January 23, 2009

LX11118

First transmission of twisted radio beams

For the first time experiments have demonstrated that radio beams can
be twisted. Beam twisting increases the capacity of radio beams to
transmit information and provides unique possibilities to study
rotating phenomena. A team led by scientists from the Swedish
Institute of Space Physics performed the experiments at the High
frequency Active Auroral Research Program (HAARP) in Alaska, USA.
Powerful radio beams were transmitted into the ionosphere, the
overhead near-Earth space environment. Several degrees of twisting
were successfully transmitted and the interaction of the twisted beams
with the ionosphere was studied. The characteristic ring-shaped cross
section of the beams could be observed by the weak optical emissions
that the beams excited in the ionosphere, a common technique used to
study the effects of radio waves in that region. Twisting radio beams
constitutes a new dimension for information transfer. The technique
works independent of the well known amplitude (AM) and frequency
modulation (FM) techniques used today, for example, for broadcasting
radio and TV channels. Further, twisted beams provide unique
possibilities for remote sensing and interaction with rotating
phenomena, such as vortices in aurora, as well as for extracting
corresponding information from distant objects in the universe in
radio astronomy.


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LW11444

A new way to see molecules

Researchers have demonstrated a new optical methodology that provides an instantaneous two-dimensional projection of molecular function. The method maps an arbitrary number of quantum couplings within or between molecules. Published this week in Physical Review Letters, the researchers demonstrate the power of the method in the mapping of electronic excitations in a photosynthetic protein. For the first time, they show the ability to instantaneously and directly distinguish coupled-electron motions from other electron motions, providing a new window on how energy is efficiently transported to drive a chemical reaction. The novel methodology, Angle Resolved Coherent (ARC) wave-mixing, works by imaging the angle of light emissions from a chemical sample that is illuminated by a combination of high power, pulsed laser beams. A two-dimensional map is captured without post-processing and in one ten thousandth of a billionth of a second (the duration of a laser pulse). Variations of the same methodology can in principle map molecular vibrations or bonds. With relevance to the molecular biosciences and spanning disciplines, this method provides a powerful new way to feedback to molecular simulation and opens up new avenues in relation to sensitive and rapid sample characterisation.


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LX10929

Thermal diffusion in polymer solutions: To the cold or to the warm?

Recent experiments on dilute polystyrene solutions in a temperature
gradient reported that long molecules, consisting of many styrene units,
diffuse to the cold, whereas monomers diffuse to the warm; thus the
transport coefficient changes sign as a function of the molecular weight
[Stadelmaier & Köhler, Macromolecules 41, 6205 (2008)].

Here we show that this change of sign arises from the competition of two
opposite mechanisms. The first one, pointed out by Brochard & de Gennes in
1981, stems from solute-solvent interactions and drives high polymers to
the cold side of the sample. In addition, we derive a novel term of
opposite sign; this directed Brownian motion is most relevant for short
molecules and drives the solute to the warm.


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LU11652

Society benefits from behavioral diversity

Cooperation is essential in every society, but puzzling from an
evolutionary perspective. In this work, we address the role of
behavioral differences – ubiquitous among Humans - on the evolution of
cooperation. We study a model in which individuals can either cooperate
or defect. They engage in a social dilemma of cooperation, interacting
along the edges of a complex network. The structure of the network
changes in time, as individuals regularly engage in new interactions
while abandoning old ones. Social interactions may be long or brief,
depending on the individuals involved. When dissatisfied, some
individuals will try to break contact as soon as possible, whereas
others will remain in touch. We show that cooperation blooms – and
society as a whole benefits – the larger the behavioral diversity in
responding to unwanted interactions. These results support the idea that
diversity, on a grand scale, is instrumental in shaping us as the most
sophisticated cooperating entities on this planet. Mathematically, we
show that taking explicitly into account the feedback between
co-evolving mechanisms – here network topology and individual strategy
and behavior - profoundly affects the outcome of those processes, a
result which has widespread consequences in many problems of natural and
social sciences.

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BX10909

Probing the Exchange Bias in Co/CoO Nanoscale Antidot Arrays using Anisotropic Magnetoresistance

There has been sustained interest in the exchange bias phenomenon, which is a magnetic proximity effect that typically occurs due to interfacial exchange coupling at ferromagnetic/antiferromagnetic (FM/AFM) interfaces. This effect has been used to pin the magnetization orientation of the FM layer, which then serves as the reference layer for key devices in magnetic sensors and high density magnetic data storage. In this work, we have probed in detail using anisotropic magnetoresistance (AMR), the exchange bias effect in nanoscale Co/CoO antidot arrays as a function of temperature and FM layer thickness. We have employed the Co/CoO system due to its Néel temperature TN (291 K), which is just below room temperature, thus enabling the exchange bias to be reset conveniently. Our results demonstrate that the asymmetry in magnetization reversal of Co/CoO bilayers is markedly modified due the presence of antidots and is strongly dependent on the FM layer thickness. We also observe that the exchange bias field in the antidot arrays can be either larger or smaller than the continuous film, depending on the temperature. The interfacial nature of the FM-AFM coupling in the exchange biased antidot arrays is further established from the dependence of HE and HC on the FM layer thickness.

January 21, 2009

LK11128A

Abraham vs. Minkowski: A centenary, and an answer at last.

The momentum of light in transparent media has been debated for 100
years, since Minkowski proposed (in 1908) that it increased, and Abraham
(in 1909) argued that it decreased, when entering a dielectric medium.
Although calculations based on Abraham's theory have greater scientific
rigour, Minkowski's theory leads to a number of popular, and very
effective, shortcuts. This paper shows for the first time why these
shortcuts work, how they may be improved, and, ultimately, when they
fail. With reference to a review of the last hundred years' research, we
see that the momentum of the light wave and the medium cannot be cleanly
separated, and hence any method based on such a separation must
eventually break down. In showing the limits of these shortcuts, we
reinforce that previous conclusion; with the determination and partial
experimental verification of their limits, we provide the shortcuts for
the first time with a firm theoretical basis, validating their
widespread empirical use.

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BX10585

The striped superconductor: (another) new state of matter

Superconductivity is characterized by a quantity called the
"superconducting order parameter", which evolves from zero in the
normal (metallic) state to a non-zero, spatially uniform value in
the superconducting state. In this paper, we describe a new state
of matter, the "striped superconductor", in which the
superconducting order parameter is non-zero, but rather than being
uniform, it is modulated in space (such that its average
vanishes). This state can explain recent transport experiments
in the original high-temperature superconductor
La_{1.875}Ba_{0.125}CuO_4, which reveal a remarkable cascade of
transitions and crossovers which occur above the superconducting
transition temperature. We study the properties of the striped superconductor
theoretically, and propose microscopic models which realize it.
Its most striking new property is that in the presence of weak
disorder (e.g. lattice imperfections), the striped superconductor
necessarily gives way to what is the superconducting analogue of a
glass state. This is in stark contrast to a regular
superconductor, in which weak disorder does not change much. We
propose that such a "superconducting glass" has already been
observed in La_{1.875}Ba_{0.125}CuO_4.

***

EX10489

Waving motion induces physical properties changes

In this paper we study a new mechanism to hasten the aggregation process of magnetic particles dispersed in mineral oils exposed simultaneously to a static magnetic field and a low amplitude oscillating magnetic field as a perturbation. The perturbation field induces a waving movement in the chains formed by the fields. These chain movements enhance lateral interactions, which induce lateral aggregation of chains to form larger chains or chains with thicker structures, in a way remarkably more intense and faster than without waving. This mechanism allows to control to some extent the characteristics of the chains and consequently to enhance changes of the physical properties of the dispersion under magnetic fields. Analogous behavior would also happen in other “dipolar fluids”.


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LX11503

Cavity Tuning of Molecule Colors

When placing a molecule into an optical nano-cavity, one changes its
fundamental quantum-mechanical interaction with the surrounding
electromagnetic field. Decades ago, this was experimentally
demonstrated by measuring the cavity-induced modification of the
fluorescence lifetime (average lifetime of a molecule's lowest-lying
excited state). Now, for the first time, it was shown that the
emission spectrum of one and the same individual molecule can be
continuously tailored by tuning the cavity size. The nano-cavity
changes the mode density of the electromagnetic field and thus its
coupling with a molecule's internal quantum-mechanical states.
Because this is sensitively dependent on the wavelength of the field
modes, it leads to a complete restructuring of a molecule's emission
spectrum as distinct wavelengths are affected differently. The effect
is well described by a semi-classical theoretical treatment, offering
the possibility to predict molecular emission properties in complex
nano-environments. This opens an exciting field for designing
fluorescent emitters with adjustable spectral emission properties.


The attached figure shows the changed emission spectrum (red dots =
measurement, blue line = fit) of a single fluorescenct dye molecule
within a silver-mirror microcavity in comparison to the dye's free
spectrum (gray shaded area).

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AY10417

Radiosensitivity of the halouracil molecules studied trough their collision with carbon ions

A strong enhancement of DNA damage through ionizing radiation may be observed by replacement of thymine by 5-bromouracil in cellular DNA and is widely employed in radiation therapy. Such behaviour is studied in this paper through the collision of C4+ carbon ion with the different halouracil molecules. Effectively, as 5-halouracils are supposed to enhance sensitivity to ionizing radiation, the collision with ions would favour fragmentation of the biomolecule. That means that, on the contrary, the charge transfer process would be less efficient with 5-halouracils compared to the uracil molecule.
In the present paper, the charge transfer has been studied theoretically by means of ab-initio quantum chemistry molecular methods followed by a semiclassical dynamical treatment. The process appears markedly less efficient, by at least a factor 100, than the corresponding charge transfer with a uracil target. This leads to an enhancement of the fragmentation process, in complete agreement with the radiosensitization properties of the 5-halouracils, in particular for 5-bromouracil. The charge transfer appears to be an anisotropic process and the preferred orientation depends on the halouracil target considered. The mechanism appears to be driven by two effects: a global electronic effect with regard to the electronegativity of the halogen atom which induces a lowering of the charge transfer cross-sections, and a more specific steric effect relied to the size of the halogen atom which favours preferred orientations for the collision reaction.


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ET10538

Speedup through recursion

Discretized effective actions are used to substantially speed up and
improve the convergence of numerical Monte Carlo calculations of
properties of physical systems. By recursively solving the underlying
Schrodinger equation, in this paper we set up an efficient systematic
approach for deriving analytic expressions for discretized effective
actions. With this we have obtained discrete short-time propagators
for both one and many particles in arbitrary dimension to orders which
have not been accessible before. Apart from Monte Carlo calculations,
our approach can also be used to systematically improve the Numerical
Matrix Diagonalization method for calculating energy eigenvalues and
eigenstates. Furthermore, the obtained discretized effective actions
are applicable to efficiently determine the statistical properties of
Bose-Einstein condensates confined in harmonic or anharmonic traps.
The presented method is also ideally suited for dealing with dilute
quantum gases in a disorder environment where the impact of two-
particle interactions upon the recently discovered phenomenon of
Anderson localization is at present studied.

***

LY11801

A Newtonian System that Mimics the Baldness of Rotating Black Holes

The rotating black hole has been described as one of nature’s most
perfect objects. As described by the Kerr solution of Einstein’s
gravitational field equations, its spacetime geometry is completely
characterized by only two numbers, mass and spin, and is sometimes
described by the aphorism ``black holes have no hair’’. A particle
orbiting a rotating black hole always conserves its energy and
angular momentum, but otherwise traces a complicated twisting rosette
pattern with no discernable regularity. But in 1968, Brandon Carter
showed that the particle’s wild gyrations nevertheless hold another
variable fixed, now called the ``Carter constant’’. The true meaning
of Carter’s constant still remains somewhat mysterious 40 years after
its discovery.

Now Clifford Will of Washington University in St. Louis and the
Institute of Astrophysics in Paris has shown that, even in Newton’s
theory of gravitation, arrangements of masses exist whose
gravitational field also admits a Carter-like constant of motion, in
addition to energy and angular momentum. What’s more, the deviation
of the field’s shape from being spherical is determined by a set of
equations that are identical to those for Kerr black holes. One
Newtonian system that exhibits this property is surprisingly simple:
two equal point masses at rest separated by a fixed distance.

One goal of this research is to build a better understanding of the
Carter constant in order to analyse the orbits of small black holes
or neutron stars revoloving around rotating supermassive black
holes. The gravitational wave signal from such events may be
detectable by the advanced LIGO-VIRGO-GEO network of ground-based
laser interferometric detectors, or by the proposed space-based
antenna LISA.

***

LU10982

Hot Atoms can Freeze Images

Any image propagating in free space undergoes a diffraction spreading and
eventually blurs out. For this reason, we have lenses in our eyes and
cameras, which reverse the diffraction and recover the original image. In
this paper, we present a novel scheme to eliminate the optical diffraction
of arbitrary images all throughout their propagation. It was recently
demonstrated that arbitrary images can be imprinted on light pulses which
are dramatically slowed when traversing a medium of room-temperature atoms.
Here, we show that by carefully tuning the light-matter interaction, the
optical diffraction of such images can be eliminated completely. This is
achieved by exploiting the random thermal-motion of the atoms, which
effectively trap the light in the plane perpendicular to the propagation
direction. In an analogy to the laser-trapping of atoms,
outwards-confronting light components couple more efficiently to inwards
moving atoms, counterbalancing the natural diffraction of the light. No
other medium suggests non-diffraction of images regardless of their position
and shape. Applications of our scheme include high-resolution imaging,
slowing and storage of images, and nonlinear optics, and the experimental
conditions for its realization are readily available.


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AY10309

Entanglement Distillation with local common reservoirs

Entanglement or correlations between quantum states is an essential
ingredient for teleportation and quantum cryptography.
It is known that these correlation are lost when the system interacts
with the environment.
In this work, a method is presented on how to protect the quantum
correlations against the damaging effects of the surrounding environment,
by using common reservoirs for Alice and Bob (the two parties trying to
communicate) and performing certain measurements.