7,670 research outputs found
Destroying Aliases from the Ground and Space: Super-Nyquist ZZ Cetis in K2 Long Cadence Data
With typical periods of order 10 minutes, the pulsation signatures of ZZ Ceti
variables (pulsating hydrogen-atmosphere white dwarf stars) are severely
undersampled by long-cadence (29.42 minutes per exposure) K2 observations.
Nyquist aliasing renders the intrinsic frequencies ambiguous, stifling
precision asteroseismology. We report the discovery of two new ZZ Cetis in
long-cadence K2 data: EPIC 210377280 and EPIC 220274129. Guided by 3-4 nights
of follow-up, high-speed (<=30 s) photometry from McDonald Observatory, we
recover accurate pulsation frequencies for K2 signals that reflected 4-5 times
off the Nyquist with the full precision of over 70 days of monitoring (~0.01
muHz). In turn, the K2 observations enable us to select the correct peaks from
the alias structure of the ground-based signals caused by gaps in the
observations. We identify at least seven independent pulsation modes in the
light curves of each of these stars. For EPIC 220274129, we detect three
complete sets of rotationally split ell=1 (dipole mode) triplets, which we use
to asteroseismically infer the stellar rotation period of 12.7+/-1.3 hr. We
also detect two sub-Nyquist K2 signals that are likely combination (difference)
frequencies. We attribute our inability to match some of the K2 signals to the
ground-based data to changes in pulsation amplitudes between epochs of
observation. Model fits to SOAR spectroscopy place both EPIC 210377280 and EPIC
220274129 near the middle of the ZZ Ceti instability strip, with Teff =
11590+/-200 K and 11810+/-210 K, and masses 0.57+/-0.03 Msun and 0.62+/-0.03
Msun, respectively.Comment: 13 pages, 9 figures, 7 tables; accepted for publication in Ap
Edge and Bulk Transport in the Mixed State of a Type-II Superconductor
By comparing the voltage-current (V-I) curves obtained before and after
cutting a sample of 2H-NbSe2, we separate the bulk and edge contributions to
the transport current at various dissipation levels and derive their respective
V- I curves and critical currents. We find that the edge contribution is
thermally activated across a current dependent surface barrier. By contrast the
bulk V-I curves are linear, as expected from the free flux flow model. The
relative importance of bulk and edge contributions is found to depend on
dissipation level and sample dimensions. We further show that the peak effect
is a sharp bulk phenomenon and that it is broadened by the edge contribution
Density-functional study of hydrogen chemisorption on vicinal Si(001) surfaces
Relaxed atomic geometries and chemisorption energies have been calculated for
the dissociative adsorption of molecular hydrogen on vicinal Si(001) surfaces.
We employ density-functional theory, together with a pseudopotential for Si,
and apply the generalized gradient approximation by Perdew and Wang to the
exchange-correlation functional. We find the double-atomic-height rebonded D_B
step, which is known to be stable on the clean surface, to remain stable on
partially hydrogen-covered surfaces. The H atoms preferentially bind to the Si
atoms at the rebonded step edge, with a chemisorption energy difference with
respect to the terrace sites of >sim 0.1 eV. A surface with rebonded single
atomic height S_A and S_B steps gives very similar results. The interaction
between H-Si-Si-H mono-hydride units is shown to be unimportant for the
calculation of the step-edge hydrogen-occupation. Our results confirm the
interpretation and results of the recent H_2 adsorption experiments on vicinal
Si surfaces by Raschke and Hoefer described in the preceding paper.Comment: 13 pages, 8 figures, submitted to Phys. Rev. B. Other related
publications can be found at http://www.rz-berlin.mpg.de/th/paper.htm
Pion photoproduction on the nucleon in the quark model
We present a detailed quark-model study of pion photoproduction within the
effective Lagrangian approach. Cross sections and single-polarization
observables are investigated for the four charge channels, , , , and .
Leaving the coupling strength to be a free parameter, we obtain a
reasonably consistent description of these four channels from threshold to the
first resonance region. Within this effective Lagrangian approach, strongly
constrainted by the quark model, we consider the issue of double-counting which
may occur if additional {\it t}-channel contributions are included.Comment: Revtex, 35 pages, 16 eps figures; version to appear on PR
Quark mean field model with density dependent couplings for finite nuclei
The quark mean field model, which describes the nucleon using the constituent
quark model, is applied to investigate the properties of finite nuclei. The
couplings of the scalar and vector mesons with quarks are made density
dependent through direct coupling to the scalar field so as to reproduce the
relativistic Brueckner-Hartree-Fock results of nuclear matter. The present
model provides satisfactory results on the properties of spherical nuclei, and
predicts an increasing size of the nucleon as well as a reduction of the
nucleon mass in the nuclear environmentComment: 8 pages, REVTeX, 8 ps figures, accepted for publication in Phys. Rev.
Influence of polymer excluded volume on the phase behavior of colloid-polymer mixtures
We determine the depletion-induced phase-behavior of hard sphere colloids and
interacting polymers by large-scale Monte Carlo simulations using very accurate
coarse-graining techniques. A comparison with standard Asakura-Oosawa model
theories and simulations shows that including excluded volume interactions
between polymers leads to qualitative differences in the phase diagrams. These
effects become increasingly important for larger relative polymer size. Our
simulations results agree quantitatively with recent experiments.Comment: 5 pages, 4 figures submitted to Physical Review Letter
Metastable Dynamics of the Hard-Sphere System
The reformulation of the mode-coupling theory (MCT) of the liquid-glass
transition which incorporates the element of metastability is applied to the
hard-sphere system. It is shown that the glass transition in this system is not
a sharp one at the special value of the density or the packing fraction, which
is in contrast to the prediction by the conventional MCT. Instead we find that
the slowing down of the dynamics occurs over a range of values of the packing
fraction. Consequently, the exponents governing the sequence of time
relaxations in the intermediate time regime are given as functions of packing
fraction with one additional parameter which describes the overall scale of the
metastable potential energy for defects in the hard-sphere system. Implications
of the present model on the recent experiments on colloidal systems are also
discussed.Comment: 21 pages, 5 figures (available upon request), RevTEX3.0, JFI
Preprint
Test of mode coupling theory for a supercooled liquid of diatomic molecules.I. Translational degrees of freedom
A molecular dynamics simulation is performed for a supercooled liquid of
rigid diatomic molecules. The time-dependent self and collective density
correlators of the molecular centers of mass are determined and compared with
the predictions of the ideal mode coupling theory (MCT) for simple liquids.
This is done in real as well as in momentum space. One of the main results is
the existence of a unique transition temperature T_c, where the dynamics
crosses over from an ergodic to a quasi-nonergodic behavior. The value for T_c
agrees with that found earlier for the orientational dynamics within the error
bars. In the beta- regime of MCT the factorization of space- and time
dependence is satisfactorily fulfilled for both types of correlations. The
first scaling law of ideal MCT holds in the von Schweidler regime, only, since
the validity of the critical law can not be confirmed, due to a strong
interference with the microscopic dynamics. In this first scaling regime a
consistent description within ideal MCT emerges only, if the next order
correction to the asymptotic law is taken into account. This correction is
almost negligible for q=q_max, the position of the main peak in the static
structure factor S(q), but becomes important for q=q_min, the position of its
first minimum. The second scaling law, i.e. the time-temperature superposition
principle, holds reasonably well for the self and collective density
correlators and different values for q. The alpha-relaxation times tau_q^(s)
and tau_q follow a power law in T-T_c over 2 -- 3 decades. The corresponding
exponent gamma is weakly q-dependent and is around 2.55. This value is in
agreement with the one predicted by MCT from the value of the von Schweidler
exponent but at variance with the corresponding exponent gammaComment: 14 pages of RevTex, 19 figure
Theory of Decoupling in the Mixed Phase of Extremely Type-II Layered Superconductors
The mixed phase of extremely type-II layered superconductors in perpendicular
magnetic field is studied theoretically via the layered XY model with uniform
frustration. A partial duality analysis is carried out in the weak-coupling
limit. It consistently accounts for both intra-layer (pancake) and inter-layer
(Josephson) vortex excitations. The main conclusion reached is that
dislocations of the two-dimensional (2D) vortex lattices within layers drive a
unique second-order melting transition at high perpendicular fields between a
low-temperature superconducting phase that displays a Josephson effect and a
high-temperature ``normal'' phase that displays no Josephson effect. The former
state is best described by weakly coupled 2D vortex lattices, while the latter
state is best characterized by a decoupled vortex liquid. It is further argued
on the basis of the duality analysis that the second-order melting transition
converts itself into a first-order one as the perpendicular field is lowered
and approaches the dimensional cross-over scale. The resulting critical
endpoint potentially accounts for the same phenomenon that is observed in the
mixed phase of clean high-temperature superconductors.Comment: 39 pgs. of PLAIN TeX, 2 postscript figs., published versio
Violation of Bell inequalities by photons more than 10 km apart
A Franson-type test of Bell inequalities by photons 10.9 km apart is
presented. Energy-time entangled photon-pairs are measured using two-channel
analyzers, leading to a violation of the inequalities by 16 standard deviations
without subtracting accidental coincidences. Subtracting them, a 2-photon
interference visibility of 95.5% is observed, demonstrating that distances up
to 10 km have no significant effect on entanglement. This sets quantum
cryptography with photon pairs as a practical competitor to the schemes based
on weak pulses.Comment: 4 pages, REVTeX, 2 postscript figures include
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