7,862 research outputs found
Evidence for the absence of regularization corrections to the partial-wave renormalization procedure in one-loop self energy calculations in external fields
The equivalence of the covariant renormalization and the partial-wave
renormaliz ation (PWR) approach is proven explicitly for the one-loop
self-energy correction (SE) of a bound electron state in the presence of
external perturbation potentials. No spurious correctio n terms to the
noncovariant PWR scheme are generated for Coulomb-type screening potentia ls
and for external magnetic fields. It is shown that in numerical calculations of
the SE with Coulombic perturbation potential spurious terms result from an
improper treatment of the unphysical high-energy contribution. A method for
performing the PWR utilizing the relativistic B-spline approach for the
construction of the Dirac spectrum in external magnetic fields is proposed.
This method is applied for calculating QED corrections to the bound-electron
-factor in H-like ions. Within the level of accuracy of about 0.1% no
spurious terms are generated in numerical calculations of the SE in magnetic
fields.Comment: 22 pages, LaTeX, 1 figur
Swedish integration policy documents: a close dialogic reading
Sweden as the great welfare state where everybody is equally welcomed and cared for has for long been the prevailing view. Although Swedish integration policy seems to confirm this view, this is far removed from many peopleâs experienced reality. I argue that part of this disharmony lies in how West European languages contain and relate to an âidentityâ construction, which perpetuates and is perpetuated through dichotomies that strengthen the social and political cogency of concepts such as âraceâ, ethnicity and culture. Based on this, I carry out a discourse analysis of Swedenâs major integration policy documents from the mid 1970s up to today.
After an eclectic reading of discourses on migration and integration terminology, âidentityâ and language, I assert the centrality of âidentityâ construction to everything we do. With this in mind, taking the dialogism promoted by the Bakhtinian Circle as the dichotomy to monologism, I carry out a close dialogic reading in the tradition of Lynn Pearce (1994) and Peter Stallybrass and Allon White (1986).
Contextualising the policy documents, I present the history of migration and integration from a Swedish perspective. Focusing on the last five decades, I divide the different historic tendencies into themes ranging from: emigration to labour migration, refugee migration and the European Union, and from immigrant policy to integration policy.
Believing that the conceptualisation and the handling of categorisation, segregation, culture, discrimination and racism are all central to a successful integration policy, I analyse the policy documents thematically accordingly. I show how the interdependence of the common âidentityâ constructions and language sometimes obscures and frequently counteracts the intention of the author. As a result, I argue that the Bakhtinian Circle holds the key to a better understanding of the invincibility of stereotyping within racialised discourses, through applying absolute âidentityâ constructions in monologic speech, and how this may be counteracted in order to strive for a dialogic approach to the world
Friction, order, and transverse pinning of a two-dimensional elastic lattice under periodic and impurity potentials
Frictional phenomena of two-dimensional elastic lattices are studied
numerically based on a two-dimensional Frenkel-Kontorova model with impurities.
It is shown that impurities can assist the depinning. We also investigate
anisotropic ordering and transverse pinning effects of sliding lattices, which
are characteristic of the moving Bragg glass state and/or transverse glass
state. Peculiar velocity dependence of the transverse pinning is observed in
the presence of both periodic and random potentials and discussed in the
relation with growing order and discommensurate structures.Comment: RevTeX, 4 pages, 5 figures. to appear in Phys. Rev. B Rapid Commu
Methanol masers reveal the magnetic field of the high-mass protostar IRAS 18089-1732
Context. The importance of the magnetic field in high-mass-star formation is
not yet fully clear and there are still many open questions concerning its role
in the accretion processes and generation of jets and outflows. In the past few
years, masers have been successfully used to probe the magnetic field
morphology and strength at scales of a few au around massive protostars, by
measuring linear polarisation angles and Zeeman splitting. The massive
protostar IRAS 18089-1732 is a well studied high-mass-star forming region,
showing a hot core chemistry and a disc-outflow system. Previous SMA
observations of polarised dust revealed an ordered magnetic field oriented
around the disc of IRAS 18089-1732. Aims. We want to determine the magnetic
field in the dense region probed by 6.7 GHz methanol maser observations and
compare it with observations in dust continuum polarisation, to investigate how
the magnetic field in the compact maser region relates to the large-scale field
around massive protostars. Methods. We reduced MERLIN observations at 6.7 GHz
of IRAS 18089-1732 and we analysed the polarised emission by methanol masers.
Results. Our MERLIN observations show that the magnetic field in the 6.7 GHz
methanol maser region is consistent with the magnetic field constrained by the
SMA dust polarisation observations. A tentative detection of circularly
polarised line emission is also presented. Conclusions. We found that the
magnetic field in the maser region has the same orientation as in the disk.
Thus the large-scale field component, even at the au scale of the masers,
dominates over any small-scale field fluctuations. We obtained, from the
circular polarisation tentative detection, a field strength along the line of
sight of 5.5 mG which appeared to be consistent with the previous estimates.Comment: 12 pages, 7 figures, accepted for publication in A&
Dynamical transitions and sliding friction in the two-dimensional Frenkel-Kontorova model
The nonlinear response of an adsorbed layer on a periodic substrate to an
external force is studied via a two dimensional uniaxial Frenkel-Kontorova
model. The nonequlibrium properties of the model are simulated by Brownian
molecular dynamics. Dynamical phase transitions between pinned solid, sliding
commensurate and incommensurate solids and hysteresis effects are found that
are qualitatively similar to the results for a Lennard-Jones model, thus
demonstrating the universal nature of these features.Comment: 11 pages, 12 figures, to appear in Phys. Rev.
Friction Laws for Elastic Nano-Scale Contacts
The effect of surface curvature on the law relating frictional forces F with
normal load L is investigated by molecular dynamics simulations as a function
of surface symmetry, adhesion, and contamination. Curved, non-adhering, dry,
commensurate surfaces show a linear dependency, F proportional to L, similar to
dry flat commensurate or amorphous surfaces and macroscopic surfaces. In
contrast, curved, non-adhering, dry, amorphous surfaces show F proportional to
L^(2/3) similar to friction force microscopes. In our model, adhesive effects
are most adequately described by the Hertz plus offset model, as the
simulations are confined to small contact radii. Curved lubricated or
contaminated surfaces show again different behavior; details depend on how much
of the contaminant gets squeezed out of the contact. Also, it is seen that the
friction force in the lubricated case is mainly due to atoms at the entrance of
the tip.Comment: 7 pages, 5 figures, submitted to Europhys. Let
The Schr\"odinger operator on an infinite wedge with a tangent magnetic field
We study a model Schr\"odinger operator with constant magnetic field on an
infinite wedge with Neumann boundary condition. The magnetic field is assumed
to be tangent to a face. We compare the bottom of the spectrum to the model
spectral quantities coming from the regular case. We are particularly motivated
by the influence of the magnetic field and the opening angle of the wedge on
the spectrum of the model operator and we exhibit cases where the bottom of the
spectrum is smaller than in the regular case. Numerical computations enlighten
the theoretical approach
ALMA reveals the magnetic field evolution in the high-mass star forming complex G9.62+0.19
Context. The role of magnetic fields during the formation of high-mass stars
is not yet fully understood, and the processes related to the early
fragmentation and collapse are largely unexplored today. The high-mass star
forming region G9.62+0.19 is a well known source, presenting several cores at
different evolutionary stages. Aims. We determine the magnetic field morphology
and strength in the high-mass star forming region G9.62+0.19, to investigate
its relation to the evolutionary sequence of the cores. Methods. We use Band 7
ALMA observations in full polarisation mode and we analyse the polarised dust
emission. We estimate the magnetic field strength via the
Davis-Chandrasekhar-Fermi and the Structure Function methods. Results. We
resolve several protostellar cores embedded in a bright and dusty filamentary
structure. The polarised emission is clearly detected in six regions. Moreover
the magnetic field is oriented along the filament and appears perpendicular to
the direction of the outflows. We suggest an evolutionary sequence of the
magnetic field, and the less evolved hot core exhibits a magnetic field
stronger than the more evolved one. We detect linear polarisation from thermal
line emission and we tentatively compared linear polarisation vectors from our
observations with previous linearly polarised OH masers observations. We also
compute the spectral index, the column density and the mass for some of the
cores. Conclusions. The high magnetic field strength and the smooth polarised
emission indicate that the magnetic field could play an important role for the
fragmentation and the collapse process in the star forming region G9.62+019 and
that the evolution of the cores can be magnetically regulated. On average, the
magnetic field derived by the linear polarised emission from dust, thermal
lines and masers is pointing in the same direction and has consistent strength.Comment: accepted by A&A, version after language editin
Transverse thermal depinning and nonlinear sliding friction of an adsorbed monolayer
We study the response of an adsorbed monolayer under a driving force as a
model of sliding friction phenomena between two crystalline surfaces with a
boundary lubrication layer. Using Langevin-dynamics simulation, we determine
the nonlinear response in the direction transverse to a high symmetry direction
along which the layer is already sliding. We find that below a finite
transition temperature, there exist a critical depinning force and hysteresis
effects in the transverse response in the dynamical state when the adlayer is
sliding smoothly along the longitudinal direction.Comment: 4 pages, 4 figures, to appear in Phys. Rev. Let
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