LM11654
Axion result all washed up
An anomolous report of a possible new particle is disproved by researchers
at Fermilab. The axion, a possible dark matter particle, was coined in the late
1970's after Axion brand detergent because it "cleaned up" some mysteries of
theoretical particle physics. In 2006, there was an anomolous positive signal that
an axion-like particle might have caused light to behave strangely in a magnetic
field. This observation motivated several world-wide experiments to try to
directly observe this possible new particle by seeing whether light could go
through the equivalent of a brick wall in a high magentic field.
In less than 1 year, with one of the smallest budget's for a recent Fermilab
experiment, a small group of researchers borrowed necessary spare parts including
a laser, powerful accelerator magnet, and circuit boards normally given away to high
schools to study cosmic rays. They have produced conclusive results that rule
out an axion interpretation of the anomolous signal reported in 2006. Their findings
appear in a Phys. Rev. Lett. article and online at gammev.fnal.gov.
***
LM11599
Engineering the mobility of molecules in small spaces
As civic planners and schoolteachers have long appreciated, the motion of cars on highways or children through hallways proceeds smoothly if lanes of traffic are formed. Engineers have now shown that a similar principle applies for the motion of fluid molecules in tiny channels. Specifically, computer simulations reveal that molecules can more easily move past one another if they first form "layers" aligned with the boundaries of the channels. Researchers have also discovered a way to determine which types of channel boundaries could promote the formation of the layers necessary for faster molecular transport. This fundamental advance could aid in the development of new technologies that rely on the flow of fluids through small spaces, including drug delivery devices, biological "lab-on-a-chip" components, and nano-imprinting tools.
***
LN11021
Measuring torsion components for the first time
For the first time, scientists have found a way to measure the size of
19 of the 24 quantities describing the peculiar warping of space and
time called torsion. This advance follows a theoretical investigation
into the effects that this spacetime warping has on particles.
Curiously, the strongest influence is on electrons, protons, and
neutrons, which have particular internal spin characteristics. The
effect on photons, which have very different spin behavior, is far
weaker. To amplify and measure the effects of torsion, one can study the
motion of large numbers of electrons with aligned spins, as has been
done using a spin-polarized torsion pendulum at the University of
Washington in Seattle. Alternatively, one can study the microwaves
emitted from atoms in a helium-xenon maser, such as the one at the
Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts.
It can discern changes in the spin orientation of neutrons in these
atoms with astounding precision. Theorist Alan Kostelecky and
collaborators from Indiana and Northern Michigan Universities used
results from their study and from these experiments to place the
first-ever bounds on 15 components of torsion, and the best-ever bounds
on the only four torsion components that had previously been measured.
***
LZ10234
Quantum mysteries for everyone!
In our letter, we present a new Bell inequality for two parties with
two measurements of arbitrary many outcomes. The inequality can be
related to the known Collins-Gisin-Linden-Massar-Popescu inequality,
however, it is slightly more general but nevertheless much simpler. It
not only has a simple form, but also its proof is so simple that it is
understandable to everyone.
We investigate the maximal violation of this new Bell inequality.
Naively one would expected that the so-called maximally entangled
states, which are in a sense the most nonclassical states, lead to a
maximal violation of the corresponding Bell inequality. However, we
give strong numerical evidence that for more than two possible
outcomes the optimal states are not maximally entangled. In contrast
to earlier work in this directing we also considered the case of
Hilbert spaces of dimension higher than the number of outcomes.
Further, the simple form of the new inequality enables us to extend
the numerical search to a very large number of measurement outcomes.
Interestingly, this gives evidence for a new “quantum Bell inequality”
which seems to be exhibited for those optimal states in the limiting
case of infinitely many outcomes.
***
LL11096
Single mode heat rectifier: Controlling energy flow between
electronic conductors
In an electric rectifier electron current between metals is restricted
to one way flow. Can we analogously rectify the energy current between
two, hot and cold, metals?
We describe here a simple model for thermal rectification between
electronic conductors, assuming the metals are coupled via an
intermediate radiation mode or through an electrically insulating
molecule, permitting only vibrational energy flow.
We analytically show that the onset of rectification in the system
is directly related to the nonlinearity of the electron gas dispersion
relation,
combined with some system asymmetry. When the metals have strictly
linear dispersion relation a Landauer type expression for the thermal
current holds, symmetric with respect to the temperature difference.
Spatial asymmetry can be practically introduced into the device by
using a metal-superconducting junction, or by applying a voltage bias
across the system.
Our electronic model can be mapped into a phononic picture
where the metals nonlinear dispersion properties translate into
anharmonic phonon-phonon couplings. Since dissipative reservoirs
typically contain nonlinear interactions, finite rectification of the
energy current between metals and dielectric surfaces is an inevitable
effect.
***
LK11746
High Capacity Hydrogen Absorption in Transition Metal -
Ethylene Complexes Observed via Nanogravimetry
Using a high resolution nanogravimetric method where mass changes on the
picogram level can be measured we have observed upto 12% by weight of
hydrogen (H2) absorption at room temperature in certain metal-carbon
complexes synthesized using ethylene as a precursor. While the maximum of
12 weight % is obtained with a titanium-ethylene complex many other
elements of the transition metal series complexed with ethylene show
significant hydrogen uptake as well. In addition to the large uptake we
also observe very rapid kinetics. The absorption process is completed
within ten to fifteen minutes of introducing hydrogen. By performing mass
spectroscopic analysis during the synthesis of these metal-ethylene
complexes we find evidence for a bound metal-ethylene species in the gas
phase which most likely is responsible for the large hydrogen absorbing
characteristic. This work is promising from the hydrogen storage
perspective since both ethylene and titanium are inexpensive and abundant.
Limited experiments performed to date where the sample has been heated to
120 degrees centigrade however have not revealed significant desorption.
Further, work along these lines as well as in nailing down the molecular
structure spectroscopically to better understand these materials is needed
before their promise for energy storage purposes can be realized.
***
LF11474
Biodiversity or Extinction?
How can cyclic dominance between three species support their survival?
Rock-Paper-Scissors is a game played worldwide by children:
Scissors cut paper, paper wraps rock, and rock crushes scissors.
This game is a fair play in the sense that the coin one
player loses, matches the gain of the winning player, so the
money is conserved and the bank neither loses or wins -
a zero-sum game in the economic language.
The game is also played by bacteria (E.coli) and
by lizards (Uta stansburiana), as has been
and studied extensively during the last decade.
But the way lizards and bacteria play in their
struggle for survival, is not fair, and,
moreover, lizards and bacteria play it differently:
While the total outcome of an interaction of
different strains is negative for the bacteria,
experiments indicate that the sum is positive
when two different types of lizards meet.
At this point a recent study in Physical Review Letters
by Jens Christian Claussen and Arne Traulsen
points out how the picture of such a game
is changed in well-mixed but finite populations.
If the sum is negative, survival would only
be possible from resource or spatial niches,
and in a well-mixed population of three species
all but one will go extinct due to fluctuations.
The picture changes if the outcome of the game
is positive, as for the lizards. Here the authors
demonstrated that for a positive-sum game
coexixtence is stabilized not only in the mathematical
limit of an infinite population, but even in a finite population:
Above a critical population size, which can be derived
analytically for several microscopic processes,
coexistence is stabilized. Such population sizes can be
of order 20 to several 100 and thus are not unrealistic.
Bacteria and lizards do not play a zero-sum game,
and in such a three-species system a positive-sum game
can sustain biodiversity.
***
LP10180
Electrons loose the face
Electrons in normal metals behave as free particles. They have effective mass and form a charged liquid known as Fermi liquid. However in new recently discovered materials, such cuprates (high-temperature superconductors) and ruthenates (rare-earth compounds) the electron’s behavior is drastically different. In the published paper it was shown that in cubic ruthenates electrons are loosing their personality. The matter arises because specifically in these materials these electrons have additional freedom, they may freely change atomic orbitals. In other words electrons are here additionally dressed by orbital fluctuations. These electrons are not anymore behaving as free individual particles but perform rather as teams, which are glued and covered in orbital dress. Electrons in such teams are acting together to form collective charge fluctuations in which individual electrons loose their faces. The evidence of this striking phenomenon and the collective behavior of electrons is presented in a detailed perusal of various experiments including Raman spectra, optical conductivity and Hall effect observed in cubic ruthenates, SrRuO3 and CaRuO3. It was also shown that as the result of this striking phenomenon a new state of matter - orbital (non-Fermi) liquid is emerging. The finding of this new state of matter is a unique manifestation of an important role of atomic degenerate orbitals in cubic ruthenates, and opens a new route to understanding of non-Fermi liquid metallic behavior in many others novel materials.
***
LG11163
HOT MOLECULES BROUGHT UNDER CONTROL WITH ULTRAFAST IONIZATION
Hot (room-temperature) molecules vibrate wildly due to thermal motion.
This vibrational motion could be harnessed to enhance or modify chemical
reactions, but since the motion is random each molecule is doing something
different. Trying to control this motion would be as difficult as herding
cats. Thus, most "coherent control" schemes start with cold molecules,
which are well behaved, but this makes possible applications much less
practical. In experiments involving room-temperature iodine molecules, we
have shown that by gently ionizing about half of the molecules, the
remaining neutral molecules are brought under control and they all vibrate
in unison. This "coherent" motion is striking revealed in data taken with
ultrafast (femtosecond) laser pulses in a "pump-probe" arrangement. (See
figure). Generally speaking, most laser interactions with molecules are
"reversible" and cannot handle thermal ensembles. In contrast, ionization
is "dissipative" which allows it to tame random motion. This method of
using ionization to control vibrational motion is closely related to a
process called "Lochfrass" which was recently demonstrated by another
group (Phys. Rev. Lett. 97, 103004 (2006)). In that work, initially cold
molecules were forced to start vibrating through weak ionization. Our
work opens up new ways to work with hot molecules.
***
LL11201
Slow dynamics in columnar discotic liquid crystals
In this work the assignment of the slow dynamics in discotic liquid
crystals pertinent to their long range organization is made. Discotic
liquid crystals based on hexa-peri-hexabenzocoronenes (HBCs)
("super-benzenes" due to their symmetry and aromatic core structure)
show record high charge carrier mobilities and find applications as
advanced electronic materials. HBCs substituted with flexible
aliphatic chains are known to self-assemble with the disc-shaped
molecules organizing into columns that further assemble into
two-dimensional arrays. This work revealed that these materials
exhibit "fast" and "slow" dynamics with intriguing temperature and
pressure dependencies associated, respectively, with the disc axial
motion and a collective re-organization of the columns. The latter
"breathing modes" are important for the stability of long-range
assemblies needed in electronics.
***
LM10902
The rough makes it smooth!
Would you believe that we should design rough surfaces to make them behave
in the smoothest possible way? In other words, would you ever imagine that
a rough surface may help in inducing a motion on the top of it, instead of
inhibiting the same? That, as a possibility, would indeed sound
unachievable, until we discovered from our recent study that specially
designed tiny water-transport channels (or pores) may achieve this
apparently impossible task by two simple mechanisms. First, confining
rough surfaces made of water-disliking materials may trigger the formation
of tiny bubbles adhering to the walls of narrow channels. This incipient
vapor layer acts as an effective smoothening blanket, by disallowing the
liquid on the top of it to be directly exposed to the rough surface
asperities. In such cases, the liquid is not likely to feel the presence
of the rough surface directly, and may instead sail smoothly over the
intervening vapor layer shield. Thus, instead of ‘sticking’ to a rough
channel surface, the liquid may effectively ‘slip’ on the same. Secondly,
the spontaneous formation of an electrically charged layer adhering to the
channel surface amplifies this tendency of slippage to a large extent, by
pumping the layer of fluid even more effectively along with the movable
charges. Based on this novel conjecture, we may design miniaturized
super-fluidic systems with an unimaginably high rate of liquid pumping,
without actually using any pumping device.
***
LL11056
The smallest crystal
Text books teach us that solid or liquid behavior are collective properties
of very large systems. But after cutting a crystal in two halfs it still
remains a crystal. So, how long can this procedure be repeated before
crystal behavior vanishes? Researchers from Kiel University in Germany have
solved this problem. The key was to pose the correct question which
adequately captures the nature of crystalline behavior and then derive the
proper quantity which one has to measure. Certainly, a crystal requires
regular arrangement of particles which have to be localized near their
lattice sites. But this is not enough: a crystal must also allow for its
abrupt destruction, when it is being heated or compressed. Thus, the
existence of a melting transition turned out to be the main criterion.
With the help of a novel sensitive quantity the researchers could clearly
analyze by computer simulations for what crystal size the melting transition
vanishes. The answer: the smallest crystal contains 5 particles.
***
LK11450
A watchful eye makes entanglement life longer
One of the most paradoxical phenomena of quantum theory, the quantum Zeno effect, allows to perform a further step in the implementation of new quantum technologies. Quantum computers and quantum networks crucially rely on exquisitely quantum properties such as entanglement. Entanglement is however very delicate. Any interaction of quantum systems with their surroundings destroys it. Our results illustrate how to fight
this deterioration using the quantum Zeno effect. The heart of the effect is that repeated and frequent measurements of the state of a quantum system, aimed at checking whether it is still in its initial state or not, freeze its dynamics. As the saying goes "A watched pot never boils".
Interestingly enough a similar conclusion holds if appropriate measurements are performed on the environment rather than on the system itself. Our results demonstrate that the entanglement of two quantum bits can be protected from the deterioration caused by the inevitable interaction with their environment simply by monitoring the environment. Instead of watching the pot we watch the stove flame. Specifically we consider two entangled two-level atoms in a lossy cavity and we prove that monitoring the population of the cavity mode leads to entanglement protection.
***
LJ11071
An exact solution for single-lane quantum Bose traffic
Everyone who has once been travelling in a car along a curved narrow road
knows the situation: the tractor ahead can not be overtaken without a
certain risk of terminating life. In a traffic jam, the 1-dimensional
character of a road is even more apparent. In the microscopic quantum
world atomic physicists knowadays create situations which resemble in many
ways the everyday rush-hour horror: Elongated, in the transverse
directions strongly confining laser traps constrain bosonic atoms to a
one-dimensional geometry to create so-called Lieb-Liniger gases studied
theoretically from the 1960s. Due to the inherent quantum nature of these
systems, the microscopic Bose-"cars" have a finite probability of passing
each other on this laser road, unless their effective mutual interactions
are in the so-called Tonks-Girardeau regime of impenetrable particles. The
authors of [LJ11071] recently found an exact solution for the
time-dependent Schrodinger equation for such gases. They described the
free expansion of an initially localised gas and showed that, as the
quantum traffic gets diluted, it approaches the "no overtaking"
Tonks-Girardeau limit - without the classical post-jam race between the
fastest roadsters.
-----------------------------------------------------
Figure:
Quantum traffic is described in terms of probabilities
of finding the "cars" at specific coordinates on the road;
figure shows one such probability in the intermediate stage
of the expansion.
***
LE11321
Universality behind Basquin's law of fatigue fracture
It has long been recognized by industry that structural components
exposed to periodic loading can fail after a certain number of
cycles even if the load amplitude is much below the safety limit. In
the everyday life the mysterious sudden breakdown of car or kitchen
equipment is a similar experience. The material seems to get
tired due to the long time usage and therefore the phenomenon is
called "fatigue". This subcritical failure typically occurs
unexpectedly and has been responsible for a large number of
airplane and railway crashes with considerable human loss. The most
striking quantitative feature of fatigue fracture is expressed by the
classical empirical Basquin law, which states that the lifetime
decreases as a power law of the load amplitude. The
Basquin law has remained unexplained since its discovery (1910) together
with the puzzling observation that the exponent of the power law
strongly depends on the material properties.
In order to understand the origin of Basquin's law, we worked out a
theoretical approach for the fatigue fracture of disordered
materials which provides a direct connection between the microscopic
fracture mechanisms and the macroscopic time evolution of fatigue. In
the model, material elements fail either due to immediate breaking or
undergo a damage accumulating ageing process. We found that
on the micro-level the competition of the two failure modes gives rise
to bursts of breakings which are characterized by universal power law
distributions. Astonishingly, the macroscopic Basquin law
appears to be the fingerprint of this scale-free microscopic bursting
activity, where material dependence enters only through the
specific damage accumulation mechanism. Furthermore we show that when
micro-cracks can heal leading to damage recovery, a threshold load
emerges below which only partial failure occurs and the material has
an infinite lifetime.
***
LG10991
Ear Reading Information from the Bumps in Your Ear
The external ears (pinnae) of many mammals - including bats and humans
- generate valuable information on the direction of an incoming sound
through direction-dependent acoustic diffraction by their intricate
shapes. In this paper, we establish an immediate and quantitative link
between the shape of the pinna and the direction information it
generates. This allows us to tell how much information a specific
pinna shape feature generates and in which way. We demonstrate the
utility of this approach by studying the role of an inconspicuous flap
on the inner wall of a bat pinna: We find that this flap alone
generates sufficient direction information to sustain - at least in
principle - the spatial accuracy observed in bats. We explain how this
is achieved through a fan-beam of sensitivity lobes which scan the
environment in a systematic fashion as the sound frequency is
changed. Since the basic principle of operation of the pinna is shared
by many mammals including humans, our methods could be applied widely
to mammalian and human hearing. Furthermore, features of comparable
relative size and geometry are common in bats as well as in other
mammals and could have similar effects to the flap studied here.
Attached figure: Fan-beam of sidelobes caused by the flap (a: flap
and sidelobes present, b: flap removed and sidelobes mostly gone).
***
LH11344
CAN DIAMOND BE A CONDUCTOR?
About twenty years ago, diamond, one of the best known insulators,
was found to exhibit substantial conductivity when exposed to air.
The origin of this intriguing phenomena has eluded explanantion and
remains uncertain. The resolution of this uncertainty is of immense
current interest since it could expand the technological use of
diamond into many new areas. The present paper makes a major headway
in this area using theoretical calculations by providing a mechanism
which could lead to the use of hydrogenated diamond surface as a
conducting material. According to our finding, the hydrogen adlayer
acts as a mediator in extracting electrons from the diamond
surface. This is achieved with the help of an additional layer of
water molecules (provided by the atmospheric air) adsorbed on the
hydrogenated diamond surface. The dipole moment generated on the water
layer adds an attractive component to the hydrogen layer which,
in turn, becomes an energetically favorable destination for the
electrons from diamond, leaving it with conduction holes.
***
LF11291
Stacking Matters
By and large, the electronic properties of LEDs and solar cells
fabricated from organic semiconducting polymers are determined by the
polymer chains themselves. However, how a bulk-heterojunction device
behaves may depend critically upon both how the chains are aligned and
how the internal vibrational motions mediate the breakup of excitons
into free carriers. Using a state-of-the-art fully quantum mechanical
time-dependent treatment of the combined electronic and vibrational
degrees of freedom for a model heterojunction system shows that subtle
shifts in the relative alignment of two pi-stacked polymers determine
how high vs. low-frequency motions within the polymer chains effectively
couple and drive the transfer of an electron from one chain to the other.
The computational treatment hinges upon the use of an electron/phonon
coupling scheme that finds the most significant vibronic interactions and
then adds in the remaining couplings through a hierarchical series of
equations. This allows what would ordinarily be an intractable computational
problem to be conveniently treated on a modest workstation. The authors
hope that their studies will spur a close investigation of the details of
the electronic couplings between molecules at the interface between
semiconducting domains, and will help to define new criteria for
material design.
***
LA11229
To buoy or not to buoy, both for a good reason
Intentional introduction of magnetic impurities or "dopants" into a
semiconductor often results in unwanted precipitates that are considered
detrimental to device performance. A conceptually intriguing doping
scheme, based on the combination of dopant trapping and surfactant
action, has now been shown to not only produce a precipitate-free
germanium semiconductor but also to transform the host germanium into a
surprisingly strong ferromagnet.
To make a semiconductor such as silicon or germanium magnetic for
spintronic applications, one intentionally introduces magnetic
impurities, such as manganese. Of course, it would be highly desirable
if the semiconductor could stay magnetic up to at least room
temperature, but this typically requires excessive manganese levels of
at least a few percent. Such high doping levels are detrimental to the
structure and properties of most semiconductors and, consequently, to
their device performance.
In Physical Review Letters of Feb. ??, 2008, Zeng and coworkers
establish subsurfactant epitaxy as a conceptually new approach for
introducing manganese as a magnetic dopant into germanium. The result is
a substantial enhancement of the magnetic transition temperature, along
with a significant reduction of the impurity levels needed for
establishing magnetism. The crux of the method is to suppress the
thermodynamically favoured formation of clusters and precipitates. The
authors devised a kinetic pathway in which manganese atoms are
intentionally trapped at low temperature below the surface of a
germanium crystal. The crystal is subsequently warmed to slightly above
room temperature and covered with additional germanium atom layers.
During this capping process, trapped manganese atoms become unleashed
and display amazing buoyancy. Their buoyancy is driven by the energetic
preference of the manganese atoms to remain one atomic layer below the
surface, as was predicted in a Physical Review Letters paper a few years
ago. In a way, the "floating" manganese atoms preferring subsurface
sites behave like the surfactant molecules in laundry detergents
floating at the water surface. This is the reason the authors dubbed
their growth method "subsurfactant epitaxy."
Not all atoms manage to stay afloat, however. Some manganese atoms
remain trapped inside the film, but luckily these settled atoms end up
being distributed evenly throughout the film, becoming magnetic dopants.
The resulting doping levels of order 0.25% are normally considered too
low for ferromagnetic ordering; nonetheless the films are still magnetic
at room temperature, an observation that is striking in its own right.
Whatever the underlying mechanism of the surprisingly strong magnetism
may turn out to be, subsurfactant epitaxy promises superior dopant
control in magnetic semiconductors and other semiconductor applications
that require doping levels above the thermodynamic solubility limit.
***
LL10927
Another promising candidate in tomorrow's quantum key distribution systems
So far, an attenuated laser is mostly used as the light source in quantum
key distribution systems. In this paper, we experimentally demonstrate
that a sub-Poissonian distributed heralded single photon source from
parametric down-conversion processes will be another promising candidate
when combined with the so called decoy state method. At first, by
comparing with other practical schemes in numerical simulations, we show
that our scheme using heralded single photon source based on decoy state
method can overcome all of them, either in a secure key generation rate or
in a tolerable total loss, and its performance can even come close to an
ideal single photon source, when a proper heralded single photon source
being used. Then we realize this scheme in experiment, and in principle
demonstrate the advantages of it with our present high lossy system.
Besides, our scheme does not pertain more costs or technological
requirements than any other practical scheme. Therefore, it should be very
competitive in the implementation of the quantum key distribution in the
near future.
***
LP10180
Extinction of electrons and formation of orbital liquid in Cubic Ruthenates.
Electrons in normal metals form a fundamental state of matter known as Fermi liquid. Inside such a liquid the Coulomb electron-electron interaction vanishes and electrons are free although have lighter or heavier effective mass. For half a century, the Fermi liquid idea played a pivotal role in understanding electronic behavior in metals. However, the discovery of new materials, such as high-temperature superconductors and rare-earth compounds with qualitatively different properties has forced the emergence of a new paradigm. In this paper, a detailed perusal of various experiments including Raman spectra, optical conductivity and Hall effect observed in cubic ruthenates, SrRuO3 and CaRuO3, has been performed. Then, deep underlying anomalies in a broad variety of their physical properties have been revealed and shown that strong, multi-orbital electron-electron interaction is leading to extinction of the electron-like quasiparticles, which are elementary excitations of the Fermi liquid and the signature of the Fermi liquid. As the result a new state of matter - quantum orbital (non-Fermi) liquid is emerging. The comparison with existing experimental data shows that the proposed orbital liquid provides a quantitatively accurate description of these data and all similarities observed in these materials. The finding of this new state of matter is a unique manifestation of an important role of electron-electron interaction and orbital fluctuations in ruthenates, and opens a new route to understanding of non-Fermi liquid metallic behavior in many others new materials.
***
LG11986
Estimation of Friction of a Molecule on a Surface by using Hammer Atoms
The friction of a single molecule chemisorbed on a surface is a very
fundamental property and plays a crucial role in diverse applications at a
nano-level or a molecular level since it affects the formation mechanism of
self-assembled molecular films and surface-bound nanostructures, as well as
the diffusion rates and hence the reaction rates of chemisorbed adsorbates
in catalytic reactions. However, studies of molecular level friction have
been limited to physisorption systems at present. In this report, in order
to investigate the friction of CO molecules on a surface, we used a newly
developed method, in which energy-controlled Ar atoms collide with CO
molecules on a stepped surface to displace CO molecules from initial
terrace sites to final step sites and the number of CO molecules at step
sites is compared with classical molecular dynamics simulation results.
Using the estimated friction, the relation with the adsorption dynamics,
the jump length and lifetime of translational motion is discussed.
***
LH10869
Carbon-Based Semiconductors Show Unique Sensitivity to Magnetic Fields
The amount of electrical current flowing through organic (carbon-based) semiconductors can be strongly modified by a magnetic field, an effect called magnetoresistance. In this paper, we report experiments demonstrating the unique property of organic semiconductors to show an inversion of magnetoresistance, i.e., in one set of conditions, the current through an organic semiconductor may increase when a magnetic field is applied, but small changes to the temperature, voltage or thickness of the organic semiconductor can cause the current to decrease, instead. Additionally, the authors apply the MIST model (magnetoresistance by the interconversion of singlets and triplets) they developed to describe the quantum mechanical phenomena responsible for the increase or decrease of current in the presence of a magnetic field. In contrast, electric currents flowing through inorganic semiconductors used in common microchips, such as silicon or gallium arsenide, are largely insensitive to magnetic fields.
***
LK11639
Interferometry: atoms get closer to photons
A step forward in high precision interferometry has been realized by employing for the first time non interacting atoms in a Bose Einstein condensate, the closest analogous to photons in a laser.
In the last 90 years optical interferometry has enormously contributed to the development of both experimental and theoretical physics. Interferometry is not only limited to photons but can be performed implementing massive particles, allowing high precision measurement of gravitational and inertial forces. The recent realization of Bose Einstein condensation, the matter wave analogous of the optical laser, has produced much excitation in the scientific community due to the diffuse expectation that this discovery would have led to a revolution similar to the one produced by lasers in the field of optical interferometry. However this didn’t happen because atoms, contrary to photons, experience mutual interaction which, at the high atomic density achieved in typical condensates, leads to a rapid destruction of the interferometric signal (A).
An experimental team at LENS, University of Florence, has now succeeded in performing an interferometry experiment with a condensate of potassium 39 atoms, where the deleterious effects of the interactions are cancelled. Applying a proper external magnetic field, the collisional properties of the atoms can be tuned at will and the interactions even cancelled. The sensitivity of the interferometer results enormously increased, achieving a higher contrast of the interference fringes (B). The technique developed in this work paves the way towards the realization of a new generation of sensors based on atom laser interferometry.
***
LL10945
Chaotic coordinates for chaotic dynamics
The classical approach to the study of (Hamiltonian) dynamical systems is to
adapt a coordinate system to the dynamics itself. This is like stretching and
squeezing a map (perhaps a map made from elastic) until all the roads and
streets are straight. This approach works when the dynamics itself is
regular; however, for chaotic behaviour one must employ chaotic coordinates.
An article to appear in Physical Review Letters adapts a coordinate system to
the invariant "signposts" of the fractal dynamics. Not even chaotic
coordinates can straighten chaotic dynamics, but chaotic coordinates can
separate out the almost-straight dynamics from the seemingly random chaos.
This effectively solves the problem of transport in chaos, with the (fractal)
devil's staircase transport profile naturally emerging.
***
LD11801
Swimming in circles
It is generally believed that in order to generate waves, a small
object (like an insect) moving at the air-water surface must exceed
the minimum wave speed (about 23 centimeters per second). In this
letter, we show that this result is only valid for a rectilinear
uniform motion, an assumption often overlooked in the literature. In
the case of a steady circular motion (a situation of particular
importance for the study of whirligig beetles), we demonstrate that no
such velocity threshold exists and that even at small velocities a
finite wave drag is experienced by the object. This wave drag
originates from the emission of a spiral-like wave pattern. The
results presented in this letter should be important for a better
understanding of the propulsion of water-walking insects. For example,
it would be very interesting to know if whirligig beetles can take
advantage of such spirals for echolocation purposes.
***
LL11172
Strain-controlled electrical behaviour in oxide thin films
In this paper, we reveal the extraordinary sensitivity of the electrical
conductivity of La0.7Sr0.3CoO3 films towards an elastic biaxial strain.
Reversible strain control of the resistance of epitaxially grown films by
about a factor of 10 at room temperature raises hopes for the application
potential of such perovskite cobaltites being related to the colossal
magnetoresistance manganites.Thin films of La0.7Sr0.3CoO3 have been
epitaxially grown on various substrates inducing tensile or compressive
in-plane strain up to 2%. A piezoelectric substrate has been employed for
reversible strain control. Strain-dependent electrical conductivity and
magnetization data of the films are presented. We find an extreme
conductivity decrease by 8 orders of magnitude under tensile strain and
prove this strain effect using measurements under reversible strain.
Further, a microscopic mechanism is proposed. We believe that this is the
first observation of the huge strain influence on the electrical nature of
doped perovskite cobaltites. It may initiate research activities in this
material belonging to the family of strongly correlated electron compounds,
which bear the promise of a new oxide electronics.
***
LL11215
Squeezing Waves through Narrow Tight Channels
Common sense suggests that it is difficult to squeeze light and other electromagnetic waves through extremely tiny bottlenecks. However, in their earlier theoretical papers [Phys. Rev. Lett., 97, 157403 (2006), Phys. Rev. B., 76, 245109 (2007)], Silveirinha and Engheta showed that these bottlenecks may be overcome, provided that the channel is filled with metamaterials with near-zero dielectric constant wherein waves passing through suddenly speed up to infinite phase velocities and squeeze through with near perfect transmission. Even more curious and counterintuitive is the fact that in this scenario the narrower and tighter the channel is, the better the wave may tunnel through! Now, two groups, Smith’s group from Duke University [Phys. Rev. Lett., 100, 023903 (2008)] and Engheta’s group from University of Pennsylvania [Phys. Rev. Lett., LL11215, to appear on Feb 1], have independently verified experimentally this anomalous phenomenon at microwave frequencies, using two different approaches. These findings confirm that materials with near-zero dielectric constant may provide useful means for connecting two waveguide sections at any angle and thus re-routing and re-directing electromagnetic energy through bending waveguides, sending waves through a very tight region, and designing low-reflective waveguide junctions, with potential applications in microwave component design, nanophotonics, and optical routing.
***
LH11449
Multiwalled carbon nanotubes: the thicker, the softer
Size matters for the mechanics of multiwalled carbon nanotubes (MWCNTs).
It has been known for some time that MWCNTs often wrinkle under
deformation exhibiting the so-called rippling deformation pattern, which
makes MWCNTs much softer. Through large-scale multiscale simulations we
have characterized with a power law the softer wrinkled response, and
showed that the transition strain between the super-stiff behavior
attributed to MWCNTs and this softer regime scales as the inverse of the
tube diameter. Thus, the tera Pascal Young’s modulus can be fully
exploited in devices and materials only for moderately sized tubes.
Similarly, in interpreting experiments or designing devices, the
classical Euler-Bernouilli beam theory can only be applied to such
tubes. The elasticity of thicker tubes is nonlinear, typically display
mixtures of wrinkled and unwrinkled sections, and often exhibit
hysteretic mechanical behavior.
***
LH11195
Sticky obstacles to intramolecular energy flow
Poincare, when defeated by the task of following the intricate way
chaotic orbits behave, especially in multidimensional systems, pinned
his hopes on periodic motions by stating farsightedly: "In fact, what
makes these periodic solutions so precious to us, is that they are, so
to speak, the only breach through which we can try to penetrate in a
place which, up to now, was thought to be inaccsssible". His serendipity
has been proven once more in a surprising context, namely chemistry.
Chemical reactions usually proceed through a complex choreography of
energy flow processes that deliver the needed vibrational energy to the
reactive mode. The manner and time in which energy travels determine the
outcome of the reaction and the properties of the products. The
conventional wisdom concerning this fundamental process is that
vibrational energy travels very fast and, well before a reaction takes
place, distributes itself statistically among the modes of the molecule,
assumed to resemble an ensemble of coupled oscillators. However, there
is increasing evidence that the approach to equilibrium usually proceeds
more slowly than predicted by statistical theories and it is also
nonuniform, showing intriguing fits and starts. If the initial energy
were concentrated in one of the periodic motions of the molecule--a very
unlikely event-- it would stay there. It turns out, however, that
periodic motions influence the approach to equilibrium nevertheless,
because in their neighborhood the system mimics the dynamics of this
periodic motion, at least for a short time. In our recent Letter, we
translate this qualitative insight into vibrational energy bottlenecks
in molecules and describe how and for how long they trap energy, and how
they release it. The bottleneck mechanism could also be operating in the
dynamical evolution of Mars-crossing asteroids, superradiant
instabilities in atomic gases, and the approach to equilibrium in
systems with long-range interactions, to name a few higher-dimensional
systems.
***
LL11260
Electron motion captured by a quantum stroboscope
We demonstrate an attosecond (1 as = 10-18 s) quantum stroboscope capable of guiding and imaging electron motion on a sub-femtosecond (1 fs = 10-15 s) time scale. Just as a conventional stroboscope can be used to freeze the beating of a hummingbird’s wings, revealing details that would normally be blurred, we use the quantum stroboscope to record the electron momentum distribution from a single ionization event. Our technique is based on a sequence of identical attosecond pulses that are used to release electrons into a moderately strong laser field exactly once per laser cycle. With this periodicity, each pulse creates an identical electron wave packet which adds coherently to the measured signal, with the result that the properties of an individual electron wave packet can be studied stroboscopically. In our paper we present an experiment in which we have used this technique to guide ionized electrons back to their parent ion and image the scattering event. We envision that coherent electron scattering from atoms, molecules and surfaces captured by the attosecond quantum stroboscope will complement more traditional scattering techniques since it provides high temporal as well as spatial resolution.
