9,164 research outputs found
A kinetic model describing the processivity of Myosin-V
The precise details of how myosin-V coordinates the biochemical reactions and mechanical motions of its two head elements to engineer effective processive molecular motion along actin filaments remain unresolved. We compare a quantitative kinetic model of the myosin-V walk, consisting of five basic states augmented by two further states to allow for futile hydrolysis and detachments, with experimental results for run lengths, velocities, and dwell times and their dependence on bulk nucleotide concentrations and external loads in both directions. The model reveals how myosin-V can use the internal strain in the molecule to synchronize the motion of the head elements. Estimates for the rate constants in the reaction cycle and the internal strain energy are obtained by a computational comparison scheme involving an extensive exploration of the large parameter space. This scheme exploits the fact that we have obtained analytic results for our reaction network, e.g., for the velocity but also the run length, diffusion constant, and fraction of backward steps. The agreement with experiment is often reasonable but some open problems are highlighted, in particular the inability of such a general model to reproduce the reported dependence of run length on ADP concentration. The novel way that our approach explores parameter space means that any confirmed discrepancies should give new insights into the reaction network model
Interlayer coherent composite Fermi liquid phase in quantum Hall bilayers
Composite fermions have played a seminal role in understanding the quantum
Hall effect, particularly the formation of a compressible `composite Fermi
liquid' (CFL) at filling factor nu = 1/2. Here we suggest that in multi-layer
systems interlayer Coulomb repulsion can similarly generate `metallic' behavior
of composite fermions between layers, even if the electrons remain insulating.
Specifically, we propose that a quantum Hall bilayer with nu = 1/2 per layer at
intermediate layer separation may host such an interlayer coherent CFL, driven
by exciton condensation of composite fermions. This phase has a number of
remarkable properties: the presence of `bonding' and `antibonding' composite
Fermi seas, compressible behavior with respect to symmetric currents, and
fractional quantum Hall behavior in the counterflow channel. Quantum
oscillations associated with the Fermi seas give rise to a new series of
incompressible states at fillings nu = p/[2(p \pm 1)] per layer (p an integer),
which is a bilayer analogue of the Jain sequence.Comment: 4 pages, 3 figure
Phase Diagram for a 2-D Two-Temperature Diffusive XY Model
Using Monte Carlo simulations, we determine the phase diagram of a diffusive
two-temperature XY model. When the two temperatures are equal the system
becomes the equilibrium XY model with the continuous Kosterlitz-Thouless (KT)
vortex-antivortex unbinding phase transition. When the two temperatures are
unequal the system is driven by an energy flow through the system from the
higher temperature heat-bath to the lower temperature one and reaches a
far-from-equilibrium steady state. We show that the nonequilibrium phase
diagram contains three phases: A homogenous disordered phase and two phases
with long range, spin-wave order. Two critical lines, representing continuous
phase transitions from a homogenous disordered phase to two phases of long
range order, meet at the equilibrium the KT point. The shape of the
nonequilibrium critical lines as they approach the KT point is described by a
crossover exponent of phi = 2.52 \pm 0.05. Finally, we suggest that the
transition between the two phases with long-range order is first-order, making
the KT-point where all three phases meet a bicritical point.Comment: 5 pages, 4 figure
Halo detection via large-scale Bayesian inference
We present a proof-of-concept of a novel and fully Bayesian methodology
designed to detect halos of different masses in cosmological observations
subject to noise and systematic uncertainties. Our methodology combines the
previously published Bayesian large-scale structure inference algorithm, HADES,
and a Bayesian chain rule (the Blackwell-Rao Estimator), which we use to
connect the inferred density field to the properties of dark matter halos. To
demonstrate the capability of our approach we construct a realistic galaxy mock
catalogue emulating the wide-area 6-degree Field Galaxy Survey, which has a
median redshift of approximately 0.05. Application of HADES to the catalogue
provides us with accurately inferred three-dimensional density fields and
corresponding quantification of uncertainties inherent to any cosmological
observation. We then use a cosmological simulation to relate the amplitude of
the density field to the probability of detecting a halo with mass above a
specified threshold. With this information we can sum over the HADES density
field realisations to construct maps of detection probabilities and demonstrate
the validity of this approach within our mock scenario. We find that the
probability of successful of detection of halos in the mock catalogue increases
as a function of the signal-to-noise of the local galaxy observations. Our
proposed methodology can easily be extended to account for more complex
scientific questions and is a promising novel tool to analyse the cosmic
large-scale structure in observations.Comment: 17 pages, 13 figures. Accepted for publication in MNRAS following
moderate correction
Algebraic vortex liquid theory of a quantum antiferromagnet on the kagome lattice
There is growing evidence from both experiment and numerical studies that low
half-odd integer quantum spins on a kagome lattice with predominant
antiferromagnetic near neighbor interactions do not order magnetically or break
lattice symmetries even at temperatures much lower than the exchange
interaction strength. Moreover, there appear to be a plethora of low energy
excitations, predominantly singlets but also spin carrying, which suggest that
the putative underlying quantum spin liquid is a gapless ``critical spin
liquid'' rather than a gapped spin liquid with topological order. Here, we
develop an effective field theory approach for the spin-1/2 Heisenberg model
with easy-plane anisotropy on the kagome lattice. By employing a vortex duality
transformation, followed by a fermionization and flux-smearing, we obtain
access to a gapless yet stable critical spin liquid phase, which is described
by (2+1)-dimensional quantum electrodynamics (QED) with an emergent
flavor symmetry. The specific heat, thermal conductivity, and
dynamical structure factor are extracted from the effective field theory, and
contrasted with other theoretical approaches to the kagome antiferromagnet.Comment: 14 pages, 8 figure
In-situ measurements of the optical absorption of dioxythiophene-based conjugated polymers
Conjugated polymers can be reversibly doped by electrochemical means. This
doping introduces new sub-bandgap optical absorption bands in the polymer while
decreasing the bandgap absorption. To study this behavior, we have prepared an
electrochemical cell allowing measurements of the optical properties of the
polymer. The cell consists of a thin polymer film deposited on gold-coated
Mylar behind which is another polymer that serves as a counterelectrode. An
infrared transparent window protects the upper polymer from ambient air. By
adding a gel electrolyte and making electrical connections to the
polymer-on-gold films, one may study electrochromism in a wide spectral range.
As the cell voltage (the potential difference between the two electrodes)
changes, the doping level of the conjugated polymer films is changed
reversibly. Our experiments address electrochromism in
poly(3,4-ethylene-dioxy-thiophene) (PEDOT) and
poly(3,4-dimethyl-propylene-dioxy-thiophene) (PProDOT-Me). This closed
electrochemical cell allows the study of the doping induced sub-bandgap
features (polaronic and bipolaronic modes) in these easily oxidized and highly
redox switchable polymers. We also study the changes in cell spectra as a
function of polymer thickness and investigate strategies to obtain cleaner
spectra, minimizing the contributions of water and gel electrolyte features
Collapse of a Molecular Cloud Core to Stellar Densities: The First Three-Dimensional Calculations
We present results from the first three-dimensional calculations ever to
follow the collapse of a molecular cloud core (~ 10^{-18} g cm^{-3}) to stellar
densities (> 0.01 g cm^{-3}). The calculations resolve structures over 7 orders
of magnitude in spatial extent (~ 5000 AU - 0.1 R_\odot), and over 17 orders of
magnitude in density contrast. With these calculations, we consider whether
fragmentation to form a close binary stellar system can occur during the second
collapse phase. We find that, if the quasistatic core that forms before the
second collapse phase is dynamically unstable to the growth of non-axisymmetric
perturbations, the angular momentum extracted from the central regions of the
core, via gravitational torques, is sufficient to prevent fragmentation and the
formation of a close binary during the subsequent second collapse.Comment: ApJ Letters, in press (will appear in Nov 20 issue; available from
the ApJ Rapid Release web page). 7 pages, incl. 5 figures. Also available at
http://www.mpia-hd.mpg.de/theory/bat
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