Transforming the Biphotons
How to shape the biphotons according to need is a hot topic. In an upcoming
issue of Physical Review Letters, researchers transform the spatial profile
of the biphotons by manipulating the domain structure of the nonlinear
crystal. The experiment results reveal that the modulation of the domain
pattern is transferred into the spatial mode of the generated biphotons.
This technique is very useful for integration of the quantum optics device
and generation of the novel entanglement state.
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LQ11802
When neutron constituents behave like a single constituent
In the first precision test of the neutron¹s so-called ³quark-hadron
duality,² physicists have found that spinning neutrons exibit quark-hadron
duality, similar to the observations on protons before. This observation may
provide insight into the particles and the force that builds nearly all of
the visible matter in the universe.
From large distances, a proton or a neutron appears as a strongly
interacting hadron, a tight cluster of quarks and gluons, which can be
excited into various quark configurations called resonances. When probed
from short distances, protons and neutrons appear as an incoherent state of
quasi-free quarks. Though seemingly far different, measurements in these two
regions show, on average, a remarkably similar behavior, a phenomenon called
³quark-hadron duality.²
In the last decade, there has been significant improvement in high-precision
duality data on the spin-averaged structure of the proton and of nuclei. It
was also shown for the first time that quark-hadron duality holds for the
proton spin-polarized structure function. But one essential experimental
result that has been missing is a test on the neutron structure.
During the early months of 2003, a group of physicists at Jefferson Lab
conducted an experiment aimed at measuring the neutron spin structure in the
resonance region at moderate energies. These resonance measurements were
then compared to the expected curve derived from the data taken on the
quasi-free quarks. It was found that, within experimental errors,
quark-hadron duality starts to work for the neutron spin structure at about
the same energy as for the proton.
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LD11686
Spindles, cusps and bifurcation for capsules in strong flows
Capsules are commonly used in a variety of industrial and biomedical
applications that require controlled release of medical agents (as in
drug delivery), aromas and flavors. However, little is currently known
at the deformation and dynamics of these membrane-enclosed fluid volumes
at strong flows owing to the complicated coupling of the fluid dynamics
with the membrane properties.
Based on computational investigation, our study shows that
strain-hardening capsules in strong extensional flows develop steady-state
shapes whose edges from spindled (or concave) become cusped with
increasing flow rate owing to a transition of the edge tensions from
tensile to compressive. A bifurcation in the steady-state shapes
is also found (i.e. existence of both spindled and cusped edges for
a range of high flow rates) by implementing different experiments,
owing to the different evolution of the membrane tensions. Thus our
present work complements the similar evolution for low-viscosity drops
which was first identified by the famous experiments of G. I. Taylor
in 1934, further explained in the 1970's and 1980's and still finds
useful applications nowadays. In addition, our study elucidates the
importance of compressive tensions in capsule mechanical deformation
that is applicable to both industrial and physiological processes.
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BU10860
Can a glass be superfluid?
Glasses are often described as just liquids
that have stopped to flow. Superfluids instead flow without any
resistance. The existence of a phase of matter characterized by both
properties at the same time seems therefore utterly impossible. In
this paper we show instead that interacting quantum particles can form
a "super-glass" phase at very low temperature and high density; this
theory of the super-glass phase confirms previous numerical
results. This purely quantum phase is indeed characterized by an
amorphous density profile, as e.g. window glasses, but at the same
time by a finite fraction of the atoms that flow without any
resistance. The properties of this new phase of matter are
investigated for a class of models that turn out to be particularly
easy to analyze. The super-glass phase is not only a dream of
theoreticians but it is very likely to be observed in
experiments. Many recent experimental results on solid Helium 4 indeed
show many evidences of a super-solid phase of matter. Super-glass
phases are also likely to appear in another very different context:
mixture of cold atoms trapped by lasers in frustrated lattices.
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BUR1079
The magnetic response of a bilayer of ferromagnetic (FM) and
antiferromagnetic (AF) thin films can be shifted along the field axis by
the exchange coupling of pinned interfacial spins with the FM. This
behavior is called exchange bias, and is widely used in the information
storage industry. We have used element-specific, soft
x-ray reflection to examine the interfacial spin configurations that
produce exchange bias in the prototypical (FM = Permalloy)/(AF = CoO)
system. In a ~1nm interfacial region, Co atoms showing a ferromagnetic
responses are present in two forms. Most of the FM Co responds to
applied fields exactly as does the Permalloy. However, about 10% of the
interfacial FM Co is pinned in a direction antiparallel to the applied
cooling field, and therefore is exchange-coupled antiferromagnetically
to the Permalloy, and is likely to be responsible for the exchange bias.
There are no measurable interfacial pinned spins in the Permalloy. The
interfacial region exhibits a temperature dependent magnetic behavior
that strongly differs from that of CoO and Permalloy.
***
LU12019
New test for mystery dark energy
How do we detect something we don't understand?
Over the last half century our understanding of the large scale
universe has gone from basically zero to a refined science driven by
the interplay between precise observations and sophisticated model
building. Of course, major questions remain such as the nature of the
big bang, and why the universe is here at all. Arguably the most
important at the moment is the most bizarre: the expansion of our
universe is speeding up, not slowing down. The 'dark energy'
apparently responsible is categorically unexpected in fundamental
physics, and is now one of the most important and far-reaching
mysteries in science today.
The critical issue facing cosmologists lies in determining if
Einstein's 'cosmological constant' is the underlying ingredient of
dark energy. Until a physically-motivated alternative is discovered,
it is imperative that we establish whether this simplest and currently
observationally favoured model is in fact correct or not. But our
total lack of understanding of the physics of dark energy - including
this
cosmological constant - places
severe constrains on our ability to say anything about its possible
dynamical nature: is this a field which changes with the cosmic
expansion? In this Letter, we present a new observational test which can
signal if the cosmological constant is the wrong answer, but,
critically, without having to invent ad hoc phenomenological
alternatives to it.
***
AP10366
NEW POSSIBILITIES FOR ELECTROMAGNETIC PULSES SLOWING IN BEC
As known, the propagation velocity of an electromagnetic pulse in medium
can be lower than in vacuum. For example, one of the most common and dense
materials in this meaning is diamond, where the light propagates slower in
2.5 times. But, the fact that the signal velocity in medium can differ
more than in million times in comparison with its value in free space is
rather unusual. The mentioned optical properties, in particular, are
typical of gases in the Bose-Einstein condensation (BEC) phase. The
authors theoretically investigate the ultra-slow light phenomenon in such
systems in the framework of the developed microscopic approach. It is
shown that for BEC of sodium atoms without a stimulated transparency the
minimum velocity for light pulses reaches the limit of 300 m/s. We find
that microwave signals can be more intensively slowed to 0.001 m/s. It is
also shown that the propagation velocity can strongly depend on the
intensity of the external magnetic field. Moreover, in some cases there
can propagate electromagnetic pulses with a negative group velocity. Thus
the investigation emphasizes one more time the uniqueness of the
properties of a new phase, which was reached at first time in 1995.
