1,925 research outputs found
The Consistent Newtonian Limit of Einstein's Gravity with a Cosmological Constant
We derive the `exact' Newtonian limit of general relativity with a positive
cosmological constant . We point out that in contrast to the case with
, the presence of a positive in Einsteins's equations
enforces, via the condition , on the potential , a range
, within which the
Newtonian limit is valid. It also leads to the existence of a maximum mass,
. As a consequence we cannot put the boundary
condition for the solution of the Poisson equation at infinity. A boundary
condition suitably chosen now at a finite range will then get reflected in the
solution of provided the mass distribution is not spherically symmetric.Comment: Latex, 15 pages, no figures, errors correcte
A Helly theorem for convexity in graphs
AbstractIt is shown that for chordless path convexity in any graph, the Helly number equals the size of a maximum clique
Vector magnetic field microscopy using nitrogen vacancy centers in diamond
The localized spin triplet ground state of a nitrogen vacancy (NV) center in
diamond can be used in atomic-scale detection of local magnetic fields. Here we
present a technique using these defects in diamond to image fields around
magnetic structures. We extract the local magnetic field vector by probing
resonant transitions of the four fixed tetrahedral NV orientations. In
combination with confocal microscopy techniques, we construct a 2-dimensional
image of the local magnetic field vectors. Measurements are done in external
fields less than 50 G and under ambient conditions.Comment: 9 pages, 3 figure
DEEP: a provenance-aware executable document system
The concept of executable documents is attracting growing interest from both academics and publishers since it is a promising technology for the dissemination of scientific results. Provenance is a kind of metadata that provides a rich description of the derivation history of data products starting from their original sources. It has been used in many different e-Science domains and has shown great potential in enabling reproducibility of scientific results. However, while both executable documents and provenance are aimed at enhancing the dissemination of scientific results, little has been done to explore the integration of both techniques. In this paper, we introduce the design and development of DEEP, an executable document environment that generates scientific results dynamically and interactively, and also records the provenance for these results in the document. In this system, provenance is exposed to users via an interface that provides them with an alternative way of navigating the executable document. In addition, we make use of the provenance to offer a document rollback facility to users and help to manage the system's dynamic resources
Deterministic Domain Wall Motion Orthogonal To Current Flow Due To Spin Orbit Torque.
Spin-polarized electrons can move a ferromagnetic domain wall through the transfer of spin angular momentum when current flows in a magnetic nanowire. Such current induced control of a domain wall is of significant interest due to its potential application for low power ultra high-density data storage. In previous reports, it has been observed that the motion of the domain wall always happens parallel to the current flow - either in the same or opposite direction depending on the specific nature of the interaction. In contrast, here we demonstrate deterministic control of a ferromagnetic domain wall orthogonal to current flow by exploiting the spin orbit torque in a perpendicularly polarized Ta/CoFeB/MgO heterostructure in presence of an in-plane magnetic field. Reversing the polarity of either the current flow or the in-plane field is found to reverse the direction of the domain wall motion. Notably, such orthogonal motion with respect to current flow is not possible from traditional spin transfer torque driven domain wall propagation even in presence of an external magnetic field. Therefore the domain wall motion happens purely due to spin orbit torque. These results represent a completely new degree of freedom in current induced control of a ferromagnetic domain wall
Spin Control of Drifting Electrons using Local Nuclear Polarization in Ferromagnet/Semiconductor Heterostructures
We demonstrate methods to locally control the spin rotation of moving
electrons in a GaAs channel. The Larmor frequency of optically-injected spins
is modulated when the spins are dragged through a region of spin-polarized
nuclei created at a MnAs/GaAs interface. The effective field created by the
nuclei is controlled either optically or electrically using the ferromagnetic
proximity polarization effect. Spin rotation is also tuned by controlling the
carrier traverse time through the polarized region. We demonstrate coherent
spin rotations exceeding 4 pi radians during transport.Comment: 15 pages, 4 figure
The influence of porosity on machinability of sintered fe foam elements
The aim of the experiment was to study the machinability of porous metal foams formed by reduction of metal oxides during sintering. The analysis focused on the machining process of metal foams with respect to their porosity and mechanical properties. The factors investigated included the geometry of the specimens, the surface condition depending on the machining parameters (milling) and the porosity of the metal foam. The metallic porous structure was obtained as a result of sintering the mixture of iron based powders ASC 100.29 and DISTALOY SE (DIST SE) with porosity from 67,9 % (SE 1) to 77,8 % (SE 2) for ASC base powder and 75,7 % to 80.3 % for DISTALOY SE were used
Measurements of Nanoscale Domain Wall Flexing in a Ferromagnetic Thin Film
We use the high spatial sensitivity of the anomalous Hall effect in the
ferromagnetic semiconductor Ga1-xMnxAs, combined with the magneto-optical Kerr
effect, to probe the nanoscale elastic flexing behavior of a single magnetic
domain wall in a ferromagnetic thin film. Our technique allows position
sensitive characterization of the pinning site density, which we estimate to be
around 10^14 cm^{-3}. Analysis of single site depinning events and their
temperature dependence yields estimates of pinning site forces (10 pN range) as
well as the thermal deactivation energy. Finally, our data hints at a much
higher intrinsic domain wall mobility for flexing than previously observed in
optically-probed micron scale measurements
Uncovering hidden modes in RR Lyrae stars
The Kepler space telescope revealed new, unexpected phenomena in RR Lyrae
stars: period doubling and the possible presence of additional modes.
Identifying these modes is complicated because they blend in the rich features
of the Fourier-spectrum. Our hydrodynamic calculations uncovered that a
'hidden' mode, the 9th overtone is involved in the period doubling phenomenon.
The period of the overtone changes by up to 10 per cent compared to the linear
value, indicating a very significant nonlinear period shift caused by its
resonance with the fundamental mode. The observations also revealed weak peaks
that may correspond to the first or second overtones. These additional modes
are often coupled with period doubling. We investigated the possibilities and
occurrences of mutual resonances between the fundamental mode and multiple
overtones in our models. These theoretical findings can help interpreting the
origin and nature of the 'hidden' modes may be found in the high quality light
curves of space observatories.Comment: In proceedings of "20th Stellar Pulsation Conference Series: Impact
of new instrumentation & new insights in stellar pulsations", 5-9 September
2011, Granada, Spai
Fourier decomposition and frequency analysis of the pulsating stars with P < 1day in the OGLE database. II. Multiperiodic RR Lyrae variables in the Galactic Bulge
We present the results of a systematic search for multiperiodic pulsators
among the Galactic Bulge RR Lyrae stars of the OGLE-1 sample. We identify one
"canonical" double-mode variable (RRd star) pulsating in two radial modes. In
38 stars we detect secondary periodicities very close to the primary pulsation
frequency. This type of multiperiodic variables constitute ~23% of RRab and ~5%
of RRc population of the Bulge. With the observed period ratios of 0.95-1.02
the secondary periods must correspond to nonradial modes of oscillation. Their
beating with the primary (radial) pulsation leads to a long-term amplitude and
phase modulation, known as the Blazhko effect. The Blazhko RRab variables occur
more frequently in the Galactic Bulge than in the LMC. The opposite tendency is
seen in case of the RRd stars. The differences of incidence rates are most
likely caused by different metallicity of the two populations. We discuss
pulsation properties of the OGLE-1 Blazhko stars and compare them with
predictions of theoretical models. We argue, that the oblique magnetic pulsator
model of Shibahashi (2000) cannot account for the observations and should be
ruled out.Comment: 11 pages, 8 figures, accepted for publication in Astronomy &
Astrophysic
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