37,281 research outputs found
A class of symplectic integrators with adaptive timestep for separable Hamiltonian systems
Symplectic integration algorithms are well-suited for long-term integrations
of Hamiltonian systems because they preserve the geometric structure of the
Hamiltonian flow. However, this desirable property is generally lost when
adaptive timestep control is added to a symplectic integrator. We describe an
adaptive-timestep symplectic integrator that can be used if the Hamiltonian is
the sum of kinetic and potential energy components and the required timestep
depends only on the potential energy (e.g. test-particle integrations in fixed
potentials). In particular, we describe an explicit, reversible, symplectic,
leapfrog integrator for a test particle in a near-Keplerian potential; this
integrator has timestep proportional to distance from the attracting mass and
has the remarkable property of integrating orbits in an inverse-square force
field with only "along-track" errors; i.e. the phase-space shape of a Keplerian
orbit is reproduced exactly, but the orbital period is in error by O(1/N^2),
where N is the number of steps per period.Comment: 24 pages, 3 figures, submitted to Astronomical Journal; minor errors
in equations and one figure correcte
Changing the Ties That Bind? The Emerging Roles and Identities of General Practitioners and Managers in the New Clinical Commissioning Groups in the English NHS
The English National Health Service (NHS) is undergoing significant reorganization following the 2012 Health and Social Care Act. Key to these changes is the shift of responsibility for commissioning services from Primary Care Trusts (PCTs) to general practitioners (GPs) working together in Clinical Commissioning Groups (CCGs). This article is based on an empirical study that examined the development of emerging CCGs in eight case studies across England between September 2011 and June 2012. The findings are based on interviews with GPs and managers, observations of meetings, and reading of related documents. Scott’s notion that institutions are constituted by three pillars—the regulative, normative, and cognitive–cultural—is explored here. This approach helps to understand the changing roles and identities of doctors and managers implicated by the present reforms. This article notes the far reaching changes in the regulative pillar and questions how these changes will affect the normative and cultural–cognitive pillars
Giant electrocaloric effect in thin film Pb Zr_0.95 Ti_0.05 O_3
An applied electric field can reversibly change the temperature of an
electrocaloric material under adiabatic conditions, and the effect is strongest
near phase transitions. This phenomenon has been largely ignored because only
small effects (0.003 K V^-1) have been seen in bulk samples such as
Pb0.99Nb0.02(Zr0.75Sn0.20Ti0.05)0.98O3 and there is no consensus on macroscopic
models. Here we demonstrate a giant electrocaloric effect (0.48 K V^-1) in 300
nm sol-gel PbZr0.95Ti0.05O3 films near the ferroelectric Curie temperature of
222oC. We also discuss a solid state device concept for electrical
refrigeration that has the capacity to outperform Peltier or magnetocaloric
coolers. Our results resolve the controversy surrounding macroscopic models of
the electrocaloric effect and may inspire ab initio calculations of
electrocaloric parameters and thus a targeted search for new materials.Comment: 5 pages, 4 figure
Growth inhibition observed following administration of an LHRH agonist to a clonal variant of the MCF-7 breast cancer cell line is accompanied by an accumulation of cells in the G0/G1 phase of the cell cycle.
Fast algorithm for detecting community structure in networks
It has been found that many networks display community structure -- groups of
vertices within which connections are dense but between which they are sparser
-- and highly sensitive computer algorithms have in recent years been developed
for detecting such structure. These algorithms however are computationally
demanding, which limits their application to small networks. Here we describe a
new algorithm which gives excellent results when tested on both
computer-generated and real-world networks and is much faster, typically
thousands of times faster than previous algorithms. We give several example
applications, including one to a collaboration network of more than 50000
physicists.Comment: 5 pages, 4 figure
Minimum orbit dimension for local unitary action on n-qubit pure states
The group of local unitary transformations partitions the space of n-qubit
quantum states into orbits, each of which is a differentiable manifold of some
dimension. We prove that all orbits of the n-qubit quantum state space have
dimension greater than or equal to 3n/2 for n even and greater than or equal to
(3n + 1)/2 for n odd. This lower bound on orbit dimension is sharp, since
n-qubit states composed of products of singlets achieve these lowest orbit
dimensions.Comment: 19 page
Hybrid phase-space simulation method for interacting Bose fields
We introduce an approximate phase-space technique to simulate the quantum
dynamics of interacting bosons. With the future goal of treating Bose-Einstein
condensate systems, the method is designed for systems with a natural
separation into highly occupied (condensed) modes and lightly occupied modes.
The method self-consistently uses the Wigner representation to treat highly
occupied modes and the positive-P representation for lightly occupied modes. In
this method, truncation of higher-derivative terms from the Fokker-Planck
equation is usually necessary. However, at least in the cases investigated
here, the resulting systematic error, over a finite time, vanishes in the limit
of large Wigner occupation numbers. We tested the method on a system of two
interacting anharmonic oscillators, with high and low occupations,
respectively. The Hybrid method successfully predicted atomic quadratures to a
useful simulation time 60 times longer than that of the positive-P method. The
truncated Wigner method also performed well in this test. For the prediction of
the correlation in a quantum nondemolition measurement scheme, for this same
system, the Hybrid method gave excellent agreement with the exact result, while
the truncated Wigner method showed a large systematic error.Comment: 13 pages; 6 figures; references added; figures correcte
Optimal Moments for the Analysis of Peculiar Velocity Surveys
We present a new method for the analysis of peculiar velocity surveys which
removes contributions to velocities from small scale, nonlinear velocity modes
while retaining information about large scale motions. Our method utilizes
Karhunen--Lo\`eve methods of data compression to construct a set of moments out
of the velocities which are minimally sensitive to small scale power. The set
of moments are then used in a likelihood analysis. We develop criteria for the
selection of moments, as well as a statistic to quantify the overall
sensitivity of a set of moments to small scale power. Although we discuss our
method in the context of peculiar velocity surveys, it may also prove useful in
other situations where data filtering is required.Comment: 25 Pages, 3 figures. Submitted to Ap
Simple theory for spin-lattice relaxation in metallic rare earth ferromagnets
The spin-lattice relaxation time is a key quantity both for the
dynamical response of ferromagnets excited by laser pulses and as the speed
limit of magneto-optical recording. Extending the theory for the electron
paramagnetic resonance of magnetic impurities to spin-lattice relaxation in
ferromagnetic rare earths we calculate for Gd and find a value of
48 ps in very good agreement with time-resolved spin-polarized photoemission
experiments. We argue that the time scale for in metals is
essentially given by the spin-orbit induced magnetocrystalline anisotropy
energy.Comment: 18 pages revtex, 5 uuencoded figure
Fast and Slow Rotators in the Densest Environments: a SWIFT IFS study of the Coma Cluster
We present integral-field spectroscopy of 27 galaxies in the Coma cluster
observed with the Oxford SWIFT spectrograph, exploring the kinematic
morphology-density relationship in a cluster environment richer and denser than
any in the ATLAS3D survey. Our new data enables comparison of the kinematic
morphology relation in three very different clusters (Virgo, Coma and Abell
1689) as well as to the field/group environment. The Coma sample was selected
to match the parent luminosity and ellipticity distributions of the early-type
population within a radius 15' (0.43 Mpc) of the cluster centre, and is limited
to r' = 16 mag (equivalent to M_K = -21.5 mag), sampling one third of that
population. From analysis of the lambda-ellipticity diagram, we find 15+-6% of
early-type galaxies are slow rotators; this is identical to the fraction found
in the field and the average fraction in the Virgo cluster, based on the
ATLAS3D data. It is also identical to the average fraction found recently in
Abell 1689 by D'Eugenio et al.. Thus it appears that the average slow rotator
fraction of early type galaxies remains remarkably constant across many
different environments, spanning five orders of magnitude in galaxy number
density. However, within each cluster the slow rotators are generally found in
regions of higher projected density, possibly as a result of mass segregation
by dynamical friction. These results provide firm constraints on the mechanisms
that produce early-type galaxies: they must maintain a fixed ratio between the
number of fast rotators and slow rotators while also allowing the total
early-type fraction to increase in clusters relative to the field. A complete
survey of Coma, sampling hundreds rather than tens of galaxies, could probe a
more representative volume of Coma and provide significantly stronger
constraints, particularly on how the slow rotator fraction varies at larger
radii.Comment: Accepted for publication in MNRA
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