Novel scheme to beat the resolution limit in optical lithography
Our work shows how the standard resolution limitations in optical
lithography can be overcome using simple laser light of arbitrarily low
intensity. This represents a major advance over existing proposals based
on multiphoton absorption processes that require highly intense light
sources.
The miniaturization of nano- and semiconductor structures generated by
optical lithography is necessary to maintain the current pace of
technological innovation. However it is limited by the physical phenomenon
of diffraction, i.e., the bending of light. Up to now, the creation of
smaller feature sizes has been achieved by progressing to light with lower
wavelengths, which, however, is becoming exceedingly challenging.
Alternatively, it has been proposed that the diffraction limit can be
overcome by making use of a resist that absorbs N light quanta
simultaneously, thereby lowering the effective wavelength by a factor of
1/N. The drawback of the latter schemes up to now has been the demand for
very high light field intensities, making an implementation difficult.
We show that the standard diffraction limit in optical lithography can be
overcome even at arbitrarily low laser intensities. This advancement is
due to the fact that our scheme only comprises resonant light-matter
interactions.
***
LH11772
Tiny tendrils are robust feature of mixing liquid layers
To help understand what structures and flows exist in the
earth’s mantle, many experiments mimic motion in the mantle by
heating layers of liquid with different composition and
tracking how the liquids become mixed over time. In these
experiments the mixing proceeds very differently from mixing
in a single liquid layer, such as occurs when a pot of water
is heated over the stove top – here, liquid from a lower layer
is mixed into the upper by forming thin and tenuous tendrils
which puncture the interface. Surprisingly, these tendrils
persist over many convection cycles, despite the presence of
fluctuations in the temperature and large-scale flow.
In our paper, we examined how such thin tendrils can be so
robust in a theoretical analysis. Our work provides an
estimate for how quickly two layers become mixed by
translating insights from previous works on encapsulation and
entrainment flows on micron scales to large-scale mantle flow.
A crucial ingredient for the tendril to persist despite
fluctuations is a sharply curved topography at the interface
between distinct layers. The curved interface anchors the
thin tendril and isolates the flow within the tendril from
disturbances in the overall convection.
***
LF10949: No-ghost theorem for the fourth-order derivative Pais-Uhlenbeck
oscillator model.
Carl M. Bender and Philip D. Mannheim, to appear in Physical Review
Letters.
FIFTY-YEAR-OLD PROBLEM SOLVED -- MIGHT LEAD TO A NONSTRING APPROACH TO
QUANTUM GRAVITY
Carl Bender and Philip Mannheim have shown that the oscillator model
introduced
by Pais and Uhlenbeck in 1950 is a consistent quantum-mechanical system. For
over 50 years this model was thought to possess negative-probability states
(ghost states) that would render the model physically unacceptable. The
recent
work of Bender and Mannheim demonstrates that the long-accepted argument that
there are ghosts has a subtle flaw, namely the unquestioned presumption
that the
Hamiltonian for the model was Dirac Hermitian. Bender and Mannheim found that
the Hamiltonian belongs to a class of non-Dirac-Hermitian Hamiltonians
that are
symmetric under combined space reflection P and time reversal T. As
established
recently by Bender and collaborators, in such PT-symmetric theories all
states
have positive probability and thus the Pais-Uhlenbeck model is physically
viable. This result might have significant consequences for constructing a
theory of quantum gravity. Attempts to quantize the standard Einstein
theory of
gravity run into problems arising from uncontrollable infinities. String
theory
cures the infinity problem at the cost of extending space-time to ten
dimensions. Conformal gravity, a four-dimensional alternative to standard
gravity, has no infinity problem and is of interest because Mannheim has
shown
that it has no cosmological-constant problem. However, conformal gravity has
long been thought to have ghost states. Because the Pais-Uhlenbeck model,
which
is a prototype of conformal gravity, is ghost free, there is now hope that
one
may be able to construct a consistent four-dimensional theory of quantum
gravity.
***
LM10882
Experiments and physical model for rotating filaments explains the
transition to helical shape and describes a mechanism for propulsion
in microorganisms
Qian Bian, Thomas Powers and Kenny Breuer[1]
Division of Engineering, Brown University
We have been engaged over several years in a program to understand the
mechanics of flagellar motility. Many organisms, such as E. coli or
S. marascens propel themselves through water by means of one or
several helical flagella, each of which are rotated by a nanoscale
motor embedded in the cell wall. One (of many) questions that arise,
is how a long, flexible rotating filament might interact with the
surrounding fluid, what shape it adopts, and what thrust is generated.
We address this in the current Phys Rev Lett paper, using a scaled-up
experimental model in which a flexible rod is rotated in a tank of
very viscous liquid. We find that the rod has two states – a splayed
conformation at low torque and, at higher torque, a more compact
helical conformation which rotates at a higher speed and generates
more thrust. We compare our results with a mathematical theory, and
lastly, we present a simple model problem which is much simpler than
the complete flexible rod system, but nevertheless captures the
essential behavior and helps to explain the relevant physics. This is
an excellent surrogate for the behavior of flexible filaments such as
cilia or flagella, although there are significant differences between
the geometries in nature and this simple model problem. The results
may have engineering applications in the design and fabrication of
propulsion systems for microscale robotic "swimmers".
More background and videos can be seen at
http://fluids.engin.brown.edu/research/bacterial_summary.html
***
LF11529
"How bumps on whale flippers delay stall: an aerodynamic model"
by Ernst A. van Nierop, Silas Alben, Michael P. Brenner
Summary for non-scientists:
"
How Bumps on Whale Flippers Delay Stall
Experiments in a wind tunnel at the Naval Academy in 2004 showed that
the bumps on humpback whale flippers makes their stall behavior much
more gradual. All wings lose lift dramatically (stall) at some angle
of attack, and gradual stalling increases the range over which a wing
(or flipper) contributes to maneuverability, while decreasing the
unpredictability of lift forces near the stall transition. In this
paper, we demonstrate that basic concepts from aerodynamics go a long
way towards explaining how bumps work. The bumps modify the flow over
the wing, so that the troughs between the bumps stall first. By
averaging over all the bumps and troughs, the overall stall is much
more gradual. Applications include "unstallable wings" for special
purpose aircraft, and special wind turbine blades for turbulent-wind areas.
"
***
LB11135
Proteins stick and slide
We reveal how proteins attach to surfaces by mimicking the immersion of artificial materials (such as medical or dental implants) in biological solutions (eg blood or saliva). By exploiting experiments and theories developed to understand how materials grow we have identified the key steps specific to protein layer formation. In particular, this work shows that proteins first stick to the surface and then slide around until they meet each other and stick together to form a cluster of proteins. Surprisingly these clusters are also capable of sliding around, albeit at a slower rate, and combining together. This movement leads to a surface covered with isolated protein islands with large areas of bare surface. If proteins were fixed at the first point of attachment they would produce both a different environment for each protein molecule and a very different surface structure to that observed here. Such differences are significant as the initial protein structure influences how cells grow and potentially the integration or rejection of an implanted material. This new model of how proteins behave when attached to a surface will aid the development of better biocompatible materials and protein handling technologies.
***
LG10957
Finding the front and the back: how living cells start to move
Living cells move by assembling a branched network of so called actin
filaments (a protein) at their front. The same process can induce the
motion of viruses or bacteria, which invade other cells and move
inside their host cells by trailing an actin tail behind.
Recently biologists have succeeded in growing an actin network around
small beads, which then started to move like bacteria. The onset of
this motion is a fundamental step, called symmetry-breaking, which has
been occupying theoreticians and experimentalists alike.
We can show, that the elastic properties of the actin gel are at the very
heart of the symmetry-breaking. Through the growth process, elastic
stresses build up in the network, which in turn lead to a biased
growth: at one side of the bead the network gets thinner, whereas it
gets thicker at the opposite side, thus reducing the stress.
Furthermore we have shown by numerical simulations, that this biased
growth leads eventually to the evolution of a comet shaped network,
which can then cause the motion of the bead.
This discovery reveals that the interplay between mechanical
properties of biological materials and their growth might be at the
origin of motion in more complex systems, like bacteria, viruses or
cells of the human immune system.
***
LA10955
A Tug-of-War Suggests a New Paradigm in DNA Replication
Error-free DNA replication is central to the integrity of our genome.
Therefore, it is of considerable interest to study the action of the
proteins known as polymerases that replicate DNA. Through computer
simulations of the dynamics of a polymerase DNA complex at atomic
resolution we suggest that an intriguing dynamical coupling between
the cooperative motion of polymerase and DNA atoms may play a
significant role in aiding and abetting the chemical reaction
(chemical step) that leads to nucleotide incorporation during DNA
replication. We also suggest that the coupling is disrupted to varying
extents in a context-specific fashion, i.e., when the inserted
nucleotide is not complementary to the template base (mispair) or when
the template base in the DNA is oxidatively damaged. The emerging
paradigm from our studies is that the dynamical coupling can represent
a possible additional molecular mechanism in the polymerase to achieve
error control during DNA replication. As a direct consequence of the
dynamical coupling, our calculations show that the rate of the
chemical step is dependent on the applied force on the DNA (template)
strand and that the force-dependence is also context-specific, which
provides a direct route to validating our paradigm through
single-molecule experiments.
***
LL11465
Truly Surface Optics Provides Marriage of Optics, Electronics
We develop an approach to design the surface analogs of lenses, prisms,
and other optical elements to steer and reshape surface plasmon
polaritons (SPPs) similar to the way used to control optical beams, a
development that may resolve the communication bottleneck in modern
electronics. The SPPs are surface optical waves that result from the
coupling between the free electrons in metals or semiconductors and
optical radiation in surrounding non-conducting media. These waves,
capable to enable negative refraction of light, super-imaging, and
optical "cloaking", are considered to be the missing link between the
electronics and optical communications. Unfortunately, in conventional
(isotropic) media the "height" of the surface wave strongly depends on
the material. Therefore, at each optical interface 10-30% of the energy
of SPP scatters away (top panel in Fig.). In this work we demonstrate
that it is possible to use carefully designed combinations of
anisotropic media to ideally match the "height" of SPPs across the
system, completely eliminating the parasitic scattering (bottom panel).
This allows creation of surface electro-optical circuits where SPP
lenses, prisms, etc. can be controlled electronically. The potential
this opens for a merger of optics and electronics, and an entirely new
range of products based on such technology, is extraordinary.
***
LL11519
Ultracold fermions as a Butterfly Net
A simple problem in quantum mechanics is the study of a particle moving
on a lattice in the presence of a magnetic field. While the separate
effects of the lattice (band structure) and the magnetic field (Landau
levels) are well understood, their combination creates a very rich
problem where the allowed energies form the fractal known as the
Hofstadter Butterfly (see figure). Furthermore, each energy gap of the
butterfly was discovered to be characterized by an integer, which
determines the value of Hall conductance. This connection is one of the
most important applications of ideas from Topology (a branch of
mathematics) to quantum mechanics. So far it has not been possible to
test these predictions experimentally, as the magnetic fields needed to
create the Butterfly spectrum in a solid state system (thousands of
Tesla) are far beyond what is available (tens of Tesla).
In this work, we argue that both the Hofstadter butterfly, and
topological quantization of Hall conductance can be tested by an
experiment using ultracold Fermi gases. In ultracold atom experiments
periodic potentials known as optical lattices are routinely created by
laser beams, and rotating such a lattice simulates the magnetic field.
We show that, measurement of the density of atoms in such a potential
gives information about both the butterfly spectrum and the Hall
conductance of the system. Our results indicate that exotic states of
matter known as topological insulators may soon be realized experimentally.
***
LL10841
Beyond Einstein?
The mystery of the “dark matter” and “dark energy” required by Einstein’s general relativity to explain the available astrophysical data has led many physicists to suggest that this theory may not be a correct description of gravity on very large (galactic) scales. However, it turned out to be extremely difficult to modify Einstein’s theory and to remain consistent with what is observed. All known modifications are quite drastic in the sense that new “particles” or, how physicists refer to them, “degrees of freedom” are introduced, and it is then a major challenge to explain why these are not seen. This paper for the first time describes a large class of gravity theories that introduce no new degrees of freedom as compared to general relativity, with Einstein’s theory itself being a member of the class. The results of this work may lead to new insights on the description of our Universe on very large scales.
***
LJ11620
CHIRAL ORDER IN HELIMAGNETS
Chirality is a fundamental property of matter that can be found in
different systems like living organisms or subatomic particles.
The term itself indicates an object which is not superimposable to its
mirror image. Some magnetic systems are known to undergo transitions to
chiral order where the magnetic moments pass from random orientation to
ordered states.
In our Letter, we report experimental results on the molecular chain
compound Gd(hfac)3NITEt that provide evidence for an intermediate state
with a "chiral" order.
At high temperatures, this system is a mixture of right and left
hands â¿ magnetic corkscrewsâ¿ , i.e. finite segments of magnetic moments
disposed along the chain and turning clockwise or anticlockwise.
Decreasing the temperature, all the â¿¿magnetic corkscrewsâ¿ become
"right" ("left") hand, but the phase angle between two subsequent
magnetic moments is still random: this is the new "chiral" order
predicted thirty years ago by Jacques Villain.
Further decrease in temperature, at the helical transition the phase
angle between nearest neighbours becomes the same.
Summarising, it is possible to freeze a semi-ordered state taking
advantage of the unique properties of molecular chain magnets. Can this
particular order be found in other biological or nuclear system?
***
LK11340
ULTRA-SENSITIVE FORCE MICROSCOPE
A new principle to enhance the
sensitivity of force microscopes has been discovered. The principle
is based on the coupling between two flexural resonances of the
microscope. The coupling is induced by the simultaneous excitation of
those resonances. In this way, the microscope increases its
information channels from two to four, and thus its the ability to
extract information on the surface properties. The second mode can
probe the tip-surface forces in a way that is not hindered by the
feedback mechanism as it happens in conventional amplitude modulation
AFM. The predicted force sensitivity in this multifrequency force
microscope is 0.2 pN, which represents an improvement of about two
orders of magnitude. The microscope can be operated at forces below
those required to unfold proteins. Consequently, this microscope
could image in a gentle manner small soft-materials. The analytical
model identifies the virial and the energy dissipated by the
tip-surface forces as the parameters responsible for the contrast.
The relevance of the discovered principle goes beyond force
microscopy. It could be applied to a wide variety of micro and
nanomechanical sensors.
***
LG11215
----------------------------------------------------------------------
The singular limit of a rough granular gas and the standard perturbative
theories
------------------------------------------------------------------------
It is well-known that the translational and rotational
motions are decoupled in a molecular gas. Using numerical simulation,
we show that there is significant correlation between translational and
rotational velocities in a sheared `granular' gas of rough particles.
The collisions between rough particles have been modelled by two restitution
coefficients: the normal restitution coefficient (which is an indicator of
the `inelasticity' of particle collisions) and the tangential restitution
coefficient (which characterizes the surface `roughness' of a particle).
Note that for `smooth' particles, the rotational velocity remains unchanged
upon a collision and hence the rotational dissipation vanishes in the
smooth limit. Interestingly, the orientational correlation between
translation
and rotation persists even in the limiting case of smooth, inelastic
particles, and hence the smooth limit of a rough granular gas is
`singular'.
This immediately raises questions about the validity of standard
perturbative theories that are used to derive constitutive models for
granular gases. This issue can only be resolved by probing the velocity
distribution functions of a rough granular gas.
Our work shows that the leading-order velocity distribution function is a
Gaussian for both `smooth' and `rough' limits in the limit of dissipation
going to zero. Hence the theoretical approaches based on
Chapman-Enskog expansion of the pertinent Boltzmann equation are
valid for a rough granular gas for both smooth and rough limits.
This clearly resolves an incorrect assertion in an earlier PRL-paper
about the validity of perturbative expansions about the `smooth' limit
of a granular gas. The key to this resolution is to approach
the `smooth' limit by allowing both translational and rotational
dissipations going to zero.
***
LB11402
What Lies Beneath.
Our life depends on water [1], yet many unique properties of water
still present a puzzle for which different interpretations have been
proposed in the past. We find a quantity that can be measured in
experiments and can distinguish between the two interpretations
commonly used to understand the anomalies of water at low temperature,
a verification that has been elusive so far. The result has possible
implications in understanding the dynamics of hydrated proteins [2],
essential for the functioning of organic cells.
Water has more than sixty anomalies, such as the increase of density
upon increasing temperature --making fishes survive under frozen
lakes-- or its extraordinary large capacity of absorbing heat,
essential for regulating our body temperature. Its heat capacity,
contrary to most of the liquids increases at low temperatures,
where other anomalies appear. For example, water can stay liquid at
very low temperatures: down to -47 Celsius in plants and -92 Celsius
in laboratory at a pressure of 2 kbars.
By numerical simulations and theoretical calculations, we study how
the dynamics of liquid water at low temperature changes with
pressure. This study provides a measure that could clarify what lies
beneath the region of water's phase diagram that we presently know.
***
LH10970
*Reducing the Molecule-Substrate Coupling in C_60 -Based Nanostructures
by Molecular Interactions*
Molecules locking into partner molecules and lifting them up from a
supporting gold surface have been shown in our recent paper in Physical
Review Letters.
Using a scanning temperature microscope, we find molecular
nanostructures composed of three saturated hydrocarbons anchoring to a
C_60 molecule by electrostatic intermolecular forces. In these
nanostructures the fullerene cage is at larger distance to the surface
than expected. While usually the properties of molecules adsorbed on
metal surfaces are perturbed by the bonding interactions, the electronic
structure of the lifted fullerene resembles the one of a free molecule.
Free-like molecules on surfaces are an interesting avenue in the field
of molecular electronics, where the electronic properties of a single
molecule are to be used in single molecule devices.
Our approach to lift a molecule from the supporting substrate by using
molecular interactions opens the possibility to design strategies that
permit to modify the degree of mixing between electronic states of
molecules with surfaces.
***
LB11611
Lipid rafts – standing room only!
If a protein in the cell membrane decides to board a lipid raft – a domain floating in the membrane – it had better prepare to face a crowded ride: our latest work suggests that lipid rafts are only about 10nm in diameter. Rafts have been implied in a wide range of biological processes – the immune response and viral entry among them - due to their potential for regulating the behavior of membrane proteins. To date, however, rafts have stubbornly defied direct observation, leading some to believe that they do not exist in living cells. To investigate the role of the membrane’s material properties in the formation of rafts, we use artificial membranes whose lipid composition mimics that of a living cell. The artificial membranes form giant vesicles in which the coexistence of two liquid phases, among which the ‘raft phase’, is observed. The interplay of two crucial material parameters - the membrane’s bending rigidity and the line tension between the phases - leads to a peanut-like vesicle shape which we capture in a fully analytical model. This allows for the first reliable determination of the physical parameters that govern the shape of composite lipid vesicle systems. These parameters, combined with existing models for raft formation, imply a size of only 10 nm for lipid rafts in living cells. Our work therefore suggests that rafts may yet exist in living systems; we just need to look more closely to see them!
***
LC11142
Spooky cheating for quantum coin flipping
Let's flip a coin to decide who get a car. It is known that a fair
coin flip is possible quantum mechanically [1,2]. What is going on if
the car is a quantum object of the Einstein-Podolsky-Rosen type? In
this paper, we considered this problem. In a quantum coin flip, it is
ensured by the laws of physics that a dishonest player (Alice) cannot
control the outcome of the coin flip. Nevertheless, it is possible for
Alice that the state of the quantum car is changed into what she
desires, whenever she loses the coin flip. Alice accomplishes this
spooky cheating by exploiting the small bias of the probability of the
outcome that a quantum coin flip generally allows. A coin flip is now
an important cryptographic primitive on a communication network. Our
result warns that, if a quantum coin flip is combined with another
quantum cryptographic protocol as a subroutine, an unexpected security
hole will occur.
***
LG11695
In their paper, "Weak values and the Leggett-Garg inequality in solid-state qubits", Williams and Jordan discuss how to implement a 'weak value', one of the strangest predictions of quantum mechanics, in nano-electronic systems. The authors predict that the electrical current generated by a detector will give markedly different results depending on whether the detector was measuring a quantum or classical system - in the quantum system the detector current could exceed the signal from an analogous classical system by hundreds or thousands. This effect can also be used as a test to demonstrate that an experimental system is a bona fide quantum mechanical device that cannot be classically mimicked. The idea of testing a single (possibly macroscopic) system for its `quantumness' was proposed in 1985 by Leggett and Garg who showed that sequential time measurements on quantum systems are correlated in a way that cannot possibly be generated by a classical system - provided that detector does not alter the system it is measuring. Jordan and Williams demonstrate that the `quantumness' tested by the Leggett-Garg criteria is actually a disguised form of the weak value effect: If the detector produces an anomalously large current, it must necessarily violate the Leggett-Garg criteria and visa-versa. This insight also shows the detector must be non-invasive for the weak value to function as a quantum mechanical test.
***
LF11321
Particle Theorists launch charm offensive
The subatomic world is undergoing a revolution
and a relatively exotic denizen,
the charm quark, is the latest target. A team
of theorists from Cornell, Glasgow and the Ohio
State University has calculated key properties of
particles known as D mesons that contain the charm
quark with an accuracy improved by a factor of 4
over previous calculations and existing experimental
results. Experimental numbers are expected to improve
significantly in the next year and this, with
the new theoretical result, will provide a stringent test
of the Standard Model of particle physics in a new arena.
This is an important part of the worldwide programme
exploring all areas of particle
physics for clues to a more complete theory than our
current Standard Model.
The theory of how quarks interact is well understood
in principle, but in practice is very difficult to handle
because the interaction is so strong. It
can be tackled in numerical calculations on a computer
using a technique known as lattice QCD. This
underwent a revolution five years ago when new methods
and increased computer power came together to make accurate
calculations possible for the first time. The new methods
focussed on the up, down and strange quarks that make
up the 'everyday' subatomic world, being constituents
of, for example, the proton and the pi meson. These
quarks are very light, the up and
down quarks in particular weighing almost nothing.
The more exotic charm quark has a mass heavier
than the proton, and this makes it much harder
to work with in lattice QCD. Initial calculations used
different methods for the charm quark than for
up, down and strange and produced less accurate results
because of this.
Now, in this paper, theorists have
produced results by improving the method for
up, down and strange so that it also works well
for charm quarks. This gives not only hugely improved
accuracy but also increased predictive power. For
example, they have been able to determine the mass
of the D meson and its cousin, the D_s, for the first time.
They obtain an accuracy of better than 0.5%, and
good agreement with experiment.
They have also calculated the decay constants
for the D, D_s and the pi meson in the same calculation.
The decay constant measures the probability of the
quark and antiquark that make up the meson being
in the same place so that they can annihilate.
The rate of annihilation can be measured by experiment
and is well known for the pi meson, where the
theorists agree to 2%, but not yet
very well known for the D or D_s. Early
results have appeared from the CLEO-c experiment
at Cornell, the BaBar experiment at Stanford and
the Belle experiment at KEK in Japan, and they will be
working hard to reduce their errors to the 2% level
that theorists have now achieved.
The theorists will now go on to more complicated
calculations that can provide additional stringent
tests of the Standard Model `jigsaw', searching for
the piece that does not fit.
Figure attached : 2 D mesons being produced in the
CLEO-c experiment and decaying. One decays to
a K meson and 3 pi mesons, the other annihilates in
the process being studied in this paper and produces
a mu particle (marked) and an invisible neutrino.
