Predicting earthquakes made possible?
A grand challenge in geophysics has been developing a method for
predicting
when a significant earthquake may occur. A team of researchers has
developed
a new method that may go a long way towards this goal. Using the method,
Manshoor {\it et al.} analyzed the fluctuations of the detrended
increments of
the time series for Earth's vertical velocity for many earthquakes.
Their
analysis reveals a significant change in the nature of the probability
density functions (PDF) of the series' increments. For a large
earthquake
the time at which the PDF undergoes a transition from a Gaussian to a
non-Gaussian is 5-10 hours. The key quantity that signals the
transition is a
parameter $\lambda_s$ that characterizes the shape of the PDF.
Far from the earthquake,
$\lambda_s\simeq 0$, but close to it $\lambda_s$ suddenly increases,
signaling
the transition. Figure 1 demonstrates this for an 7.1 earthquake that
occurred on May 21, 2003 in Spain.
The trends can also be seen in the PDF's flatness but not in the
signal itself.
Thus, the transition in the PDF, and the changes in
$\lambda_s$ and the PDF's flatness, all happening at the same time,
represent a new precursor for detecting impending significant
earthquakes.
A key insight is that, due to localization of elastic waves, only
stations
close to the epicenter provide the alert.
***
LT11399
Tuneable spin-polarized transport in amorphous CoFeB alloys
Relevant to the field of spintronics, a correlation between the spin polarization of tunnelling electrons (TSP) in AlOx/CoFeB tunnel junctions and the magnetic moments of amorphous Co80-xFexB20 alloys is reported. Such a correlation is surprising since the TSP involves s-like electrons close to the Fermi level (EF), while the magnetic moment mainly arises due to all d-electrons below EF.
The foundation of spintronics is based on the fact that s-like conduction electrons in transition metal ferromagnets get highly spin-polarized as a consequence of their interaction with localized d-electrons. Naturally, the extent to which this interaction influences the electronic structure of the conduction electrons, and the possibility of controlling the transport properties of these metals through this interaction, is an issue of vital importance. Consequently, we believe that this observed correlation and direct insight into the magnetic, electronic and transport properties of CoFeB alloys open several new possibilities to control, engineer and enhance the performance of spintronic devices
We find that the origin of this correlation is the change in the d-band exchange splitting and spin & orbital moments which force a transition from weak to strong ferromagnetism on the iron atoms. Ensuing this transition in the d-bands and due to the s-d interaction, a marked influence on the s-electron dominated TSP is observed which leads to the correlated behaviour of two entities primarily evolving from different aspects of the alloy electronic structure.
***
LY8951
A Double Look at the Resonant States of the Proton
The proton is not only the building block of atomic nuclei. As a
composite particle made of quarks and gluons, it can go to
excited states, or resonances of short lifetime. In this paper,
high-energy electron scattering at the Jefferson Laboratory
was used to study the excitation of the proton into resonances,
with their subsequent decay in two different channels: one
produces a photon, while the other produces a pion. Therefore
the first channel is governed by the electromagnetic interaction,
while the second one is governed by the strong interaction.
This double look allows new insight into the resonance behavior,
namely which physical features are similar or different between
both decays. The experiment provides an amount of consistent data
for constraining the theoretical models, which deal with the
very complex structure of the proton. At the upper end of the
measured excitation spectrum, we may even have seen hints of
a direct interaction with the quarks inside the proton. However,
to test this conjecture more experimental data will be needed,
in a specific kinematical domain that can be reached at the
future JLab 12 GeV upgrade.
***
LW10993
Attracted by repulsion: Exotic superfluidity in an expanding atomic cloud
Cold quantum particles can display frictionless flow, known as
superfluidity. For fermions this happens when pairs are formed by an
attractive interaction. Here we propose and study a paradoxical situation
where superfluidity of fermions occurs as a result of a very strong
repulsion, instead of attraction. The resulting superfluid state is
exotic, because the total momentum of each pair is non-zero, in contrast
to conventional superfluids. The recipe for creating such a state relies
on recent advances in atomic physics and is deceptively simple: If a dense
cold cloud of fermionic atoms is slowly expanded in the presence of a
lattice created by laser beams, then a superfluid state of doubly occupied
lattice sites emerges naturally. The reason behind this miracle is that
the pairs, although high in energy, are metastable because energy
conservation prohibits them to decay.
***
LT11699
Alfven instability in a compressible flow
A previously unknown macroscopic instability in flowing plasmas is
presented. Macroscopic instabilities modify the global structure and dynamics
in laboratory and space plasmas. Well-known examples include the
Rayleigh-Taylor and the Kelvin-Helmholtz (wind over water) instabilities. The
new instability does not have an analogue in hydrodynamics. It may only arise
in the presence of a compressible plasma flow embedded in an ambient magnetic
field. The kinetic energy of the flow is extracted and fed into transverse
disturbances that propagate along the magnetic field. The existence of such
transverse disturbances known as Alfven waves was established over sixty years
ago in laboratory conditions. There is an increasing volume of evidence to
suggest the presence of Alfven waves in space plasmas. However, so far little
has been known about their origin. The energetic and dynamic significance of
such waves cannot be underestimated. Possible implications include the heating
of the solar corona and the acceleration of the solar wind. The presented new
instability mechanism offers a unique and efficient way for the generation of
large amplitude Alfven waves. No flow shears or super-Alfvenic flow speeds are
required.
***
LQ11377B
Are the low-energy spectral features of the cuprate superconductors universal?
One of the hindrances in the general understanding of the cuprate superconductors
as a whole is the lack of evidences showing the universality of the low-energy spectral
features exhibited by the different cuprate superconductor families. In this paper, we
show that the low-energy spectral features like the asymmetry of the coherence peaks,
dips and humps beyond the peaks observed at the superconducting state of a 123 cuprate
system, viz., NdBa$_2$Cu$_3$O$_{7-\delta}$ are qualitatively similar to the ones repeatedly observed for
a different system, viz. Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ (2212 system) [1].
The scanning tunneling spectroscopy (STS) experiments and angle resolved photoemission
experiments on cuprates have established that the dips and humps beyond the coherence
peaks are observed when the CuO$_2$ plane layer is probed [1,2]. However, several
tunneling experiments performed on a different cuprate system like YBa$_2$Cu$_3$O$_{7-\delta}$ (123 family)
do not exhibit these features [1], raising the speculation that the CuO$_$2 planes of these systems might be
influenced by the existence of the other quasi one-dimensional CuO chain
layers in the unit cell of these cuprates. Our work indicates that a direct tunneling to the CuO$_2$
plane layer (avoiding the CuO chain layer in the tunneling path) would reveal the true features of the CuO$_2$
layer in the 123 systems. Similarity of the features as observed for NdBa$_2$Cu$_3$O$_{7-\delta}$ in the present work to the ones
for the 2212 system reported by others point towards the universality of these features. From the STS data,
we also observed a very clear signature of a comparatively lower energy bosonic mode being excited by the
tunneling electrons. The work would be important for further understanding of the high transition temperature
cuprate superconductors in general.
***
LS11002A
Efficient Frequency Conversion
Frequency conversion, or conversion of light colors, is a key concept in
the field of nonlinear optics. In this process, light of two colors is
mixed in a nonlinear crystal, resulting in the generation of a third
color with their sum or difference frequency. However, there is usually
a tradeoff between the bandwidth, which is the range of frequencies that
can be converted, and the efficiency of the conversion process. In this
research, we present a novel way to achieve both. Efficient conversion
for a bandwidth that is up 100 times wider than in conventional
conversion schemes is demonstrated. This scheme was discussed after
showing that the problem of frequency conversion can be mathematically
formulated and geometrically visualized in complete analogy with the
physics of two-level systems, as pioneered by Bloch and Feynman in NMR
and atomic physics. Using this analogy, the concept of “rapid adiabatic
passage” for robust efficient population transfer in two-level systems
is applied in the context of color conversion, and the requirements of
high efficiency and broad bandwidth were reconciled. This analogy and
its geometrical visualization, can bring new physical insight into the
process of frequency conversion and to better understanding of nonlinear
optical processes.
Image caption
"Adiabatic sum frequency conversion" scheme. a) Adiabatic (very slow)
variation of the periodicity along a nonlinear crystal is required for
efficient broadband color conversion. The adiabatic constraints are
discussed in the article. b) The dynamics of the process can be
visualized geometrically on a sphere surface, where the south-pole
represents the input color, and the north-pole represents the converted
color (Efficient conversion=reaching the north pole). Usually, one can
reach the north pole only for limited colors (in a conventional
conversion scheme). Here we present the adiabatic trajectory, where
large band of frequencies can reach close enough to the north pole (we
can send an avi-movie to demonstrate this dynamics). (c) Experimental
results - conversion efficiency as a function of input color (measured
in optical wavelength).
***
ER10481
When Engineering meets Physics: a new model for semi-flexible polymers.
The mechanical properties of cells and tissue is controlled by a network of relatively stiff polymer filaments like actin, collagen, etc. In polymer physics,these filaments are descibed as semi-flexible: flexible enough to fluctuate under Brownian forces, but stiff enough for the bending energy to limit the Brownian conformations that can be assumed. This semi-flexible character causes networks of these filaments to demonstrate previously-unseen mechanical behavior, but it also necessitates the use of a computer model since the mathematical equations descibing semi-flexible behavior are complex and difficult to solve. Our idea was to simulate a semi-flexible filament as a continuous string of tiny engineering beams, as opposed to the more traditional and computationally-intensive way of simulating it as a string of beads. We showed how the Brownian forces on a curved beam can be resolved by simple statics force balances, and how engineering Beam Theory can be used to solve the bending. By such an approach, we were able to avoid the common approximation that bending deflections be small, and did not need to impose an artificial constraint to prevent filaments from lengthening while bending. However, the most
important advantage of our approach is that one beam replaces multiple beads as the modeling unit, lowering the computational cost without compromising the physics.
***
LU12009
Ferroelectricity defies high pressure
Pressure has long been considered as the enemy of ferroelectricity, the
existence of a switchable polarization in piezoelectric materials. In this
article we show experimentally that the model ferroelectric perovskite,
lead titanate, exhibits a much more fascinating behavior under pressure: a
complex succession of phase transitions leads to the re-entrance of
ferroelectricity at high pressure. This behavior was predicted
theoretically through an original electronic effect and has been supported
by our preliminary experimental results. Here, combining synchrotron x-ray
diffraction and Raman spectroscopy under pressure we show that the
accommodation of the pressure-induced strain is done through rotation of
the oxygen octahedra and/or cation displacement in successive tetragonal
phases. These results open the way to new concepts for explaining this
intruiging form of high pressure ferroelectricity.
***
LU11406
MATTER WAVES PLAY THE PHOTON GAME
In the last years, scientists have engineered materials in which light
behaves in a very strange way. In the so-called photonic crystals, photons
- the quantum particles of light - may propagate or not, depending on their
energy. The presence of this "stop"-energy band for photons, severely
affects the light matter interaction within the crystal.
We have proposed a system in which matter waves behave like photons within
a photonic crystal. This idea seems reasonable, since Quantum Mechanics
teaches us that given some conditions, matter may also propagate as waves.
But in practice in order to observe matter-waves in full performance, one
needs very well controlled environmental conditions. In our proposal, this
is achieved by considering atoms frozen in an optical lattice - an
artificial atomic crystal created by laser beams. The release of atoms out
of the optical lattice turns out to be a process analogous to the emission
of photons inside a photonic crystal. In this way, the plethora of
phenomena observed with light-matter interaction in those crystals find a
matter-wave counterpart. On the other hand, contrary to photonic crystals,
optical lattices are highly controllable devices where experimentalists can
tune the parameters to drive the system into many different regimes. This
would allow to observe in particular two extreme regimes: One in which
emitted atoms get blocked and can not propagate, and another one in which
interference effects between the matter waves give rise to an enhancement
of the emission rate.
