LK11352
A MECHANISM FOR BACTERIAL CELL DIVISION
In this work we proposed a new molecular mechanism which may
be responsible for driving cell division in bacteria. It is
well known that in a common bacteria like Eucherichia coli,
during its division, the cell wall at the division site grows
radially inward like the shutter of a camera and guillotins
the cell into two daughters. The wall is believed to be pulled
upon from inside by a polymeric ring which itself shrinks in
radius. The ring is made of short filaments of an intracellular
protein called FtsZ, but it is not understood why the ring
contracts or equivalently how it generates the required
contractile force. We propose a theoretical model and simulate
it to show how force generation may result from the natural
curvature of the FtsZ filaments, lateral attraction among
them and conversion of GTP into GDP (hydrolysis), which is
ubuiquitous in most cellular phenomena. Understanding the
detailed mechanism that drives the contraction of the ring
may help design new class of antibiotics which can block the
contrcation and thereby halt bacterial proliferation.
Friday, October 3, 2008
Thursday, October 2, 2008
10-2-08
AQ10395
Diffraction in low-energy electron scattering from DNA: bridging gas
phase and solid state theory
Can the microscopic physics of an electron colliding with a small
molecule be scaled up to help understand DNA strand breakage and
mutations induced by high energy radiation? A recent theoretical
breakthrough suggests that this might be possible. When radiation hits a
cell, it generates copious amounts of low-energy electrons, which in
turn can damage DNA and cause mutations. In this paper, calculations
demonstrate that collective effects between the DNA bases are just as
important as single base properties, and that the overall structure of
DNA influences the energies at which damage can occur. Starting from an
accurate model for low-energy electron collisions with the DNA bases in
the gas phase, followed by assembly of the microscopic theoretical
pieces to simulate solid-state behavior, the authors explore factors
that might influence radiation damage, such as several structural
parameters and the sequence and range of the interactions. Comparison
with DNA thin-film experiments shows a good match with calculations for
A-form DNA.
***
BUR1112
The impact of atomic intermixing in ferromagnet/semiconductor hybrid
structures on the performance of spintronic devices
Semiconductor spintronics comprises a new class of devices which, in
contrast to conventional electronics, do not only utilize the charge but
also the spin, a purely quantum physical property of electrons. One
important building block for such devices is the generation of electrons
with a defined spin orientation in a semiconductor. In this respect, the
combination of semiconductors with ferromagnetic metals turns out to be
the most promising approach. In such hybrid structures, spin-oriented
electrons are introduced into the semiconductor by electrical injection from a
ferromagnetic layer, which is called spin injection. One possible drawback
for spintronic applications is the fact, that intermixing during the
preparation of ferromagnet/semiconductor structures is difficult to avoid. We
observed for one promising material system, Co_2FeSi on (Al,Ga)As, that under
certain preparation conditions the electronic and magnetic properties of the
semiconductor part in devices are strongly modified due to intermixing during
the deposition of a ferromagnetic layer. In some cases, the desired spin
orientation of the electrons injected from the ferromagnetic layer gets
completely lost due to the presence of magnetic impurities in the
semiconductor part of the devices, which have been introduced there by
intermixing processes. Nevertheless, we found preparation conditions for
which the successful generation of spin oriented electrons can be
observed with an spin injection efficiency of at least 50%. Our results
demonstrate the importance of further studies on intermixing processes from
which we expect the development of preparation concepts leading to even
larger spin-generation efficiencies, which is 100% in the ideal case. For the
investigated Heusler alloy Co_2FeSi a 100% efficiency appears not to be
unrealistic because of the predicted halfmetallic characteristics. Halfmetals
are ideal candidates for spin injection since in such materials mobile
electrons exhibit only one defined spin orientation. Finally, our findings
indicate that so-called tunneling processes might be not necessarily
important for the spin generation, in contrast to what is commonly believed.
***
BV10868
Optics under extreme conditions : one step beyond.
Studying the properties of condensed matter under extreme conditions has
yielded many striking discoveries such as Quantum Hall Effect and Giant
Magnetoresistance. It is also of overwhelming interest to understand the
properties of materials such as superconductors, nanomaterials or
semiconductors, very promising for a huge variety of applications.
Recently, a team from the High Magnetic Field Laboratory (LNCMP) in
Toulouse has largely extended the experimentally achievable domain by
combining pressure of 100 000 atmospheres (10 GPa) and pulsed magnetic
field 600 000 times stronger than the earth magnetic field (60 T). Using
optical fibers and a Diamond Anvil Cell, they have measured the
photoluminescence of tiny crystals of ruby and explored their microscopic
properties through a careful analysis of the Zeeman and Paschen-Back
effects leading to a better comprehension of this largely explored gem.
***
LU11070
ON THE STRUCTURE OF THE VORTEX LATTICE IN SUPERCONDUCTORS
What does the vortex lattice in superconductors look like ? The answer to
this question is of prime importance for designing current carrying devices
using superconductors. Abrikosov proposed in the fifties that the vortex
lattice is formed by flux tubes that order in a hexagonal lattice. Two
decades ago, researchers at Lucent Technologies studied the material NbSe2,
considered by many as a prototypical superconductor. They found that the
hexagonal vortex lattice is formed by star shaped, rather than round, tubes.
The question if these six-fold star shaped tubes are specific of the
material studied, NbSe2, or a more general feature of superconductors
remained since then without answer. In a recent work it has been found,
using a scanning tunneling microscope cooled to millikelvin temperatures,
that the star shape of the flux tubes is produced by a charge density wave,
specific to NbSe2. The vortex lattice in superconductors generally looks
like round flux tubes ordered in a hexagonal lattice, although some
materials properties may considerably influence the cylindrical nature of
the flux tubes.
***
LT11977
Can the cosmic acceleration be explained without dark energy?
Astronomers have discovered that we live in an accelerating Universe.
It has been suggested that this acceleration is being caused by
a mysterious substance which has negative pressure, and which has
been dubbed `Dark Energy'. Some researchers believe that it may not
be necessary to invoke dark energy to explain the acceleration, and that
cosmic inhomogeneities may do the job. What this means is the following.
The observed acceleration is a property of an idealized homogeneous
Universe - this acceleration has been inferred by comparing observations
with the equations of general relativity applied to the homogeneous
Universe. However the actual Universe we live in is highly inhomogeneous,
and it appears homogeneous only when averaged on sufficiently large
length scales. The process of averaging the equations of general relativity
produces correction terms which modify the equations for the
homogeneous case. Could these correction terms be large enough to
mimic the contribution of a dark energy, and hence explain
the acceleration? In this paper we investigate a reasonably general
model to show this not to be the case. The model considers the growth
and formation of nonlinear cosmic structures and voids, and applies a
well-defined averaging scheme to show that averaging over such realistic
inhomogeneities produces only a very tiny correction to the
homogeneous equations. Hence we conclude that the averaging
of cosmic inhomogeneities cannot explain the observed cosmic acceleration.
***
LU11729
Crystalline ice -- a quantum chemist's view
Simulating a material's properties in a computer is a formidable
challenge for scientists at the best of times. The workhorse of solid
state physicists, density functional theory, generally works well, but
has its limitations. Would it not be great to make use of quantum
chemists' accurate yet costly methods also for large, periodic systems?
In this paper, for the first time quantum chemical methods are used to
describe ground state properties of ice, one of the most intriguing and
fascinating materials on Earth. The authors achieve unprecedented
agreement with experimental data, from ab-initio calculations that do
not include any fitted parameters. Besides predicting the elastic
properties of ice at very low temperatures, their work also suggests a
new approach for rather inexpensive accurate simulations of liquid
water -- which would be of tremendous help to shed light on the
anomalous properties of this ubiquitous substance.
***
LT11582
Exact Calculations of the Force of the Quantum Vacuum
Our group at the University of Oklahoma has discovered that
in many cases exact explicit formulas can be found for so-called
Casimir forces between distinct bodies. These forces can be viewed
as the summation of van der Waals interactions between molecules that
make up the bodies. In the past, the only exact results were for the
forces between infinite parallel plates, so various approximation
methods were employed to estimate forces between bodies with curvature
or finite size. Our results prove that these approximations are often
unreliable. The techniques developed in our work, together with those
of other workers, will enable application of quantum vacuum forces to
nanotechnology; elsewhere we have already proposed nano-gears, for
which we can now calculate reliable results.
Diffraction in low-energy electron scattering from DNA: bridging gas
phase and solid state theory
Can the microscopic physics of an electron colliding with a small
molecule be scaled up to help understand DNA strand breakage and
mutations induced by high energy radiation? A recent theoretical
breakthrough suggests that this might be possible. When radiation hits a
cell, it generates copious amounts of low-energy electrons, which in
turn can damage DNA and cause mutations. In this paper, calculations
demonstrate that collective effects between the DNA bases are just as
important as single base properties, and that the overall structure of
DNA influences the energies at which damage can occur. Starting from an
accurate model for low-energy electron collisions with the DNA bases in
the gas phase, followed by assembly of the microscopic theoretical
pieces to simulate solid-state behavior, the authors explore factors
that might influence radiation damage, such as several structural
parameters and the sequence and range of the interactions. Comparison
with DNA thin-film experiments shows a good match with calculations for
A-form DNA.
***
BUR1112
The impact of atomic intermixing in ferromagnet/semiconductor hybrid
structures on the performance of spintronic devices
Semiconductor spintronics comprises a new class of devices which, in
contrast to conventional electronics, do not only utilize the charge but
also the spin, a purely quantum physical property of electrons. One
important building block for such devices is the generation of electrons
with a defined spin orientation in a semiconductor. In this respect, the
combination of semiconductors with ferromagnetic metals turns out to be
the most promising approach. In such hybrid structures, spin-oriented
electrons are introduced into the semiconductor by electrical injection from a
ferromagnetic layer, which is called spin injection. One possible drawback
for spintronic applications is the fact, that intermixing during the
preparation of ferromagnet/semiconductor structures is difficult to avoid. We
observed for one promising material system, Co_2FeSi on (Al,Ga)As, that under
certain preparation conditions the electronic and magnetic properties of the
semiconductor part in devices are strongly modified due to intermixing during
the deposition of a ferromagnetic layer. In some cases, the desired spin
orientation of the electrons injected from the ferromagnetic layer gets
completely lost due to the presence of magnetic impurities in the
semiconductor part of the devices, which have been introduced there by
intermixing processes. Nevertheless, we found preparation conditions for
which the successful generation of spin oriented electrons can be
observed with an spin injection efficiency of at least 50%. Our results
demonstrate the importance of further studies on intermixing processes from
which we expect the development of preparation concepts leading to even
larger spin-generation efficiencies, which is 100% in the ideal case. For the
investigated Heusler alloy Co_2FeSi a 100% efficiency appears not to be
unrealistic because of the predicted halfmetallic characteristics. Halfmetals
are ideal candidates for spin injection since in such materials mobile
electrons exhibit only one defined spin orientation. Finally, our findings
indicate that so-called tunneling processes might be not necessarily
important for the spin generation, in contrast to what is commonly believed.
***
BV10868
Optics under extreme conditions : one step beyond.
Studying the properties of condensed matter under extreme conditions has
yielded many striking discoveries such as Quantum Hall Effect and Giant
Magnetoresistance. It is also of overwhelming interest to understand the
properties of materials such as superconductors, nanomaterials or
semiconductors, very promising for a huge variety of applications.
Recently, a team from the High Magnetic Field Laboratory (LNCMP) in
Toulouse has largely extended the experimentally achievable domain by
combining pressure of 100 000 atmospheres (10 GPa) and pulsed magnetic
field 600 000 times stronger than the earth magnetic field (60 T). Using
optical fibers and a Diamond Anvil Cell, they have measured the
photoluminescence of tiny crystals of ruby and explored their microscopic
properties through a careful analysis of the Zeeman and Paschen-Back
effects leading to a better comprehension of this largely explored gem.
***
LU11070
ON THE STRUCTURE OF THE VORTEX LATTICE IN SUPERCONDUCTORS
What does the vortex lattice in superconductors look like ? The answer to
this question is of prime importance for designing current carrying devices
using superconductors. Abrikosov proposed in the fifties that the vortex
lattice is formed by flux tubes that order in a hexagonal lattice. Two
decades ago, researchers at Lucent Technologies studied the material NbSe2,
considered by many as a prototypical superconductor. They found that the
hexagonal vortex lattice is formed by star shaped, rather than round, tubes.
The question if these six-fold star shaped tubes are specific of the
material studied, NbSe2, or a more general feature of superconductors
remained since then without answer. In a recent work it has been found,
using a scanning tunneling microscope cooled to millikelvin temperatures,
that the star shape of the flux tubes is produced by a charge density wave,
specific to NbSe2. The vortex lattice in superconductors generally looks
like round flux tubes ordered in a hexagonal lattice, although some
materials properties may considerably influence the cylindrical nature of
the flux tubes.
***
LT11977
Can the cosmic acceleration be explained without dark energy?
Astronomers have discovered that we live in an accelerating Universe.
It has been suggested that this acceleration is being caused by
a mysterious substance which has negative pressure, and which has
been dubbed `Dark Energy'. Some researchers believe that it may not
be necessary to invoke dark energy to explain the acceleration, and that
cosmic inhomogeneities may do the job. What this means is the following.
The observed acceleration is a property of an idealized homogeneous
Universe - this acceleration has been inferred by comparing observations
with the equations of general relativity applied to the homogeneous
Universe. However the actual Universe we live in is highly inhomogeneous,
and it appears homogeneous only when averaged on sufficiently large
length scales. The process of averaging the equations of general relativity
produces correction terms which modify the equations for the
homogeneous case. Could these correction terms be large enough to
mimic the contribution of a dark energy, and hence explain
the acceleration? In this paper we investigate a reasonably general
model to show this not to be the case. The model considers the growth
and formation of nonlinear cosmic structures and voids, and applies a
well-defined averaging scheme to show that averaging over such realistic
inhomogeneities produces only a very tiny correction to the
homogeneous equations. Hence we conclude that the averaging
of cosmic inhomogeneities cannot explain the observed cosmic acceleration.
***
LU11729
Crystalline ice -- a quantum chemist's view
Simulating a material's properties in a computer is a formidable
challenge for scientists at the best of times. The workhorse of solid
state physicists, density functional theory, generally works well, but
has its limitations. Would it not be great to make use of quantum
chemists' accurate yet costly methods also for large, periodic systems?
In this paper, for the first time quantum chemical methods are used to
describe ground state properties of ice, one of the most intriguing and
fascinating materials on Earth. The authors achieve unprecedented
agreement with experimental data, from ab-initio calculations that do
not include any fitted parameters. Besides predicting the elastic
properties of ice at very low temperatures, their work also suggests a
new approach for rather inexpensive accurate simulations of liquid
water -- which would be of tremendous help to shed light on the
anomalous properties of this ubiquitous substance.
***
LT11582
Exact Calculations of the Force of the Quantum Vacuum
Our group at the University of Oklahoma has discovered that
in many cases exact explicit formulas can be found for so-called
Casimir forces between distinct bodies. These forces can be viewed
as the summation of van der Waals interactions between molecules that
make up the bodies. In the past, the only exact results were for the
forces between infinite parallel plates, so various approximation
methods were employed to estimate forces between bodies with curvature
or finite size. Our results prove that these approximations are often
unreliable. The techniques developed in our work, together with those
of other workers, will enable application of quantum vacuum forces to
nanotechnology; elsewhere we have already proposed nano-gears, for
which we can now calculate reliable results.
Tuesday, September 30, 2008
9-29-08
LS11321
Agglomerates do it better
How do exotic particles in condensed-matter systems organize
in order to traverse nearly impenetrable barriers? Will they line
up quietly and cross one by one, or will they wait until a little
chattering crowd forms before embarking noisily on the perilous
quest? Previously unexplained experiments in the quantum Hall
regime [1] tell us that these particles may clump together when...
it is cold enough.
In this paper LS11321 we provide a microscopic explanation of this
peculiar behaviour. At the lowest attainable temperatures of a few
milliKelvins, we prove that it is fractional statistics that makes
it easier for agglomerates of quasiparticles to cross the tunnel
barrier. This tendency to bunch together can be
detected by "hearing" the racket of the tunneling agglomerates
through current noise measurements.
The scenario we depict accounts for a broader spectrum
of experimental data, shedding new light on fractional statistics
particles and their intricate collective habits.
***
BT11071
Finding Needles in a Haystack
We found unusually narrow, needle-like features in the bilayer manganite
phase diagram, as a function of strontium substitution for lanthanum, that
were completely missed previously. At the heart of our achievement was
synthesis of crystals with highly uniform strontium substitution and a
method to verify that uniformity. A small gradient of strontium
concentration, intrinsic to our growth technique, is used to map out the
qualitative shape of these needle-like charge-ordered phases and that shape
agreed with the predictions of our simple entropy argument. Prominent
correlated electron materials often result from a similar substitution
(dopant) in a parent compound. In many cases there is an intense
competition among the possible electronic or magnetic phases that depends
sensitively on the dopant concentration. The homogeneity we achieved and
verified is invaluable for any doped system to isolate phases from their
neighbors. It thus allowed us to rule out an erroneous conclusion of local
coexistence of antiferromagnetic phases with the charge-ordered phases that
had been ubiquitously reported in numerous previous studies.
***
BR 10675
Klein tunnelling and photon assisted transport in Graphene
Charge carriers in a single layer of graphite, graphene, exhibit unique
properties. They can travel unimpeded through high and wide potential
barriers- Klein tunneling. In this work, we show that Klein tunneling
occurs even for oscillating barriers. Charge carriers traversing the
oscillating potential barrier exchange energy in discrete quanta
resulting in photon assisted transport in graphene.
***
LU12059
Correlated electron tunneling through two separate quantum dot systems with strong capacitive interdot coupling
Using a pseudo-spin description is a very common concept in physics
which shows that - at first look - different systems obey the
same physics. It is known that the degree of freedom in occupying
a localized state by an electron of either spin up or down from an
electron reservoir containing both spin orientations lead to the
formation of a highly correlated many-electron state.
Despite the simplicity of the arrangement the resulting state is highly
non-trivial and one of the most fundamental effects in theoretical solid
state physics, coined after the Japanese physicist Jun Kondo.
The Kondo effect has been measured in electrical transport through
single quantum dot systems where it can overcome the Coulomb blockade
of the conductance at low temperatures. In our work we demonstrate that
two quantum dot systems which, individually and mutually, are in the
Coulomb blockade regime become both conducting even though the interaction
between them is purely electrostatic. This is because of the
aforementioned Kondo effect, albeit in terms of a pseudo-spin
associated with the two
quantum dot systems. A big advantage of our pseudo-spin realization is
the much better experimental control over the system.
***
EU10307
Slow changes in mixtures of glass
How does a glass made from two different components behave?
The physical properties of glasses change over long times.
This is known as aging. In this paper we show that it is the
particles which are most mobile when in a pure sample that set
the aging agenda for both species. We study a mixture composed
of tiny plastic particles of two sizes suspended in a liquid.
This model system acts like a glass when the particle concentration
is increased. Using an optical confocal microscope, we directly
view particle motions in 3D. We observe three key features of
aging in two-component mixtures: 1) particles move in cooperative
groups, just as had been observed in previous work; 2) These mobile
groups tend to be richer in small particles; and 3) these small
particles facilitate the motion of nearby particles of both sizes.
Our work is a step towards the microscopic understanding of real
world glasses which are often complex multi-component materials
and which remain a deep puzzle.
***
LR10980
To reduce drag, flag in front!
Racing cars and bicyclists can reduce air resistance by following closely
behind a leader, but we find that this conventional fluid drafting or
slip-streaming is reversed for undulating objects. Inspired by schooling
fish and flocking birds, we studied how flapping flags change the fluid
drag forces on one another when grouped together in a flowing fluid. To
our surprise, we discovered that the leading flag enjoys a drag reduction
(up to 50%) while its downstream neighbor suffers a significant drag
increase. If this effect applies to fish schools and bird flocks, the
leaders would also have a reduced burden and spend less energy as they
swim or fly.
***
LP11389
Cosmological magnetic fields from matter genesis
The genesis of matter is shown to be accompanied by the creation
of a cosmic magnetic field that can provide a window to the very
early universe. Particle physics models of cosmic matter-genesis
rely on transitions through exotic intermediate states which decay
and produce helical magnetic fields as a by-product. The strength,
coherence and helicity of the cosmic magnetic field today depends
on the physics of matter-genesis nano seconds after the big bang.
Detection of such cosmic magnetic fields would give
information about the origin of matter that is complementary to
that from particle accelerators, and may be within reach of planned
observations.
***
LS11507
Using sound reinforcement amplifiers to cool and trap atoms
for new types of collision studies.
In this work we use high power sound reinforcement amplifiers (the
same as are used at rock concerts) to cool and trap atoms to
temperatures only 250 millionths of a degree above absolute zero in a
novel type of atom trap - the AC-MOT. By driving the magnetic and
laser fields required to trap the atoms at audio frequencies, we can
switch the trap on and off in only a few millionths of a second -
allowing us to efficiently fire charged particles into these cold
targets and study the subsequent excitation and ionization that
occurs. This new technique opens up cold atom research to the field
of collision physics, and this allows us to produce new and precise
data about these interactions. We have demonstrated this technique
using electron ionization of cold potassium atoms, and expect this
new method to be widely adopted by cold atom and collision physicists
in the near future.
***
LT11400
Strong Excitonic Effect in Cuprates
Another important ingredient to explain the physics of cuprates is suggested. The high temperature superconductivity (HTSC) in cuprates is obtained by carrier doping in its insulating parent materials. These undoped materials, so called Mott insulators, become insulating due to a strong Coulomb interaction between carriers. It is important to understand the parent compound to understand the HTSC. In this paper, temperature dependent optical spectra of a one dimensional chain compound Sr2CuO3 were presented. For one dimensional systems, interactions not only between carriers at a same site but also between carriers at neighboring sites have been considered. Interestingly, the obtained spectrum at low temperature showed narrow bound exciton peaks which can exist only with a sizable inter-site interaction. The inter-site interaction has seldom been considered in the physics of two dimensional cuprates because a weak inter-site interaction is believed to play a minor role to renormalize the intra-site interaction strength. However, this study suggests that the inter-site interaction is strong enough even to form bound excitons. Moreover, it demands the long range Coulomb interaction to be considered. This result should encourage theorists to take the inter-site interaction into account to explain the physics of cuprates and its HTSC.
***
LU11781
Tracks tracked: revealing the structure of an ion track in glass
Heavy ions at high velocities can leave permanent trails of damage
termed ion tracks as they traverse a solid - but the mechanisms
behind this have remained controversial for decades. Now an
Australian-European Research team has come a big step closer to
solving the mystery by measuring and simulating the structure of an
ion track in glass (amorphous SiO2). The finding is a milestone in
understanding the interaction of highly energetic ions with solids
and has ramifications for materials science, nuclear physics,
geochronology, archaeology and interplanetary science. Using advanced
synchrotron techniques, the researchers have now resolved subtle
changes in density at length scales of tens of atoms across an ion
track. Calculations and simulations using a local heat spike about
the ion trajectory demonstrate the track structure is consistent with
a frozen-in nano-scale acoustic shock wave. This shock wave is
generated by the sudden thermal expansion at the ion track center as
the ion passes through the solid yielding lattice temperatures
greater than that required for melting. Upon rapid quenching of this
molten track, the research team has experimentally and theoretically
established that ion tracks are actually comprised of a less-dense
core surrounded by a more-dense shell (relative to unirradiated material).
***
LT11544
NanoDominoes
A longitudinal domino wave can be developed in a single-walled carbon nanotube. Domino phenomenon, which originally refers to the successive toppling of a row of dominoes when the first one is knocked over, widely exists in natural systems (e.g., avalanche of snow). It has been shown that a molecular domino cascade may be used to perform mechanical calculation on the nanometer length scale (Heinrich et. al., Science 298, 1381, 2002). In this paper, we demonstrate that after a collapse of a nanotube cross section a longitudinal domino wave is produced. The wave is driven by van der Waals potential energy and its natural speed may be up to 1 km/s. Molecules inside a SWCNT can be accelerated by the domino wave and finally shot out, allowing a SWCNT to be an energy supplier. The finding provides opportunities for designing new concept (domino-driven) NEMS devices, such as a nano gun (with a muzzle velocity 10 times of a Desert Eagle pistol).
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