10,481 research outputs found
Current Induced Excitations in Cu/Co/Cu Single Ferromagnetic Layer Nanopillars
Current-induced magnetic excitations in Cu/Co/Cu single layer nanopillars
(~50 nm in diameter) have been studied experimentally as a function of Co layer
thickness at low temperatures for large applied fields perpendicular to the
layers. For asymmetric junctions current induced excitations are observed at
high current densities for only one polarity of the current and are absent at
the same current densities in symmetric junctions. These observations confirm
recent predictions of spin-transfer torque induced spin wave excitations in
single layer junctions with a strong asymmetry in the spin accumulation in the
leads.Comment: 4 pages, 3 figures, submitted to Phys. Rev. Let
Current-Induced Effective Magnetic Fields in Co/Cu/Co Nanopillars
We present a method to measure the effective field contribution to
spin-transfer-induced interactions between the magnetic layers in a trilayer
nanostructure, which enables spin-current effects to be distinguished from the
usual charge-current-induced magnetic fields. This technique is demonstrated on
submicron Co/Cu/Co nanopillars. The hysteresis loop of one of the magnetic
layers in the trilayer is measured as a function of current while the direction
of magnetization of the other layer is kept fixed, first in one direction and
then in the opposite direction. These measurements show a current-dependent
shift of the hysteresis loop which, based on the symmetry of the magnetic
response, we associate with spin-transfer. The observed loop-shift with applied
current at room temperature is reduced in measurements at 4.2 K. We interprete
these results both in terms of a spin-current dependent effective activation
barrier for magnetization reversal and a spin-current dependent effective
magnetic field. From data at 4.2 K we estimate the magnitude of the
spin-transfer induced effective field to be Oe
cm/A, about a factor of 5 less than the spin-transfer torque.Comment: 6 pages, 4 figure
The age of the Milky Way inner stellar spheroid from RR Lyrae population synthesis
The central kiloparsecs of the Milky Way are known to host an old, spheroidal
stellar population, whose spatial and kinematical properties set it apart from
the boxy/peanut structure that constitutes most of the central stellar mass.
The nature of this spheroidal population, whether a small classical bulge, the
innermost stellar halo or a population of disk stars with large initial
velocity dispersion, remains unclear. This structure is also a promising
candidate to host some of the oldest stars in the Galaxy. Here we address the
topic of the inner stellar spheroid age, using spectroscopic and photometric
metallicities for a sample of 935 RR Lyrae stars that are constituents of this
component. By means of stellar population synthesis, we derive an
age-metallicity relation for RR Lyrae populations. We infer, for the RR Lyrae
stars in the bulge spheroid, an extremely ancient age of Gyr
and conclude they were among the first stars to form in what is now the Milky
Way galaxy. Our age estimate for the central spheroid shows remarkable
agreement with the age profile that has been inferred for the Milky Way stellar
halo, suggesting a connection between the two structures. However, we find mild
evidence for a transition in the halo properties at ~kpc. We
also investigate formation scenarios for metal-rich RR Lyrae stars, such as
binarity and helium variations, and whether they can provide alternative
explanations for the properties of our sample. We conclude that, within our
framework, the only viable alternative is to have younger, slightly
helium-rich, RR Lyrae stars, a hypothesis that would open intriguing questions
for the formation of the inner stellar spheroid.Comment: Accepted for publication in Astronomy & Astrophysics. 17 pages, 15
figure
Correlated quantum percolation in the lowest Landau level
Our understanding of localization in the integer quantum Hall effect is
informed by a combination of semi-classical models and percolation theory.
Motivated by the effect of correlations on classical percolation we study
numerically electron localization in the lowest Landau level in the presence of
a power-law correlated disorder potential. Careful comparisons between
classical and quantum dynamics suggest that the extended Harris criterion is
applicable in the quantum case. This leads to a prediction of new localization
quantum critical points in integer quantum Hall systems with power-law
correlated disorder potentials. We demonstrate the stability of these critical
points to addition of competing short-range disorder potentials, and discuss
possible experimental realizations.Comment: 15 pages, 12 figure
Baryon Resonance Analysis from SAID
We discuss the analysis of data from piN elastic scattering and single pion
photo- and electroproduction. The main focus is a study of low-lying
non-strange baryon resonances. Here we concentrate on some difficulties
associated with resonance identification, in particular the Roper and higher
P11 states.Comment: 4 pages, 6 figures; Nstar2009 Conf Proceedings; small revisio
Screened and Unscreened Phases in Sedimenting Suspensions
A coarse-grained stochastic hydrodynamical description of velocity and
concentration fluctuations in steadily sedimenting suspensions is constructed,
and analyzed using self-consistent and renormalization group methods. We find
that there exists a dynamical, non-equilibrium phase transition from an
"unscreened" phase in which we recover the Caflisch-Luke (R.E. Caflisch and
J.H.C. Luke, Phys. Fluids 28, 759 (1985)) divergence of the velocity variance
to a "screened" phase where the velocity fluctuations have a finite correlation
length growing as where is the particle volume fraction,
in agreement with Segr\`e et. al. (Phys. Rev. Lett. 79, 2574 (1997)) and the
velocity variance is independent of system size. Detailed predictions are made
for the correlation function in both phases and at the transition.Comment: 4 pages, revtex 1 figur
Current-Induced Magnetization Reversal in High Magnetic Fields in Co/Cu/Co Nanopillars
Current-induced magnetization dynamics in Co/Cu/Co trilayer nanopillars
(~100nm in diameter) has been studied experimentally for large applied fields
perpendicular to the layers. An abrupt and hysteretic increase in dynamic
resistance is observed at high current densities for one polarity of the
current, comparable to the giant magnetoresistance effect observed at low
fields. A micromagnetic model, that includes a spin-transfer torque, suggests
that the current induces a complete reversal of the thin Co layer to alignment
antiparallel to the applied field-that is, to a state of maximum magnetic
energy.Comment: 11 pages, 3 figures, (submitted to Phys. Rev. Lett.), added missing
figure caption of fig. 3, updated to published versio
Configuration Mixing within the Energy Density Functional Formalism: Removing Spurious Contributions from Non-Diagonal Energy Kernels
Multi-reference calculations along the lines of the Generator Coordinate
Method or the restoration of broken symmetries within the nuclear Energy
Density Functional (EDF) framework are becoming a standard tool in nuclear
structure physics. These calculations rely on the extension of a
single-reference energy functional, of the Gogny or the Skyrme types, to
non-diagonal energy kernels. There is no rigorous constructive framework for
this extension so far. The commonly accepted way proceeds by formal analogy
with the expressions obtained when applying the generalized Wick theorem to the
non-diagonal matrix element of a Hamilton operator between two product states.
It is pointed out that this procedure is ill-defined when extended to EDF
calculations as the generalized Wick theorem is taken outside of its range of
applicability. In particular, such a procedure is responsible for the
appearance of spurious divergences and steps in multi-reference EDF energies,
as was recently observed in calculations restoring particle number or angular
momentum. In the present work, we give a formal analysis of the origin of this
problem for calculations with and without pairing, i.e. constructing the
density matrices from either Slater determinants or quasi-particle vacua. We
propose a correction to energy kernels that removes the divergences and steps,
and which is applicable to calculations based on any symmetry restoration or
generator coordinate. The method is formally illustrated for particle number
restoration and is specified to configuration mixing calculations based on
Slater determinants.Comment: 27 pages, 1 figure, accepted for publication in PR
Thermodynamic and Tunneling Density of States of the Integer Quantum Hall Critical State
We examine the long wave length limit of the self-consistent Hartree-Fock
approximation irreducible static density-density response function by
evaluating the charge induced by an external charge. Our results are consistent
with the compressibility sum rule and inconsistent with earlier work that did
not account for consistency between the exchange-local-field and the disorder
potential. We conclude that the thermodynamic density of states is finite, in
spite of the vanishing tunneling density of states at the critical energy of
the integer quantum Hall transition.Comment: 5 pages, 4 figures, minor revisions, published versio
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