1,841 research outputs found
Relativistic wave functions and energies for nonzero angular momentum states in light-front dynamics
Light-front dynamics (LFD) is a powerful approach to the theory of
relativistic composite systems (hadrons in the quark models and relativistic
nucleons in nuclei). Its explicitly covariant version has been recently applied
with success to describe the new CEBAF/TJNAF data on the deuteron
electromagnetic form factors. The solutions used in were however not obtained
from solving exactly the LFD equations but by means of a perturbative
calculation with respect to the non relativistic wave function. Since, a
consequent effort has been made to obtain exact solutions of LFD equations. The
first results concerning J=0 states in a scalar model have been published in
nucl-th/9912050. The construction of states in LFD is complicated by
the two following facts. First, the generators of the spatial rotations contain
interaction and are thus difficult to handle. Second, one is always forced to
work in a truncated Fock space, and consequently, the Poincar\'e group
commutation relations between the generators -- ensuring the correct properties
of the state vector under rotation -- are in practice destroyed. In the
standard approach, with the light-front plane defined as , this
violation of rotational invariance manifests by the fact that the energy
depends on the angular momentum projection on -axis.
We present here a method to construct states in the explicitly
covariant formulation of LFD and show how it leads to a restoration of
rotational invariance.Comment: To appear in Nucl.Phys.B, 3 pages, 2 figures, .tar.gz fil
Two-Body Bound States in Light-Front Dynamics
We present the main features of the explicitly covariant Light-Front Dynamics
formalism and a summary of our recent works on this topic. They concern the
bound states of two scalar particles in the Wick-Cutkosky model and of two
fermions interacting via the usual OBEP ladder kernels.Comment: Invited contribution to the XIVth International School on Nuclear
Physics, Varna, Bulgaria, Sept 25-30. 20 pages, 23 figure
Stability of bound states in the light-front Yukawa model
We show that in the system of two fermions interacting by scalar exchange,
the solutions for J= bound states are stable without any cutoff
regularization for coupling constant below some critical value.Comment: 5 pages, 2 figures, submitted to publicatio
Electromagnetic form factor via Bethe-Salpeter amplitude in Minkowski space
For a relativistic system of two scalar particles, we find the Bethe-Salpeter
amplitude in Minkowski space and use it to compute the electromagnetic form
factor. The comparison with Euclidean space calculation shows that the Wick
rotation in the form factor integral induces errors which increase with the
momentum transfer Q^2. At JLab domain (Q^2=10 GeV^2/c^2), they are about 30%.
Static approximation results in an additional and more significant error. On
the contrary, the form factor calculated in light-front dynamics is almost
indistinguishable from the Minkowski space one.Comment: 8 pages, 7 figures, to be published in Eur. Phys. J. A; Reference
[15] is adde
Current-induced Pinwheel Oscillations in Perpendicular Magnetic Anisotropy Spin Valve Nanopillars
Nanopillar spin valve devices are typically comprised of two ferromagnetic
layers: a reference layer and a free layer whose magnetic orientation can be
changed by both an external magnetic field and through the introduction of
spin-polarized electric current. Here we report the continuous repeated
switching behavior of both the reference and free layers of a perpendicular
spin valve made of Co/Pd and Co/Ni multilayers that arises for sufficiently
large DC currents. This periodic switching of the two layers produces an
oscillating signal in the MHz regime but is only observed for one sign of the
applied current. The observed behavior agrees well with micromagnetic
simulations
Temperature dependent nucleation and propagation of domain walls in a sub-100 nm perpendicularly magnetized Co/Ni multilayer
We present a study of the temperature dependence of the switching fields in
Co/Ni-based perpendicularly magnetized spin-valves. While magnetization
reversal of all-perpendicular Co/Ni spin valves at ambient temperatures is
typically marked by a single sharp step change in resistance, low temperature
measurements can reveal a series of resistance steps, consistent with
non-uniform magnetization configurations. We propose a model that consists of
domain nucleation, propagation and annihilation to explain the temperature
dependence of the switching fields. Interestingly, low temperature (<30 K) step
changes in resistance that we associate with domain nucleation, have a bimodal
switching field and resistance step distribution, attributable to two competing
nucleation pathways.Comment: 5 pages, 4 figure
Temperature dependence of the switching field distributions in all-perpendicular spin-valve nanopillars
We present temperature dependent switching measurements of the Co/Ni
multilayered free element of 75 nm diameter spin-valve nanopillars. Angular
dependent hysteresis measurements as well as switching field measurements taken
at low temperature are in agreement with a model of thermal activation over a
perpendicular anisotropy barrier. However, the statistics of switching (mean
switching field and switching variance) from 20 K up to 400 K are in
disagreement with a N\'{e}el-Brown model that assumes a temperature independent
barrier height and anisotropy field. We introduce a modified N\'{e}el-Brown
model thats fit the experimental data in which we take a dependence
to the barrier height and the anisotropy field due to the temperature dependent
magnetization and anisotropy energy.Comment: 5 pages, 4 figure
Time-Resolved Magnetic Relaxation of a Nanomagnet on Subnanosecond Time Scales
We present a two-current-pulse temporal correlation experiment to study the
intrinsic subnanosecond nonequilibrium magnetic dynamics of a nanomagnet during
and following a pulse excitation. This method is applied to a model
spin-transfer system, a spin valve nanopillar with perpendicular magnetic
anisotropy. Two-pulses separated by a short delay (< 500 ps) are shown to lead
to the same switching probability as a single pulse with a duration that
depends on the delay. This demonstrates a remarkable symmetry between magnetic
excitation and relaxation and provides a direct measurement of the magnetic
relaxation time. The results are consistent with a simple finite temperature
Fokker-Planck macrospin model of the dynamics, suggesting more coherent
magnetization dynamics in this short time nonequilibrium limit than near
equilibrium
Bimodal switching field distributions in all-perpendicular spin-valve nanopillars
Switching field measurements of the free layer element of 75 nm diameter
spin-valve nanopillars reveal a bimodal distribution of switching fields at low
temperatures (below 100 K). This result is inconsistent with a model of thermal
activation over a single perpendicular anisotropy barrier. The correlation
between antiparallel to parallel and parallel to antiparallel switching fields
increases to nearly 50% at low temperatures. This reflects random fluctuation
of the shift of the free layer hysteresis loop between two different
magnitudes, which may originate from changes in the dipole field from the
polarizing layer. The magnitude of the loop shift changes by 25% and is
correlated to transitions of the spin-valve into an antiparallel configuration.Comment: 3 pages, 4 figures. Submitted to JAP for 58th MMM Proceeding
Distortion of the Stoner-Wohlfarth astroid by a spin-polarized current
The Stoner-Wohlfarth astroid is a fundamental object in magnetism. It
separates regions of the magnetic field space with two stable magnetization
equilibria from those with only one stable equilibrium and it characterizes the
magnetization reversal of nano-magnets induced by applied magnetic fields. On
the other hand, it was recently demonstrated that transfer of spin angular
momentum from a spin-polarized current provides an alternative way of switching
the magnetization. Here, we examine the astroid of a nano-magnet with uniaxial
magnetic anisotropy under the combined influence of applied fields and
spin-transfer torques. We find that spin-transfer is most efficient at
modifying the astroid when the external field is applied along the easy-axis of
magnetization. On departing from this situation, a threshold current appears
below which spin-transfer becomes ineffective yielding a current-induced dip in
the astroid along the easy-axis direction. An extension of the Stoner-Wohlfarth
model is outlined which accounts for this phenomenon.Comment: 8 pages, 6 figure
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