1,327 research outputs found
Equation of the field lines of an axisymmetric multipole with a source surface
Optical spectropolarimeters can be used to produce maps of the surface magnetic fields of stars and hence to determine how stellar magnetic fields vary with stellar mass, rotation rate, and evolutionary stage. In particular, we now can map the surface magnetic fields of forming solar-like stars, which are still contracting under gravity and are surrounded by a disk of gas and dust. Their large scale magnetic fields are almost dipolar on some stars, and there is evidence for many higher order multipole field components on other stars. The availability of new data has renewed interest in incorporating multipolar magnetic fields into models of stellar magnetospheres. I describe the basic properties of axial multipoles of arbitrary degree ℓ and derive the equation of the field lines in spherical coordinates. The spherical magnetic field components that describe the global stellar field topology are obtained analytically assuming that currents can be neglected in the region exterior to the star, and interior to some fixed spherical equipotential surface. The field components follow from the solution of Laplace’s equation for the magnetostatic potential
Bridging the Divide in Work and Occupational Psychology: Evidence From Practice
This study explores the extent to which work and organizational (W&O) psychology practitioners use evidence, how they apply it to the everyday contexts in which they work and the types of barriers they encounter in so doing. It adopts a mixed methods approach involving the administration of a survey to a UK sample (N=163) of W&O psychologists and a series of semi-structured interviews (N=25) exploring in greater depth how evidence is applied in practice. Findings reveal that practitioners consult a wide range of different types of evidence which they employ at various stages of engagement with client organisations and that this evidence is pressed into service in the pursuit of solutions which are both acceptable from the client perspective and consistent with the scientific standards underpinning professional knowledge and expertise in W&O psychology. Barriers to evidence-use were mainly practical in nature, concerning issues around managing the client-consultant relationship and the particularities of implementation context, both of which were shown to influence evidence utilisation. The study contributes to current debate on the extent to which W&O psychologists adopt an evidence-based approach and provides a valuable and much called-for empirical insight into the enactment of the scientist-practitioner model in W&O psycholog
Interaction of vortices in thin superconducting films and Berezinskii-Kosterlitz-Thouless transition
The precondition for the BKT transition in thin superconducting films, the
logarithmic intervortex interaction, is satisfied at distances short relative
to , is the London penetration depth of the
bulk material and is the film thickness. For this reason, the search for
the transition has been conducted in samples of the size . It is
argued below that film edges turn the interaction into near exponential
(short-range) thus making the BKT transition impossible. If however the
substrate is superconducting and separated from the film by an insulated layer,
the logarithmic intervortex interaction is recovered and the BKT transition
should be observable.Comment: 4 pages, no figure
On the uniqueness of the surface sources of evoked potentials
The uniqueness of a surface density of sources localized inside a spatial
region and producing a given electric potential distribution in its
boundary is revisited. The situation in which is filled with various
metallic subregions, each one having a definite constant value for the electric
conductivity is considered. It is argued that the knowledge of the potential in
all fully determines the surface density of sources over a wide class of
surfaces supporting them. The class can be defined as a union of an arbitrary
but finite number of open or closed surfaces. The only restriction upon them is
that no one of the closed surfaces contains inside it another (nesting) of the
closed or open surfaces.Comment: 16 pages, 5 figure
Gradient of the Casimir force between Au surfaces of a sphere and a plate measured using atomic force microscope in a frequency shift technique
We present measurement results for the gradient of the Casimir force between
an Au-coated sphere and an Au-coated plate obtained by means of an atomic force
microscope operated in a frequency shift technique. This experiment was
performed at a pressure of 3x10^{-8} Torr with hollow glass sphere of 41.3 mcm
radius. Special attention is paid to electrostatic calibrations including the
problem of electrostatic patches. All calibration parameters are shown to be
separation-independent after the corrections for mechanical drift are included.
The gradient of the Casimir force was measured in two ways with applied
compensating voltage to the plate and with different applied voltages and
subsequent subtraction of electric forces. The obtained mean gradients are
shown to be in mutual agreement and in agreement with previous experiments
performed using a micromachined oscillator. The obtained data are compared with
theoretical predictions of the Lifshitz theory including corrections beyond the
proximity force approximation. An independent comparison with no fitting
parameters demonstrated that the Drude model approach is excluded by the data
at a 67% confidence level over the separation region from 235 to 420 nm. The
theoretical approach using the generalized plasma-like model is shown to be
consistent with the data over the entire measurement range. Corrections due to
the nonlinearity of oscillator are calculated and the application region of the
linear regime is determined. A conclusion is made that the results of several
performed experiments call for a thorough analysis of the basics of the theory
of dispersion forces.Comment: 35 pages, 14 figures, 1 table; to appear in Phys. Rev.
Electrostatics of ions inside the nanopores and trans-membrane channels
A model of a finite cylindrical ion channel through a phospholipid membrane
of width separating two electrolyte reservoirs is studied. Analytical
solution of the Poisson equation is obtained for an arbitrary distribution of
ions inside the trans-membrane pore. The solution is asymptotically exact in
the limit of large ionic strength of electrolyte on the two sides of membrane.
However, even for physiological concentrations of electrolyte, the
electrostatic barrier sizes found using the theory are in excellent agreement
with the numerical solution of the Poisson equation. The analytical solution is
used to calculate the electrostatic potential energy profiles for pores
containing charged protein residues. Availability of a semi-exact interionic
potential should greatly facilitate the study of ionic transport through
nanopores and ion channels
Modifying the Casimir force between indium tin oxide film and Au sphere
We present complete results of the experiment on measuring the Casimir force
between an Au-coated sphere and an untreated or, alternatively, UV-treated
indium tin oxide film deposited on a quartz substrate. Measurements were
performed using an atomic force microscope in a high vacuum chamber. The
measurement system was calibrated electrostatically. Special analysis of the
systematic deviations is performed, and respective corrections in the
calibration parameters are introduced. The corrected parameters are free from
anomalies discussed in the literature. The experimental data for the Casimir
force from two measurement sets for both untreated and UV-treated samples are
presented. The experimental errors are determined at a 95% confidence level. It
is demonstrated that the UV treatment of an I TO plate results in a significant
decrease in the magnitude of the Casimir force (from 21% to 35% depending on
separation). However, ellipsometry measurements of the imaginary parts of
dielectric permittivities of the untreated and UV-treated samples did not
reveal any significant differences. The experimental data are compared with
computations in the framework of the Lifshitz theory. It is found that the data
for the untreated sample are in a very good agreement with theoretical results
taking into account the free charge carriers in an ITO film. For the UV-treated
sample the data exclude the theoretical results obtained with account of free
charge carriers. These data are in a very good agreement with computations
disregarding the contribution of free carriers. According to the explanation
provided, this is caused by the phase transition of the ITO film from metallic
to dielectric state caused by the UV treatment. Possible applications of the
discovered phenomenon in nanotechnology are discussed.Comment: 30 pages, 19 figures, 1 tabl
Steplike electric conduction in a classical two-dimensional electron system through a narrow constriction in a microchannel
Using molecular dynamics simulation, we investigate transport properties of a
classical two-dimensional electron system confined in a microchannel with a
narrow constriction. As a function of the confinement strength of the
constriction, the calculated conductance in the simulations exhibits steplike
increases as reported in a recent experiment [D. G. Rees et al., Phys. Rev.
Lett. 106, 026803 (2011)]. It is confirmed that the number of the steps
corresponds to the number of stream lines of electrons through the
constriction. We verify that density fluctuation plays a major role in
smoothing the steps in the conductance.Comment: 11 pages, 9 figure
Generators for the hyperelliptic Torelli group and the kernel of the Burau representation at t = -1
We prove that the hyperelliptic Torelli group is generated by Dehn twists about
separating curves that are preserved by the hyperelliptic involution. This verifies a
conjecture of Hain. The hyperelliptic Torelli group can be identified with the kernel
of the Burau representation evaluated at t = −1 and also the fundamental group of
the branch locus of the period mapping, and so we obtain analogous generating sets
for those. One application is that each component in Torelli space of the locus of
hyperelliptic curves becomes simply connected when curves of compact type are added
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