4,891 research outputs found
Self-learning Multiscale Simulation for Achieving High Accuracy and High Efficiency Simultaneously
We propose a new multi-scale molecular dynamics simulation method which can
achieve high accuracy and high sampling efficiency simultaneously without
aforehand knowledge of the coarse grained (CG) potential and test it for a
biomolecular system. Based on the resolution exchange simulations between
atomistic and CG replicas, a self-learning strategy is introduced to
progressively improve the CG potential by an iterative way. Two tests show
that, the new method can rapidly improve the CG potential and achieve efficient
sampling even starting from an unrealistic CG potential. The resulting free
energy agreed well with exact result and the convergence by the method was much
faster than that by the replica exchange method. The method is generic and can
be applied to many biological as well as non-biological problems.Comment: 14 pages, 6 figure
Systematic Errors in Future Weak Lensing Surveys: Requirements and Prospects for Self-Calibration
We study the impact of systematic errors on planned weak lensing surveys and
compute the requirements on their contributions so that they are not a dominant
source of the cosmological parameter error budget. The generic types of error
we consider are multiplicative and additive errors in measurements of shear, as
well as photometric redshift errors. In general, more powerful surveys have
stronger systematic requirements. For example, for a SNAP-type survey the
multiplicative error in shear needs to be smaller than 1%(fsky/0.025)^{-1/2} of
the mean shear in any given redshift bin, while the centroids of photometric
redshift bins need to be known to better than 0.003(fsky/0.025)^{-1/2}. With
about a factor of two degradation in cosmological parameter errors, future
surveys can enter a self-calibration regime, where the mean systematic biases
are self-consistently determined from the survey and only higher-order moments
of the systematics contribute. Interestingly, once the power spectrum
measurements are combined with the bispectrum, the self-calibration regime in
the variation of the equation of state of dark energy w_a is attained with only
a 20-30% error degradation.Comment: 20 pages, 9 figures, to be submitted to MNRAS. Comments are welcom
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A point mutation of integrin beta 1 subunit blocks binding of alpha 5 beta 1 to fibronectin and invasin but not recruitment to adhesion plaques.
A point mutation in a highly conserved region of the beta 1 subunit, Asp130 to Ala (D130A) substitution, abrogates the Arg-Gly-Asp (RGD)-dependent binding of alpha 5 beta 1 to fibronectin (FN) without disrupting gross structure or heterodimer assembly. The D130A mutation also interferes with binding to invasin, a ligand that lacks RGD sequence. In spite of the lack of detectable FN binding by alpha 5 beta 1(D130A), it was recruited to adhesion plaques formed on FN by endogenous hamster receptors. Thus, intact ligand binding function is not required for recruitment of alpha 5 beta 1 to adhesion plaques. Overexpression of beta 1(D130A) partially interfered with endogenous alpha 5 beta 1 function, thus defining a dominant negative beta 1 integrin mutation
Charge-fluctuation contribution to the Raman response in superconducting cuprates
We calculate the Raman response contribution due to collective modes, finding
a strong dependence on the photon polarizations and on the characteristic
wavevectors of the modes. We compare our results with recent Raman spectroscopy
experiments in underdoped cuprates, and
, where anomalous low-energy peaks are
observed, which soften upon lowering the temperature. We show that the specific
dependence on doping and on photon polarizations of these peaks is only
compatible with charge collective excitations at finite wavelength.Comment: 5 pages, 3 figure
Measurement of the Transmission Phase of an Electron in a Quantum Two-Path Interferometer
A quantum two-path interferometer allows for direct measurement of the
transmission phase shift of an electron, providing useful information on
coherent scattering problems. In mesoscopic systems, however, the two-path
interference is easily smeared by contributions from other paths, and this
makes it difficult to observe the \textit{true} transmission phase shift. To
eliminate this problem, multi-terminal Aharonov-Bohm (AB) interferometers have
been used to derive the phase shift by assuming that the relative phase shift
of the electrons between the two paths is simply obtained when a smooth shift
of the AB oscillations is observed. Nevertheless the phase shifts using such a
criterion have sometimes been inconsistent with theory. On the other hand, we
have used an AB ring contacted to tunnel-coupled wires and acquired the phase
shift consistent with theory when the two output currents through the coupled
wires oscillate with well-defined anti-phase. Here, we investigate thoroughly
these two criteria used to ensure a reliable phase measurement, the anti-phase
relation of the two output currents and the smooth phase shift in the AB
oscillation. We confirm that the well-defined anti-phase relation ensures a
correct phase measurement with a quantum two-path interference. In contrast we
find that even in a situation where the anti-phase relation is less
well-defined, the smooth phase shift in the AB oscillation can still occur but
does not give the correct transmission phase due to contributions from multiple
paths. This indicates that the phase relation of the two output currents in our
interferometer gives a good criterion for the measurement of the \textit{true}
transmission phase while the smooth phase shift in the AB oscillation itself
does not.Comment: 5 pages, 4 figure
Implication of Omega_m through the Morphological Analysis of Weak Lensing Fields
We apply the morphological descriptions of two-dimensional contour map, the
so-called Minkowski functionals (the area fraction, circumference, and Euler
characteristics), to the convergence field of the
large-scale structure reconstructed from the shear map produced by the
ray-tracing simulations. The perturbation theory of structure formation has
suggested that the non-Gaussian features on the Minkowski functionals with
respect to the threshold in the weakly nonlinear regime are induced by the
three skewness parameters of that are sensitive to the density
parameter of matter, . We show that, in the absence of noise
due to the intrinsic ellipticities of source galaxies with which the
perturbation theory results can be recovered, the accuracy of
determination is improved by using the Minkowski functionals
compared to the conventional method of using the direct measure of skewness.Comment: 4 pages, 3 figures, to appear in ApJ Lette
Electronic Collective Modes and Superconductivity in Layered Conductors
A distinctive feature of layered conductors is the presence of low-energy
electronic collective modes of the conduction electrons. This affects the
dynamic screening properties of the Coulomb interaction in a layered material.
We study the consequences of the existence of these collective modes for
superconductivity. General equations for the superconducting order parameter
are derived within the strong-coupling phonon-plasmon scheme that account for
the screened Coulomb interaction. Specifically, we calculate the
superconducting critical temperature Tc taking into account the full
temperature, frequency and wave-vector dependence of the dielectric function.
We show that low-energy plasmons may contribute constructively to
superconductivity. Three classes of layered superconductors are discussed
within our model: metal-intercalated halide nitrides, layered organic materials
and high-Tc oxides. In particular, we demonstrate that the plasmon contribution
(electronic mechanism) is dominant in the first class of layered materials. The
theory shows that the description of so-called ``quasi-two-dimensional
superconductors'' cannot be reduced to a purely 2D model, as commonly assumed.
While the transport properties are strongly anisotropic, it remains essential
to take into account the screened interlayer Coulomb interaction to describe
the superconducting state of layered materials.Comment: Final version (minor changes) 14 pages, 6 figure
Cross-correlating the Thermal Sunyaev-Zel'dovich Effect and the Distribution of Galaxy Clusters
We present the analytical formulas, derived based on the halo model, to
compute the cross-correlation between the thermal Sunyaev-Zel'dovich (SZ)
effect and the distribution of galaxy clusters. By binning the clusters
according to their redshifts and masses, this cross-correlation, the so-called
stacked SZ signal, reveals the average SZ profile around the clusters. The
stacked SZ signal is obtainable from a joint analysis of an
arcminute-resolution cosmic microwave background (CMB) experiment and an
overlapping optical survey, which allows for detection of the SZ signals for
clusters whose masses are below the individual cluster detection threshold. We
derive the error covariance matrix for measuring the stacked SZ signal, and
then forecast for its detection from ongoing and forthcoming combined
CMB-optical surveys. We find that, over a wide range of mass and redshift, the
stacked SZ signal can be detected with a significant signal to noise ratio
(total S/N \gsim 10), whose value peaks for the clusters with intermediate
masses and redshifts. Our calculation also shows that the stacking method
allows for probing the clusters' SZ profiles over a wide range of scales, even
out to projected radii as large as the virial radius, thereby providing a
promising way to study gas physics at the outskirts of galaxy clusters.Comment: 11 pages, 6 figures, 3 tables, minor revisions reflect PRD published
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