Inside Aerogels in Nanoscale 3-D
Until now, no one has ever seen inside an aerogel to determine its
structure on the nanoscale. By performing high-resolution diffraction
imaging of an aerogel with laser-like soft xrays, a research team has
revealed its three-dimensional bulk lattice structure down to features
measured in mere billionths of a meter.
The x-ray beam passed through the aerogel sample and was diffracted onto
a CCD detector screen; some 150 diffraction patterns were stored as the
sample moved and rotated. Fast computers and improved algorithms allowed
millions of calculations to position each measured photon precisely in
three-dimensional space, a key advance that made the 3-D nanoscale
imaging possible.
¿Seeing inside bulk porous materials has never been done before at this
resolution,¿ says Stefano Marchesini, who led the research at beamline
9.0.1 of the Department of Energy¿s Advanced Light Source at Lawrence
Berkeley National Laboratory. ¿This one of the first applications of
x-ray diffractive microscopy to a real problem.¿
The strength of foam-like structures typically scales with their
density, but aerogels, less than two-percent dense, are orders of
magnitude weaker than expected. The structure revealed in the new 3-D
images reveals why, and suggests changes in aerogel preparation that
might improve their strength.
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LU11168
The lowest energy state of the "bottomonium"
Collaborators working on the BaBar experiment at the Stanford Linear
Accelerator Center
have discovered the lowest energy state of the "bottomonium" family
whose members are
bound states of a bottom quark with an anti-bottom quark held
together in different angular
momentum configurations. This spin-0 particle, named eta_b, was
observed for the first time
some thirty years after the discovery of the spin-1 tower of states
of the bottomonium spectrum
called Upsilon(nS). The precise determination of the hyperfine mass
splitting between the eta_b
and the Upsilon(1S) plays a key role in understanding the effect of
spin-spin interactions in these
systems as well as fundamental properties of the strong force. In the
BaBar detector, the
highly-energetic collision of an electron and a positron can result
in the production of an
Upsilon (3S) particle, which in turn can decay to an eta_b by
emitting a gamma ray. This eta_b
production mechanism occurs just once for every two to three thousand
Upsilon(3S) decays.
To identify a significant sample of eta_b's in the presence of
background noise it was necessary
to record more than 100 million of such Upsilon(3S) events. This
achievement was made possible
by the excellent luminosity of the PEP-II accelerator and the
advanced technology of the BaBar detector.
***
BPJ1079BR
Size-specific cracking shakes out at the nanoscale
As the size of a structure is reduced towards the nanoscale, atomic
vibrations (phonons) begin to feel its size and shape, an effect
called phonon confinement. While these size effects on phonons are
known to play an important role in thermal transport, electronic
processes, and thermodynamic stability, little is known about their
role in fracture. In this article, we report that for plates of
cerium hydride there is a characteristic thickness at which the
entropy of these confined vibrations leads to a minimum in the
fracture energy, resulting size specific fracture. While this idea
was developed to explain the behavior of cerium hydride, it is much
more general and has important ramifications for the design of
nanostructures. For example, it implies that certain sizes may be
more susceptible to failure by fracture than others. It may also
prove useful in the deliberate creation of large quantities of stable
nanostructures by fracture.
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LN11150
Tuning in on ultracold molecules
Ultra-cold molecular gases (at a temperature typically below one
micro-Kelvin) are the coolest molecular system in the universe.
Nowadays, the most successful route to form them is to associate
previously cooled atoms by using either laser or magnetic
fields. Unfortunately, only highly excited molecules can be
efficiently populated leading to unstable and short-lived samples.
In our article, we proposed a novel method to efficiently produce
ultra-cold stable molecules in their lowest vibrational levels.
This work solves many of the challenges to produce such molecules by
using giant formation rates obtained with a tunable magnetic field
and a laser field. Consequently, it opens the door to a broad variety of
applications ranging from quantum computing to high
resolution spectroscopy at a frontier between physics and chemistry.
***
LN11381BR
Oscillatory Hall effect in high-mobility two dimensional electron
gases
Summary: The formation of magnetically ordered states in initially
non-magnetic materials makes research on spontaneous spin polarization
exciting. An excellent tool to investigate such spin phenomena is the
measurement of the zero-field Hall coefficient: In the presence of
localized spins, electron scattering is spin-selective and leads to an
anomalous contribution to the Hall voltage. This manuscript reports an
unexpected anomaly in the zero-field Hall coefficient of two dimensional
electron systems (2DES). At very low temperature, both positive and
negative deviations from the non-interacting Hall coefficient
$\gamma_{\rm H}^0$ are observed, which can be twice as large as
$\gamma_{\rm H}^0$ itself. A distinct regularity in the deviations and
their temperature dependence are interpreted as the spontaneous
formation of localized spins and their mutual indirect interaction,
indicating predominantly anti-ferromagnetic spontaneous magnetic
interactions in 2DES at low temperature.
***
LN11510
Nanoearthquakes and MicroTornados Form and Wipe Out Colloidal Islands
Just like earthquakes potentially lead to the generation of tsunami waves, tiny vibrations on a plate on which a small water drop sits can create large waves at the drop surface---with peculiar consequences. If the drop were to contain nanoparticles, Australian scientists Drs Haiyan Li, James Friend and Leslie Yeo at Monash University's Micro/Nanophysics Research Laboratory have discovered a peculiar phenomenon in which these waves could result in the formation, evolution and destruction of island-like clusters of these particles on the water surface.
The tiny vibrations are surface acoustic waves or SAWs, which are megahertz frequency sound waves about ten nanometers in amplitude. A nanometer is one billionth of a meter---the width of a human hair is therefore about one hundred thousand nanometers. SAWs have, for decades, been employed in the telecommunications industry for signal processing. A typical mobile phone would therefore have several SAW devices.
Though they may be small, SAWs have large surface accelerations---typically, millions of g's. This is the reason why they can create large waves on the surface of a water drop measuring several millimeters in diameter sitting above them. These waves, in turn, bounce the nanoparticles around on the drop surface to form island clusters in a manner similar to the patterns formed when sand is sprinkled onto the surface of a vibrating metal plate.
As the islands form at nodal positions of the standing wave, i.e., points where the waves cancel each other due to destructive interference, which depend on the size of the drop, the number of islands decrease successively as the drop evaporates and hence reduces in size. In their work, the researchers show how the number of islands and their positions on the drop can be predicted.
Curiously though, the islands are wiped out intermittently when the SAWs trigger an instability which causes the liquid in the drop to recirculate rapidly like miniature tornados. This anomalous behavior, attributed to chaotic flow, only lasts a few seconds. When the recirculation stops, the islands reform and the cycle is repeated.
This research, which is to be published in Physical Review Letters, has important implications in microfluidics, which is the science of how fluids and particles can be manipulated at tiny scales. Already, the researchers are using SAWs to make tiny pumps, reactors, centrifuges and separators, as part of a larger effort to develop portable medical diagnostic kits and drug delivery systems.
***
LL11129
Subtleties in the Quantum Coupling of Light and Matter in a Semiconductor
Cavity Quantum Electrodynamics harnesses the quantum theory of
light an matter in the confine of a cavity. For decades, optical
macrocavities have been used to trap light, into which Rydberg atoms
were sent as probes of "the best of our theories". The quantum (or
"strong") coupling (SC) of a photon with an atom results in the notion
of light and matter to vanish away. New quantum states substitute
them, combining their properties. With atoms, the signature of SC is
the observation of anticrossing of the photon and atom energies: when
they are brought together, the emergence of new quantum objects give
rise to two new energies. This is seen as two peaks in, e.g., an
emission spectrum. This strong qualitative effect has been regarded as
the landmark of SC, but is not. We found that in a semiconductor, the
situation is more subtle: the physics remains the same, but can reveal
differently. Anticrossing can be overlooked, or spurious interferences
can mimic it. By considering self-consistently pumping and
decoherence, we obtained excellent agreement with the experimental
data, that so far were content to observe the qualitative effect of
anticrossing. We confirmed quantitatively the reports of SC in
semiconductors, but found that it manifests here with unexpected
subtleties.
