded due to strain buildup. Alternatively,
sometimes surface steps help create bilayer islands by allowing direct
access to the ¿second floor¿. However, in this study, we deposit Ag on
NiAl(110) where strain is not significant, yet bilayer islands form on
broad terraces away from steps even down to very low temperatures of
130K. How can upward mass transport be so facile? We propose that
strongly anisotropic interactions between Ag atoms on NiAl(110) are key.
Weak interactions in one direction allow easy climbing up one side of
monolayer islands. In addition, Ag atoms located at first-layer kink
sites at island edges are not effectively trapped by multiple strong
bonds (as in isotropic systems), but can escape by breaking just one
strong bond and then climb upstairs. Simulation of a realistic atomistic
model confirms this picture.

***

LL11760

Converting sound into light: the brightest sonoluminescent bubble ever.

Researchers from Instituto Balseiro/CAB, Argentina obtained the brightest bubble ever reported in sonoluminescence. The phenomena known as single bubble sonoluminescence was discovered by Gaitan in 1989. In his experiment a bubble was levitated at the center of an acoustic resonator and it emits light pulses at the frequency of an acoustic field. In this work Urteaga and Bonetto were able to get light intensities 300 times higher than in Gaitan´s experiments. They used as a resonator a spherical glass shell 250microns in thickness (approximately twice the thickness of a human hair) and 90mm in diameter filled with almost pure sulfuric acid. They were able to fix the bubble at the resonator center using two synchronized acoustic waves (one with a frequency of 33kHz and the other of 165kHz). They also present a state of the art theoretical model to explain the main features of the experimental results. The paper will be published in the February, 22nd 2008 issue of Physical Review Letter.

***



LL11279

Cooper pairs of entangled atoms

We propose a method to create large Cooper pairs of bosonic atoms in an optical lattice.
Cooper pairs are responsible for the effect of superconductivity. To date, they have been only produced using fermions, either electrons or fermionic atoms.

Our proposal starts with an optical lattice, a periodic potential created by light, in which tightly bound entangled pairs of bosonic atoms are loaded (setup.jpg). This quantum state has been realized experimentally in 2007. We propose to lower the strength of the optical lattice that traps the pairs, which makes the atoms spread across the lattice while pairing and entanglement between them is preserved.
This spreading is shown in spreadpairs.jpg, where the pair wavefunction is plotted as a function of time.
The final pairs have a size of a few microns, about 10,000 times larger than the atom diameter.

***

LJ11271

Nonlinearities make a quantum system more insensitive to
environment when thermal fluctuations are increased.

Nanoscopic devices could in the next future keep advantage of the
quantum peculiarities in order to significantly improve the
computational performances. In spite of these potentialities,
the noise degradation in quantum computers, known as quantum
decoherence, has devastating consequences and makes their
application impossible at present. Important sources of noise
are the thermal fluctuations. Since such fluctuations are reduced
by decreasing the temperature, one could conclude that the quantum
decoherence could be minimized by sufficiently cooling the
environment of the quantum device.

In our work, we study two simple models of nonlinear environment and
show that the increase of temperature can reduce the decoherence
rate. In fact, although the fluctuations grow with the temperature,
the increase of thermal energy changes the frequency spectrum shape of
these fluctuations via the nonlinearities and makes the environment
off-resonant with the device. Thus, although the fluctuation amplitude
grows with temperature, the device becomes more insensitive to
them. The nonlinearities at nanoscopic scale could be relevant for the
development of quantum computers.


***

LE11622

Deciphering the relation between geometry and time-delay
for harmonic light emission, molecular alignment and orbital symmetry.

In this paper a quantum theory is developed that sucessfully provides a
unified interpretation of the hitherto unexplained intensity modulations of
harmonic radiations from coherently rotating linear molecules and their
instantaneous alignment. This has become possible by correlating for
the first time the simultaneous effects of the two control parameters that are
used in the experiments, namely, the relative polarization angle and the
time-delay between a "pump" and a "probe" pulse. The first of these two
femtosecond pulses sets an initially thermal ensemble of linear molecules
in quantum coherent rotations, while the second pulse induces the harmonic
light emission from the rotating molecules. A control of the alignment of
the molecular axes is of direct interest, for example, in sterioscopic
chemical reactions. Our theory also correlates the molecular orbital
symmetry with the presence (or absence) of a "magic" polarization angle. This
signature can now be used directly to identify the orbital symmetry of unknown
linear molecules, both inorganic and organic. Such knowledge coupled with the
explicitly anlytic form of the theory, provides a qualitative advancement
toward a full solution of the currently hotly pursued inverse problem of
"molecular imaging" i.e. construction of the molecular orbitals of an unknown
probe molecule from the experimental data.

***

LG11498
Ultra-shallow molecular potential

Molecular vibrational states supported by a potential curve
with a sub-GHz well depth have been experimentally observed.
This ultra-shallow potential is formed due to
avoided crossings of potentials among a hyperfine structure.
This state is a novel kind of so-called purely long-range states,
which have the inner classical
turning points at long internuclear distances of a few nanometer.
So far, purely long-range states have been experimentally observed for
alkali-metal atoms and helium. These states are formed due to
fine structures, and the depths of the potential wells are much deeper.
In this Letter, ytterbium (Yb) atoms are used to observe this
hyperfine-structure-induced purely long-range state.
Yb is the only atom for which a spinless
Bose-Einstein condensation has been realized, and
attracts considerable attention as a future optical frequency standard.
High-resolution photoassociation spectroscopy of the intercombination
line ($^1S_0 - ^3P_1$) of laser-cooled $^{171}$Yb atoms has been demonstrated,
and four rovibrational levels of $^{171}$Yb$_2$ are successfully
assigned to this state.


***

LH11691

- Testing gravity with ultracold atoms -

Experiments investigating gravity using quantum probes are rare. In two
experiments conducted by the group led by Guglielmo Tino in Firenze
(Italy), atomic probes based on quantum interferometry with laser-cooled
atoms are used to determine the value of the Newtonian gravitational
constant and to measure gravity with micrometer resolution. Recent results
were just published in two articles in Physical Review Letters [1,2].

The Newtonian constant of gravity G is one of the most measured
fundamental physical constants but still the least precisely known. The
weakness of the gravitational force and the impossibility of shielding it
make a precision measurement of G extremely difficult. Improving the
knowledge of G has a metrological interest and is very important for the
key role that it plays in several theories in physics. From the time of
Cavendish experiment in 1798, the preferred method has been the torsion
balance, in which the restoring force of a twisted fibre balances the weak
gravitational torque produced by the attraction between macroscopic test
masses. In the new experiment [1] in Firenze (MAGIA, funded by INFN),
microscopic atomic probes and an atom interferometry detection scheme are
used to measure this elusive fundamental constant. Freely falling samples
of laser-cooled rubidium atoms are used in an atomic gravity gradiometer
to probe the field generated by nearby tungsten source masses. The
achieved precision is of the order of a part per thousand and
uncertainties below the 100 ppm level, which is the present limit, are
within reach.

The second experiment [2] is addressing the possibility of measuring
gravity with high precision at extremely small distances, that can also be
interpreted as a test of the constancy of the value of G over different
spatial scales. Using laser-cooled strontium atoms in a modulated vertical
optical lattice, Wannier-Stark intraband transitions producing a coherent
delocalization of atomic wavepackets were detected for the first time
with high resolution. This is an important result on its own and for the
implications to experiments in different fields. Gravity was indeed
determined with ppm precision and micrometer resolution. This enables new
tests on small-scale forces and gravity can be investigated in regions so
far unexplored searching for deviations from Newtonian gravity predicted
by theories beyond the standard model.