LR12462

Molecules can superflow

Some liquids such as helium exhibit peculiar properties: at very low temperatures, the liquid seems to lose all friction as it flows; spontaneous fountains can form; or the liquid could creep up the wall of its container. These behaviors are manifestations of a property called superfluidity. Scientists have long searched for such phenomena in other condensed substances to see if this behavior extends to other substances. We present the first direct experimental evidence of superfluidity in a molecular system, namely nano-clusters of hydrogen molecules. The laboratory observation is supported by theoretical analysis from computer simulations that predicted the presence of superfluidity. We observed the nearly free rotation of a carbon dioxide molecule embedded in the hydrogen nano-clusters. The carbon dioxide acts as a "paddle" to test wether the cluster presents any resistance to motion. The lack of resistance, or viscosity, due to the motion of the "paddle" indicates that superfluidity in the nano-cluster has been established. We have attributed the superfluidity to the delocalized nature of the hydrogen molecules, which means that at very low temperatures, the superfluid nano-cluster behaves as a new single entity and no longer as a liquid with independent molecules.

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LN12202BR

Geminate recombination absent in efficient organic solar cells

Experiments show that an expected loss mechanism is absent in the most efficiency organic solar cells. Light absorbed in an organic semiconductor creates a tightly bound electron and hole pair – an exciton – that cannot separate except at very high electric fields. The bulk heterojunction solar cell solves the problem with a nanoscale mixture of two semiconductors, which allows the exciton to split at internal interfaces and enables solar cells of steadily improving efficiency. However the split electron and hole are still close together and bound by their Coulomb energy. Many scientists expected that their immediate recombination – known as geminate recombination – limits the solar cell performance. We were able to show that transient photoconductivity measurements distinguish geminate from non-geminate recombination. Carriers undergoing geminate recombination do not contribute to the photoconductivity while carriers that separate before they recombine do contribute. We tested two commonly studied high efficiency organic solar cell and found no detectable geminate recombination in either. This result is good for solar cells as it removes one fundamental limitation on the efficiency.


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LS12743

Controlling the temperature of quantum objects by measurements

The act of measuring is one of the most enigmatic phenomena in quantum physics since, as opposed to classical observations, it can in and of itself change the properties of the observed quantum object. A new twist to this paradox, due to appear in Phys. Rev. Lett. [1], shows that the state purity or polarization in a quantum object may change as a result of how frequent it is observed. This would be akin to taking the temperature of an object and finding that this depends on the rate at which it is taken: when the rate is very high the temperature goes up, and when it is taken more slowly it goes down. Counter intuitive effects of this kind have now been experimentally observed in experiments designed at controlling the polarization of nuclear spins in an ensemble of carbon atoms placed in contact with a "bath" consisting of proton spins. This study confirms theoretical predictions first described in an article that appeared in Nature in 2008. These effects defy the standard rules of macroscopic thermodynamics, whereby the interaction between a large heat source (“heat bath”) and a much smaller system must bring them to thermal equilibrium; that is towards a common, fixed temperature, unaffected by measurements. On the fundamental side, the present results help us bridge the long-standing gap between the quantum dynamics typically characterizing small (“nano”) systems, and the thermodynamics that generally governs the behavior of macrosystems. On the applied side, these results indicate the possibility of principally novel heating and cooling schemes, that may be faster and more robust than the existing ones.

LM12492

The road not taken by a traveling drop

We have successfully explained the time-dependent behavior of the traffic of discrete objects within a simple network. Although our study focuses on the dynamic behavior of micron-sized liquid droplets at a fork, the reasoning behind the model we derived may be employed in confronting other challenges, from planning vehicular traffic in modern cities to routing data over the internet. Our results concerning droplet traffic at a bifurcation should provide a breakthrough for the development of the type of lab-on-a-chip technology used nowadays. For example, in high-throughput screening for drug development, the use of passive control, sorting, and storage of individual droplets, would eliminate the need for complicated micromechanical valves or pumps. The model derived in our paper, which describes the periodic dynamic behavior of droplets travelling in a splitting channel, provides rules for the "choice" of which path to follow. These selection rules are successfully compared to simulations and experiments.


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LT12935

The Clapping Book: wind-driven oscillations in a stack of elastic sheets

We present a hybrid experimental and theoretical study on the oscillatory behavior exhibited by multiple thin sheets under aerodynamic loading. Our clapping book consists of a stack of paper, clamped at the downstream end, placed in a wind tunnel with steady flow. As pages lift off , they accumulate onto a bent stack held up by the wind. The book collapses shut once the elasticity and weight of the pages overcome the aerodynamic force. The process then repeats periodically. We develop a theoretical model that predictively describes this robust oscillatory behavior.

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LU12503

Hawking radiation from ultrashort laser pulse filaments

Event horizons of astrophysical black holes and gravitational analogues have been predicted to excite the quantum vacuum and give rise to the emission of quanta, known as Hawking radiation. We experimentally create such a gravitational analogue using ultrashort laser pulse filaments and our measurements demonstrate a spontaneous emission of photons that confirms theoretical predictions.

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LS12383

Modeling Online Popularity

The purpose of this paper is to offer a massive quantitative, global, and longitudinal analysis of the processes driving the acquisition of popularity in the online world. To this end the study analyzes large-scale datasets that are representative of the Web by capturing the online behaviors of millions of people and sheds light for the first time onto the manner in which collective attention manifest itself through bursts which are both violent in magnitude and unpredictable – much like online earthquakes. However, the model provided is capable of reproducing all of the observed dynamics of online popularity through a mechanism that causes sudden, nonlinear bursts of collective attention. Given the increasing number of activities carried out online and the effect that large-scale opinion formation has on important phenomena like public policy, culture, and advertising profits, such a quantitative picture of online human activity will only become more pertinent in the coming years.

ES10664


ADAPTIVE NETWORKS, OR HANGING OUT WITH HEALTHY PEOPLE

A novel analytic formalism is introduced to track epidemic dynamics in populations featuring an adaptive contact network. Under the threat of an emerging disease, people naturally tend to avoid interactions with the infectious in order to reduce their own chances of getting sick. This behavioral response to the disease can be captured in the framework of adaptive networks, where the contact network and the state of the nodes evolve in an intricate manner, allowing healthy individuals to cut ties with those who are infectious: in other words, hang out with healthy people. Being the first to correctly reproduce the time evolution of both dynamical elements, disease and topology, the approach presented in this paper is able to make accurate predictions on the conditions under which a disease will invade a population under the adaptive contact network hypothesis. This represents an important step forward in the inclusion of more realistic features in existing epidemic models. This work is part of a collective effort made by network epidemiologists to provide policy makers with an analytic toolbox that will help guiding the development of future prevention and intervention strategies.


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LT12730

Physics of peeling

The phenomenon of peeling or delamination i.e., a spatial separation
of rigid bodies at an interface occurs commonly in nature. It ranges
from geological processes, such as lithospheres separating from
tectonic plates to biological ones, involving blastoderms in cells
forming a gastrula. It is also a common failure-mode in many everyday
processes such as peeling of paint etc.. In a single imaging
experiment using colloidal crystals of varying rigidity, we have
captured this entire process in a laboratory scale measurement. We
found that the origin of the observed diversity of the process is in
the competing effects of the rigidity of the medium, the strength of
adhesion to the substrate from which it peels and the externally
applied stress. The images obtained in the experiment - vapor-like
evaporation of individual particles occurs in the soft films, while
solid-like collective delamination of large chunks, triggered by
nucleation and propagation of cracks occurs for rigid films - provide
a simple and intuitive understanding of the otherwise extremely
complex phenomenon. This understanding is important for applications
such as adhesion, patterning, lithography and soft electronics, in
addition to the prevention of catastrophic mechanical failure in
materials.


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EU10655

Tunable solid-like behavior in fluids

Yield stress is one of the distinguishing properties of solids, and
ordinary (Newtonian) fluids are not known to exhibit this feature.
However, some complex (nonNewtonian) fluids under external driving do
exhibit a solid-like behavior, so that they can support a shear stress
without flowing. A group of Brazilian scientists has investigated the
influence of a tunable magnetic yield stress on the morphology of fluid
droplets subjected to external magnetic and centrifugal forces. A sizable
magnetic field-dependent yield stress appears on highly nonNewtonian fluid
suspensions of magnetic micronsized particles, known as magnetorheological
(MR) fluids. In contrast, colloidal suspensions of magnetic nanoparticles
called ferrofluids (FF) present negligible yield stress. By confining
magnetic fluid droplets between narrowly spaced parallel glass plates, an
exact balance between the forces involved is achieved, leading to the
emergence of novel pattern forming structures. One particularly noteworthy
aspect is the material dependence that clearly enters into the resulting
shapes. The manipulation of yield stress properties via magnetic means
opens up the possibility of unveiling a number of still unexplored pattern
morphologies and new dynamic behaviors. This offers a stimulating
challenge to soft matter researchers.

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LT12324E

Crowding or Sticking: How do breast cancer cells move?

Understanding how breast cancer cells interact with each other and move through host tissues
is the first critical step in understanding how breast cancer can spread through our body.
To examine the motion of breast cancer cells we present new data describing cell movement in a series
of carefully designed experiments. Various experimental conditions lead to a variety of responses
which cannot be interpreted intuitively. We anticipate that the experimental results could be explained
by cell-to-cell adhesion (stickiness) or cell-to-cell crowding effects. By replicating the experimental
data with an appropriate discrete random walk model, we show that a low value of cell-to-cell adhesion
strength provides the best explanation of the experimental data suggesting that cell crowding effects plays
a more important role than cell-to-cell adhesion. This is a critical result since cell crowding effects are
typically neglected in standard models describing the motion of cell populations

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LS12522

Positrons Spy on Defects

The worlds most intense positron beam was used to track
down fast processes of atomic defects in solids.
Novel materials often gain their advanced superior
properties not only by specific compositions but also by
structural modifications. Processing metallic materials,
e.g., by extreme plastic deformation produces structures
on a nanoscale which lead to superior mechanical properties.
It may seem like a paradox that the enhanced mechanical
properties are based on defects of the lattice. However,
these defects are on an atomic scale whereas defects on
a micro scale such as cracks or pores often deteriorate
materials.
Now, for the first time, fast kinetic processes of
atomic defects in materials were investigated by making
use of the worlds most intense positron beam provided by
NEPOMUC at the research reactor FRM II of the Technische
Universität München at Garching (Germany) in collaboration
with Austrian researchers from the Technical University
of Graz, the Montan University Leoben and the University
of Vienna. These studies are essential for the development
of novel structural materials and the antiparticle-particle
annihilation of the positron-electron reaction is ideally
suited for such kind of studies.


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LS12130

Optical control of the refractive index of a single atom

The effects of matter on light are known from our everyday life: It passes through transparent air, is refracted by glass or is absorbed by soot. These optical properties usually don't change for a certain material. However, in some cases, they can be controlled and drastically changed using laser light with the right color. In this paper, we have demonstrated that a single cesium atom can be tuned continuously from absorbing to transparent. Because the effect of a single atom on a light beam is very small, we have placed the atom inside an optical cavity using optical tweezers. The cavity is formed by two highly-reflecting mirrors, such that our probing light passes the atom up to 300,000 times. With a control-laser beam shone on it from the side, the absorption of the probe light passing the atom has been strongly suppressed. An optical switch based on these mechanisms could prove useful in quantum communication. Surprisingly, we observed that the atoms are also cooled when they are rendered transparent. Since colder atoms stay longer in optical tweezers, the new cooling effect enabled us to experiment with one atom about twenty times longer than before.

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LU12566

Simple crystals show exotic negative refraction effects in the far infrared

In this letter we describe how a simple quartz crystal may be used to achieve negative refraction for all angles of incidence, positive or negative. Research on refraction, the change of direction suffered by a light ray as it passes from one medium to another, can be traced back as far as the ancient Greeks. In conventional (positive) refraction, the ray has to cross the normal, an imaginary line drawn perpendicular to the interface where the incident ray hits it. However, in the case of negative refraction, a phenomenon which has come to prominence over the last decade, the change of direction is sufficient for the ray to bend back to the same side of the normal. Most studies of negative refraction have concentrated on materials formed from intricate artificial structures. However, in this work we show how, at certain far infrared frequencies, all-angle negative refraction may be achieved using a natural anisotropic crystal such as quartz. The secret
lies in the interaction of the far infrared radiation with phonons – natural vibrations of the crystal lattice. Reflection and transmission measurements both point to the occurrence of negative refraction and show that, although absorption exists, transmission efficiency is relatively high.