Structural transition in compressed amorphous sulfur.
Two forms of amorphous sulfur (a-S), with different structures and
densities, have been observed under high pressures. Pressure-induced
amorphization and polyamorphic transitions retain a growing interest in
both fundamental and applied physics in the search for new families of
useful materials. In this paper, we report in situ x-ray diffraction
data on a-S between 50 to 100 GPa and 40 to 175 K, and an implemented
method to extract the density of non-crystalline materials at such
extreme conditions. Synthesized from pressurizing Sulfur I (S-I), a-S
undergoes an abrupt structural transition above 65 GPa, accompanied by a
density discontinuity of 7 %. These results show that this is a
polyamorphic transition, from a low density (LDA) to a high density
(HDA) form. Densities and structures of LDA and HDA forms are similar to
those of S-III and S-IV phases respectively, arguing in favor of their
nanocrystalline nature. The results cast light on the nature of pressure
amorphization, and provide a potential route for the synthesis of new
nano-materials.
***
LH11658
HIGH-SPIN MOLECULES FORMED IN SUPERFLUID HELIUM NANODROPLETS
In our research group, we investigate how to use helium droplets
made of only a few thousand He atoms (diameter of some ten nanometers,
and temperature of 0.4 Kelvin) to assemble high-spin molecules from
individual alkali-metal atoms.
In most materials, electrons pair up and their spins cancel each other.
Under special circumstances, some electrons remain unpaired and align
their spins, which are the source of magnetism in virtually every
material. Their study has very important practical applications,
most notably magnetic storage. Electron and nuclear spins can also
be very sensitive to their environment. Nuclear spins are thus a
routine diagnostic tool in medicine (MRI); both nuclear and electron
spin are of common use in research laboratories to learn about the
structure of molecules and materials. Electron spins are also
candidate Qubits for quantum computers. Finally, electron spins may
be the factor determining the outcome of a chemical reaction
(as well as the reactivity of the products) or the electrical
conductivity of a material.
Superfluid helium is a great environment: being cold and weakly
interacting, it simplifies the spectra of the atoms and molecules
under study. We make it into a jet of droplets in vacuum, easy to
load with the species of interest, in our case rubidium and potassium
atoms. Two or more atoms on the same droplet meet as if in a
nanometer-sized test tube, and form a weakly bound complex. We
concentrate on the high-spin
trimers, which we excite with laser radiation. The many excitation spectra
need to be unfolded from one another, which requires a variety of lasers,
plenty of ingenuity, and patience.
The spectra are assigned by matching them to our own highly accurate
electronic-structure calculations, that is: computer simulations
of the electrons' behavior in these molecules. By looking
at all possible combinations of three atoms, we uncover a regular pattern
which allows us to explain our observation, and in general to predict
what to expect from this class of interesting molecular systems.
We find that they share many interesting properties with quantum dots.
***
LG11386
"Breaking the sound barrier, tuning acoustic resonances, and zero-differential resistance induced by electric current"
If a 2D metal, also known as a quantum Hall system (QHS), is subject to a magnetic field, electrons’ motion becomes quantized into equally spaced levels. If it is further exposed to monochromatic radiation, the system will absorb the radiation when the level spacing matches the photon energy. This absorption can make electrical resistance to increase or virtually vanish [Nature02, PRL03, Physics Today 04].
When instead a QHS is subject to elevated temperatures, sound waves (phonons) become excited. Surprisingly, even though phonons of all different energies below thermal energy are present, electrons chose to predominantly respond to phonons of a certain energy making acoustic resonances possible [PRL 02].
In this paper we study these acoustic resonances in QHS driven by high electrical current. Remarkably, current can both tune [panel (a)] and enhance [panel (b)] phonon resonances, making them evident even at low temperatures. Further, as electrons are accelerated to the speed of sound a prominent peak [arrow in panel (a)] emerges in resistance indicating phonon emission when the sound barrier is broken. Finally, current induces a novel state with zero-differential resistance [panel (c)] which appears to be an analog to radiation-induced zero resistance states [Nature02, PRL03].
***
LK11392
Is Ball Lightning a Dusty Plasma?
In a Physical Review Letter, published in 2006, Eli Jerby and Vladimir
Dikhtyar of the University of Tel-Aviv
announced the laboratory formation of a floating, glowing plasma ball,
closely resembling the phenomenon known
as « Ball Lightning ». This fireball, several centimetres in diameter,
was produced in air by the detachment of
a microwave created discharge from a glass surface. In a follow-up
experiment, Jerby and co-workers, Brian
Mitchell and Jean-Luc LeGarrec from the University of Rennes I and
Theyencheri Narayanan and Michael Sztucki
from the European Synchrotron Radiation Facility (ESRF) in France have
reproduced this fireball and have used
Small Angle X-ray Scattering (SAXS) to demonstrate that the ball
consists of a dusty plasma containing glowing
nanoparticles with a mean diameter of 50 nm. (Physical Review Letters,
in press, 2008). The SAXS technique has
been used recently to map the growth of soot and other nanoparticles in
hydrocarbon flames but this is believed
to be the first time that it has been used to study particles in a
plasma. The dusty plasma fireball, that could
be sustained by microwave energy for up to 15 minutes, was shown to
decay in a time of 2 seconds after the
microwave power was cut off. Future experiments will seek to extend this
decay time by using reactive, but
slowly oxidizing precursor mixtures that have been proposed by
Abrahamson (Nature, 2002) to explain the fact
that Ball Lightning can survive for up to several tens of seconds.
***
LJ11443
Looking Inside a Spin Resonance
The SPIN@COSY polarized beam team found striking new results [1] while studying
the spin-manipulation of polarized deuterons, at the COSY 3.5 GeV/c proton and
deuteron storage ring at the Forschungszentrum in Julich.
The team - from Michigan, COSY, Bonn, J-PARC, Indiana and Groningen, led by
Alan Krisch - used a new RFsolenoid magnet (see Fig. 1) to manipulate the spins
of stored 1.85 GeV/c deuterons, which are spin-1 bosons.
The new RF-solenoid was designed by Michigan graduate student Maria Leonova and
J-PARC electrical engineer Alexander Schnase, and built by Dieter Prasuhn and
his accelerator team at COSY. It used the same sophisticated RF high-voltage
supply as its predecessor RF-dipole. However, the RF solenoid produced a
longitudinal RF magnetic field rather than a radial magnetic field.
The goal of the experiment was to precisely test a new analytic matrix
formalism [2] developed by a theoretical member of the SPIN@COSY team,
Alexander Chao of SLAC. The Chao formalism is the first generalization of the
famed 1960 Froissart-Stora formula [3], which allows one to calculate the beam
polarization after passing through a spin resonance. However, as Froissart and
Stora correctly wrote, their formula is only valid if one measures the initial
beam polarization long before crossing the spin resonance and the final beam
polarization long after crossing it. As polarized beam hardware and the
understanding of spin dynamics improved, polarized beam enthusiasts became
eager to learn what happens very near or even inside a spin resonance.
Thus, Michigan PostDoc Vasily Morozov used the Chao formalism to calculate in
detail what might happen in a new type of experiment, where a 1 MHz RF-magnet's
frequency is swept by a fixed range of 400 Hz, while its end-frequency f_end is
stepped through many different values near and inside spin resonance (see Fig.
2). The Chao-Morozov calculations predicted that, if the magnets resonance
strength was not strong enough to fully flip the spin, then there would be
large oscillations in the final polarization. These oscillations seem so
sensitive to the resonance strength and other parameters, such as the beam's
momentum spread delta-p/p, and the resonances central frequency f_r, that the
oscillations might provide a new way to precisely measure such parameters.
The data from this new type of experiment showed striking oscillations that
agreed very well with these calculations (see Fig. 3). The experiment's data
also verified the polarization's extreme sensitivity to the resonances
strength, the resonance's frequency spread (due to the beam's momentum spread),
and the resonance's central frequency f_r. Moreover, the data [1, 4] clearly
demonstrated that the oscillation's size increased rapidly as the beam's
momentum spread decreased.
These new experimental results also confirm the validity of the Chao matrix
formalism. Thus, it may now be used to better understand the behaviour of the
100-250 GeV polarized protons stored in Brookhavens RHIC and perhaps someday
polarized antiprotons in FAIR's 15 GeV HESR at GSI, or polarized protons stored
in Japan's 30-50 GeV J-PARC or even in CERN's 7 TeV Large Hadron Collider.
***
lg11105
Electric sand findings could lead to better climate models
ANN ARBOR, Mich.--- Wind isn’t acting alone in the geological process
behind erosion, sand dunes and airborne dust particles called aerosols.
The other culprit is electricity. By taking both factors into account,
researchers at the University of Michigan have developed a new model
that matches real-world measurements of “saltation” better than the
decades-old classical theory.
Saltation is the process of wind blowing grains of sand across a
landscape, sending them bouncing against the ground and each other. The
bouncing motion of the saltating grains on the soil bed kicks dust
aerosols into the air.
This new knowledge could lead to better climate models because it helps
scientists understand how aerosols are released, U-M researchers say.
Dust is one type of aerosol. Burning fossil fuels releases another
type. They are known to affect Earth’s climate by blocking and
absorbing sunlight and seeding clouds.
Nilton Renno, associate professor in the Department of Atmospheric,
Oceanic and Space Sciences, and doctoral student Jasper Kok have
demonstrated that saltation creates a field of static electricity that
can be strong enough to double the concentration of bouncing sand
particles, compared to previous assumptions. A paper on their findings
will be published in an upcoming issue of Physical Review Letters.
“The effect of aerosols is one of the most uncertain processes in
climate change modeling,” Kok said. “We now know more of the physics of
how dust aerosols get into the atmosphere, so we should be able to
improve on the way that climate models account for their emission.”
Saltation itself has never been fully understood. Only recently have
detailed measurements been made in nature, as opposed to in a wind
tunnel. And those natural measurements disagreed with classical theory.
Renno first noticed that electricity might be missing from the equation
while studying dust devils in Arizona years ago. The devils had a
strong electric field.
“I was surprised at how large the field was,” Renno said.
Others had suggested that electricity may be involved in saltation, but
Renno said no one determined the extent of that role and created a
model to describe the process including electricity, until now.
“What we discovered is as these particles bounce and rub against each
other, the surface of the ground gets a positive charge and the
particles get a negative charge,” Renno said. “The electric field can
become strong enough to directly lift sand from the surface.”
The surface of the ground acts as a conductor, Kok explained, because
it has a thin film of water on top.
The researchers say this model can accurately reproduce observations.
“It’s a fundamental change in our understanding of the physics of
saltation,” Renno said.
Renno, who is a co-investigator on NASA’s Phoenix and Mars Science
Laboratory missions to Mars, speculates that these saltation electric
fields get so large on the Red Planet they produce ground-level sparks.
The paper is called “Electrostatics in wind-blown sand.”
Kok and Renno's research on the basic physics of saltation and its
implications to climate has been supported by the National Science
Foundation’s Physical and Dynamic Meteorology Program.
