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2015-08-19
|
Variations in solar wind fractionation as seen by ACE/SWICS over a solar cycle and the implications for Genesis Mission results
|
We use ACE/SWICS elemental composition data to compare the variations in
solar wind fractionation as measured by SWICS during the last solar maximum
(1999-2001), the solar minimum (2006-2009) and the period in which the Genesis
spacecraft was collecting solar wind (late 2001 - early 2004). We differentiate
our analysis in terms of solar wind regimes (i.e. originating from interstream
or coronal hole flows, or coronal mass ejecta). Abundances are normalized to
the low-FIP ion magnesium to uncover correlations that are not apparent when
normalizing to high-FIP ions. We find that relative to magnesium, the other
low-FIP elements are measurably fractionated, but the degree of fractionation
does not vary significantly over the solar cycle. For the high-FIP ions,
variation in fractionation over the solar cycle is significant: greatest for
Ne/Mg and C/Mg, less so for O/Mg, and the least for He/Mg. When abundance
ratios are examined as a function of solar wind speed, we find a strong
correlation, with the remarkable observation that the degree of fractionation
follows a mass-dependent trend. We discuss the implications for correcting the
Genesis sample return results to photospheric abundances.
|
1508.04566v1
|
2015-08-28
|
Cosmic variance in the nanohertz gravitational wave background
|
We use large N-body simulations and empirical scaling relations between dark
matter halos, galaxies, and supermassive black holes to estimate the formation
rates of supermassive black hole binaries and the resulting low-frequency
stochastic gravitational wave background (GWB). We find this GWB to be
relatively insensitive ($\lesssim10\%$) to cosmological parameters, with only
slight variation between WMAP5 and Planck cosmologies. We find that uncertainty
in the astrophysical scaling relations changes the amplitude of the GWB by a
factor of $\sim 2$. Current observational limits are already constraining this
predicted range of models. We investigate the Poisson variance in the amplitude
of the GWB for randomly-generated populations of supermassive black holes,
finding a scatter of order unity per frequency bin below 10 nHz, and increasing
to a factor of $\sim 10$ near 100 nHz. This variance is a result of the rarity
of the most massive binaries, which dominate the signal, and acts as a
fundamental uncertainty on the amplitude of the underlying power law spectrum.
This Poisson uncertainty dominates at $\gtrsim 20$ nHz, while at lower
frequencies the dominant uncertainty is related to our poor understanding of
the astrophysical scaling relations, although very low frequencies may be
dominated by uncertainties related to the final parsec problem and the
processes which drive binaries to the gravitational wave dominated regime.
Cosmological effects are negligible at all frequencies.
|
1508.07336v2
|
2015-09-01
|
Magnon-driven longitudinal spin Seebeck effect in F|N and N|F|N structures: role of asymmetric in-plane magnetic anisotropy
|
The influence of an asymmetric in-plane magnetic anisotropy on the thermally
activated spin current is studied theoretically for two different systems; (i)
the system consisting of a ferromagnetic insulator in a direct contact with a
nonmagnetic metal, and the sandwich structure consisting of a ferromagnetic
insulating part sandwiched between two nonmagnetic metals. It is shown that
when the difference between the temperatures of the two nonmagnetic metals in a
structure is not large, the spin pumping currents from the magnetic part to the
nonmagnetic ones are equal in amplitude and have opposite directions, so only
the spin torque current contributes to the total spin current. The spin current
flows then from the nonmagnetic metal with the higher temperature to the
nonmagnetic metal having a lower temperature. Its amplitude varies linearly
with the difference in temperatures. In addition, we have found that if the
magnetic anisotropy is in the layer plane, then the spin current increases with
the magnon temperature, while in the case of an out-of-plane magnetic
anisotropy the spin current decreases when the magnon temperature enhances.
Enlarging the difference between the temperatures of the nonmagnetic metals,
the linear response becomes important, as confirmed by analytical expressions
inferred from the Fokker-Planck approach and by the results obtained upon a
full numerical integration of the stochastic Landau-Lifshitz-Gilbert equation.
|
1509.00376v1
|
2015-09-08
|
Stellar Dynamics around a Massive Black Hole II: Resonant Relaxation
|
We present a first-principles theory of Resonant Relaxation (RR) of a low
mass stellar system orbiting a more massive black hole (MBH). We first extend
the kinetic theory of Gilbert (1968) to include the Keplerian field of a black
hole of mass $M_\bullet$. Specializing to a Keplerian stellar system of mass $M
\ll M_\bullet$, we use the orbit-averaging method of Sridhar & Touma (2015;
Paper I) to derive a kinetic equation for RR. This describes the collisional
evolution of a system of $N \gg 1$ Gaussian Rings in a reduced 5-dim space,
under the combined actions of self-gravity, 1 PN and 1.5 PN relativistic
effects of the MBH and an arbitrary external potential. In general geometries
RR is driven by both apsidal and nodal resonances, so the distinction between
scalar-RR and vector-RR disappears. The system passes through a sequence of
quasi-steady secular collisionless equilibria, driven by irreversible 2-Ring
correlations that accrue through gravitational interactions, both direct and
collective. This correlation function is related to a `wake function', which is
the linear response of the system to the perturbation of a chosen Ring. The
wake function is easier to appreciate, and satisfies a simpler equation, than
the correlation function. We discuss general implications for the interplay of
secular dynamics and non-equilibrium statistical mechanics in the evolution of
Keplerian stellar systems toward secular thermodynamic equilibria, and set the
stage for applications to the RR of axisymmetric discs in Paper III.
|
1509.02401v2
|
2015-10-11
|
End-to-End Error-Correcting Codes on Networks with Worst-Case Symbol Errors
|
The problem of coding for networks experiencing worst-case symbol errors is
considered. We argue that this is a reasonable model for highly dynamic
wireless network transmissions. We demonstrate that in this setup prior network
error-correcting schemes can be arbitrarily far from achieving the optimal
network throughput. A new transform metric for errors under the considered
model is proposed. Using this metric, we replicate many of the classical
results from coding theory. Specifically, we prove new Hamming-type,
Plotkin-type, and Elias-Bassalygo-type upper bounds on the network capacity. A
commensurate lower bound is shown based on Gilbert-Varshamov-type codes for
error-correction. The GV codes used to attain the lower bound can be
non-coherent, that is, they do not require prior knowledge of the network
topology. We also propose a computationally-efficient concatenation scheme. The
rate achieved by our concatenated codes is characterized by a Zyablov-type
lower bound. We provide a generalized minimum-distance decoding algorithm which
decodes up to half the minimum distance of the concatenated codes. The
end-to-end nature of our design enables our codes to be overlaid on the
classical distributed random linear network codes [1]. Furthermore, the
potentially intensive computation at internal nodes for the link-by-link
error-correction is un-necessary based on our design.
|
1510.03060v1
|
2015-10-12
|
Analysis of laser shock experiments on precompressed samples using a quartz reference and application to warm dense hydrogen and helium
|
Megabar (1 Mbar = 100 GPa) laser shocks on precompressed samples allow
reaching unprecedented high densities and moderately high 10000-100000K
temperatures. We describe here a complete analysis framework for the
velocimetry (VISAR) and pyrometry (SOP) data produced in these experiments.
Since the precompression increases the initial density of both the sample of
interest and the quartz reference for pressure-density, reflectivity and
temperature measurements, we describe analytical corrections based on available
experimental data on warm dense silica and density-functional-theory based
molecular dynamics computer simulations. Using our improved analysis framework
we report a re-analysis of previously published data on warm dense hydrogen and
helium, compare the newly inferred pressure, density and temperature data with
most advanced equation of state models and provide updated reflectivity values.
|
1510.03301v1
|
2015-11-09
|
Simulations of the Pairwise Kinematic Sunyaev-Zel'dovich Signal
|
The pairwise kinematic Sunyaev-Zel'dovich (kSZ) signal from galaxy clusters
is a probe of their line-of-sight momenta, and thus a potentially valuable
source of cosmological information. In addition to the momenta, the amplitude
of the measured signal depends on the properties of the intra-cluster gas and
observational limitations such as errors in determining cluster centers and
redshifts. In this work we simulate the pairwise kSZ signal of clusters at z<1,
using the output from a cosmological N-body simulation and including the
properties of the intra-cluster gas via a model that can be varied in
post-processing. We find that modifications to the gas profile due to star
formation and feedback reduce the pairwise kSZ amplitude of clusters by ~50%,
relative to the naive 'gas traces mass' assumption. We demonstrate that
mis-centering can reduce the overall amplitude of the pairwise kSZ signal by up
to 10%, while redshift errors can lead to an almost complete suppression of the
signal at small separations. We confirm that a high-significance detection is
expected from the combination of data from current-generation, high-resolution
CMB experiments, such as the South Pole Telescope, and cluster samples from
optical photometric surveys, such as the Dark Energy Survey. Furthermore, we
forecast that future experiments such as Advanced ACTPol in conjunction with
data from the Dark Energy Spectroscopic Instrument will yield detection
significances of at least 20{\sigma}, and up to 57{\sigma} in an optimistic
scenario. Our simulated maps are publicly available at:
http://www.hep.anl.gov/cosmology/ksz.html
|
1511.02843v2
|
2015-11-11
|
Magnetization switching by current and microwaves
|
We propose a theoretical model of magnetization switching in a ferromagnetic
multilayer by both electric current and microwaves. The electric current gives
a spin transfer torque on the magnetization, while the microwaves induce a
precession of the magnetization around the initial state. Based on numerical
simulation of the Landau-Lifshitz-Gilbert (LLG) equation, it is found that the
switching current is significantly reduced compared with the switching caused
solely by the spin transfer torque when the microwave frequency is in a certain
range. We develop a theory of switching from the LLG equation averaged over a
constant energy curve. It was found that the switching current should be
classified into four regions, depending on the values of the microwave
frequency. Based on the analysis, we derive an analytical formula of the
optimized frequency minimizing the switching current, which is smaller than the
ferromagnetic resonance frequency. We also derive an analytical formula of the
minimized switching current. Both the optimized frequency and the minimized
switching current decrease with increasing the amplitude of the microwave
field. The results will be useful to achieve high thermal stability and low
switching current in spin torque systems simultaneously.
|
1511.03366v2
|
2015-11-13
|
Time-domain numerical modeling of brass instruments including nonlinear wave propagation, viscothermal losses, and lips vibration
|
A time-domain numerical modeling of brass instruments is proposed. On one
hand, outgoing and incoming waves in the resonator are described by the
Menguy-Gilbert model, which incorporates three key issues: nonlinear wave
propagation, viscothermal losses, and a variable section. The non-linear
propagation is simulated by a TVD scheme well-suited to non-smooth waves. The
fractional derivatives induced by the viscothermal losses are replaced by a set
of local-in-time memory variables. A splitting strategy is followed to couple
optimally these dedicated methods. On the other hand, the exciter is described
by a one-mass model for the lips. The Newmark method is used to integrate the
nonlinear ordinary differential equation so-obtained. At each time step, a
coupling is performed between the pressure in the tube and the displacement of
the lips. Finally, an extensive set of validation tests is successfully
completed. In particular, self-sustained oscillations of the lips are simulated
by taking into account the nonlinear wave propagation in the tube. Simulations
clearly indicate that the nonlinear wave propagation has a major influence on
the timbre of the sound, as expected. Moreover, simulations also highlight an
influence on playing frequencies, time envelopes and on the playability of the
low frequencies in the case of a variable lips tension.
|
1511.04247v1
|
2015-11-24
|
Planetary Candidates from the First Year of the K2 Mission
|
The Kepler Space Telescope is currently searching for planets transiting
stars along the ecliptic plane as part of its extended K2 mission. We processed
the publicly released data from the first year of K2 observations (Campaigns 0,
1, 2, and 3) and searched for periodic eclipse signals consistent with
planetary transits. Out of 59,174 targets we searched, we detect 234 planetary
candidates around 208 stars. These candidates range in size from gas giants to
smaller than the Earth, and range in orbital periods from hours to over a
month. We conducted initial reconnaissance spectroscopy of 68 of the brighter
candidate host stars, and present high resolution optical spectra for these
stars. We make all of our data products, including light curves, spectra, and
vetting diagnostics available to users online.
|
1511.07820v2
|
2015-11-25
|
A Search for Water in the Atmosphere of HAT-P-26b Using LDSS-3C
|
The characterization of a physically-diverse set of transiting exoplanets is
an important and necessary step towards establishing the physical properties
linked to the production of obscuring clouds or hazes. It is those planets with
identifiable spectroscopic features that can most effectively enhance our
understanding of atmospheric chemistry and metallicity. The newly-commissioned
LDSS-3C instrument on Magellan provides enhanced sensitivity and suppressed
fringing in the red optical, thus advancing the search for the spectroscopic
signature of water in exoplanetary atmospheres from the ground. Using data
acquired by LDSS-3C and the Spitzer Space Telescope, we search for evidence of
water vapor in the transmission spectrum of the Neptune-mass planet HAT-P-26b.
Our measured spectrum is best explained by the presence of water vapor, a lack
of potassium, and either a high-metallicity, cloud-free atmosphere or a
solar-metallicity atmosphere with a cloud deck at ~10 mbar. The emergence of
multi-scale-height spectral features in our data suggests that future
observations at higher precision could break this degeneracy and reveal the
planet's atmospheric chemical abundances. We also update HAT-P-26b's transit
ephemeris, t_0 = 2455304.65218(25) BJD_TDB, and orbital period, p =
4.2345023(7) days.
|
1511.08226v2
|
2015-12-22
|
Induced voltage in an open wire
|
A puzzle arising from Faraday's law is considered and solved concerning the
question which voltage is induced in an open wire with a time-varying
homogeneous magnetic field. In contrast to closed wires where the voltage is
determined by the time variance of magnetic field and enclosed area, in an open
wire we have to integrate the electric field along the wire. It is found that
the longitudinal electric field contributes with 1/3 and the transverse field
with 2/3 to the induced voltage. In order to find the electric fields the
sources of the magnetic fields are necessary to know. The representation of a
homogeneous and time-varying magnetic field implies unavoidably a certain
symmetry point or symmetry line which depend on the geometry of the source. As
a consequence the induced voltage of an open wire is found to be the area
covered with respect to this symmetry line or point perpendicular to the
magnetic field. This in turn allows to find the symmetry points of a magnetic
field source by measuring the voltage of an open wire placed with different
angles in the magnetic field. We present exactly solvable models for a symmetry
point and for a symmetry line, respectively. The results are applicable to open
circuit problems like corrosion and for astrophysical applications.
|
1512.07133v3
|
2015-12-22
|
Charge transport and vector meson dissociation across the thermal phase transition in lattice QCD with two light quark flavors
|
We compute and analyze correlation functions in the isovector vector channel
at vanishing spatial momentum across the deconfinement phase transition in
lattice QCD. The simulations are carried out at temperatures $T/T_c=0.156, 0.8,
1.0, 1.25$ and $1.67$ with $T_c\simeq203$MeV for two flavors of Wilson-Clover
fermions with a zero-temperature pion mass of $\simeq270$MeV. Exploiting exact
sum rules and applying a phenomenologically motivated ansatz allows us to
determine the spectral function $\rho(\omega,T)$ via a fit to the lattice
correlation function data. From these results we estimate the electrical
conductivity across the deconfinement phase transition via a Kubo formula and
find evidence for the dissociation of the $\rho$ meson by resolving its
spectral weight at the available temperatures. We also apply the Backus-Gilbert
method as a model-independent approach to this problem. At any given frequency,
it yields a local weighted average of the true spectral function. We use this
method to compare kinetic theory predictions and previously published
phenomenological spectral functions to our lattice study.
|
1512.07249v1
|
2016-01-18
|
Search for transiting exoplanets and variable stars in the open cluster NGC 7243
|
We report results of the first five observing campaigns for the open stellar
cluster NGC 7243 in the frame of project Young Exoplanet Transit Initiative
(YETI). The project focuses on the monitoring of young and nearby stellar
clusters, with the aim to detect young transiting exoplanets, and to study
other variability phenomena on time-scales from minutes to years. After five
observing campaigns and additional observations during 2013 and 2014, a clear
and repeating transit-like signal was detected in the light curve of
J221550.6+495611. Furthermore, we detected and analysed 37 new eclipsing binary
stars in the studied region. The best fit parameters and light curves of all
systems are given. Finally, we detected and analysed 26 new, presumably
pulsating variable stars in the studied region. The follow-up investigation of
these objects, including spectroscopic measurements of the exoplanet candidate,
is currently planned.
|
1601.04562v1
|
2016-01-21
|
Basker: A Threaded Sparse LU Factorization Utilizing Hierarchical Parallelism and Data Layouts
|
Scalable sparse LU factorization is critical for efficient numerical
simulation of circuits and electrical power grids. In this work, we present a
new scalable sparse direct solver called Basker. Basker introduces a new
algorithm to parallelize the Gilbert-Peierls algorithm for sparse LU
factorization. As architectures evolve, there exists a need for algorithms that
are hierarchical in nature to match the hierarchy in thread teams, individual
threads, and vector level parallelism. Basker is designed to map well to this
hierarchy in architectures. There is also a need for data layouts to match
multiple levels of hierarchy in memory. Basker uses a two-dimensional
hierarchical structure of sparse matrices that maps to the hierarchy in the
memory architectures and to the hierarchy in parallelism. We present
performance evaluations of Basker on the Intel SandyBridge and Xeon Phi
platforms using circuit and power grid matrices taken from the University of
Florida sparse matrix collection and from Xyce circuit simulations. Basker
achieves a geometric mean speedup of 5.91x on CPU (16 cores) and 7.4x on Xeon
Phi (32 cores) relative to KLU. Basker outperforms Intel MKL Pardiso (PMKL) by
as much as 53x on CPU (16 cores) and 13.3x on Xeon Phi (32 cores) for low
fill-in circuit matrices. Furthermore, Basker provides 5.4x speedup on a
challenging matrix sequence taken from an actual Xyce simulation.
|
1601.05725v1
|
2016-02-16
|
JSJ decompositions of groups
|
This is an account of the theory of JSJ decompositions of finitely generated
groups, as developed in the last twenty years or so.
We give a simple general definition of JSJ decompositions (or rather of their
Bass-Serre trees), as maximal universally elliptic trees. In general, there is
no preferred JSJ decomposition, and the right object to consider is the whole
set of JSJ decompositions, which forms a contractible space: the JSJ
deformation space (analogous to Outer Space).
We prove that JSJ decompositions exist for any finitely presented group,
without any assumption on edge groups. When edge groups are slender, we
describe flexible vertices of JSJ decompositions as quadratically hanging
extensions of 2-orbifold groups.
Similar results hold in the presence of acylindricity, in particular for
splittings of torsion-free CSA groups over abelian groups, and splittings of
relatively hyperbolic groups over virtually cyclic or parabolic subgroups.
Using trees of cylinders, we obtain canonical JSJ trees (which are invariant
under automorphisms).
We introduce a variant in which the property of being universally elliptic is
replaced by the more restrictive and rigid property of being universally
compatible. This yields a canonical compatibility JSJ tree, not just a
deformation space. We show that it exists for any finitely presented group.
We give many examples, and we work throughout with relative decompositions
(restricting to trees where certain subgroups are elliptic).
|
1602.05139v2
|
2016-03-28
|
Write error rate of spin-transfer-torque random access memory including micromagnetic effects using rare event enhancement
|
Spin-transfer-torque random access memory (STT-RAM) is a promising candidate
for the next-generation of random-access-memory due to improved scalability,
read-write speeds and endurance. However, the write pulse duration must be long
enough to ensure a low write error rate (WER), the probability that a bit will
remain unswitched after the write pulse is turned off, in the presence of
stochastic thermal effects. WERs on the scale of 10$^{-9}$ or lower are
desired. Within a macrospin approximation, WERs can be calculated analytically
using the Fokker-Planck method to this point and beyond. However, dynamic
micromagnetic effects within the bit can affect and lead to faster switching.
Such micromagnetic effects can be addressed via numerical solution of the
stochastic Landau-Lifshitz-Gilbert-Slonczewski (LLGS) equation. However,
determining WERs approaching 10$^{-9}$ would require well over 10$^{9}$ such
independent simulations, which is infeasible. In this work, we explore
calculation of WER using "rare event enhancement" (REE), an approach that has
been used for Monte Carlo simulation of other systems where rare events
nevertheless remain important. Using a prototype REE approach tailored to the
STT-RAM switching physics, we demonstrate reliable calculation of a WER to
10$^{-9}$ with sets of only approximately 10$^{3}$ ongoing stochastic LLGS
simulations, and the apparent ability to go further.
|
1603.08512v2
|
2016-04-04
|
Probing unconventional superconductivity in inversion symmetric doped Weyl semimetal
|
Unconventional superconductivity has been predicted to arise in the
topologically non-trivial Fermi surface of doped inversion symmetric Weyl
semimetals (WSM). In particular, Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) and
nodal BCS states are theoretically predicted to be possible superconductor
pairing states in inversion symmetric doped WSM. In an effort to resolve
preferred pairing state, we theoretically study two separate four terminal
quantum transport methods that each exhibit a unique electrical signature in
the presence of FFLO and nodal BCS states in doped WSMs. We first introduce a
Josephson junction that consists of a doped WSM and an s-wave superconductor in
which we show that the application of a transverse uniform current in s-wave
superconductor effectively cancels the momentum carried by FFLO states in doped
WSM. From our numerical analysis, we find a peak in Josephson current amplitude
at finite uniform current in s-wave superconductor that serves as an indicator
of FFLO states in doped WSMs. Furthermore, we show using a four terminal
measurement configuration that the nodal points may be shifted by an
application of transverse uniform current in doped WSM. We analyze the
topological phase transitions induced by nodal pair annihilation in
non-equilibrium by constructing the phase diagram and we find a characteristic
decrease in the density of states that serves as a signature of the quantum
critical point in the topological phase transition, thereby identifying nodal
BCS states in doped WSM.
|
1604.01040v1
|
2016-04-22
|
Opt: A Domain Specific Language for Non-linear Least Squares Optimization in Graphics and Imaging
|
Many graphics and vision problems can be expressed as non-linear least
squares optimizations of objective functions over visual data, such as images
and meshes. The mathematical descriptions of these functions are extremely
concise, but their implementation in real code is tedious, especially when
optimized for real-time performance on modern GPUs in interactive applications.
In this work, we propose a new language, Opt (available under
http://optlang.org), for writing these objective functions over image- or
graph-structured unknowns concisely and at a high level. Our compiler
automatically transforms these specifications into state-of-the-art GPU solvers
based on Gauss-Newton or Levenberg-Marquardt methods. Opt can generate
different variations of the solver, so users can easily explore tradeoffs in
numerical precision, matrix-free methods, and solver approaches. In our
results, we implement a variety of real-world graphics and vision applications.
Their energy functions are expressible in tens of lines of code, and produce
highly-optimized GPU solver implementations. These solver have performance
competitive with the best published hand-tuned, application-specific GPU
solvers, and orders of magnitude beyond a general-purpose auto-generated
solver.
|
1604.06525v3
|
2016-05-06
|
Spin orbit alignment for KELT-7b and HAT-P-56b via Doppler tomography with TRES
|
We present Doppler tomographic analyses for the spectroscopic transits of
KELT-7b and HAT-P-56b, two hot-Jupiters orbiting rapidly rotating F-dwarf host
stars. These include analyses of archival TRES observations for KELT-7b, and a
new TRES transit observation of HAT-P-56b. We report spin-orbit aligned
geometries for KELT-7b (2.7 +/- 0.6 deg) and HAT-P-56b (8 +/- 2 deg). The host
stars KELT-7 and HAT-P-56 are among some of the most rapidly rotating
planet-hosting stars known. We examine the tidal re-alignment model for the
evolution of the spin-orbit angle in the context of the spin rates of these
stars. We find no evidence that the rotation rates of KELT-7 and HAT-P-56 have
been modified by star-planet tidal interactions, suggesting that the spin-orbit
angle of systems around these hot stars may represent their primordial
configuration. In fact, KELT-7 and HAT-P-56 are two of three systems in
super-synchronous, spin-orbit aligned states, where the rotation periods of the
host stars are faster than the orbital periods of the planets.
|
1605.01991v1
|
2016-06-18
|
Mathematical Foundations of the GraphBLAS
|
The GraphBLAS standard (GraphBlas.org) is being developed to bring the
potential of matrix based graph algorithms to the broadest possible audience.
Mathematically the Graph- BLAS defines a core set of matrix-based graph
operations that can be used to implement a wide class of graph algorithms in a
wide range of programming environments. This paper provides an introduction to
the mathematics of the GraphBLAS. Graphs represent connections between vertices
with edges. Matrices can represent a wide range of graphs using adjacency
matrices or incidence matrices. Adjacency matrices are often easier to analyze
while incidence matrices are often better for representing data. Fortunately,
the two are easily connected by matrix mul- tiplication. A key feature of
matrix mathematics is that a very small number of matrix operations can be used
to manipulate a very wide range of graphs. This composability of small number
of operations is the foundation of the GraphBLAS. A standard such as the
GraphBLAS can only be effective if it has low performance overhead. Performance
measurements of prototype GraphBLAS implementations indicate that the overhead
is low.
|
1606.05790v2
|
2016-07-15
|
Influence of grain size and exchange interaction on the LLB modeling procedure
|
Reliably predicting bit-error rates in realistic heat-assisted magnetic
recording simulations is a challenging task. Integrating the
Landau-Lifshitz-Bloch (LLB) equation can reduce the computational effort to
determine the magnetization dynamics in the vicinity of the Curie temperature.
If one aims that these dynamics coincide with trajectories calculated from the
atomistic Landau-Lifshitz-Gilbert equation, one has to carefully model required
temperature dependent material functions such as the zero-field equilibrium
magnetization as well as the parallel and normal susceptibilities. We present
an extensive study on how these functions depend on grain size and exchange
interactions. We show that, if the size or the exchange constant of a reference
grain is modified, the material functions can be scaled, according to the
changed Curie temperature, yielding negligible errors. This is shown to be
valid for volume changes of up to $\pm 40$ % and variations of the exchange
constant of up to $\pm10$ %. Besides the temperature dependent material curves,
computed switching probabilities also agree well with probabilities separately
determined for each system. Our study suggest that there is no need to
recalculate the required LLB input functions for each particle. Within the
presented limits it is sufficient to scale them to the Curie temperature of the
altered system.
|
1607.04480v1
|
2016-08-23
|
Thermal stability and irreversibility of skyrmion-lattice phases in Cu$_2$OSeO$_3$
|
Small angle neutron scattering measurements have been performed to study the
thermodynamic stability of skyrmion-lattice phases in Cu$_2$OSeO$_3$. We found
that the two distinct skyrmion-lattice phases [SkX(1) and SkX(2) phases] can be
stabilized through different thermal histories; by cooling from the
paramagnetic phase under finite magnetic field, the SkX(2) phase is selected.
On the other hand, the 30$^{\circ}$-rotated SkX(1) phase becomes dominant by
heating the sample from the ordered conical phase under finite field. This
difference in stabilization is surprisingly similar to the irreversibility
observed in spin glasses. The zero-field cooling results in the co-existence of
the two phases. It is further found that once one of the skyrmion-lattice
phases is formed, it is hardly destabilized. This indicates unusual thermal
stability of the two skyrmion-lattice phases originating from an unexpectedly
large energy barrier between them.
|
1608.06359v2
|
2016-08-24
|
Carbon Stars in the Satellites and Halo of M31
|
We spectroscopically identify a sample of carbon stars in the satellites and
halo of M31 using moderate-resolution optical spectroscopy from the
Spectroscopic and Photometric Landscape of Andromeda's Stellar Halo survey. We
present the photometric properties of our sample of 41 stars, including their
brightness with respect to the tip of the red giant branch (TRGB) and their
distributions in various color-color spaces. This analysis reveals a bluer
population of carbon stars fainter than the TRGB and a redder population of
carbon stars brighter than the TRGB. We then apply principal component analysis
to determine the sample's eigenspectra and eigencoefficients. Correlating the
eigencoefficients with various observable properties reveals the spectral
features that trace effective temperature and metallicity. Putting the
spectroscopic and photometric information together, we find the carbon stars in
the satellites and halo of M31 to be minimally impacted by dust and internal
dynamics. We also find that while there is evidence to suggest that the
sub-TRGB stars are extrinsic in origin, it is also possible that they are are
particularly faint members of the asymptotic giant branch.
|
1608.06714v1
|
2016-09-12
|
Discovery and Precise Characterization by the MEarth Project of LP 661-13, an Eclipsing Binary Consisting of Two Fully Convective Low-mass Stars
|
We report the detection of stellar eclipses in the LP 661-13 system. We
present the discovery and characterization of this system, including high
resolution spectroscopic radial velocities and a photometric solution spanning
two observing seasons. LP 661-13 is a low mass binary system with an orbital
period of $4.7043512^{+0.0000013}_{-0.0000010}$ days at a distance of $24.9 \pm
1.3$ parsecs. LP 661-13A is a $0.30795 \pm 0.00084$ $M_\odot$ star while LP
661-13B is a $0.19400 \pm 0.00034$ $M_\odot$ star. The radius of each component
is $0.3226 \pm 0.0033$ $R_\odot$ and $0.2174 \pm 0.0023$ $R_\odot$,
respectively. We detect out of eclipse modulations at a period slightly shorter
than the orbital period, implying that at least one of the components is not
rotating synchronously. We find that each component is slightly inflated
compared to stellar models, and that this cannot be reconciled through age or
metallicity effects. As a nearby eclipsing binary system where both components
are near or below the full-convection limit, LP 661-13 will be a valuable test
of models for the structure of cool dwarf stars.
|
1609.03591v1
|
2016-09-14
|
Topological superconductivity in an ultrathin, magnetically-doped topological insulator proximity coupled to a conventional superconductor
|
As a promising candidate system to realize topological superconductivity, the
system of a 3D topological insulator (TI) grown on top of the s-wave
superconductor has been extensively studied. To access the topological
superconductivity experimentally, the 3D TI sample must be thin enough to allow
for Cooper pair tunneling to the exposed surface of TI. The use of magnetically
ordered dopants to break time-reversal symmetry may allow the surface of a TI
to host Majorana fermion, which are believed to be a signature of topological
superconductivity. In this work, we study a magnetically-doped thin film
TI-superconductor hybrid systems. Considering the proximity induced order
parameter in thin film of TI, we analyze the gap closing points of the
Hamiltonian and draw the phase diagram as a function of relevant parameters:
the hybridization gap, Zeeman energy, and chemical potential of the TI system.
Our findings provide a useful guide in choosing relevant parameters to
facilitate the observation of topological superconductivity in thin film
TI-superconductor hybrid systems. In addition, we further perform numerical
analysis on a TI proximity coupled to a s-wave superconductor and find that,
due to the spin-momentum locked nature of the surface states in TI, the induced
s-wave order parameter of the surface states persists even at large magnitude
of the Zeeman energy.
|
1609.04129v2
|
2016-09-16
|
Convex separation from convex optimization for large-scale problems
|
We present a scheme, based on Gilbert's algorithm for quadratic minimization
[SIAM J. Contrl., vol. 4, pp. 61-80, 1966], to prove separation between a point
and an arbitrary convex set $S\subset\mathbb{R}^{n}$ via calls to an oracle
able to perform linear optimizations over $S$. Compared to other methods, our
scheme has almost negligible memory requirements and the number of calls to the
optimization oracle does not depend on the dimensionality $n$ of the underlying
space. We study the speed of convergence of the scheme under different promises
on the shape of the set $S$ and/or the location of the point, validating the
accuracy of our theoretical bounds with numerical examples. Finally, we present
some applications of the scheme in quantum information theory. There we find
that our algorithm out-performs existing linear programming methods for certain
large scale problems, allowing us to certify nonlocality in bipartite scenarios
with upto $42$ measurement settings. We apply the algorithm to upper bound the
visibility of two-qubit Werner states, hence improving known lower bounds on
Grothendieck's constant $K_G(3)$. Similarly, we compute new upper bounds on the
visibility of GHZ states and on the steerability limit of Werner states for a
fixed number of measurement settings.
|
1609.05011v2
|
2016-09-18
|
Narrowly distributed crystal orientation in biomineral vaterite
|
Biominerals formed by animals provide skeletal support, and many other
functions. They were previously shown to grow by aggregation of amorphous
nanoparticles, but never to grow ion-by-ion from solution, which is a common
growth mechanism for abiotic crystals. We analyze vaterite CaCO3 multi
crystalline spicules from the solitary tunicate Herdmania momus, with
Polarization dependent Imaging Contrast PIC mapping, scanning and aberration
corrected transmission electron microscopies. The first fully quantitative PIC
mapping data, presented here, measured 0{\deg} 30{\deg} angle spreads between
immediately adjacent crystals. Such narrowly distributed crystal orientations
demonstrate that crystallinity does not propagate from one crystal to another
0{\deg} angle spreads, nor that new crystals with random orientation 90{\deg}
nucleate. There are no organic layers at the interface between crystals, hence
a new, unknown growth mechanism must be invoked, with crystal nucleation
constrained within 30{\deg}. Two observations are consistent with crystal
growth from solution: vaterite microcrystals express crystal faces, and are
smooth at the nanoscale after cryo fracture. The observation of 30{\deg} angle
spreads, lack of interfacial organic layers, and smooth fracture figures
broadens the range of known biomineralization mechanisms and may inspire novel
synthetic crystal growth strategies. Spherulitic growth from solution is one
possible mechanism consistent with all these observations.
|
1609.05449v1
|
2016-09-29
|
Multiscale Model Approach for Magnetization Dynamics Simulations
|
Simulations of magnetization dynamics in a multiscale environment enable
rapid evaluation of the Landau-Lifshitz-Gilbert equation in a mesoscopic sample
with nanoscopic accuracy in areas where such accuracy is required. We have
developed a multiscale magnetization dynamics simulation approach that can be
applied to large systems with spin structures that vary locally on small length
scales. To implement this, the conventional micromagnetic simulation framework
has been expanded to include a multiscale solving routine. The software
selectively simulates different regions of a ferromagnetic sample according to
the spin structures located within in order to employ a suitable discretization
and use either a micromagnetic or an atomistic model. To demonstrate the
validity of the multiscale approach, we simulate the spin wave transmission
across the regions simulated with the two different models and different
discretizations. We find that the interface between the regions is fully
transparent for spin waves with frequency lower than a certain threshold set by
the coarse scale micromagnetic model with no noticeable attenuation due to the
interface between the models. As a comparison to exact analytical theory, we
show that in a system with Dzyaloshinskii-Moriya interaction leading to spin
spiral, the simulated multiscale result is in good quantitative agreement with
the analytical calculation.
|
1609.09295v1
|
2016-12-21
|
Geometric generalised Lagrangian mean theories
|
Many fluctuation-driven phenomena in fluids can be analysed effectively using
the generalised Lagrangian mean (GLM) theory of Andrews & McIntyre (1978). This
theory relies on particle-following averaging to incorporate the constraints
imposed by the material conservations. It relies implicitly on an Euclidean
structure; as a result, it does not have a geometrically intrinsic
interpretation and suffers from undesirable features, including the divergence
of the Lagrangian-mean velocity for incompressible fluids. Motivated by this,
we develop a geometric generalisation of GLM that we formulate intrinsically.
The theory applies to arbitrary Riemannian manifolds; it also establishes a
clear distinction between results that stem directly from geometric consistency
and those that depend on particular choices. We show that the Lagrangian mean
momentum -- the average of the pull-back of the momentum one-form -- obeys a
simple equation which guarantees the conservation of Kelvin's circulation,
irrespective of the mean-flow definition. We discuss four possible definitions
of the mean flow: a direct extension of standard GLM, a definition based on
optimal transportation, a definition based on a geodesic distance in the group
of volume-preserving diffeomorphisms, and the glm definition proposed by Soward
& Roberts (2010). Assuming small-amplitude perturbations, we carry out
order-by-order calculations to obtain explicit expressions for the mean flow
and pseudomomentum at leading order. We also show how the wave-action
conservation of GLM extends to the geometric setting. To make the paper
self-contained, we introduce the tools of differential geometry and main ideas
of geometric fluid dynamics on which we rely. We mostly focus on the Euler
equations for incompressible inviscid fluids but sketch out extensions to the
rotating-stratified Boussinesq, compressible Euler and magnetohydrodynamic
equations.
|
1612.07111v3
|
2017-01-13
|
A Multi-Planet System Transiting the $V$ = 9 Rapidly Rotating F-Star HD 106315
|
We report the discovery of a multi-planet system orbiting HD 106315, a
rapidly rotating mid F-type star, using data from the K2 mission. HD 106315
hosts a $2.51\pm0.12\,R_\oplus$ sub-Neptune in a 9.5 day orbit, and a
$4.31_{-0.27}^{+0.24}\,R_\oplus$ super-Neptune in a 21 day orbit. The projected
rotational velocity of HD 106315 (12.9 km s$^{-1}$) likely precludes precise
measurements of the planets' masses, but could enable a measurement of the
sky-projected spin-orbit obliquity for the outer planet via Doppler tomography.
The eccentricities of both planets were constrained to be consistent with 0,
following a global modeling of the system that includes a Gaia distance and
dynamical arguments. The HD 106315 system is one of few multi-planet systems
hosting a Neptune-sized planet for which orbital obliquity measurements are
possible, making it an excellent test-case for formation mechanisms of
warm-Neptunian systems. The brightness of the host star also makes HD 106315 c
a candidate for future transmission spectroscopic follow-up studies.
|
1701.03807v2
|
2017-01-17
|
The Landau-Lifshitz equation, the NLS, and the magnetic rogue wave as a by-product of two colliding regular "positons"
|
In this article we present a new method for construction of exact solutions
of the Landau-Lifshitz-Gilbert equation (LLG) for ferromagnetic nanowires. The
method is based on the established relationship between the LLG and the
nonlinear Schr\"odinger equation (NLS), and is aimed at resolving an old
problem: how to produce multiple-rogue wave solutions of NLS using just the
Darboux-type transformations. The solutions of this type - known as P-breathers
- have been proven to exist by Dubard and Matveev, but their technique heavily
relied on using the solutions of yet another nonlinear equation,
Kadomtsev-Petviashvili I equation (KP-I), and its relationship with NLS. We
have shown that in fact one doesn't have to use KP-I but can instead reach the
same results just with NLS solutions, but only if they are dressed via the
binary Darboux transformation. In particular, our approach allows to construct
all the Dubard-Matveev P-breathers. Furthermore, the new method can lead to
some completely new, previously unknown solutions. One particular solution that
we have constructed describes two positon-like waves, colliding with each other
and in the process producing a new, short-lived rogue wave. We called this
unusual solution (rogue wave begotten after the impact of two solitons) the
"impacton".
|
1701.04903v3
|
2017-01-24
|
Partial Bridging of Vaccine Efficacy to New Populations
|
Suppose one has data from one or more completed vaccine efficacy trials and
wishes to estimate the efficacy in a new setting. Often logistical or ethical
considerations make running another efficacy trial impossible. Fortunately, if
there is a biomarker that is the primary modifier of efficacy, then the
biomarker-conditional efficacy may be identical in the completed trials and the
new setting, or at least informative enough to meaningfully bound this
quantity. Given a sample of this biomarker from the new population, we might
hope we can bridge the results of the completed trials to estimate the vaccine
efficacy in this new population. Unfortunately, even knowing the true
conditional efficacy in the new population fails to identify the marginal
efficacy due to the unknown conditional unvaccinated risk. We define a curve
that partially identifies (lower bounds) the marginal efficacy in the new
population as a function of the population's marginal unvaccinated risk, under
the assumption that one can identify bounds on the conditional unvaccinated
risk in the new population. Interpreting the curve only requires identifying
plausible regions of the marginal unvaccinated risk in the new population. We
present a nonparametric estimator of this curve and develop valid lower
confidence bounds that concentrate at a parametric rate. We use vaccine
terminology throughout, but the results apply to general binary interventions
and bounded outcomes.
|
1701.06739v1
|
2017-02-07
|
Resonant spin transfer torque nano-oscillators
|
Spin transfer torque nano-oscillators are potential candidates for replacing
the traditional inductor based voltage controlled oscillators in modern
communication devices. Typical oscillator designs are based on trilayer
magnetic tunnel junctions which are disadvantaged by low power outputs and poor
conversion efficiencies. In this letter, we theoretically propose to use
resonant spin filtering in pentalayer magnetic tunnel junctions as a possible
route to alleviate these issues and present device designs geared toward a high
microwave output power and an efficient conversion of the d.c. input power. We
attribute these robust qualities to the resulting non-trivial spin current
profiles and the ultra high tunnel magnetoresistance, both arising from
resonant spin filtering. The device designs are based on the nonequilibrium
Green's function spin transport formalism self-consistently coupled with the
stochastic Landau-Lifshitz-Gilbert-Slonczewski's equation and the Poisson's
equation. We demonstrate that the proposed structures facilitate oscillator
designs featuring a large enhancement in microwave power of around $775\%$ and
an efficiency enhancement of over $1300\%$ in comparison with typical trilayer
designs. We also rationalize the optimum operating regions via an analysis of
the dynamic and static device resistances. This work sets stage for pentalyer
spin transfer torque nano-oscillator device designs that extenuate most of the
issues faced by the typical trilayer designs.
|
1702.01869v1
|
2017-03-17
|
Communication Primitives in Cognitive Radio Networks
|
Cognitive radio networks are a new type of multi-channel wireless network in
which different nodes can have access to different sets of channels. By
providing multiple channels, they improve the efficiency and reliability of
wireless communication. However, the heterogeneous nature of cognitive radio
networks also brings new challenges to the design and analysis of distributed
algorithms.
In this paper, we focus on two fundamental problems in cognitive radio
networks: neighbor discovery, and global broadcast. We consider a network
containing $n$ nodes, each of which has access to $c$ channels. We assume the
network has diameter $D$, and each pair of neighbors have at least $k\geq 1$,
and at most $k_{max}\leq c$, shared channels. We also assume each node has at
most $\Delta$ neighbors. For the neighbor discovery problem, we design a
randomized algorithm CSeek which has time complexity
$\tilde{O}((c^2/k)+(k_{max}/k)\cdot\Delta)$. CSeek is flexible and robust,
which allows us to use it as a generic "filter" to find "well-connected"
neighbors with an even shorter running time. We then move on to the global
broadcast problem, and propose CGCast, a randomized algorithm which takes
$\tilde{O}((c^2/k)+(k_{max}/k)\cdot\Delta+D\cdot\Delta)$ time. CGCast uses
CSeek to achieve communication among neighbors, and uses edge coloring to
establish an efficient schedule for fast message dissemination.
Towards the end of the paper, we give lower bounds for solving the two
problems. These lower bounds demonstrate that in many situations, CSeek and
CGCast are near optimal.
|
1703.06130v1
|
2017-03-22
|
Magnetization induced dynamics of a Josephson junction coupled to a nanomagnet
|
We study the superconducting current of a Josephson junction (JJ) coupled to
an external nanomagnet driven by a time dependent magnetic field both without
and in the presence of an external AC drive. We provide an analytic, albeit
perturbative, solution for the Landau-Lifshitz (LL) equations governing the
coupled JJ-nanomagnet system in the presence of a magnetic field with arbitrary
time-dependence oriented along the easy axis of the nanomagnet's magnetization
and in the limit of weak dimensionless coupling $\epsilon_0$ between the JJ and
the nanomagnet. We show the existence of Shapiro-like steps in the I-V
characteristics of the JJ subjected to a voltage bias for a constant or
periodically varying magnetic field and explore the effect of rotation of the
magnetic field and the presence of an external AC drive on these steps. We
support our analytic results with exact numerical solution of the LL equations.
We also extend our results to dissipative nanomagnets by providing a
perturbative solution to the Landau-Lifshitz-Gilbert (LLG) equations for weak
dissipation. We study the fate of magnetization-induced Shapiro steps in the
presence of dissipation both from our analytical results and via numerical
solution of the coupled LLG equations. We discuss experiments which can test
our theory.
|
1703.07717v3
|
2017-04-19
|
Integrating optimization with thermodynamics and plant physiology for crop ideotype design
|
A computational framework integrating optimization algorithms, parallel
computing and plant physiology was developed to explore crop ideotype design.
The backbone of the framework is a plant physiology model that accurately
tracks water use (i.e. a plant hydraulic model) coupled with mass transport
(CO2 exchange and transport), energy conversion (leaf temperature due to
radiation, convection and mass transfer) and photosynthetic biochemistry of an
adult maize plant. For a given trait configuration, soil parameters and hourly
weather data, the model computes water use and photosynthetic output over the
life of an adult maize plant. We coupled this validated model with a parallel,
meta-heuristic optimization algorithm, specifically a genetic algorithm (GA),
to identify trait sets (ideotypes) that resulted in desired water use behavior
of the adult maize plant. We detail features of the model as well as the
implementation details of the coupling with the optimization framework and
deployment on high performance computing platforms. We illustrate a
representative result of this framework by identifying maize ideotypes with
optimized photosynthetic yields using weather and soil conditions corresponding
to Davis, CA. Finally, we show how the framework can be used to identify broad
ideotype trends that can inform breeding efforts. The developed presented tool
has the potential to inform the development of future climate-resilient crops.
|
1704.05885v1
|
2017-04-28
|
From deep inelastic scattering to heavy-flavor semi-leptonic decays: Total rates into multi-hadron final states from lattice QCD
|
We present a new technique for extracting decay and transition rates into
final states with any number of hadrons. The approach is only sensitive to
total rates, in which all out-states with a given set of QCD quantum numbers
are included. For processes involving photons or leptons, differential rates
with respect to the non-hadronic kinematics may also be extracted. Our method
involves constructing a finite-volume Euclidean four-point function, whose
corresponding spectral function measures the decay and transition rates in the
infinite-volume limit. This requires solving the inverse problem of extracting
the spectral function from the correlator and also necessitates a smoothing
procedure so that a well-defined infinite-volume limit exists. Both of these
steps are accomplished by the Backus-Gilbert method and, as we show with a
numerical example, reasonable precision can be expected in cases with multiple
open decay channels. Potential applications include nucleon structure functions
and the onset of the deep inelastic scattering regime, as well as semi-leptonic
$D$ and $B$ decay rates.
|
1704.08993v2
|
2017-05-01
|
Measuring galaxy cluster masses with CMB lensing using a Maximum Likelihood estimator: Statistical and systematic error budgets for future experiments
|
We develop a Maximum Likelihood estimator (MLE) to measure the masses of
galaxy clusters through the impact of gravitational lensing on the temperature
and polarization anisotropies of the cosmic microwave background (CMB). We show
that, at low noise levels in temperature, this optimal estimator outperforms
the standard quadratic estimator by a factor of two. For polarization, we show
that the Stokes Q/U maps can be used instead of the traditional E- and B-mode
maps without losing information. We test and quantify the bias in the recovered
lensing mass for a comprehensive list of potential systematic errors. Using
realistic simulations, we examine the cluster mass uncertainties from
CMB-cluster lensing as a function of an experiment's beam size and noise level.
We predict the cluster mass uncertainties will be 3 - 6% for SPT-3G, AdvACT,
and Simons Array experiments with 10,000 clusters and less than 1% for the
CMB-S4 experiment with a sample containing 100,000 clusters. The mass
constraints from CMB polarization are very sensitive to the experimental beam
size and map noise level: for a factor of three reduction in either the beam
size or noise level, the lensing signal-to-noise improves by roughly a factor
of two.
|
1705.00411v2
|
2017-05-03
|
Current driven second harmonic domain wall resonance in ferromagnetic metal/ nonmagnetic metal bilayer: a field-free method for spin Hall angle measurements
|
We study the ac current-driven domain wall motion in bilayer ferromagnetic
metal (FM)/nonmagnetic metal (NM) nanowire. The solution of the modified
Landau-Lifshitz-Gilbert equation including all the spin transfer torques is
used to describe motion of the domain wall in presence of the spin Hall effect.
We show that the domain wall center has second harmonic frequency response in
addition to the known first harmonic excitation. In contrast to the
experimentally observed second harmonic response in harmonic Hall measurements
of spin-orbit torque in magnetic thin films, this second harmonic response
directly originates from spin-orbit torque driven domain wall dynamics. Based
on the spin current generated by domain wall dynamics, the longitudinal spin
motive force generated voltage across the length of the nanowire is determined.
The second harmonic response introduces additionally a new practical field-free
and all-electrical method to probe the effective spin Hall angle for FM/NM
bilayer structures that could be applied in experiments. Our results also
demonstrate the capability of utilizing FM/NM bilayer structure in domain wall
based spin torque signal generators and resonators.
|
1705.01355v5
|
2017-05-20
|
SVM via Saddle Point Optimization: New Bounds and Distributed Algorithms
|
We study two important SVM variants: hard-margin SVM (for linearly separable
cases) and $\nu$-SVM (for linearly non-separable cases). We propose new
algorithms from the perspective of saddle point optimization. Our algorithms
achieve $(1-\epsilon)$-approximations with running time $\tilde{O}(nd+n\sqrt{d
/ \epsilon})$ for both variants, where $n$ is the number of points and $d$ is
the dimensionality. To the best of our knowledge, the current best algorithm
for $\nu$-SVM is based on quadratic programming approach which requires
$\Omega(n^2 d)$ time in worst case~\cite{joachims1998making,platt199912}. In
the paper, we provide the first nearly linear time algorithm for $\nu$-SVM. The
current best algorithm for hard margin SVM achieved by Gilbert
algorithm~\cite{gartner2009coresets} requires $O(nd / \epsilon )$ time. Our
algorithm improves the running time by a factor of $\sqrt{d}/\sqrt{\epsilon}$.
Moreover, our algorithms can be implemented in the distributed settings
naturally. We prove that our algorithms require $\tilde{O}(k(d
+\sqrt{d/\epsilon}))$ communication cost, where $k$ is the number of clients,
which almost matches the theoretical lower bound. Numerical experiments support
our theory and show that our algorithms converge faster on high dimensional,
large and dense data sets, as compared to previous methods.
|
1705.07252v4
|
2017-06-15
|
On the Maximum Size of Block Codes Subject to a Distance Criterion
|
We establish a general formula for the maximum size of finite length block
codes with minimum pairwise distance no less than $d$. The achievability
argument involves an iterative construction of a set of radius-$d$ balls, each
centered at a codeword. We demonstrate that the number of such balls that cover
the entire code alphabet cannot exceed this maximum size. Our approach can be
applied to codes $i)$ with elements over arbitrary code alphabets, and $ii)$
under a broad class of distance measures, thereby ensuring the generality of
our formula. Our formula indicates that the maximum code size can be fully
characterized by the cumulative distribution function of the distance measure
evaluated at two independent and identically distributed random codewords. When
the two random codewords assume a uniform distribution over the entire code
alphabet, our formula recovers and obtains a natural generalization of the
Gilbert-Varshamov (GV) lower bound. We also establish a general formula for the
zero-error capacity of any sequence of channels. Finally, we extend our study
to the asymptotic setting, where we establish first- and second-order bounds on
the asymptotic code rate subject to a normalized minimum distance constraint.
|
1706.04709v2
|
2017-06-19
|
Capability of Detecting Ultra-Violet Counterparts of Gravitational Waves with GLUV
|
With the discovery of gravitational waves (GW), attention has turned towards
detecting counterparts to these sources. In discussions on counterpart
signatures and multi-messenger follow-up strategies to GW detections,
ultra-violet (UV) signatures have largely been neglected, due to UV facilities
being limited to SWIFT, which lacks high-cadence UV survey capabilities. In
this paper, we examine the UV signatures from merger models for the major GW
sources, highlighting the need for further modelling, while presenting
requirements and a design for an effective UV survey telescope. Using $u'$-band
models as an analogue, we find that a UV survey telescope requires a limiting
magnitude of m$_{u'}\rm (AB)\approx 24$ to fully complement the aLIGO range and
sky localisation. We show that a network of small, balloon-based UV telescopes
with a primary mirror diameter of 30~cm could be capable of covering the aLIGO
detection distance from $\sim$60--100\% for BNS events and $\sim$40\% for BHNS
events. The sensitivity of UV emission to initial conditions suggests that a UV
survey telescope would provide a unique dataset, that can act as an effective
diagnostic to discriminate between models.
|
1706.06106v2
|
2017-07-27
|
LCD codes over ${\mathbb F}_q $ are as good as linear codes for q at least four
|
The hull $H(C)$ of a linear code $C$ is defined by $H(C)=C \cap C^\perp$. A
linear code with a complementary dual (LCD) is a linear code with $H(C)=\{0\}$.
The dimension of the hull of a code is an invariant under permutation
equivalence. For binary and ternary codes the dimension of the hull is also
invariant under monomial equivalence and we show that this invariant is
determined by the extended weight enumerator of the code.\\ The hull of a code
is not invariant under monomial equivalence if $q\geq 4$. We show that every
${\mathbb F}_q $-linear code is monomial equivalent with an LCD code in case $q
\geq 4$. The proof uses techniques from Gr\"obner basis theory. We conclude
that if there exists an ${\mathbb F}_q $-linear code with parameters
$[n,k,d]_q$ and $q \geq 4$, then there exists also a LCD code with the same
parameters. Hence this holds for optimal and MDS codes. In particular there
exist LCD codes that are above the Gilbert-Varshamov bound if $q$ is a square
and $q\geq 49$ by the existence of such codes that are algebraic geometric.\\
Similar results are obtained with respect to Hermitian LCD codes.
|
1707.08856v1
|
2017-08-04
|
Energy release in the solar atmosphere from a stream of infalling prominence debris
|
Recent high-resolution and high-cadence EUV imaging has revealed a new
phenomenon, impacting prominence debris, where prominence material from failed
or partial eruptions can impact the lower atmosphere, releasing energy. We
report a clear example of energy release and EUV brightening due to infalling
prominence debris that occurred on 2011 September 7-8. The initial eruption of
material was associated with an X1.8-class flare from AR11283, occurring at
22:30 UT on 2011 September 7. Subsequently, a semi-continuous stream of this
material returned to the solar surface with a velocity v > 150 km/s, impacting
a region remote from the original active region between 00:20 - 00:40 UT on
2011 September 8. Using SDO/AIA, the differential emission measure of the
plasma was estimated throughout this brightening event. We found that the
radiated energy of the impacted plasma was L_rad ~10^27 ergs, while the thermal
energy peaked at ~10^28 ergs. From this we were able to determine the mass
content of the debris to be in the range 2x10^14 < m < 2x10^15 g. Given typical
promimence masses, the likely debris mass is towards the lower end of this
range. This clear example of a prominence debris event shows that significant
energy release takes place during these events, and that such impacts may be
used as a novel diagnostic tool for investigating prominence material
properties.
|
1708.01555v2
|
2017-08-16
|
Magneto Acoustic Spin Hall Oscillators
|
This paper introduces a novel oscillator that combines the tunability of spin
Hall-driven nano oscillators with the high quality factor (Q) of high overtone
bulk acoustic wave resonators (HBAR), integrating both reference and tunable
oscillators on the same chip with CMOS. In such magneto acoustic spin Hall
(MASH) oscillators, voltage oscillations across the magnetic tunnel junction
(MTJ) that arise from a spin-orbit torque (SOT) are shaped by the transmission
response of the HBAR that acts as a multiple peak-bandpass filter and a delay
element due to its large time constant, providing delayed feedback. The
filtered voltage oscillations can be fed back to the MTJ via a) strain, b)
current, or c) magnetic field. We develop a SPICE-based circuit model by
combining experimentally benchmarked models including the stochastic
Landau-Lifshitz-Gilbert (sLLG) equation for magnetization dynamics and the
Butterworth Van Dyke (BVD) circuit for the HBAR. Using the self-consistent
model, we project up to $\sim$ 50X enhancement in the oscillator linewidth with
Q reaching up to 52825 at 3 GHz, while preserving the tunability by locking the
STNO to the nearest high Q peak of the HBAR. We expect that our results will
inspire MEMS-based solutions to spintronic devices by combining attractive
features of both fields for a variety of applications.
|
1708.04735v2
|
2017-09-01
|
An order optimal policy for exploiting idle spectrum in cognitive radio networks
|
In this paper a spectrum sensing policy employing recency-based exploration
is proposed for cognitive radio networks. We formulate the problem of finding a
spectrum sensing policy for multi-band dynamic spectrum access as a stochastic
restless multi-armed bandit problem with stationary unknown reward
distributions. In cognitive radio networks the multi-armed bandit problem
arises when deciding where in the radio spectrum to look for idle frequencies
that could be efficiently exploited for data transmission. We consider two
models for the dynamics of the frequency bands: 1) the independent model where
the state of the band evolves randomly independently from the past and 2) the
Gilbert-Elliot model, where the states evolve according to a 2-state Markov
chain. It is shown that in these conditions the proposed sensing policy attains
asymptotically logarithmic weak regret. The policy proposed in this paper is an
index policy, in which the index of a frequency band is comprised of a sample
mean term and a recency-based exploration bonus term. The sample mean promotes
spectrum exploitation whereas the exploration bonus encourages for further
exploration for idle bands providing high data rates. The proposed recency
based approach readily allows constructing the exploration bonus such that it
will grow the time interval between consecutive sensing time instants of a
suboptimal band exponentially, which then leads to logarithmically increasing
weak regret. Simulation results confirming logarithmic weak regret are
presented and it is found that the proposed policy provides often improved
performance at low complexity over other state-of-the-art policies in the
literature.
|
1709.00237v1
|
2017-09-08
|
EndoSensorFusion: Particle Filtering-Based Multi-sensory Data Fusion with Switching State-Space Model for Endoscopic Capsule Robots
|
A reliable, real time multi-sensor fusion functionality is crucial for
localization of actively controlled capsule endoscopy robots, which are an
emerging, minimally invasive diagnostic and therapeutic technology for the
gastrointestinal (GI) tract. In this study, we propose a novel multi-sensor
fusion approach based on a particle filter that incorporates an online
estimation of sensor reliability and a non-linear kinematic model learned by a
recurrent neural network. Our method sequentially estimates the true robot pose
from noisy pose observations delivered by multiple sensors. We experimentally
test the method using 5 degree-of-freedom (5-DoF) absolute pose measurement by
a magnetic localization system and a 6-DoF relative pose measurement by visual
odometry. In addition, the proposed method is capable of detecting and handling
sensor failures by ignoring corrupted data, providing the robustness expected
of a medical device. Detailed analyses and evaluations are presented using
ex-vivo experiments on a porcine stomach model prove that our system achieves
high translational and rotational accuracies for different types of endoscopic
capsule robot trajectories.
|
1709.03401v3
|
2017-09-12
|
Distributed Scheduling in Time Dependent Environments: Algorithms and Analysis
|
Consider the problem of a multiple access channel in a time dependent
environment with a large number of users. In such a system, mostly due to
practical constraints (e.g., decoding complexity), not all users can be
scheduled together, and usually only one user may transmit at any given time.
Assuming a distributed, opportunistic scheduling algorithm, we analyse the
system's properties, such as delay, QoS and capacity scaling laws.
Specifically, we start with analyzing the performance while \emph{assuming the
users are not necessarily fully backlogged}, focusing on the queueing problem
and, especially, on the \emph{strong dependence between the queues}. We first
extend a known queueing model by Ephremides and Zhu, to give new results on the
convergence of the probability of collision to its average value (as the number
of users grows), and hence for the ensuing system performance metrics, such as
throughput and delay. This model, however, is limited in the number of users
one can analyze. We thus suggest a new model, which is much simpler yet can
accurately describes the system behaviour when the number of users is large.
We then proceed to the analysis of this system under the assumption of time
dependent channels. Specifically, we assume each user experiences a different
channel state sequence, expressing different channel fluctuations
(specifically, the Gilbert-Elliott model). The system performance under this
setting is analysed, along with the channel capacity scaling laws.
|
1709.04361v1
|
2017-09-16
|
A differential memristive synapse circuit for on-line learning in neuromorphic computing systems
|
Spike-based learning with memristive devices in neuromorphic computing
architectures typically uses learning circuits that require overlapping pulses
from pre- and post-synaptic nodes. This imposes severe constraints on the
length of the pulses transmitted in the network, and on the network's
throughput. Furthermore, most of these circuits do not decouple the currents
flowing through memristive devices from the one stimulating the target neuron.
This can be a problem when using devices with high conductance values, because
of the resulting large currents. In this paper we propose a novel circuit that
decouples the current produced by the memristive device from the one used to
stimulate the post-synaptic neuron, by using a novel differential scheme based
on the Gilbert normalizer circuit. We show how this circuit is useful for
reducing the effect of variability in the memristive devices, and how it is
ideally suited for spike-based learning mechanisms that do not require
overlapping pre- and post-synaptic pulses. We demonstrate the features of the
proposed synapse circuit with SPICE simulations, and validate its learning
properties with high-level behavioral network simulations which use a
stochastic gradient descent learning rule in two classification tasks.
|
1709.05484v1
|
2017-09-21
|
Impacts of Surface Depletion on the Plasmonic Properties of Doped Semiconductor Nanocrystals
|
Degenerately doped semiconductor nanocrystals (NCs) exhibit a localized
surface plasmon resonance (LSPR) in the infrared range of the electromagnetic
spectrum. Unlike metals, semiconductor NCs offer tunable LSPR characteristics
enabled by doping, or via electrochemical or photochemical charging. Tuning
plasmonic properties through carrier density modulation suggests potential
applications in smart optoelectronics, catalysis, and sensing. Here, we
elucidate fundamental aspects of LSPR modulation through dynamic carrier
density tuning in Sn-doped Indium Oxide NCs. Monodisperse Sn-doped Indium Oxide
NCs with various doping level and sizes were synthesized and assembled in
uniform films. NC films were then charged in an in situ electrochemical cell
and the LSPR modulation spectra were monitored. Based on spectral shifts and
intensity modulation of the LSPR, combined with optical modeling, it was found
that often-neglected semiconductor properties, specifically band structure
modification due to doping and surface states, strongly affect LSPR modulation.
Fermi level pinning by surface defect states creates a surface depletion layer
that alters the LSPR properties; it determines the extent of LSPR frequency
modulation, diminishes the expected near field enhancement, and strongly
reduces sensitivity of the LSPR to the surroundings.
|
1709.07136v2
|
2017-10-05
|
Transport theory for femtosecond laser-induced spin-transfer torques
|
Ultrafast demagnetization of magnetic layers pumped by a femtosecond laser
pulse is accompanied by a nonthermal spin-polarized current of hot electrons.
These spin currents are studied here theoretically in a spin valve with
noncollinear magnetizations. To this end, we introduce an extended model of
superdiffusive spin transport that enables to treat noncollinear magnetic
configurations, and apply it to the perpendicular spin valve geometry. We show
how spin-transfer torques arise due to this mechanism and calculate their
action on the magnetization present, as well as how the latter depends on the
thicknesses of the layers and other transport parameters. We demonstrate that
there exists a certain optimum thickness of the out-of-plane magnetized
spin-current polarizer such that the torque acting on the second magnetic layer
is maximal. Moreover, we study the magnetization dynamics excited by the
superdiffusive spin-transfer torque due to the flow of hot electrons employing
the Landau-Lifshitz-Gilbert equation. Thereby we show that a femtosecond laser
pulse applied to one magnetic layer can excite small-angle precessions of the
magnetization in the second magnetic layer. We compare our calculations with
recent experimental results.
|
1710.02083v2
|
2017-10-12
|
A critical comparison of methods for the determination of the ageing sensitivity in biomedical grade yttria stabilized zirconia
|
Since the recent failure events of two particular series of zirconia femoral
heads for total hip replacement prosthesis, a large decrease in the use of
zirconia ceramics for orthopaedic implants has been observed. In spite of the
biomedical success of this material during the last ten years, this was
required for safety reasons, until the cause of the failures is known. It has
been shown that these failures were related to the low temperature hydrothermal
degradation (also known as ageing). Thus it is crucial to better understand the
ageing behaviour, in order to be able to assess its importance and then control
it if required. In this paper, various techniques relevant to assess the
hydrothermal degradation sensitivity of biomedical grade yttria stabilized
zirconia are discussed and compared. The expected outputs of conventional
methods, i.e. X-Ray diffraction and scanning electron microscopy are examined.
More recent methods like optical interferometry and atomic force microscopy are
presented, with their respective benefits and drawbacks. An up to date
comparison of these different techniques is provided, and their use for
ensuring the long term reliability of a particular batch of zirconia in terms
of ageing degradation is demonstrated.
|
1710.04449v1
|
2017-10-26
|
Evaluation of Treatment Effect Modification by Biomarkers Measured Pre- and Post-randomization in the Presence of Non-monotone Missingness
|
In vaccine studies, investigators are often interested in studying effect
modifiers of clinical treatment efficacy by biomarker-based principal strata,
which is useful for selecting biomarker study endpoints for evaluating
treatments in new trials, exploring biological mechanisms of clinical treatment
efficacy, and studying mediators of clinical treatment efficacy. However, in
trials where participants may enter the study with prior exposure therefore
with variable baseline biomarker values, clinical treatment efficacy may depend
jointly on a biomarker measured at baseline and measured at a fixed time after
vaccination. Therefore, it is of interest to conduct a bivariate effect
modification analysis by biomarker-based principal strata and baseline
biomarker values. Previous methods allow this assessment if participants who
have the biomarker measured at the the fixed time point post randomization
would also have the biomarker measured at baseline. However, additional
complications in study design could happen in practice. For example, in the
Dengue correlates study, baseline biomarker values were only available from a
fraction of participants who have biomarkers measured post-randomization. How
to conduct the bivariate effect modification analysis in these studies remains
an open research question. In this article, we propose an estimated likelihood
method to utilize the sub-sampled baseline biomarker in the effect modification
analysis and illustrate our method with datasets from two dengue phase 3
vaccine efficacy trials.
|
1710.09923v1
|
2017-10-29
|
If it ain't broke, don't fix it: Sparse metric repair
|
Many modern data-intensive computational problems either require, or benefit
from distance or similarity data that adhere to a metric. The algorithms run
faster or have better performance guarantees. Unfortunately, in real
applications, the data are messy and values are noisy. The distances between
the data points are far from satisfying a metric. Indeed, there are a number of
different algorithms for finding the closest set of distances to the given ones
that also satisfy a metric (sometimes with the extra condition of being
Euclidean). These algorithms can have unintended consequences, they can change
a large number of the original data points, and alter many other features of
the data.
The goal of sparse metric repair is to make as few changes as possible to the
original data set or underlying distances so as to ensure the resulting
distances satisfy the properties of a metric. In other words, we seek to
minimize the sparsity (or the $\ell_0$ "norm") of the changes we make to the
distances subject to the new distances satisfying a metric. We give three
different combinatorial algorithms to repair a metric sparsely. In one setting
the algorithm is guaranteed to return the sparsest solution and in the other
settings, the algorithms repair the metric. Without prior information, the
algorithms run in time proportional to the cube of the number of input data
points and, with prior information we can reduce the running time considerably.
|
1710.10655v1
|
2017-12-06
|
Monitoring the orientation of rare-earth-doped nanorods for flow shear tomography
|
Rare-earth phosphors exhibit unique luminescence polarization features
originating from the anisotropic symmetry of the emitter ion's chemical
environment. However, to take advantage of this peculiar property, it is
necessary to control and measure the ensemble orientation of the host particles
with a high degree of precision. Here, we show a methodology to obtain the
photoluminescence polarization of Eu-doped LaPO4 nano rods assembled in an
electrically modulated liquid-crystalline phase. We measure Eu3+ emission
spectra for the three main optimal configurations ({\sigma}, {\pi} and
{\alpha}, depending on the direction of observation and the polarization axes)
and use them as a reference for the nano rod orientation analysis. Based on the
fact that flowing nano rods tend to orient along the shear strain profile, we
use this orientation analysis to measure the local shear rate in a flowing
liquid. The potential of this approach is then demonstrated through tomographic
imaging of the shear rate distribution in a microfluidic system.
|
1712.02191v1
|
2017-12-08
|
Shrewd Selection Speeds Surfing: Use Smart EXP3!
|
In this paper, we explore the use of multi-armed bandit online learning
techniques to solve distributed resource selection problems. As an example, we
focus on the problem of network selection. Mobile devices often have several
wireless networks at their disposal. While choosing the right network is vital
for good performance, a decentralized solution remains a challenge. The
impressive theoretical properties of multi-armed bandit algorithms, like EXP3,
suggest that it should work well for this type of problem. Yet, its real-word
performance lags far behind. The main reasons are the hidden cost of switching
networks and its slow rate of convergence. We propose Smart EXP3, a novel
bandit-style algorithm that (a) retains the good theoretical properties of
EXP3, (b) bounds the number of switches, and (c) yields significantly better
performance in practice. We evaluate Smart EXP3 using simulations, controlled
experiments, and real-world experiments. Results show that it stabilizes at the
optimal state, achieves fairness among devices and gracefully deals with
transient behaviors. In real world experiments, it can achieve 18% faster
download over alternate strategies. We conclude that multi-armed bandit
algorithms can play an important role in distributed resource selection
problems, when practical concerns, such as switching costs and convergence
time, are addressed.
|
1712.03038v3
|
2017-12-08
|
Qatar Exoplanet Survey: Qatar-6b -- a grazing transiting hot Jupiter
|
We report the discovery of Qatar-6b, a new transiting planet identified by
the Qatar Exoplanet Survey (QES). The planet orbits a relatively bright
(V=11.44), early-K main-sequence star at an orbital period of P~3.506 days. An
SED fit to available multi-band photometry, ranging from the near-UV to the
mid-IR, yields a distance of d = 101 +/- 6 pc to the system. From a global fit
to follow-up photometric and spectroscopic observations, we calculate the mass
and radius of the planet to be Mp = 0.67 +/- 0.07 Mjup and Rp = 1.06 +/- 0.07
Rjup, respectively. We use multi-color photometric light curves to show that
the transit is grazing, making Qatar-6b one of the few exoplanets known in a
grazing transit configuration. It adds to the short list of targets that offer
the best opportunity to look for additional bodies in the host planetary system
through variations in the transit impact factor and duration.
|
1712.03216v1
|
2018-01-25
|
Generating survival times using Cox proportional hazards models with cyclic time-varying covariates, with application to a multiple-dose monoclonal antibody clinical trial
|
In two harmonized efficacy studies to prevent HIV infection through multiple
infusions of the monoclonal antibody VRC01, a key objective is to evaluate
whether the serum concentration of VRC01, which changes cyclically over time
along with the infusion schedule, is associated with the rate of HIV infection.
Simulation studies are needed in the development of such survival models. In
this paper, we consider simulating event time data with a continuous
time-varying covariate whose values vary with time through multiple drug
administration cycles, and whose effect on survival changes differently before
and after a threshold within each cycle. The latter accommodates settings with
a zero-protection biomarker threshold above which the drug provides a varying
level of protection depending on the biomarker level, but below which the drug
provides no protection. We propose two simulation approaches: one based on
simulating survival data under a single-dose regimen first before data are
aggregated over multiple doses, and another based on simulating survival data
directly under a multiple-dose regimen. We generate time-to-event data
following a Cox proportional hazards model based on inverting the cumulative
hazard function and a log link function for relating the hazard function to the
covariates. The method's validity is assessed in two sets of simulation
experiments. The results indicate that the proposed procedures perform well in
producing data that conform to their cyclic nature and assumptions of the Cox
proportional hazards model.
|
1801.08248v1
|
2018-01-29
|
Band-pass superlattice magnetic tunnel junctions
|
Significant scientific and technological progress in the field of spintronics
is based on trilayer magnetic tunnel junction devices which principally rely on
the physics of single barrier tunneling. While technologically relevant devices
have been prototyped, the physics of single barrier tunneling poses ultimate
limitations on the performance of magnetic tunnel junction devices. Here, we
propose a fresh route toward high performance magnetic tunnel junctions by
making electronic analogs of optical phenomena such as anti-reflections and
Fabry-P\`erot resonances. The devices we propose feature anti-reflection
enabled superlattice heterostructures sandwiched between the fixed and the free
ferromagnets of the magnetic tunnel junction structure. Our predictions are
based on the non-equilibrium Green's function spin transport formalism coupled
self-consistently with the Landau-Lifshitz-Gilbert-Slonczewski equation. Owing
to the physics of bandpass spin filtering in the bandpass superlattice magnetic
tunnel junction device, we demonstrate an ultra-high boost in the tunnel
magneto-resistance (TMR$\approx5\times10^4\%$) and nearly 92% suppression of
spin transfer torque switching bias in comparison to a traditional trilayer
magnetic tunnel junction device. We rationalize improvised spin transfer torque
switching via analysis of the Slonczewski spin current transmission spectra.
The proof of concepts presented here can lead to next-generation spintronics
device design harvesting the rich physics of superlattice heterostructures and
exploiting spintronic analogs of optical phenomena.
|
1801.09409v2
|
2018-01-29
|
Theory of AC quantum transport with fully electrodynamic coupling
|
With the continued scaling of microelectronic devices along with the growing
demand of high-speed wireless telecommunications technologies, there is
increasing need for high-frequency device modeling techniques that accurately
capture the quantum mechanical nature of charge transport in nanoscale devices
along with the dynamic fields that are generated. In an effort to fill this
gap, we develop a simulation methodology that self-consistently couples AC
non-equilibrium Green functions (NEGF) with the full solution of Maxwell's
equations in the frequency domain. We apply this technique to simulate
radiation from a quantum-confined, quarter-wave, monopole antenna where the
length $L$ is equal to one quarter of the wavelength, $\lambda_0$. Classically,
such an antenna would have a narrower, more directed radiation pattern compared
to one with $L \ll \lambda_0$, but we find that a quantum quarter-wave antenna
has no directivity gain compared to the classical solution. We observe that the
quantized wave function within the antenna significantly alter the charge and
current density distribution along the length of the wire, which in turn
modifies the far-field radiation pattern from the antenna. These results show
that high-frequency radiation from quantum systems can be markedly different
from classical expectations. Our method, therefore, will enable accurate
modeling of the next generation of high-speed nanoscale electronic devices.
|
1801.09611v1
|
2018-02-17
|
Design and Implementation of the Andromeda Proof Assistant
|
Andromeda is an LCF-style proof assistant where the user builds derivable
judgments by writing code in a meta-level programming language AML. The only
trusted component of Andromeda is a minimalist nucleus (an implementation of
the inference rules of an object-level type theory), which controls
construction and decomposition of type-theoretic judgments.
Since the nucleus does not perform complex tasks like equality checking
beyond syntactic equality, this responsibility is delegated to the user, who
implements one or more equality checking procedures in the meta-language. The
AML interpreter requests witnesses of equality from user code using the
mechanism of algebraic operations and handlers. Dynamic checks in the nucleus
guarantee that no invalid object-level derivations can be constructed. %even if
the AML code (or interpreter) is untrusted.
To demonstrate the flexibility of this system structure, we implemented a
nucleus consisting of dependent type theory with equality reflection. Equality
reflection provides a very high level of expressiveness, as it allows the user
to add new judgmental equalities, but it also destroys desirable meta-theoretic
properties of type theory (such as decidability and strong normalization).
The power of effects and handlers in AML is demonstrated by a standard
library that provides default algorithms for equality checking, computation of
normal forms, and implicit argument filling. Users can extend these new
algorithms by providing local "hints" or by completely replacing these
algorithms for particular developments. We demonstrate the resulting system by
showing how to axiomatize and compute with natural numbers, by axiomatizing the
untyped $\lambda$-calculus, and by implementing a simple automated system for
managing a universe of types.
|
1802.06217v1
|
2018-03-02
|
Broadband spectroscopy of thermodynamic magnetization fluctuations through a ferromagnetic spin-reorientation transition
|
We use scanning optical magnetometry to study the broadband frequency spectra
of spontaneous magnetization fluctuations, or "magnetization noise", in an
archetypal ferromagnetic film that can be smoothly tuned through a spin
reorientation transition (SRT). The SRT is achieved by laterally varying the
magnetic anisotropy across an ultrathin Pt/Co/Pt trilayer, from the
perpendicular to in-plane direction, via graded Ar$^+$ irradiation. In regions
exhibiting perpendicular anisotropy, the power spectrum of the magnetization
noise, $S(\nu)$, exhibits a remarkably robust $\nu^{-3/2}$ power law over
frequencies $\nu$ from 1~kHz to 1~MHz. As the SRT region is traversed, however,
$S(\nu)$ spectra develop a steadily-increasing critical frequency, $\nu_0$,
below which the noise power is spectrally flat, indicating an evolving
low-frequency cutoff for magnetization fluctuations. The magnetization noise
depends strongly on applied in- and out-of-plane magnetic fields, revealing
local anisotropies and also a field-induced emergence of fluctuations in
otherwise stable ferromagnetic films. Finally, we demonstrate that higher-order
correlators can be computed from the noise. These results highlight broadband
spectroscopy of thermodynamic fluctuations as a powerful tool to characterize
the interplay between thermal and magnetic energy scales, and as a means of
characterizing phase transitions in ferromagnets.
|
1803.00962v1
|
2018-03-06
|
On Simple Back-Off in Unreliable Radio Networks
|
In this paper, we study local and global broadcast in the dual graph model,
which describes communication in a radio network with both reliable and
unreliable links. Existing work proved that efficient solutions to these
problems are impossible in the dual graph model under standard assumptions. In
real networks, however, simple back-off strategies tend to perform well for
solving these basic communication tasks. We address this apparent paradox by
introducing a new set of constraints to the dual graph model that better
generalize the slow/fast fading behavior common in real networks. We prove that
in the context of these new constraints, simple back-off strategies now provide
efficient solutions to local and global broadcast in the dual graph model. We
also precisely characterize how this efficiency degrades as the new constraints
are reduced down to non-existent, and prove new lower bounds that establish
this degradation as near optimal for a large class of natural algorithms. We
conclude with a preliminary investigation of the performance of these
strategies when we include additional generality to the model. These results
provide theoretical foundations for the practical observation that simple
back-off algorithms tend to work well even amid the complicated link dynamics
of real radio networks.
|
1803.02216v3
|
2018-04-12
|
Connectivity in Random Annulus Graphs and the Geometric Block Model
|
We provide new connectivity results for {\em vertex-random graphs} or {\em
random annulus graphs} which are significant generalizations of random
geometric graphs. Random geometric graphs (RGG) are one of the most basic
models of random graphs for spatial networks proposed by Gilbert in 1961,
shortly after the introduction of the Erd\H{o}s-R\'{en}yi random graphs. They
resemble social networks in many ways (e.g. by spontaneously creating cluster
of nodes with high modularity). The connectivity properties of RGG have been
studied since its introduction, and analyzing them has been significantly
harder than their Erd\H{o}s-R\'{en}yi counterparts due to correlated edge
formation.
Our next contribution is in using the connectivity of random annulus graphs
to provide necessary and sufficient conditions for efficient recovery of
communities for {\em the geometric block model} (GBM). The GBM is a
probabilistic model for community detection defined over an RGG in a similar
spirit as the popular {\em stochastic block model}, which is defined over an
Erd\H{o}s-R\'{en}yi random graph. The geometric block model inherits the
transitivity properties of RGGs and thus models communities better than a
stochastic block model. However, analyzing them requires fresh perspectives as
all prior tools fail due to correlation in edge formation. We provide a simple
and efficient algorithm that can recover communities in GBM exactly with high
probability in the regime of connectivity.
|
1804.05013v3
|
2018-04-19
|
Equilibrium magnetization of a quasispherical cluster of single-domain particles
|
Equilibrium magnetization curve of a rigid finite-size spherical cluster of
single-domain particles is investigated both numerically and analytically. The
spatial distribution of particles within the cluster is random. Dipole-dipole
interactions between particles are taken into account. The particles are
monodisperse. It is shown, using the stochastic Landau-Lifshitz-Gilbert
equation that the magnetization of such clusters is generally lower than
predicted by the classical Langevin model. In a broad range of dipolar coupling
parameters and particle volume fractions, the cluster magnetization in the weak
field limit can be successfully described by the modified mean-field theory,
which was originally proposed for the description of concentrated ferrofluids.
In moderate and strong fields, the theory overestimates the cluster
magnetization. However, predictions of the theory can be improved by adjusting
the corresponding mean-field parameter. If magnetic anisotropy of particles is
additionally taken into account and if the distribution of the particles' easy
axes is random and uniform, then the cluster equilibrium response is even
weaker. The decrease of the magnetization with increasing anisotropy constant
is more pronounced at large applied fields. The phenomenological generalization
of the modified mean-field theory, that correctly describes this effect for
small coupling parameters, is proposed.
|
1804.07196v2
|
2018-05-28
|
Starbug fibre positioning robots: performance and reliability enhancements
|
Starbugs are miniature piezoelectric walking robots that can be operated in
parallel to position many payloads like optical fibers across a telescopes
focal plane. They consist of two concentric piezoelectric ceramic tubes that
walk with micron step size. In addition to individual optical fibers, Starbugs
have moved a payload of 0.75kg at several millimeters per second. The
Australian Astronomical Observatory previously developed prototype devices and
tested them in the laboratory. Now we are optimizing the Starbug design for
production and deployment in the TAIPAN instrument, which will be capable of
configuring 300 optical fibers over a six degree field-of-view on the UK
Schmidt Telescope within a few minutes. The TAIPAN instrument will demonstrate
the technology and capability for MANIFEST (Many Instrument Fiber-System)
proposed for the Giant Magellan Telescope. Design is addressing: connector
density and voltage limitations, mechanical reliability and construction
repeatability, field plate residues and scratching, metrology stability, and
facilitation of improved motion in all aspects of the design for later
evaluation. Here we present the new design features of the AAO TAIPAN Starbug.
|
1805.10761v1
|
2018-06-14
|
Resolving interfacial charge transfer in titanate superlattices using resonant X-ray reflectometry
|
Charge transfer in oxide heterostructures can be tuned to promote emergent
interfacial states, and accordingly, has been the subject of intense study in
recent years. However, accessing the physics at these interfaces, which are
often buried deep below the sample surface, remains difficult. Addressing this
challenge requires techniques capable of measuring the local electronic
structure with high-resolution depth dependence. Here, we have used
linearly-polarized resonant X-ray reflectometry (RXR) as a means to visualize
charge transfer in oxide superlattices with atomic layer precision. From our
RXR measurements, we extract valence depth profiles of SmTiO$_3$
(SmTO)/SrTiO$_3$ (STO) heterostructures with STO quantum wells varying in
thickness from 5 SrO planes down to a single, atomically thin SrO plane. At the
polar-nonpolar SmTO/STO interface, an electrostatic discontinuity leads to
approximately half an electron per areal unit cell transferred from the
interfacial SmO layer into the neighboring STO quantum well. We observe this
charge transfer as a suppression of the t$_{2g}$ absorption peaks that
minimizes contrast with the neighboring SmTO layers at those energies and leads
to a pronounced absence of superlattice peaks in the reflectivity data. Our
results demonstrate the sensitivity of RXR to electronic reconstruction at the
atomic scale, and establish RXR as a powerful means of characterizing charge
transfer at buried oxide interfaces.
|
1806.05733v1
|
2018-06-18
|
Formation Timescales for High-Mass X-ray Binaries in M33
|
We have identified 55 candidate high-mass X-ray binaries (HMXBs) in M33 using
available archival {\it HST} and {\it Chandra} imaging to find blue stars
associated with X-ray positions. We use the {\it HST} photometric data to model
the color-magnitude diagrams in the vicinity of each candidate HMXB to measure
a resolved recent star formation history (SFH), and thus a formation timescale,
or age for the source. Taken together, the SFHs for all candidate HMXBs in M33
yield an age distribution that suggests preferred formation timescales for
HMXBs in M33 of $<$ 5 Myr and $\sim$ 40 Myr after the initial star formation
episode. The population at 40 Myr is seen in other Local Group galaxies, and
can be attributed to a peak in formation efficiency of HMXBs with neutron stars
as compact objects and B star secondary companions. This timescale is preferred
as neutron stars should form in abundance from $\sim$ 8 M$_{\odot}$
core-collapse progenitors on these timescales, and B stars are shown
observationally to be most actively losing mass around this time. The young
population at $<$ 5 Myr has not be observed in other Local Group HMXB
population studies, but may be attributed to a population of very massive
progenitors forming black holes very early on. We discuss these results in the
context of massive binary evolution, and the implications for compact object
binaries and gravitational wave sources.
|
1806.06863v1
|
2018-06-24
|
Nanoscopic time crystal obtained by nonergodic spin dynamics
|
We study the far-from-equilibrium properties of quenched magnetic nanoscopic
classical spin systems. In particular, we focus on the interplay between
lattice vibrations and magnetic frustrations induced by surface effects typical
of an antiferromagnet. We use a combination of Monte Carlo simulations and
explore the dynamical behaviours by solving the stochastic
Landau-Lifshitz-Gilbert equation at finite temperature. The Monte Carlo
approach treats both the ionic degrees of freedom and spin variables on the
same footing, via an extended Lennard-Jones Hamiltonian with a spin-lattice
coupling. The zero temperature phase diagram of the finite size nanoscopic
systems with respect to the range of the Heisenberg interaction and the
Lennard-Jones coupling constant shows two main structures with non-trivial
magnetisation triggered by antiferromagnetism: a simple cubic and a
body-centred cubic. At non zero temperature, the competition between spins and
the ionic vibrations considerably affects the magnetization of the system.
Exploring the dynamics reveals a non-trivial structural induced behaviour in
the spin relaxation with a concomitant memory of the initially applied
ferromagnetic quench. We report the observation of a non-trivial dynamical
scenario, obtained after a ferromagnetic magnetic quench at low temperature.
Furthermore, we observe long-lived non-thermal states which could open new
avenues for nano-technology.
|
1806.09130v4
|
2018-06-29
|
The warm Neptunes around HD 106315 have low stellar obliquities
|
We present the obliquity of the warm Neptune HD 106315c measured via a series
of spectroscopic transit observations. HD 106315c is a 4.4 REarth warm Neptune
orbiting a moderately rotating late F-star with a period of 21.05 days. HD
106315 also hosts a 2.5 REarth super-Earth on a 9.55 day orbit. Our Doppler
tomographic analyses of four transits observed by the Magellan/MIKE, HARPS, and
TRES facilities find HD 106315c to be in a low stellar obliquity orbit,
consistent with being well aligned with the spin axis of the host star at
lambda = -10 +3.6/-3.8 deg. We suggest, via dynamical N-body simulations, that
the two planets in the system must be co-planar, and thus are both well aligned
with the host star. HD 106315 is only the fourth warm Neptune system with
obliquities measured. All warm Neptune systems have been found in well aligned
geometries, consistent with the interpretation that these systems are formed
in-situ in the inner protoplanetary disk, and also consistent with the majority
of Kepler multi-planet systems that are in low obliquity orbits. With a transit
depth of 1.02 mmag, HD 106315c is among the smallest planets to have been
detected in transit spectroscopy, and we discuss its detection in the context
of TESS and the next generations of spectrographs.
|
1807.00024v1
|
2018-07-13
|
BFORE: A CMB Balloon Payload to Measure Reionization, Neutrino Mass, and Cosmic Inflation
|
BFORE is a high-altitude ultra-long-duration balloon mission to map the
cosmic microwave background (CMB). During a 28-day mid-latitude flight launched
from Wanaka, New Zealand, the instrument will map half the sky to improve
measurements of the optical depth to reionization tau. This will break
parameter degeneracies needed to detect neutrino mass. BFORE will also hunt for
the gravitational wave B-mode signal, and map Galactic dust foregrounds. The
mission will be the first near-space use of TES/mSQUID multichroic detectors
(150/217 GHz and 280/353 GHz bands) with low-power readout electronics.
|
1807.05215v1
|
2018-07-19
|
Magnetization nutation induced by surface effects in nanomagnets
|
We investigate the magnetization dynamics of ferromagnetic nanoparticles in
the atomistic approach taking account of surface anisotropy and the spin
misalignment it causes. We demonstrate that such inhomogeneous spin
configurations induce nutation in the dynamics of the particle's magnetization.
More precisely, in addition to the ordinary precessional motion with frequency
$f_{p}\sim10\,{\rm GHz}$, we find that the dynamics of the net magnetic moment
exhibits two more resonance peaks with frequencies $f_{c}$ and $f_{n}$ which
are higher than the frequency $f_{p} : f_{c}=4\times f_{p}\sim40\,{\rm GHz}$ is
related with the oscillations of the particle's magnetic moment between the
minima of the effective potential induced by weak surface anisotropy. On the
other hand, the much higher frequency $f_{n}\sim1\,{\rm THz}$ is attributed to
the magnetization fluctuations at the atomic level driven by exchange
interaction. We have compared our results on nutation induced by surface
effects with those rendered by the macroscopic approach based on the
Landau-Lifshitz-Gilbert equation augmented by an inertial term (proportional to
the second-order time derivative of the macroscopic moment) with a
phenomenological coefficient. The good agreement between the two models have
allowed us to estimate the latter coefficient in terms of the atomistic
parameters such as the surface anisotropy constant. We have thus proposed a new
origin for the magnetization nutations as being induced by surface effects and
have interpreted the corresponding resonance peaks and their frequencies.
|
1807.07392v1
|
2018-07-19
|
Unsupervised Metric Learning in Presence of Missing Data
|
For many machine learning tasks, the input data lie on a low-dimensional
manifold embedded in a high dimensional space and, because of this
high-dimensional structure, most algorithms are inefficient. The typical
solution is to reduce the dimension of the input data using standard dimension
reduction algorithms such as ISOMAP, LAPLACIAN EIGENMAPS or LLES. This
approach, however, does not always work in practice as these algorithms require
that we have somewhat ideal data. Unfortunately, most data sets either have
missing entries or unacceptably noisy values. That is, real data are far from
ideal and we cannot use these algorithms directly. In this paper, we focus on
the case when we have missing data. Some techniques, such as matrix completion,
can be used to fill in missing data but these methods do not capture the
non-linear structure of the manifold. Here, we present a new algorithm
MR-MISSING that extends these previous algorithms and can be used to compute
low dimensional representation on data sets with missing entries. We
demonstrate the effectiveness of our algorithm by running three different
experiments. We visually verify the effectiveness of our algorithm on synthetic
manifolds, we numerically compare our projections against those computed by
first filling in data using nlPCA and mDRUR on the MNIST data set, and we also
show that we can do classification on MNIST with missing data. We also provide
a theoretical guarantee for MR-MISSING under some simplifying assumptions.
|
1807.07610v3
|
2018-08-09
|
Four new eclipsing mid M-dwarf systems from the New Luyten Two Tenths catalog
|
Using data from the MEarth-North and MEarth-South transit surveys, we present
the detection of eclipses in four mid M-dwarf systems: LP 107-25, LP 261-75, LP
796-24, and LP 991-15. Combining the MEarth photometry with spectroscopic
follow-up observations, we show that LP 107-25 and LP 796-24 are short-period
(1.388 and 0.523 day, respectively) eclipsing binaries in triple-lined systems
with substantial third light contamination from distant companions. LP 261-75
is a short-period (1.882 day) single-lined system consisting of a mid M-dwarf
eclipsed by a probable brown dwarf secondary, with another distant visual brown
dwarf companion. LP 991-15 is a long-period (29.3 day) double-lined eclipsing
binary on an eccentric orbit with a geometry which produces only primary
eclipses. A spectroscopic orbit is given for LP 991-15, and initial orbits for
LP 107-25 and LP 261-75.
|
1808.03243v1
|
2018-08-14
|
Addressing Johnson graphs, complete multipartite graphs, odd cycles and other graphs
|
Graham and Pollak showed that the vertices of any graph $G$ can be addressed
with $N$-tuples of three symbols, such that the distance between any two
vertices may be easily determined from their addresses. An addressing is
optimal if its length $N$ is minimum possible.
In this paper, we determine an addressing of length $k(n-k)$ for the Johnson
graphs $J(n,k)$ and we show that our addressing is optimal when $k=1$ or when
$k=2, n=4,5,6$, but not when $n=6$ and $k=3$. We study the addressing problem
as well as a variation of it in which the alphabet used has more than three
symbols, for other graphs such as complete multipartite graphs and odd cycles.
We also present computations describing the distribution of the minimum length
of addressings for connected graphs with up to $10$ vertices. Motivated by
these computations we settle a problem of Graham, showing that most graphs on
$n$ vertices have an addressing of length at most $n-(2-o(1))\log_2 n$.
|
1808.04757v2
|
2018-09-26
|
On Bioelectric Algorithms: A Novel Application of Theoretical Computer Science to Core Problems in Developmental Biology
|
Cellular bioelectricity describes the biological phenomenon in which cells in
living tissue generate and maintain patterns of voltage gradients induced by
differing concentrations of charged ions. A growing body of research suggests
that bioelectric patterns represent an ancient system that plays a key role in
guiding many important developmental processes including tissue regeneration,
tumor suppression, and embryogenesis. Understanding the relationship between
high-level bioelectric patterns and low-level biochemical processes might also
enable powerful new forms of synthetic biology. A key open question in this
area is understanding how a collection of cells, interacting with each other
and the extracellular environment only through simple ligand bindings and ion
fluxes, can compute non-trivial patterns and perform non-trivial information
processing tasks. The standard approach to this question is to model a given
bioelectrical network as a system of differential equations and then explore
its behavior using simulation techniques. In this paper, we propose applying a
computational approach. In more detail, we present the cellular bioelectric
model (CBM), a new computational model that captures the primary capabilities
and constraints of bioelectric interactions between cells and their
environment. We use this model to investigate several important topics in
cellular bioelectricity, including symmetry breaking and information
processing. Among other results, we describe and analyze a basic bioelectric
strategy the efficiently stabilizes arbitrary cell networks into maximal
independent sets (a structure known to play a role in the nervous system
development of flys), and prove cells in our model are Turing complete in their
ability to process information encoded in their initial voltage potential.
|
1809.10046v1
|
2018-10-02
|
Floquet engineering of classical systems
|
We develop the Floquet-Magnus expansion for a classical equation of motion
under a periodic drive that is applicable to both isolated and open systems.
For classical systems, known approaches based on the Floquet theorem fail due
to the nonlinearity and the stochasticity of their equations of motion (EOMs)
in contrast to quantum ones. Here, employing their master equation, we
successfully extend the Floquet methodology to classical EOMs to obtain their
Floquet-Magnus expansions, thereby overcoming this difficulty. Our method has a
wide range of application from classical to quantum as long as they are
described by differential equations including the Langevin equation, the
Gross-Pitaevskii equation, and the time-dependent Ginzburg-Landau equation. By
analytically evaluating the higher-order terms of the Floquet-Magnus expansion,
we find that it is, at least asymptotically, convergent and well approximates
the relaxation to their prethermal or non-equilibrium steady states. To support
these analytical findings, we numerically analyze two examples: (i) the Kapitza
pendulum with friction and (ii) laser-driven magnets described by the
stochastic Landau-Lifshitz-Gilbert equation. In both cases, the effective EOMs
obtained from their Floquet-Magnus expansions correctly reproduce their exact
time evolution for a long time up to their non-equilibrium steady states. In
the example of driven magnets, we demonstrate the controlled generations of a
macroscopic magnetization and a spin chirality by laser and discuss possible
applications to spintronics.
|
1810.01103v2
|
2018-10-02
|
Geodesic motion on the groups of diffeomorphisms with $H^1$ metric as geometric generalised Lagrangian mean theory
|
Generalized Lagrangian mean theories are used to analyze the interactions
between mean flows and fluctuations, where the decomposition is based on a
Lagrangian description of the flow. A systematic geometric framework was
recently developed by Gilbert and Vanneste (J. Fluid Mech., 2018) who cast the
decomposition in terms of intrinsic operations on the group of volume
preserving diffeomorphism or on the full diffeomorphism group. In this setting,
the mean of an ensemble of maps can be defined as the Riemannian center of mass
on either of these groups. We apply this decomposition in the context of
Lagrangian averaging where equations of motion for the mean flow arise via a
variational principle from a mean Lagrangian, obtained from the kinetic energy
Lagrangian of ideal fluid flow via a small amplitude expansion for the
fluctuations.
We show that the Euler-$\alpha$ equations arise as Lagrangian averaged Euler
equations when using the $L^2$-geodesic mean on the volume preserving
diffeomorphism group of a manifold without boundaries, imposing a `Taylor
hypothesis', which states that first order fluctuations are transported as a
vector field by the mean flow, and assuming that fluctuations are statistically
isotropic. Similarly, the EPDiff equations arise as the Lagrangian averaged
Burgers' equations using the same argument on the full diffeomorphism group.
These results generalize an earlier observation by Oliver (Proc. R. Soc. A,
2017) to manifolds in geometrically fully intrinsic terms.
|
1810.01377v1
|
2018-10-07
|
Training Convolutional Neural Networks and Compressed Sensing End-to-End for Microscopy Cell Detection
|
Automated cell detection and localization from microscopy images are
significant tasks in biomedical research and clinical practice. In this paper,
we design a new cell detection and localization algorithm that combines deep
convolutional neural network (CNN) and compressed sensing (CS) or sparse coding
(SC) for end-to-end training. We also derive, for the first time, a
backpropagation rule, which is applicable to train any algorithm that
implements a sparse code recovery layer. The key observation behind our
algorithm is that cell detection task is a point object detection task in
computer vision, where the cell centers (i.e., point objects) occupy only a
tiny fraction of the total number of pixels in an image. Thus, we can apply
compressed sensing (or, equivalently sparse coding) to compactly represent a
variable number of cells in a projected space. Then, CNN regresses this
compressed vector from the input microscopy image. Thanks to the SC/CS recovery
algorithm (L1 optimization) that can recover sparse cell locations from the
output of CNN. We train this entire processing pipeline end-to-end and
demonstrate that end-to-end training provides accuracy improvements over a
training paradigm that treats CNN and CS-recovery layers separately. Our
algorithm design also takes into account a form of ensemble average of trained
models naturally to further boost accuracy of cell detection. We have validated
our algorithm on benchmark datasets and achieved excellent performances.
|
1810.03075v1
|
2018-11-16
|
Asymmetric Drift in the Andromeda Galaxy (M31) as a Function of Stellar Age
|
We analyze the kinematics of Andromeda's disk as a function of stellar age by
using photometry from the Panchromatic Hubble Andromeda Treasury (PHAT) survey
and spectroscopy from the Spectroscopic and Photometric Landscape of
Andromeda's Stellar Halo (SPLASH) survey. We use HI 21-cm and CO ($\rm J=1
\rightarrow 0$) data to examine the difference between the deprojected rotation
velocity of the gas and that of the stars. We divide the stars into four
stellar age bins, from shortest lived to longest lived: massive main sequence
stars (0.03 Gyr), more luminous intermediate mass asymptotic giant branch (AGB)
stars (0.4 Gyr), less luminous intermediate mass AGB stars (2 Gyr), and low
mass red giant branch stars (4 Gyr). There is a clear correlation between the
offset of the stellar and the gas rotation velocity, or the asymmetric drift:
the longer lived populations lag farther behind the gas than short lived
populations. We also examine possible causes of the substructure in the
rotation curves and find that the most significant cause of scatter in the
rotation curves comes from the tilted ring model being an imperfect way to
account for the multiple warps in Andromeda's disk.
|
1811.07037v2
|
2018-11-21
|
Exploring interfacial exchange coupling and sublattice effect in heavy metal/ferrimagnetic insulator heterostructures using Hall measurements, x-ray magnetic circular dichroism, and neutron reflectometry
|
We use temperature-dependent Hall measurements to identify contributions of
spin Hall, magnetic proximity, and sublattice effects to the anomalous Hall
signal in heavy metal/ferrimagnetic insulator heterostructures with
perpendicular magnetic anisotropy. This approach enables detection of both the
magnetic proximity effect onset temperature and the magnetization compensation
temperature and provides essential information regarding the interfacial
exchange coupling. Onset of a magnetic proximity effect yields a local extremum
in the temperature-dependent anomalous Hall signal, which occurs at higher
temperature as magnetic insulator thickness increases. This magnetic proximity
effect onset occurs at much higher temperature in Pt than W. The magnetization
compensation point is identified by a sharp anomalous Hall sign change and
divergent coercive field. We directly probe the magnetic proximity effect using
x-ray magnetic circular dichroism and polarized neutron reflectometry, which
reveal an antiferromagnetic coupling between W and the magnetic insulator.
Finally, we summarize the exchange-coupling configurations and the anomalous
Hall-effect sign of the magnetized heavy metal in various heavy metal/magnetic
insulator heterostructures.
|
1811.08574v2
|
2018-12-13
|
Qatar Exoplanet Survey: Qatar-7b -- A Very Hot Jupiter Orbiting a Metal Rich F-Star
|
We present the discovery of Qatar-7b --- a very hot and inflated giant gas
planet orbiting close its parent star. The host star is a relatively massive
main sequence F-star with mass and radius Mstar = 1.41 +/- 0.03 Msun and Rstar
= 1.56 +/- 0.02 Rsun, respectively, at a distance d = 726 +/- 26 pc, and an
estimated age ~1 Gyr. With its orbital period of P = 2.032 days the planet is
located less than 5 stellar radii from its host star and is heated to a high
temperature Teq ~ 2100 K. From a global solution to the available photometric
and radial velocity observations, we calculate the mass and radius of the
planet to be Mpl = 1.88 +/- 0.25 Mjup and Rpl = 1.70 +/- 0.03 Rjup,
respectively. The planet radius and equilibrium temperature put Qatar-7b in the
top 6% of the hottest and largest known exoplanets. With its large radius and
high temperature Qatar-7b is a valuable addition to the short list of targets
that offer the best opportunity for studying their atmospheres through
transmission spectroscopy.
|
1812.05601v1
|
2019-01-02
|
Leader Election in Well-Connected Graphs
|
In this paper, we look at the problem of randomized leader election in
synchronous distributed networks with a special focus on the message
complexity. We provide an algorithm that solves the implicit version of leader
election (where non-leader nodes need not be aware of the identity of the
leader) in any general network with $O(\sqrt{n} \log^{7/2} n \cdot t_{mix})$
messages and in $O(t_{mix}\log^2 n)$ time, where $n$ is the number of nodes and
$t_{mix}$ refers to the mixing time of a random walk in the network graph $G$.
For several classes of well-connected networks (that have a large conductance
or alternatively small mixing times e.g. expanders, hypercubes, etc), the above
result implies extremely efficient (sublinear running time and messages) leader
election algorithms. Correspondingly, we show that any substantial improvement
is not possible over our algorithm, by presenting an almost matching lower
bound for randomized leader election. We show that
$\Omega(\sqrt{n}/\phi^{3/4})$ messages are needed for any leader election
algorithm that succeeds with probability at least $1-o(1)$, where $\phi$ refers
to the conductance of a graph. To the best of our knowledge, this is the first
work that shows a dependence between the time and message complexity to solve
leader election and the connectivity of the graph $G$, which is often
characterized by the graph's conductance $\phi$. Apart from the $\Omega(m)$
bound in [Kutten et al., J.ACM 2015] (where $m$ denotes the number of edges of
the graph), this work also provides one of the first non-trivial lower bounds
for leader election in general networks.
|
1901.00342v1
|
2019-01-23
|
Cooperation Speeds Surfing: Use Co-Bandit!
|
In this paper, we explore the benefit of cooperation in adversarial bandit
settings. As a motivating example, we consider the problem of wireless network
selection. Mobile devices are often required to choose the right network to
associate with for optimal performance, which is non-trivial. The excellent
theoretical properties of EXP3, a leading multi-armed bandit algorithm, suggest
that it should work well for this type of problem. Yet, it performs poorly in
practice. A major limitation is its slow rate of stabilization. Bandit-style
algorithms perform better when global knowledge is available, i.e., when
devices receive feedback about all networks after each selection. But,
unfortunately, communicating full information to all devices is expensive.
Therefore, we address the question of how much information is adequate to
achieve better performance. We propose Co-Bandit, a novel cooperative bandit
approach, that allows devices to occasionally share their observations and
forward feedback received from neighbors; hence, feedback may be received with
a delay. Devices perform network selection based on their own observation and
feedback from neighbors. As such, they speed up each other's rate of learning.
We prove that Co-Bandit is regret-minimizing and retains the convergence
property of multiplicative weight update algorithms with full information.
Through simulation, we show that a very small amount of information, even with
a delay, is adequate to nudge each other to select the right network and yield
significantly faster stabilization at the optimal state (about 630x faster than
EXP3).
|
1901.07768v1
|
2019-01-31
|
Still out there: Modeling and Identifying Russian Troll Accounts on Twitter
|
There is evidence that Russia's Internet Research Agency attempted to
interfere with the 2016 U.S. election by running fake accounts on Twitter -
often referred to as "Russian trolls". In this work, we: 1) develop machine
learning models that predict whether a Twitter account is a Russian troll
within a set of 170K control accounts; and, 2) demonstrate that it is possible
to use this model to find active accounts on Twitter still likely acting on
behalf of the Russian state. Using both behavioral and linguistic features, we
show that it is possible to distinguish between a troll and a non-troll with a
precision of 78.5% and an AUC of 98.9%, under cross-validation. Applying the
model to out-of-sample accounts still active today, we find that up to 2.6% of
top journalists' mentions are occupied by Russian trolls. These findings imply
that the Russian trolls are very likely still active today. Additional analysis
shows that they are not merely software-controlled bots, and manage their
online identities in various complex ways. Finally, we argue that if it is
possible to discover these accounts using externally - accessible data, then
the platforms - with access to a variety of private internal signals - should
succeed at similar or better rates.
|
1901.11162v1
|
2019-02-11
|
Efficient Randomized Test-And-Set Implementations
|
We study randomized test-and-set (TAS) implementations from registers in the
asynchronous shared memory model with n processes. We introduce the problem of
group election, a natural variant of leader election, and propose a framework
for the implementation of TAS objects from group election objects. We then
present two group election algorithms, each yielding an efficient TAS
implementation. The first implementation has expected max-step complexity
$O(\log^\ast k)$ in the location-oblivious adversary model, and the second has
expected max-step complexity $O(\log\log k)$ against any read/write-oblivious
adversary, where $k\leq n$ is the contention. These algorithms improve the
previous upper bound by Alistarh and Aspnes [2] of $O(\log\log n)$ expected
max-step complexity in the oblivious adversary model. We also propose a
modification to a TAS algorithm by Alistarh, Attiya, Gilbert, Giurgiu, and
Guerraoui [5] for the strong adaptive adversary, which improves its space
complexity from super-linear to linear, while maintaining its $O(\log n)$
expected max-step complexity. We then describe how this algorithm can be
combined with any randomized TAS algorithm that has expected max-step
complexity $T(n)$ in a weaker adversary model, so that the resulting algorithm
has $O(\log n)$ expected max-step complexity against any strong adaptive
adversary and $O(T(n))$ in the weaker adversary model. Finally, we prove that
for any randomized 2-process TAS algorithm, there exists a schedule determined
by an oblivious adversary such that with probability at least $(1/4)^t$ one of
the processes needs at least t steps to finish its TAS operation. This
complements a lower bound by Attiya and Censor-Hillel [7] on a similar problem
for $n\geq 3$ processes.
|
1902.04002v1
|
2019-03-11
|
Evidence for the formation of nanoprecipitates with magnetically disordered regions in bulk $\mathrm{Ni}_{50}\mathrm{Mn}_{45}\mathrm{In}_{5}$ Heusler alloys
|
Shell ferromagnetism is a new functional property of certain Heusler alloys
which has been recently observed in
$\mathrm{Ni}_{50}\mathrm{Mn}_{45}\mathrm{In}_{5}$. We report the results of a
comparative study of the magnetic microstructure of bulk
$\mathrm{Ni}_{50}\mathrm{Mn}_{45}\mathrm{In}_{5}$ Heusler alloys using
magnetometry, synchrotron x-ray diffraction, and magnetic small-angle neutron
scattering (SANS). By combining unpolarized and spin-polarized SANS (POLARIS)
we demonstrate that a number of important conclusions regarding the mesoscopic
spin structure can be made. In particular, the analysis of the magnetic neutron
data suggests that nanoprecipitates with an effective ferromagnetic component
form in an antiferromagnetic matrix on field annealing at $700 \, \mathrm{K}$.
These particles represent sources of perturbation, which seem to give rise to
magnetically disordered regions in the vicinity of the particle-matrix
interface. Analysis of the spin-flip SANS cross section via the computation of
the correlation function yields a value of $\sim 55 \, \mathrm{nm}$ for the
particle size and $\sim 20 \, \mathrm{nm}$ for the size of the spin-canted
region.
|
1903.04183v1
|
2019-03-14
|
Low Field-size, Rate-Optimal Streaming Codes for Channels With Burst and Random Erasures
|
In this paper, we design erasure-correcting codes for channels with burst and
random erasures, when a strict decoding delay constraint is in place. We
consider the sliding-window-based packet erasure model proposed by Badr et al.,
where any time-window of width $w$ contains either up to $a$ random erasures or
an erasure burst of length at most $b$. One needs to recover any erased packet,
where erasures are as per the channel model, with a strict decoding delay
deadline of $\tau$ time slots. Presently existing rate-optimal constructions in
the literature require, in general, a field-size which grows exponential in
$\tau$, for a constant $\frac{a}{\tau}$. In this work, we present a new
rate-optimal code construction covering all channel and delay parameters, which
requires an $O(\tau^2)$ field-size. As a special case, when $(b-a)=1$, we have
a field-size linear in $\tau$. We also present three other constructions having
linear field-size, under certain constraints on channel and decoding delay
parameters. As a corollary, we obtain low field-size, rate-optimal
convolutional codes for any given column distance and column span. Simulations
indicate that the newly proposed streaming code constructions offer lower
packet-loss probabilities compared to existing schemes, for selected instances
of Gilbert-Elliott and Fritchman channels.
|
1903.06210v1
|
2019-03-21
|
Emergent topology and symmetry-breaking order in correlated quench dynamics
|
Quenching a quantum system involves three basic ingredients: the initial
phase, the post-quench target phase, and the non-equilibrium dynamics which
carries the information of the former two. Here we propose a dynamical theory
to characterize both the topology and symmetry-breaking order in correlated
quantum system, through quenching the Haldane-Hubbard model from an initial
magnetic phase to topologically nontrivial regime. The equation of motion for
the complex pseudospin dynamics is obtained with the flow equation method, with
the pseudospin evolution shown to obey a microscopic
Landau-Lifshitz-Gilbert-Bloch equation. We find that the correlated quench
dynamics exhibit robust universal behaviors on the so-called band-inversion
surfaces (BISs), from which the nontrivial topology and magnetic order can be
extracted. In particular, the topology of the post-quench regime can be
characterized by an emergent dynamical topological pattern of quench dynamics
on BISs, which is robust against dephasing and heating induced by interactions;
the pre-quench symmetry-breaking orders is read out from a universal scaling
behavior of the quench dynamics emerging on the BIS, which is valid beyond the
mean-field regime. This work opens a way to characterize both the topology and
symmetry-breaking orders by correlated quench dynamics.
|
1903.09144v3
|
2019-03-22
|
Advanced Non-Destructive in Situ Characterization of Metals with the French Collaborating Research Group D2AM/BM02 Beamline at the European Synchrotron Radiation Facility
|
The ability to non-destructively measure the structural properties of
devices, either in situ or operando, are now possible using an intense X-ray
synchrotron source combined with specialized equipment. This tool attracted
researchers, in particular metallurgists, to attempt more complex and ambitious
experiments aimed at answering unresolved questions in formation mechanisms,
phase transitions, and magnetism complex alloys for industrial applications. In
this paper, we introduce the diffraction diffusion anomale multi-longueur
d'onde (D2AM) beamline, a French collaborating research group (CRG) beamline at
the European Synchrotron Radiation Facility (ESRF), partially dedicated to in
situ X-ray scattering experiments. The design of the beamline combined with the
available equipment (two-dimensional fast photon counting detectors,
sophisticated high precision kappa diffractometer, a variety of sample
environments, continuous scanning for X-ray imaging, and specific software for
data analysis) has made the D2AM beamline a highly efficient tool for advanced,
in situ synchrotron characterization in materials science, e.g., single crystal
or polycrystalline materials, powders, liquids, thin films, or epitaxial
nanostructures. This paper gathers the main elements and equipment available at
the beamline and shows its potential and flexibility in performing a wide
variety of temporally, spatially, and energetically resolved X-ray synchrotron
scattering measurements in situ.
|
1903.09390v1
|
2019-03-31
|
Relaxation to equilibrium in models of classical spins with long-range interactions
|
For a model long-range interacting system of classical Heisenberg spins, we
study how fluctuations, such as those arising from having a finite system size
or through interaction with the environment, affect the dynamical process of
relaxation to Boltzmann-Gibbs equilibrium. Under deterministic spin
precessional dynamics, we unveil the full range of quasistationary behavior
observed during relaxation to equilibrium, whereby the system is trapped in
nonequilibrium states for times that diverge with the system size. The
corresponding stochastic dynamics, modeling interaction with the environment
and constructed in the spirit of the stochastic Landau-Lifshitz-Gilbert
equation, however shows a fast relaxation to equilibrium on a size-independent
timescale and no signature of quasistationarity, provided the noise is strong
enough. Similar fast relaxation is also seen in Glauber Monte Carlo dynamics of
the model, thus establishing the ubiquity of what has been reported earlier in
particle dynamics (hence distinct from the spin dynamics considered here) of
long-range interacting systems, that quasistationarity observed in
deterministic dynamics is washed away by fluctuations induced through contact
with the environment.
|
1904.00432v2
|
2019-04-27
|
Blue-Light-Emitting Color Centers in High-Quality Hexagonal Boron Nitride
|
Light emitters in wide band gap semiconductors are of great fundamental
interest and have potential as optically addressable qubits. Here we describe
the discovery of a new color center in high-quality hexagonal boron nitride
(h-BN) with a sharp emission line at 435 nm. The emitters are activated and
deactivated by electron beam irradiation and have spectral and temporal
characteristics consistent with atomic color centers weakly coupled to lattice
vibrations. The emitters are conspicuously absent from commercially available
h-BN and are only present in ultra-high-quality h-BN grown using a
high-pressure, high-temperature Ba-B-N flux/solvent, suggesting that these
emitters originate from impurities or related defects specific to this unique
synthetic route. Our results imply that the light emission is activated and
deactivated by electron beam manipulation of the charge state of an
impurity-defect complex.
|
1904.12107v6
|
2019-04-30
|
Realization of Ordered Magnetic Skyrmions in Thin Films at Ambient Conditions
|
Magnetic skyrmions present interesting physics due to their topological
nature and hold significant promise for future information technologies. A key
barrier to realizing skyrmion devices has been stabilizing these spin
structures under ambient conditions. In this manuscript, we exploit the tunable
magnetic properties of amorphous Fe/Gd mulitlayers to realize skyrmion lattices
which are stable over a large temperature and magnetic field parameter space,
including room temperature and zero magnetic field. These hybrid skyrmions have
both Bloch-type and N\'eel-type character and are stabilized by dipolar
interactions rather than Dzyaloshinskii-Moriya interactions, which are
typically considered required for the generation of skyrmions. Small angle
neutron scattering (SANS) was used in combination with soft X-ray microscopy to
provide a unique, multi-scale probe of the local and long-range order of these
structures. These results identify a pathway to engineer controllable skyrmion
phases in thin film geometries which are stable at ambient conditions.
|
1904.13274v1
|
2019-05-16
|
Ultralow-loss domain wall motion driven by magnetocrystalline anisotropy gradient in antiferromagnetic nanowire
|
Searching for new methods controlling antiferromagnetic (AFM) domain wall is
one of the most important issues for AFM spintronic device operation. In this
work, we study theoretically the domain wall motion of an AFM nanowire, driven
by the axial anisotropy gradient generated by external electric field, allowing
the electro control of AFM domain wall motion in the merit of ultra-low energy
loss. The domain wall velocity depending on the anisotropy gradient magnitude
and intrinsic material properties is simulated based on the
Landau-Lifshitz-Gilbert equation and also deduced using the energy dissipation
theorem. It is found that the domain wall moves at a nearly constant velocity
for small gradient, and accelerates for large gradient due to the enlarged
domain wall width. The domain wall mobility is independent of lattice dimension
and types of domain wall, while it is enhanced by the Dzyaloshinskii-Moriya
interaction. In addition, the physical mechanism for much faster AFM wall
dynamics than ferromagnetic wall dynamics is qualitatively explained. This work
unveils a promising strategy for controlling the AFM domain walls, benefiting
to future AFM spintronic applications.
|
1905.06695v2
|
2019-05-22
|
Constraining level densities using spectral data
|
Several models of level densities exist and they often make simplified
assumptions regarding the overall behavior of the total level densities (LD)
and the intrinsic spin and parity distributions of the excited states.
Normally, such LD models are constrained only by the measured $D_0$, i.e. the
density of levels at the neutron separation energy of the compound nucleus
(target plus neutron), and the sometimes subjective extrapolation of discrete
levels. In this work we use microscopic Hartree-Fock-Bogoliubov (HFB) level
densities, which intrinsically provide more realistic spin and parity
distributions, and associate variations predicted by the HFB model with the
observed double-differential cross sections at low outgoing neutron energy,
region that is dominated by the LD input. With this approach we are able to
perform fits of the LD based on actual experimental data, constraining the
model and ensuring its consistency. This approach can be particularly useful in
extrapolating the LD to nuclei for which high-excited discrete levels and/or
values of $D_0$ are unknown. It also predicts inelastic gamma
(n,n$^{\prime}\gamma$) cross sections that in some cases can differ
significantly from more standard LD models such as Gilbert-Cameron.
|
1905.09194v1
|
2019-05-23
|
The Kepler Smear Campaign: Light curves for 102 Very Bright Stars
|
We present the first data release of the Kepler Smear Campaign, using
collateral 'smear' data obtained in the Kepler four-year mission to reconstruct
light curves of 102 stars too bright to have been otherwise targeted. We
describe the pipeline developed to extract and calibrate these light curves,
and show that we attain photometric precision comparable to stars analyzed by
the standard pipeline in the nominal Kepler mission. In this paper, aside from
publishing the light curves of these stars, we focus on 66 red giants for which
we detect solar-like oscillations, characterizing 33 of these in detail with
spectroscopic chemical abundances and asteroseismic masses as benchmark stars.
We also classify the whole sample, finding nearly all to be variable, with
classical pulsations and binary effects. All source code, light curves, TRES
spectra, and asteroseismic and stellar parameters are publicly available as a
Kepler legacy sample.
|
1905.09831v1
|
2019-06-06
|
A Hot Saturn Near (but unassociated with) the Open Cluster NGC 1817
|
We report on the discovery of a hot Saturn-sized planet (9.916 +/- 0.985
R_earth) around a late F star, EPIC 246865365, observed in Campaign 13 of the
K2 mission. We began studying this planet candidate because prior to the
release of Gaia DR2, the host star was thought to have been a member (> 90%
membership probability) of the approximately 1 Gyr open cluster NGC 1817 based
on its kinematics and photometric distance. We identify the host star (among
three stars within the K2 photometric aperture) using seeing-limited photometry
and rule out false positive scenarios using adaptive optics imaging and radial
velocity observations. We statistically validate EPIC 246865365b by calculating
a false positive probability rate of 0.01%. However, we also show using new
kinematic measurements provided by Gaia DR2 and our measured radial velocity of
the system that EPIC 246865365 is unassociated with the cluster NGC 1817.
Therefore, the long-running search for a giant transiting planet in an open
cluster remains fruitless. Finally, we note that our use of seeing-limited
photometry is a good demonstration of similar techniques that are already being
used to follow up TESS planet candidates, especially in crowded regions.
|
1906.02395v1
|
2019-06-12
|
Towards the nucleon hadronic tensor from lattice QCD
|
We present the first calculation of the hadronic tensor on the lattice for
the nucleon. The hadronic tensor can be used to extract the structure functions
in deep inelastic scatterings and also provide information for the
neutrino-nucleon scattering which is crucial to the neutrino-nucleus scattering
experiments at low energies. The most challenging part in the calculation is to
solve an inverse problem. We have implemented and tested three algorithms using
mock data, showing that the Bayesian Reconstruction method has the best
resolution in extracting peak structures while the Backus-Gilbert and Maximum
Entropy methods are somewhat more stable for the flat spectral function.
Numerical results are presented for both the elastic case (clover fermions on
domain wall configuration with $m_\pi\sim$ 370 MeV and $a\sim$ 0.06 fm) and a
case (anisotropic clover lattice with $m_\pi\sim$ 380 MeV and $a_t\sim$ 0.035
fm) with large momentum transfer. For the former case, the reconstructed
Minkowski hadronic tensor gives precisely the vector charge which proves the
feasibility of the approach. While for the latter case, the nucleon resonances
and possibly shallow inelastic scattering contributions around $\nu=1$ GeV are
clearly observed but no information is obtained for higher excited states with
$\nu>2$ GeV. A check of the effective masses of $\rho$ meson with different
lattice setups indicates that, in order to reach higher energy transfers, using
lattices with smaller lattice spacings is essential.
|
1906.05312v1
|
2019-06-17
|
Non-equilibrium Green's Function and First Principle Approach to Modeling of Multiferroic Tunnel Junctions
|
Recently, multiferroic tunnel junctions (MFTJs) have gained significant
spotlight in the literature due to its high tunneling electro-resistance
together with its non-volatility. In order to analyze such devices and to have
insightful understanding of its characteristics, there is a need for developing
a multi-physics modeling and simulation framework. The simulation framework
discussed in this paper is motivated by the scarcity of such multi-physics
studies in the literature. In this study, a theoretical analysis of MFTJs is
demonstrated using self-consistent analysis of spin-based non-equilibrium
Green's function (NEGF) method to estimate the tunneling current,
Landau-Khalatnikov (LK) equation to model the ferroelectric polarization
dynamics, together with landau-Lifshitz-Gilbert's (LLG) equations to capture
the magnetization dynamics. The spin-based NEGF method is equipped with a
magnetization dependent Hamiltonian that eases the modeling of the tunneling
electro-resistance (TER), tunneling magneto-resistance (TMR), and the
magnetoelectric effect (ME) in MFTJs. Moreover, we apply the first principle
calculations to estimate the screening lengths of the MFTJ electrodes that are
necessary for estimation of tunneling current. The simulation results of the
proposed framework are in good agreement with the experimental results.
Finally, a comprehensive analysis of TER and TMR of MFTJs and their dependence
on various device parameters is illustrated.
|
1906.06986v1
|
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