670 research outputs found
Hard rod gas with long-range interactions: Exact predictions for hydrodynamic properties of continuum systems from discrete models
One-dimensional hard rod gases are explicitly constructed as the limits of
discrete systems: exclusion processes involving particles of arbitrary length.
Those continuum many-body systems in general do not exhibit the same
hydrodynamic properties as the underlying discrete models. Considering as
examples a hard rod gas with additional long-range interaction and the
generalized asymmetric exclusion process for extended particles (-ASEP),
it is shown how a correspondence between continuous and discrete systems must
be established instead. This opens up a new possibility to exactly predict the
hydrodynamic behaviour of this continuum system under Eulerian scaling by
solving its discrete counterpart with analytical or numerical tools. As an
illustration, simulations of the totally asymmetric exclusion process
(-TASEP) are compared to analytical solutions of the model and applied to
the corresponding hard rod gas. The case of short-range interaction is treated
separately.Comment: 19 pages, 8 figure
Structural Diversity of Lithium N âMesitylâ P , P âdiphenylphosphinimidate of the type [(L)Li{OâPPh 2 =NâMes] n Depending on Lewis Base L
Abstract Metalation of N âmesitylâ P,P âdiphenylphosphinic amide with n BuLi in toluene yields tetranuclear lithium N âmesitylâ P , P âdiphenylphosphinimidate ([Ph 2 P(OLi)=NâMes] 4 , 1 ). Metalation of Ph 2 P(O)âN(H)Mes with a mixture of dibutylmagnesium and butyllithium in DME leads to formation of dinuclear [Ph 2 P{OLi(dme)}=NâMes] 2 ( 2 ). Excess of Ph 2 P(O)âN(H)Mes gives dinuclear [Li(OâPPh 2 =NâMes){Ph 2 P(=O)âN(H)âMes}] 2 ( 3 ) with threeâcoordinate alkali ions. The metathetical approach via reaction of 1 with anhydrous magnesium bromide in ethereal solution yields [{(thf)LiBr} 2 {(thf)Li(OâPPh 2 =NMes)(Et 2 O)Li(OâPPh 2 =NMes)}] ( 4 ). Heterobimetallic Li/Mg compounds are not accessible by these protocols. Reactions of 1 with DME, with excess of Ph 2 P(O)âN(H)Mes or with LiBr allows the straightforward conversion to compounds 2 , 3 and 4 .imag
Directed diffusion of reconstituting dimers
We discuss dynamical aspects of an asymmetric version of assisted diffusion
of hard core particles on a ring studied by G. I. Menon {\it et al.} in J. Stat
Phys. {\bf 86}, 1237 (1997). The asymmetry brings in phenomena like kinematic
waves and effects of the Kardar-Parisi-Zhang nonlinearity, which combine with
the feature of strongly broken ergodicity, a characteristic of the model. A
central role is played by a single nonlocal invariant, the irreducible string,
whose interplay with the driven motion of reconstituting dimers, arising from
the assisted hopping, determines the asymptotic dynamics and scaling regimes.
These are investigated both analytically and numerically through
sector-dependent mappings to the asymmetric simple exclusion process.Comment: 10 pages, 6 figures. Slight corrections, one added reference. To
appear in J. Phys. Cond. Matt. (2007). Special issue on chemical kinetic
Formation of phase lags at the cyclotron energies in the pulse profiles of magnetized, accreting neutron stars
Context: Accretion-powered X-ray pulsars show highly energy-dependent and
complex pulse-profile morphologies. Significant deviations from the average
pulse profile can appear, in particular close to the cyclotron line energies.
These deviations can be described as energy-dependent phase lags, that is, as
energy-dependent shifts of main features in the pulse profile. Aims: Using a
numerical study we explore the effect of cyclotron resonant scattering on
observable, energy-resolved pulse profiles. Methods: We generated the
observable emission as a function of spin phase, using Monte Carlo simulations
for cyclotron resonant scattering and a numerical ray-tracing routine
accounting for general relativistic light-bending effects on the intrinsic
emission from the accretion columns. Results: We find strong changes in the
pulse profile coincident with the cyclotron line energies. Features in the
pulse profile vary strongly with respect to the average pulse profile with the
observing geometry and shift and smear out in energy additionally when assuming
a non-static plasma. Conclusions: We demonstrate how phase lags at the
cyclotron energies arise as a consequence of the effects of angular
redistribution of X-rays by cyclotron resonance scattering in a strong magnetic
field combined with relativistic effects. We also show that phase lags are
strongly dependent on the accretion geometry. These intrinsic effects will in
principle allow us to constrain a system's accretion geometry.Comment: 4 pages, 4 figures; updated reference lis
Cyclotron resonant scattering feature simulations. I. Thermally averaged cyclotron scattering cross sections, mean free photon-path tables, and electron momentum sampling
Electron cyclotron resonant scattering features (CRSFs) are observed as
absorption-like lines in the spectra of X-ray pulsars. A significant fraction
of the computing time for Monte Carlo simulations of these quantum mechanical
features is spent on the calculation of the mean free path for each individual
photon before scattering, since it involves a complex numerical integration
over the scattering cross section and the (thermal) velocity distribution of
the scattering electrons.
We aim to numerically calculate interpolation tables which can be used in
CRSF simulations to sample the mean free path of the scattering photon and the
momentum of the scattering electron. The tables also contain all the
information required for sampling the scattering electron's final spin.
The tables were calculated using an adaptive Simpson integration scheme. The
energy and angle grids were refined until a prescribed accuracy is reached. The
tables are used by our simulation code to produce artificial CRSF spectra. The
electron momenta sampled during these simulations were analyzed and justified
using theoretically determined boundaries.
We present a complete set of tables suited for mean free path calculations of
Monte Carlo simulations of the cyclotron scattering process for conditions
expected in typical X-ray pulsar accretion columns (0.01<B/B_{crit}<=0.12,
where B_{crit}=4.413x10^{13} G and 3keV<=kT<15keV). The sampling of the tables
is chosen such that the results have an estimated relative error of at most
1/15 for all points in the grid. The tables are available online at
http://www.sternwarte.uni-erlangen.de/research/cyclo.Comment: A&A, in pres
The XMM-Newton view of the Crab
Aims. We discuss the current X-ray view of the Crab Nebula and Pulsar, summarising our analysis of observations of the source with the EPIC-pn camera on board the XMM-Newton observatory. Different modes of EPIC-pn were combined in order to yield a complete scenario of the spectral properties of the Crab resolved in space and time (pulse phase). In addition we give a description of the special
EPIC-pn Burst mode and guidance for data reduction in that mode.
Methods. We analysed spectra for the nebula and pulsar separately in the 0.6â12.0 keV energy band. All data were processed with the
SAS 6.0.0 XMM-Newton Scientific Analysis System package; models were fitted to the data with XSPEC 11. The high time resolution of EPIC-pn in its Burst mode (7 ÎŒs) was used for a phase resolved analysis of the pulsar spectrum, after determination of the period with epoch folding techniques. Data from the SmallWindow mode were processed and corrected for pile-up allowing for spectroscopy
simultaneously resolved in space and time.
Results. The spatial variation of the spectrum over the entire region of the Crab shows a gradual spectral softening from the inner pulsar region to the outer nebula region with a variation in photon index, Î, from 2.0 to 2.4. Pulse phase resolved spectroscopy of the Crab Pulsar reveals a phase dependent modulation of the photon index in form of a significant hardening of the spectrum in the
inter-peak phase from Î = 1.7 during the pulse peak to Î = 1.5
Participation and satisfaction after spinal cord injury: results of a vocational and leisure outcome study
Study design: Survey. Objectives: Insight in (1) the changes in participation in vocational and leisure activities and (2) satisfaction with the current participation level of people with spinal cord injuries (SCIs) after reintegration in society. Design: Descriptive analysis of data from a questionnaire. Setting: Rehabilitation centre with special department for patients with SCIs, Groningen, The Netherlands. Subjects: A total of 57 patients with traumatic SCI living in the community, who were admitted to the rehabilitation centre two to 12 years before the current assessment. Main outcome measures: Changes in participation in activities; current life satisfaction; support and unmet needs. Results: Participation expressed in terms of hours spent on vocational and leisure activities changed to a great extent after the SCI. This was mainly determined by a large reduction of hours spent on paid work. While 60% of the respondents successfully reintegrated in work, many changes took place in the type and extent of the job. Loss of work was partially compensated with domestic and leisure activities. Sports activities were reduced substantially. The change in participation level and compensation for the lost working hours was not significantly associated with the level of SCI-specific health problems and disabilities. As was found in other studies, most respondents were satisfied with their lives. Determinants of a negative life satisfaction several years following SCI were not easily indicated. Reduced quality of life was particularly related to an unsatisfactory work and leisure situation. Conclusions: Most people with SCI in this study group were able to resume work and were satisfied with their work and leisure situation
A simple vapor-diffusion method enables protein crystallization inside the HARE serial crystallography chip
Fixed-target serial crystallography has become an important method for the study of protein structure and dynamics at synchrotrons and X-ray free-electron lasers. However, sample homogeneity, consumption and the physical stress on samples remain major challenges for these high-throughput experiments, which depend on high-quality protein microcrystals. The batch crystallization procedures that are typically applied require time- and sample-intensive screening and optimization. Here, a simple protein crystallization method inside the features of the HARE serial crystallography chips is reported that circumvents batch crystallization and allows the direct transfer of canonical vapor-diffusion conditions to in-chip crystallization. Based on conventional hanging-drop vapor-diffusion experiments, the crystallization solution is distributed into the wells of the HARE chip and equilibrated against a reservoir with mother liquor. Using this simple method, high-quality microcrystals were generated with sufficient density for the structure determination of four different proteins. A new protein variant was crystallized using the protein concentrations encountered during canonical crystallization experiments, enabling structure determination from âŒ55â
”g of protein. Additionally, structure determination from intracellular crystals grown in insect cells cultured directly in the features of the HARE chips is demonstrated. In cellulo crystallization represents a comparatively unÂexplored space in crystallization, especially for proteins that are resistant to crystallization using conventional techniques, and eliminates any need for laborious protein purification. This in-chip technique avoids harvesting the sensitive crystals or any further physical handling of the crystal-containing cells. These proof-of-principle experiments indicate the potential of this method to become a simple alternative to batch crystallization approaches and also as a convenient extension to canonical crystallization screens
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