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Multistate current-induced magnetization switching in Au/Fe/MgO(001) epitaxial heterostructures
Magnetization switching using in-plane charge current recently has been widely investigated in heavy metal/ferromagnet bilayers with the switching mechanism usually attributed to the action of the spin-orbit coupling. Here we study in-plane current induced magnetization switching in model epitaxial bilayers that consist of Au(001) and Fe(001) grown on MgO(001). We use the planar Hall effect combined with magnetooptical Kerr effect (MOKE) microscopy to investigate magnetic properties of the bilayers and current-induced switching. We show that a current density beyond 1.4×107 A/cm can be employed for reproducible electrical switching of the magnetization between multiple stable states that correspond to different arrangements of magnetic domains with magnetization direction along one of the in-plane easy magnetization axes of the Fe(001) film. Lower current densities result in stable intermediate transversal resistances which are interpreted based on MOKE-microscopy investigations as resulting from the current-induced magnetic domain structure that is formed in the area of the Hall cross. We find that the physical mechanism of the current-induced magnetization switching of the Au/Fe/MgO(001) system at room temperature can be fully explained by the Oersted field, which is generated by the charge current flowing mostly through the Au layer
Testing gravity to second post-Newtonian order: a field-theory approach
A new, field-theory-based framework for discussing and interpreting tests of
gravity, notably at the second post-Newtonian (2PN) level, is introduced.
Contrary to previous frameworks which attempted at parametrizing any
conceivable deviation from general relativity, we focus on the best motivated
class of models, in which gravity is mediated by a tensor field together with
one or several scalar fields. The 2PN approximation of these
"tensor-multi-scalar" theories is obtained thanks to a diagrammatic expansion
which allows us to compute the Lagrangian describing the motion of N bodies. In
contrast with previous studies which had to introduce many phenomenological
parameters, we find that the 2PN deviations from general relativity can be
fully described by only two new 2PN parameters, epsilon and zeta, beyond the
usual (Eddington) 1PN parameters beta and gamma. It follows from the basic
tenets of field theory, notably the absence of negative-energy excitations,
that (beta-1), epsilon and zeta (as well as any new parameter entering higher
post-Newtonian orders) must tend to zero with (gamma-1). It is also found that
epsilon and zeta do not enter the 2PN equations of motion of light. Therefore,
light-deflection or time-delay experiments cannot probe any theoretically
motivated 2PN deviation from general relativity, but they can give a clean
access to (gamma-1), which is of greatest significance as it measures the basic
coupling strength of matter to the scalar fields. Because of the importance of
self-gravity effects in neutron stars, binary-pulsar experiments are found to
constitute a unique testing ground for the 2PN structure of gravity. A
simplified analysis of four binary pulsars already leads to significant
constraints: |epsilon| < 7x10^-2, |zeta| < 6x10^-3.Comment: 63 pages, 11 figures.ps.tar.gz.uu, REVTeX 3.
Towards mirror symmetry \`a la SYZ for generalized Calabi-Yau manifolds
Fibrations of flux backgrounds by supersymmetric cycles are investigated. For
an internal six-manifold M with static SU(2) structure and mirror \hat{M}, it
is argued that the product M x \hat{M} is doubly fibered by supersymmetric
three-tori, with both sets of fibers transverse to M and \hat{M}. The mirror
map is then realized by T-dualizing the fibers. Mirror-symmetric properties of
the fluxes, both geometric and non-geometric, are shown to agree with previous
conjectures based on the requirement of mirror symmetry for Killing
prepotentials. The fibers are conjectured to be destabilized by fluxes on
generic SU(3)xSU(3) backgrounds, though they may survive at type-jumping
points. T-dualizing the surviving fibers ensures the exchange of pure spinors
under mirror symmetry.Comment: 30 pages, 3 figures, LaTeX; v2: references adde
Search for dark photons as candidates for Dark Matter with FUNK
An additional U(1) symmetry predicted in theories beyond the Standard Model of particle physics can give rise to hidden (dark) photons. Depending on the mass and density of these hidden photons, they could account for a large fraction of the Dark Matter observed in the Universe. When passing through an interface of materials with different dielectric properties, hidden photons are expected to produce a tiny flux of photons. The wavelength of these photons is directly related to the mass of the hidden photons. In this contribution we report on measurements covering the visible and near-UV spectrum, corresponding to a dark photon mass in the eV range. The data were taken with the FUNK experiment using a spherical mirror of ~14m2 total area built up of 36 aluminum segments
Role of baryonic resonances in the dilepton emission in nucleon-nucleon collisions
Within an effective Lagrangian model, we present calculations for cross
sections of the dilepton production in proton-proton and proton-neutron
collisions at laboratory kinetic energies in 1-5 GeV range. Production
amplitudes include contributions from the nucleon-nucleon bremsstrahlung as
well as from the mechanism of excitation, propagation, and radiative decay of
Delta(1232) and N*(1520) intermediate baryonic resonances. It is found that the
delta isobar terms dominate the cross sections in the entire considered beam
energy range. Our calculations are able to explain the data of the DLS
collaboration on the dilepton production in proton-proton collisions for beam
energies below 1.3 GeV. However, for incident energies higher than this the
inclusion of contributions from other dilepton sources like Dalitz decay of pi0
and eta mesons, and direct decay of rho and omega mesons is necessary to
describe the data.Comment: 22 pages, 7 figures, more details of the calculations added, version
to appear in Phys. Rev
Time-symmetric initial data for binary black holes in numerical relativity
We look for physically realistic initial data in numerical relativity which
are in agreement with post-Newtonian approximations. We propose a particular
solution of the time-symmetric constraint equation, appropriate to two
momentarily static black holes, in the form of a conformal decomposition of the
spatial metric. This solution is isometric to the post-Newtonian metric up to
the 2PN order. It represents a non-linear deformation of the solution of Brill
and Lindquist, i.e. an asymptotically flat region is connected to two
asymptotically flat (in a certain weak sense) sheets, that are the images of
the two singularities through appropriate inversion transformations. The total
ADM mass M as well as the individual masses m_1 and m_2 (when they exist) are
computed by surface integrals performed at infinity. Using second order
perturbation theory on the Brill-Lindquist background, we prove that the
binary's interacting mass-energy M-m_1-m_2 is well-defined at the 2PN order and
in agreement with the known post-Newtonian result.Comment: 27 pages, to appear in Phys. Rev.
Equation of motion for relativistic compact binaries with the strong field point particle limit: Third post-Newtonian order
An equation of motion for relativistic compact binaries is derived through
the third post-Newtonian (3 PN) approximation of general relativity. The strong
field point particle limit and multipole expansion of the stars are used to
solve iteratively the harmonically relaxed Einstein equations. We take into
account the Lorentz contraction on the multipole moments defined in our
previous works. We then derive a 3 PN acceleration of the binary orbital motion
of the two spherical compact stars based on a surface integral approach which
is a direct consequence of local energy momentum conservation. Our resulting
equation of motion admits a conserved energy (neglecting the 2.5 PN radiation
reaction effect), is Lorentz invariant and is unambiguous: there exist no
undetermined parameter reported in the previous works. We shall show that our 3
PN equation of motion agrees physically with the Blanchet and Faye 3 PN
equation of motion if , where is the parameter
which is undetermined within their framework. This value of is
consistent with the result of Damour, Jaranowski, and Sch\"afer who first
completed a 3 PN iteration of the ADM Hamiltonian in the ADMTT gauge using the
dimensional regularization.Comment: 52 pages, no figure, Appendices B and D added. Phys. Rev. D in pres
Post-Newtonian Gravitational Radiation
1 Introduction 2 Multipole Decomposition 3 Source Multipole Moments 4
Post-Minkowskian Approximation 5 Radiative Multipole Moments 6 Post-Newtonian
Approximation 7 Point-Particles 8 ConclusionComment: 46 pages, in Einstein's Field Equations and Their Physical
Implications, B. Schmidt (Ed.), Lecture Notes in Physics, Springe
Dilepton Production at SPS Energies
We present a nonperturbative dynamical study of production in
proton- nucleus and nucleus-nucleus collisions at SPS energies on the basis of
a covariant transport approach. For p + A reactions the dilepton yield for
invariant masses m 1.2 GeV is found to be dominated by the decays of
the and mesons in line with the findings of the
CERES collaboration. For S + Au collisions at 200 GeV/A the dilepton yield is,
however, dominated by annihilation due to the high pion densities
achieved. Whereas for `free' meson masses and form factors the experimental
cross section is slightly underestimated for 0.3 GeV m 0.45 GeV,
different medium modifications of the -meson appear compatible with
current CERES data.Comment: 1 compressed uuencoded postscript file with 6 figures included, 18
page
Meson Cloud of the Nucleon in Polarized Semi-Inclusive Deep-Inelastic Scattering
We investigate the possibility of identifying an explicit pionic component of
the nucleon through measurements of polarized baryon fragments
produced in deep-inelastic leptoproduction off polarized protons, which may
help to identify the physical mechanism responsible for the breaking of the
Gottfried sum rule. The pion-exchange model predicts highly correlated
polarizations of the and target proton, in marked contrast with
the competing diquark fragmentation process. Measurement of asymmetries in
polarized production may also reveal the presence of a kaon cloud in
the nucleon.Comment: 23 pages REVTeX, 7 uuencoded figures, accepted for publication in
Zeit. Phys.
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