AN10175
MICROCAVITIES PROBE FUNDAMENTAL OPTICAL PROCESS IN NANOMETER SIZED
SILICON
Silicon, the flesh and blood of the microprocessor, suffers from a
major drawback when it comes to photonic functionality: it cannot emit
light. Efficient light emission from silicon will enable functionality
hitherto unobtainable in semiconductor chips and will forever change
the face of electronic industry as we know it. In 1990, it was
observed in several laboratories across the world, that silicon does
emit light when it is diced up into nanometer sized chunks - popularly
known as nanocrystals. In spite of these advances, the dream of a
silicon laser has remained elusive for nearly two decades. Is it even
possible in theory? This is the question we seek to answer in our paper.
In doing so, we have demonstrated the use of optical microcavities to
probe fundamental optical losses in silicon nanocrystal doped devices.
Microcavities are not only an essential component of any laser but can
also serve as excellent diagnostic tools for optical processes. This
analysis has enabled us to fabricate state-of-the-art resonators
having quality factors exceeding 1,400 -- nearly 4-5 times larger than
reported previously. More importantly, we have been able to estimate
the limiting performance silicon nanocrystal-doped cavities. Our study
indicates that, even at low temperatures, it will be extremely
difficult to overcome the optical losses in a continuous-wave
operation. Owing to the prevalence of fast optical loss processes, our
only hope may be to resort to pulsed operation.
Over the past few years, microcavities have been at the center of
attention in the photonics community. Our work has highlighted their
use as extremely sensitive diagnostic tools for exploring fundamental
optical processes at the materials and device levels.
***
LL11012ER
Hot Transformers: Chemical Oscillations Found in Collapsing Bubbles
The mysterious slow modulations of an air bubble in water
oscillating nonlinearly by high frequency ultrasound driving
are shown to have their reason in chemical oscillations.
Collapsing bubbles are known for a phenomenon called sonoluminescence:
A very short light flash is emitted at the time when its volume is
compressed a million times from its biggest size.
And this process was thought to be very stable. Until the team of
Thomas and Holt of Boston University measured
large periodical variations.
Nobody found a definite explanation by that time.
In this paper it has been shown for the first time
that a bubble may transform slowly
from a big and relatively cold nitrogen bubble
to a smaller but hotter argon bubble.
But this process may get unstable - the bubble gets too small
and colder and transforms back into a nitrogen containing sphere.
One single chemical oscillation usually takes 2 - 5 seconds to complete
until it repeats all over. And these oscillations are stable,
they only change when an external parameter is varied.
The chemistry happening during the brief moments of collapse
is normally of interest only for rocket scientists:
At 10,000 deg Kelvin under pressures
exceeding 10,000 bars the gases in the bubble are compressed nearly to
liquid density. The calculations in this paper help to explore
reactions in the same extreme environmental conditions,
only for a much smaller approx. 10 micrometer in diameter sized bubble.
***
ER10316
Hierarchical Structure in Branching Patterns
Branching objects appearing in nature have hierarchical structures.
Regarding a river network, for example, merging of two streams forms a broader stream and all streams finally flow into the main stream.
To measure the hierarchical structures quantitatively, a method of branch-numbering, named Horton-Strahler ordering, was introduced.
A stream originating from a source has order 1, joining of two streams of order 1 arises a stream of order 2, and so on.
And a simple scaling law (called Horton's law of stream numbers) has been confirmed empirically and analytically.
In this paper, we consider the simplest case in which every type of branching patterns is appeared randomly.
We introduce "hierarchical hanging method" and calculate the average of the number of streams of the same order.
This is expressed in the form of a recursive equation about the order of streams.
It is noted that for the streams of order 2, the exact solution is derived.
By using this recursive equation, Horton's law is proved numerically and analytically.
Furthermore, we also derive a relation for the k-th moment, which is regarded as the extended form of Horton's law.
***
BS10524
Exciton magnetic properties very strongly dependent on the kinetic energy
Excitons are quanta of electronic excitation traveling through condensed
matter and are central to a huge range of optical and energy transfer
phenomena in the physical and life sciences. In semiconductors, excitons
are of the Wannier type, in which an electron is bound to a hole by the
Coulomb interaction, the resulting energy states being analogous to those
of a hydrogen atom. Motion of excitons in bulk material can therefore be
thought of in terms of hydrogen-like particles or, alternatively, in terms
of waves. However, little has been known about how excitons behave when
they are actually moving, largely because there are very few experiments
which enable them to be ‘caught’ whilst under motion. In the present work,
we investigated how the magnetic properties of excitons depend on kinetic
energy in quantum wells which are very wide, but in which the exciton
energies (and hence those of the optical transitions) are nevertheless
quantized. By studying each transition individually, we find the magnetic
properties to be very strongly dependent on the kinetic energy. This
unusual behavior is ascribed to mixing between the 1S ground state and
higher-lying states. The mixing is induced by the exciton’s motion and
appears to be a universal feature for semiconductors.
***
AM10316
Interference of Subwavelength Light Beams Enhances the Light Energy
The wave nature of light causes the light waves passing through slits in
screen to interfere in the observation plane, creating an interference pattern
of bright and dark bands on the screen. Normally, the interference (addition)
of two or more waves causes enhancement or suppression of the light intensity,
but not the light energy. We show that the waves (beams) generated by
multiple, subwavelength-wide slits can have similar phases and can enhance the
light energy. If the spacing of the slits is smaller than the optical
wavelength, then the phases of the beams at the detector are nearly the same
and beams add coherently (the light energy increases as the number of
light-sources squared, regardless of periodicity). If the spacing is larger,
then the addition is not so efficient, but still leads to enhancements and
resonances versus wavelength in the total energy transmitted. The anomalies in
transmission spectra of gratings, a long standing problem in optics, follow
naturally from the interference properties of the model. The mechanism could
be interpreted as a non-quantum analog of the superradiance emission of a
subwavelength ensemble of atoms (the light energy scales as the number of
light-sources squared) predicted by the Dicke quantum model.
Tuesday, July 29, 2008
Friday, July 25, 2008
7-25-08
LQ11930
Cold atoms are lasing.
A new laser has been developed at the Institut Non
Linéaire de Nice (France). This laser uses a cloud of laser-cooled rubidium
atoms (~100 µK) as the gain medium, placed inside a cavity. As there is no
stationary population inversion in such a medium, gain is obtained with
trickier mechanisms, corresponding to multi-photons transitions. The team
led by Robin Kaiser has demonstrated lasing action with three different
mechanisms, namely Mollow gain, Raman gain, and four-wave-mixing. Up to 300
µW laser power have been produced. The gain mechanism that produces the
laser depends on the pumping parameters (detuning, intensity,
polarization…). The laser can thus be tuned continuously from one regime to
another. This laser could find applications in quantum optics or in the
study of laser dynamics, but the primary goal of the Nice team is to
combine one of these gain mechanisms with multiple scattering in the atom
cloud. This should lead to the first realization of a random laser (when
the feedback is due to multiple scattering in the gain medium itself) with
cold atoms. The present result, published in Phys. Rev. Lett. (W. Guerin,
F. Michaud and R. Kaiser, …) is the first step towards this goal. The next
step is thus to preserve the laser... without the cavity !
***
BSR1107
Spin-polarized current plays an important role in
nano spin-electronics because it can change the direction of
magnetizations or move magnetic walls in magnetic nano structures.
However, the ratio between the coefficients of adiabatic and
non-adiabatic components of the torque exerted by the spin-polarized
current on magnetizations has been a controversial issue for many years.
In this paper, the authors derived the equation for the magnetization
dynamics in the presence of a spin current based on the non-equilibrium
thermodynamics. They showed that the coefficient of non-adiabatic
torque is not equal to the damping coefficient called the Gilbert
damping constant, in general. The equality holds when the relaxation
time of the fluctuating magnetic field is very short compared to the
time scale of magnetization dynamics. They applied the theory to
current-induced magnetization reversal in magnetic multi-layers and
showed that the switching time is a decreasing function of the
relaxation time.
***
EQ10326
Tracing hidden signal inside plasma
First time, we have shown that the plasma medium can be useful to detect
untraceable subthreshold signal applied externally in the plasma medium,
by applying external noise, whereas, previously only autonomous dynamics
of the plasma was detectable using noise. Usually noises are disturbance
for the modern communication devices. In this experiment it is shown that
for suitable parametric regions and configurations of the system, untraceable
signal can be traced using noise and inherent nonlinearity of the plasma.
This technique may be used in plasma antenna to detect hidden signal.
We have also devised very simple statistical tool (AMD) to quantify stochastic
resonance that is free from computational difficulties of usual statistical
tools like Signal to noise ration (SNR) or cross-correlation techniques.
Vedio 1: Oscilloscopic view of stochastic resonance in plasma. Lower trace
is the applied subthreshold signal plus noise and upper trace is the output
signal. The video shows that initially for low amplitude of noise applied
signal was not traceable (noise has been increased continuously) and with
increase in the noise system detects the hidden signal. Maximum transmission
is at optimum noise level and for high level noise, transmission becomes noisy.
Cold atoms are lasing.
A new laser has been developed at the Institut Non
Linéaire de Nice (France). This laser uses a cloud of laser-cooled rubidium
atoms (~100 µK) as the gain medium, placed inside a cavity. As there is no
stationary population inversion in such a medium, gain is obtained with
trickier mechanisms, corresponding to multi-photons transitions. The team
led by Robin Kaiser has demonstrated lasing action with three different
mechanisms, namely Mollow gain, Raman gain, and four-wave-mixing. Up to 300
µW laser power have been produced. The gain mechanism that produces the
laser depends on the pumping parameters (detuning, intensity,
polarization…). The laser can thus be tuned continuously from one regime to
another. This laser could find applications in quantum optics or in the
study of laser dynamics, but the primary goal of the Nice team is to
combine one of these gain mechanisms with multiple scattering in the atom
cloud. This should lead to the first realization of a random laser (when
the feedback is due to multiple scattering in the gain medium itself) with
cold atoms. The present result, published in Phys. Rev. Lett. (W. Guerin,
F. Michaud and R. Kaiser, …) is the first step towards this goal. The next
step is thus to preserve the laser... without the cavity !
***
BSR1107
Spin-polarized current plays an important role in
nano spin-electronics because it can change the direction of
magnetizations or move magnetic walls in magnetic nano structures.
However, the ratio between the coefficients of adiabatic and
non-adiabatic components of the torque exerted by the spin-polarized
current on magnetizations has been a controversial issue for many years.
In this paper, the authors derived the equation for the magnetization
dynamics in the presence of a spin current based on the non-equilibrium
thermodynamics. They showed that the coefficient of non-adiabatic
torque is not equal to the damping coefficient called the Gilbert
damping constant, in general. The equality holds when the relaxation
time of the fluctuating magnetic field is very short compared to the
time scale of magnetization dynamics. They applied the theory to
current-induced magnetization reversal in magnetic multi-layers and
showed that the switching time is a decreasing function of the
relaxation time.
***
EQ10326
Tracing hidden signal inside plasma
First time, we have shown that the plasma medium can be useful to detect
untraceable subthreshold signal applied externally in the plasma medium,
by applying external noise, whereas, previously only autonomous dynamics
of the plasma was detectable using noise. Usually noises are disturbance
for the modern communication devices. In this experiment it is shown that
for suitable parametric regions and configurations of the system, untraceable
signal can be traced using noise and inherent nonlinearity of the plasma.
This technique may be used in plasma antenna to detect hidden signal.
We have also devised very simple statistical tool (AMD) to quantify stochastic
resonance that is free from computational difficulties of usual statistical
tools like Signal to noise ration (SNR) or cross-correlation techniques.
Vedio 1: Oscilloscopic view of stochastic resonance in plasma. Lower trace
is the applied subthreshold signal plus noise and upper trace is the output
signal. The video shows that initially for low amplitude of noise applied
signal was not traceable (noise has been increased continuously) and with
increase in the noise system detects the hidden signal. Maximum transmission
is at optimum noise level and for high level noise, transmission becomes noisy.
Thursday, July 24, 2008
APS Physics Tip Sheet: Superfluid insight into rogue waves; and lumpy life
---------------------------
Origin of Monster Ocean Waves
A.N. Ganshin, V.B. Efimov, G.V. Kolmakov, L.P. Mezhov-Deglin, and P.V.E. McClintock
Physical Review Letters (LS11531)
Physicists at the University of Lancaster, UK and Institute of Solid State Physics, Russia have for the first time discovered that wave energy can rapidly concentrate to create monster waves under some conditions, in contrast to previous work suggesting that it requires hundreds of miles of open sea for the waves to appear and disappear.
By using superfluid helium to model complex wave processes, researchers observed that monster waves in the laboratory developed by what is called an inverse wave energy cascade, whereby energy is forced to flow in a way that drives the rapid formation of abnormally large waves. Their results confirm theoretical predictions by the Russian mathematician Vladimir Zakharov of the University of Arizona and Landau Institute Russia.
For centuries monster waves (also known as rogue waves) have been blamed for occasional mysterious disappearances of ships and sailors. These giant, violent waves are believed to reach 100 feet or more in height, and are often described as a towering "wall of water." What makes them so dangerous is their tendency to emerge unexpectedly from relatively tranquil seas. It is the unpredictable nature of rogue waves that has prompted many to investigate their origins.
There is much interest in the origin of rogue waves, as they have huge adverse affects on maritime commerce and industry. If this new understanding can be exploited to explain how rogue waves arise on the ocean, it may be possible to predict them. -NR
----------------------------------
Lumpy Life
B. Houchmandzadeh
Physical Review Letters (LQ11828)
Even in a perfect world life would be imperfectly distributed, according to a new experiment performed at the Centre National de la Recherche Scientifique (CNRS) and Grenoble University in France.
Plants and animals tend to live near their relatives, rather than spreading out uniformly across the planet. Some of the clustering is due to variations in the environment, because species are generally better adapted for certain types of conditions than for others. But physicist Bahram Houchmandzadeh discovered that even in a nearly perfect experimental environment, such as a petri dish with nutrients and living conditions that are uniform from place to place, amoeba clump together.
Houchmandzadeh attributes some of the clumping in his amoeba colony to the fact that newborn creatures start life close to their siblings. Although the microbes move about randomly, which smoothes out some of the lumpy distribution, it's not enough to erase the uneven distributions entirely. The experiment followed robust amoeba colonies as they grew, and did not address the effects of death or population fluctuations that occur in the real world. Houchmandzadeh, however, argues that such things would amplify the tendency of populations to cluster in groups.
The natural inclination toward clustering of plants and animals only partly explains population distributions in nature, but the new experiment shows that it is likely to be much more important than many researchers had previously suspected. -JR
----------------------------------
50 Years of PRL
Martin Blume
Physical Review Letters turns 50 this year. Martin Blume is celebrating the green journal's birthday by summarizing the most intriguing papers to appear in PRL each year since 1958. To see past editions of Marty's Milestone PRL project, visit http://prl.aps.org/50years/milestones
This week, Marty is taking a look at milestone papers from 1981 and 1982 showing that no hidden variables can be responsible for weird quantum effects.
Experimental Tests of Realistic Local Theories via Bell's Theorem
Alain Aspect, Philippe Grangier, and Gérard Roger
Phys. Rev. Lett. 47, 460 (1981)
Experimental Realization of Einstein-Podolsky-Rosen-Bohm Gedankenexperiment: A New Violation of Bell's Inequalities
Alain Aspect, Philippe Grangier, and Gérard Roger
Phys. Rev. Lett. 49, 91 (1982)
Experimental Test of Bell's Inequalities Using Time-Varying Analyzers
Alain Aspect, Jean Dalibard, and Gérard Roger
Phys. Rev. Lett. 49, 1804 (1982)
In these Letters, Aspect and collaborators experimentally tested the spacetime behavior of an entangled system. (While two of the Letters were published in 1982, they are included here as part of the 1981 Milestone selection.) According to quantum mechanics, strong correlations are to be expected between measurements performed on systems that have interacted, even though they are separated at the time of measurement. Consideration of these predictions had led A. Einstein, B. Podolsky, and N. Rosen [Phys. Rev. 47, 777 (1935); see also Physical Review Focus 16, story 10] to argue that quantum mechanics cannot be a complete description of reality. Nearly 30 years later, John S. Bell proved that certain inequalities must hold among polarization measurements performed on two separated particles which had previously interacted if, as Einstein and collaborators felt, quantum mechanics is underlain by local "hidden variables"; these inequalities can be violated in a purely quantum-mechanical system.
Aspect and collaborators tested Bell's inequalities as generalized by J.F. Clauser, M.A. Horne, A.Shimony, and R.A. Holt [Phys. Rev. Lett. 23, 880 (1969)], in a series of experiments which approached ever closer to the ideal experiment first envisioned by David Bohm, based on the concerns of Einstein, Podolsky, and Rosen. These experiments used pairs of correlated photons produced by laser excitations of an atomic radiative cascade. In the first Letter, the use of single-channel analyzers did not allow the direct measurement of all polarization states; two-channel analyzers were used in the second Letter to overcome this limitation. In the third Letter, variable polarizers were used to eliminate the possibility that unknown interactions among the measuring instruments could evade the inequalities. In these Letters, and in subsequent work by a number of groups, the results showed violations of the generalized Bell inequalities, in accordance with quantum mechanics and in disagreement with local hidden-variable theories. Thus, more than 45 years after the paper of Einstein, Podolsky, and Rosen, the gedankenexperiment suggested by their work was performed, with results that would have disappointed them. Further research on entanglement has led to the new field of quantum information.
---------------------------------------------------------
Nadia Ramlagan and James Riordon contributed to this Tip Sheet
Journal articles and preprints are available to journalists on request.
Contact: James Riordon
American Physical Society
riordon@aps.org
301-209-3238
Origin of Monster Ocean Waves
A.N. Ganshin, V.B. Efimov, G.V. Kolmakov, L.P. Mezhov-Deglin, and P.V.E. McClintock
Physical Review Letters (LS11531)
Physicists at the University of Lancaster, UK and Institute of Solid State Physics, Russia have for the first time discovered that wave energy can rapidly concentrate to create monster waves under some conditions, in contrast to previous work suggesting that it requires hundreds of miles of open sea for the waves to appear and disappear.
By using superfluid helium to model complex wave processes, researchers observed that monster waves in the laboratory developed by what is called an inverse wave energy cascade, whereby energy is forced to flow in a way that drives the rapid formation of abnormally large waves. Their results confirm theoretical predictions by the Russian mathematician Vladimir Zakharov of the University of Arizona and Landau Institute Russia.
For centuries monster waves (also known as rogue waves) have been blamed for occasional mysterious disappearances of ships and sailors. These giant, violent waves are believed to reach 100 feet or more in height, and are often described as a towering "wall of water." What makes them so dangerous is their tendency to emerge unexpectedly from relatively tranquil seas. It is the unpredictable nature of rogue waves that has prompted many to investigate their origins.
There is much interest in the origin of rogue waves, as they have huge adverse affects on maritime commerce and industry. If this new understanding can be exploited to explain how rogue waves arise on the ocean, it may be possible to predict them. -NR
----------------------------------
Lumpy Life
B. Houchmandzadeh
Physical Review Letters (LQ11828)
Even in a perfect world life would be imperfectly distributed, according to a new experiment performed at the Centre National de la Recherche Scientifique (CNRS) and Grenoble University in France.
Plants and animals tend to live near their relatives, rather than spreading out uniformly across the planet. Some of the clustering is due to variations in the environment, because species are generally better adapted for certain types of conditions than for others. But physicist Bahram Houchmandzadeh discovered that even in a nearly perfect experimental environment, such as a petri dish with nutrients and living conditions that are uniform from place to place, amoeba clump together.
Houchmandzadeh attributes some of the clumping in his amoeba colony to the fact that newborn creatures start life close to their siblings. Although the microbes move about randomly, which smoothes out some of the lumpy distribution, it's not enough to erase the uneven distributions entirely. The experiment followed robust amoeba colonies as they grew, and did not address the effects of death or population fluctuations that occur in the real world. Houchmandzadeh, however, argues that such things would amplify the tendency of populations to cluster in groups.
The natural inclination toward clustering of plants and animals only partly explains population distributions in nature, but the new experiment shows that it is likely to be much more important than many researchers had previously suspected. -JR
----------------------------------
50 Years of PRL
Martin Blume
Physical Review Letters turns 50 this year. Martin Blume is celebrating the green journal's birthday by summarizing the most intriguing papers to appear in PRL each year since 1958. To see past editions of Marty's Milestone PRL project, visit http://prl.aps.org/50years/milestones
This week, Marty is taking a look at milestone papers from 1981 and 1982 showing that no hidden variables can be responsible for weird quantum effects.
Experimental Tests of Realistic Local Theories via Bell's Theorem
Alain Aspect, Philippe Grangier, and Gérard Roger
Phys. Rev. Lett. 47, 460 (1981)
Experimental Realization of Einstein-Podolsky-Rosen-Bohm Gedankenexperiment: A New Violation of Bell's Inequalities
Alain Aspect, Philippe Grangier, and Gérard Roger
Phys. Rev. Lett. 49, 91 (1982)
Experimental Test of Bell's Inequalities Using Time-Varying Analyzers
Alain Aspect, Jean Dalibard, and Gérard Roger
Phys. Rev. Lett. 49, 1804 (1982)
In these Letters, Aspect and collaborators experimentally tested the spacetime behavior of an entangled system. (While two of the Letters were published in 1982, they are included here as part of the 1981 Milestone selection.) According to quantum mechanics, strong correlations are to be expected between measurements performed on systems that have interacted, even though they are separated at the time of measurement. Consideration of these predictions had led A. Einstein, B. Podolsky, and N. Rosen [Phys. Rev. 47, 777 (1935); see also Physical Review Focus 16, story 10] to argue that quantum mechanics cannot be a complete description of reality. Nearly 30 years later, John S. Bell proved that certain inequalities must hold among polarization measurements performed on two separated particles which had previously interacted if, as Einstein and collaborators felt, quantum mechanics is underlain by local "hidden variables"; these inequalities can be violated in a purely quantum-mechanical system.
Aspect and collaborators tested Bell's inequalities as generalized by J.F. Clauser, M.A. Horne, A.Shimony, and R.A. Holt [Phys. Rev. Lett. 23, 880 (1969)], in a series of experiments which approached ever closer to the ideal experiment first envisioned by David Bohm, based on the concerns of Einstein, Podolsky, and Rosen. These experiments used pairs of correlated photons produced by laser excitations of an atomic radiative cascade. In the first Letter, the use of single-channel analyzers did not allow the direct measurement of all polarization states; two-channel analyzers were used in the second Letter to overcome this limitation. In the third Letter, variable polarizers were used to eliminate the possibility that unknown interactions among the measuring instruments could evade the inequalities. In these Letters, and in subsequent work by a number of groups, the results showed violations of the generalized Bell inequalities, in accordance with quantum mechanics and in disagreement with local hidden-variable theories. Thus, more than 45 years after the paper of Einstein, Podolsky, and Rosen, the gedankenexperiment suggested by their work was performed, with results that would have disappointed them. Further research on entanglement has led to the new field of quantum information.
---------------------------------------------------------
Nadia Ramlagan and James Riordon contributed to this Tip Sheet
Journal articles and preprints are available to journalists on request.
Contact: James Riordon
American Physical Society
riordon@aps.org
301-209-3238
7-24-08
LH11797
Inside Aerogels in Nanoscale 3-D
Until now, no one has ever seen inside an aerogel to determine its
structure on the nanoscale. By performing high-resolution diffraction
imaging of an aerogel with laser-like soft xrays, a research team has
revealed its three-dimensional bulk lattice structure down to features
measured in mere billionths of a meter.
The x-ray beam passed through the aerogel sample and was diffracted onto
a CCD detector screen; some 150 diffraction patterns were stored as the
sample moved and rotated. Fast computers and improved algorithms allowed
millions of calculations to position each measured photon precisely in
three-dimensional space, a key advance that made the 3-D nanoscale
imaging possible.
¿Seeing inside bulk porous materials has never been done before at this
resolution,¿ says Stefano Marchesini, who led the research at beamline
9.0.1 of the Department of Energy¿s Advanced Light Source at Lawrence
Berkeley National Laboratory. ¿This one of the first applications of
x-ray diffractive microscopy to a real problem.¿
The strength of foam-like structures typically scales with their
density, but aerogels, less than two-percent dense, are orders of
magnitude weaker than expected. The structure revealed in the new 3-D
images reveals why, and suggests changes in aerogel preparation that
might improve their strength.
***
LU11168
The lowest energy state of the "bottomonium"
Collaborators working on the BaBar experiment at the Stanford Linear
Accelerator Center
have discovered the lowest energy state of the "bottomonium" family
whose members are
bound states of a bottom quark with an anti-bottom quark held
together in different angular
momentum configurations. This spin-0 particle, named eta_b, was
observed for the first time
some thirty years after the discovery of the spin-1 tower of states
of the bottomonium spectrum
called Upsilon(nS). The precise determination of the hyperfine mass
splitting between the eta_b
and the Upsilon(1S) plays a key role in understanding the effect of
spin-spin interactions in these
systems as well as fundamental properties of the strong force. In the
BaBar detector, the
highly-energetic collision of an electron and a positron can result
in the production of an
Upsilon (3S) particle, which in turn can decay to an eta_b by
emitting a gamma ray. This eta_b
production mechanism occurs just once for every two to three thousand
Upsilon(3S) decays.
To identify a significant sample of eta_b's in the presence of
background noise it was necessary
to record more than 100 million of such Upsilon(3S) events. This
achievement was made possible
by the excellent luminosity of the PEP-II accelerator and the
advanced technology of the BaBar detector.
***
BPJ1079BR
Size-specific cracking shakes out at the nanoscale
As the size of a structure is reduced towards the nanoscale, atomic
vibrations (phonons) begin to feel its size and shape, an effect
called phonon confinement. While these size effects on phonons are
known to play an important role in thermal transport, electronic
processes, and thermodynamic stability, little is known about their
role in fracture. In this article, we report that for plates of
cerium hydride there is a characteristic thickness at which the
entropy of these confined vibrations leads to a minimum in the
fracture energy, resulting size specific fracture. While this idea
was developed to explain the behavior of cerium hydride, it is much
more general and has important ramifications for the design of
nanostructures. For example, it implies that certain sizes may be
more susceptible to failure by fracture than others. It may also
prove useful in the deliberate creation of large quantities of stable
nanostructures by fracture.
***
LN11150
Tuning in on ultracold molecules
Ultra-cold molecular gases (at a temperature typically below one
micro-Kelvin) are the coolest molecular system in the universe.
Nowadays, the most successful route to form them is to associate
previously cooled atoms by using either laser or magnetic
fields. Unfortunately, only highly excited molecules can be
efficiently populated leading to unstable and short-lived samples.
In our article, we proposed a novel method to efficiently produce
ultra-cold stable molecules in their lowest vibrational levels.
This work solves many of the challenges to produce such molecules by
using giant formation rates obtained with a tunable magnetic field
and a laser field. Consequently, it opens the door to a broad variety of
applications ranging from quantum computing to high
resolution spectroscopy at a frontier between physics and chemistry.
***
LN11381BR
Oscillatory Hall effect in high-mobility two dimensional electron
gases
Summary: The formation of magnetically ordered states in initially
non-magnetic materials makes research on spontaneous spin polarization
exciting. An excellent tool to investigate such spin phenomena is the
measurement of the zero-field Hall coefficient: In the presence of
localized spins, electron scattering is spin-selective and leads to an
anomalous contribution to the Hall voltage. This manuscript reports an
unexpected anomaly in the zero-field Hall coefficient of two dimensional
electron systems (2DES). At very low temperature, both positive and
negative deviations from the non-interacting Hall coefficient
$\gamma_{\rm H}^0$ are observed, which can be twice as large as
$\gamma_{\rm H}^0$ itself. A distinct regularity in the deviations and
their temperature dependence are interpreted as the spontaneous
formation of localized spins and their mutual indirect interaction,
indicating predominantly anti-ferromagnetic spontaneous magnetic
interactions in 2DES at low temperature.
***
LN11510
Nanoearthquakes and MicroTornados Form and Wipe Out Colloidal Islands
Just like earthquakes potentially lead to the generation of tsunami waves, tiny vibrations on a plate on which a small water drop sits can create large waves at the drop surface---with peculiar consequences. If the drop were to contain nanoparticles, Australian scientists Drs Haiyan Li, James Friend and Leslie Yeo at Monash University's Micro/Nanophysics Research Laboratory have discovered a peculiar phenomenon in which these waves could result in the formation, evolution and destruction of island-like clusters of these particles on the water surface.
The tiny vibrations are surface acoustic waves or SAWs, which are megahertz frequency sound waves about ten nanometers in amplitude. A nanometer is one billionth of a meter---the width of a human hair is therefore about one hundred thousand nanometers. SAWs have, for decades, been employed in the telecommunications industry for signal processing. A typical mobile phone would therefore have several SAW devices.
Though they may be small, SAWs have large surface accelerations---typically, millions of g's. This is the reason why they can create large waves on the surface of a water drop measuring several millimeters in diameter sitting above them. These waves, in turn, bounce the nanoparticles around on the drop surface to form island clusters in a manner similar to the patterns formed when sand is sprinkled onto the surface of a vibrating metal plate.
As the islands form at nodal positions of the standing wave, i.e., points where the waves cancel each other due to destructive interference, which depend on the size of the drop, the number of islands decrease successively as the drop evaporates and hence reduces in size. In their work, the researchers show how the number of islands and their positions on the drop can be predicted.
Curiously though, the islands are wiped out intermittently when the SAWs trigger an instability which causes the liquid in the drop to recirculate rapidly like miniature tornados. This anomalous behavior, attributed to chaotic flow, only lasts a few seconds. When the recirculation stops, the islands reform and the cycle is repeated.
This research, which is to be published in Physical Review Letters, has important implications in microfluidics, which is the science of how fluids and particles can be manipulated at tiny scales. Already, the researchers are using SAWs to make tiny pumps, reactors, centrifuges and separators, as part of a larger effort to develop portable medical diagnostic kits and drug delivery systems.
***
LL11129
Subtleties in the Quantum Coupling of Light and Matter in a Semiconductor
Cavity Quantum Electrodynamics harnesses the quantum theory of
light an matter in the confine of a cavity. For decades, optical
macrocavities have been used to trap light, into which Rydberg atoms
were sent as probes of "the best of our theories". The quantum (or
"strong") coupling (SC) of a photon with an atom results in the notion
of light and matter to vanish away. New quantum states substitute
them, combining their properties. With atoms, the signature of SC is
the observation of anticrossing of the photon and atom energies: when
they are brought together, the emergence of new quantum objects give
rise to two new energies. This is seen as two peaks in, e.g., an
emission spectrum. This strong qualitative effect has been regarded as
the landmark of SC, but is not. We found that in a semiconductor, the
situation is more subtle: the physics remains the same, but can reveal
differently. Anticrossing can be overlooked, or spurious interferences
can mimic it. By considering self-consistently pumping and
decoherence, we obtained excellent agreement with the experimental
data, that so far were content to observe the qualitative effect of
anticrossing. We confirmed quantitatively the reports of SC in
semiconductors, but found that it manifests here with unexpected
subtleties.
Inside Aerogels in Nanoscale 3-D
Until now, no one has ever seen inside an aerogel to determine its
structure on the nanoscale. By performing high-resolution diffraction
imaging of an aerogel with laser-like soft xrays, a research team has
revealed its three-dimensional bulk lattice structure down to features
measured in mere billionths of a meter.
The x-ray beam passed through the aerogel sample and was diffracted onto
a CCD detector screen; some 150 diffraction patterns were stored as the
sample moved and rotated. Fast computers and improved algorithms allowed
millions of calculations to position each measured photon precisely in
three-dimensional space, a key advance that made the 3-D nanoscale
imaging possible.
¿Seeing inside bulk porous materials has never been done before at this
resolution,¿ says Stefano Marchesini, who led the research at beamline
9.0.1 of the Department of Energy¿s Advanced Light Source at Lawrence
Berkeley National Laboratory. ¿This one of the first applications of
x-ray diffractive microscopy to a real problem.¿
The strength of foam-like structures typically scales with their
density, but aerogels, less than two-percent dense, are orders of
magnitude weaker than expected. The structure revealed in the new 3-D
images reveals why, and suggests changes in aerogel preparation that
might improve their strength.
***
LU11168
The lowest energy state of the "bottomonium"
Collaborators working on the BaBar experiment at the Stanford Linear
Accelerator Center
have discovered the lowest energy state of the "bottomonium" family
whose members are
bound states of a bottom quark with an anti-bottom quark held
together in different angular
momentum configurations. This spin-0 particle, named eta_b, was
observed for the first time
some thirty years after the discovery of the spin-1 tower of states
of the bottomonium spectrum
called Upsilon(nS). The precise determination of the hyperfine mass
splitting between the eta_b
and the Upsilon(1S) plays a key role in understanding the effect of
spin-spin interactions in these
systems as well as fundamental properties of the strong force. In the
BaBar detector, the
highly-energetic collision of an electron and a positron can result
in the production of an
Upsilon (3S) particle, which in turn can decay to an eta_b by
emitting a gamma ray. This eta_b
production mechanism occurs just once for every two to three thousand
Upsilon(3S) decays.
To identify a significant sample of eta_b's in the presence of
background noise it was necessary
to record more than 100 million of such Upsilon(3S) events. This
achievement was made possible
by the excellent luminosity of the PEP-II accelerator and the
advanced technology of the BaBar detector.
***
BPJ1079BR
Size-specific cracking shakes out at the nanoscale
As the size of a structure is reduced towards the nanoscale, atomic
vibrations (phonons) begin to feel its size and shape, an effect
called phonon confinement. While these size effects on phonons are
known to play an important role in thermal transport, electronic
processes, and thermodynamic stability, little is known about their
role in fracture. In this article, we report that for plates of
cerium hydride there is a characteristic thickness at which the
entropy of these confined vibrations leads to a minimum in the
fracture energy, resulting size specific fracture. While this idea
was developed to explain the behavior of cerium hydride, it is much
more general and has important ramifications for the design of
nanostructures. For example, it implies that certain sizes may be
more susceptible to failure by fracture than others. It may also
prove useful in the deliberate creation of large quantities of stable
nanostructures by fracture.
***
LN11150
Tuning in on ultracold molecules
Ultra-cold molecular gases (at a temperature typically below one
micro-Kelvin) are the coolest molecular system in the universe.
Nowadays, the most successful route to form them is to associate
previously cooled atoms by using either laser or magnetic
fields. Unfortunately, only highly excited molecules can be
efficiently populated leading to unstable and short-lived samples.
In our article, we proposed a novel method to efficiently produce
ultra-cold stable molecules in their lowest vibrational levels.
This work solves many of the challenges to produce such molecules by
using giant formation rates obtained with a tunable magnetic field
and a laser field. Consequently, it opens the door to a broad variety of
applications ranging from quantum computing to high
resolution spectroscopy at a frontier between physics and chemistry.
***
LN11381BR
Oscillatory Hall effect in high-mobility two dimensional electron
gases
Summary: The formation of magnetically ordered states in initially
non-magnetic materials makes research on spontaneous spin polarization
exciting. An excellent tool to investigate such spin phenomena is the
measurement of the zero-field Hall coefficient: In the presence of
localized spins, electron scattering is spin-selective and leads to an
anomalous contribution to the Hall voltage. This manuscript reports an
unexpected anomaly in the zero-field Hall coefficient of two dimensional
electron systems (2DES). At very low temperature, both positive and
negative deviations from the non-interacting Hall coefficient
$\gamma_{\rm H}^0$ are observed, which can be twice as large as
$\gamma_{\rm H}^0$ itself. A distinct regularity in the deviations and
their temperature dependence are interpreted as the spontaneous
formation of localized spins and their mutual indirect interaction,
indicating predominantly anti-ferromagnetic spontaneous magnetic
interactions in 2DES at low temperature.
***
LN11510
Nanoearthquakes and MicroTornados Form and Wipe Out Colloidal Islands
Just like earthquakes potentially lead to the generation of tsunami waves, tiny vibrations on a plate on which a small water drop sits can create large waves at the drop surface---with peculiar consequences. If the drop were to contain nanoparticles, Australian scientists Drs Haiyan Li, James Friend and Leslie Yeo at Monash University's Micro/Nanophysics Research Laboratory have discovered a peculiar phenomenon in which these waves could result in the formation, evolution and destruction of island-like clusters of these particles on the water surface.
The tiny vibrations are surface acoustic waves or SAWs, which are megahertz frequency sound waves about ten nanometers in amplitude. A nanometer is one billionth of a meter---the width of a human hair is therefore about one hundred thousand nanometers. SAWs have, for decades, been employed in the telecommunications industry for signal processing. A typical mobile phone would therefore have several SAW devices.
Though they may be small, SAWs have large surface accelerations---typically, millions of g's. This is the reason why they can create large waves on the surface of a water drop measuring several millimeters in diameter sitting above them. These waves, in turn, bounce the nanoparticles around on the drop surface to form island clusters in a manner similar to the patterns formed when sand is sprinkled onto the surface of a vibrating metal plate.
As the islands form at nodal positions of the standing wave, i.e., points where the waves cancel each other due to destructive interference, which depend on the size of the drop, the number of islands decrease successively as the drop evaporates and hence reduces in size. In their work, the researchers show how the number of islands and their positions on the drop can be predicted.
Curiously though, the islands are wiped out intermittently when the SAWs trigger an instability which causes the liquid in the drop to recirculate rapidly like miniature tornados. This anomalous behavior, attributed to chaotic flow, only lasts a few seconds. When the recirculation stops, the islands reform and the cycle is repeated.
This research, which is to be published in Physical Review Letters, has important implications in microfluidics, which is the science of how fluids and particles can be manipulated at tiny scales. Already, the researchers are using SAWs to make tiny pumps, reactors, centrifuges and separators, as part of a larger effort to develop portable medical diagnostic kits and drug delivery systems.
***
LL11129
Subtleties in the Quantum Coupling of Light and Matter in a Semiconductor
Cavity Quantum Electrodynamics harnesses the quantum theory of
light an matter in the confine of a cavity. For decades, optical
macrocavities have been used to trap light, into which Rydberg atoms
were sent as probes of "the best of our theories". The quantum (or
"strong") coupling (SC) of a photon with an atom results in the notion
of light and matter to vanish away. New quantum states substitute
them, combining their properties. With atoms, the signature of SC is
the observation of anticrossing of the photon and atom energies: when
they are brought together, the emergence of new quantum objects give
rise to two new energies. This is seen as two peaks in, e.g., an
emission spectrum. This strong qualitative effect has been regarded as
the landmark of SC, but is not. We found that in a semiconductor, the
situation is more subtle: the physics remains the same, but can reveal
differently. Anticrossing can be overlooked, or spurious interferences
can mimic it. By considering self-consistently pumping and
decoherence, we obtained excellent agreement with the experimental
data, that so far were content to observe the qualitative effect of
anticrossing. We confirmed quantitatively the reports of SC in
semiconductors, but found that it manifests here with unexpected
subtleties.
Tuesday, July 22, 2008
7-22-08
LM11397
Why spark branches sometimes reconnect
When sparks and lightning strokes pave their way, they frequently form
characteristically branched trees, but sometimes branches also
reconnect. This was observed between tens of kilometers huge sprite
discharges in the thin air high above thunderclouds, and also in
much smaller discharges in normal room air in the lab. In our
Physical Review Letter, we explain how this can
happen. Generically, one expects and observes that the branches repel
each other. This is because the discharge channels penetrate into the
non-ionized air through a strong local enhancement of the electric
field at their heads which is created through local electric
charges. However, we show that this electrostatic repulsion can be
overcome by a nonlocal ionization reaction that lets the channel heads
merge; the same reaction is also responsible for the fact that
positive sparks in air can extend with approximately the electron
drift velocity (free electrons are the fastest particles in the
discharge), but against the direction of electron drift. Whether the
discharge channels attract or repel each other depends actually on the
gas density as well as on the oxygen-nitrogen ratio. Discharge merging
therefore gives us a new handle to investigate this little studied,
but important nonlocal interaction of discharges in our atmosphere.
Discharge branching and reconnection are intrinsically
three-dimensional phenomena while pictures show only two-dimensional
projections. We now can study the full three dimensional structure of
the discharge trees in experiments with stereo-photography, and in
simulations with the computational technique developed in.
***
LQ11425
A passive quantum error correction strategy is presented that can be used
to design intrinsically-robust Bell inequalities. Most quantum error
correction schemes use active quantum information processing to remove
deleterious errors due to the inevitable coupling to the environment. In
this paper we show that quantum error correction may be achieved in a
completely passive manner, without quantum processing or feed-forward, for
at least one application. Our passive quantum error correction strategy
is almost as efficient as the optimal combination of active quantum error
correction and a Bell inequality test.
***
EP10347
Manipulation of nano/micro magnetic/nonmagnetic inorganic/organic materials
An economical, efficient yet environmentally friendly manipulation method was developed utilising tumbling motions of particles. Nano/micro objects can be precisely positioned anywhere on a 3-D substrate. The manipulation of nano- and micro-scale objects is an important task across the board in science and technology, and various manipulation techniques have already been developed. The dynamic control and precise positioning of nanomaterials such as nanoparticles, nanowires, fullerenes and carbon nanotubes are key technologies in nanoelectronics, and the development of efficient methods of transporting and sorting biological cells is urgently required for the design of biological and biomedical devices such as micro-total analysis systems. In this paper, a new method of manipulating both magnetic and nonmagnetic particles is shown using a rotational magnetic field. The method proposed is simple, low-cost and kind to the environment. Indeed, no extreme conditions are required for the operation. The necessary intensity and frequency of external magnetic fields are relatively low, that is, ~ 10 mT (100 Gauss) and several Hz, respectively. Furthermore, there is no need for either nano/micro fabrications or any other complex devices. A variety of new manipulation methodologies of particles, fullerenes, nanotubes, biomolecules and cells can easily be developed utilising the present principle. These could be used instead of or in combination with the conventional methods.
***
LS11491
Liquid anti-diamond: onset of core chemistry in compressed Lithium
New computer simulations of liquid Lithium predict that at high
pressure and
temperature, the behavior of the lightest metal will be much closer to
that of
molten Carbon than an ordinary liquid metal. These new results
describe a
unique state of matter where an element's chemical and physical
properties are
determined by interactions between both core and valence electrons. At
ambient
pressure, only the outermost, valence electrons are responsible for
chemistry. It
is for this reason that the Periodic Table is made up of families of
atoms –
members have the same number of valence electrons. Under high pressure,
however, this paradigm breaks down, and core electrons begin to
overlap and
interact with one another. In molten Lithium, such interactions result
in the
formation of short-lived tetrahedral Li5 clusters. These clusters can
be found at
temperatures as high as 1000 K – a phenomenon that has been previously
observed only in strongly bonded materials such as molten diamond.
However,
unlike diamond where the valence electrons localize between adjacent
atoms to
form attractive bonds, in Lithium they occupy the voids of a
tetrahedral network
while the core electrons reside between neighboring atoms. The
resulting “anti-
diamond” structure is thus stabilized by predominantly repulsive
interactions.
Why spark branches sometimes reconnect
When sparks and lightning strokes pave their way, they frequently form
characteristically branched trees, but sometimes branches also
reconnect. This was observed between tens of kilometers huge sprite
discharges in the thin air high above thunderclouds, and also in
much smaller discharges in normal room air in the lab. In our
Physical Review Letter, we explain how this can
happen. Generically, one expects and observes that the branches repel
each other. This is because the discharge channels penetrate into the
non-ionized air through a strong local enhancement of the electric
field at their heads which is created through local electric
charges. However, we show that this electrostatic repulsion can be
overcome by a nonlocal ionization reaction that lets the channel heads
merge; the same reaction is also responsible for the fact that
positive sparks in air can extend with approximately the electron
drift velocity (free electrons are the fastest particles in the
discharge), but against the direction of electron drift. Whether the
discharge channels attract or repel each other depends actually on the
gas density as well as on the oxygen-nitrogen ratio. Discharge merging
therefore gives us a new handle to investigate this little studied,
but important nonlocal interaction of discharges in our atmosphere.
Discharge branching and reconnection are intrinsically
three-dimensional phenomena while pictures show only two-dimensional
projections. We now can study the full three dimensional structure of
the discharge trees in experiments with stereo-photography, and in
simulations with the computational technique developed in.
***
LQ11425
A passive quantum error correction strategy is presented that can be used
to design intrinsically-robust Bell inequalities. Most quantum error
correction schemes use active quantum information processing to remove
deleterious errors due to the inevitable coupling to the environment. In
this paper we show that quantum error correction may be achieved in a
completely passive manner, without quantum processing or feed-forward, for
at least one application. Our passive quantum error correction strategy
is almost as efficient as the optimal combination of active quantum error
correction and a Bell inequality test.
***
EP10347
Manipulation of nano/micro magnetic/nonmagnetic inorganic/organic materials
An economical, efficient yet environmentally friendly manipulation method was developed utilising tumbling motions of particles. Nano/micro objects can be precisely positioned anywhere on a 3-D substrate. The manipulation of nano- and micro-scale objects is an important task across the board in science and technology, and various manipulation techniques have already been developed. The dynamic control and precise positioning of nanomaterials such as nanoparticles, nanowires, fullerenes and carbon nanotubes are key technologies in nanoelectronics, and the development of efficient methods of transporting and sorting biological cells is urgently required for the design of biological and biomedical devices such as micro-total analysis systems. In this paper, a new method of manipulating both magnetic and nonmagnetic particles is shown using a rotational magnetic field. The method proposed is simple, low-cost and kind to the environment. Indeed, no extreme conditions are required for the operation. The necessary intensity and frequency of external magnetic fields are relatively low, that is, ~ 10 mT (100 Gauss) and several Hz, respectively. Furthermore, there is no need for either nano/micro fabrications or any other complex devices. A variety of new manipulation methodologies of particles, fullerenes, nanotubes, biomolecules and cells can easily be developed utilising the present principle. These could be used instead of or in combination with the conventional methods.
***
LS11491
Liquid anti-diamond: onset of core chemistry in compressed Lithium
New computer simulations of liquid Lithium predict that at high
pressure and
temperature, the behavior of the lightest metal will be much closer to
that of
molten Carbon than an ordinary liquid metal. These new results
describe a
unique state of matter where an element's chemical and physical
properties are
determined by interactions between both core and valence electrons. At
ambient
pressure, only the outermost, valence electrons are responsible for
chemistry. It
is for this reason that the Periodic Table is made up of families of
atoms –
members have the same number of valence electrons. Under high pressure,
however, this paradigm breaks down, and core electrons begin to
overlap and
interact with one another. In molten Lithium, such interactions result
in the
formation of short-lived tetrahedral Li5 clusters. These clusters can
be found at
temperatures as high as 1000 K – a phenomenon that has been previously
observed only in strongly bonded materials such as molten diamond.
However,
unlike diamond where the valence electrons localize between adjacent
atoms to
form attractive bonds, in Lithium they occupy the voids of a
tetrahedral network
while the core electrons reside between neighboring atoms. The
resulting “anti-
diamond” structure is thus stabilized by predominantly repulsive
interactions.
Monday, July 21, 2008
7-21-08
ER10469
Complex Networks Made Simpler
A novel elegant way to construct connected graphs from a limited set
of types and a set of connection rules is offering new insights into
complex networks. Complex networks, such as the Internet and the WWW,
networks of flight connections, or networks of social contacts have
been the subject of much recent scrutiny, due to their ubiquity in
everyday life and in numerous science disciplines. However, as their
name implies, the structure and topology of complex nets is intricate
and difficult to analyze, hindering their understanding. In this
paper, "sequence" graphs, or networks, are constructed from ordered
sequences of just a few types of nodes and a simple set of connection
rules between the various types. The mechanistic rules allow for
rigorous analysis of sequence nets. Despite their ostensible
simplicity, sequence nets serve as suitable models of everyday life
complex nets, exhibiting many of their hallmark features .
***
LP11098ER
A set of equations which describes "liquid rope coiling" effect ---
When we pour honey over toast, we can see that the thread of honey
folds to make a coil.
This phenomenon is called "liquid rope coiling" and have been studied
several decades.
In this paper, we found a set of differential equations, which is
very simple and successfully
describe the coiling behavior. We theoretically obtained a state
diagram for coiling and uncoiling
for the first time, and predicted there may be a strong hysteresis
effect. Experiments have been
revealed three distinct scaling laws for the frequency of coiling.
Our mathematical model
reproduces all the scaling laws. Thus, we conclude the rich and
complex dynamics of
"liquid rope coiling" comes out only due to the compressional viscous
stress in the liquid thread.
Liquid rope coiling simulation movies
Complex Networks Made Simpler
A novel elegant way to construct connected graphs from a limited set
of types and a set of connection rules is offering new insights into
complex networks. Complex networks, such as the Internet and the WWW,
networks of flight connections, or networks of social contacts have
been the subject of much recent scrutiny, due to their ubiquity in
everyday life and in numerous science disciplines. However, as their
name implies, the structure and topology of complex nets is intricate
and difficult to analyze, hindering their understanding. In this
paper, "sequence" graphs, or networks, are constructed from ordered
sequences of just a few types of nodes and a simple set of connection
rules between the various types. The mechanistic rules allow for
rigorous analysis of sequence nets. Despite their ostensible
simplicity, sequence nets serve as suitable models of everyday life
complex nets, exhibiting many of their hallmark features .
***
LP11098ER
A set of equations which describes "liquid rope coiling" effect ---
When we pour honey over toast, we can see that the thread of honey
folds to make a coil.
This phenomenon is called "liquid rope coiling" and have been studied
several decades.
In this paper, we found a set of differential equations, which is
very simple and successfully
describe the coiling behavior. We theoretically obtained a state
diagram for coiling and uncoiling
for the first time, and predicted there may be a strong hysteresis
effect. Experiments have been
revealed three distinct scaling laws for the frequency of coiling.
Our mathematical model
reproduces all the scaling laws. Thus, we conclude the rich and
complex dynamics of
"liquid rope coiling" comes out only due to the compressional viscous
stress in the liquid thread.
Liquid rope coiling simulation movies
Friday, July 18, 2008
7-18-08
EN10253
How rigid are viruses?
Viruses have traditionally been considered dangerous entities capable of infecting and mutating plant and animal genetic codes. In recent years, however, viruses have been explored for applications ranging from drug delivery and gene therapy to nano-technology. Monodispersity, surface chemistry, temperature and pH stability make viruses versatile tools for photonic and electronic templates. Although structures of many viruses are known, their properties remain largely unexplored. In this paper we employ light scattering to analyze the mechanical rigidity and inter-virion coupling of Wiseana iridovirus (WIV). The measurements indicate unexpectedly high Young's modulus ~7 GPa, a surprising value for a biological objects that we traditionally consider in the class of 'Soft Materials'. This modulus is higher than the modulus of usual hard plastics. We show that the hard DNA core is the reason for so high WIV rigidity. The results also indicate a strong mechanical coupling between individual virion particles. The obtained results and the developed technique for analysis of virus rigidity might be very helpful for various bio- and nano- technologies and for studies of other nanoscale objects.
***
BR10615
Microstructure and oxygen concentration of YBCO
The microstructure and oxygen concentration profoundly affect the
electronic properties of the high-temperature superconductor
YBa2Cu3Odelta-7 (YBCO). Using quantitative high-resolution electron
microscopy, we measured the local atomic structure associated with
twinning in several YBCO samples. We observed inhomogeneities in the
lattice parameter ratios on the scale of the twin domains that are
ubiquitous in YBCO. This result was surprising because YBCO is
generally considered to have well defined lattice parameters. Our
measurements reveal that features commonly considered to be twin
boundaries often do not have a true twinning symmetry and are more
accurately described as phase boundaries. Furthermore, the
lattice-parameter ratios in a single sample tend to oscillate back and
forth between two different values from domain to domain, apparently
due to the local variation of oxygen concentration. As a result, we
expect these samples to have locally variable superconducting
properties. Furthermore, one can easily imagine a material with a
network of superconducting channels dispersed within an insulating
matrix. Such a network could have profound implications for theories of
flux pinning and superconductivity in YBCO.
***
LS11726
Fluctuations of a liquid surface: from the molecular scale to
the scale of capillary waves
The fluctuations in the local density of molecules in a liquid
depend sensitively on whether the density is measured close to the
liquid's surface. Far from the surface the density fluctuations are
mild and molecular in range but close to it they are dominated by
long-wavelength "capillary" waves along the surface, with the
interfacial tension and gravity acting as the dominant restoring
forces.
In this paper we show that to describe the full spectrum of
density fluctuations from the molecular scale up to the scale
of capillary waves, one must take account of the energy associated
with *bending* the surface as described by the interface's
bending rigidity. Two of the important results are (1) that the
bending rigidity for the interface between phase-separated
colloid-polymer mixtures is *negative*, with the result that
capillary waves are "more violent", less restricted by interfacial
tension, at smaller wavelengths, and (2) that on approach to the
critical point it vanishes proportionally to the interfacial tension
rather than, as had often been supposed, varying proportionally to
the product of the tension and the square of the correlation length,
thereby approaching a finite, non-zero limit. Both features are shown
to be in accord with what is found in computer simulations.
***
BM10751
Stone Wales defects are unstable in planar graphene
Defects in graphene are extremely important because they alter its
electronic and magnetic properties. We have predicted that Stone
Wales defects in planar graphene are unstable [1]. This is surprising
because the opposite was believed based on Molecular Dynamics
simulations and experiments in carbon nanotubes [2]. Despite this
previous belief, our prediction is natural. Stone Wales defects
comprise two pentagon-heptagon defects whose heptagons share one
side. Each pentagon-heptagon defect is the core of an edge
dislocation and the Stone Wales defect is a dipole formed by two edge
dislocations that have opposite Burgers vectors and share the same
gliding line. In an unstressed sample, planar linear elasticity
implies that the two dislocations glide towards each other and
annihilate. To show this, we have numerically solved the dynamics of
a novel periodized discrete elasticity model on the hexagonal lattice
and obtained the stable cores of different edge dislocations and
dipoles. Recent experimental results by Meyer et al at Lawrence
Berkeley National Laboratories confirm our prediction. They use a
novel imaging technique based on a TEM microscope to observe the
evolution of Stone Wales and other defects created by irradiation of
a graphene membrane [3]. These defects annihilate within a few
seconds thereby leaving a perfect lattice. These time scales are
several orders of magnitude longer than typical ones in Molecular
Dynamics simulations (pico to nanoseconds).
***
LQ11682A
Antimatter is seen to bounce from a matter wall
If you were to ask scientists what antimatter does
when it comes into contact with ordinary matter, all of them
will reply: "annihilation".
This is actually also the layman's general idea of the
matter-antimatter interaction: they are expected to cancel out
each other immediately in a burst of energy.
However, what we report in this paper is that this is only true
half the time. Our measurements show that, when a bunch of low energy
antiprotons hits an aluminum wall, around half of them are reflected and,
in our experimental conditions, can still travel tens of centimeters
from the reflecting wall.
What's the trick? Actually, none. What we observe is connected to
an effect known since the century-old Rutherford-Geiger-Marsden
experiments: when a charged particle approaches an atom,
and steals into its electron cloud, the Coulomb force exerted by
the nucleus is able to deflect its trajectory.
Through interactions with many atoms, the incident (anti)particle
can eventually be pushed backwards.
So why was this not seen before with antimatter? It's because most studies
were done at relatively high energies, where the annihilation probability
is much higher than the reflection probability.
But this is not true at very low energies, where reflection is
greatly enhanced.
The principal difficulty was to find out a set of data taken at very, very
low energy, in the heaps of data taken by the OBELIX experiment, around
a decade ago, at CERN.
While the antiprotons with "normal" energies annihilated soon after
impacting on the wall, the ones with 1000 times less energy showed
this unexpected behaviour.
***
LS11568
Does the expansion of spacetime do the same thing to physical
theories that scaling up lengths and times does in flat space?
If so, we could understand some issues in cosmology the same
way we do when the temperature and pressure of water is tuned
to eliminate the distinction between liquid and vapor. This
idea would effect such a vast simplification that it has found
many adherants. On the other hand, there has never been a clear
demonstration that the formalism --- known as the Renormaliztion
Group --- either should or should not apply to cosmology. In
this paper I analyze the model that is the paradigm for all
systems with a one dimensional order parameter and I show that
its cosmological evolution is not correctly described by the
Renormalization Group. My analysis makes use of a nonperturbative
resummation technique due to the Russian cosmologist Alexei
Starobinsky. Although the Renormalization Group does not describe
the effects of cosmological expansion, the good news is that
Starobinsky's technique does, and it is even simpler to use.
How rigid are viruses?
Viruses have traditionally been considered dangerous entities capable of infecting and mutating plant and animal genetic codes. In recent years, however, viruses have been explored for applications ranging from drug delivery and gene therapy to nano-technology. Monodispersity, surface chemistry, temperature and pH stability make viruses versatile tools for photonic and electronic templates. Although structures of many viruses are known, their properties remain largely unexplored. In this paper we employ light scattering to analyze the mechanical rigidity and inter-virion coupling of Wiseana iridovirus (WIV). The measurements indicate unexpectedly high Young's modulus ~7 GPa, a surprising value for a biological objects that we traditionally consider in the class of 'Soft Materials'. This modulus is higher than the modulus of usual hard plastics. We show that the hard DNA core is the reason for so high WIV rigidity. The results also indicate a strong mechanical coupling between individual virion particles. The obtained results and the developed technique for analysis of virus rigidity might be very helpful for various bio- and nano- technologies and for studies of other nanoscale objects.
***
BR10615
Microstructure and oxygen concentration of YBCO
The microstructure and oxygen concentration profoundly affect the
electronic properties of the high-temperature superconductor
YBa2Cu3Odelta-7 (YBCO). Using quantitative high-resolution electron
microscopy, we measured the local atomic structure associated with
twinning in several YBCO samples. We observed inhomogeneities in the
lattice parameter ratios on the scale of the twin domains that are
ubiquitous in YBCO. This result was surprising because YBCO is
generally considered to have well defined lattice parameters. Our
measurements reveal that features commonly considered to be twin
boundaries often do not have a true twinning symmetry and are more
accurately described as phase boundaries. Furthermore, the
lattice-parameter ratios in a single sample tend to oscillate back and
forth between two different values from domain to domain, apparently
due to the local variation of oxygen concentration. As a result, we
expect these samples to have locally variable superconducting
properties. Furthermore, one can easily imagine a material with a
network of superconducting channels dispersed within an insulating
matrix. Such a network could have profound implications for theories of
flux pinning and superconductivity in YBCO.
***
LS11726
Fluctuations of a liquid surface: from the molecular scale to
the scale of capillary waves
The fluctuations in the local density of molecules in a liquid
depend sensitively on whether the density is measured close to the
liquid's surface. Far from the surface the density fluctuations are
mild and molecular in range but close to it they are dominated by
long-wavelength "capillary" waves along the surface, with the
interfacial tension and gravity acting as the dominant restoring
forces.
In this paper we show that to describe the full spectrum of
density fluctuations from the molecular scale up to the scale
of capillary waves, one must take account of the energy associated
with *bending* the surface as described by the interface's
bending rigidity. Two of the important results are (1) that the
bending rigidity for the interface between phase-separated
colloid-polymer mixtures is *negative*, with the result that
capillary waves are "more violent", less restricted by interfacial
tension, at smaller wavelengths, and (2) that on approach to the
critical point it vanishes proportionally to the interfacial tension
rather than, as had often been supposed, varying proportionally to
the product of the tension and the square of the correlation length,
thereby approaching a finite, non-zero limit. Both features are shown
to be in accord with what is found in computer simulations.
***
BM10751
Stone Wales defects are unstable in planar graphene
Defects in graphene are extremely important because they alter its
electronic and magnetic properties. We have predicted that Stone
Wales defects in planar graphene are unstable [1]. This is surprising
because the opposite was believed based on Molecular Dynamics
simulations and experiments in carbon nanotubes [2]. Despite this
previous belief, our prediction is natural. Stone Wales defects
comprise two pentagon-heptagon defects whose heptagons share one
side. Each pentagon-heptagon defect is the core of an edge
dislocation and the Stone Wales defect is a dipole formed by two edge
dislocations that have opposite Burgers vectors and share the same
gliding line. In an unstressed sample, planar linear elasticity
implies that the two dislocations glide towards each other and
annihilate. To show this, we have numerically solved the dynamics of
a novel periodized discrete elasticity model on the hexagonal lattice
and obtained the stable cores of different edge dislocations and
dipoles. Recent experimental results by Meyer et al at Lawrence
Berkeley National Laboratories confirm our prediction. They use a
novel imaging technique based on a TEM microscope to observe the
evolution of Stone Wales and other defects created by irradiation of
a graphene membrane [3]. These defects annihilate within a few
seconds thereby leaving a perfect lattice. These time scales are
several orders of magnitude longer than typical ones in Molecular
Dynamics simulations (pico to nanoseconds).
***
LQ11682A
Antimatter is seen to bounce from a matter wall
If you were to ask scientists what antimatter does
when it comes into contact with ordinary matter, all of them
will reply: "annihilation".
This is actually also the layman's general idea of the
matter-antimatter interaction: they are expected to cancel out
each other immediately in a burst of energy.
However, what we report in this paper is that this is only true
half the time. Our measurements show that, when a bunch of low energy
antiprotons hits an aluminum wall, around half of them are reflected and,
in our experimental conditions, can still travel tens of centimeters
from the reflecting wall.
What's the trick? Actually, none. What we observe is connected to
an effect known since the century-old Rutherford-Geiger-Marsden
experiments: when a charged particle approaches an atom,
and steals into its electron cloud, the Coulomb force exerted by
the nucleus is able to deflect its trajectory.
Through interactions with many atoms, the incident (anti)particle
can eventually be pushed backwards.
So why was this not seen before with antimatter? It's because most studies
were done at relatively high energies, where the annihilation probability
is much higher than the reflection probability.
But this is not true at very low energies, where reflection is
greatly enhanced.
The principal difficulty was to find out a set of data taken at very, very
low energy, in the heaps of data taken by the OBELIX experiment, around
a decade ago, at CERN.
While the antiprotons with "normal" energies annihilated soon after
impacting on the wall, the ones with 1000 times less energy showed
this unexpected behaviour.
***
LS11568
Does the expansion of spacetime do the same thing to physical
theories that scaling up lengths and times does in flat space?
If so, we could understand some issues in cosmology the same
way we do when the temperature and pressure of water is tuned
to eliminate the distinction between liquid and vapor. This
idea would effect such a vast simplification that it has found
many adherants. On the other hand, there has never been a clear
demonstration that the formalism --- known as the Renormaliztion
Group --- either should or should not apply to cosmology. In
this paper I analyze the model that is the paradigm for all
systems with a one dimensional order parameter and I show that
its cosmological evolution is not correctly described by the
Renormalization Group. My analysis makes use of a nonperturbative
resummation technique due to the Russian cosmologist Alexei
Starobinsky. Although the Renormalization Group does not describe
the effects of cosmological expansion, the good news is that
Starobinsky's technique does, and it is even simpler to use.
Tuesday, July 15, 2008
7-15-08
LQ11550
Zero Sum Game
Scattering of energetic electrons from neutrons reveals a series of
excited states of the neutron, providing clear evidence of the neutron's
quark-gluon substructure. Calculating the individual response of each
resonant state is presently beyond the reach of theoretical calculations
in all but the most simplified circumstances. However, when the spin of
the neutron is polarized perpendicular to the spin of the incident
electron, a relation known as the Burkhardt--Cottingham sum rule
predicts that regardless of the complexity of the individual responses,
the sum total of all possible states of the neutron vanishes. This
relation was strongly supported by experimental evidence presented in
2004 by the Jefferson Lab E94-010 collaboration.
The collaboration has now tested this same relation for the first time
with a nuclear target : He-3. Since this nucleus consists of two protons
and a neutron, its excitation response is more complicated and includes
several reaction states not present in the single neutron case. But even
when these additional states are included, the total sum still vanishes,
indicating an intriguing level of correlation among the separate states.
In this sense, the He-3 nucleus behaves very similarly to the neutron
located within it.
***
LQ11834
Brain Dynamics Motivate New Mathematics
To understand the brain and its functions, researchers often focus on
the individual activity of specific neurons. We have done so here and
discovered that models of the brain's electrical activity motivate a new
type of mathematics. Abrupt transitions (or bifurcations) typically
characterize two obviously different states (for example, the
bifurcation in behavior many of us experience each evening from "wake"
to "sleep"). In the particular neurons we studied (Purkinje cells ---
some of the largest neurons in the brain) we discovered a novel
bifurcation between two distinct types of behavior. In one type, the
neuron exhibited rapid, incessant activity. In the other, extended
quiescent intervals separated the active states. We found that, to
understand the transition between these behaviors required a new type of
mathematics that we labeled a ¿torus canard¿. A mathematical canard
exhibits quite surprising behavior. Like a tightrope walker, the
dynamics follow temporarily one narrow, unstable region of space before
falling to another, more stable region. For the Purkinje cell model,
this space is five-dimensional, and the tightrope walker navigates
complicated, donut-shaped surfaces that change under his or her feet.
Understanding this complex, torus canard, behavior --- perhaps
fundamental to Purkinje cell activity --- motivates new types of
mathematics.
Zero Sum Game
Scattering of energetic electrons from neutrons reveals a series of
excited states of the neutron, providing clear evidence of the neutron's
quark-gluon substructure. Calculating the individual response of each
resonant state is presently beyond the reach of theoretical calculations
in all but the most simplified circumstances. However, when the spin of
the neutron is polarized perpendicular to the spin of the incident
electron, a relation known as the Burkhardt--Cottingham sum rule
predicts that regardless of the complexity of the individual responses,
the sum total of all possible states of the neutron vanishes. This
relation was strongly supported by experimental evidence presented in
2004 by the Jefferson Lab E94-010 collaboration.
The collaboration has now tested this same relation for the first time
with a nuclear target : He-3. Since this nucleus consists of two protons
and a neutron, its excitation response is more complicated and includes
several reaction states not present in the single neutron case. But even
when these additional states are included, the total sum still vanishes,
indicating an intriguing level of correlation among the separate states.
In this sense, the He-3 nucleus behaves very similarly to the neutron
located within it.
***
LQ11834
Brain Dynamics Motivate New Mathematics
To understand the brain and its functions, researchers often focus on
the individual activity of specific neurons. We have done so here and
discovered that models of the brain's electrical activity motivate a new
type of mathematics. Abrupt transitions (or bifurcations) typically
characterize two obviously different states (for example, the
bifurcation in behavior many of us experience each evening from "wake"
to "sleep"). In the particular neurons we studied (Purkinje cells ---
some of the largest neurons in the brain) we discovered a novel
bifurcation between two distinct types of behavior. In one type, the
neuron exhibited rapid, incessant activity. In the other, extended
quiescent intervals separated the active states. We found that, to
understand the transition between these behaviors required a new type of
mathematics that we labeled a ¿torus canard¿. A mathematical canard
exhibits quite surprising behavior. Like a tightrope walker, the
dynamics follow temporarily one narrow, unstable region of space before
falling to another, more stable region. For the Purkinje cell model,
this space is five-dimensional, and the tightrope walker navigates
complicated, donut-shaped surfaces that change under his or her feet.
Understanding this complex, torus canard, behavior --- perhaps
fundamental to Purkinje cell activity --- motivates new types of
mathematics.
Monday, July 14, 2008
7-14-08
LR11808
Connecting Qubits to Physics
Quantum Communication is the research area that exploits quantum mechanic to complete tasks not acessible in a standard classical communication world. The most prominent example is quantum cryptography. While the design of such quantum communication protocols is usually done efficiently in the abstract language of qubits, any physical realisation will resort to physical systems that are not directly qubits. For optical communication, light pulses have a much richer structure than qubits and also photo-detectors do not operate on a qubit level. We have been able to develop a powerful tool which helps us to connect the qubit language with the language of optical implementations. This way, one can design protocols in the qubit language and be assured that they fit the physical optical implementation.
***
LS11513
MANIPULATING SPINS WITH A LASER, IN SUPERFLUID HELIUM NANODROPLETS
In our research group we investigate, via their electron spin (in the
future nuclear spin too), atoms and molecules assembled on helium
droplets. Helium droplets consist of only a few thousand He atoms,
have a diameter of some ten nanometers, and a temperature of 0.4 Kelvin;
we dope them with exactly one rubidium atom each, which contributes
an electron spin of 1/2.
Electron spins are the source of magnetism in virtually every
material, thus their study has very important practical applications
(most notably magnetic storage). Electron and nuclear spins can also
be very sensitive to their environment. Nuclear spins are thus a
routine diagnostic tool in medicine (MRI); both nuclear and electron
spin are of common use in research laboratories to learn about the
structure of molecules and materials. Electron spins are also
candidate Qubits for quantum computers.
In all these applications, the ability to probe and control the spin
alignment is crucial; this is often accomplished via optical excitation.
Superfluid helium is a great environment: being cold, weakly interacting,
and nonmagnetic, it preserves the spin polarization of the dopant under
study for a long time. We excite rubidium atoms with a polarized laser in
a strong magnetic field, and are able to find a "sweet spot" where the the
strong laser does not destroy these fragile systems but simply flips one
electron spin. This is the first time that electron spin manipulation
in helium droplets is ever achieved; the way to spin resonance spectroscopy
(in progress in our group) is now open.
***
BR10825
The structure of a large family of TS lattices is equivalent
In (J. Appl. Phys. 102, 093511 ), Torquato and Stillinger explain the most
dilute known way to pack spheres in a crystal lattice.
The result is a family of heretofore unknown lattices (which we call TS
lattices) whose physical properties present an interesting case-study.
Our work elucidates the magnetic properties of a large family of TS
lattices. These are geometrically frustrated: there is no way to
arrange microscopic magnetic moments (or spins) on all of the lattice
sites so that each is anti-parallel to all of its neighbors. In such
cases, a rich variety of phenomena can occur at very low temperatures.
We find two general possibilities, on the lattices we study: if each spin is free to
orient itself within a plane, there is a unique optimal arrangement, in
which all spins point along one of three directions. If the spin can
choose between only two directions, however, there are infinitely many
configurations which have the same energy. It is thus not obvious
which configuration(s) will 'win' at very low temperatures. We
explain, using a combination of mathematical and numerical arguments,
the expected behavior in this case. The significance of these results extends
beyond the expected material properties of the compounds in question: we show that,
at least from the point of view of simple magnetic models, the structure of a large family
of TS lattices is equivalent, and moreover very similar to a well-studied lattice structure.
This perspective greatly simplifies our analysis, and gives a powerful tool for understanding
and classifying the structures of the TS lattices.
***
LF11807
Electrical control of nano-pendulum motions
The motion of a movable superconducting island coupled to
superconducting contacts, can be controlled by electric currents. By
application of specific voltages between the contacts, the island can
be forced to oscillate between the contact with predetermined
frequencies. Scientists in Los Alamos, NM, and Uppsala, Sweden,
provides a theoretical prediction of the influence from the electric
current on the motion of a movable superconducting island, when the
island is coupled to superconducting leads. In absence of the electric
current, the motion of the island is determined by its mass and spring
constant (Newtonian mechanics). Turning on the current, modifies the
island's motion, and this modification is controllable by adjusting
the size of the current. Signatures of the island motions are fed back
into the current, which would make it possible to read-out the
specific frequency of the island's oscillations.
***
LP11401 (Embargoed until August 5, 2008)
Chaotic Dance of Nuclear Spins
An experimental study of atomic nuclei in a substance used widely for
medical imaging of human lungs has revealed a new fundamental property of
interacting nuclear spins in solids. Radically different signals measured
by nuclear magnetic resonance (NMR) exhibit identical long-time behavior.
It has been proposed that this universality is related to the chaotic
motion of the nuclear spins, which erases the memory of the initial spin
state. Such universal behavior is extremely challenging both to establish
experimentally and to understand theoretically and had remained
undiscovered in the 60 years since the advent of NMR. In the experiment,
nuclei of xenon were "hyperpolarized" with a laser in the gas phase,
liquefied and then solidified. The resulting enormous nuclear polarization
made it possible to track the spin signal with great sensitivity. The
experiment focuses attention on an unsolved 20th-Century problem--the role
and the implications of chaos in the behavior of large ensembles of
quantum particles. The observed universal behavior indicates that,
contrary to conventional wisdom, collective quantum dynamics exhibits
extreme randomness even when the individual behavior of quantum particles
is not yet randomized.
***
LG11893 (Embargoed until August 6, 2008)
Shaping up graphite using light
Can we make diamond from graphite without resorting to extreme heat and
pressure as diamond was made in the Earth in geological time? The group
of scientist in Michigan State University raises such a possibility by
shining a short burst of intense femtosecond laser pulse on graphite and
watching using ultrafast diffraction the movements of some loosely
separated carbon atoms in graphene layers undergoing rehybridization,
forming more tightly bound diamond-like bonds. Whereas this intriguing
intermediate structure is short-lived, approximately for 30 picoseconds,
this experiment shows the possibility of a structural transformation
purely induced by light in this very versatile class of material.
Assisted by density functional theory calculation, the cause for such a
transformation is attributed to the electronic structure changes and the
Coulomb field buildup following the photoexcitation.
***
LS11409
Spin torque breaks the speed limit
New spin torque speed record
A collaboration of researchers has realized spin torque switching of a
nanomagnet as fast as the fundamental speed limit allows [1]. These
findings are important for a new generation of ultra fast magnetic memory
chips.
Spin torque is an effect that allows programming of a magnetic memory cell
simply by application of a current pulse. It can be used for a novel high
density, non-volatile magnetic memory chip. Several major semiconductor
producers have demonstrated spin torque memory prototypes and market
introduction is expected, soon.
In a spin torque memory cell the current pulse excites a rotational motion
of the magnetization ? the so-called precession. Normally, the
magnetization has to undergo several precessional turns before
magnetization reversal takes place. Reliable programming of a memory cell
can therefore only be achieved by rather long current pulses of several
nanoseconds duration which limits the speed of the memory chip. In an
experiment carried out at PTB Braunschweig spin torque magnetization
reversal has now been realized by a single precessional turn, only. This
so called ?ballistic? spin torque magnetization reversal corresponds to
the ultra short physical limit of magnetization reversal time. It was
achieved by precise tailoring of the spin torque pulse parameters in
combination with a small magnetic field. Ballistic spin torque reversal
could allow future non-volatile magnetic memories operating with GHz clock
rates and thus faster than the fastest volatile computer memories
available.
***
ZJ10029
Efficient acceleration of electrons by launching two weakly relativistic laser pulses, one behind the other, in plasma
If one asks a strong, well built individual to lift a heavy carton and then ask two moderately built persons to perform the same task, which of the two processes would be performed in a more stable and smooth manner? The answer is obvious - two men (though moderately built) working together would give a better performance. This is exactly the idea projected in the present paper, wherein a novel concept of accelerating fundamental particles, such as electrons, to high energies (multi MeV and) has been proposed, using two mildly intense laser beams propagating in plasma. Earlier studies and experiments have shown that an ultraintense laser beam propagating in plasma leads to the generation of large amplitude wakefields which are used to accelerate electrons to ultrahigh energies. However, in the process, many laser-plasma instabilities arise and lead to depletion in efficiency of the acceleration process. Plasma based laser wakefield accelerators (LWFA) are important because they can accelerate particles to energies that cannot be attained by conventional accelerators such as rf linacs which can produce electrons of only a few MeV. These accelerated electrons have important applications in chemical and biological spectroscopy, medical imaging and radiation therapy.
***
BM10877
We study the phase diagram of ferri-feroelectric mixed crystals
by broadband dielectric spectroscopy. The phase diagram of investigated crystals
is strongly asymmetric - the decreasing of ferroelectric phase transition
temperatures by doping is much more flat that the corresponding decreasing of
ferrielectric phase transition temperatures. In the middle part of the phase diagram
the dipolar glass phase has been observed. In boundary region between ferroelectric
order and dipolar glass phases at low temperatures the nonergodic relaxor phase appears.
***
Lq11479
Polariton Light-Matter Particles Moving in Step
It is shown that condensed light-matter particles in a solid-state resonator remain locked together over times much longer than their lifetime. In such a Bose-Einstein Condensate (BEC) a large number of particles accumulate in a single state. A fundamental property of these condensed particles is that they move in phase, forming a single coherent whole. In this work it is demonstrated that polariton condensates exhibit long phase memory times, two order of magnitude longer than the particle lifetime. Such long times permit the fundamental mechanisms determining the phase memory times to be revealed.
A key characteristic of the polariton particles, which arise in semiconductor solids due to coupling between electronic excitations and light, is that they can be manipulated by light beams on length scales of hundredths of millimetres and exhibit condensation at high temperatures (~20 K). In contrast to atomic condensates the polariton system is non-equilibrium: it loses particles as fast as they fall into the condensate. Nevertheless, we are able to show that polariton condensates exhibit properties expected for an equilibrium system, and like atomic condensates, have potential for use in quantum information processing experiments.
Connecting Qubits to Physics
Quantum Communication is the research area that exploits quantum mechanic to complete tasks not acessible in a standard classical communication world. The most prominent example is quantum cryptography. While the design of such quantum communication protocols is usually done efficiently in the abstract language of qubits, any physical realisation will resort to physical systems that are not directly qubits. For optical communication, light pulses have a much richer structure than qubits and also photo-detectors do not operate on a qubit level. We have been able to develop a powerful tool which helps us to connect the qubit language with the language of optical implementations. This way, one can design protocols in the qubit language and be assured that they fit the physical optical implementation.
***
LS11513
MANIPULATING SPINS WITH A LASER, IN SUPERFLUID HELIUM NANODROPLETS
In our research group we investigate, via their electron spin (in the
future nuclear spin too), atoms and molecules assembled on helium
droplets. Helium droplets consist of only a few thousand He atoms,
have a diameter of some ten nanometers, and a temperature of 0.4 Kelvin;
we dope them with exactly one rubidium atom each, which contributes
an electron spin of 1/2.
Electron spins are the source of magnetism in virtually every
material, thus their study has very important practical applications
(most notably magnetic storage). Electron and nuclear spins can also
be very sensitive to their environment. Nuclear spins are thus a
routine diagnostic tool in medicine (MRI); both nuclear and electron
spin are of common use in research laboratories to learn about the
structure of molecules and materials. Electron spins are also
candidate Qubits for quantum computers.
In all these applications, the ability to probe and control the spin
alignment is crucial; this is often accomplished via optical excitation.
Superfluid helium is a great environment: being cold, weakly interacting,
and nonmagnetic, it preserves the spin polarization of the dopant under
study for a long time. We excite rubidium atoms with a polarized laser in
a strong magnetic field, and are able to find a "sweet spot" where the the
strong laser does not destroy these fragile systems but simply flips one
electron spin. This is the first time that electron spin manipulation
in helium droplets is ever achieved; the way to spin resonance spectroscopy
(in progress in our group) is now open.
***
BR10825
The structure of a large family of TS lattices is equivalent
In (J. Appl. Phys. 102, 093511 ), Torquato and Stillinger explain the most
dilute known way to pack spheres in a crystal lattice.
The result is a family of heretofore unknown lattices (which we call TS
lattices) whose physical properties present an interesting case-study.
Our work elucidates the magnetic properties of a large family of TS
lattices. These are geometrically frustrated: there is no way to
arrange microscopic magnetic moments (or spins) on all of the lattice
sites so that each is anti-parallel to all of its neighbors. In such
cases, a rich variety of phenomena can occur at very low temperatures.
We find two general possibilities, on the lattices we study: if each spin is free to
orient itself within a plane, there is a unique optimal arrangement, in
which all spins point along one of three directions. If the spin can
choose between only two directions, however, there are infinitely many
configurations which have the same energy. It is thus not obvious
which configuration(s) will 'win' at very low temperatures. We
explain, using a combination of mathematical and numerical arguments,
the expected behavior in this case. The significance of these results extends
beyond the expected material properties of the compounds in question: we show that,
at least from the point of view of simple magnetic models, the structure of a large family
of TS lattices is equivalent, and moreover very similar to a well-studied lattice structure.
This perspective greatly simplifies our analysis, and gives a powerful tool for understanding
and classifying the structures of the TS lattices.
***
LF11807
Electrical control of nano-pendulum motions
The motion of a movable superconducting island coupled to
superconducting contacts, can be controlled by electric currents. By
application of specific voltages between the contacts, the island can
be forced to oscillate between the contact with predetermined
frequencies. Scientists in Los Alamos, NM, and Uppsala, Sweden,
provides a theoretical prediction of the influence from the electric
current on the motion of a movable superconducting island, when the
island is coupled to superconducting leads. In absence of the electric
current, the motion of the island is determined by its mass and spring
constant (Newtonian mechanics). Turning on the current, modifies the
island's motion, and this modification is controllable by adjusting
the size of the current. Signatures of the island motions are fed back
into the current, which would make it possible to read-out the
specific frequency of the island's oscillations.
***
LP11401 (Embargoed until August 5, 2008)
Chaotic Dance of Nuclear Spins
An experimental study of atomic nuclei in a substance used widely for
medical imaging of human lungs has revealed a new fundamental property of
interacting nuclear spins in solids. Radically different signals measured
by nuclear magnetic resonance (NMR) exhibit identical long-time behavior.
It has been proposed that this universality is related to the chaotic
motion of the nuclear spins, which erases the memory of the initial spin
state. Such universal behavior is extremely challenging both to establish
experimentally and to understand theoretically and had remained
undiscovered in the 60 years since the advent of NMR. In the experiment,
nuclei of xenon were "hyperpolarized" with a laser in the gas phase,
liquefied and then solidified. The resulting enormous nuclear polarization
made it possible to track the spin signal with great sensitivity. The
experiment focuses attention on an unsolved 20th-Century problem--the role
and the implications of chaos in the behavior of large ensembles of
quantum particles. The observed universal behavior indicates that,
contrary to conventional wisdom, collective quantum dynamics exhibits
extreme randomness even when the individual behavior of quantum particles
is not yet randomized.
***
LG11893 (Embargoed until August 6, 2008)
Shaping up graphite using light
Can we make diamond from graphite without resorting to extreme heat and
pressure as diamond was made in the Earth in geological time? The group
of scientist in Michigan State University raises such a possibility by
shining a short burst of intense femtosecond laser pulse on graphite and
watching using ultrafast diffraction the movements of some loosely
separated carbon atoms in graphene layers undergoing rehybridization,
forming more tightly bound diamond-like bonds. Whereas this intriguing
intermediate structure is short-lived, approximately for 30 picoseconds,
this experiment shows the possibility of a structural transformation
purely induced by light in this very versatile class of material.
Assisted by density functional theory calculation, the cause for such a
transformation is attributed to the electronic structure changes and the
Coulomb field buildup following the photoexcitation.
***
LS11409
Spin torque breaks the speed limit
New spin torque speed record
A collaboration of researchers has realized spin torque switching of a
nanomagnet as fast as the fundamental speed limit allows [1]. These
findings are important for a new generation of ultra fast magnetic memory
chips.
Spin torque is an effect that allows programming of a magnetic memory cell
simply by application of a current pulse. It can be used for a novel high
density, non-volatile magnetic memory chip. Several major semiconductor
producers have demonstrated spin torque memory prototypes and market
introduction is expected, soon.
In a spin torque memory cell the current pulse excites a rotational motion
of the magnetization ? the so-called precession. Normally, the
magnetization has to undergo several precessional turns before
magnetization reversal takes place. Reliable programming of a memory cell
can therefore only be achieved by rather long current pulses of several
nanoseconds duration which limits the speed of the memory chip. In an
experiment carried out at PTB Braunschweig spin torque magnetization
reversal has now been realized by a single precessional turn, only. This
so called ?ballistic? spin torque magnetization reversal corresponds to
the ultra short physical limit of magnetization reversal time. It was
achieved by precise tailoring of the spin torque pulse parameters in
combination with a small magnetic field. Ballistic spin torque reversal
could allow future non-volatile magnetic memories operating with GHz clock
rates and thus faster than the fastest volatile computer memories
available.
***
ZJ10029
Efficient acceleration of electrons by launching two weakly relativistic laser pulses, one behind the other, in plasma
If one asks a strong, well built individual to lift a heavy carton and then ask two moderately built persons to perform the same task, which of the two processes would be performed in a more stable and smooth manner? The answer is obvious - two men (though moderately built) working together would give a better performance. This is exactly the idea projected in the present paper, wherein a novel concept of accelerating fundamental particles, such as electrons, to high energies (multi MeV and) has been proposed, using two mildly intense laser beams propagating in plasma. Earlier studies and experiments have shown that an ultraintense laser beam propagating in plasma leads to the generation of large amplitude wakefields which are used to accelerate electrons to ultrahigh energies. However, in the process, many laser-plasma instabilities arise and lead to depletion in efficiency of the acceleration process. Plasma based laser wakefield accelerators (LWFA) are important because they can accelerate particles to energies that cannot be attained by conventional accelerators such as rf linacs which can produce electrons of only a few MeV. These accelerated electrons have important applications in chemical and biological spectroscopy, medical imaging and radiation therapy.
***
BM10877
We study the phase diagram of ferri-feroelectric mixed crystals
by broadband dielectric spectroscopy. The phase diagram of investigated crystals
is strongly asymmetric - the decreasing of ferroelectric phase transition
temperatures by doping is much more flat that the corresponding decreasing of
ferrielectric phase transition temperatures. In the middle part of the phase diagram
the dipolar glass phase has been observed. In boundary region between ferroelectric
order and dipolar glass phases at low temperatures the nonergodic relaxor phase appears.
***
Lq11479
Polariton Light-Matter Particles Moving in Step
It is shown that condensed light-matter particles in a solid-state resonator remain locked together over times much longer than their lifetime. In such a Bose-Einstein Condensate (BEC) a large number of particles accumulate in a single state. A fundamental property of these condensed particles is that they move in phase, forming a single coherent whole. In this work it is demonstrated that polariton condensates exhibit long phase memory times, two order of magnitude longer than the particle lifetime. Such long times permit the fundamental mechanisms determining the phase memory times to be revealed.
A key characteristic of the polariton particles, which arise in semiconductor solids due to coupling between electronic excitations and light, is that they can be manipulated by light beams on length scales of hundredths of millimetres and exhibit condensation at high temperatures (~20 K). In contrast to atomic condensates the polariton system is non-equilibrium: it loses particles as fast as they fall into the condensate. Nevertheless, we are able to show that polariton condensates exhibit properties expected for an equilibrium system, and like atomic condensates, have potential for use in quantum information processing experiments.
Friday, July 11, 2008
7-11-08
LP11401
Chaotic Dance of Nuclear Spins
An experimental study of atomic nuclei in a substance used widely for
medical imaging of human lungs has revealed a new fundamental property of
interacting nuclear spins in solids. Radically different signals measured
by nuclear magnetic resonance (NMR) exhibit identical long-time behavior.
It has been proposed that this universality is related to the chaotic
motion of the nuclear spins, which erases the memory of the initial spin
state. Such universal behavior is extremely challenging both to establish
experimentally and to understand theoretically and had remained
undiscovered in the 60 years since the advent of NMR. In the experiment,
nuclei of xenon were "hyperpolarized" with a laser in the gas phase,
liquefied and then solidified. The resulting enormous nuclear polarization
made it possible to track the spin signal with great sensitivity. The
result of the experiment focuses attention on an unsolved 20th-Century
problem--the role and the implications of chaos in the behavior of large
ensembles of quantum particles. The result of this experiment suggests
that, contrary to conventional wisdom, collective quantum dynamics
exhibits extreme randomness even when the individual behavior of quantum
particles is not yet randomized.
***
LP11113 (Embargoed until July 23, 2008)
PREDICTION AND UNDERSTANDING OF NOVEL MATERIALS TO BE USED FOR NEW, HIGH
EFFICIENCY PHOTOVOLTAIC CELLS
A new type of material which can provide highly efficiency solar cells has
been developed. This material will have an intermediate energy band which is
one possible way of enhancing the efficiency of photovoltaic cells. The basic
operation of a conventional photovoltaic cell relies on the electron
promotion from a lower energy state (valence band) to a higher energy one
(conduction band) through absorption of photons with sufficient energy. The
intermediate band is located between these bands and helps to absorb, not
only the sun’s photons with energy higher than that of the gap width, but
also the lower energy ones. This will enhance the photovoltaic conversion
efficiency in these new cells in relation to the conventional ones. In this
paper, using quantum calculations, we have obtained structural, electronic
and optical properties of a new system based on an indium sulphide
semiconductor substituted with transition metal atoms. The computed optical
absorption of these compounds compared to the corresponding undoped material,
predicts a significant absorption below the band-gap of the parent
semiconductor and an enhancement of the optical absorption across the whole
solar-spectrum range. These systems seem promising for developing more
efficient novel optoelectronic devices. Their experimental synthesis has
already been reported.
***
BR10591
The solid solution hardening SSH is a long standing problem of statistical physics.
The early time of the SSH theory sends us back to the analytical works of the late
Sir Nevill Francis Mott and Frank Reginald Nunes Nabarro. The strength of a single
crystal was then determined through some continuous models and it was predicted
to vary as a fractional power law of the solute concentration. An important number
of experimental studies were performed and the compiling of their results reveals
the conundrums left aside by the early theory, as for instance the temperature effect.
The current revival of the SSH theory is mainly supported by of the development of
the 3 dimensional atomistic simulations. The atomistic simulations allows to shed a new
light on the SSH and to advance on questions that remain.
In our paper, we studied throughout numerical simulations an alloy extensively used
in aeronautics as matrix for wings and engines, i.e. the Nickel-Aluminum system.
This allowed us to emphasize that the common belief for the weakness
of the screw dislocation pinning strength does not hold for the solid solution we studied.
We determined which version of the SSH theory is the more adapted to predict the strength
of our system. Such a result should serve as a guide to further the theoretical
developments on the dislocation mobility in random media.
***
LR11643
Colorful approach to the fractional quantum Hall effect
The different types of fractional quantum Hall effect may be described in
terms of multi-color quantum liquids. This is the result of a trial wave
function approach proposed in a recent paper by Regnault, Goerbig, and
Jolicoeur (CNRS France), which yields a complementary vision of the
fractional quantum Hall effect. Indeed, each color group of two-dimensional
particles in a strong magnetic field forms a Laughlin liquid. However, in a
typical system, there is only one type of particles, say gray, and the colors
need to be viewed as an artificial marking. In order to get rid of these
artificial colors, the authors have proposed a procedure which renders all
particles gray again. This may be viewed as taking a black-and-white photo of
a colorful painting. Amazingly, the black-and-white photo reveals an internal
structure of the quantum liquid - it consists of distinct droplets the number
of particles of which is that of the original colors. The performed numerical
calculations indicate that this structure may be a common feature of the
different types of fractional quantum Hall states, such as the
composite-fermion states or else the 5/2 state with its exotic excitations.
***
LS11531
ORIGIN OF GIANT OCEAN WAVES
Scientists at Lancaster University have made a discovery that
illuminates the origin of rogue waves -- the giant waves that
occasionally appear on the ocean and are suspected of being
responsible for many unexplained losses of large ships. These
waves are quite different from the tsunami created by undersea
earthquakes, which in the open sea are usually so low that they
are almost invisible. In contrast, survivors describe a giant wave
as being like ``a wall of water'', perhaps 100 feet or more or
more in height. There is intense interest in the origin of giant
waves on account of the commercial importance of this
extraordinary phenomenon. The Lancaster team is studying nonlinear
wave interactions through experiments on superfluid helium. These
enable fundamental wave processes to be studied under controlled
conditions, in contrast to giant ocean waves which require
hundreds of miles of open sea to appear and disappear. The
scientists were astonished to discover that wave energy could
sometimes concentrate to create giant waves in the laboratory. If
the new understanding can be exploited to explain how rogue waves
arise on the ocean, it may be possible to predict them. If so,
there will be many grateful mariners and insurance companies.
***
LP10938A
Confined light could rotate microscopic rods on a chip
Laser light can apply mechanical pressure on microscopic objects, causing them to be trapped and even rotated. In this paper, we theoretically demonstrate that light that is being guided by a tiny rod and confined around it, could rotate the rod itself, forming a new type of microscopic machines that could be on a chip. The rod, possibly made of a glass-like material, could be a fraction of a micrometer in diameter and only a few micrometers long. Laser light with a "rotating" wave front needs to be injected into the rod, and then if the rod absorbs some the light, it will tend to rotate. This is similar to a plastic collision in mechanics, such that the rotating light particles "stick" to the rod and consequently apply a rotational force. Another effect that results from the interaction of the light with the absorptive rod is that the light tends to push the rod forward. Several light-driven motors may be conceived based on this concept, such as a microscopic drill that is being pushed forward and rotated by the guided and confined light.
***
LE10914AR
FROG-CRAB catches the light
Researchers at JILA, University of Colorado at Boulder, have made a giant leap
in simplifying the generation of some of the shortest light flashes achievable to date:
x-ray pulses that are just about one femtosecond (a millionth of a billionth of a second)
long can now be generated by simply shining high-power laser pulses into a gas-filled hollow
capillary. Now, if you generate an event this short, then not only making it, but also
measuring it is a challenge. The researchers accomplished this using a method called
FROG-CRAB. The ultrashort x-ray flash is scanned across the electric field oscillations
of a second, much longer infrared light pulse, and the combined electric field of the long
and short pulse knocks out photoelectrons from a noble-gas. The recorded “spectrogram”,
shown in Figure 1, contains all the information required to know the shape and duration of
the ultrashort x-ray flash (shown in Figure 2). This kind of ultrafast radiation can now
serve as a flashlight to track some of the fastest events occurring in our everyday world,
as they happen e.g. in molecular and materials dynamics, or in the motion of electrons in a
chemical reaction.
***
LP11298
Crackling dynamics of cracks
From broken dishes to collapsing buildings, materials failure bothers,
damages or devastates our life. While the failure of homogeneous solids
is well described by Linear Elastic Fracture Mechanics, the case of
heterogeneous materials remains far more complex. In particular, the
cracks propagation displays there an intermittent dynamics -- so-called
crackling dynamics -- with seemingly random discrete jumps of a variety
of sizes. Indeed, the distribution of energy released through these
jumps forms a power law with no characteristic size scale, as observed
for instance in the acoustic emission accompagnying the failure of
various materials or - at a much larger scale - in the seismic activity
associated with earthquakes. Here we derive - and confront to
experiments - a simple stochastic description for crack growth in
heterogeneous media which suggests that this crackling dynamics observed
in fracture exhibits statistical features insensitive to the mechanistic
details. This "universality" proves that model experiments in
laboratories and more complex failure phenomena as earthquakes share
common, and to some extent predictable, features.
Chaotic Dance of Nuclear Spins
An experimental study of atomic nuclei in a substance used widely for
medical imaging of human lungs has revealed a new fundamental property of
interacting nuclear spins in solids. Radically different signals measured
by nuclear magnetic resonance (NMR) exhibit identical long-time behavior.
It has been proposed that this universality is related to the chaotic
motion of the nuclear spins, which erases the memory of the initial spin
state. Such universal behavior is extremely challenging both to establish
experimentally and to understand theoretically and had remained
undiscovered in the 60 years since the advent of NMR. In the experiment,
nuclei of xenon were "hyperpolarized" with a laser in the gas phase,
liquefied and then solidified. The resulting enormous nuclear polarization
made it possible to track the spin signal with great sensitivity. The
result of the experiment focuses attention on an unsolved 20th-Century
problem--the role and the implications of chaos in the behavior of large
ensembles of quantum particles. The result of this experiment suggests
that, contrary to conventional wisdom, collective quantum dynamics
exhibits extreme randomness even when the individual behavior of quantum
particles is not yet randomized.
***
LP11113 (Embargoed until July 23, 2008)
PREDICTION AND UNDERSTANDING OF NOVEL MATERIALS TO BE USED FOR NEW, HIGH
EFFICIENCY PHOTOVOLTAIC CELLS
A new type of material which can provide highly efficiency solar cells has
been developed. This material will have an intermediate energy band which is
one possible way of enhancing the efficiency of photovoltaic cells. The basic
operation of a conventional photovoltaic cell relies on the electron
promotion from a lower energy state (valence band) to a higher energy one
(conduction band) through absorption of photons with sufficient energy. The
intermediate band is located between these bands and helps to absorb, not
only the sun’s photons with energy higher than that of the gap width, but
also the lower energy ones. This will enhance the photovoltaic conversion
efficiency in these new cells in relation to the conventional ones. In this
paper, using quantum calculations, we have obtained structural, electronic
and optical properties of a new system based on an indium sulphide
semiconductor substituted with transition metal atoms. The computed optical
absorption of these compounds compared to the corresponding undoped material,
predicts a significant absorption below the band-gap of the parent
semiconductor and an enhancement of the optical absorption across the whole
solar-spectrum range. These systems seem promising for developing more
efficient novel optoelectronic devices. Their experimental synthesis has
already been reported.
***
BR10591
The solid solution hardening SSH is a long standing problem of statistical physics.
The early time of the SSH theory sends us back to the analytical works of the late
Sir Nevill Francis Mott and Frank Reginald Nunes Nabarro. The strength of a single
crystal was then determined through some continuous models and it was predicted
to vary as a fractional power law of the solute concentration. An important number
of experimental studies were performed and the compiling of their results reveals
the conundrums left aside by the early theory, as for instance the temperature effect.
The current revival of the SSH theory is mainly supported by of the development of
the 3 dimensional atomistic simulations. The atomistic simulations allows to shed a new
light on the SSH and to advance on questions that remain.
In our paper, we studied throughout numerical simulations an alloy extensively used
in aeronautics as matrix for wings and engines, i.e. the Nickel-Aluminum system.
This allowed us to emphasize that the common belief for the weakness
of the screw dislocation pinning strength does not hold for the solid solution we studied.
We determined which version of the SSH theory is the more adapted to predict the strength
of our system. Such a result should serve as a guide to further the theoretical
developments on the dislocation mobility in random media.
***
LR11643
Colorful approach to the fractional quantum Hall effect
The different types of fractional quantum Hall effect may be described in
terms of multi-color quantum liquids. This is the result of a trial wave
function approach proposed in a recent paper by Regnault, Goerbig, and
Jolicoeur (CNRS France), which yields a complementary vision of the
fractional quantum Hall effect. Indeed, each color group of two-dimensional
particles in a strong magnetic field forms a Laughlin liquid. However, in a
typical system, there is only one type of particles, say gray, and the colors
need to be viewed as an artificial marking. In order to get rid of these
artificial colors, the authors have proposed a procedure which renders all
particles gray again. This may be viewed as taking a black-and-white photo of
a colorful painting. Amazingly, the black-and-white photo reveals an internal
structure of the quantum liquid - it consists of distinct droplets the number
of particles of which is that of the original colors. The performed numerical
calculations indicate that this structure may be a common feature of the
different types of fractional quantum Hall states, such as the
composite-fermion states or else the 5/2 state with its exotic excitations.
***
LS11531
ORIGIN OF GIANT OCEAN WAVES
Scientists at Lancaster University have made a discovery that
illuminates the origin of rogue waves -- the giant waves that
occasionally appear on the ocean and are suspected of being
responsible for many unexplained losses of large ships. These
waves are quite different from the tsunami created by undersea
earthquakes, which in the open sea are usually so low that they
are almost invisible. In contrast, survivors describe a giant wave
as being like ``a wall of water'', perhaps 100 feet or more or
more in height. There is intense interest in the origin of giant
waves on account of the commercial importance of this
extraordinary phenomenon. The Lancaster team is studying nonlinear
wave interactions through experiments on superfluid helium. These
enable fundamental wave processes to be studied under controlled
conditions, in contrast to giant ocean waves which require
hundreds of miles of open sea to appear and disappear. The
scientists were astonished to discover that wave energy could
sometimes concentrate to create giant waves in the laboratory. If
the new understanding can be exploited to explain how rogue waves
arise on the ocean, it may be possible to predict them. If so,
there will be many grateful mariners and insurance companies.
***
LP10938A
Confined light could rotate microscopic rods on a chip
Laser light can apply mechanical pressure on microscopic objects, causing them to be trapped and even rotated. In this paper, we theoretically demonstrate that light that is being guided by a tiny rod and confined around it, could rotate the rod itself, forming a new type of microscopic machines that could be on a chip. The rod, possibly made of a glass-like material, could be a fraction of a micrometer in diameter and only a few micrometers long. Laser light with a "rotating" wave front needs to be injected into the rod, and then if the rod absorbs some the light, it will tend to rotate. This is similar to a plastic collision in mechanics, such that the rotating light particles "stick" to the rod and consequently apply a rotational force. Another effect that results from the interaction of the light with the absorptive rod is that the light tends to push the rod forward. Several light-driven motors may be conceived based on this concept, such as a microscopic drill that is being pushed forward and rotated by the guided and confined light.
***
LE10914AR
FROG-CRAB catches the light
Researchers at JILA, University of Colorado at Boulder, have made a giant leap
in simplifying the generation of some of the shortest light flashes achievable to date:
x-ray pulses that are just about one femtosecond (a millionth of a billionth of a second)
long can now be generated by simply shining high-power laser pulses into a gas-filled hollow
capillary. Now, if you generate an event this short, then not only making it, but also
measuring it is a challenge. The researchers accomplished this using a method called
FROG-CRAB. The ultrashort x-ray flash is scanned across the electric field oscillations
of a second, much longer infrared light pulse, and the combined electric field of the long
and short pulse knocks out photoelectrons from a noble-gas. The recorded “spectrogram”,
shown in Figure 1, contains all the information required to know the shape and duration of
the ultrashort x-ray flash (shown in Figure 2). This kind of ultrafast radiation can now
serve as a flashlight to track some of the fastest events occurring in our everyday world,
as they happen e.g. in molecular and materials dynamics, or in the motion of electrons in a
chemical reaction.
***
LP11298
Crackling dynamics of cracks
From broken dishes to collapsing buildings, materials failure bothers,
damages or devastates our life. While the failure of homogeneous solids
is well described by Linear Elastic Fracture Mechanics, the case of
heterogeneous materials remains far more complex. In particular, the
cracks propagation displays there an intermittent dynamics -- so-called
crackling dynamics -- with seemingly random discrete jumps of a variety
of sizes. Indeed, the distribution of energy released through these
jumps forms a power law with no characteristic size scale, as observed
for instance in the acoustic emission accompagnying the failure of
various materials or - at a much larger scale - in the seismic activity
associated with earthquakes. Here we derive - and confront to
experiments - a simple stochastic description for crack growth in
heterogeneous media which suggests that this crackling dynamics observed
in fracture exhibits statistical features insensitive to the mechanistic
details. This "universality" proves that model experiments in
laboratories and more complex failure phenomena as earthquakes share
common, and to some extent predictable, features.
Wednesday, July 9, 2008
7-9-08
LE10865A
An interferometer-free experiment is proposed aiming at violating a
Franson-type Bell inequality using photons generated from four-wave mixing
in an ensemble of two-level atoms. Previous research showed that when a
retro-reflected pump is passed through the ensemble, pairs of
counter-propagating photons emerge off of the pump axis. These photons can
either be produced at the same wavelength as the pump beam or at
wavelengths differing from the pump by the Rabi frequency that the pump
induces. In this paper, we show that interference (Figure 1) between these
two processes leads to oscillations in the photon correlation time since
photons emitted at the same wavelength will experience the same group
delay in the ensemble whereas those emitted at different wavelengths will
experience different group delays. The violation of Bell¿s inequality
further implies the result for non-locality. Since the energy separation
between photons with different wavelengths is tunable, this experiment
might be an interesting way of probing the quantum nature of the detection
process. The interference (Figure 1) will disappear when the separation
approaches the fundamental timescales for photon absorption in the
detector.
Monday, July 7, 2008
7-7-08
LF11471
Superconducting silane is layered
An international research team has predicted from first principles the
superconducting properties of silicon-based hydrogen rich alloy ┐ silane
(SiH4) with a layered structure, fueling up the possible realization of
metallization and superconductivity in dense hydrogen which has long been
a major driving force in high-pressure physics and remains an important
challenge in modern physics and astrophysics.
Silane offers a source for high purity silicon which is at the base of
electronics and microdevices. It is gas at ambient condition. The new
structure having layered network is formed when silane is subjected to
pressures above 600,000 times atmospheric pressure at sea level. The
researchers obtained the superconducting transition temperature in the
range of 20 and 75 K in the layered metallic phase. They demonstrated that
silane is a good example to metallize and superconduct hydrogen at modest
pressure much lower than necessary for solid hydrogen because it has been
already chemically precompressed.
The research, publishing in Physical Review Letters, suggested that the
layered feature could be essential for superconductivity in other
hydrogen-dominant compounds.
***
LQ11339
Light rulers on a microchip
A frequency comb is a laser source that emits a spectrum of many different discrete frequencies (corresponding to different colors), which are perfectly uniformly spaced and can serve as a ruler to measure optical frequencies. Frequency combs have become a universal tool for optical frequency metrology, spectrometer calibration, gas sensing and arbitrary optical waveform generation within the last decade and part of the Nobel price in physics in 2005 has been dedicated to this invention.
Recently we presented frequency comb generation in circular microresonators made of fused silica. These sub-millimeter resonators are fabricated on microchips using processes that are well known from computer chip production and are promising elements for integrated photonic computers. However, to generate a uniform frequency comb, the time a photon needs for one round-trip in these resonators has to be stabilized. In our work we present in a remarkably simple approach how this stabilization can be done by controlling the optical power sent into the resonator and using a thermal effect that controls its effective size. Using this approach we were able control the average photon round-trip time during one second down to a level of 10-23 seconds (0.00000000000000000000001 s). This represents an important step towards phase stabilized on-chip frequency comb generators.
***
EQ10358
More Bandgaps in Periodic Structures
Periodic structures featuring forbidden bandgaps have found a great many
applications in diverse areas of physics and engineering. Almost all
bandgaps found hitherto are induced by the well-known Bragg resonance which
occurs between waves of identical field profiles transverse to propagation
direction. Physically, resonance can also occur between waves of distinct
transverse wave profiles. However, this non-Bragg nature resonance was
usually overlooked until its existence was proved in electromagnetic waves
[1]. Extension was recently made to sound waves in an axially hard-walled
duct [2]. In the present work we make a further extension, both
theoretically and experimentally, to surface waves in a water trough with
corrugated sidewalls. Our results demonstrate the coexistence of both types
of resonances in water-waves, thus concluding the ubiquity of the phenomenon
for classic waves. We also find that the bandgaps are highly tunable by a
simple geometric arrangement, and the non-Bragg bandgap can even be made as
wide as the Bragg one. What surprises us most, just as in the case for sound
waves, is the impressively greater transmission loss within the non-Bragg
gap, which shows the higher efficiency in localizing wave energy. The
richness of transverse modes in waveguides implies the feasibility of
implementing more bandgaps with improved techniques, thus opening a new
avenue of control over band structures.
***
LQ11818
A Hot Union
The phenomenon of superconductivity requires two essential ingredients: the
binding of electrons into pairs and the establishment of coherence between
the phases of the pairs' wave-functions. Unfortunately, systems in which
pairing is strong typically exhibit a low transition temperature due to
significant phase fluctuations. On the other hand, large phase stiffness is
commonly accompanied by weak pairing. An intriguing question then arises:
Is it possible for a composite system, made of a strong-pairing component
and a phase-stiff element, to inherit the best of the two worlds and posses
a transition temperature higher than those of both its constituents? We have
demonstrated that such an enhancement indeed takes place at the interface
between underdoped and heavily overdoped cuprate high-temperature
superconductors. Our results corroborate the notion that the underdopd
regime of the cuprates is governed by a high pairing scale and strong phase
fluctuations, while the overdoped region is more conventional in the sense
that pairing and phase order occur simultaneously. Interestingly, by varying
the doping level of the underdoped layer we found that the maximal
transition temperature of the bilayer is obtained at the same doping level
where the transition temperature of the bare underdoped films exhibits an
anomalous suppression. This suppression is usually associated with the
spontaneous segregation of the electrons into quasi-one-dimensional
"stripes", and our findings may shed new light on their role in the
mechanism of high-temperature superconductivity. From a practical point of
view, the approach pursued in this study may offer guidance for the design
of higher temperature superconductors.
***
LE11370BR
Watching superconductivity to appear in copper oxides
Summary:
High temperature superconductivity occurs when an
antiferromagnetic insulator is doped with charge carriers beyond a certain
level by
slightly modifying its chemical composition. In this paper we show
that, while the magnetic order disappears continuously, there
are discontinuous changes in the
electronic, lattice and magnetic properties at the onset
of superconductivity. This is not at all
obvious since the new state emerges at zero absolute temperature
where transitions are often governed by quantum fluctuations
favoring continuous variations. In our light scattering
experiments we follow the evolution of spin, charge, and lattice
excitations with doping. We find that all vary continuously
up to the onset point of superconductivity. Here, all of a
sudden spin excitations and one of the lattice vibrations become
strongly damped and the charges start behaving as those of a normal metal.
In many respects this is reminiscent of a first order phase transition
such as the one between ice and water.
***
LQ11301
Biodiversity in ecological systems is often maintained by the
self-arrangement of the interacting individuals into spatial patterns.
In our article, theoretically and generally, we investigate the effects
of such self-organizing spatial patterns, and find that, in most cases,
they support species diversity. However, we also identify a situation
where an instability of patterns results in rapid species extinction.
Our work builds on recent microbial experiments. There, three strains of
colicinogenic Escherichia coli display a competition similar to the
children's game "rock-paper-scissors". Growing on a (essentially
two-dimensional) Petri dish, spatial patterns form. In each spatial
region, one of the three strains dominates. These patterns help to
protect one strain from the others, and therefore enable stable
coexistence, in other words, the diversity. In our theoretical work, we
investigate a more general situation where three species exhibit cyclic
dominance. We demonstrate that, in most cases, the formation of spatial
patterns helps the maintenance of biodiversity (see Figure). In
contrast, and for the first time, we also show that in a certain regime,
the opposite is true: the self-formation of patterns leads to rapid
extinction of all but one species. We provide a fundamental
understanding of both effects in terms of an analytical description via
a complex Ginzburg-Landau equation.
***
LQ11182
RELATIVISTIC CONDITIONS FROM LONG-WAVELENGTH LASERS
It is shown in a recent PRL paper (Manuscript LQ11182) that
long-wavelength, strong-field lasers can provide access to
a relativistic domain of phenomena that has not previously
been examined. In particular, the transverse fields created
by a long-wavelength laser can produce an environment in
which the magnetic field of the laser becomes important.
With increased wavelength, true relativistic conditions can
be established. This is contrary to expectations based on a
theoretical analysis using a so-called "tunneling method",
thought to be valid for long wavelengths. The new PRL
article: 'Limits on tunneling theories of strong-field
ionization', shows that a tunneling method is not
applicable when the laser has a sufficiently long
wavelength. Instead, powerful long-wavelength lasers - such
as existing infrared FELs - can approach and enter a
low-frequency relativistic domain about which almost
nothing is now known.
***
BN10869
Large-scale Atomistic Simulation for Ferromagnetic Materials at Elevated Temperatures
Magnetic materials, on both nano- and macro- scales, have been used for a growing number of applications of increasing sophistication, ranging from the storage, recovery, communication, manipulation and processing of information, to quantum computing, and to irradiation damage-resistant ferritic-martensitic steels for nuclear reactors. However, modeling dynamical processes in these materials at high temperature and/or under irradiation proves difficult because the correlated dynamics of motion of atoms and spins is characterized by broadly similar timescales, making the evolution of the corresponding variables inseparable. The spin-lattice dynamics approach developed in this paper links the real-space motion of the atoms and the precession of their spins in one time-dependent simulation, yielding an interactive description of the lattice and spin subsystems.
The example simulations described in the paper include spin-lattice relaxation of domain walls, equilibrium and time-dependent spin correlation functions, temperature dependent magnetization curves for both the infinite periodic and finite-size atomic systems, the analysis of short range spin order below and above the Curie temperature, and the effect of magnetism on elastic modulus and thermal expansion of the material. The results show that the method will likely replace conventional molecular dynamics simulations in iron-based alloys, steels and other materials where magnetism strongly affects structural, mechanical, and various other properties relevant to applications.
***
LR11184
Do we know what it really means by alloy?
Alloying effect is generally considered well understood, and the subject
is obviously of less interest than nanomaterials these days.
Conventional wisdom tells us that an alloy state of a disordered
structure can be connected to a well defined Bloch state belonging to an
ordered structure in a so-called virtual-crystal approximation. In this
paper, we have revealed that such a understanding is invalid for alloy
states in general, based on a first ever performed systematic
examination of a prototype semiconductor alloy system Ga(x)In(1-x)P
throughout the whole composition range (0 < x < 1) and in a broad
spectral range. This material happens to be of one of the few most
important semiconductor alloy systems, if not the most important one, in
major technology applications: telecommunications, photovoltaics, and
solid-state lighting. It is the key component that leads to the recent
breakthrough in > 40% multijunction solar cell efficiency and offers a
realistic potential to exceed 45%.
***
LQ11940BR
THEORETICAL STUDIES OF ORGANIC INTERFACES AID THE DEVELOPMENT OF
ORGANIC ELECTRONICS
The strong push to use organic materials in the electronics and
optoelectronics industries means that an in-depth understanding of the
physical processes underlying the performance of novel devices such as
flexible displays and organic solar cells is required. In the present
work, the interface formed between one of the most widely used organic
semiconductors, pentacene, and the surface of a carbon electrode
(graphite) was studied using quantum-mechanical calculations.
Understanding such interfaces is critical since electronic devices
present multiple layers of materials and the interfaces between these
layers strongly control the motion of electrical charges across the
device and directly impact device performance. A novel methodology
capable of describing interfacial electronic processes was developed
based on a molecular-level picture; its validity was assessed through a
comparison with available experimental data. Subtle effects due to the
weak electronic interactions between pentacene and graphite were
uncovered and have significant implications for charge transport. The
current work is expected to help in the development of novel
optoelectronic devices by providing a toolbox of computational methods
allowing the description of key electronic processes.
Superconducting silane is layered
An international research team has predicted from first principles the
superconducting properties of silicon-based hydrogen rich alloy ┐ silane
(SiH4) with a layered structure, fueling up the possible realization of
metallization and superconductivity in dense hydrogen which has long been
a major driving force in high-pressure physics and remains an important
challenge in modern physics and astrophysics.
Silane offers a source for high purity silicon which is at the base of
electronics and microdevices. It is gas at ambient condition. The new
structure having layered network is formed when silane is subjected to
pressures above 600,000 times atmospheric pressure at sea level. The
researchers obtained the superconducting transition temperature in the
range of 20 and 75 K in the layered metallic phase. They demonstrated that
silane is a good example to metallize and superconduct hydrogen at modest
pressure much lower than necessary for solid hydrogen because it has been
already chemically precompressed.
The research, publishing in Physical Review Letters, suggested that the
layered feature could be essential for superconductivity in other
hydrogen-dominant compounds.
***
LQ11339
Light rulers on a microchip
A frequency comb is a laser source that emits a spectrum of many different discrete frequencies (corresponding to different colors), which are perfectly uniformly spaced and can serve as a ruler to measure optical frequencies. Frequency combs have become a universal tool for optical frequency metrology, spectrometer calibration, gas sensing and arbitrary optical waveform generation within the last decade and part of the Nobel price in physics in 2005 has been dedicated to this invention.
Recently we presented frequency comb generation in circular microresonators made of fused silica. These sub-millimeter resonators are fabricated on microchips using processes that are well known from computer chip production and are promising elements for integrated photonic computers. However, to generate a uniform frequency comb, the time a photon needs for one round-trip in these resonators has to be stabilized. In our work we present in a remarkably simple approach how this stabilization can be done by controlling the optical power sent into the resonator and using a thermal effect that controls its effective size. Using this approach we were able control the average photon round-trip time during one second down to a level of 10-23 seconds (0.00000000000000000000001 s). This represents an important step towards phase stabilized on-chip frequency comb generators.
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EQ10358
More Bandgaps in Periodic Structures
Periodic structures featuring forbidden bandgaps have found a great many
applications in diverse areas of physics and engineering. Almost all
bandgaps found hitherto are induced by the well-known Bragg resonance which
occurs between waves of identical field profiles transverse to propagation
direction. Physically, resonance can also occur between waves of distinct
transverse wave profiles. However, this non-Bragg nature resonance was
usually overlooked until its existence was proved in electromagnetic waves
[1]. Extension was recently made to sound waves in an axially hard-walled
duct [2]. In the present work we make a further extension, both
theoretically and experimentally, to surface waves in a water trough with
corrugated sidewalls. Our results demonstrate the coexistence of both types
of resonances in water-waves, thus concluding the ubiquity of the phenomenon
for classic waves. We also find that the bandgaps are highly tunable by a
simple geometric arrangement, and the non-Bragg bandgap can even be made as
wide as the Bragg one. What surprises us most, just as in the case for sound
waves, is the impressively greater transmission loss within the non-Bragg
gap, which shows the higher efficiency in localizing wave energy. The
richness of transverse modes in waveguides implies the feasibility of
implementing more bandgaps with improved techniques, thus opening a new
avenue of control over band structures.
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LQ11818
A Hot Union
The phenomenon of superconductivity requires two essential ingredients: the
binding of electrons into pairs and the establishment of coherence between
the phases of the pairs' wave-functions. Unfortunately, systems in which
pairing is strong typically exhibit a low transition temperature due to
significant phase fluctuations. On the other hand, large phase stiffness is
commonly accompanied by weak pairing. An intriguing question then arises:
Is it possible for a composite system, made of a strong-pairing component
and a phase-stiff element, to inherit the best of the two worlds and posses
a transition temperature higher than those of both its constituents? We have
demonstrated that such an enhancement indeed takes place at the interface
between underdoped and heavily overdoped cuprate high-temperature
superconductors. Our results corroborate the notion that the underdopd
regime of the cuprates is governed by a high pairing scale and strong phase
fluctuations, while the overdoped region is more conventional in the sense
that pairing and phase order occur simultaneously. Interestingly, by varying
the doping level of the underdoped layer we found that the maximal
transition temperature of the bilayer is obtained at the same doping level
where the transition temperature of the bare underdoped films exhibits an
anomalous suppression. This suppression is usually associated with the
spontaneous segregation of the electrons into quasi-one-dimensional
"stripes", and our findings may shed new light on their role in the
mechanism of high-temperature superconductivity. From a practical point of
view, the approach pursued in this study may offer guidance for the design
of higher temperature superconductors.
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LE11370BR
Watching superconductivity to appear in copper oxides
Summary:
High temperature superconductivity occurs when an
antiferromagnetic insulator is doped with charge carriers beyond a certain
level by
slightly modifying its chemical composition. In this paper we show
that, while the magnetic order disappears continuously, there
are discontinuous changes in the
electronic, lattice and magnetic properties at the onset
of superconductivity. This is not at all
obvious since the new state emerges at zero absolute temperature
where transitions are often governed by quantum fluctuations
favoring continuous variations. In our light scattering
experiments we follow the evolution of spin, charge, and lattice
excitations with doping. We find that all vary continuously
up to the onset point of superconductivity. Here, all of a
sudden spin excitations and one of the lattice vibrations become
strongly damped and the charges start behaving as those of a normal metal.
In many respects this is reminiscent of a first order phase transition
such as the one between ice and water.
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LQ11301
Biodiversity in ecological systems is often maintained by the
self-arrangement of the interacting individuals into spatial patterns.
In our article, theoretically and generally, we investigate the effects
of such self-organizing spatial patterns, and find that, in most cases,
they support species diversity. However, we also identify a situation
where an instability of patterns results in rapid species extinction.
Our work builds on recent microbial experiments. There, three strains of
colicinogenic Escherichia coli display a competition similar to the
children's game "rock-paper-scissors". Growing on a (essentially
two-dimensional) Petri dish, spatial patterns form. In each spatial
region, one of the three strains dominates. These patterns help to
protect one strain from the others, and therefore enable stable
coexistence, in other words, the diversity. In our theoretical work, we
investigate a more general situation where three species exhibit cyclic
dominance. We demonstrate that, in most cases, the formation of spatial
patterns helps the maintenance of biodiversity (see Figure). In
contrast, and for the first time, we also show that in a certain regime,
the opposite is true: the self-formation of patterns leads to rapid
extinction of all but one species. We provide a fundamental
understanding of both effects in terms of an analytical description via
a complex Ginzburg-Landau equation.
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LQ11182
RELATIVISTIC CONDITIONS FROM LONG-WAVELENGTH LASERS
It is shown in a recent PRL paper (Manuscript LQ11182) that
long-wavelength, strong-field lasers can provide access to
a relativistic domain of phenomena that has not previously
been examined. In particular, the transverse fields created
by a long-wavelength laser can produce an environment in
which the magnetic field of the laser becomes important.
With increased wavelength, true relativistic conditions can
be established. This is contrary to expectations based on a
theoretical analysis using a so-called "tunneling method",
thought to be valid for long wavelengths. The new PRL
article: 'Limits on tunneling theories of strong-field
ionization', shows that a tunneling method is not
applicable when the laser has a sufficiently long
wavelength. Instead, powerful long-wavelength lasers - such
as existing infrared FELs - can approach and enter a
low-frequency relativistic domain about which almost
nothing is now known.
***
BN10869
Large-scale Atomistic Simulation for Ferromagnetic Materials at Elevated Temperatures
Magnetic materials, on both nano- and macro- scales, have been used for a growing number of applications of increasing sophistication, ranging from the storage, recovery, communication, manipulation and processing of information, to quantum computing, and to irradiation damage-resistant ferritic-martensitic steels for nuclear reactors. However, modeling dynamical processes in these materials at high temperature and/or under irradiation proves difficult because the correlated dynamics of motion of atoms and spins is characterized by broadly similar timescales, making the evolution of the corresponding variables inseparable. The spin-lattice dynamics approach developed in this paper links the real-space motion of the atoms and the precession of their spins in one time-dependent simulation, yielding an interactive description of the lattice and spin subsystems.
The example simulations described in the paper include spin-lattice relaxation of domain walls, equilibrium and time-dependent spin correlation functions, temperature dependent magnetization curves for both the infinite periodic and finite-size atomic systems, the analysis of short range spin order below and above the Curie temperature, and the effect of magnetism on elastic modulus and thermal expansion of the material. The results show that the method will likely replace conventional molecular dynamics simulations in iron-based alloys, steels and other materials where magnetism strongly affects structural, mechanical, and various other properties relevant to applications.
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LR11184
Do we know what it really means by alloy?
Alloying effect is generally considered well understood, and the subject
is obviously of less interest than nanomaterials these days.
Conventional wisdom tells us that an alloy state of a disordered
structure can be connected to a well defined Bloch state belonging to an
ordered structure in a so-called virtual-crystal approximation. In this
paper, we have revealed that such a understanding is invalid for alloy
states in general, based on a first ever performed systematic
examination of a prototype semiconductor alloy system Ga(x)In(1-x)P
throughout the whole composition range (0 < x < 1) and in a broad
spectral range. This material happens to be of one of the few most
important semiconductor alloy systems, if not the most important one, in
major technology applications: telecommunications, photovoltaics, and
solid-state lighting. It is the key component that leads to the recent
breakthrough in > 40% multijunction solar cell efficiency and offers a
realistic potential to exceed 45%.
***
LQ11940BR
THEORETICAL STUDIES OF ORGANIC INTERFACES AID THE DEVELOPMENT OF
ORGANIC ELECTRONICS
The strong push to use organic materials in the electronics and
optoelectronics industries means that an in-depth understanding of the
physical processes underlying the performance of novel devices such as
flexible displays and organic solar cells is required. In the present
work, the interface formed between one of the most widely used organic
semiconductors, pentacene, and the surface of a carbon electrode
(graphite) was studied using quantum-mechanical calculations.
Understanding such interfaces is critical since electronic devices
present multiple layers of materials and the interfaces between these
layers strongly control the motion of electrical charges across the
device and directly impact device performance. A novel methodology
capable of describing interfacial electronic processes was developed
based on a molecular-level picture; its validity was assessed through a
comparison with available experimental data. Subtle effects due to the
weak electronic interactions between pentacene and graphite were
uncovered and have significant implications for charge transport. The
current work is expected to help in the development of novel
optoelectronic devices by providing a toolbox of computational methods
allowing the description of key electronic processes.
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