Quantum Electromagnetically Induced Transparency

This article reports the realization of a powerful quantum optical
effect -known as Electromagnetically Induced Transparency- with an ensemble
of electrons in a semiconductor. The study used optical transitions of
electrons bound at donor sites, and a resonant laser field is usually
absorbed in this medium. This absorption was suppressed by driving a
parallel optical transition at the same time with another laser. The
absorption disappears because destructive quantum interference in the
systems' dynamics prohibits excitation, which can occur when the electrons
are driven in a quantum superposition of spin-up and spin-down that is just
right for avoiding absorption. Realizing this effect in an ensemble with
many electrons removes the need for using optical cavities for controlling
strong interactions between spin states and optical fields. Work in the
field of atomic physics (with vapors of Rubidium or Cesium) showed that this
gives access to very robust quantum optical control, and realizations of
slow-light, storage of light, quantum communication, and preparing quantum
entanglement between spins that are separated by a large distance. By
implementing this physics in a semiconductor these applications can be
strongly miniaturized.


***

LT12358

Measuring Qubits

In this paper, we have discovered a device that can be probed at both extremely low powers, where transitions between single quantum levels dominate, and high powers, where the system responds classically. Surprisingly, the macroscopic response of the system for large drive powers is dictated by the quantum state occupied by the system immediately before the measurement. Intrinsically quantum effects are thus easily observed in a classical signal. Moreover, the system is extremely simple: it requires only a single Josephson junction coupled to a resonator with no extraneous parts needed to control the transition to the classical regime, as in other circuits; essentially, it is a "qubit", or artificial atom, which acts as its own amplifier. We have also shown how this novel behavior easily lends itself to joint measurements of three or more qubits simultaneously. This measurement scheme is a significant improvement over prior readout schemes, taking readout fidelity from ~5% in this device to almost 90%. The phenomenon we report is unique in the simplicity and elegance that intrinsically quantum effects can express themselves macroscopically.

***

LT12770

Quantum plasticity and supersolidity

We have discovered that a helium-4 crystal with no impurity at all is anomalously soft. Its plasticity is large, due to quantum effects. This is because it contains dislocations which can move macroscopic distances (a fraction of a millimeter) at high speed (several meters per second). Dislocations are lines running through the crystal, where the stacking of atoms is disordered compared to the rest of the crystal. In classical crystals all atoms are completely frozen at low temperature. But in quantum crystals such as helium-4, quantum fluctuations are large and atoms can jump by quantum tunneling from site to site, especially along dislocations where the packing is not as compact as elsewhere. Quantum tunneling is a well documented process in which particles go through energy barriers without any dissipation because of their wave-like character. Highly mobile dislocations are able to reduce the stiffness of helium-4 crystals by one order of magnitude.
However, very tiny traces of helium-3 impurities are sufficient to stop the motion of dislocations when they attach to them below temperatures of order 100 millikelvin. Apparently, this is what drives these crystals to a Òsupersolid stateÓ, a highly debated new state of matter which may be the consequence of mass flow along the core of dislocations when they stop moving.


***

LF12863B

'Dirty' Device Offers New Perspective on Electron Transport


The transistors that have enabled the information age have been so successful in part because of the near-perfection with which they can be made. Some effects, however, require a little dirt. Taking an unconventional approach, researchers have now designed a semiconductor device to be extremely sensitive to material imperfections (and, for good measure, they deliberately added impurities to the sample). In the resulting device, electrons confined to a single layer, called a quantum well, became a hundred times more likely to escape from the layer if they are scattered by a defect. The researchers then showed that this unusual behavior could be used like a periscope to probe how the electrons were moving in the layer simply by measuring how quickly they escaped. This demonstration is interesting not only because it highlights a relatively unexplored type of electron transport but also because it offers a fundamentally new approach to investigating the many peculiar quantum effects, such as the quantum Hall effect, that occur when electrons are trapped in a single layer of a semiconductor.

***

LQ12519

The unbearable weight of the vacuum

Modern physics has unveiled an incredibly rich structure for what
was thought in classical physics to be the most uninteresting of
the states: the vacuum. From the early Dirac sea of negative-energy
states to the picture of virtual particles constantly being created
and annihilated, the vacuum has acquired conceptual importance for a
consistent description of nature. Yet, from the observational
point of view, its existence continues to be almost as evasive as in
classical physics, demanding carefully designed experiments in order
to detect its subtle effects (e.g., the Casimir effect). Here, in
contrast, we use the recently unveiled “vacuum awakening” mechanism
[Lima and Vanzella, Phys. Rev. Lett. 104, 161102 (2010)] to show
that the gravitational field of some neutron stars may force the
quantum vacuum to become, in the lapse of a few milliseconds, as
dense in energy as the stars themselves (which are already the most
dense objects known to exist). This "extra weight" would destabilize
the star, possibly leading to events of astrophysical proportions
fueled by the vacuum. Reversing the argument, the mere observation
of stable neutron-star configurations may be used to rule out the
existence of certain kinds of fields in nature (those for which the
vacuum awakening effect would have been triggered). In a Universe
where 95% of the energy content is unknown, such a simple method to
discard fields provides a great deal of information.

LR12215

Optics in Curved Space

The impact of space-time curvature on the spatial evolution of light as
stated by general relativity becomes significant only on astronomic
scales or in the vicinity of large masses, making a direct experimental
observation in the laboratory impossible. In our paper we demonstrate
the effects of space curvature on light propagation in a table-top
experiment and, hence, develop optics beyond the limits of flat space.
Our approach is to abandon one spatial dimension and to confine light
propagation to two-dimensional curved surfaces. On a positively curved
surface like that of a sphere beams seem to attract each other, giving
rise to a lensing feature of space itself. In contrast, on a negatively
curved surface like a saddle beams look like repelling each other and
light spreads exponentially as it propagates. This is not only a new
kind of tool kit for integrated optics, but may serve as an intuitive
picture of the complex dynamics of light in general relativity.

***

LT12266

The benefits of probing continuously: It helps to measure fluxes while
maintaining validity of Fluctuation Theorems

According to quantum mechanics any measurement disturbs the measured
object. In
our research we focused on the flux of electrons through a nanoscopic
junction
connected to the electrodes of a battery, and discovered that important
properties, known as fluctuation theorems, continue to hold unaltered, even
when the flux of electrons is continuously measured. This means that certain
aspects of the dynamics of a quantum system remain unchanged even though the
state of the system is strongly altered by a measurement process. In the
case
of electron transport through a nano-junction the fluctuation theorems say
that some of the electrons go to the negative electrode while most of them
behave in the "normal" way and go to the positive one. These theorems also
quantify the number of exceptional events of electrons flowing in the
"wrong"
direction. Our discovery corroborates recent experimental findings
[Physical Review B, 81, 125331 (2010)], and makes the theory of quantum
fluctuation theorems applicable to a wide class of experiments, enabling
the --otherwise not contemplated-- advantageous continuous measurement
of fluxes.


***

LS12715


How nanostructures healthily cope with stress

We have discovered that, by fabricating self-assembled nanostructures on
pre-patterned substrates, we can "feed them" with material and let them
grow in size far beyond what is possible on conventional planar
substrates. For the considered nanostructures (self-assembled
semiconductor quantum dots), elastic stress is the main driving force
governing their "birth and fate". Once formed, dots eagerly compete for
the externally provided material. In this way some of them are able to
rapidly grow in size and need to find ways to release the increasing
amount of stress. In the end, when a dot reaches a certain critical
size, it usually relaxes by crystal defect introduction, which is
analogous to its "death", since defects are deleterious for dot
applications. Now, by guiding the formation of the dots at well defined
positions on a periodically patterned substrate and thus imposing an
equal sharing of the provided material, we find that dots are able to
cope with the increasing stress by cyclically incorporating large
amounts of material from the substrate and corresponding changes of
their shape. This allows them to keep growing in size while delaying
relaxation mediated by defects.

***
BS11331


Element-selective insights into Magnetism within reach of any Laboratory

In our work we present an experimental technique for element-selective and ultra-fast investigations of magnetism on the nanometer scale, which is going to be available in any optical laboratory in the near future. It overcomes the restrictions of previous existing methods that either require highly specialized and rare infrastructure, such as large-scale synchrotron radiation facilities, or lack in performance, such as laser-based techniques. Our method is based on reflectivity experiments in the extreme ultraviolet (XUV) range and benefits from recently developed table-top soft x-ray sources that manage to produce ultra-fast and coherent XUV light with moderate effort.
In our publication we investigate magnetic switching processes in multilayer systems consisting of thin magnetic films in the XUV region that have already been studied extensively in the visible range as well as soft x-ray region during the past decades, because of their high relevance for both fundamental research and technology. Just as this region of the electro-magnetic spectrum is located in between the visible and soft x-ray range, its interaction with matter combines the characteristics of both spectral regions potentially contributing an additional perspective. A deeper understanding of such materials paves the road to technologies for faster, smaller and more economical data processing and storage devices, for example.

DR10770L

Quasar alignment tests cosmic string hypothesis

An unusual observed alignment of quasars may be the first observation of the effects of defects in the early universe known as
cosmic strings. In 2005 Damien Hutsemekers at the University of Liege reported that polarized light from a large sample of
quasars (highly energetic regions in the center of forming active galaxies) tended to be aligned with polarized light of other
nearby quasars. Naively one would expect the direction of polarization to be random. Furthermore, the average direction of
polarization tended to rotate with distance. The Hutsemekers team has also recently shown that the direction of polarization is
correlated with the orientation of the galaxy itself.

Now, physicists have offered an explanation of this unusual observation tracing back to defects from when the universe was 10^-12 seconds
old. Cosmic strings (which are different from the fundamental strings of string theory) are a defect from a phase transition in
the early universe (much like cracks forming in an ice skeet as a lake freezes over from a liquid to a solid phase). These
strings imprint a magnetic field in the universe. Objects (such as quasars) that form in the vicinity of these strings will be
affected by and tend to align with the leftover magnetic field. Models of two-looped cosmic strings were
able to reproduce the observation of the Hutsemekers group quite well.

***

LT12893

Which way is up in empty space?

Current cosmological theories assume that Universe is the same in
all directions, but how good is this assumption? To look for a
possible preferred direction, we use an intrinsic arrow associated
with elementary particles called spin. Does the energy of a particle
change when the spin points in different directions? To answer this
question experimentally we have constructed an apparatus containing
two kinds of spins associated with helium-3 and potassium atoms, so
any mundane effects like magnetic fields can be canceled. The whole
1500 lb apparatus is rotated every 22 seconds to look for
orientation dependent effects. Because the Earth is moving relative
to the rest of the Universe, we can also test the equivalence
between space and time, a fundamental aspect of Einstein's theory of
relativity. Our results are 30 times more sensitive than previous
experiments but have yet to find any preferred direction. At our
present level of sensitivity, they are consistent with
present assumptions that there is no up in empty space, and time and
space are indeed equivalent.


***

LU12754
Trapped and stored: atomic nuclei reach for the stars

Exotic atomic nuclei, trapped in previously unknown excited states, have been discovered using an ion-storage ring. This is the first such application of a storage ring and represents the unique capabilities of the heavy-ion accelerator laboratory at GSI in Darmstadt, Germany. First, gold ions were accelerated to three-quarters the speed of light and fired at a beryllium target. Then individual fragments from the collisions were stored and monitored for periods of up to 30 minutes to give precise identifications of each nuclear species, and to measure their stability. The kind of trapped nuclear state found in this way only exists naturally in exploding stars, and laboratory observations may be useful in pinning down the explosion conditions. It is also hoped that the trapped states will be useful for the investigation unknown processes at the borderland between atomic and nuclear physics, perhaps leading to novel energy-storage devices. For the time being, however, the newly developed experimental techniques will be focussed on the exploration of the limits of nuclear binding, thus providing key information about the forces of nature that hold together the matter that we are made of.

ET10629

Bose-Einstein condensation in evolution of sexual populations

Driven by new experimental genetic information on epistatic interactions occurring between genetic loci (single-nucleotide-polymorphisms) researchers have formulated a new model for evolution of multi-loci sexual and diploid populations: Surprising relations with quantum mechanics emerge from this work.

First of all, the evolutionary dynamics can be captured by an effective “Feynman diagram of evolution of diploid populations” depicting the information flow encoded in the “gametic life-cycle”. Secondly, the system might undergo a phase transition in the Bose-Einstein condensation universality. Below this phase transition, at sufficiently high selection pressure, a finite fraction of pairs of genetic loci is fixed in the population.

Previous works limited to asexual populations, have shown that another phase transition in the Bose-Einstein universality class also occurs in asexual populations. Below this phase transition most of the population has the same genotype.

On the light of their results, the authors of the paper propose that condensation transitions in the Bose-Einstein universality class occur in evolution of haploid and diploid populations when there is a competition between processes enhancing genetic variation in the population(i.e. mutations or recombination process) and selection processes.

***

ES10702

Predicting Patchy Populations

Developing accurate tools to model population dynamics is very important if we wish to predict whether a population will become extinct or flourish. The logistic model, proposed by Pierre Verhulst in 1838, is a paradigm for representing population dynamics in a system limited by competition for natural resources. However, the logistic model fails to predict population patchiness which can be an important and common feature of biological and ecological populations. Our work shows that when births and deaths take place rapidly compared to movement, the population becomes patchy. These patches have a negative impact on the overall growth of the population and the classical logistic model fails to predict the population size. We provide a new correction to the logistic model, by taking patchiness into account, and show that we can accurately predict patchy population levels.

***

AU10551

Shedding new light on ultracold gases

In this paper, for the first time, we combine the rapidly advancing fields of quantum imaging and ultracold atoms. Quantum imaging uses special states of light with multiple photons to examine a system with greater accuracy than allowed by classical imaging. Here we apply these techniques to imaging of ultracold atoms. This is important as these systems of ultracold atoms are now an indispensable part of studies of many-body physics and may be used as building blocks for quantum computing, but to fully use these systems techniques must be developed that reveal more about their state. This paper shows how quantum imaging leads to greater resolution of the spatial state of the ultracold atoms, giving experimenters a way to gain more and better information about the state. We also introduce a novel technique using pairs of photon to determine correlations between the ultracold atoms in space, making it possible to clearly distinguish different states.

LM12449

Atoms now drive the light

In the interaction between light and atom, it is usually a strong light
field that pumps or drives the atoms from lower energy states to the
higher ones and back forth, an effect best-known as atomic Rabi
oscillation. Now a group of scientists from East China Normal
University in Shanghai, China reversed the roles of light and atoms and
achieved photonic Rabi oscillation by using a strong collective atomic
excitation from a Rubidium cell to coherently drive the frequency
change of light, an equivalent of energy state change for light.

Collective atomic excitation (CAE), also dubbed atomic spin wave, is a
form of coherent superposition state for a group of atoms. But
different from a Bose-Einstein condensate, which is a coherent state of
atomic motion for a group of atoms, CAE is about atomic inner states.
In fact, the concept of CAE first appeared in the phenomena of
Electromagnetically Induced Transparency (EIT) as atomic coherence [1]
and was later used for atomic storage, quantum memory and atomic
entanglement [2]. Now it is used to control the light field by inducing
transitions between photonic states with different energies. The
technique can be used to create frequency entangled state of photons
for quantum wavelength multiplexing.

***

LT12301

Epilayers Take Turns

You can park your car into a spot two ways: forward and backward. In the
language of crystallography this corresponds to two rotation domains.
How many equivalent ways can you fit a square on a hexagon? In crystal
growth this problem arises for cubic ferroelectric barium titanate
deposited on hexagonal zinc oxide, a prototype system for novel
transparent, non-volatile memory cells. The answer is three; the
material on top occurs in three different orientations, forming three
rotation domains. The inverse problem, hexagon on a square, yields a
different number of only two possibilities. All material combinations
fall into one of twentyfive categories, depending on the rotational
symmetries of the lower and upper layer. The number of rotation domains
has been derived now for all cases using group theory, helping in the
design of novel heterostructures. And nature follows the rule.
Sometimes, however, additional rotation domains occur - kind of as if
Nature occasionally also allows parallel parking.

***

LT12467

High T_c superconductivity down to the mesoscopic scale

In this paper we give the first evidence of a minigap imprinted in high
voltage conductance oscillations of self-assembled nanoscale YBaCuO grain
boundary (GB) Josephson junctions. Minigap is a code to unveil coherence
in nanostructures. The nature of high critical temperature superconductors
(HTS) promotes an intriguing ‘length scale hierarchy’ where the normal
state coherence prevails over the superconducting order induced in the
barrier of the junction. GB nano-contacts guarantee secure channels where
high energy nodal quasi-particles coherently interfere.
Equivalent conditions can be met by properly designed hybrid systems,
incorporating low critical temperature superconductors.
More is to expect from the confluence of HTS and nanophysics. HTS
Josephson junctions can significantly benefit of being adjusted at the
nano scale, promoting novel insights on the physics of strongly correlated
systems and guaranteeing novel functionalities in a fully hybrid platform.

***

LR12425

QUANTUM GRAVITY EFFECTS IN THE LABORATORY

In a paper to be published in Physical Review Letters a novel proposal
for simulating quantum gravity effects in the laboratory is outlined.
Quantum gravity is the "marriage" of the general theory of relativity
and quantum mechanics. So far the marriage has not yet achieved its
plenitude and is defying physicists for almost a century. It would
enable physicists to predict the outcome of situations where effects
of both quantum mechanics and gravity are important. One feature of
quantum gravity is that the speed of light fluctuates from point to
point in space. Direct experimental verification of such fluctuations
in a cosmological setting is virtually impossible. But, as pointed out
in the paper, effects analogous to those due to a fluctuating speed of
light in quantum gravity can be studied in controllable experiments
employing fluids with intrinsic randomness, as the speed of sound in
this type of fluids also fluctuates from point to point. Detection of
the predicted effects in fluids would open unexplored directions and
provide deeper insight into quantum gravity effects within a
laboratory setting with available technology.

LS12121

The smallest possible refrigerator


When it comes to refrigerators, size matters. Who hasn’t at least once in their life wished for a bigger fridge? However, who can say that they have wished for the extreme opposite – the smallest conceivable one? In our new paper, this is exactly what we do. It’s not a question of engineering, of how small can one build a refrigerator, but about the fundamental limitations that Nature may impose on the size of refrigerators. Is there a minimum size below which no refrigerator can work? We found there is none, and, using quantum mechanics, designed what is arguably the smallest possible refrigerator. It works extremely well too: it can cool as close as we like towards absolute zero. And while our main motivation was understanding fundamental limitations of Nature and not possible applications, as high-tech devices get smaller and smaller – nano-technology, quantum computers and so on - the smallest possible refrigerator may still find its way into your home.

***

Localizing from space the highest energy explosions made in our atmosphere

In our paper we report the first localization from space of the elusive
geophysical phenomena known as Terrestrial Gamma-Ray Flashes (TGFs).
These are very brief (up to a few milliseconds) but very energetic
bursts of radiation (photons and electrons) associated with violent
thunderstorm activity. For this work we used the gamma-ray imaging
detector on-board the Italian AGILE satellite, which is normally used to
image and study celestial sources in high energy gamma-rays. Instead of
celestial sources, we used an original data analysis technique to
observe the Earth and for the first time we measured the direction and
energy of photons associated with TGFs with energies above 20
mega-electronvolts, almost a factor 1000 larger than the energies used
in common medical radiography. Our results are important because: 1)
they confirm that TGFs are typically detected within 400 km from the
satellite footprint; 2) TGF photons can reach energies of 40
mega-electronvolt and above; 3) these findings pose constraints on the
not-yet clear production models, especially concerning the maximum
potential energy involved and the shape of the electric field at the source.

***


CUR1019

From exploding stars to meteorites: one atom’s tale

In everyday life, the element sulphur calls to mind lead-acid batteries for cars and rotten eggs, but other fascinating and far less understood connections happen in the universe at large. Consider the case of a particular kind of sulphur, "sulphur-30", which is built from 16 protons and 14 neutrons. In a wonderful example of the microscopic influencing the astronomical, the specific ways that these protons and neutrons stick together in sulphur-30 -also known as its "structure"- are connected to the evolution and outcome of distant stellar explosions, including the inclusion of their ejected "stardust" into meteorites that eventually arrive on Earth. Here on Earth, we have produced and studied sulphur-30 at Yale University, using a technique that gave a much clearer and more complete picture of its structure than
previously known. This new picture in turn now paves the way for an
improved understanding of the detailed role of sulphur-30 in exploding
stars and in the composition of meteoritic material.

LN11763

First discovery of light amplification in graphene

A phenomenon of light amplification was firstly found from hybrid structures of graphene/ZnO semiconductor and attributed to the coupling between graphene plasmon (plasmon: quasiparticle resulting from collective oscillations of free electrons in graphene or metal) and light from ZnO by a Korean research team led by Prof. Suk-Ho Choi at Global campus of Kyung Hee University in South Korea. Related paper was published today in internet edition of Physical Review Letters, one of the world-class physics journals. Graphene, a one-atom-thick planar-sheet crystal composed of carbon atoms, has recently received strong attention as next-generation unique nanomaterial in global science communities. Since graphene was discovered in 2004, most research has been concentrated on its structural and electrical properties, and large-area fabrication. Prof. Choi said, "This time, by the finding and the demonstration of the plasmon-light coupling effect through a series of experiments and a simple calculation, graphene became very promising for potential applications in highly-efficient optical devices such as LEDs and solar cells". The research output is now under review for a patent.

LU12552

Formation of spontaneous patterns in magnetic Bose-Einstein condensates

The spontaneous formation of spatial patterns has many examples in physics, most
prominently in the early universe. Now atoms at ultralow temperatures allow a
particularly detailed investigation of this process. While ultracold atoms
typically accumulate in the lowest energy state during the formation of a
Bose-Einstein condensate, it was now shown that magnetic Bose-Einstein
condensates can spontaneously produce matter waves in excited states.

In the experiments, a trapped, ultracold cloud of atoms is prepared such that
the magnetization of all individual atoms is zero. As time goes by, pairs of
atoms with zero magnetization may flip and produce a pair with magnetizations
oriented up and down. These flipped atoms now accumulate in states that
experience a box-like external confinement. Hence their shape resembles the
waves in a round glass of water, described by the well-known Bessel functions.
Moreover the formation of these patterns can turn a spatially symmetric and
unmagnetized cloud into an asymmetric, magnetized cloud. This process resembles
the early universe which started perfectly homogeneous after the big bang, and
developed all complicated structures by a similar spontaneous formation of
spatial patterns.




***

LU12735

New method to investigate light-dressed atoms and molecules and
instantaneous geometrical structures of molecules

Laser-assisted electron scattering (LAES) in a femtosecond intense laser
field was observed. The LAES is a characteristic electron-atom
scattering process in a laser field in which electrons can gain or lose
their energy through the scattering with a unit of the photon energy of
laser light. However, because of experimental difficulties, LAES
experiments in such intense laser fields whose intensity is high enough
for formation of the light-dressed atoms have not been explored over 30
years from the first observation of LAES in 1976. Our success in
measuring the LAES process in intense laser fields (~2 TW/cm^2) opened
up a new possibility to investigate light-dressed atoms and molecules
though the electron scattering. Furthermore, we propose to use this LAES
phenomena for time-resolved gas electron diffraction for probing the
change in the geometrical structure of molecules with high precision of
the order of 0.01 angstrom with extremely high temporal resolution,
which could be of the order of 1 fs if 1 fs laser pulses are employed,
which could be three-orders of magnitude shorter than the previously
achieved temporal resolution of gas electron diffraction experiments.
Ultrafast intense laser field science provided ultimate experimental
tools for atomic and molecular sciences.

***

LP12337

Quantum entangled "light beams" beyond entangled "photons"

While the entangled-photon pair is a key element in quantum information technologies, nowadays it also attracts much attention as a new type of excitation light source that leads to unconventional chemical reaction, nano-fabrication, and so on.
However, the generation rate of entangled photons is currently very low, and hence the generation of high-power and high-quality entangled photons has not been reported so far. Although simply the "high-power" generation has been reported in 2004 by using a resonant two-photon excitation in semiconductor, one of the entangled photons frequently disappears due to the inevitable absorption in the semiconductor, and it seems an unavoidable obstacle to realize the "high-power" and "high-quality" generation.
In this report, we have theoretically revealed that this problem can be overcome by a new type of super-radiance in a semiconductor nano-film, which significantly shorten the escape time of photons from the semiconductor film while maintaining the high generation power.
This is certainly an innovative scheme to generate the extremely high-power and high-quality entangled photons, and this result would open new research fields using entangled photons for a variety of purposes such as the next-generation technologies of nano-fabrication and chemical reactions.

LR12617

How does water get hot on cold comets?

Our recent laboratory study focused on at water molecules produced by splitting off a
proton from hydronium ions (H3O+) by addition of a slow electron - the
main process that forms H2O in dilute gas under space conditions at
temperatures as low as 10 to 100 K. The study showed that extremely hot
water molecules are created, corresponding to temperatures of 60 000 K or
more. This helps to understand previous observations by British
astronomers, who are speculating about the origin of spectral lines from
hot water molecules they find the gaseous cloud around comets, the coma.
For water formation from H3O+ ions, electrons also present in the cold
medium play a crucial role. In a cold collision, they first attach to
hydonium ions forming an unstable neutral intermediate, from which a
hydrogen atom splits off to leave H2O. These reactions were generated in
the laboratory study, using the institute's ion storage ring TSR to
project the breakup products from individual reactions onto a particle
counting detector. With a new technique, the water-producing reaction
channel was isolated and the pattern of breakup products revealed an
unsually high amount of energy in the generated H2O molecules.

***

ES10666

Visually checking if a time series is consistent with a mathematical
definition of chaos

Deterministic chaos has been researched for more than 30 years. It
attracts a lot of attentions since a simple rule generates complex
phenomena while its deterministic rule makes possible many applications
including short-term prediction and chaos control. One of its
mathematical definitions is Devaney’s one. Although it has been
investigated well mathematically, Devaney’s chaos has not been applied
to real observations of time series so far since its definition uses
arbitrary neighborhoods. In this paper, we relax the conditions of
Devaney’s chaos by using a fixed size of neighborhoods and interpret
these conditions by using recurrence plots, which visualize time series
data. Using the proposed conditions, one can tell whether a given time
series is consistent with deterministic chaos in the Devaney’s sense or
not by visually inspecting its recurrence plot. For example, we showed
in the paper that a real dataset obtained from a squid giant axon is
consistent with Devaney’s chaos (see the attached figure). The set of
the proposed conditions provides a quick and easy way for non-experts to
analyze the chaotic nature of time series.

***

BM11326

Chiral Symmetry-Breaking of a Platinum-Covered Surface

Formation of a chiral state of matter rests on twisted atoms such that left- and right-rotated variants exist, a principle which is also fundamental to many biological molecules. In finding a chiral surface that contains the catalytically active atom platinum, this may open the door to selective adsorption and processing of chiral molecules for only one of the two symmetry variants.
Recent studies of a single atomic layer of platinum on a silicon surface now unveil that the platinum atoms arrange themselves as trimers. Unexpectedly, the trimer orientation is twisted by 30° with respect to the substrate lattice. This can be seen by scanning tunneling microscopy at the atomic scale. The symmetry-breaking spreads over the entire surface, and thus forms a chiral system. It is a novel basis for studies aiming at adsorption of a mixture of molecules where only one chiral type docks onto the surface, with bearing for a wide range of chemical and pharmaceutical applications.

***

EQ10643

Bose-Einstein condensation in people's daily lives

There is an interested phenomenon in quantum world that a lot of Bosons (such as photon, helium-4 atom, etc) would condense into the lowest energy level once this Boson system approaches absolute zero. This phenomenon is called the Bose-Einstein condensation (BEC), which was firstly predicted by Einstein in 1920s. Nevertheless, our study shows that there also exists BEC even in people's daily lives, that is, the economic crisis, which has puzzled human for near 200 years. It is well known that the economic crisis almost always starts from economy overheated extreme market, which can be described by perfect competitive market. Our study shows that the perfect competitive market not only obeys Bose-Einstein statistics but also shall lead to BEC as society reaches full employment, that is, a lot of firms condense into the zero output level. As use of this study, early warning of economic crises can be achieved.

***

LP12424

Light microscopy with axial nanometer resolution

In recent years super-resolution methods based on fluorescence have broken the diffraction
barrier that limited the resolution of conventional optical microscopy. Due to the wave nature
of light the lateral image resolution was limited to about 200 nm and the axial resolution to
about 500 nm for visible wavelengths. In this letter, we introduce 3-Dimensional Supercritical
Angle Fluorescence Microscopy (3D-SAFM) − a novel optical microscopy method to achieve
nanometer resolution along the optical axis. The method utilizes the distance-dependent
influence of the microscope glass slide on the angular distribution of fluorescence emission. It
is accomplished straightforwardly by the parallel detection of fluorescence emitted into two
angular regions. The proportion of fluorescence measured below and above the critical angle
for total internal reflection of the sample/glass interface is used to determine the axial position
of emitters with an accuracy of up to one nanometer.

LR12229

COSMIC ACCELERATORS DISCOVERED IN OUR OWN GALAXY

Analyzing the data from the largest cosmic ray detector in history, scientists from UCLA and Japan have discovered evidence of natural nuclear accelerators at work in our own Milky Way galaxy.

Cosmic rays of the highest energies were believed to come from remote galaxies hosting gigantic black holes capable of consuming stars and accelerating protons to "macroscopic" energies, comparable to that of a bullet from a modern rifle. However, earlier this year, Pierre Auger Observatory has published a surprising discovery: many of the energetic cosmic rays are, in fact, nuclei, not protons. And the higher the energy, the more nuclei per proton the observers detect. This was totally unexpected because the nuclei, more fragile than protons, tend to disintegrate into protons on their long journey through space. Moreover, it is very unlikely that a cosmic accelerator of any kind would accelerate nuclei better than protons at these high energies.

The resolution of the paradox came from the analysis to be published in an upcoming issue of Physical Review Letters. Stellar explosions in our own Galaxy can accelerate both protons and nuclei, but, while the protons leave the Galaxy promptly, the heavier and less mobile nuclei get trapped in the turbulent magnetic field and linger longer than protons. As a result, the local density of nuclei is increased, and they bombard Earth in greater numbers, as seen by the Pierre Auger Observatory.

Stellar explosions capable of accelerating particles to ultra‑high energies have been seen in other galaxies, where they produce gamma‑ray bursts. The new analysis provides evidence that such powerful explosions took place in our Galaxy as well, at least a few times per million years.

The ultra‑high energy nuclei observed today have been trapped in the web of Galactic magnetic fields for millions of years, and their arrival directions have been completely randomized by the numerous twists and turns in the tangled field. However, the researchers predict, the protons escaping from other galaxies should still be seen at the highest energies and should point back to their sources, providing Pierre Auger Observatory with a valuable data for charged‑particle astronomy.


***


LR12740

Ultimate Laser power is limited by physical laws

During the last decades, the world’s maximal available intensity
of laser radiation has been growing impressively up to 10^22W/cm^2.
Now days there are some projects under discussion and funding which
are capable of its further increase in the nearest future. But are
there any natural fundamental limits for intensity of optical lasers?
In our paper we are showing that such limitations not only exist, but
will be very probably faced with the next generation of high‑intensity
laser facilities. The origin of limitation we are discussing in the
paper is related to electron‑positron pair production. It was believed
for a long time that such process in vacuum may become important only
at critical intensity level of 10^29W/cm^2, which is beyond the scope
of either present or perspective facilities. However, it was shown
recently, that since the focal spot of optical laser systems exceeds
the length scale, characteristic for QED, by many orders of magnitude,
the threshold for pair production from vacuum is generally lower than
it was commonly accepted before. Under special set‑up, this effect can
become observable already at the intensity level of 10^25‑10^26W/cm^2.
As intensity is increasing further, the pair production yield is growing
very sharply. Moreover, according to our paper, a new mechanism of pair
production, development of electron‑positron avalanche, must simultaneously
come into play. The latter effect can be viewed qualitatively in analogy
with the known phenomenon of breakdown in dielectric slab. As a result,
the avalanche of electron‑positron pair production must blow up the
focused laser field at intensities 10^26‑10^28W/cm^2 (depending on the
experimental set‑up), so that critical intensity becomes completely
inaccessible with optical lasers. Our conclusion confirms the early
conjecture of N. Bohr that critical QED electric field can never be created.


***

LT12953

TESTABLE PREDICTIONS OF STRING THEORY

String theory has yet to make readily testable predictions in high energy physics or cosmology, but in this paper we invoke the ''stringy black hole/qubit correspondence'' to derive new results in the field of quantum information theory, that can in principle be tested in the laboratory. Einstein called the entanglement between two quantum bits (qubits) ''spooky action at a distance''. Qubit entanglement finds application in quantum computing, teleportation, cryptography and communication. In the apparently separate world of quantum gravity, the Hawking effect of radiating black holes has also occupied center stage. Despite their apparent differences, recent work by the authors has established a correspondence between the two, in which the Bekenstein-Hawking black hole entropy is related to the amount of three-qubit entanglement. In this paper, we take things one step further and use the classification of black holes in string theory to solve an outstanding problem in quantum information theory, namely the classification of four-qubit entanglement, a subject of recent interest to experimentalists.

LP11980


Capturing Electrons and Asteroids

What do asteroid capture and double ionization have in common? A great deal, it turns out: A circularly polarized (CP) laser field hurls ionized electrons back at the core in the same way that comets and interplanetary debris make their way to planets. According to conventional wisdom, a CP field suppresses collision-induced double ionization and high harmonic generation since ionized electrons spiral away and therefore cannot revisit the core. A few experiments carried out with rare gas atoms in the past confirmed this belief, and the matter would rest there if it weren't for conflicting experiments showing the signature of electron-electron correlation in the double ionization of magnesium. We reconcile these seemingly contradictory results by finding the conditions for an ionized electron to revisit the core to ionize more electrons (or recombine to generate high harmonics). Ionized electrons can return through a moving saddle point which arises from the joint actions of the Coulomb potential and the laser field. Our results imply that the so-called "recollision" or "three-step" model, which is the keystone of strong field physics in linearly polarized fields, can also be valid in circularly polarized ones.

***

EP10647

To queue or not to queue? A social paradigm under the lens of
computational physics.

Why do certain cultures privilege ordered, one‑dimensional queues
before a ticket
counter while others tend to prefer two‑dimensional chaotic crowding:
is it only a matter of
social conventions? In this paper we introduce a simple agent‑based
Monte Carlo model
for assessing quantitatively issues of the like in crowd dynamics. Our
simulations show that,
while on average the two queuing habits yield equivalent series of
waiting times, in crowd‑queuing
inclined cultures your size is a plus ‑ the smaller agents get served
first. This effect may be
thought of as the equivalent of the Brazil nut effect as the agents
keep crowding and
redistributing round the counter. Besides the current application, our
model provides a simple,
yet powerful, alternative to the current molecular dynamics schemes
for the investigation of many
issues in crowd dynamics.

***

CSR1022


PUTTING THE MASS BACK INTO THE PROTON

It has long been widely held that the ground-state of the strong-interaction piece of the Standard Model; namely, quantum chromodynamics, is enormously complicated, populated e.g., by a sea of quark-antiquark pairs. This so-called vacuum quark condensate is 5-times more dense than matter at the core of a neutron star. In this paper we show that there is an alternative to this conventional picture -- the ground state is empty! Owing to the remarkable property of quark and gluon confinement, the quark condensate is entirely contained within the pions, protons and other hadrons that constitute the strong-interaction's experimentally observed spectrum. Within quantum chromodynamics we demonstrate that there is no leakage from the hadrons, and thus, contrary to conventional wisdom, there are no space-time-independent condensates permeating the universe. Amongst its many consequences, this paradigmatic shift has a huge impact on the cosmological constant paradox: it resolves a 45-orders-of-magnitude conflict between quantum chromodynamics and experiment. The zero-point energy of the universe just got a lot smaller.

LS12441

High‑precision directional and localized membrane poration on a single
cell surface by laser‑generated tandem microbubble

The collapse of two laser‑generated microbubbles is controlled with high
precision to produce directional microjets and associated microstreaming
and vortices in the surrounding fluid that can open up
transiently nano‑ to micro‑meter pores on a cell surface nearby.
Cavitation (i.e., bubble formation and oscillation induced by ultrasound
or laser) plays a vital role in many therapeutic applications. Yet, the
exact mechanism of action is not completely known. Researchers at Duke
University have now developed a novel method to investigate cavitation
bubble‑cell interaction at single cell level in a microfluidics channel
that can capture, for the first time, the entire process of
bubble‑jet‑cell interaction with pin‑point opening of cell membrane and
subsequent progressive diffusion of macromolecules into the target
cell. This method has great potential in directional and localized
gene, siRNA and drug delivery to single cells with a diverse range of
applications in bioengineering, drug screening and lab‑on‑a‑chip
devices, as well as in basic studies to better understand the mechanisms
of cavitation‑induced bioeffects.


***

ER10715

Contactless pumping of magnetic fluids

Medical drugs can be attached to magnetic particles. Magnetic fluids are
made from magnetic particles, and can be manipulated without touching
them by applying magnetic fields. In this paper we describe a new method
of continuosly pumping a magnetic liquid, using a periodically modulated
magnetic field. In this way, a flow can be maintained inside a closed
system, where moving components are not wanted or not possible. To
improve the efficiency by more than an order of magnitude, we make use
of a surface instability. Although the resulting pressure is far below
one bar, it is sufficient to drive for example some microfluidic devices
or lab‑on‑a‑chip applications. By utilizing our pumping mechanism, these
devices can be sealed from the ambient, which would be a requirement for
health care applications. Our publication points out the physics that
makes the pump work.

***

LR12478

Twisting Space‑Time by Relativistic Mechanism: Origin of Cosmic Magnetic
Field and Vorticity

The universe is filled with "vortexes" (such as galaxies, accretion disks,
stars and planetary systems, etc.) that clump and wind‑up with magnetic
fields. Strikingly absent in this rich narrative of growth and evolution of
the cosmic systems, is a satisfactory "universal" mechanism that could have
generated the original seed magnetic field. Because the explosive expansion
of the universe must immensely dilute the magnetic field strength, very
strong fields must have originated in the early universe. Exploiting the
space‑time distortion inherent in relativistic dynamics, we have unearthed
just the mechanism that, by breaking the topological constraint forbidding
the emergence of magnetic fields (vortexes), allows "general vorticities"
‑‑naturally coupled vortexes of matter motion and magnetic fields‑‑ to be
created in an ideal fluid. The newly postulated relativistic mechanism,
arising from the interaction between the inhomogeneous flow fields and
inhomogeneous entropy, may be an attractive universal solution to the
origin problem.

***

LT12743

Visualizing quantum effects in jellium with x‑rays

It has already been known for a century that electrons in metals can
only be described correctly by using quantum mechanics. However, until
now an accurate measurement of the underlying quantum state has proved
extremely challenging. In this article, we report how the extremely
bright x‑rays from a synchrotron light source can provide information
on how electrons correlate with each other in jellium, the simplest
type of electron gas.

Electrons are elementary particles obeying Fermi statistics, meaning
that each one must behave slightly differently from the others. This
property is directly reflected in the electron distribution as a
discontinuity at the so‑called Fermi surface. We report the first
quantitative experiment measuring the magnitude of this discontinuity
in jellium and compare the experimental value with the most accurate
theoretical predictions available. The study gives important
information for understanding electron correlation effects, which are
believed to play a key role in most of the unanswered questions in
condensed matter physics today, such as high‑temperature
superconductivity.

***

LG12460

1/f noise found optimal for information transport.

1/f noise, also known as pink noise or flicker noise, is often referred to as ubiquitous in nature.
Since its first discovery in semiconductor diodes it is in fact been found in numerous physical, biological, socio-economical complex systems, from electron devices to internet traffic.
It is characterized by its spectral properties, it is increasingly stronger at lower frequencies differently from the more common uniform white noise.
Why is it so widespread in nature?
Much work has been devoted to pinpoint the universal character of 1/f-noise,
in this paper we analyse the information transport properties of 1/f signals in the context of renewal processes and show that the ideal condition of 1/f noise corresponds to maximal information transmission rate.
More precisely we show that complex networks generating 1/f noise, known to be insensitive to harmonic perturbations, respond instead to similar 1/f-type stimuli and we argue that adaptive complex networks might therefore naturally be led to the ideal 1/f noise condition by optimization of shared information.

LS12752

Wire through Ice

In this work, the motion of a weighted wire encased in ice ‑ Thomson's
famous regulation experiment from the 19th century ‑ is simulated at the
molecular level. It is found that as the wire is set in motion by an
external force, the transition from a static to moving state changes
depending on the hydrophilicity of the wire, i.e., how easily water
covers the wire surface.
Ice has the peculiar property that it melts under high pressure even at
freezing temperatures. Therefore, a wire can be pushed through ice as
pressure in front of the wire locally melts the ice. The liquid water
produced by this pressure melting spontaneously surrounds a hydrophilic
('water‑loving') wire in a bubble but not a hydrophobic
('water‑fearing') one. Therefore, in order to move a hydrophobic wire,
one needs to push it harder than a similar hydrophilic one. This is an
unexpected feature of ice friction as typically hydrophobic surfaces are
the ones to slide easily on water.


***

LR12112B

Select desired mechanical and electical properties for carbon nanotubes
‑ It's all about geometry !

In this paper we found ‑ with the help of easily understandable
geometric arguments ‑ how the arrangement of carbon atoms at the edge of
the tubes influences the distribution of chiralities during the growth
of carbon nanotubes. Carbon nanotubes can be described as a sheet of
carbon atoms which are arranged in hexagons like in the board game
"Settlers of Catan" and then rolled up into a tube. They have
fascinating mechanical and electrical properties depending on the way
the carbon atoms of the nanotubes are arranged after rolling the sheet,
which we call chirality. Producing nanotubes nowadays always yields a
large number of different chiralities leading to a mixture of tubes with
different properties in a nanotube sample. The superior properties of
nanotubes can be strongly enhanced if only a specific chirality with
desired properties could be chosen. Many applications are even
impossible without using only a specific chirality. The paper will help
to understand how growth conditions need to be modified to grow a
specific chirality and might also be interesting as it opens up a whole
new view on hexagons and structures constructed from them.

***

ES10675

Peculiar properties of an evaporation process of globular clusters

We show that, in an escape of stars from globular clusters (the so‑called
evaporation process), the standard thermodynamic properties such as stellar
polytropes are not valid. In this paper, to examine the evaporation process,
we consider self‑gravitating open systems enclosed in a spherical container
with semipermeable reflecting walls, by means of N‑body simulations. Here we
clearly show that, in a lower‑energy region or for a rapid evaporation
process, the thermodynamic properties deviate from those for the stellar
polytrope. Nevertheless, we found that a negative specific heat occurs even
in such conditions.


***

BR11446

Suppressing the magnetism of Chromium

Chromium has fascinated physicists since it was discovered in 1953 to be antiferromagnetic, a magnetism where the magnetic field of each atom is opposite to its neighbors. Although an understanding of the electrons that conduct and cause the magnetism was worked out over 30 years ago, its’ behavior when a different element, such as vanadium, replaced some of the chromium and the temperature at which the transition from nonmagnetic to magnetic is suppressed from just above room temperature to absolute zero (-459.67o Fahrenheit) is much less understood. This scenario is known as quantum criticality and recent investigations of more complex quantum critical antiferromagnets discovered highly unusual properties, such as unconventional superconductivity. These discoveries make the understanding of this quite simple metal more urgent. In our paper we demonstrate that the relatively modest changes that Chromium undergoes near criticality are due mainly to the presence of a small group of electrically conducting electrons which are relatively unchanged by the quantum critical transition. These mobile electrons are scattered much less frequently than those electrons most closely associated with the magnetism and dominate many of the physical properties. Furthermore, we demonstrated that even for vanadium concentrations well beyond that required to quench the magnetism in chromium, quantum fluctuations, finite time and spatial extent where the magnetic state appears, tend to effectively scatter the electrons responsible for the magnetism in pure Cr demonstrating the importance of quantum criticality over wide regions of composition and temperature.

***

LR12115

Refrigeration using quantum statistics

It was known since the 1970’s that quantum statistics differing from the usual +/- 1 of Bose and Fermi Statistics is possible in 2D. Such “fractional” or “anyon” statistics is realized by quasiparticle excitations of fractional quantum Hall states formed by a two dimensional electron gas subject to a strong magnetic field. Among various quantum Hall states, the one with Landau level filling factor 5/2 is predicted to be particularly unusual, namely its quasiparticle statistics is “non-abelian”. For such non-abelian quantum states, there is a ground state degeneracy (and associated entropy) that grows with quasiparticle number, making this state ideal for fault-tolerant topological quantum computation. In spite of extensive research, non-abelian quantum states have not yet been confirmed in the laboratory, in part due to the limited experiments that can probe the 5/2 fractional quantum Hall effect in the bulk. In their recent Letter, Gervais and Yang have shown that if a quantum state is non-abelian, it should be possible to use its non-abelian entropy to achieve quantum refrigeration. What they have found for such state is that adiabatically increasing the number of particles in the system gives rise to a counter-intuitive cooling effect, as opposed to heating in a normal classical gas. While the authors hope it can be used to identify fractional quantum Hall states that are non-abelian, they note that the underlying principle of this refrigeration is general, and should exist in any systems that support excitations that gives rise to ground state degeneracy, including those that Majorana fermion or Ising anyon modes that have drawn significant attention in other contexts, including topological insulators and p+ip superconductors/superfluids.

LP12055

Exploiting the Cheerios effect to study heterogeneous matter

Cheerios floating in a bowl of milk are usually seen to aggregate due to
surface tension. We exploit this effect to study the structure of
heterogeneous agglomerates formed by floating spheres by changing the
level of the liquid in a conical container and thus uniformly
compressing the system. Using this trick we can study aggregates formed
over a wide range of density. We find that the structures are not random
as one may expect for objects brought together randomly, but rather have
local hexagonal structure at short scale and pockets of empty space or
voids. We show with statistical measures that this anomalous behavior is
particularly dramatic at low densities and the system becomes
increasingly homogeneous as the density is increased because of
crowding. Agglomeration due to cohesive interactions occurs broadly in
nature including in dust and soot due to electrostatic interactions,
inter‑galactic dust under gravitation, and in powders due to humidity.


***



LP12468
Materials yield to creep stresses... but not uniformly


If a bridge or a piece of paper is stressed, it will start slowly to yield, or
creep, by accumulating irreversible deformation. A simple experiment done with
ubiquitous copy paper demonstrates that a sample does not creep uniformly. As
time goes on, the yielding slows down, but fluctuations ‑ differences between
local creep rates and the total one of the sample ‑ become meanwhile larger
and larger relative to the average yield rate. The experiment and simulations
of simple metal plasticity tell us that all this is a result of hidden
complexity inside the material, and related here to a phase transition: if an
object is loaded enough, it will creep until brought to failure, whereas
stresses smaller than a critical value will bring the deformation eventually
to a stop. The results will have an impact on our understanding of the most
important properties of materials, as such variations in creep have a novel
interpretation via a phase transition. They also show how simple, almost toy
experiments, such as pulling a piece of paper, may contain complex, yet
beautiful physics.

Image caption: A copy paper sample under creep due to a constant stress applied
in the vertical direction. The color code shows how the sample deformed over
100 second period of time: from small creep deformation (blue) over to large
deformation (red). In the background one may discern the fibrous,
heterogeneous structure of paper. The non‑uniform deformation shows a signature
of a phase transition, in the collective dynamics of the material.


***

LT12557

Molecules whittled out by x‑rays

The LCLS x‑ray laser at the SLAC National Accelerator Laboratory
is proving its mettle as a machine for discovery in strong field
molecular physics. The first observations of double core
vacancies (DCH) ‑‑ so‑called “hollow atoms,” which have lost
their core electrons ‑‑ were reported in early July in both
Physical Review Letters [Hoener et al.] and in Nature [Young et
al.]. These exotic states should be particularly sensitive
indicators of the chemical environment inside a molecule.

Now barely a month after these first LCLS results, two new
Physical Review Letters are showing the first detailed pictures of
electrons in hollowed out molecules. The Auger electrons, which
boil away from a core‑excited atom with much of the energy
deposited by the x‑rays, are more energetic when there is a
double vacancy. They also retain information about the shape of
the molecule and the distribution of electrons that form the
chemical bonds.

***

LS11930

How to mix immiscible materials down to the atomic scale

The presence and the conditions for the formation of surface alloys on bulk materials or thin films is of utmost scientific and technological importance in various fields of science, such as metallurgy, catalysis or magnetism. Of particular significance is the formation and the understanding of new type of surface alloys from components that are immiscible in the bulk, because new materials with potential for applications may be found.

On a surface, foreign atoms are often stressed due to the atomic size differences with substrate atoms. They can be too much or not enough packed (like apples and cherries in an egg carton). But if we mix big AND small atoms together, we can obtain a good packing if they are correctly ordered. If we make the good choice of constituents and composition of the alloy, we may force two immiscible metals in the bulk to make an ordered surface alloy.

In a recent experiment reported in Physical Review letters (July 2010), two immiscible metal, iron and gold, have been evaporated on a ruthenium surface (which mimics the egg carton) and by choosing a precise ratio of 1/3 a long range ordered bidimensional alloy, has been created and observed with scanning tunneling microscopy and electron diffraction. It was generally accepted that such mixing might be favored because of the strain imposed by the corrugation of the substrate.

This work goes one step further, since the authors have performed state-of-the-art spin-polarized ab initio density functional theory calculations for their system for various atomic configurations and compositions. These calculations show that the driving force for the stability of the ordered alloy is the development of the high Fe atomic moment, i.e. magnetism, while stress relief plays no major role, which is a significant discovery.

These results open up new directions in surface alloying and magnetism, and may lead to the development of technologically relevant alloys for magnetic storage and catalysis.


***

LP12487BR

Probing energy levels of massless electrons at the surface

We present the first observation of magnetic‑field‑quantized energy
levels of a highly unusual electronic state that is formed at the
surface of a newly discovered kind of insulator. Like conventional
insulators, these new insulators are electronically inert in the bulk;
however, so‑called topological insulators can support electrical
conduction ‑ but only at the surface. This surface state is predicted
theoretically to have a number of unusual properties, a striking one of
which is that, just like photons, the mobile electrons at the surface do
not have a mass. Quantum‑mechanical properties of massless electrons are
distinct from those of conventional electrons and this should be
manifest in their quantized energy levels in a magnetic field. Although
it was experimentally verified that the surface electrons are indeed
massless, there has been a lack of studies exploring their quantum
properties due to the inevitable contribution from the bulk electrons in
a real material. Using the surface‑sensitive scanning tunneling
spectroscopy technique in a magnetic field, we selectively probed the
surface massless electrons and successfully detected the quantized
energy levels, with exactly the indexing and field dependence expected
for topological insulators. Present results may provide basis for
understanding topological insulators as well as open several new avenues
for technological exploitation.

***



LQ12706

Attosecond electron interferometry

Ultrafast motion can be captured using ultrashort pulses, which, when it comes to electrons, means that attosecond pulses (1 as = 10-18 s) are necessary. These extremely short pulses have intrinsically very broad bandwidths, which leads to a fundamental problem: they cannot be used directly to resolve the spectral content of an electron wave packet. In this letter we present experimentally a novel technique to overcome this obstacle by utilizing a basic principle of quantum mechanics that continues to fascinate us all: the fact that a coherent addition of different possible electron paths results in high contrast interference fringes. In our experiment these fringes contain state specific amplitude and phase information. We obtain this interference structure by coherently launching a reference wave packet simultaneously with the excitation of the electron wave packet that we want to study. At a later time we recombine the wave packets using a few cycle infrared probe pulse and the interference fringes are recorded as a function of the delay between the two pulses. The presented technique combines high spectral and temporal resolution simultaneously, thus providing an increased precision when doing attosecond experiments.

Please note that in a theoretical work (LR11894) that will be published back to back with our letter the technique has been used to study wave packets created from doubly excited autoionizing states.


***

BRR1130BR

Modeling an unconventional oscillator

In free space, an electron subjected to a static electric field becomes
uniformly accelerated. In a crystalline solid instead, as predicted by
Bloch in 1928, it performs oscillations with a time frequency
proportional to the electric field strength. This spectacular but very
counterintuitive behavior remained a tantalizing concept till Esaki and
Tsu pointed out that a periodic stacking of semiconductor layers of
nanometric dimension (a superlattice) would allow realizing it in
solids. Its implementation has since then been achieved in various
experiments, which unambiguously demonstrate that a Bloch oscillator
excited in a biased superlattice emits light in the THz range, a
frequency domain where reliable and tunable sources are badly lacking.
In our paper we present a modeling of superlattice’s Bloch oscillations
that combines accuracy and flexibility. It is based on a comprehensive
quantum‑optical like description of the superlattice response to a fast
optical pulse and results in the finding of the stronger THz emission.
Such a theoretical strategy fully responds to the present demand of a
dedicated analysis tool. It paves the way to optimizing the emission
characteristics of such fascinating solid‑state oscillators.

EM10670

Reverse color sequence in the diffraction of white light by the wing of the male butterfly Pierella luna (Nymphalidae: Satyrinae)

The butterfly Pierella luna (Nymphalidae) shows an intriguing rainbow iridescence effect: the forewings of the male, when illuminated along the axis from the body to the wing tip, decompose a white light beam as a diffraction grating would do. Violet light, however, emerges along a grazing angle, near the wing surface, while the other colors, from blue to red, exit respectively at angles progressively closer to the direction perpendicular to the wing plane. This sequence is the reverse of the usual decomposition of light by a grating with a periodicity parallel to the wing surface. It is shown that this effect is produced by a macroscopic deformation of the entire scale, which curls in such a way that it forms a vertical" grating, perpendicular to the wing surface, and functions in transmission instead of
reflection.

LP11880E

Sudden changes of direction of Marching Locust groups

One may find in nature many examples of animals which possess an
undeniable collective behavior, such as ants or bees. In some cases,
this collective behavior manifests itself in patterns of movement in
large groups of animals like fishes, insects or birds. Strikingly,
many of these groups show sudden changes of direction in their
displacements in the absence of any external stimuli. Movement in
groups is nowadays understood as an advantageous strategy in the
search of food or as protection against predation. But the mechanistic
underpinning of these sudden changes of direction has thus far
remained largely unresolved. In this work we analyze experimental data
on the collective movement of a group of marching locusts in an
experimental setup. Our results point to the Poissonian character of
the change of direction stochastic process. This implies that this
process is purely random and as such it may well be the consequence of
the accumulation of errors made by the individuals within the group as
they try to imitate the velocity of their neighbors. These small
errors cancel each other for short times; however there is a small
probability that they may add up to produce a change of direction, and
this small probability becomes indeed significant for long times. We
have also noted the similarity of this effect with the sudden
magnetization changes that appear in magnetic materials, which
apparently share a comparable mechanism.

***

CR10222


Alpha-gas state in heavier nuclear systems

A dilute gas-like state of alpha particles can exist in heavier nuclear systems.
In the last decade, alpha clustering in nuclei has become a topic of great interest,
involving conglomerates of alpha-particles forming inside the atomic nucleus.
Recently, the extension to systems termed nuclear molecules has broadened this interest.
One atomic analogy is Bose-Einstein condensation. Recent advances in nuclear
theory indicate that there are a class of alpha particles (4He nuclei) condense out into
a dilute gas-like state. These states have a considerably extended nuclear radius and
radically different properties to other nuclear states close-by. One of the most well known
examples in light nuclei is the second 0+ state in Carbon-12, the so called Hoyle state,
responsible for our existence, due to its role in nucleosynthesis in the stars.
We have gone a step further by proving from a theoretical point of view that such
a dilute state of alpha particles can exist in heavier systems. We have shown that
a three-alpha cluster around a Calcium-40 nucleus can have a dilute gas-like structure,
which is consistence with experimental results by Kokalova et al.

Picometer‑sized magnets in free‑electron‑laser light

Atomic clusters represent an ideal model system to study size‑driven
effects in solids. Indeed, such clusters provide a self‑contained
`laboratory' where, as a function of the number of constituent atoms,
condensed matter effects gradually appear. Particularly intriguing is
the evolution of the geometric and electronic structure, and their role
in the appearance of magnetic ordering in increasing the size of the
system from a single atom to a macroscopic unit.

Knowing the structure of a cluster is thus the first important step in
understanding more complex properties. This is a challenging task since
none of the usual spectroscopy techniques work on low‑density gas‑phase
clusters. However, we prove it possible using intense tunable infrared
light from a free electron laser. The laser radiation was used to
resonantly shake a small messenger molecule off the cluster surface.
Thus obtained vibrational spectrum was proven to be a unique fingerprint
of not only the geometric, but also the magnetic configuration of the
clusters. For example, a tiny object with only four Fe atoms was shown
to be a ferrimagnet with a large exchange energy.


***

LT12529

How to make water ‘bounce’ at the nanoscale

In our cells, water is stuck between molecules with only a few nanometers to spare. Such ‘nanoconfined’ water has long been suspected of having unique properties. Now a team of physicists at Wayne State University has measured the mechanical properties of water squeezed down to just a few molecules, and found that water can be switched from being a liquid to a bouncy solid by small changes in external conditions. Using a new Atomic Force Microscope technique developed at Wayne State, the team probed the mechanical properties of confined water layers without disturbing them. Oscillating a tiny probe, immersed in the liquid, with amplitudes the size of a hydrogen atom (0.1 nm), they recorded the response as the probe squeezed the water at extremely low speeds. Once squeezed to a layer four molecules thick or less, the water behaved like honey: more viscous than in bulk, but still liquid. However, at squeeze speeds of 0.8 nm/s and above, water became elastic. This speed is so slow, it would take 12 years to move one foot, yet it is enough to change the behavior of water drastically.