6-18-08

BQR1070
Spins in ensembles of quantum dots offer one possible pathway to implementing quantum information technologies in a solid-state environment.

Unfortunately, the spin interaction with the host lattice
leads to coherence lost of the quantum bit. In this work the authors
reported the first measurement of the temperature dependence of the
electron spin decoherence time T2 in semiconductor quantum dots and
they compared their experimental results to recent theoretical
calculations. It has been possible thanks to: (i) a refocusing
technique using laser pulses called "mode-locking technique" developed
by this research group in order to avoid the inhomogeneities within an
ensemble, (ii) the investigation of InAs quantum dots where the
electronic confinement is much larger than GaAs dots (gate-defined
GaAs dots which allow only very low temperature measurements). It was
found that T2 remains constant up to 20 K and then it presents a sharp
drop due fluctuations of the electron spin interaction with the
lattice nuclei spins. The topic of the paper is of timely nature. The
results are quite important for the corresponding science community
considering the actual debate in the community about the mechanisms
responsible for spin decoherence.


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LP11210E
Rich-club vs rich-multipolarization phenomena in weighted networks

Large scale hierarchies characterize many complex networks describing
systems in different domains. Elements at their top, the rich, are
usually recognized as the most central or influential. In what manner
do these hubs relate to each other, in particular whether they are
multipolarized or on the contrary tend to club forming elites or
backbones, is an open question. The rich-club phenomenon quantifies
this tendency to form tightly intertwined communities based on
unweighted network representations. Here, we define this metric for
weighted networks. We show that in some real systems the results
provided by the unweighted and weighted approaches may differ
dramatically, implying oligarchies of rich nodes that despite being
mutually joined by a considerable number of links are nevertheless
loosely interconnected in terms of weight. We also argue that an
exhaustive assessment of the property requires the scanning of the
weighted subgraphs formed by the hubs. This examination is able to
unveil features contrary to the average behavior: the formation of
local alliances in rich-multipolarized environments, or a lack of
cohesion even in the presence of rich-club ordering. Beyond structure,
this analysis matters for understanding functionalities and dynamical
processes relying on hub interconnectedness and, in a broader context,
may help explain how primary forces such as competition and
cooperation influence collective behavior.


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EK10349
Cliques in networks: When the friend of my friend is also my friend.

A group of closely knitted pals is denoted a clique, every member of a
clique of friends is befriended with each other. In network theory
cliques are maximal sets of vertices mutually interconnected, where
`maximal' means that there are no out-of-clique vertices connected to
all members of the clique. We propose and study a hierarchical algorithm
to generate graphs having a predetermined distribution of cliques. We
evaluate the statistical properties of the graphs generated, such as the
degree distribution and network diameters, and compare them to some
real-world graphs, such as protein-protein interaction networks. This
work is part of a general research effort aiming to understand the
emergence of non-trivial structural network properties by studying
appropriate graph evolution rules.

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AR10377E
Microwaves, tunneling faster than light?

The mystery of superluminality in the radio range, manifested by
tunneling of GHz microwaves through the wave barrier, and the
accompanying effect ¿ complete transmission of tunneling wave through
this ¿forbidden zone¿ are discussed in our paper. The traditional
observations of tunneling electromagnetic waves are known to be impeded
by very weak percolation of these waves through an opaque barrier. To
avoid this obstacle, we propose a setup, consisting of two equal
segments of a coaxial transmission line. One segment is empty and the
other contains a thin gradient dielectric diaphragm, characterized by a
special profile of dielectric susceptibility across the diaphragm. The
microwave in an empty coaxial is travelling with a free space light
velocity c, which is considered, according to Einsteinian causality, to
be the highest limit of propagation speed of any physical signal. The
wave, tunneling through such gradient diaphragm, possesses a jump-like
phase shift, which is shown to exceed in some cases the phase shift,
continuously accumulated by the freely propagating wave. The possibility
to measure the difference between phase path lengths of tunneling and
travelling modes is expected to shed light on the fundamental physical
problem: does the Einsteinian limit remain valid for tunneling waves
too, or is there a hope to send short tunneling signals ¿Yes!¿ and ¿No!¿
faster than by ordinary light carriers in vacuum?


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LR10945
A Surprising Connection Between Black Holes and Superconductivity

We demonstrate that the physics of black holes can describe the physics
of superconductivity. Black holes, which were among the outlandish
predictions of Einstein's equations of general relativity, are
regions of space where the gravitational field is so strong that not
even light can escape. Superconductors, on the other hand, are
materials of great technological interest that conduct electricity
without any resistance at low temperatures. Electrical currents can
persist for years in metals that have been cooled below the
temperature at which they become superconducting. Until recently, it
would have appeared impossible that there could be a connection
between these two very different physical systems. Our demonstration
builds upon a discovery in string theory called the 'anti-de Sitter/
conformal field theory correspondence', or AdS/CFT correspondence for
short, first proposed by Maldacena, Gubser, Klebanov, Polyakov, and
Witten ten years ago.

While most metals become superconducting only at temperatures below
minus 420 degrees Fahrenheit, certain copper based compounds remain
superconducting at the boiling point of liquid nitrogen, a
comparatively balmy minus 320 degrees Fahrenheit. The mechanism that
allows these 'high temperature' superconductors to work is poorly
understood because the current carrying electrons interact strongly
amongst themselves. By showing that a new approach to
superconductivity is possible, we hope that this work will ultimately
contribute to a better understanding of high temperature
superconductivity.


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BR10647
Resonant Switching of Magnetic Devices Assisted by Microwave Current Pulses

We present experimental results of a new strategy to achieve magnetic
switching in nanoscale devices that can be used as magnetic random
access memory (MRAM) elements. The strategy employs the use of
spin-polarized electrical currents to apply a torque to the
magnetization of a small magnetic sample. Unlike previous studies of
this "spin-transfer torque effect" which employed simple square-wave
current pulses, we explore the use of an oscillating applied current
whose frequency (in the microwave range) is matched to the natural
precession frequency of the magnet. We find by using low-temperature
proof-of-principle experiments that this resonant alternating current
can efficiently drive the nanomagnet to large precession angles,
enabling switching to occur faster and more reproducibly, with less
energy consumption, than when using square-wave pulses alone. The
phenomenon is somewhat analogous to pushing a child on a swing -- it
is easier to achieve a large amplitude of swinging by using a
periodic push matched to the natural frequency of the swing, rather
than by applying a single long push in one direction. In addition to
enabling greater speed and efficiency, resonantly-excited magnetic
switching may also enable alternative, more compact MRAM circuit
architectures.



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BSR1064

Strong correlations and order in the iron pnictide superconductors.

One of the key questions raised by the discovery of the new iron based
high temperature superconductors is whether their physics is akin
to the copper based high temperature superconductors discovered over 2
decades ago.
In our paper we argue that structural and spin ordering transitions
observed
in recent neutron scattering experiments suggest that these two
classes do
indeed share important features of ``strong correlation physics''. We
show
that a model of localized, and strongly correlated, electrons naturally
exhibits a transition at relatively high temperatures where the square
lattice symmetry of the
Fe ions is reduced to a rectangular symmetry, just as is observed.
We extend our model to describe a number of properties of the
superconducting
and metallic states, including the pairing symmetry of the Cooper pairs.



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LQ11753

Even water cannot escape quantum mechanics

The importance of water in environmental and biological systems
arises from the unique behavior of the hydrogen bonds between
adjacent water molecules. Typically, in chemistry, quantum mechanics
is only considered important when describing the electrons. However,
due to the light hydrogen atom, nuclear quantum effects have a
significant impact upon the nature of water. Properties such as the
melting point show a dependence upon the hydrogen isotope present.
Moreover, recent experiments showed that the momenta of protons in
water are at great variance with classical behavior. Here, we
report a computer simulation in which the nuclei are treated within
quantum theory while their interactions are derived on the fly from
the quantum mechanical ground state of the electrons. These
computations were carried out on massively parallel IBM Blue Gene/L
hardware utilizing recently developed algorithms that facilitate the
calculation of the proton momentum distribution. Our results show
that the momenta of protons in water are entangled with the position,
as dictated by the Uncertainty Principle. Therefore, broadening the
position distributions corresponds to narrowing the momentum
distributions. Due to this entanglement, differences in the hydrogen
bond structure between the solid and liquid phases are reflected in
the momentum distribution. Additionally, we confirm that nuclear
quantum effects soften the structure of the liquid, in agreement with
the isotope effect on the melting point. These results suggest that
effects such as delocalization and tunneling, that are associated to
the quantum character of the hydrogen nuclei, may play a role in
biological settings.

6-16-08

AN10173
Quantum honeycomb

A new structure resembling the packed hexagonal form of honeycomb has
been predicted and observed in computer models of groups of atoms that
are cooled down to a point where they lose their individual identities.
The honeycomb in this case is built not from wax but from a regular
arrangement of spinning vortices that form holes in the pancake-shaped
cloud of atoms. The wavelike behavior of such cold atom clouds has
previously been tested by splitting atomic clouds into two parts and
allowing the parts to collide, in experiments that are analogous to
Thomas Young's famous 200 year-old experiment with light. In this paper
we show that if the cloud is instead split into three parts, the pattern
of dark stripes observed by Young and cold atom researchers changes
dramatically into the lattice of swirling vortices. For cold atom clouds
to form requires them to be magnetically confined. We show that this
confinement leads the lattice to melt as the vortices interact
chaotically, clustering into structures that migrate and scatter
throughout the cloud. Computer generated movies of the lattice formation
and vortex interactions have been produced showing the whole process.


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EP10337
JUST FOLLOW YOUR NOSE

Levy walks named after the French mathematician Paul Pierre Levy have
been observed in a bewildering range of organisms, including plankton,
honeybees, jackals and even sharks. In this paper I show how such
movement patterns will rise in predators that locate their prey by
simply following their noses. This is a significant finding because the
key to prediction and understanding lies in the elucidation of
mechanisms underlying observed patterns. The new result indicates that
Levy movements are common place but runs counter to the long-standing
notion that Levy movements arise from the execution of an optimal
searching strategy. The analysis and interpretation of animal movement
data is, however, not wholly straightforward and some of the analyses
claiming Levy walk behaviour have recently been called into question.
This issue is hotly contested, and arguments about the reliability of
methods used to test for the presence of Levy movement patterns may
continue for some time. Central to a resolution, though, is the
determination of underlying mechanisms that can give rise to such
patterns.


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LR11055
A Non Accelerator Probe of New Physics

Motion reversal is known in physics parlance as time-reversal, as it
can be realized mathematically by changing time, t to -t. The origin
of time-reversal violation is one of the least understood of all the
profound issues in physics. The observation of an electric dipole
moment (EDM) of any fundamental particle or of a composite system like
atom or a molecule is a direct signature of the violation of
time-reversal symmetry in Nature. Open-shell atoms will have two
dominant sources of intrinsic electric dipole moments; one due to the
intrinsic EDM of its constituent electrons and the other due to a
time-reversal violating interaction between the electrons and the
nucleus mediated by spin zero particles. Despite the relentless
experimental search for EDMs in elementary particles and as well as in
composite systems for more than five decades no conclusive result has
been obtained. However, many ongoing high precision atomic EDM
experiments are aiming to achieve better detection limits, a few
orders of magnitude lower than the current experimental limits. In
order to obtain a limit for the electron EDM, one needs both the
enhancement factors (ratios of the atomic to the electron EDMs) and
the experimental atomic EDMs to a high precision. A rigorous
relativistic quantum mechanical calculation has been carried out to
predict the EDM enhancement factors for Rubidium (Rb) and Cesium (Cs)
with a sub 1% accuracy for the first time. One of the unique features
of this work is that it deals with the interplay of two very different
interactions—the long range Coulomb interaction and the short range
time-reversal violating interaction. The new results for the
enhancement factors when combined with those of the proposed non
accelerator EDM experiments on Rb and Cs when they achieve their
desired sensitivities could open up a novel direction for finding new
physics beyond the much celebrated model of particle physics, the
Standard Model, which indeed is quite significant, in the era of the
Large Hadron Collider. This could also serve as stringent tests of
many unification models, including Super-symmetry, and provide
insights into one of the most important but unresolved questions in
cosmology: the matter-antimatter asymmetry in the Universe to which we
owe our existence today.


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LH11279B

Electronic orbital currents and polarization in Mott insulators

Systems with correlated electrons are now at the forefront of research in solid
state physics. The standard point of view is that at low energies Mott
insulators exhibit only magnetic properties, while charge degrees of freedom are
frozen because electrons are localized. But are they really localized? We
demonstrate this is not true in general: for certain spin textures on frustrated
lattices (v.g. triangular) there exist nontrivial charge effects in the ground
and lowest excited states. We show that in some cases \textit{spontaneous
circular electric currents} exist in the ground state \textit{of Mott
insulators} and are proportional to the \textit{scalar spin chirality}. Such
persistent currents can run on the surface of these insulators. We also show
that other spin structures generate \textit{spontaneous charge redistribution}.
In some cases, this charge ordering results in a net \textit{electric
polarization}. This is a novel, purely electronic mechanism for
\textit{multiferroic behavior}. We also discuss some dynamic consequences of the
effects discovered such as dipole-active "ESR" transitions, rotation of electric
polarization by spins and the possibility of having a negative refraction.