348 research outputs found
Theory of Spin-Transfer Torque in the Current-in-Plane Geometries
Two alternative current-induced switching geometries, in which the current
flows parallel to the magnet/nonmagnet interface, are investigated
theoretically using the nonequilibrium Keldysh theory. In the first geometry,
the current is perpendicular to the polarizing magnet/nonmagnet interface but
parallel to the nonmagnet/switching magnet interface (CPIP). In the second
geometry, the current is parallel to both the polarizing magnet/nonmagnet and
nonmagnet/switching magnet interfaces (CIP). Calculations for a single-orbital
tight binding model indicate that the spin current flowing parallel to the
switching magnet/nonmagnet interface can be absorbed by a lateral switching
magnet as efficiently as in the traditional current-perpendicular-to-plane
(CPP) geometry. The results of the model calculations are shown to be valid
also for experimentally relevant Co/Cu CPIP system described by fully realistic
tight binding bands fitted to an ab initio band structure. It is shown that
almost complete absorption of the incident spin current by a lateral switching
magnet occurs when the lateral dimensions of the switching magnet are of the
order of 50-100 interatomic distances, i.e., about 20nm and its height as small
as a few atomic planes. It is also demonstratedthat strong spin current
absorption in the CPIP/CIP geometry is not spoilt by the presence of a rough
interface between the switching magnet and nonmagnetic spacer. Polarization
achieved using a lateral magnet in the CIP geometry is found to be about 25% of
that in the traditional CPP geometry. The present CPIP calculations of the spin
transfer torque are also relevant to the so called pure-spin-current-induced
magnetization switching that had been recently observed.Comment: 9 pages 8 figure
Spin-Transfer Torque in Helical Spin-Density Waves
The current driven magnetisation dynamics of a helical spin-density wave is
investigated. Expressions for calculating the spin-transfer torque of real
systems from first principles density functional theory are presented. These
expressions are used for calculating the spin-transfer torque for the spin
spirals of Er and fcc Fe at two different lattice volumes. It is shown that the
calculated torque induces a rigid rotation of the order parameter with respect
to the spin spiral axis. The torque is found to depend on the wave vector of
the spin spiral and the spin-polarisation of the Fermi surface states. The
resulting dynamics of the spin spiral is also discussed.Comment: 6 pages 2 figure
The quantum-mechanical basis of an extended Landau-Lifshitz-Gilbert equation for a current-carrying ferromagnetic wire
An extended Landau-Lifshitz-Gilbert (LLG) equation is introduced to describe
the dynamics of inhomogeneous magnetization in a current-carrying wire. The
coefficients of all the terms in this equation are calculated
quantum-mechanically for a simple model which includes impurity scattering.
This is done by comparing the energies and lifetimes of a spin wave calculated
from the LLG equation and from the explicit model. Two terms are of particular
importance since they describe non-adiabatic spin-transfer torque and damping
processes which do not rely on spin-orbit coupling. It is shown that these
terms may have a significant influence on the velocity of a current-driven
domain wall and they become dominant in the case of a narrow wall.Comment: 19 pages, 1 figur
Coherent Pair Production by Photons in the 20-170 GeV Energy Range Incident on Crystals and Birefringence
The cross section for coherent pair production by linearly polarised photons
in the 20-170 GeV energy range was measured for photon aligned incidence on
ultra-high quality diamond and germanium crystals. The theoretical description
of coherent bremsstrahlung and coherent pair production phenomena is an area of
active theoretical debate and development. However, under our experimental
conditions, the theory predicted the combined cross section and polarisation
experimental observables very well indeed. In macroscopic terms, our experiment
measured a birefringence effect in pair production in a crystal. This study of
this effect also constituted a measurement of the energy dependent linear
polarisation of photons produced by coherent bremsstrahlung in aligned
crystals. New technologies for manipulating high energy photon beams can be
realised based on an improved understanding of QED phenomena at these energies.
In particular, this experiment demonstrates an efficient new polarimetry
technique. The pair production measurements were done using two independent
methods simultaneously. The more complex method using a magnet spectrometer
showed that the simpler method using a multiplicity detector was also viable.Comment: 10 pages, 13 figures, 1 table, REVTeX4 two column, Version for
publicatio
Linear to Circular Polarisation Conversion using Birefringent Properties of Aligned Crystals for Multi-GeV Photons
We present the first experimental results on the use of a thick aligned Si
crystal acting as a quarter wave plate to induce a degree of circular
polarisation in a high energy linearly polarised photon beam. The linearly
polarised photon beam is produced from coherent bremsstrahlung radiation by 178
GeV unpolarised electrons incident on an aligned Si crystal, acting as a
radiator. The linear polarisation of the photon beam is characterised by
measuring the asymmetry in electron-positron pair production in a Ge crystal,
for different crystal orientations. The Ge crystal therefore acts as an
analyser. The birefringence phenomenon, which converts the linear polarisation
to circular polarisation, is observed by letting the linearly polarised photons
beam pass through a thick Si quarter wave plate crystal, and then measuring the
asymmetry in electron-positron pair production again for a selection of
relative angles between the crystallographic planes of the radiator, analyser
and quarter wave plate. The systematics of the difference between the measured
asymmetries with and without the quarter wave plate are predicted by theory to
reveal an evolution in the Stokes parameters from which the appearance of a
circularly polarised component in the photon beam can be demonstrated. The
measured magnitude of the circularly polarised component was consistent with
the theoretical predictions, and therefore is in indication of the existence of
the birefringence effect.Comment: 12 pages, 12 figures, 1 table, REVTeX4 two column, Version for
publicatio
Results on the Coherent Interaction of High Energy Electrons and Photons in Oriented Single Crystals
The CERN-NA-59 experiment examined a wide range of electromagnetic processes
for multi-GeV electrons and photons interacting with oriented single crystals.
The various types of crystals and their orientations were used for producing
photon beams and for converting and measuring their polarisation.
The radiation emitted by 178 GeV unpolarised electrons incident on a 1.5 cm
thick Si crystal oriented in the Coherent Bremsstrahlung (CB) and the
String-of-Strings (SOS) modes was used to obtain multi-GeV linearly polarised
photon beams.
A new crystal polarimetry technique was established for measuring the linear
polarisation of the photon beam. The polarimeter is based on the dependence of
the Coherent Pair Production (CPP) cross section in oriented single crystals on
the direction of the photon polarisation with respect to the crystal plane.
Both a 1 mm thick single crystal of Germanium and a 4 mm thick multi-tile set
of synthetic Diamond crystals were used as analyzers of the linear
polarisation.
A birefringence phenomenon, the conversion of the linear polarisation of the
photon beam into circular polarisation, was observed. This was achieved by
letting the linearly polarised photon beam pass through a 10 cm thick Silicon
single crystal that acted as a "quarter wave plate" (QWP) as suggested by N.
Cabibbo et al.Comment: Presented at International workshop "Relativistic Channeling and
Related Coherent Phenomena", Frascati (Rome) 23-26 March 200
A gene signature for post-infectious chronic fatigue syndrome
Background: At present, there are no clinically reliable disease markers for chronic fatigue syndrome. DNA chip microarray technology provides a method for examining the differential expression of mRNA from a large number of genes. Our hypothesis was that a gene expression signature, generated by microarray assays, could help identify genes which are dysregulated in patients with post-infectious CFS and so help identify biomarkers for the condition. Methods: Human genome-wide Affymetrix GeneChip arrays (39,000 transcripts derived from 33,000 gene sequences) were used to compare the levels of gene expression in the peripheral blood mononuclear cells of male patients with post-infectious chronic fatigue (n = 8) and male healthy control subjects (n = 7). Results: Patients and healthy subjects differed significantly in the level of expression of 366 genes. Analysis of the differentially expressed genes indicated functional implications in immune modulation, oxidative stress and apoptosis. Prototype biomarkers were identified on the basis of differential levels of gene expression and possible biological significance Conclusion: Differential expression of key genes identified in this study offer an insight into the possible mechanism of chronic fatigue following infection. The representative biomarkers identified in this research appear promising as potential biomarkers for diagnosis and treatment
Coherent bremsstrahlung, boherent pair production, birefringence and polarimetry in the 20-170 GeV energy range using aligned crystals
The processes of coherent bremsstrahlung (CB) and coherent pair production
(CPP) based on aligned crystal targets have been studied in the energy range
20-170 GeV. The experimental arrangement allowed for measurements of single
photon properties of these phenomena including their polarization dependences.
This is significant as the theoretical description of CB and CPP is an area of
active theoretical debate and development. With the theoretical approach used
in this paper both the measured cross sections and polarization observables are
predicted very well. This indicates a proper understanding of CB and CPP up to
energies of 170 GeV. Birefringence in CPP on aligned crystals is applied to
determine the polarization parameters in our measurements. New technologies for
high energy photon beam optics including phase plates and polarimeters for
linear and circular polarization are demonstrated in this experiment. Coherent
bremsstrahlung for the strings-on-strings (SOS) orientation yields a larger
enhancement for hard photons than CB for the channeling orientations of the
crystal. Our measurements and our calculations indicate low photon
polarizations for the high energy SOS photons.Comment: 23 pages, 27 figures, 2 tables, REVTeX4 two column
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