publicationDate stringlengths 1 2.79k | title stringlengths 1 36.5k ⌀ | abstract stringlengths 1 37.3k ⌀ | id stringlengths 9 47 |
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1998-09-04 | Cluster-Cluster Strong Lensing: Expectations and Detection Methods | We calculate the all-sky number of galaxy clusters that are expected to be
gravitationally lensed by foreground massive clusters. We describe the redshift
and number distributions of clusters using a Press-Schechter analysis, and
model the foreground lensing clusters as singular isothermal spheres. If
Omega_m=0.3 and Omega_Lambda=0.7, we expect ~ 30 cluster-cluster strong lensing
events that involve foreground X-ray luminous clusters with total mass greater
than 7.5 x 10^14 h^-1 M_sun, or X-ray luminosity L_x (2-10 keV) 8 x 10^44 h^-2
ergs s^-1, and background clusters with total mass greater than 10^14 h^-1
M_sun. The number expected in an open universe with Omega_m = 0.3 is less than
\~ 4. Because of uncertainty in sigma_8, the root-mean-square density
fluctuations in spheres of radius 8 h^-1 Mpc, the exact number of such lensing
events is uncertain by a factor of about 5. We examine methods to detect
cluster-cluster lensing events based on optical, X-ray, and Sunyaev-Zel'dovich
effect observations. | 9809062v3 |
2000-04-14 | Source Reconstruction as an Inverse Problem | Inverse Problem techniques offer powerful tools which deal naturally with
marginal data and asymmetric or strongly smoothing kernels, in cases where
parameter-fitting methods may be used only with some caution. Although they are
typically subject to some bias, they can invert data without requiring one to
assume a particular model for the source. The Backus-Gilbert method in
particular concentrates on the tradeoff between resolution and stability, and
allows one to select an optimal compromise between them. We use these tools to
analyse the problem of reconstructing features of the source star in a
microlensing event, show that it should be possible to obtain useful
information about the star with reasonably obtainable data, and note that the
quality of the reconstruction is more sensitive to the number of data points
than to the quality of individual ones. | 0004200v1 |
2000-04-18 | Galaxy Cluster Baryon Fractions, Cluster Surveys and Cosmology | The properties of nearby galaxy clusters limit the range of cosmological
parameters consistent with our universe. We describe the limits which arise
from studies of the intracluster medium (ICM) mass fraction fICM and
consideration of the possible sources of systematic error:
Omega_M<0.44h_{50}^{-1/2} at 95% confidence. We emphasize that independent of
Type Ia supernovae (SNe Ia) observations, this cluster study, taken together
with published cosmic microwave background (CMB) anisotropy studies, indicates
a non-zero quintessence or dark energy component Omega_Q>0.
We then discuss future galaxy cluster surveys which will probe the abundance
of galaxy clusters to intermediate and high redshift. We investigate the
sensitivity of these surveys to the cosmological density parameter Omega_M and
the equation of state parameter w of any quintessence component. In particular,
we show that cluster survey constraints from a proposed large solid angle X-ray
survey are comparable in precision and complementary in nature to constraints
expected from future CMB anisotropy and SNe Ia studies. | 0004244v1 |
2000-05-11 | Measurement of [OIII] Emission in Lyman Break Galaxies | Measurements of [OIII] emission in Lyman Break galaxies (LBGs) at z>3 are
presented. Four galaxies were observed with narrow-band filters using the
Near-IR Camera on the Keck I 10-m telescope. A fifth galaxy was observed
spectroscopically during the commissioning of NIRSPEC, the new infrared
spectrometer on Keck II. The emission-line spectrum is used to place limits on
the metallicity. Comparing these new measurements with others available from
the literature, we find that strong oxygen emission in LBGs may suggest
sub-solar metallicity for these objects. The [OIII]5007 line is also used to
estimate the star formation rate (SFR) of the LBGs. The inferred SFRs are
higher than those estimated from the UV continuum, and may be evidence for dust
extinction. | 0005254v1 |
2001-03-02 | Clusters in the Precision Cosmology Era | Over the coming decade, the observational samples available for studies of
cluster abundance evolution will increase from tens to hundreds, or possibly to
thousands, of clusters. Here we assess the power of future surveys to determine
cosmological parameters. We quantify the statistical differences among
cosmologies, including the effects of the cosmic equation of state parameter w,
in mock cluster catalogs simulating a 12 sq. deg Sunyaev-Zeldovich Effect
survey and a deep 10^4 sq. deg X-ray survey. The constraints from clusters are
complementary to those from studies of high-redshift Supernovae (SNe), CMB
anisotropies, or counts of high-redshift galaxies. Our results indicate that a
statistical uncertainty of a few percent on both Omega_m and w can be reached
when cluster surveys are used in combination with any of these other datasets. | 0103049v1 |
2002-07-05 | New Tests of the Cluster Entropy Floor Hypothesis | Recent efforts to account for the observed X-ray luminosity - temperature
relation of galaxy clusters has led to suggestions that the ICM has an apparent
``entropy floor'' at or above the level of 300 keV cm^2. Here, we propose new
tests based on the thermal Sunyaev-Zeldovich effect and on the cluster gas mass
- temperature trend (from X-ray data) to probe the level of excess entropy in
the ICM. We show that these new tests lend further support to the case for a
high entropy floor in massive clusters. | 0207147v1 |
2003-06-18 | Kinematic Masses of Super Star Clusters in M82 from High-Resolution Near-Infrared Spectroscopy | Using high-resolution (R~22,000) near-infrared (1.51 -- 1.75 microns) spectra
from Keck Observatory, we measure the kinematic masses of two super star
clusters in M82. Cross-correlation of the spectra with template spectra of cool
evolved stars gives stellar velocity dispersions of sigma_r=15.9 +/- 0.8 km/s
for MGG-9 and sigma_r=11.4 +/- 0.8 km/s for MGG-11. The cluster spectra are
dominated by the light of red supergiants, and correlate most closely with
template supergiants of spectral types M0 and M4.5. We fit King models to the
observed profiles of the clusters in archival HST/NICMOS images to measure the
half-light radii. Applying the virial theorem, we determine masses of 1.5 +/-
0.3 x 10^6 M_sun for MGG-9 and 3.5 +/- 0.7 x 10^5 M_sun for MGG-11. Population
synthesis modelling suggests that MGG-9 is consistent with a standard initial
mass function, whereas MGG-11 appears to be deficient in low-mass stars
relative to a standard IMF. There is, however, evidence of mass segregation in
the clusters, in which case the virial mass estimates would represent lower
limits. | 0306373v1 |
2003-09-10 | The CMB Quadrupole in a Polarized Light | The low quadrupole of the cosmic microwave background (CMB), measured by COBE
and confirmed by WMAP, has generated much discussion recently. We point out
that the well-known correlation between temperature and polarization
anisotropies of the CMB further constrains the low multipole anisotropy data.
This correlation originates from the fact that the low-multipole polarization
signal is sourced by the CMB quadrupole as seen by free electrons during the
relatively recent cosmic history. Consequently, the large-angle temperature
anisotropy data make restrictive predictions for the large-angle polarization
anisotropy, which depend primarily on the optical depth for electron scattering
after cosmological recombination, tau. We show that if current cosmological
models for the generation of large angle anisotropy are correct and the
COBE/WMAP data are not significantly contaminated by non-CMB signals, then the
observed C_te amplitude on the largest scales is discrepant at the 99.8% level
with the observed C_tt for the concordance LCDM model with tau=0.10. Using
tau=0.17, the preferred WMAP model-independent value, the discrepancy is at the
level of 98.5%. | 0309281v2 |
2003-10-11 | Statistics of Giant Arcs in Galaxy Clusters | We study the expected properties and statistics of giant arcs produced by
galaxy clusters in a LambdaCDM universe and investigate how the characteristics
of CDM clusters determine the properties of the arcs they generate. Due to the
triaxiality and substructure of CDM halos, the giant arc cross section for
individual clusters varies by more than an order of magnitude as a function of
viewing angle. In addition, the shallow density cusps and triaxiality of CDM
clusters cause systematic alignments of giant arcs which should be testable
with larger samples from forthcoming lensing surveys. We compute the predicted
statistics of giant arcs for the LambdaCDM model and compare to results from
previous surveys. The predicted arc statistics are in excellent agreement with
the numbers of giant arcs observed around low redshift (0.2 < z < 0.6) clusters
from the EMSS sample, however there are hints of a possible excess of arcs
observed around high redshift z > 0.6 clusters. This excess, if real, appears
to be due to the presence of highly massive or concentrated clusters at high
redshifts. | 0310306v1 |
2004-01-23 | Gravitational Lensing of the Microwave Background by Galaxy Clusters | Galaxy clusters will distort the pattern of temperature anisotropies in the
microwave background via gravitational lensing. We create lensed microwave
background maps using clusters drawn from numerical cosmological simulations. A
distinctive dipole-like temperature fluctuation pattern is formed aligned with
the underlying microwave temperature gradient. For a massive cluster, the
characteristic angular size of the temperature distortion is a few arcminutes
and the characteristic amplitude a few micro-Kelvin. We demonstrate a simple
technique for estimating the lensing deflection induced by the cluster;
microwave background lensing measurements have the potential to determine the
mass distribution for some clusters with good accuracy on angular scales up to
a few arcminutes. Future high-resolution and high-sensitivity microwave
background maps will have the capability to detect lensing by clusters; we
discuss various systematic limitations on probing cluster masses using this
technique. | 0401519v2 |
2005-08-04 | Gravitino, Axino, Kaluza-Klein Graviton Warm and Mixed Dark Matter and Reionisation | Stable particle dark matter may well originate during the decay of long-lived
relic particles, as recently extensively examined in the cases of the axino,
gravitino, and higher-dimensional Kaluza-Klein (KK) graviton. It is shown that
in much of the viable parameter space such dark matter emerges naturally
warm/hot or mixed. In particular, decay produced gravitinos (KK-gravitons) may
only be considered cold for the mass of the decaying particle in the several
TeV range, unless the decaying particle and the dark matter particle are almost
degenerate. Such dark matter candidates are thus subject to a host of
cosmological constraints on warm and mixed dark matter, such as limits from a
proper reionisation of the Universe, the Lyman-alpha forest, and the abundance
of clusters of galaxies.. It is shown that constraints from an early
reionsation epoch, such as indicated by recent observations, may potentially
limit such warm/hot components to contribute only a very small fraction to the
dark matter. | 0508141v2 |
1999-08-10 | Magnetic relaxation in a classical spin chain as model for nanowires | With decreasing particle size, different mechanisms dominate the thermally
activated magnetization reversal in ferromagnetic particles. We investigate
some of these mechanisms for the case of elongated, single-domain nanoparticles
which we describe by a classical Heisenberg spin chain driven by an external
magnetic field. For sufficiently small system size the magnetic moments rotate
coherently. With increasing size a crossover to a reversal due to
soliton-antisoliton nucleation sets in. For even larger systems many of these
soliton-antisoliton pairs nucleate at the same time. These effects give rise to
a complex size dependence of the energy barriers and characteristic time scales
of the relaxation. We study these quantities using Monte Carlo simulations as
well as a direct integration of the Landau-Lifshitz-Gilbert equation of motion
with Langevin dynamics and we compare our results with asymptotic solutions for
the escape rate following from the Fokker-Planck equation. Also, we investigate
the crossover from coherent rotation to soliton-antisoliton nucleation and
multi-droplet nucleation, especially its dependence on the system size, the
external field and the anisotropy of the system. | 9908150v1 |
2000-07-17 | Fine-grid Simulations of Thermally Activated Switching in Nanoscale Magets | Numerical integration of the Landau-Lifshitz-Gilbert equation with thermal
fluctuations is used to study the dynamic response of single-domain nanomagnets
to rapid changes in the applied magnetic field. The simulation can resolve
magnetization patterns within nanomagnets and uses the Fast Multipole method to
calculate dipole-dipole interactions efficiently. The thermal fluctuations play
an essential part in the reversal process whenever the applied field is less
than the zero-temperature coercive field. In this situation pillar-shaped
nanomagnets are found to reverse through a local curling mode that involves the
formation and propagation of a domain wall. Tapering the ends of the pillars to
reduce pole-avoidance effects changes the energies involved but not the
fundamental process. The statistical distribution of switching times is well
described by the independent nucleation and subsequent growth of regions of
reversed magnetization at both ends of the pillar. | 0007279v1 |
2001-01-31 | Langevin Simulation of Thermally Activated Magnetization Reversal in Nanoscale Pillars | Numerical solutions of the Landau-Lifshitz-Gilbert micromagnetic model
incorporating thermal fluctuations and dipole-dipole interactions (calculated
by the Fast Multipole Method) are presented for systems composed of nanoscale
iron pillars of dimension 9 nm x 9 nm x 150 nm. Hysteresis loops generated
under sinusoidally varying fields are obtained, while the coercive field is
estimated to be 1979 $\pm$ 14 Oe using linear field sweeps at T=0 K. Thermal
effects are essential to the relaxation of magnetization trapped in a
metastable orientation, such as happens after a rapid reversal of an external
magnetic field less than the coercive value. The distribution of switching
times is compared to a simple analytic theory that describes reversal with
nucleation at the ends of the nanomagnets. Results are also presented for
arrays of nanomagnets oriented perpendicular to a flat substrate. Even at a
separation of 300 nm, where the field from neighboring pillars is only $\sim$ 1
Oe, the interactions have a significant effect on the switching of the magnets. | 0101477v2 |
2001-05-04 | On a common circle: natural scenes and Gestalt rules | To understand how the human visual system analyzes images, it is essential to
know the structure of the visual environment. In particular, natural images
display consistent statistical properties that distinguish them from random
luminance distributions. We have studied the geometric regularities of oriented
elements (edges or line segments) present in an ensemble of visual scenes,
asking how much information the presence of a segment in a particular location
of the visual scene carries about the presence of a second segment at different
relative positions and orientations. We observed strong long-range correlations
in the distribution of oriented segments that extend over the whole visual
field. We further show that a very simple geometric rule, cocircularity,
predicts the arrangement of segments in natural scenes, and that different
geometrical arrangements show relevant differences in their scaling properties.
Our results show similarities to geometric features of previous physiological
and psychophysical studies. We discuss the implications of these findings for
theories of early vision. | 0105097v1 |
2002-10-11 | Fluctuations and Dissipation of Coherent Magnetization | A quantum mechanical model is used to derive a generalized Landau-Lifshitz
equation for a magnetic moment, including fluctuations and dissipation. The
model reproduces the Gilbert-Brown form of the equation in the classical limit.
The magnetic moment is linearly coupled to a reservoir of bosonic degrees of
freedom. Use of generalized coherent states makes the semiclassical limit more
transparent within a path-integral formulation. A general
fluctuation-dissipation theorem is derived. The magnitude of the magnetic
moment also fluctuates beyond the Gaussian approximation. We discuss how the
approximate stochastic description of the thermal field follows from our
result. As an example, we go beyond the linear-response method and show how the
thermal fluctuations become anisotropy-dependent even in the uniaxial case. | 0210273v2 |
2002-11-18 | Field dependence of magnetization reversal by spin transfer | We analyse the effect of the applied field (Happl) on the current-driven
magnetization reversal in pillar-shaped Co/Cu/Co trilayers, where we observe
two different types of transition between the parallel (P) and antiparallel
(AP) magnetic configurations of the Co layers. If Happl is weaker than a rather
small threshold value, the transitions between P and AP are irreversible and
relatively sharp. For Happl exceding the threshold value, the same transitions
are progressive and reversible. We show that the criteria for the stability of
the P and AP states and the experimentally observed behavior can be precisely
accounted for by introducing the current-induced torque of the spin transfer
models in a Landau-Lifschitz-Gilbert equation. This approach also provides a
good description for the field dependence of the critical currents. | 0211371v1 |
2003-10-18 | NMR Investigation of the Organic Conductor lambda-(BETS)2FeCl4 | The two-dimensional organic conductor lambda-(BETS)2FeCl4 has an unusual
phase diagram as a function of temperature and magnetic field that includes a
paramagnetic metal (PM) phase, an antiferromagnetic insulating (AFI) phase, and
a field-induced superconducting phase [S. Uji, H. Kobayashi, L. Balicas, and
James S. Brooks, Adv. Mater. 14, 243 (2002), and cited references]. Here, we
report a preliminary investigation of the PM and AFI phases at 9.0 T over the
temperature range 2.0-180 K that uses proton NMR measurements of the spectrum,
the spin-lattice relaxation rate (1/T1), and the spin echo decay rate (1/T2).
The sample is asmall single crystal whose mass is approximately 3 micrograms
(approximately 2E16 protons). Its small size creates several challenges that
include detecting small signals and excluding parasitic proton signals that are
not from the sample [H. N. Bachman and I. F. Silvera, J. Mag. Res. 162, 417
(2003)]. These strategies and other techniques used to obtain viable signals
are described. | 0310433v1 |
2004-04-22 | Non-collinear magnetic structures: a possible cause for current induced switching | Current induced switching in Co/Cu/Co trilayers is described in terms of
ab-initio determined magnetic twisting energies and corresponding sheet
resistances. In viewing the twisting energy as an energy flux the
characteristic time thereof is evaluated by means of the
Landau-Lifshitz-Gilbert equation using ab-initio parameters. The obtained
switching times are in very good agreement with available experimental data. In
terms of the calculated currents, scalar quantities since a classical Ohm's law
is applied, critical currents needed to switch magnetic configurations from
parallel to antiparallel and vice versa can unambiguously be defined. It is
found that the magnetoresistance viewed as a function of the current is
essentially determined by the twisting energy as a function of the relative
angle between the orientations of the magnetization in the magnetic slabs,
which in turn can also explain in particular cases the fact that after having
switched off the current the system remains in the switched magnetic
configuration. For all ab-initio type calculations the fully relativistic
Screened Korringa-Kohn-Rostoker method and the corresponding Kubo-Greenwood
equation in the context of density functional theory are applied. | 0404534v1 |
2004-06-21 | Basic considerations for magnetization dynamics in the combined presence of spin-transfer torques and thermal fluctuations | This article reviews basic theoretical features of Gilbert magnetization
dynamics of a single domain magnetic film in the presence of Slonczewski
spin-transfer torques, with and without thermal fluctuations taken into
account. Rather than showing results of detailed numerical calculations, the
discussion here is restricted to basic analytical results and conclusions which
can mostly be derived from simply the form of the equations of motion, as well
as elementary considerations based on classical stability analysis and the
fluctuation-dissipation theorem. The presents work describes how interesting
features of spin-transfer may be viewed as arising from non-equilibrium
thermodynamics that are a direct consequence of the nonreciprocal nature of
spin-transfer torques. The present article discusses fairly general results for
spin-torque induced instability without thermal fluctuations, as well as the
case of thermally activated magnetization reversal in uniaxial devices in the
combined presence of external fields, thermal fluctuations, and spin-transfer
torques. The results will be discussed and briefly compared and contrasted with
that of prior work. | 0406486v1 |
2004-06-24 | Thermal Effects on the Magnetic Field Dependence of Spin Transfer Induced Magnetization Reversal | We have developed a self-aligned, high-yield process to fabricate CPP
(current perpendicular to the plane) magnetic sensors of sub 100 nm dimensions.
A pinned synthetic antiferromagnet (SAF) is used as the reference layer which
minimizes dipole coupling to the free layer and field induced rotation of the
reference layer. We find that the critical currents for spin transfer induced
magnetization reversal of the free layer vary dramatically with relatively
small changes the in-plane magnetic field, in contrast to theoretical
predictions based on stability analysis of the Gilbert equations of
magnetization dynamics including Slonczewski-type spin-torque terms. The
discrepancy is believed due to thermal fluctuations over the time scale of the
measurements. Once thermal fluctuations are taken into account, we find good
quantitative agreement between our experimental results and numerical
simulations. | 0406574v1 |
2004-07-23 | Micromagnetic understanding of current-driven domain wall motion in patterned nanowires | In order to explain recent experiments reporting a motion of magnetic domain
walls (DW) in nanowires carrying a current, we propose a modification of the
spin transfer torque term in the Landau-Lifchitz-Gilbert equation. We show that
it explains, with reasonable parameters, the measured DW velocities as well as
the variation of DW propagation field under current. We also introduce
coercivity by considering rough wires. This leads to a finite DW propagation
field and finite threshold current for DW propagation, hence we conclude that
threshold currents are extrinsic. Some possible models that support this new
term are discussed. | 0407628v2 |
2004-08-07 | Hysteresis multicycles in nanomagnet arrays | We predict two new physical effects in arrays of single-domain nanomagnets by
performing simulations using a realistic model Hamiltonian and physical
parameters. First, we find hysteretic multicycles for such nanomagnets. The
simulation uses continuous spin dynamics through the Landau-Lifshitz-Gilbert
(LLG) equation. In some regions of parameter space, the probability of finding
a multicycle is as high as ~0.6. We find that systems with larger and more
anisotropic nanomagnets tend to display more multicycles. This result
demonstrates the importance of disorder and frustration for multicycle
behavior. We also show that there is a fundamental difference between the more
realistic vector LLG equation and scalar models of hysteresis, such as Ising
models. In the latter case, spin and external field inversion symmetry is
obeyed but in the former it is destroyed by the dynamics, with important
experimental implications. | 0408158v1 |
2004-12-03 | High frequency magnetic permeability of nanocomposite film | The high frequency magnetic permeability of nanocomposite film consisting of
the single-domain spherical ferromagnetic particles in the dielectric matrix is
studied. The permeability is assumed to be determined by rotation of the
ferromagnetic inclusion magnetic moments around equilibrium direction in AC
magnetic field. The composite is modeled by a cubic array of ferromagnetic
particles. The magnetic permeability tensor is calculated by solving the
Landau-Lifshits-Gilbert equation accounting for the dipole interaction of
magnetic particles. The permeability tensor components are found as functions
of the frequency, temperature, ferromagnetic inclusions density and magnetic
anisotropy. The obtained results show that nanocomposite films could have
rather high value of magnetic permeability in the microwave range. | 0412073v1 |
2005-01-07 | Dielectric resonances of ordered passive arrays | The electrical and optical properties of ordered passive arrays, constituted
of inductive and capacitive components, are usually deduced from Kirchhoff's
rules. Under the assumption of periodic boundary conditions, comparable results
may be obtained via an approach employing transfer matrices. In particular,
resonances in the dielectric spectrum are demonstrated to occur if all
eigenvalues of the transfer matrix of the entire array are unity. The latter
condition, which is shown to be equivalent to the habitual definition of a
resonance in impedance for an array between electrodes, allows for a convenient
and accurate determination of the resonance frequencies, and may thus be used
as a tool for the design of materials with a specific dielectric response. For
the opposite case of linear arrays in a large network, where periodic boundary
condition do not apply, several asymptotic properties are derived. Throughout
the article, the derived analytic results are compared to numerical models,
based on either Exact Numerical Renormalisation or the spectral method. | 0501137v1 |
2005-07-27 | "Stochastic Modeling of Coercivity " - A Measure of Non-equilibrium State | A typical coercivity versus particle size curve for magnetic nanoparticles
has been explained by using the Gilbert equation followed by the corresponding
Fokker Plank equation. Kramer's treatment has been employed to explain the
increase in coercivity in the single domain region. The single to multi-domain
transformation has been assumed to explain the decrease in coercive field
beyond a certain particle size. The justification for using Langevin theory of
paramagnetism (including anisotropy energy) to fit the M vs H curve is
discussed. The super-symmetric Hamiltonian approach is used to find out the
relaxation time for the spins (making an angle greater than $90^0$ with applied
field) at domain wall. The main advantage of our technique is that we can
easily take into account the time of measurement as we usually do in realistic
measurement. | 0507640v1 |
2005-09-13 | Synchronization of spin-transfer oscillators driven by stimulated microwave currents | We have simulated the non-linear dynamics of networks of spin-transfer
oscillators. The oscillators are magnetically uncoupled but electrically
connected in series. We use a modified Landau-Lifschitz- Gilbert equation to
describe the motion of each oscillator in the presence of the oscillations of
all the others. We show that the oscillators of the network can be synchronized
not only in frequency but also in phase. The coupling is due to the microwave
components of the current induced in each oscillator by the oscillations in all
the other oscillators. Our results show how the emitted microwave power of
spin-transfer oscillators can be considerably enhanced by current-induced
synchronization in an electrically connected network. We also discuss the
possible application of our synchronization mechanism to the interpretation of
the surprisingly narrow microwave spectrum in some isolated spin-transfer
oscillators. | 0509326v2 |
2005-11-04 | Synchronized Magnetization Oscillations in F/N/F Nanopillars | Current-induced magnetization dynamics in a trilayer structure composed of
two ferromagnetic free layers and a nonmagnetic spacer is examined. Both free
layers are treated as a monodomain magnetic body with an uniform agnetization.
The dynamics of the two magnetizations is modeled by modified
Landau-Lifshitz-Gilbert equations with spin-transfer torque terms. By solving
the equations simultaneously, we discuss their various solutions in detail. We
show that there exists the synchronous motion of two magnetizations among the
various solutions; the magnetizations are resonantly coupled via spin-transfer
torques and perform precessional motions with the same period. The condition to
excite the synchronous motion depends on the difference between the intrinsic
frequencies of the two ferromagnetic free layers as well as the magnitude of
current. | 0511095v1 |
2006-01-27 | Dynamics of thin-film spin-flip transistors with perpendicular source-drain magnetizations | A "spin-flip transistor" is a lateral spin valve consisting of ferromagnetic
source drain contacts to a thin-film normal-metal island with an electrically
floating ferromagnetic base contact on top. We analyze the
\emph{dc}-current-driven magnetization dynamics of spin-flip transistors in
which the source-drain contacts are magnetized perpendicularly to the device
plane by magnetoelectronic circuit theory and the macrospin
Landau-Lifshitz-Gilbert equation. Spin flip scattering and spin pumping effects
are taken into account. We find a steady-state rotation of the base
magnetization at GHz frequencies that is tuneable by the source-drain bias. We
discuss the advantages of the lateral structure for high-frequency generation
and actuation of nanomechanical systems over recently proposed nanopillar
structures. | 0601630v1 |
2007-03-17 | Large-amplitude coherent spin waves exited by spin-polarized current in nanoscale spin valves | We present spectral measurements of spin-wave excitations driven by direct
spinpolarized current in the free layer of nanoscale
Ir20Mn80/Ni80Fe20/Cu/Ni80Fe20 spin valves. The measurements reveal that
large-amplitude coherent spin wave modes are excited over a wide range of bias
current. The frequency of these excitations exhibits a series of jumps as a
function of current due to transitions between different localized nonlinear
spin wave modes of the Ni80Fe20 nanomagnet. We find that micromagnetic
simulations employing the Landau-Lifshitz-Gilbert equation of motion augmented
by the Slonczewski spin torque term (LLGS) accurately describe the frequency of
the current-driven excitations including the mode transition behavior. However
LLGS simulations give qualitatively incorrect predictions for the amplitude of
excited spin waves as a function of current. | 0703458v2 |
2001-12-11 | A Data Mining Framework for Optimal Product Selection in Retail Supermarket Data: The Generalized PROFSET Model | In recent years, data mining researchers have developed efficient association
rule algorithms for retail market basket analysis. Still, retailers often
complain about how to adopt association rules to optimize concrete retail
marketing-mix decisions. It is in this context that, in a previous paper, the
authors have introduced a product selection model called PROFSET. This model
selects the most interesting products from a product assortment based on their
cross-selling potential given some retailer defined constraints. However this
model suffered from an important deficiency: it could not deal effectively with
supermarket data, and no provisions were taken to include retail category
management principles. Therefore, in this paper, the authors present an
important generalization of the existing model in order to make it suitable for
supermarket data as well, and to enable retailers to add category restrictions
to the model. Experiments on real world data obtained from a Belgian
supermarket chain produce very promising results and demonstrate the
effectiveness of the generalized PROFSET model. | 0112013v1 |
2005-02-22 | The QuarkNet/Grid Collaborative Learning e-Lab | We describe a case study that uses grid computing techniques to support the
collaborative learning of high school students investigating cosmic rays.
Students gather and upload science data to our e-Lab portal. They explore those
data using techniques from the GriPhyN collaboration. These techniques include
virtual data transformations, workflows, metadata cataloging and indexing, data
product provenance and persistence, as well as job planners. Students use web
browsers and a custom interface that extends the GriPhyN Chiron portal to
perform all of these tasks. They share results in the form of online posters
and ask each other questions in this asynchronous environment. Students can
discover and extend the research of other students, modeling the processes of
modern large-scale scientific collaborations. Also, the e-Lab portal provides
tools for teachers to guide student work throughout an investigation.
http://quarknet.uchicago.edu/elab/cosmic | 0502089v1 |
2006-10-11 | Properties of codes in rank metric | We study properties of rank metric and codes in rank metric over finite
fields. We show that in rank metric perfect codes do not exist. We derive an
existence bound that is the equivalent of the Gilbert--Varshamov bound in
Hamming metric. We study the asymptotic behavior of the minimum rank distance
of codes satisfying GV. We derive the probability distribution of minimum rank
distance for random and random $\F{q}$-linear codes. We give an asymptotic
equivalent of their average minimum rank distance and show that random
$\F{q}$-linear codes are on GV bound for rank metric.
We show that the covering density of optimum codes whose codewords can be
seen as square matrices is lower bounded by a function depending only on the
error-correcting capability of the codes. We show that there are quasi-perfect
codes in rank metric over fields of characteristic 2. | 0610057v1 |
1995-12-28 | MSSM radiative contributions to the WW$γ$ and WWZ form factors | We evaluate one-loop contributions to the C and P conserving $WW\gamma, WWZ$
form factors in the Minimal Supersymmetric Standard Model (MSSM), and in a more
constrained Supergravity Grand Unified Theory (SUGRA-GUT). A systematic search
of maximal effects in the available parameter space, shows that at LEP2 energy
MSSM contributions can hardly reach the border of the most optimistic accuracy
expected on those couplings, even for particles close to their production
thresholds. At NLC energies, the effects are more comfortably of the order of
the expected sensitivity, and may therefore provide useful information on MSSM
parameter values which will not be available from direct particle production.
We also discuss briefly some variance with other studies. | 9512437v2 |
1998-07-10 | Inverting the Supersymmetric Standard Model Spectrum: from Physical to Lagrangian Ino Parameters | We examine the possibility of recovering the supersymmetric (and soft
supersymmetry breaking) Lagrangian parameters as direct {\em analytical}
expressions of appropriate physical masses, for the unconstrained (but CP and
R-parity conserving) minimal supersymmetric standard model. We concentrate
mainly on the algebraically non-trivial "inversion" for the ino parameters, and
obtain, for given values of $\tan\beta$, simple analytical expressions for the
$\mu$, $M_1$ and $M_2$ parameters in terms of three arbitrary input physical
masses, namely either two chargino and one neutralino masses, or alternatively
one chargino and two neutralino masses. We illustrate and discuss in detail the
possible occurrence of ambiguities in this reconstruction. The dependence of
the resulting ino Lagrangian parameters upon physical masses is illustrated,
and some simple generic behaviour uncovered in this way. We finally briefly
sketch generalizing such an inversion to the full set of MSSM Lagrangian
parameters. | 9807336v2 |
1999-07-01 | A convergent scheme for one-loop evolutions of the Yukawa couplings in the MSSM | Integrated forms of the one-loop evolution equations are given for the Yukawa
couplings in the MSSM, valid for any value of $\tan \beta$, generalizable to
virtually any number of Yukawa fermions, and including all gauge couplings.
These forms turn out to have nice mathematical convergence properties which we
prove, and we determine the ensuing convergence criteria. Furthermore, they
allow to write down general sufficient and necessary conditions to avoid
singularities in the evolution of the Yukawa couplings over physically relevant
energy ranges. We also comment briefly on the possible use of these features
for physics issues and give a short numerical illustration. | 9907204v1 |
1999-07-14 | Phases in the gaugino sector: direct reconstruction of the basic parameters and impact on the neutralino pair production | We consider recovering analytically the (generally complex) parameters $\mu$,
$M_1$ and $M_2$ of the gaugino and Higgsino Lagrangian, from appropriate
physical input in the chargino and neutralino sectors. For given $\tan\beta$,
we obtain very simple analytic solutions for $M_2$, $| \mu|$, $Arg[\mu]$ in the
chargino sector and a twofold $| M_1 |$, $Arg[M_1]$ analytic solution in the
neutralino sector, assuming two chargino, two neutralino masses, and one of the
chargino mixing angles as physical input. The twofold ambiguity in the
neutralino parameters reconstruction may be essentially resolved by measuring
the $e^+e^- \to \chi^0_1 \chi^0_2$ production cross-section at future linear
collider energies, which we study explicitly with the phase dependences. Some
salient features and specific properties of this complex case gaugino "spectrum
inversion" are illustrated and compared with the similar inversion in the real
case. In particular, our algorithms exhibit in a direct and transparent way the
non-trivial theoretical correlation among the chargino and neutralino
parameters, and the resulting allowed domains when only a subset of the
required physical input masses and production cross-sections is known. | 9907360v1 |
1999-12-08 | Analytical Study of Non-Universality of the Soft Terms in the MSSM | We obtain general analytical forms for the solutions of the one-loop
renormalization group equations in the top/bottom/$\tau$ sector of the MSSM.
These solutions are valid for any value of $\tan \beta$ as well as any
non-universal initial conditions for the soft SUSY breaking parameters and
non-unification of the Yukawa couplings. We establish analytically a generic
screening effect of non-universality, in the vicinity of the infrared quasi
fixed point, which allows to determine sector-wise a hierarchy of sensitivity
to initial conditions. We give also various numerical illustrations of this
effect away from the quasi fixed point and assess the sensitivity of the Higgs
and sfermion spectra to the non-universality of the various soft breaking
sectors. As a by-product, a typical anomaly-mediated non-universality of the
gaugino sector would have marginal influence on the scalar spectrum. | 9912271v1 |
2001-01-22 | General one-loop renormalization group evolutions and electroweak symmetry breaking in the (M+1)SSM | We study analytically the general features of electroweak symmetry breaking
in the context of the Minimal Supersymmetric Standard Model extended by one
Higgs singlet. The exact analytical forms of the renormalization group
evolutions of the Yukawa couplings and of the soft supersymmetry breaking
parameters are derived to one-loop order. They allow on one hand controllable
approximations in closed analytical form, and on the other a precise study of
the behaviour of infrared quasi fixed point regimes which we carry out. Some of
these regimes are shown to be phenomenologically inconsistent, leading to too
small an effective $\mu$-parameter. The remaining ones serve as a suitable
benchmark to understand analytically some salient aspects, often noticed
numerically in the literature, in relation to the electroweak symmetry breaking
in this model. The study does not need any specific assumption on $\tan \beta$
or on boundary conditions for the soft supersymmetry breaking parameters, thus
allowing a general insight into the sensitivity of the low energy physics to
high energy assumptions. | 0101237v1 |
2001-12-28 | Infrared Quasi Fixed Point Structure in Extended Yukawa Sectors and Application to R-parity Violation | We investigate one-loop renormalization group evolutions of extended sectors
of Yukawa type couplings. It is shown that Landau Poles which usually provide
necessary low energy upper bounds that saturate quickly with increasing initial
value conditions, lead in some cases to the opposite behaviour: some of the low
energy couplings decrease and become vanishingly small for increasingly large
initial conditions. We write down the general criteria for this to happen in
typical situations, highlighting a concept of {\sl repulsive} quasi-fixed
points, and illustrate the case both within a two-Yukawa toy model as well as
in the minimal supersymmetric standard model with R-parity violation. In the
latter case we consider the theoretical upper bounds on the various couplings,
identifying regimes where $\lambda_{kl3}, \lambda'_{kkk}, \lambda''_{3kl}$ are
dynamically suppressed due to the Landau Pole. We stress the importance of
considering a large number of couplings simultaneously. This leads altogether
to a phenomenologically interesting seesaw effect in the magnitudes of the
various R-parity violating couplings, complementing and in some cases improving
the existing limits. | 0112353v1 |
2005-04-04 | Natural gravitino dark matter in SO(10) gauge mediated supersymmetry breaking | It is shown that gravitinos with mass m_{3/2} ~ 0.1-1 MeV may provide
suitable cold dark matter candidates in scenarios of gauge mediated
supersymmetry breaking (GMSB) under SO(10) grand unification coupled to
supergravity, which accommodate a messenger sector of mass scale M_X ~ 10^6
GeV. This is due to the combined effects of renormalizable loop-suppressed
operators and generic non-renormalizable ones governing the dilution of a
pre-existing equilibrium gravitino abundance via messenger decay. The above
range of gravitino and messenger masses can be accommodated in indirect GMSB
scenarios. The gravitino abundance does not depend on the post-inflationary
reheat temperature and it is shown that leptogenesis can generate successfully
the baryon asymmetry. | 0504021v2 |
2005-06-14 | Gravitino dark matter in gauge mediated supersymmetry breaking | This paper investigates the parameter space of theories with gauge mediated
supersymmetry breaking leading to gravitino (cold) dark matter with mass
m_{3/2}= 1keV - 10MeV. We pay particular attention to the cosmological role of
messenger fields. Cosmology requires that these messengers decay to the visible
sector if the lightest messenger mass M_X > 30TeV. We then examine the various
possible messenger number violating interactions allowed by the symmetries of
the theory and by phenomenology. Late messenger decay generally results in
entropy production hence in the dilution of pre-existing gravitinos. We find
that in SU(5) grand unification only specific messenger-matter couplings allow
to produce the required amount of gravitino dark matter particles. Gravitino
dark matter with the correct abundance is however expected in larger gauge
groups such as SO(10) for generic non-renormalizable messenger-matter
interactions and for arbritrarily high post-inflationary reheating
temperatures. | 0506129v2 |
1998-04-24 | Enumeration of m-ary cacti | The purpose of this paper is to enumerate various classes of cyclically
colored m-gonal plane cacti, called m-ary cacti. This combinatorial problem is
motivated by the topological classification of complex polynomials having at
most m critical values, studied by Zvonkin and others. We obtain explicit
formulae for both labelled and unlabelled m-ary cacti, according to i) the
number of polygons, ii) the vertex-color distribution, iii) the vertex-degree
distribution of each color. We also enumerate m-ary cacti according to the
order of their automorphism group. Using a generalization of Otter's formula,
we express the species of m-ary cacti in terms of rooted and of pointed cacti.
A variant of the m-dimensional Lagrange inversion is then used to enumerate
these structures. The method of Liskovets for the enumeration of unrooted
planar maps can also be adapted to m-ary cacti. | 9804119v2 |
2002-08-21 | Toric codes over finite fields | In this note, a class of error-correcting codes is associated to a toric
variety associated to a fan defined over a finite field $\fff_q$, analogous to
the class of Goppa codes associated to a curve. For such a ``toric code''
satisfying certain additional conditions, we present an efficient decoding
algorithm for the dual of a Goppa code. Many examples are given. For small $q$,
many of these codes have parameters beating the Gilbert-Varshamov bound. In
fact, using toric codes, we construct a $(n,k,d)=(49,11,28)$ code over
$\fff_8$, which is better than any other known code listed in Brouwer's on-line
tables for that $n$ and $k$. | 0208155v2 |
2002-12-05 | Automorphisms of hyperbolic groups and graphs of groups | Using the canonical JSJ splitting, we describe the outer automorphism group
$\Out(G)$ of a one-ended word hyperbolic group $G$. In particular, we discuss
to what extent $\Out(G)$ is virtually a direct product of mapping class groups
and a free abelian group, and we determine for which groups $\Out(G)$ is
infinite. We also show that there are only finitely many conjugacy classes of
torsion elements in $\Out(G)$, for $G$ any torsion-free hyperbolic group.
More generally, let $\Gamma $ be a finite graph of groups decomposition of an
arbitrary group $G$ such that edge groups $G_e$ are rigid (i.e\. $\Out(G_e)$ is
finite). We describe the group of automorphisms of $G$ preserving $\Gamma $, by
comparing it to direct products of suitably defined mapping class groups of
vertex groups. | 0212088v1 |
2004-06-08 | The structure and labelled enumeration of K_{3,3}-subdivision-free projective-planar graphs | We consider the class F of 2-connected non-planar K_{3,3}-subdivision-free
graphs that are embeddable in the projective plane. We show that these graphs
admit a unique decomposition as a graph K_5 (the core) where the edges are
replaced by two-pole networks constructed from 2-connected planar graphs. A
method to enumerate these graphs in the labelled case is described. Moreover,
we enumerate the homeomorphically irreducible graphs in F and homeomorphically
irreducible 2-connected planar graphs. Particular use is made of two-pole
directed series-parallel networks. We also show that the number m of edges of
graphs in F with n vertices satisfies the bound m <=3n-6, for n >= 6. | 0406140v5 |
2004-09-16 | Translation equivalence in free groups | Motivated by the work of Leininger on hyperbolic equivalence of homotopy
classes of closed curves on surfaces, we investigate a similar phenomenon for
free groups. Namely, we study the situation when two elements $g,h$ in a free
group $F$ have the property that for every free isometric action of $F$ on an
$\mathbb{R}$-tree $X$ the translation lengths of $g$ and $h$ on $X$ are equal.
We give a combinatorial characterization of this phenomenon, called translation
equivalence, in terms of Whitehead graphs and exhibit two difference sources of
it. The first source of translation equivalence comes from representation
theory and $SL_2$ trace identities. The second source comes from geometric
properties of groups acting on real trees and a certain power redistribution
trick. We also analyze to what extent these are applicable to the tree actions
of surface groups that occur in the Thurston compactification of the
Teichmuller space. | 0409284v2 |
2004-11-16 | Characterization and enumeration of toroidal K_{3,3}-subdivision-free graphs | We describe the structure of 2-connected non-planar toroidal graphs with no
K_{3,3}-subdivisions, using an appropriate substitution of planar networks into
the edges of certain graphs called toroidal cores. The structural result is
based on a refinement of the algorithmic results for graphs containing a fixed
K_5-subdivision in [A. Gagarin and W. Kocay, "Embedding graphs containing
K_5-subdivisions'', Ars Combin. 64 (2002), 33-49]. It allows to recognize these
graphs in linear-time and makes possible to enumerate labelled 2-connected
toroidal graphs containing no K_{3,3}-subdivisions and having minimum vertex
degree two or three by using an approach similar to [A. Gagarin, G. Labelle,
and P. Leroux, "Counting labelled projective-planar graphs without a
K_{3,3}-subdivision", submitted, arXiv:math.CO/0406140, (2004)]. | 0411356v1 |
2005-01-19 | The outer space of a free product | We associate a contractible ``outer space'' to any free product of groups
G=G_1*...*G_q. It equals Culler-Vogtmann space when G is free,
McCullough-Miller space when no G_i is Z. Our proof of contractibility (given
when G is not free) is based on Skora's idea of deforming morphisms between
trees.
Using the action of Out(G) on this space, we show that Out(G) has finite
virtual cohomological dimension, or is VFL (it has a finite index subgroup with
a finite classifying space), if the groups G_i and Out(G_i) have similar
properties. We deduce that Out(G) is VFL if G is a torsion-free hyperbolic
group, or a limit group (finitely generated fully residually free group). | 0501288v3 |
2005-07-28 | Free-group automorphisms, train tracks and the beaded decomposition | We study the automorphisms \phi of a finitely generated free group F.
Building on the train-track technology of Bestvina, Feighn and Handel, we
provide a topological representative f:G\to G of a power of \phi that behaves
very much like the realization on the rose of a positive automorphism. This
resemblance is encapsulated in the Beaded Decomposition Theorem which describes
the structure of paths in G obtained by repeatedly passing to f-images of an
edge and taking subpaths. This decomposition is the key to adapting our proof
of the quadratic isoperimetric inequality for $F\rtimes_\phi\mathbb Z$, with
\phi positive, to the general case. To illustrate the wider utility of our
topological normal form, we provide a short proof that for every w in F, the
function $n\mapsto |\phi^n(w)|$ grows either polynomially or exponentially. | 0507589v2 |
2007-02-02 | Line-of-sight percolation | Given $\omega\ge 1$, let $Z^2_{(\omega)}$ be the graph with vertex set $Z^2$
in which two vertices are joined if they agree in one coordinate and differ by
at most $\omega$ in the other. (Thus $Z^2_{(1)}$ is precisely $Z^2$.) Let
$p_c(\omega)$ be the critical probability for site percolation in
$Z^2_{(\omega)}$. Extending recent results of Frieze, Kleinberg, Ravi and
Debany, we show that $\lim_{\omega\to\infty} \omega\pc(\omega)=\log(3/2)$. We
also prove analogues of this result on the $n$-by-$n$ grid and in higher
dimensions, the latter involving interesting connections to Gilbert's continuum
percolation model. To prove our results, we explore the component of the origin
in a certain non-standard way, and show that this exploration is well
approximated by a certain branching random walk. | 0702061v2 |
2000-03-06 | Entropy Production, Fractals, and Relaxation to Equilibrium | The theory of entropy production in nonequilibrium, Hamiltonian systems,
previously described for steady states using partitions of phase space, is here
extended to time dependent systems relaxing to equilibrium. We illustrate the
main ideas by using a simple multibaker model, with some nonequilibrium initial
state, and we study its progress toward equilibrium. The central results are
(i) the entropy production is governed by an underlying, exponentially decaying
fractal structure in phase space, (ii) the rate of entropy production is
largely independent of the scale of resolution used in the partitions, and
(iii) the rate of entropy production is in agreement with the predictions of
nonequilibrium thermodynamics. | 0003012v2 |
2002-03-21 | Entropy production of diffusion in spatially periodic deterministic systems | This paper presents an {\it ab initio} derivation of the expression given by
irreversible thermodynamics for the rate of entropy production for different
classes of diffusive processes. The first class are Lorentz gases, where
non-interacting particles move on a spatially periodic lattice, and collide
elastically with fixed scatterers. The second class are periodic systems where
$N$ particles interact with each other, and one of them is a tracer particle
which diffuses among the cells of the lattice. We assume that, in either case,
the dynamics of the system is deterministic and hyperbolic, with positive
Lyapunov exponents. This work extends methods originally developed for a
chaotic two-dimensional model of diffusion, the multi-baker map, to higher
dimensional, continuous time dynamical systems appropriate for systems with one
or more moving particles. Here we express the rate of entropy production in
terms of hydrodynamic measures that are determined by the fractal properties of
microscopic hydrodynamic modes that describe the slowest decay of the system to
an equilibrium state. | 0203046v1 |
2001-11-09 | Spatial diffusion in a periodic optical lattice: revisiting the Sisyphus effect | We numerically study the spatial diffusion of an atomic cloud experiencing
Sisyphus cooling in a three-dimensional lin$\bot$lin optical lattice in a broad
range of lattice parameters. In particular, we investigate the dependence on
the size of the lattice sites which changes with the angle between the laser
beams. We show that the steady-state temperature is largely independent of the
lattice angle, but that the spatial diffusion changes significantly. It is
shown that the numerical results fulfil the Einstein relations of Brownian
motion in the jumping regime as well as in the oscillating regime. We finally
derive an effective Brownian motion model from first principles which gives
good agreement with the simulations. | 0111070v2 |
2006-07-24 | Use of specific Green's functions for solving direct problems involving a heterogeneous rigid frame porous medium slab solicited by acoustic waves | A domain integral method employing a specific Green's function (i.e.,
incorporating some features of the global problem of wave propagation in an
inhomogeneous medium) is developed for solving direct and inverse scattering
problems relative to slab-like macroscopically inhomogeneous porous obstacles.
It is shown how to numerically solve such problems, involving both
spatially-varying density and compressibility, by means of an iterative scheme
initialized with a Born approximation. A numerical solution is obtained for a
canonical problem involving a two-layer slab. | 0607212v1 |
2005-12-12 | Cloning, expression and purification of the general stress protein Yhbo from Escherichia coli | We cloned, expressed and purified the Escherichia coli yhbO gene product,
which is homolog to the Bacillus subtilis general stress protein 18 (the yfkM
gene product), the Pyrococcus furiosus intracellular protease PfpI, and the
human Parkinson disease protein DJ-1. The gene coding for YhbO was generated by
amplifying the yhbO gene from E. coli by polymerase chain reaction. It was
inserted in the expression plasmid pET-21a, under the transcriptional control
of the bacteriophage T7 promoter and lac operator. A BL21(DE3) E. coli strain
transformed with the YhbO-expression vector pET-21a-yhbO, accumulates large
amounts of a soluble protein of 20 kDa in SDS-PAGE that matches the expected
YhbO molecular weight. YhbO was purified to homogeneity by HPLC DEAE ion
exchange chromatography and hydroxylapatite chromatography and its identity was
confirmed by N-terminal sequencing and mass spectrometry analysis. The native
protein exists in monomeric, trimeric and hexameric forms. | 0512028v1 |
2003-01-24 | Rayleigh Scattering and Atomic Dynamics in Dissipative Optical Lattices | We investigate Rayleigh scattering in dissipative optical lattices. In
particular, following recent proposals (S. Guibal {\it et al}, Phys. Rev. Lett.
{\bf 78}, 4709 (1997); C. Jurczak {\it et al}, Phys. Rev. Lett. {\bf 77}, 1727
(1996)), we study whether the Rayleigh resonance originates from the
diffraction on a density grating, and is therefore a probe of transport of
atoms in optical lattices. It turns out that this is not the case: the Rayleigh
line is instead a measure of the cooling rate, while spatial diffusion
contributes to the scattering spectrum with a much broader resonance. | 0301139v1 |
2006-05-09 | Communicating over adversarial quantum channels using quantum list codes | We study quantum communication in the presence of adversarial noise. In this
setting, communicating with perfect fidelity requires using a quantum code of
bounded minimum distance, for which the best known rates are given by the
quantum Gilbert-Varshamov (QGV) bound. By asking only for arbitrarily high
fidelity and allowing the sender and reciever to use a secret key with length
logarithmic in the number of qubits sent, we achieve a dramatic improvement
over the QGV rates. In fact, we find protocols that achieve arbitrarily high
fidelity at noise levels for which perfect fidelity is impossible. To achieve
such communication rates, we introduce fully quantum list codes, which may be
of independent interest. | 0605086v2 |
2007-05-16 | Quantization Bounds on Grassmann Manifolds of Arbitrary Dimensions and MIMO Communications with Feedback | This paper considers the quantization problem on the Grassmann manifold with
dimension n and p. The unique contribution is the derivation of a closed-form
formula for the volume of a metric ball in the Grassmann manifold when the
radius is sufficiently small. This volume formula holds for Grassmann manifolds
with arbitrary dimension n and p, while previous results are only valid for
either p=1 or a fixed p with asymptotically large n. Based on the volume
formula, the Gilbert-Varshamov and Hamming bounds for sphere packings are
obtained. Assuming a uniformly distributed source and a distortion metric based
on the squared chordal distance, tight lower and upper bounds are established
for the distortion rate tradeoff. Simulation results match the derived results.
As an application of the derived quantization bounds, the information rate of a
Multiple-Input Multiple-Output (MIMO) system with finite-rate channel-state
feedback is accurately quantified for arbitrary finite number of antennas,
while previous results are only valid for either Multiple-Input Single-Output
(MISO) systems or those with asymptotically large number of transmit antennas
but fixed number of receive antennas. | 0705.2272v1 |
2007-05-24 | Local spin dynamic arising from the non-perturbative SU(2) gauge field of the spin orbit effect | We use the non-perturbative gauge field approach to study the effects of spin
orbit coupling on the dynamic of magnetic moment. We present a general equation
of motion (EOM) which unifies i) the spin orbit coupling effect derived from
the SU(2) spin gauge field, and ii) the moment chirality effect previously
derived from the topological U(1)xU(1) rotation gauge under the adiabatic
condition. We present a modified Landau-Liftshitz-Gilbert equation and discuss
the implication of the modified EOM in various technological applications, such
as current-induced switching and trajectory of magnetic moments in spin-valve
multilayers, magnetic memory and diluted magnetic semiconductor. | 0705.3502v1 |
2007-06-07 | $^{77}$Se NMR measurements of the $π-d$ exchange field in the organic conductor $λ-$(BETS)$_{2}$FeCl$_{4}$ | $^{77}$Se-NMR spectrum and frequency shift measurements in the paramagnetic
metal (PM) and antiferromagnetic insulating (AFI) phases are reported for a
small single crystal of the organic conductor $\lambda-$(BETS)$_{2}$FeCl$_{4}$
as a function of temperature ($T$) and field alignment for an applied magnetic
field $B_{0}$ = 9 T. The results show that in the low $T$ limit, where the
localized Fe$^{3+}$ spins ($S_{d}$ = 5/2) are almost fully polarized, the
conduction electrons (Se $\pi$-electrons, spin $s_{\pi}$ = 1/2) in the BETS
molecules experience an exchange field ($\bf{B}$$_{\pi d}$) from the Fe$^{3+}$
spins with a value of $-$ 32.7 $\pm$ 1.5 T at 5 K and 9 T aligned opposite to
$\bf{B}$$_{0}$. This large negative value of $\bf{B}$$_{\pi d}$ is consistent
with that predicted by the resistivity measurements and supports the
Jaccarino-Peter internal field-compensation mechanism being responsible for the
origin of field-induced superconductivity. | 0706.0933v1 |
2007-11-05 | Feedback Capacity of the Compound Channel | In this work we find the capacity of a compound finite-state channel with
time-invariant deterministic feedback. The model we consider involves the use
of fixed length block codes. Our achievability result includes a proof of the
existence of a universal decoder for the family of finite-state channels with
feedback. As a consequence of our capacity result, we show that feedback does
not increase the capacity of the compound Gilbert-Elliot channel. Additionally,
we show that for a stationary and uniformly ergodic Markovian channel, if the
compound channel capacity is zero without feedback then it is zero with
feedback. Finally, we use our result on the finite-state channel to show that
the feedback capacity of the memoryless compound channel is given by
$\inf_{\theta} \max_{Q_X} I(X;Y|\theta)$. | 0711.0705v1 |
2007-11-12 | Mutual phase-locking in high frequency microwave nanooscillators as function of field angle | We perform a qualitative analysis of phase locking in a double point-contact
spinvalve system by solving the Landau-Lifshitz-Gilbert-Slonzewski equation
using a hybrid-finite-element method. We show that the phase-locking behaviour
depends on the applied field angle. Starting from a low field angle, the
locking-current difference between the current through contact A and B
increases with increasing angle up to a maximum of 14 mA at 30 degree and it
decreases thereafter until it reaches a minimum of 1 mA at 75 degree. The
tunability of the phase-lock frequency with current decreases linearly with
increasing out of plane angle from 45 to 21 MHz/mA. | 0711.1770v2 |
2007-11-14 | Emergent singular solutions of non-local density-magnetization equations in one dimension | We investigate the emergence of singular solutions in a non-local model for a
magnetic system. We study a modified Gilbert-type equation for the
magnetization vector and find that the evolution depends strongly on the length
scales of the non-local effects. We pass to a coupled density-magnetization
model and perform a linear stability analysis, noting the effect of the length
scales of non-locality on the system's stability properties. We carry out
numerical simulations of the coupled system and find that singular solutions
emerge from smooth initial data. The singular solutions represent a collection
of interacting particles (clumpons). By restricting ourselves to the
two-clumpon case, we are reduced to a two-dimensional dynamical system that is
readily analyzed, and thus we classify the different clumpon interactions
possible. | 0711.2177v1 |
2007-11-27 | Nonequilibrium interacting electrons in a ferromagnet | Dynamics of the magnetization in ferromagnets is examined in the presence of
transport electrons allowing the latter to interact. It is found that the
existence of inhomogeneities such as domain wall (DW) structures, leads to
changes that affect the dynamical structure of the equations of motion for the
magnetization. Only in the limit of uniform magnetizations or sufficiently wide
DW's, the equations of motion maintain the form they have in the noninteracting
case. In this limit, results like the spin torques, the Gilbert parameter, and
the DW velocities become renormalized. However the length scale that defines
such a limit depends on the strength of the interaction. It is shown that if
large ferromagnetic fluctuations exist in the metallic band then the range for
which conformity with the noninteracting case holds extends to the limit of
arbitrarily narrow DW's. | 0711.4170v2 |
2007-12-03 | Kinetic models of heterogeneous dissipation | We suggest kinetic models of dissipation for an ensemble of interacting
oriented particles, for example, moving magnetized particles. This is achieved
by introducing a double bracket dissipation in kinetic equations using an
oriented Poisson bracket, and employing the moment method to derive continuum
equations for magnetization and density evolution. We show how our continuum
equations generalize the Debye-Hueckel equations for attracting round
particles, and Landau-Lifshitz-Gilbert equations for spin waves in magnetized
media. We also show formation of singular solutions that are clumps of aligned
particles (orientons) starting from random initial conditions. Finally, we
extend our theory to the dissipative motion of self-interacting curves. | 0712.0397v1 |
2007-12-13 | Euler equation of the optimal trajectory for the fastest magnetization reversal of nano-magnetic structures | Based on the modified Landau-Lifshitz-Gilbert equation for an arbitrary
Stoner particle under an external magnetic field and a spin-polarized electric
current, differential equations for the optimal reversal trajectory, along
which the magnetization reversal is the fastest one among all possible reversal
routes, are obtained. We show that this is a Euler-Lagrange problem with
constrains. The Euler equation of the optimal trajectory is useful in designing
a magnetic field pulse and/or a polarized electric current pulse in
magnetization reversal for two reasons. 1) It is straightforward to obtain the
solution of the Euler equation, at least numerically, for a given magnetic
nano-structure characterized by its magnetic anisotropy energy. 2) After
obtaining the optimal reversal trajectory for a given magnetic nano-structure,
finding a proper field/current pulse is an algebraic problem instead of the
original nonlinear differential equation. | 0712.2101v1 |
2007-12-18 | Effect of Edge Roughness on Electronic Transport in Graphene Nanoribbon Channel Metal Oxide Semiconductor Field-Effect Transistors | Results of quantum mechanical simulations of the influence of edge disorder
on transport in graphene nanoribbon metal oxide semiconductor field-effect
transistors (MOSFETs) are reported. The addition of edge disorder significantly
reduces ON-state currents and increases OFF-state currents, and introduces wide
variability across devices. These effects decrease as ribbon widths increase
and as edges become smoother. However the bandgap decreases with increasing
width, thereby increasing the band-to-band tunneling mediated subthreshold
leakage current even with perfect nanoribbons. These results suggest that
without atomically precise edge control during fabrication, MOSFET performance
gains through use of graphene will be difficult to achieve. | 0712.3068v1 |
2007-12-22 | Explicit Non-Adaptive Combinatorial Group Testing Schemes | Group testing is a long studied problem in combinatorics: A small set of $r$
ill people should be identified out of the whole ($n$ people) by using only
queries (tests) of the form "Does set X contain an ill human?". In this paper
we provide an explicit construction of a testing scheme which is better
(smaller) than any known explicit construction. This scheme has $\bigT{\min[r^2
\ln n,n]}$ tests which is as many as the best non-explicit schemes have. In our
construction we use a fact that may have a value by its own right: Linear
error-correction codes with parameters $[m,k,\delta m]_q$ meeting the
Gilbert-Varshamov bound may be constructed quite efficiently, in $\bigT{q^km}$
time. | 0712.3876v5 |
2007-12-31 | Risk management for analytical methods: conciliating objectives of methods, validation phase and routine decision rules | In the industries that involved either chemistry or biology, such as
pharmaceutical industries, chemical industries or food industry, the analytical
methods are the necessary eyes and hear of all the material produced or used.
If the quality of an analytical method is doubtful, then the whole set of
decision that will be based on those measures is questionable. For those
reasons, being able to assess the quality of an analytical method is far more
than a statistical challenge; it's a matter of ethic and good business
practices. Many regulatory documents have been releases, primarily ICH and FDA
documents in the pharmaceutical industry (FDA, 1995, 1997, 2001) to address
that issue. | 0801.0207v1 |
2008-01-06 | An Efficient Method for Quantum Transport Calculations in Nanostructures using Full Band Structure | Scaling of semiconductor devices has reached a stage where it has become
absolutely imperative to consider the quantum mechanical aspects of transport
in these ultra small devices. In these simulations, often one excludes a
rigorous band structure treatment, since it poses a huge computational
challenge. We have proposed here an efficient method for calculating full
three-dimensionally coupled quantum transport in nanowire transistors including
full band structure. We have shown the power of the method by simulating hole
transport in p-type Ge nanowire transistors. The hole band structure obtained
from our nearest neighbor sp3s* tight binding Hamiltonian agrees well
qualitatively with more complex and accurate calculations that take third
nearest neighbors into account. The calculated I-V results show how shifting of
the energy bands due to confinement can be accurately captured only in a full
band full quantum simulation. | 0801.0880v1 |
2008-01-07 | Magnetization reversal driven by spin-injection : a mesoscopic spin-transfer effect | A mesoscopic description of spin-transfer effect is proposed, based on the
spin-injection mechanism occurring at the junction with a ferromagnet. The
effect of spin-injection is to modify locally, in the ferromagnetic
configuration space, the density of magnetic moments. The corresponding
gradient leads to a current-dependent diffusion process of the magnetization.
In order to describe this effect, the dynamics of the magnetization of a
ferromagnetic single domain is reconsidered in the framework of the
thermokinetic theory of mesoscopic systems. Assuming an Onsager
cross-coefficient that couples the currents, it is shown that spin-dependent
electric transport leads to a correction of the Landau-Lifshitz-Gilbert
equation of the ferromagnetic order parameter with supplementary diffusion
terms. The consequence of spin-injection in terms of activation process of the
ferromagnet is deduced, and the expressions of the effective energy barrier and
of the critical current are derived. Magnetic fluctuations are calculated: the
correction to the fluctuations is similar to that predicted for the activation.
These predictions are consistent with the measurements of spin-transfer
obtained in the activation regime and for ferromagnetic resonance under
spin-injection. | 0801.1019v1 |
2008-01-25 | New Lower Bounds on Sizes of Permutation Arrays | A permutation array(or code) of length $n$ and distance $d$, denoted by
$(n,d)$ PA, is a set of permutations $C$ from some fixed set of $n$ elements
such that the Hamming distance between distinct members
$\mathbf{x},\mathbf{y}\in C$ is at least $d$. Let $P(n,d)$ denote the maximum
size of an $(n,d)$ PA. This correspondence focuses on the lower bound on
$P(n,d)$. First we give three improvements over the Gilbert-Varshamov lower
bounds on $P(n,d)$ by applying the graph theorem framework presented by Jiang
and Vardy. Next we show another two new improved bounds by considering the
covered balls intersections. Finally some new lower bounds for certain values
of $n$ and $d$ are given. | 0801.3986v1 |
2008-01-30 | Femtosecond Control of the Magnetization in Ferromagnetic Semiconductors | We develop a theory of collective spin dynamics triggered by ultrafast
optical excitation of ferromagnetic semiconductors. Using the density matrix
equations of motion in the mean field approximation and including magnetic
anisotropy and hole spin dephasing effects, we predict the development of a
light--induced magnetization tilt during ultra--short time intervals comparable
to the pulse duration. This femtosecond dynamics in the coherent temporal
regime is governed by the interband nonlinear optical polarizations and is
followed by a second temporal regime governed by the magnetic anisotropy of the
Fermi sea. We interpret our numerical results by deriving a
Landau--Gilbert--like equation for the collective spin, which demonstrates an
ultrafast correction to the magnetic anisotropy effective field due to second
order coherent nonlinear optical processes. Using the Lindblad semigroup
method, we also derive a contribution to the interband polarization dephasing
determined by the Mn spin and the hole spin dephasing. Our predicted
magnetization tilt and subsequent nonlinear dynamics due to the magnetic
anisotropy can be controlled by varying the optical pulse intensity, duration,
and helicity and can be observed with pump--probe magneto--optical
spectroscopy. | 0801.4641v1 |
2008-02-01 | Vortex Ferroelectric Domains | We show experimental switching data on microscale capacitors of
lead-zirconate-titanate (PZT), which reveal time-resolved domain behavior
during switching on a 100-ns scale. For small circular capacitors, an
unswitched domain remains in the center while complete switching is observed in
square capacitors. The observed effect is attributed to the formation of vortex
domain during polarization switching in circular capacitors. This dynamical
behavior is modeled using the Landau-Liftshitz-Gilbert equations and found to
be in detailed agreement with experiment. This simulation implies rotational
motion of polarization in the xy-plane, a Heisenberg-like result supported by
the recent model of Naumov and Fu [Phys. Rev. Lett. 98, 077603 (2007)],
although not directly measurable by the present quasi-static measurements. | 0802.0186v1 |
2008-02-11 | Structure and Optimality of Myopic Policy in Opportunistic Access with Noisy Observations | A restless multi-armed bandit problem that arises in multichannel
opportunistic communications is considered, where channels are modeled as
independent and identical Gilbert-Elliot channels and channel state
observations are subject to errors. A simple structure of the myopic policy is
established under a certain condition on the false alarm probability of the
channel state detector. It is shown that the myopic policy has a semi-universal
structure that reduces channel selection to a simple round-robin procedure and
obviates the need to know the underlying Markov transition probabilities. The
optimality of the myopic policy is proved for the case of two channels and
conjectured for the general case based on numerical examples. | 0802.1379v2 |
2008-02-12 | Domain walls in (Ga,Mn)As diluted magnetic semiconductor | We report experimental and theoretical studies of magnetic domain walls in an
in-plane magnetized (Ga,Mn)As dilute moment ferromagnetic semiconductor. Our
high-resolution electron holography technique provides direct images of domain
wall magnetization profiles. The experiments are interpreted based on
microscopic calculations of the micromagnetic parameters and
Landau-Lifshitz-Gilbert simulations. We find that the competition of uniaxial
and biaxial magnetocrystalline anisotropies in the film is directly reflected
in orientation dependent wall widths, ranging from approximately 40 nm to 120
nm. The domain walls are of the N\'eel type and evolve from near-$90^{\circ}$
walls at low-temperatures to large angle [1$\bar{1}$0]-oriented walls and small
angle [110]-oriented walls at higher temperatures. | 0802.1574v1 |
2008-02-21 | Solving functional reliability issue for an optical electrostatic switch | In this paper, we report the advantage of using AC actuating signal for
driving MEMS actuators instead of DC voltages. The study is based upon micro
mirror devices used in digital mode for optical switching operation. When the
pull-in effect is used, charge injection occurs when the micro mirror is
maintained in the deflected position. To avoid this effect, a geometrical
solution is to realize grounded landing electrodes which are electro-statically
separated from the control electrodes. Another solution is the use of AC signal
which eliminates charge injection particularly if a bipolar signal is used.
Long term experiments have demonstrated the reliability of such a signal
command to avoid injection of electric charges. | 0802.3075v1 |
2008-02-21 | First principles calculation of spin-interactions and magnetic ground states of Cr trimers on Au(111) | We present calculations of the magnetic ground states of Cr trimers in
different geometries on top of a Au(111) surface. By using a least square fit
method based on a fully relativistic embedded-cluster Green's function method
first we determined the parameters of a classical vector-spin model consisting
of second and fourth order interactions. The newly developed method requires no
symmetry constraints, therefore, it is throughout applicable for small
nanoparticles of arbitrary geometry. The magnetic ground states were then found
by solving the Landau-Lifshitz-Gilbert equations. In all considered cases the
configurational energy of the Cr trimers is dominated by large
antiferromagnetic nearest neighbor interactions, whilst biquadratic
spin-interactions have the second largest contributions to the energy. We find
that an equilateral Cr trimer exhibits a frustrated 120$^\circ$ N\'eel type of
ground state with a small out-of-plane component of the magnetization and we
show that the Dzyaloshinsky-Moriya interactions determine the chirality of the
magnetic ground state. In cases of a linear chain and an isosceles trimer
collinear antiferromagnetic ground states are obtained with a magnetization
lying parallel to the surface. | 0802.3113v1 |
2008-04-07 | Woven Graph Codes: Asymptotic Performances and Examples | Constructions of woven graph codes based on constituent block and
convolutional codes are studied. It is shown that within the random ensemble of
such codes based on $s$-partite, $s$-uniform hypergraphs, where $s$ depends
only on the code rate, there exist codes satisfying the Varshamov-Gilbert (VG)
and the Costello lower bound on the minimum distance and the free distance,
respectively. A connection between regular bipartite graphs and tailbiting
codes is shown. Some examples of woven graph codes are presented. Among them an
example of a rate $R_{\rm wg}=1/3$ woven graph code with $d_{\rm free}=32$
based on Heawood's bipartite graph and containing $n=7$ constituent rate
$R^{c}=2/3$ convolutional codes with overall constraint lengths $\nu^{c}=5$ is
given. An encoding procedure for woven graph codes with complexity proportional
to the number of constituent codes and their overall constraint length
$\nu^{c}$ is presented. | 0804.0996v2 |
2008-05-26 | Photometric Follow-up Observations of the Transiting Neptune-Mass Planet GJ 436b | This paper presents multi-band photometric follow-up observations of the
Neptune-mass transiting planet GJ 436b, consisting of 5 new ground-based
transit light curves obtained in May 2007. Together with one already published
light curve we have at hand a total of 6 light curves, spanning 29 days. The
analysis of the data yields an orbital period P = 2.64386+-0.00003 days,
mid-transit time T_c [HJD] =2454235.8355+-0.0001, planet mass M_p = 23.1+-0.9
M_{\earth} = 0.073+-0.003 M_{Jup}, planet radius R_p = 4.2+-0.2 R_{\earth} =
0.37+-0.01 R_{Jup} and stellar radius R_s = 0.45+-0.02 R_{\sun}. Our typical
precision for the mid transit timing for each transit is about 30 seconds. We
searched the data for a possible signature of a second planet in the system
through transit timing variations (TTV) and variation of the impact parameter.
The analysis could not rule out a small, of the order of a minute, TTV and a
long-term modulation of the impact parameter, of the order of +0.2 year^{-1}. | 0805.3915v2 |
2008-06-04 | Broadband electrical detection of spin excitations in (Ga,Mn)As using a photovoltage technique | We report on microwave photovoltage and simultaneous magnetotransport
measurements in a (Ga,Mn)As film oriented normal to the magnetic field. We
detect the ferromagnetic resonance over a broad frequency range of 2 GHz to
18.5 GHz and determine the spectroscopic g-factor and separate the Gilbert from
the inhomogeneous contribution to magnetization relaxation. Temperature
dependent measurements below the saturation magnetization indicate that the
photovoltage signal can serve as a sensitive tool to study the crystal
anisotropy. We demonstrate that the combination of spin dynamics with charge
transport is a promising tool to study microstructured ferromagnetic
semiconductor samples. | 0806.0785v1 |
2008-08-05 | A Proof of George Andrews' and Dave Robbins' q-TSPP Conjecture (modulo a finite amount of routine calculations) | In the historic conference Combinatoire Enumerative[LL] wonderfully organized
by Gilbert Labelle and Pierre Leroux there were many stimulating lectures,
including a very interesting one by Pierre Leroux himself, who talked about his
joint work with Xavier Viennot[LV], on solving differential equations
combinatorially! During the problem session of that very same colloque, chaired
by Pierre Leroux, Richard Stanley raised some intriguing problems about the
enumeration of plane partitions, that he later expanded into a fascinating
article[Sta1]. Most of these problems concerned the enumeration of symmetry
classes of plane partitions, that were discussed in more detail in another
article of Stanley[Sta2]. All of the conjectures in the latter article have
since been proved (see Dave Bressoud's modern classic[B]), except one, that, so
far, resisted the efforts of the greatest minds in enumerative combinatorics.
It concerns the proof of an explicit formula for the q-enumeration of totally
symmetric plane partitions, conjectured independently by George Andrews and
Dave Robbins([Sta2],[Sta1](conj. 7), [B](conj. 13)). In this tribute to Pierre
Leroux, we describe how to prove that last stronghold. | 0808.0571v2 |
2008-08-09 | On some deterministic dictionaries supporting sparsity | We describe a new construction of an incoherent dictionary, referred to as
the oscillator dictionary, which is based on considerations in the
representation theory of finite groups. The oscillator dictionary consists of
order of p^5 unit vectors in a Hilbert space of dimension p, where p is an odd
prime, whose pairwise inner products have magnitude of at most 4/sqrt(p). An
explicit algorithm to construct a large portion of the oscillator dictionary is
presented. | 0808.1368v2 |
2008-08-13 | Spin Filter, Spin Amplifier and Other Spintronic Applications in Graphene Nanodisks | Graphene nanodisk is a graphene derivative with a closed edge. The trigonal
zigzag nanodisk with size $N$ has $N$-fold degenerated zero-energy states. A
nanodisk can be interpletted as a quantum dot with an internal degree of
freedom. The grand state of nanodisk has been argued to be a quasi-ferromagnet,
which is a ferromagnetic-like states with a finite but very long life time. We
investigate the spin-filter effects in the system made of nanodisks and leads
based on the master equation. The finite-size effect on spin filter is
intriguing due to a reaction from the polarization of incoming current to a
quasi-ferromagnet. Analyzing the relaxation process with the use of the
Landau-Lifshitz-Gilbert equation, we explore the response to four types of
incoming currents, namely, unpolarized current, perfectly polarized current,
partially polarized current and pulse polarized current. We propose some
applications for spintronics, such as spin memory, spin amplifier, spin valve,
spin-field-effect transistor and spin diode. | 0808.1779v1 |
2008-09-26 | Ultra-fast spin dynamics: the effect of colored noise | Recent experimental results have pushed the limits of magnetization dynamics
to pico- and femtosecond timescales. This ultra-fast spin dynamics occurs in
extreme conditions of strong and rapidly varying fields and high temperatures.
This situation requires new description of magnetization dynamics, even on a
phenomenological level of the atomistic Landau-Lifshitz-Gilbert equation,
taking into account that the correlation time for electron system could be of
the order of the inverse characteristic spin frequency. For this case we
introduce the thermodynamically correct phenomenological approach for spin
dynamics based on the Landau-Lifshitz-Miyasaki-Seki equation. The influence of
the noise correlation time on longitudinal and transverse magnetization
relaxation is investigated. We also demonstrate the effect of the noise
correlation time on demagnetisation rate of different materials during
laser-induced dynamics. | 0809.4595v1 |
2008-10-08 | Magnetic particle hyperthermia: Neel relaxation in magnetic nanoparticles under circularly polarized field | The mechanism of magnetization reversal in single-domain ferromagnetic
particles is of interest in many applications, in most of which losses must be
minimized. In cancer therapy by hyperthermia the opposite requirement prevails:
the specific loss power should be maximized. Of the mechanisms of dissipation,
here we study the effect of Neel relaxation on magnetic nanoparticles unable to
move or rotate and compare the losses in linearly and circularly polarized
field. We present exact analytical solutions of the Landau-Lifshitz equation as
derived from the Gilbert equation and use the calculated time-dependent
magnetizations to find the energy loss per cycle. In frequencies lower than the
Larmor frequency linear polarization is found to be the better source of heat
power, at high frequencies (beyond the Larmor frequency) circular polarization
is preferable. | 0810.1455v2 |
2008-10-10 | SOSEMANUK: a fast software-oriented stream cipher | Sosemanuk is a new synchronous software-oriented stream cipher, corresponding
to Profile 1 of the ECRYPT call for stream cipher primitives. Its key length is
variable between 128 and 256 bits. It ac- commodates a 128-bit initial value.
Any key length is claimed to achieve 128-bit security. The Sosemanuk cipher
uses both some basic design principles from the stream cipher SNOW 2.0 and some
transformations derived from the block cipher SERPENT. Sosemanuk aims at
improv- ing SNOW 2.0 both from the security and from the efficiency points of
view. Most notably, it uses a faster IV-setup procedure. It also requires a
reduced amount of static data, yielding better performance on several
architectures. | 0810.1858v1 |
2008-11-14 | Trees of cylinders and canonical splittings | Let T be a tree with an action of a finitely generated group G. Given a
suitable equivalence relation on the set of edge stabilizers of T (such as
commensurability, co-elementarity in a relatively hyperbolic group, or
commutation in a commutative transitive group), we define a tree of cylinders
T_c. This tree only depends on the deformation space of T; in particular, it is
invariant under automorphisms of G if T is a JSJ splitting. We thus obtain
Out(G)-invariant cyclic or abelian JSJ splittings. Furthermore, T_c has very
strong compatibility properties (two trees are compatible if they have a common
refinement). | 0811.2383v2 |
2008-11-21 | Numerical Study of Current-Induced Domain-Wall Dynamics: Crossover from Spin Transfer to Momentum Transfer | We study current-induced dynamics of a magnetic domain wall by solving a
time-dependent Schr\"{o}dinger equation combined with Landau-Lifshitz-Gilbert
equation in a one-dimensional electron system coupled to localized spins. Two
types of domain-wall motions are observed depending on the hard-axis
anisotropy, $K_{\perp}$, of the localized spin system. For small values of
$K_{\perp}$, the magnetic domain wall shows a streaming motion driven by spin
transfer. In contrast, for large values of $K_{\perp}$, a stick-slip motion
driven by momentum transfer is obtained. We clarify the origin of these
characters of domain-wall motions in terms of the dynamics of one-particle
energy levels and distribution functions. | 0811.3545v2 |
2008-12-02 | Application of Conformal Mapping to the determination of Magnetic Moment Distributions in typical Antidot Film Nanostructures | There has been an increasing technological interest on magnetic thin films
containing antidot arrays of hexagonal or square symmetry. Part of this
interest is related to the possibility of domain formation and pinning at the
antidots boundaries. In this paper, we develop a method for the calculation of
the magnetic moment distribution for such arrays which concentrates on the
immediate vicinity of each antidot. For each antidot distribution (square or
hexagonal) a suitable system of coordinates is defined to exploit the shape of
the unit-cells of the overall nanostructure. The Landau-Lifshitz-Gilbert-Brown
equations that govern the distribution of moments are rewritten in terms of
these coordinates. The equilibrium moments orientation is calculated for each
position in a Cartesian grid defined for these new coordinate systems, and then
a conformal transformation is applied to insert the moment vectors into the
actual unit-cell. The resulting vector maps display quite clearly regions of
different moment orientation around the antidots, which can be associated with
nanoscale domains. These results are similar to the ones obtained by other
authors[1-4] using the NIST oommf method. | 0812.0566v1 |
2009-01-09 | A Better Way to Deal the Cards | This paper considers the effect of riffle shuffling on decks of cards,
allowing for some cards to be indistinguishable from other cards. The dual
problem of dealing a game with hands, such as bridge or poker, is also
considered. The Gilbert-Shannon-Reeds model of card shuffling is used, along
with variation distance for measuring how close to uniform a deck has become.
The surprising results are that for a deck with only two types of cards (such
as red and black), the shuffler can greatly improve the randomness of the deck
by insuring that the top and bottom cards are the same before shuffling. And in
the case of dealing cards for a game with "hands", such as bridge or poker, the
normal method of dealing cyclically around the table is very far from optimal.
In the case of a well-shuffled bridge deck, changing to another dealing method
is as good as doing 3.7 extra shuffles. How the deck is cut in poker affects
its randomness as well. | 0901.1324v4 |
2009-03-12 | Accuracy thresholds of topological color codes on the hexagonal and square-octagonal lattices | Accuracy thresholds of quantum error correcting codes, which exploit
topological properties of systems, defined on two different arrangements of
qubits are predicted. We study the topological color codes on the hexagonal
lattice and on the square-octagonal lattice by the use of mapping into the spin
glass systems. The analysis for the corresponding spin glass systems consists
of the duality, and the gauge symmetry, which has succeeded in deriving
locations of special points, which are deeply related with the accuracy
thresholds of topological error correcting codes. We predict that the accuracy
thresholds for the topological color codes would be $1-p_c = 0.1096-8 $ for the
hexagonal lattice and $1-p_c = 0.1092-3$ for the square-octagonal lattice,
where $1-p$ denotes the error probability on each qubit. Hence both of them are
expected to be slightly lower than the probability $1-p_c = 0.110028$ for the
quantum Gilbert-Varshamov bound with a zero encoding rate. | 0903.2102v5 |
2009-03-12 | Designing Optimal Flow Networks | We investigate the problem of designing a minimum cost flow network
interconnecting n sources and a single sink, each with known locations and
flows. The network may contain other unprescribed nodes, known as Steiner
points. For concave increasing cost functions, a minimum cost network of this
sort has a tree topology, and hence can be called a Minimum Gilbert
Arborescence (MGA). We characterise the local topological structure of Steiner
points in MGAs for linear cost functions. This problem has applications to the
design of drains, gas pipelines and underground mine access. | 0903.2124v1 |
2009-04-13 | Refined Coding Bounds and Code Constructions for Coherent Network Error Correction | Coherent network error correction is the error-control problem in network
coding with the knowledge of the network codes at the source and sink nodes.
With respect to a given set of local encoding kernels defining a linear network
code, we obtain refined versions of the Hamming bound, the Singleton bound and
the Gilbert-Varshamov bound for coherent network error correction. Similar to
its classical counterpart, this refined Singleton bound is tight for linear
network codes. The tightness of this refined bound is shown by two construction
algorithms of linear network codes achieving this bound. These two algorithms
illustrate different design methods: one makes use of existing network coding
algorithms for error-free transmission and the other makes use of classical
error-correcting codes. The implication of the tightness of the refined
Singleton bound is that the sink nodes with higher maximum flow values can have
higher error correction capabilities. | 0904.1897v2 |
2009-04-27 | Effect of resistance feedback on spin torque-induced switching of nanomagnets | In large magnetoresistance devices spin torque-induced changes in resistance
can produce GHz current and voltage oscillations which can affect magnetization
reversal. In addition, capacitive shunting in large resistance devices can
further reduce the current, adversely affecting spin torque switching. Here, we
simultaneously solve the Landau-Lifshitz-Gilbert equation with spin torque and
the transmission line telegrapher's equations to study the effects of
resistance feedback and capacitance on magnetization reversal of both spin
valves and magnetic tunnel junctions. While for spin valves parallel (P) to
anti-parallel (AP) switching is adversely affected by the resistance feedback
due to saturation of the spin torque, in low resistance magnetic tunnel
junctions P-AP switching is enhanced. We study the effect of resistance
feedback on the switching time of MTJ's, and show that magnetization switching
is only affected by capacitive shunting in the pF range. | 0904.4159v2 |
2009-04-30 | A microscopic model for current-induced switching of magnetization for half-metallic leads | We study the behaviour of the magnetization in a half-metallic
ferromagnet/nonmagnetic insulator/ferromagnetic metal/paramagnetic metal
(FM1/NI/FM2/PM) tunnel junction. It is calculated self-consistently within the
nonequilibrium Keldysh formalism. The magnetic regions are treated as band
ferromagnets and are described by the single-band Hubbard model. We developed a
nonequilibrium spectral density approach to solve the Hubbard model
approximately in the switching magnet. By applying a voltage to the junction it
is possible to switch between antiparallel (AP) and parallel (P) alignment of
the magnetizations of the two ferromagnets. The transition from AP to P occurs
for positive voltages while the inverse transition from P to AP can be induced
by negative voltages only. This behaviour is in agreement with the Slonczewski
model of current-induced switching and appears self-consistently within the
model, i.e. without using half-classical methods like the
Landau-Lifshitz-Gilbert equation. | 0904.4821v1 |
2009-05-26 | Scattering Theory of Charge-Current Induced Magnetization Dynamics | In ferromagnets, charge currents can excite magnons via the spin-orbit
coupling. We develop a novel and general scattering theory of charge current
induced macrospin magnetization torques in normal metal$|$ferromagnet$|$normal
metal layers. We apply the formalism to a dirty GaAs$|$(Ga,Mn)As$|$GaAs system.
By computing the charge current induced magnetization torques and solving the
Landau-Lifshitz-Gilbert equation, we find magnetization switching for current
densities as low as $ 5\times 10^{6}$~A/cm$^2$. Our results are in agreement
with a recent experimental observation of charge-current induced magnetization
switching in (Ga,Mn)As. | 0905.4170v2 |
2009-05-28 | Riffle shuffles of a deck with repeated cards | We study the Gilbert-Shannon-Reeds model for riffle shuffles and ask
'How many times must a deck of cards be shuffled for the deck to be in close
to random order?'. In 1992, Bayer and Diaconis gave a solution which gives
exact and asymptotic results for all decks of practical interest, e.g. a deck
of 52 cards. But what if one only cares about the colors of the cards or
disregards the suits focusing solely on the ranks? More generally, how does the
rate of convergence of a Markov chain change if we are interested in only
certain features? Our exploration of this problem takes us through random walks
on groups and their cosets, discovering along the way exact formulas leading to
interesting combinatorics, an 'amazing matrix', and new analytic methods which
produce a completely general asymptotic solution that is remarkable accurate. | 0905.4698v1 |
2009-07-20 | Correcting Limited-Magnitude Errors in the Rank-Modulation Scheme | We study error-correcting codes for permutations under the infinity norm,
motivated by a novel storage scheme for flash memories call rank modulation. In
this scheme, a set of $n$ flash cells are combined to create a single virtual
multi-level cell. Information is stored in the permutation induced by the cell
charge levels. Spike errors, which are characterized by a limited-magnitude
change in cell charge levels, correspond to a low-distance change under the
infinity norm.
We define codes protecting against spike errors, called limited-magnitude
rank-modulation codes (LMRM codes), and present several constructions for these
codes, some resulting in optimal codes. These codes admit simple recursive, and
sometimes direct, encoding and decoding procedures.
We also provide lower and upper bounds on the maximal size of LMRM codes both
in the general case, and in the case where the codes form a subgroup of the
symmetric group. In the asymptotic analysis, the codes we construct out-perform
the Gilbert-Varshamov-like bound estimate. | 0907.3387v2 |
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