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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 | 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 |
2019-06-27 | Indications for Dzyaloshinskii-Moriya Interaction at the Pd/Fe Interface Studied by \textit{In Situ} Polarized Neutron Reflectometry | Using \textit{in situ} polarized neutron reflectometry, the depth resolved
evolution of the magnetism and structure in a Pd/Fe/Pd trilayer thin-film is
measured during growth. The initial film structure of Pd/Fe shows a small
proximity induced magnetism in the underlayer and a magnetization in the Fe
layer of $\approx1.6$\,$\mu_{\text{B}}$ per Fe atom, less than the expected
bulk value of $2.2$\,$\mu_{\text{B}}$. Deposition of the Pd capping layer
initially follows an island-like growth mode with subsequent coalescence. With
increasing Pd deposition the Fe moment and the proximity-induced magnetism in
the Pd capping layer decrease. After final deposition of the Pd capping layer,
the magnetic profile is structurally and magnetically symmetric across the Fe
layer, with magnetism induced in Pd up to 0.92 \,nm from the interface.
Throughout the Pd deposition the Pd/Fe/Pd trilayer structure is becoming
increasingly symmetric, a fact which points to a Dzyaloshinskii-Moriya
interaction as a likely cause of the observed magnetic behavior. | 1906.11532v1 |
2019-07-01 | Robust Formation of Ultrasmall Room-Temperature Neél Skyrmions in Amorphous Ferrimagnets from Atomistic Simulations | Ne\'el skyrmions originate from interfacial Dzyaloshinskii Moriya interaction
(DMI). Recent studies have explored using thin-film ferromagnets and
ferrimagnets to host Ne\'el skyrmions for spintronic applications. However, it
is unclear if ultrasmall (10 nm or less) skyrmions can ever be stabilized at
room temperature for practical use in high density parallel racetrack memories.
While thicker films can improve stability, DMI decays rapidly away from the
interface. As such, spins far away from the interface would experience
near-zero DMI, raising question on whether or not unrealistically large DMI is
needed to stabilize skyrmions, and whether skyrmions will also collapse away
from the interface. To address these questions, we have employed atomistic
stochastic Landau-Lifshitz-Gilbert simulations to investigate skyrmions in
amorphous ferrimagnetic GdCo. It is revealed that a significant reduction in
DMI below that of Pt is sufficient to stabilize ultrasmall skyrmions even in
films as thick as 15 nm. Moreover, skyrmions are found to retain a uniform
columnar shape across the film thickness despite the decaying DMI. Our results
show that increasing thickness and reducing DMI in GdCo can further reduce the
size of skyrmions at room temperature, which is crucial to improve the density
and energy efficiency in skyrmion based devices. | 1907.00647v1 |
2019-07-03 | Effect of Zeeman coupling on the Majorana vortex modes in iron-based topological superconductors | In the superconducting regime of FeTe$_{(1-x)}$Se$_x$, there exist two types
of vortices which are distinct by the presence or absence of zero energy states
in their core. To understand their origin, we examine the interplay of Zeeman
coupling and superconducting pairings in three-dimensional metals with band
inversion. Weak Zeeman fields are found to suppress the intra-orbital
spin-singlet pairing, known to localize the states at the ends of the vortices
on the surface. On the other hand, an orbital-triplet pairing is shown to be
stable against Zeeman interactions, but leads to delocalized zero-energy
Majorana modes which extend through the vortex. In contrast, the finite-energy
vortex modes remain localized at the vortex ends even when the pairing is of
orbital-triplet form. Phenomenologically, this manifests as an observed
disappearance of zero-bias peaks within the cores of topological vortices upon
increase of the applied magnetic field. The presence of magnetic impurities in
FeTe$_{(1-x)}$Se$_x$, which are attracted to the vortices, would lead to such
Zeeman-induced delocalization of Majorana modes in a fraction of vortices that
capture a large enough number of magnetic impurities. Our results provide an
explanation to the dichotomy between topological and non-topological vortices
recently observed in FeTe$_{(1-x)}$Se$_x$. | 1907.02077v2 |
2019-07-10 | Increasing Gender Diversity and Inclusion in Scientific Committees and Related Activities at STScI | We present a new initiative by the Women in Astronomy Forum at Space
Telescope Science Institute (STScI) to increase gender diversity and inclusion
in STScI's scientific committees and the activities they generate. This
initiative offers new and uniform guidelines on binary gender representation
goals for each committee and recommendations on how to achieve them in a
homogeneous way, as well as metrics and tools to track progress towards defined
goals. While the new guidelines presented in the paper focus on binary gender
representation, they can be adapted and implemented to support all minority
groups. By creating diverse committees and making them aware of, and trained on
implicit bias, we expect to create a diverse outcome in the activities they
generate, which, in turn, will advance science further and faster. | 1907.04880v1 |
2019-07-19 | Sparse Recovery for Orthogonal Polynomial Transforms | In this paper we consider the following sparse recovery problem. We have
query access to a vector $\vx \in \R^N$ such that $\vhx = \vF \vx$ is
$k$-sparse (or nearly $k$-sparse) for some orthogonal transform $\vF$. The goal
is to output an approximation (in an $\ell_2$ sense) to $\vhx$ in sublinear
time. This problem has been well-studied in the special case that $\vF$ is the
Discrete Fourier Transform (DFT), and a long line of work has resulted in
sparse Fast Fourier Transforms that run in time $O(k \cdot \mathrm{polylog}
N)$. However, for transforms $\vF$ other than the DFT (or closely related
transforms like the Discrete Cosine Transform), the question is much less
settled.
In this paper we give sublinear-time algorithms---running in time $\poly(k
\log(N))$---for solving the sparse recovery problem for orthogonal transforms
$\vF$ that arise from orthogonal polynomials. More precisely, our algorithm
works for any $\vF$ that is an orthogonal polynomial transform derived from
Jacobi polynomials. The Jacobi polynomials are a large class of classical
orthogonal polynomials (and include Chebyshev and Legendre polynomials as
special cases), and show up extensively in applications like numerical analysis
and signal processing. One caveat of our work is that we require an assumption
on the sparsity structure of the sparse vector, although we note that vectors
with random support have this property with high probability.
Our approach is to give a very general reduction from the $k$-sparse sparse
recovery problem to the $1$-sparse sparse recovery problem that holds for any
flat orthogonal polynomial transform; then we solve this one-sparse recovery
problem for transforms derived from Jacobi polynomials. | 1907.08362v1 |
2019-08-28 | Interplay of spin and mass superfluidity in antiferromagnetic spin-1 BEC and bicirculation vortices | The paper investigates the coexistence and interplay of spin and mass
superfluidity in the antiferromagnetic spin-1 BEC. The hydrodynamical theory
describes the spin degree of freedom by the equations similar to the
Landau--Lifshitz--Gilbert theory for bipartite antiferromagnetic insulator. The
variables in the spin space are two subspins with absolute value $\hbar/2$,
which play the role of two sublattice spins in the antiferromagnetic
insulators. As well as in bipartite antiferromagnetic insulators, in the
antiferromagnetic spin-1 BEC there are two spin-wave modes, one is a gapless
Goldstone mode, another is gapped. The Landau criterion shows that in limit of
small total spin (two subspins are nearly antiparallel) instability of
supercurrents starts from the gapped mode. In the opposite limit of large total
spin (two subspins are nearly parallel) the gapless modes become unstable
earlier than the gapped one. Mass and spin supercurrents decay via phase slips,
when vortices cross streamlines of supercurrent. The vortices participating in
phase slips are nonsingular bicirculation vortices. They are characterized by
two topological charges, which are winding numbers describing circulations of
two angles around the vortex axis. The winding numbers can be half-integer. A
particular example of a half-integer vortex is a half-quantum vortex with the
superfluid velocity circulation $h/2m$. But the superfluid velocity circulation
is not a topological charge, and in general the quantum of this circulation can
be continuously tuned from 0 to $h/2m$. | 1908.10633v2 |
2019-09-23 | The NASA Probe space mission concept, Cosmic Evolution Through UV Surveys (CETUS) | The mission concept, Cosmic Origins Through UV Surveys (CETUS) is an all-UV
space mission concept that was selected and funded by NASA for study in 2017.
The main capabilities of CETUS that even Hubble doesn't have are: (1)
wide-field (17.4'x17.4') imaging and spectroscopy of astronomical sources with
<0.5'' resolution; (2) spectral sensitivity to UV radiation at wavelengths as
short as 1000 {\AA}; (3) near-UV multi-object slit spectroscopy; and (4)
rapid-response UV spectroscopy and deep imaging of transients like GW 170817;
and (5) 23 times higher sensitivity to extended sources.
The main purposes of this CETUS Final Report are to describe the CETUS
scientific program and to demonstrate the maturity of its instrumentation,
which forms the basis of its estimated cost. While there are similarities of
this Final Report to that submitted to NASA in March 2019 by the Goddard Space
Flight Center, there are important differences including the following. *
Science. The science case has been refreshed, deepened, and expanded as a
result of ideas and recommendations expressed in the Astro2020 science white
papers. * Instrumentation. Detailed investigations including a high-level error
budget for focus with implications for thermal management, target acquisition
in the MOS micro-shutter array, contamination control have been carried out. *
Mission Design. The spacecraft and mission operations concepts as developed by
NGIS Gilbert (formerly Orbital ATK) rather than the output of Goddard's Mission
Design Lab have been adopted.. * Technology. Technology maturation plans have
been updated. | 1909.10437v1 |
2019-09-25 | Towards an improved understanding of molecular evolution: the relative roles of selection, drift, and everything in between | A major goal of molecular evolutionary biology is to identify loci or regions
of the genome under selection versus those evolving in a neutral manner.
Correct identification allows accurate inference of the evolutionary process
and thus comprehension of historical and contemporary processes driving
phenotypic change and adaptation. A fundamental difficulty lies in
distinguishing sites targeted by selection from both sites linked to these
targets and sites fully independent of selection. These three categories of
sites necessitate attention in light of the debate over the relative importance
of selection versus neutrality and the neutral theory. Modern genomic insights
have proved that complex processes such as linkage, demography, and biased gene
conversion complicate our understanding of the role of neutral versus selective
processes in evolution. In this perspective, we first highlight the importance
of the genomic and (a)biotic context of new mutations to identify the targets
of natural selection. We then present mechanisms that may constrain the
evolution of genomes and bias the inference of selection. We discuss these
mechanisms within the two critical levels that they occur: the population level
and the molecular level. We highlight that they should be taken into account to
correctly distinguish sites across the genome subject to selective or
non-selective forces and stress that a major current field-wide goal is to
quantify the absolute importance of these mechanisms. | 1909.11490v4 |
2019-10-08 | Correlated fluctuations in spin orbit torque-coupled perpendicular nanomagnets | Low barrier nanomagnets have attracted a lot of research interest for their
use as sources of high quality true random number generation. More recently,
low barrier nanomagnets with tunable output have been shown to be a natural
hardware platform for unconventional computing paradigms such as probabilistic
spin logic. Efficient generation and tunability of high quality random bits is
critical for these novel applications. However, current spintronic random
number generators are based on superparamagnetic tunnel junctions (SMTJs) with
tunability obtained through spin transfer torque (STT), which unavoidably leads
to challenges in designing concatenated networks using these two terminal
devices. The more recent development of utilizing spin orbit torque (SOT)
allows for a three terminal device design, but can only tune in-plane
magnetization freely, which is not very energy efficient due to the needs of
overcoming a large demagnetization field. In this work, we experimentally
demonstrate for the first time, a stochastic device with perpendicular magnetic
anisotropy (PMA) that is completely tunable by SOT without the aid of any
external magnetic field. Our measurements lead us to hypothesize that a tilted
anisotropy might be responsible for the observed tunability. We carry out
stochastic Landau-Lifshitz-Gilbert (sLLG) simulations to confirm our
experimental observation. Finally, we build an electrically coupled network of
two such stochastic nanomagnet based devices and demonstrate that finite
correlation or anti-correlation can be established between their output
fluctuations by a weak interconnection, despite having a large difference in
their natural fluctuation time scale. Simulations based on a newly developed
dynamical model for autonomous circuits composed of low barrier nanomagnets
show close agreement with the experimental results. | 1910.03184v1 |
2019-10-09 | Prophets, Secretaries, and Maximizing the Probability of Choosing the Best | Suppose a customer is faced with a sequence of fluctuating prices, such as
for airfare or a product sold by a large online retailer. Given distributional
information about what price they might face each day, how should they choose
when to purchase in order to maximize the likelihood of getting the best price
in retrospect? This is related to the classical secretary problem, but with
values drawn from known distributions. In their pioneering work, Gilbert and
Mosteller [\textit{J. Amer. Statist. Assoc. 1966}] showed that when the values
are drawn i.i.d., there is a thresholding algorithm that selects the best value
with probability approximately $0.5801$. However, the more general problem with
non-identical distributions has remained unsolved.
In this paper we provide an algorithm for the case of non-identical
distributions that selects the maximum element with probability $1/e$, and we
show that this is tight. We further show that if the observations arrive in a
random order, this barrier of $1/e$ can be broken using a static threshold
algorithm, and we show that our success probability is the best possible for
any single-threshold algorithm under random observation order. Moreover, we
prove that one can achieve a strictly better success probability using more
general multi-threshold algorithms, unlike the non-random-order case. Along the
way, we show that the best achievable success probability for the random-order
case matches that of the i.i.d.\ case, which is approximately $0.5801$, under a
"no-superstars" condition that no single distribution is very likely ex ante to
generate the maximum value. We also extend our results to the problem of
selecting one of the $k$ best values. | 1910.03798v1 |
2019-10-24 | Order and Information in the Patterns of Spinning Magnetic Micro-disks at the Air-water Interface | The application of the Shannon entropy to study the relationship between
information and structures has yielded insights into molecular and material
systems. However, the difficulty in directly observing and manipulating atoms
and molecules hampers the ability of these systems to serve as model systems
for further exploring the links between information and structures. Here, we
use, as a model experimental system, hundreds of spinning magnetic micro-disks
self-organizing at the air-water interface to generate various spatiotemporal
patterns with varying degrees of orders. Using the neighbor distance as the
information-bearing variable, we demonstrate the links among information,
structure, and interactions. Most importantly, we establish a direct link
between information and structure without using explicit knowledge of
interactions. Finally, we show that the Shannon entropy by neighbor distances
is a powerful observable in characterizing structural changes. Our findings are
relevant for analyzing natural self-organizing systems and for designing
collective robots. | 1910.11226v3 |
2019-11-17 | Interfacial-Redox-Induced Tuning of Superconductivity in YBa$_{2}$Cu$_{3}$O$_{7-δ}$ | Solid state ionic approaches for modifying ion distributions in getter/oxide
heterostructures offer exciting potentials to control material properties. Here
we report a simple, scalable approach allowing for total control of the
superconducting transition in optimally doped YBa$_{2}$Cu$_{3}$O$_{7-{\delta}}$
(YBCO) films via a chemically-driven ionic migration mechanism. Using a thin Gd
capping layer of up to 20 nm deposited onto 100 nm thick epitaxial YBCO films,
oxygen is found to leach from deep within the YBCO. Progressive reduction of
the superconducting transition is observed, with complete suppression possible
for a sufficiently thick Gd layer. These effects arise from the combined impact
of redox-driven electron doping and modification of the YBCO microstructure due
to oxygen migration and depletion. This work demonstrates an effective ionic
control of superconductivity in oxides, an interface induced effect that goes
well into the quasi-bulk regime, opening up possibilities for electric field
manipulation. | 1911.07275v1 |
2019-12-10 | Integration of Neural Network-Based Symbolic Regression in Deep Learning for Scientific Discovery | Symbolic regression is a powerful technique that can discover analytical
equations that describe data, which can lead to explainable models and
generalizability outside of the training data set. In contrast, neural networks
have achieved amazing levels of accuracy on image recognition and natural
language processing tasks, but are often seen as black-box models that are
difficult to interpret and typically extrapolate poorly. Here we use a neural
network-based architecture for symbolic regression called the Equation Learner
(EQL) network and integrate it with other deep learning architectures such that
the whole system can be trained end-to-end through backpropagation. To
demonstrate the power of such systems, we study their performance on several
substantially different tasks. First, we show that the neural network can
perform symbolic regression and learn the form of several functions. Next, we
present an MNIST arithmetic task where a separate part of the neural network
extracts the digits. Finally, we demonstrate prediction of dynamical systems
where an unknown parameter is extracted through an encoder. We find that the
EQL-based architecture can extrapolate quite well outside of the training data
set compared to a standard neural network-based architecture, paving the way
for deep learning to be applied in scientific exploration and discovery. | 1912.04825v2 |
2019-12-17 | New search for mirror neutron regeneration | The possibility of relatively fast neutron oscillations into a mirror neutron
state is not excluded experimentally when a mirror magnetic field is
considered. Direct searches for the disappearance of neutrons into mirror
neutrons in a controlled magnetic field have previously been performed using
ultracold neutrons, with some anomalous results reported. We describe a
technique using cold neutrons to perform a disappearance and regeneration
search, which would allow us to unambiguously identify a possible oscillation
signal. An experiment using the existing General Purpose-Small Angle Neutron
Scattering instrument at the High Flux Isotope Reactor at Oak Ridge National
Laboratory will have the sensitivity to fully explore the parameter space of
prior ultracold neutron searches and confirm or refute previous claims of
observation. This instrument can also conclusively test the validity of
recently suggested oscillation-based explanations for the neutron lifetime
anomaly. | 1912.08264v1 |
2020-01-06 | Highly efficient spin orbit torque in Pt/Co/Ir multilayers with antiferromagnetic interlayer exchange coupling | We have studied the spin orbit torque (SOT) in Pt/Co/Ir multilayers with 3
repeats of the unit structure. As the system exhibits oscillatory interlayer
exchange coupling (IEC) with varying Ir layer thickness, we compare the SOT of
films when the Co layers are coupled ferromagnetically and
antiferromagnetically. SOT is evaluated using current induced shift of the
anomalous Hall resistance hysteresis loops. A relatively thick Pt layer,
serving as a seed layer to the multilayer, is used to generate spin current via
the spin Hall effect. In the absence of antiferromagnetic coupling, the SOT is
constant against the applied current density and the corresponding spin torque
efficiency (i.e. the effective spin Hall angle) is $\sim$0.09, in agreement
with previous reports. In contrast, for films with antiferromagnetic coupling,
the SOT increases with the applied current density and eventually saturates.
The SOT at saturation is a factor of $\sim$15 larger than that without the
antiferromagnetic coupling. The spin torque efficiency is $\sim$5 times larger
if we assume the net total magnetization is reduced by a factor of 3 due to the
antiferromagnetic coupling. Model calculations based on the Landau Lifshitz
Gilbert equation show that the presence of antiferromagnetic coupling can
increase the SOT but the degree of enhancement is limited, in this case, to a
factor of 1.2-1.4. We thus consider there are other sources of SOT, possibly at
the interfaces, which may account for the highly efficient SOT in the
uncompensated synthetic anti-ferromagnet (SAF) multilayers. | 2001.01454v1 |
2019-11-24 | Cybernetical Concepts for Cellular Automaton and Artificial Neural Network Modelling and Implementation | As a discipline cybernetics has a long and rich history. In its first
generation it not only had a worldwide span, in the area of computer modelling,
for example, its proponents such as John von Neumann, Stanislaw Ulam, Warren
McCulloch and Walter Pitts, also came up with models and methods such as
cellular automata and artificial neural networks, which are still the
foundation of most modern modelling approaches. At the same time, cybernetics
also got the attention of philosophers, such as the Frenchman Gilbert Simondon,
who made use of cybernetical concepts in order to establish a metaphysics and a
natural philosophy of individuation, giving cybernetics thereby a philosophical
interpretation, which he baptised allagmatic. In this paper, we emphasise this
allagmatic theory by showing how Simondon's philosophical concepts can be used
to formulate a generic computer model or metamodel for complex systems
modelling and its implementation in program code, according to generic
programming. We also present how the developed allagmatic metamodel is capable
of building simple cellular automata and artificial neural networks. | 2001.02037v3 |
2020-02-12 | Competition between magnetic order and charge localization in Na$_2$IrO$_3$ thin crystal devices | Spin orbit assisted Mott insulators such as sodium iridate (Na$_2$IrO$_3$)
have been an important subject of study in the recent years. In these
materials, the interplay of electronic correlations, spin-orbit coupling,
crystal field effects and a honeycomb arrangement of ions bring exciting ground
states, predicted in the frame of the Kitaev model. The insulating character of
Na$_2$IrO$_3$ has hampered its integration to an electronic device, desirable
for applications, such as the manipulation of quasiparticles interesting for
topological quantum computing. Here we show through electronic transport
measurements supported by Angle Resolved Photoemission Spectroscopy (ARPES)
experiments, that electronic transport in Na$_2$IrO$_3$ is ruled by variable
range hopping and it is strongly dependent on the magnetic ordering transition
known for bulk Na$_2$IrO$_3$, as well as on external electric fields.
Electronic transport measurements allow us to deduce a value for the
localization length and the density of states in our Na$_2$IrO$_3$ thin
crystals devices, offering an alternative approach to study insulating layered
materials. | 2002.04785v1 |
2020-02-13 | Electron Beam-Induced Nanopores in Bernal-Stacked Hexagonal Boron Nitride | Controlling the size and shape of nanopores in two-dimensional materials is a
key challenge in applications such as DNA sequencing, sieving, and quantum
emission in artificial atoms. We here investigate experimentally and
theoretically triangular vacancies in (unconventional) Bernal-stacked AB-h-BN
formed using a high-energy electron beam. Due to the geometric configuration of
AB-h-BN, triangular pores in different layers are aligned, and their sizes are
controlled by the duration of the electron irradiation. Interlayer covalent
bonding at the vacancy edge is not favored, as opposed to what occurs in the
more common AA'-stacked BN. A variety of monolayer, concentric and bilayer
pores in bilayer AB-h-BN are observed in high-resolution transmission electron
microscopy and characterized using ab initio simulations. Bilayer pores in
AB-h-BN are commonly formed, and grow without breaking the bilayer character.
Nanopores in AB-h-BN exhibit a wide range of electronic properties, ranging
from half-metallic to non-magnetic and magnetic semiconducting. Therefore,
because of the controllability of the pore size, the electronic structure is
also highly controllable in these systems, and can potentially be tuned for
particular applications. | 2002.05795v3 |
2020-02-26 | Effect of chemical substitution on the skyrmion phase in Cu$_2$OSeO$_3$ | Magnetic skyrmions have been the focus of intense research due to their
unique qualities which result from their topological protections. Previous work
on Cu$_2$OSeO$_3$, the only known insulating multiferroic skyrmion material,
has shown that chemical substitution alters the skyrmion phase. We chemically
substitute Zn, Ag, and S into powdered Cu$_2$OSeO$_3$ to study the effect on
the magnetic phase diagram. In both the Ag and the S substitutions, we find
that the skyrmion phase is stabilized over a larger temperature range, as
determined via magnetometry and small-angle neutron scattering (SANS).
Meanwhile, while previous magnetometry characterization suggests two high
temperature skyrmion phases in the Zn-substituted sample, SANS reveals the high
temperature phase to be skyrmionic while we are unable to distinguish the other
from helical order. Overall, chemical substitution weakens helical and skyrmion
order as inferred from neutron scattering of the $|$q$| \approx$ 0.01
$\r{A}^{-1}$ magnetic peak. | 2002.11827v1 |
2020-03-10 | Smart City IoT Services Creation through Large Scale Collaboration | Smart cities solutions are often monolithically implemented, from sensors
data handling through to the provided services. The same challenges are
regularly faced by different developers, for every new solution in a new city.
Expertise and know-how can be re-used and the effort shared. In this article we
present the methodologies to minimize the efforts of implementing new smart
city solutions and maximizing the sharing of components. The final target is to
have a live technical community of smart city application developers. The
results of this activity comes from the implementation of 35 city services in
27 cities between Europe and South Korea. To share efforts, we encourage
developers to devise applications using a modular approach. Single-function
components that are re-usable by other city services are packaged and published
as standalone components, named Atomic Services. We identify 15 atomic services
addressing smart city challenges in data analytics, data evaluation, data
integration, data validation, and visualization. 38 instances of the atomic
services are already operational in several smart city services. We detail in
this article, as atomic service examples, some data predictor components.
Furthermore, we describe real-world atomic services usage in the scenarios of
Santander and three Danish cities. The resulting atomic services also generate
a side market for smart city solutions, allowing expertise and know-how to be
re-used by different stakeholders. | 2003.04843v1 |
2020-03-23 | Low Power Unsupervised Anomaly Detection by Non-Parametric Modeling of Sensor Statistics | This work presents AEGIS, a novel mixed-signal framework for real-time
anomaly detection by examining sensor stream statistics. AEGIS utilizes Kernel
Density Estimation (KDE)-based non-parametric density estimation to generate a
real-time statistical model of the sensor data stream. The likelihood estimate
of the sensor data point can be obtained based on the generated statistical
model to detect outliers. We present CMOS Gilbert Gaussian cell-based design to
realize Gaussian kernels for KDE. For outlier detection, the decision boundary
is defined in terms of kernel standard deviation ($\sigma_{Kernel}$) and
likelihood threshold ($P_{Thres}$). We adopt a sliding window to update the
detection model in real-time. We use time-series dataset provided from Yahoo to
benchmark the performance of AEGIS. A f1-score higher than 0.87 is achieved by
optimizing parameters such as length of the sliding window and decision
thresholds which are programmable in AEGIS. Discussed architecture is designed
using 45nm technology node and our approach on average consumes $\sim$75 $\mu$W
power at a sampling rate of 2 MHz while using ten recent inlier samples for
density estimation. \textcolor{red}{Full-version of this research has been
published at IEEE TVLSI} | 2003.10088v1 |
2020-03-30 | Efficient nonparametric inference on the effects of stochastic interventions under two-phase sampling, with applications to vaccine efficacy trials | The advent and subsequent widespread availability of preventive vaccines has
altered the course of public health over the past century. Despite this
success, effective vaccines to prevent many high-burden diseases, including
HIV, have been slow to develop. Vaccine development can be aided by the
identification of immune response markers that serve as effective surrogates
for clinically significant infection or disease endpoints. However, measuring
immune response marker activity is often costly, which has motivated the usage
of two-phase sampling for immune response evaluation in clinical trials of
preventive vaccines. In such trials, the measurement of immunological markers
is performed on a subset of trial participants, where enrollment in this second
phase is potentially contingent on the observed study outcome and other
participant-level information. We propose nonparametric methodology for
efficiently estimating a counterfactual parameter that quantifies the impact of
a given immune response marker on the subsequent probability of infection.
Along the way, we fill in theoretical gaps pertaining to the asymptotic
behavior of nonparametric efficient estimators in the context of two-phase
sampling, including a multiple robustness property enjoyed by our estimators.
Techniques for constructing confidence intervals and hypothesis tests are
presented, and an open source software implementation of the methodology, the
txshift R package, is introduced. We illustrate the proposed techniques using
data from a recent preventive HIV vaccine efficacy trial. | 2003.13771v2 |
2020-04-05 | Effects of the Affordable Care Act Dependent Coverage Mandate on Health Insurance Coverage for Individuals in Same-Sex Couples | A large body of research documents that the 2010 dependent coverage mandate
of the Affordable Care Act was responsible for significantly increasing health
insurance coverage among young adults. No prior research has examined whether
sexual minority young adults also benefitted from the dependent coverage
mandate, despite previous studies showing lower health insurance coverage among
sexual minorities and the fact that their higher likelihood of strained
relationships with their parents might predict a lower ability to use parental
coverage. Our estimates from the American Community Surveys using
difference-in-differences and event study models show that men in same-sex
couples age 21-25 were significantly more likely to have any health insurance
after 2010 compared to the associated change for slightly older 27 to
31-year-old men in same-sex couples. This increase is concentrated among
employer-sponsored insurance, and it is robust to permutations of time periods
and age groups. Effects for women in same-sex couples and men in different-sex
couples are smaller than the associated effects for men in same-sex couples.
These findings confirm the broad effects of expanded dependent coverage and
suggest that eliminating the federal dependent mandate could reduce health
insurance coverage among young adult sexual minorities in same-sex couples. | 2004.02296v1 |
2020-04-07 | A general framework for inference on algorithm-agnostic variable importance | In many applications, it is of interest to assess the relative contribution
of features (or subsets of features) toward the goal of predicting a response
-- in other words, to gauge the variable importance of features. Most recent
work on variable importance assessment has focused on describing the importance
of features within the confines of a given prediction algorithm. However, such
assessment does not necessarily characterize the prediction potential of
features, and may provide a misleading reflection of the intrinsic value of
these features. To address this limitation, we propose a general framework for
nonparametric inference on interpretable algorithm-agnostic variable
importance. We define variable importance as a population-level contrast
between the oracle predictiveness of all available features versus all features
except those under consideration. We propose a nonparametric efficient
estimation procedure that allows the construction of valid confidence
intervals, even when machine learning techniques are used. We also outline a
valid strategy for testing the null importance hypothesis. Through simulations,
we show that our proposal has good operating characteristics, and we illustrate
its use with data from a study of an antibody against HIV-1 infection. | 2004.03683v2 |
2020-04-15 | Magic DIAMOND: Multi-Fascicle Diffusion Compartment Imaging with Tensor Distribution Modeling and Tensor-Valued Diffusion Encoding | Diffusion tensor imaging provides increased sensitivity to microstructural
tissue changes compared to conventional anatomical imaging but also presents
limited specificity. To tackle this problem, the DIAMOND model subdivides the
voxel content into diffusion compartments and draws from diffusion-weighted
data to estimate compartmental non-central matrix-variate Gamma distribution of
diffusion tensors, thereby resolving crossing fascicles while accounting for
their respective heterogeneity. Alternatively, tensor-valued diffusion encoding
defines new acquisition schemes tagging specific features of the intra-voxel
diffusion tensor distribution directly from the outcome of the measurement.
However, the impact of such schemes on estimating brain microstructural
features has only been studied in a handful of parametric single-fascicle
models. In this work, we derive a general Laplace transform for the non-central
matrix-variate Gamma distribution, which enables the extension of DIAMOND to
tensor-valued encoded data. We then evaluate this "Magic DIAMOND" model in
silico and in vivo on various combinations of tensor-valued encoded data.
Assessing uncertainty on parameter estimation via stratified bootstrap, we
investigate both voxel-based and fixel-based metrics by carrying out multi-peak
tractography. We show that our estimated metrics can be mapped along tracks
robustly across regions of fiber crossing, which opens new perspectives for
tractometry and microstructure mapping along specific white-matter tracts. | 2004.07340v2 |
2020-04-16 | Measuring Human and Economic Activity from Satellite Imagery to Support City-Scale Decision-Making during COVID-19 Pandemic | The COVID-19 outbreak forced governments worldwide to impose lockdowns and
quarantines to prevent virus transmission. As a consequence, there are
disruptions in human and economic activities all over the globe. The recovery
process is also expected to be rough. Economic activities impact social
behaviors, which leave signatures in satellite images that can be automatically
detected and classified. Satellite imagery can support the decision-making of
analysts and policymakers by providing a different kind of visibility into the
unfolding economic changes. In this work, we use a deep learning approach that
combines strategic location sampling and an ensemble of lightweight
convolutional neural networks (CNNs) to recognize specific elements in
satellite images that could be used to compute economic indicators based on it,
automatically. This CNN ensemble framework ranked third place in the US
Department of Defense xView challenge, the most advanced benchmark for object
detection in satellite images. We show the potential of our framework for
temporal analysis using the US IARPA Function Map of the World (fMoW) dataset.
We also show results on real examples of different sites before and after the
COVID-19 outbreak to illustrate different measurable indicators. Our code and
annotated high-resolution aerial scenes before and after the outbreak are
available on GitHub (https://github.com/maups/covid19-satellite-analysis). | 2004.07438v4 |
2020-04-16 | Subjectifying Objectivity: Delineating Tastes in Theoretical Quantum Gravity Research | Research in Theoretical Quantum Gravity has continued expansively even as it
has become detached from classic arbiters of research such as direct empirical
falsification. This makes it an interesting test case for social-scientific
theories of what motivates and mediates contemporary scientific research and
the nature of scientific objectivity. For our empirical investigation, we
conducted 50 semi-structured interviews with researchers in the rival camps of
String Theory and Loop Quantum Gravity, coded a subset for reoccurring themes,
and subjected the resulting data to statistical analysis. Theoretically, we
mobilize aspects of Daston and Galison's depiction of the scientific self and
its relation to epistemic virtues, Pierre Bourdieu's field-centered account of
social space, and Kantian notions of aesthetics in order to delineate the
subjective tastes and the related process of collective consensus-making in
contemporary quantum gravity research. We make two key contributions. First,
our analysis sheds light on the inner workings of the field by connecting its
internal epistemic struggles with relevant social-scientific theories. For
example, we are able to suggest an explanation for how one approach, String
Theory, has become so dominant. Second, our application of theories of social
reproduction to the substance of scientific inquiry merits some substantive
generalizations to Daston and Galison's framework. Most significantly, we
propose as an addendum to their progression the notion of objectivity through
intersubjectivity: objectivity obtained not through the suppression of the self
but by its (regulated) pluralistic expression and performance. | 2004.07450v2 |
2020-04-22 | Excitation of high-frequency magnon modes in magnetoelastic films by short strain pulses | Development of energy efficient techniques for generation of spin waves
(magnons) is important for implementation of low-dissipation spin-wave-based
logic circuits and memory elements. A promising approach to achieve this goal
is based on the injection of short strain pulses into ferromagnetic films with
a strong magnetoelastic coupling between spins and strains. Here we report
micromagnetoelastic simulations of the magnetization and strain dynamics
excited in Fe$_{81}$Ga$_{19}$ films by picosecond and nanosecond acoustic
pulses created in a GaAs substrate by a transducer subjected to an optical or
electrical impulse. The simulations performed via the numerical solution of the
coupled Landau-Lifshitz-Gilbert and elastodynamic equations show that the
injected strain pulse induces an inhomogeneous magnetization precession in the
ferromagnetic film. The precession lasts up to 1 ns and can be treated as a
superposition of magnon modes having the form of standing spin waves. For
Fe$_{81}$Ga$_{19}$ films with nanoscale thickness, up to seven (six) distinct
modes have been revealed under free-surface (pinning) magnetic boundary
conditions. Remarkably, magnon modes with frequencies over 1 THz can be excited
by acoustic pulses with an appropriate shape and duration in the films
subjected to a moderate external magnetic field. This finding shows that short
strain pulses represent a promising tool for the generation of THz spin waves
necessary for the implementation of high-speed magnonic devices. | 2004.10838v1 |
2020-04-23 | Correlation-driven eightfold magnetic anisotropy in a two-dimensional oxide monolayer | Engineering magnetic anisotropy in two-dimensional systems has enormous
scientific and technological implications. The uniaxial anisotropy universally
exhibited by two-dimensional magnets has only two stable spin directions,
demanding 180 degrees spin switching between states. We demonstrate a novel
eightfold anisotropy in magnetic SrRuO3 monolayers by inducing a spin
reorientation in (SrRuO3)1/(SrTiO3)N superlattices, in which the magnetic easy
axis of Ru spins is transformed from uniaxial <001> direction (N = 1 and 2) to
eightfold <111> directions (N = 3, 4 and 5). This eightfold anisotropy enables
71 and 109 degrees spin switching in SrRuO3 monolayers, analogous to 71 and 109
degrees polarization switching in ferroelectric BiFeO3. First-principle
calculations reveal that increasing the SrTiO3 layer thickness induces an
emergent correlation-driven orbital ordering, tuning spin-orbit interactions
and reorienting the SrRuO3 monolayer easy axis. Our work demonstrates that
correlation effects can be exploited to substantially change spin-orbit
interactions, stabilizing unprecedented properties in two-dimensional magnets
and opening rich opportunities for low-power, multi-state device applications. | 2004.10939v1 |
2020-04-27 | Dynamic Predictions of Postoperative Complications from Explainable, Uncertainty-Aware, and Multi-Task Deep Neural Networks | Accurate prediction of postoperative complications can inform shared
decisions regarding prognosis, preoperative risk-reduction, and postoperative
resource use. We hypothesized that multi-task deep learning models would
outperform random forest models in predicting postoperative complications, and
that integrating high-resolution intraoperative physiological time series would
result in more granular and personalized health representations that would
improve prognostication compared to preoperative predictions. In a longitudinal
cohort study of 56,242 patients undergoing 67,481 inpatient surgical procedures
at a university medical center, we compared deep learning models with random
forests for predicting nine common postoperative complications using
preoperative, intraoperative, and perioperative patient data. Our study
indicated several significant results across experimental settings that suggest
the utility of deep learning for capturing more precise representations of
patient health for augmented surgical decision support. Multi-task learning
improved efficiency by reducing computational resources without compromising
predictive performance. Integrated gradients interpretability mechanisms
identified potentially modifiable risk factors for each complication. Monte
Carlo dropout methods provided a quantitative measure of prediction uncertainty
that has the potential to enhance clinical trust. Multi-task learning,
interpretability mechanisms, and uncertainty metrics demonstrated potential to
facilitate effective clinical implementation. | 2004.12551v2 |
2020-05-08 | Tree! I am no Tree! I am a Low Dimensional Hyperbolic Embedding | Given data, finding a faithful low-dimensional hyperbolic embedding of the
data is a key method by which we can extract hierarchical information or learn
representative geometric features of the data. In this paper, we explore a new
method for learning hyperbolic representations by taking a metric-first
approach. Rather than determining the low-dimensional hyperbolic embedding
directly, we learn a tree structure on the data. This tree structure can then
be used directly to extract hierarchical information, embedded into a
hyperbolic manifold using Sarkar's construction \cite{sarkar}, or used as a
tree approximation of the original metric. To this end, we present a novel fast
algorithm \textsc{TreeRep} such that, given a $\delta$-hyperbolic metric (for
any $\delta \geq 0$), the algorithm learns a tree structure that approximates
the original metric. In the case when $\delta = 0$, we show analytically that
\textsc{TreeRep} exactly recovers the original tree structure. We show
empirically that \textsc{TreeRep} is not only many orders of magnitude faster
than previously known algorithms, but also produces metrics with lower average
distortion and higher mean average precision than most previous algorithms for
learning hyperbolic embeddings, extracting hierarchical information, and
approximating metrics via tree metrics. | 2005.03847v4 |
2020-07-08 | On the production of He$^+$ of solar origin in the solar wind | Solar wind measurements in the heliosphere are predominantly comprised of
protons, alphas, and minor elements in a highly ionized state. The majority of
low charge states, such as He$^{+}$, measured in situ are often attributed to
pick up ions of non-solar origin. However, through inspection of the velocity
distribution functions of near Earth measurements, we find a small but
significant population of He$^+$ ions in the normal solar wind whose properties
indicate that it originated from the Sun and has evolved as part of the normal
solar wind. Current ionization models, largely governed by electron impact and
radiative ionization and recombination processes, underestimate this population
by several orders of magnitude. Therefore, to reconcile the singly ionized He
observed, we investigate recombination of solar He$^{2+}$ through charge
exchange with neutrals from circumsolar dust as a possible formation mechanism
of solar He$^{+}$. We present an empirical profile of neutrals necessary for
charge exchange to become an effective vehicle to recombine He$^{2+}$ to
He$^{+}$ such that it meets observational He$^{+}$ values. We find the
formation of He$^{+}$ is not only sensitive to the density of neutrals but also
to the inner boundary of the neutral distribution encountered along the solar
wind path. However, further observational constraints are necessary to confirm
that the interaction between solar $\alpha$ particles and dust neutrals is the
primary source of the He$^{+}$ observations. | 2007.04402v2 |
2020-07-28 | Towers and the first-order theory of hyperbolic groups | This paper is devoted to the first-order theory of torsion-free hyperbolic
groups. One of its purposes is to review some results and to provide precise
and correct statements and definitions, as well as some proofs and new results.
A key concept is that of a tower (Sela) or NTQ system
(Kharlampovich-Myasnikov). We discuss them thoroughly.
We state and prove a new general theorem which unifies several results in the
literature: elementarily equivalent torsion-free hyperbolic groups have
isomorphic cores (Sela); if $H$ is elementarily embedded in a torsion-free
hyperbolic group $G$, then $G$ is a tower over $H$ relative to $H$ (Perin);
free groups (Perin-Sklinos, Ould-Houcine), and more generally free products of
prototypes and free groups, are homogeneous.
The converse to Sela and Perin's results just mentioned is true. This follows
from the solution to Tarski's problem on elementary equivalence of free groups,
due independently to Sela and Kharlampovich-Myasnikov, which we treat as a
black box throughout the paper.
We present many examples and counterexamples, and we prove some new
model-theoretic results. We characterize prime models among torsion-free
hyperbolic groups, and minimal models among elementarily free groups. Using
Fra\"iss\'e's method, we associate to every torsion-free hyperbolic group $H$ a
unique homogeneous countable group $\mathcal{M}$ in which any hyperbolic group
$H'$ elementarily equivalent to $H$ has an elementary embedding.
In an appendix we give a complete proof of the fact, due to Sela, that towers
over a torsion-free hyperbolic group $H$ are $H$-limit groups. | 2007.14148v1 |
2020-08-13 | Prediction of magnetization dynamics in a reduced dimensional feature space setting utilizing a low-rank kernel method | We establish a machine learning model for the prediction of the magnetization
dynamics as function of the external field described by the
Landau-Lifschitz-Gilbert equation, the partial differential equation of motion
in micromagnetism. The model allows for fast and accurate determination of the
response to an external field which is illustrated by a thin-film standard
problem. The data-driven method internally reduces the dimensionality of the
problem by means of nonlinear model reduction for unsupervised learning. This
not only makes accurate prediction of the time steps possible, but also
decisively reduces complexity in the learning process where magnetization
states from simulated micromagnetic dynamics associated with different external
fields are used as input data. We use a truncated representation of kernel
principal components to describe the states between time predictions. The
method is capable of handling large training sample sets owing to a low-rank
approximation of the kernel matrix and an associated low-rank extension of
kernel principal component analysis and kernel ridge regression. The approach
entirely shifts computations into a reduced dimensional setting breaking down
the problem dimension from the thousands to the tens. | 2008.05986v3 |
2020-07-20 | Artificial Intelligence is stupid and causal reasoning won't fix it | Artificial Neural Networks have reached Grandmaster and even super-human
performance across a variety of games: from those involving perfect-information
(such as Go) to those involving imperfect-information (such as Starcraft). Such
technological developments from AI-labs have ushered concomitant applications
across the world of business - where an AI brand tag is fast becoming
ubiquitous. A corollary of such widespread commercial deployment is that when
AI gets things wrong - an autonomous vehicle crashes; a chatbot exhibits racist
behaviour; automated credit scoring processes discriminate on gender etc. -
there are often significant financial, legal and brand consequences and the
incident becomes major news. As Judea Pearl sees it, the underlying reason for
such mistakes is that, 'all the impressive achievements of deep learning amount
to just curve fitting'. The key, Judea Pearl suggests, is to replace reasoning
by association with causal-reasoning - the ability to infer causes from
observed phenomena. It is a point that was echoed by Gary Marcus and Ernest
Davis in a recent piece for the New York Times: 'we need to stop building
computer systems that merely get better and better at detecting statistical
patterns in data sets - often using an approach known as Deep Learning - and
start building computer systems that from the moment of their assembly innately
grasp three basic concepts: time, space and causality'. In this paper,
foregrounding what in 1949 Gilbert Ryle termed a category mistake, I will offer
an alternative explanation for AI errors: it is not so much that AI machinery
cannot grasp causality, but that AI machinery - qua computation - cannot
understand anything at all. | 2008.07371v1 |
2020-08-19 | Dynamical decoupling in interacting systems: applications to signal-enhanced hyperpolarized readout | Methods that preserve coherence broadly impact all quantum information
processing and metrology applications. Dynamical decoupling methods accomplish
this by protecting qubits in noisy environments but are typically constrained
to the limit where the qubits themselves are non-interacting. Here we consider
the alternate regime wherein the inter-qubit couplings are of the same order as
dephasing interactions with the environment. We propose and demonstrate a
multi-pulse protocol that protects transverse spin states by suitably
Hamiltonian engineering the inter-spin coupling while simultaneously
suppressing dephasing noise on the qubits. We benchmark the method on 13C
nuclear spin qubits in diamond, dipolar coupled to each other and embedded in a
noisy electronic spin bath, and hyperpolarized via optically pumped NV centers.
We observe effective state lifetimes of 13C nuclei $T_2^{\prime}\approx$2.5s at
room temperature, an extension of over 4700-fold over the conventional
$T_2^{\ast}$ free induction decay. The spins are continuously interrogated
during the applied quantum control, resulting in 13C NMR line narrowing and an
$>$500-fold boost in SNR due to the lifetime extension. Together with
hyperpolarization spin interrogation is accelerated by $>10^{11}$ over
conventional 7T NMR. This work suggests strategies for the dynamical decoupling
of coupled qubit systems with applications in a variety of experimental
platforms. | 2008.08323v1 |
2020-08-30 | Microwave and spin transfer torque driven coherent control in ferromagnets | Coherent control is a method used to manipulate the state of matter using
oscillatory electromagnetic radiation which relies on the non-adiabatic
interaction. It is commonly applied in quantum processing applications. This
technique is interesting in the context of ferromagnetic materials because of
the ability to combine it with spintronics for the purpose of fundamental spin
transport research, low-power information processing, and potentially future
quantum bit (Qubit) applications. In this work we address the theoretical
grounds of coherent manipulation in practical ferromagnetic systems. We study
electromagnetic radiation driven interaction that is enhanced in the presence
of spin polarized currents and map the conditions that allow coherent
manipulation for which Rabi oscillations take place. The role of the magnetic
anisotropy field is shown to act as an additional oscillatory driving field. We
discuss the Gilbert losses in the context of effective coherence decay rates
and show that it is possible to control these rates by application of a static
spin current. The case of coherent manipulation using oscillatory spin currents
that is free of radiation is discussed as well. Our work paves the way towards
spin current amplification as well as radiation-free coherent control schemes
that may potentially lead to novel Qubits that are robust and scalable. | 2008.13139v3 |
2020-08-31 | Philosophy-Guided Modelling and Implementation of Adaptation and Control in Complex Systems | Control was from its very beginning an important concept in cybernetics.
Later on, with the works of W. Ross Ashby, for example, biological concepts
such as adaptation were interpreted in the light of cybernetic systems theory.
Adaptation is the process by which a system is capable of regulating or
controlling itself in order to adapt to changes of its inner and outer
environment maintaining a homeostatic state. In earlier works we have developed
a system metamodel that on the one hand refers to cybernetic concepts such as
structure, operation, and system, and on the other to the philosophy of
individuation of Gilbert Simondon. The result is the so-called allagmatic
method that is capable of creating concrete models of systems such as
artificial neural networks and cellular automata starting from abstract
building blocks. In this paper, we add to our already existing method the
cybernetic concepts of control and especially adaptation. In regard to the
system metamodel, we rely again on philosophical theories, this time the
philosophy of organism of Alfred N. Whitehead. We show how these new
meta-theoretical concepts are described formally and how they are implemented
in program code. We also show what role they play in simple experiments. We
conclude that philosophical abstract concepts help to better understand the
process of creating computer models and their control and adaptation. In the
outlook we discuss how the allagmatic method needs to be extended in order to
cover the field of complex systems and Norbert Wiener's ideas on control. | 2009.00110v4 |
2020-09-02 | X-ray linear dichroic ptychography | Biominerals such as seashells, corals skeletons, bone, and enamel are
optically anisotropic crystalline materials with unique nano- and micro-scale
organization that translates into exceptional macroscopic mechanical
properties, providing inspiration for engineering new and superior biomimetic
structures. Here we use particles of Seriatopora aculeata coral skeleton as a
model and demonstrate, for the first time, x-ray linear dichroic ptychography.
We map the aragonite (CaCO3) crystal c-axis orientations in coral skeleton with
35 nm spatial resolution. Linear dichroic phase imaging at the O K-edge energy
shows strong polarization-dependent contrast and reveals the presence of both
narrow (< 35{\deg}) and wide (> 35{\deg}) c-axis angular spread in
sub-micrometer coral particles. These x-ray ptychography results were
corroborated using 4D scanning transmission electron nano-diffraction on the
same particles. Evidence of co-oriented but disconnected corallite sub-domains
indicates jagged crystal boundaries consistent with formation by amorphous
nanoparticle attachment. Looking forward, we anticipate that x-ray linear
dichroic ptychography can be applied to study nano-crystallites, interfaces,
nucleation and mineral growth of optically anisotropic materials with sub-ten
nanometers spatial resolution in three dimensions. | 2009.01093v1 |
2020-09-18 | The effect of the surface magnetic anisotropy of the neodymium atoms on the coercivity in the neodymium permanent magnet | The Nd permanent magnet (Nd$_{2}$Fe$_{14}$B) is an indispensable material
used in modern energy conversion devices. The realization of high coercivity at
finite temperatures is a burning issue. One of the important ingredients for
controlling the coercive force is the surface property of magnetic grains. It
has been reported by first-principles studies that the Nd atoms in the first
(001) surface layer facing the vacuum have in-plane anisotropy perpendicular to
the $c$ axis, which may decrease the coercivity. Focusing on the surface
anisotropy effect on the coercivity, we examine the coercivity at zero and
finite temperatures by using an atomistic model reflecting the lattice
structure of the Nd magnet with a stochastic Landau-Lifshitz-Gilbert equation
method. We study general three cases, in which the Nd atoms in surface layers
have (1) no anisotropy, (2) in-plane anisotropy, and (3) reinforced anisotropy
for two types of surfaces, (001) and (100) surfaces. We find that in contrast
to the zero-temperature case, due to the thermal fluctuation effect, the
modification of only the first surface layer has little effect on the
coercivity at finite temperatures. However, the modification of a few layers
results in significant effects. We discuss the details of the dependence of the
coercivity on temperature, type of surface, and modified layer depth, and also
the features of domain growth in magnetization reversal. | 2009.08572v1 |
2020-09-28 | Precise control of $J_\mathrm{eff}=1/2$ magnetic properties in Sr$_2$IrO$_4$ epitaxial thin films by variation of strain and thin film thickness | We report on a comprehensive investigation of the effects of strain and film
thickness on the structural and magnetic properties of epitaxial thin films of
the prototypal $J_\mathrm{eff}=1/2$ compound Sr$_2$IrO$_4$ by advanced X-ray
scattering. We find that the Sr$_2$IrO$_4$ thin films can be grown fully
strained up to a thickness of 108 nm. By using X-ray resonant scattering, we
show that the out-of-plane magnetic correlation length is strongly dependent on
the thin film thickness, but independent of the strain state of the thin films.
This can be used as a finely tuned dial to adjust the out-of-plane magnetic
correlation length and transform the magnetic anisotropy from two-dimensional
(2D) to three-dimensional (3D) behavior by incrementing film thickness. These
results provide a clearer picture for the systematic control of the magnetic
degrees of freedom in epitaxial thin films of Sr$_2$IrO$_4$ and bring to light
the potential for a rich playground to explore the physics of $5d$-transition
metal compounds. | 2009.13185v1 |
2020-10-03 | WinterLab: Developing a low-cost, portable experiment platform to encourage engagement in the electronics lab | Encouraging student engagement is a key aim in any educational setting, and
allowing students the freedom to pursue their own methods of solving problems
through independent experimentation has been shown to markedly improve this. In
many contexts, however, allowing students this flexibility in their learning is
hampered by constraints of the material itself, such as in the electronics
laboratory, where expensive and bulky equipment confines the learning
environment to the laboratory room. Finding ourselves in the position of
teaching one such laboratory course at the undergraduate level, we sought to
encourage students to learn through independent investigation and the pursuit
of personal projects, by providing a more flexible and inquiry-based learning
environment and allowing them to take their measurement equipment -- and their
learning -- beyond the laboratory itself. We present this project as a case of
design both for and by students, with the lead designer undertaking the project
after attending the course in question, and pursuing its development as a
foundational step in their graduate career. We discuss the challenges and
opportunities we encountered over the course of the design and development
process, and the eventual key output of the project: a portable, low-cost,
integrated electronics experimentation platform called the Winterlab board. | 2010.01426v2 |
2020-10-16 | Hyperspectral interference tomography of nacre | Structural characterization of biologically formed materials is essential for
understanding biological phenomena and their environment, and generating new
bio-inspired engineering concepts. For example, nacre -- formed by mollusks in
the ocean -- encodes local environmental conditions throughout its formation
and has exceptional strength due to its nanoscale brick-and-mortar structure.
This layered structure, comprising transparent aragonite tablets bonded with an
ultra-thin organic polymer, also results in stunning interference colors.
Existing methods of structural characterization of nacre rely on some form of
cross-sectional analysis, such as scanning electron microscopy or
polarization-dependent imaging contrast (PIC) mapping. However, these
techniques are destructive and too time- and resource-intensive to analyze
large sample areas. Here we present an all-optical, rapid, and non-destructive
imaging technique -- hyperspectral interference tomography (HIT) -- to
spatially map the structural parameters of nacre and other disordered layered
materials. We combined hyperspectral imaging with optical-interference modeling
to infer the mean tablet thickness and disordering of nacre layers across
entire mollusk shells at various stages of development, observing a previously
unknown relationship between the growth of the mollusk and tablet thickness.
Our rapid, inexpensive, and nondestructive method can be readily applied to
in-field studies. | 2010.08170v1 |
2020-11-03 | Recent results for the Landau-Lifshitz equation | We give a survey on some recent results concerning the Landau-Lifshitz
equation, a fundamental nonlinear PDE with a strong geometric content,
describing the dynamics of the magnetization in ferromagnetic materials. We
revisit the Cauchy problem for the anisotropic Landau-Lifshitz equation,
without dissipation, for smooth solutions, and also in the energy space in
dimension one. We also examine two approximations of the Landau-Lifshitz
equation given by of the Sine-Gordon equation and cubic Schr\"odinger
equations, arising in certain singular limits of strong easy-plane and
easy-axis anisotropy, respectively.
Concerning localized solutions, we review the orbital and asymptotic
stability problems for a sum of solitons in dimension one, exploiting the
variational nature of the solitons in the hydrodynamical framework.
Finally, we survey results concerning the existence, uniqueness and stability
of self-similar solutions (expanders and shrinkers) for the isotropic
Landau-Lifshitz equation with Gilbert term. Since expanders are associated with
a singular initial condition with a jump discontinuity, we also review their
well-posedness in spaces linked to the BMO space. | 2011.01692v3 |
2020-11-10 | The Virtual Goniometer: A new method for measuring angles on 3D models of fragmentary bone and lithics | The contact goniometer is a commonly used tool in lithic and
zooarchaeological analysis, despite suffering from a number of shortcomings due
to the physical interaction between the measuring implement, the object being
measured, and the individual taking the measurements. However, lacking a simple
and efficient alternative, researchers in a variety of fields continue to use
the contact goniometer to this day. In this paper, we present a new goniometric
method that we call the virtual goniometer, which takes angle measurements
virtually on a 3D model of an object. The virtual goniometer allows for rapid
data collection, and for the measurement of many angles that cannot be
physically accessed by a manual goniometer. We compare the intra-observer
variability of the manual and virtual goniometers, and find that the virtual
goniometer is far more consistent and reliable. Furthermore, the virtual
goniometer allows for precise replication of angle measurements, even among
multiple users, which is important for reproducibility of goniometric-based
research. The virtual goniometer is available as a plug-in in the open source
mesh processing packages Meshlab and Blender, making it easily accessible to
researchers exploring the potential for goniometry to improve archaeological
methods and address anthropological questions. | 2011.04898v2 |
2020-11-17 | Competing energy scales in topological superconducting heterostructures | Artificially engineered topological superconductivity has emerged as a viable
route to create Majorana modes, exotic quasiparticles which have raised great
expectations for storing and manipulating information in topological quantum
computational schemes. The essential ingredients for their realization are spin
non-degenerate metallic states proximitized to an s-wave superconductor. In
this context, proximity-induced superconductivity in materials with a sizable
spin-orbit coupling has been heavily investigated in recent years. Although
there is convincing evidence that superconductivity may indeed be induced, it
has been difficult to elucidate its topological nature. In this work, we
systematically engineer an artificial topological superconductor by
progressively introducing superconductivity (Nb) into metals with strong
spin-orbital coupling (Pt) and 3D topological surface states (Bi2Te3). Through
a longitudinal study of the character of superconducting vortices within s-wave
superconducting Nb and proximity-coupled Nb/Pt and Nb/Bi2Te3, we detect the
emergence of a zero-bias peak that is directly linked to the presence of
topological surface states. Supported by a detailed theoretical model, our
results are rationalized in terms of competing energy trends which are found to
impose an upper limit to the size of the minigap separating Majorana and
trivial modes, its size being ultimately linked to fundamental materials
properties. | 2011.08812v1 |
2020-12-01 | Phase-field modeling of biomineralization in mollusks and corals: Microstructure vs. formation mechanism | While biological crystallization processes have been studied on the
microscale extensively, models addressing the mesoscale aspects of such
phenomena are rare. In this work, we investigate whether the phase-field theory
developed in materials science for describing complex polycrystalline
structures on the mesoscale can be meaningfully adapted to model
crystallization in biological systems. We demonstrate the abilities of the
phase-field technique by modeling a range of microstructures observed in
mollusk shells and coral skeletons, including granular, prismatic,
sheet/columnar nacre, and sprinkled spherulitic structures. We also compare two
possible micromechanisms of calcification: the classical route via ion-by-ion
addition from a fluid state and a non-classical route, crystallization of an
amorphous precursor deposited at the solidification front. We show that with
appropriate choice of the model parameters microstructures similar to those
found in biomineralized systems can be obtained along both routes, though the
timescale of the non-classical route appears to be more realistic. The
resemblance of the simulated and natural biominerals suggests that, underneath
the immense biological complexity observed in living organisms, the underlying
design principles for biological structures may be understood with simple math,
and simulated by phase-field theory. | 2012.00666v1 |
2020-12-02 | Symmetry of the Magnetoelastic Interaction of Rayleigh and Shear Horizontal Magnetoacoustic Waves in Nickel Thin Films on LiTaO$_3$ | We study the interaction of Rayleigh and shear horizontal surface acoustic
waves (SAWs) with spin waves in thin Ni films on a piezoelectric LiTaO$_3$
substrate, which supports both SAW modes simultaneously. Because Rayleigh and
shear horizontal modes induce different strain components in the Ni thin films,
the symmetries of the magnetoelastic driving fields, of the magnetoelastic
response, and of the transmission nonreciprocity differ for both SAW modes. Our
experimental findings are well explained by a theoretical model based on a
modified Landau--Lifshitz--Gilbert approach. We show that the symmetries of the
magnetoelastic response driven by Rayleigh- and shear horizontal SAWs
complement each other, which makes it possible to excite spin waves for any
relative orientation of magnetization and SAW propagation direction and,
moreover, can be utilized to characterize surface strain components of unknown
acoustic wave modes. | 2012.01055v2 |
2020-12-03 | Localization of Malaria Parasites and White Blood Cells in Thick Blood Smears | Effectively determining malaria parasitemia is a critical aspect in assisting
clinicians to accurately determine the severity of the disease and provide
quality treatment. Microscopy applied to thick smear blood smears is the de
facto method for malaria parasitemia determination. However, manual
quantification of parasitemia is time consuming, laborious and requires
considerable trained expertise which is particularly inadequate in highly
endemic and low resourced areas. This study presents an end-to-end approach for
localisation and count of malaria parasites and white blood cells (WBCs) which
aid in the effective determination of parasitemia; the quantitative content of
parasites in the blood. On a dataset of slices of images of thick blood smears,
we build models to analyse the obtained digital images. To improve model
performance due to the limited size of the dataset, data augmentation was
applied. Our preliminary results show that our deep learning approach reliably
detects and returns a count of malaria parasites and WBCs with a high precision
and recall. We also evaluate our system against human experts and results
indicate a strong correlation between our deep learning model counts and the
manual expert counts (p=0.998 for parasites, p=0.987 for WBCs). This approach
could potentially be applied to support malaria parasitemia determination
especially in settings that lack sufficient Microscopists. | 2012.01994v1 |
2020-12-05 | Age-Optimal Low-Power Status Update over Time-Correlated Fading Channel | In this paper, we consider transmission scheduling in a status update system,
where updates are generated periodically and transmitted over a Gilbert-Elliott
fading channel. The goal is to minimize the long-run average age of information
(AoI) at the destination under an average energy constraint. We consider two
practical cases to obtain channel state information (CSI): (i) \emph{without
channel sensing} and (ii) \emph{with delayed channel sensing}. For case (i),
the channel state is revealed when an ACK/NACK is received at the transmitter
following a transmission, but when no transmission occurs, the channel state is
not revealed. Thus, we have to design schemes that balance tradeoffs across
energy, AoI, channel exploration, and channel exploitation. The problem is
formulated as a constrained partially observable Markov decision process
problem (POMDP). To reduce algorithm complexity, we show that the optimal
policy is a randomized mixture of no more than two stationary deterministic
policies each of which is of a threshold-type in the belief on the channel. For
case (ii), (delayed) CSI is available at the transmitter via channel sensing.
In this case, the tradeoff is only between the AoI and energy consumption and
the problem is formulated as a constrained MDP. The optimal policy is shown to
have a similar structure as in case (i) but with an AoI associated threshold.
Finally, the performance of the proposed structure-aware algorithms is
evaluated numerically and compared with a Greedy policy. | 2012.02958v2 |
2020-11-30 | Procode: the Swiss Multilingual Solution for Automatic Coding and Recoding of Occupations and Economic Activities | Objective. Epidemiological studies require data that are in alignment with
the classifications established for occupations or economic activities. The
classifications usually include hundreds of codes and titles. Manual coding of
raw data may result in misclassification and be time consuming. The goal was to
develop and test a web-tool, named Procode, for coding of free-texts against
classifications and recoding between different classifications. Methods. Three
text classifiers, i.e. Complement Naive Bayes (CNB), Support Vector Machine
(SVM) and Random Forest Classifier (RFC), were investigated using a k-fold
cross-validation. 30 000 free-texts with manually assigned classification codes
of French classification of occupations (PCS) and French classification of
activities (NAF) were available. For recoding, Procode integrated a workflow
that converts codes of one classification to another according to existing
crosswalks. Since this is a straightforward operation, only the recoding time
was measured. Results. Among the three investigated text classifiers, CNB
resulted in the best performance, where the classifier predicted accurately
57-81% and 63-83% classification codes for PCS and NAF, respectively. SVM lead
to somewhat lower results (by 1-2%), while RFC coded accurately up to 30% of
the data. The coding operation required one minute per 10 000 records, while
the recoding was faster, i.e. 5-10 seconds. Conclusion. The algorithm
integrated in Procode showed satisfactory performance, since the tool had to
assign the right code by choosing between 500-700 different choices. Based on
the results, the authors decided to implement CNB in Procode. In future, if
another classifier shows a superior performance, an update will include the
required modifications. | 2012.07521v1 |
2020-12-16 | Dynamic clay microstructures emerge via ion complexation waves | Clays control carbon, water and nutrient transport in the lithosphere,
promote cloud formation5 and lubricate fault slip through interactions among
hydrated mineral interfaces. Clay mineral properties are difficult to model
because their structures are disordered, curved and dynamic. Consequently,
interactions at the clay mineral-aqueous interface have been approximated using
electric double layer models based on single crystals of mica and atomistic
simulations. We discover that waves of complexation dipoles at dynamically
curving interfaces create an emergent long-range force that drives exfoliation
and restacking over time- and length-scales that are not captured in existing
models. Curvature delocalizes electrostatic interactions in ways that
fundamentally differ from planar surfaces, altering the ratio of ions bound to
the convex and concave sides of a layer. Multiple-scattering reconstruction of
low-dose energy-filtered cryo electron tomography enabled direct imaging of ion
complexes and electrolyte distributions at hydrated and curved mineral
interfaces with {\aa}ngstrom resolution over micron length scales. Layers
exfoliate and restack abruptly and repeatedly over timescales that depend
strongly on the counterion identity, demonstrating that the strong coupling
between elastic, electrostatic and hydration forces in clays promote collective
reorganization previously thought to be a feature only of active matter. | 2012.09295v1 |
2020-12-17 | Age-optimal Scheduling over Hybrid Channels | We consider the problem of minimizing the age of information when a source
can transmit status updates over two heterogeneous channels. Our work is
motivated by recent developments in 5G mmWave technology, where transmissions
may occur over an unreliable but fast (e.g., mmWave) channel or a slow reliable
(e.g., sub-6GHz) channel. The unreliable channel is modeled as a
time-correlated Gilbert-Elliot channel at a high rate when the channel is in
the 'ON' state. The reliable channel provides a deterministic but lower data
rate. The scheduling strategy determines the channel to be used for
transmission in each time slot, aiming to minimize the time-average age of
information (AoI). The optimal scheduling problem is formulated as a Markov
Decision Process (MDP), which is challenging to solve because super-modularity
does not hold in a part of the state space. We address this challenge and show
that a multi-dimensional threshold-type scheduling policy is optimal for
minimizing the age. By exploiting the structure of the MDP and analyzing the
discrete-time Markov chains (DTMCs) of the threshold-type policy, we devise a
low-complexity bisection algorithm to compute the optimal thresholds. We
compare different scheduling policies using numerical simulations. | 2012.09403v6 |
2020-12-21 | Variations on the Maiani-Testa approach and the inverse problem | We discuss a method to construct hadronic scattering and decay amplitudes
from Euclidean correlators, by combining the approach of a regulated inverse
Laplace transform with the work of Maiani and Testa. Revisiting the original
result, we observe that the key observation, i.e. that only threshold
scattering information can be extracted at large separations, can be understood
by interpreting the correlator as a spectral function, $\rho(\omega)$,
convoluted with the Euclidean kernel, $e^{- \omega t}$, which is sharply peaked
at threshold. We therefore consider a modification in which a smooth step
function, equal to one above a target energy, is inserted in the spectral
decomposition. This can be achieved either through Backus-Gilbert-like methods
or more directly using the variational approach. The result is a shifted
resolution function, such that the large $t$ limit projects onto scattering or
decay amplitudes above threshold. The utility of this method is highlighted
through large $t$ expansions of both three- and four-point functions that
include leading terms proportional to the real and imaginary parts (separately)
of the target observable. This work also presents new results relevant for the
un-modified correlator at threshold, including expressions for extracting the
$N \pi$ scattering length from four-point functions and a new strategy to
organize the large $t$ expansion that exhibits better convergence than the
expansion in powers of $1/t$. | 2012.11488v1 |
2021-01-13 | PID passivity-based droop control of power converters: Large-signal stability, robustness and performance | We present a full review of PID passivity-based controllers (PBC) applied to
power electronic converters, discussing limitations, unprecedented merits and
potential improvements in terms of large-signal stability, robustness and
performance. We provide four main contributions. The nominal case is first
considered and it is shown, under the assumption of perfect knowledge of the
system parameters, that the PID-PBC is able to guarantee global exponential
stability of a desired operating point for any positive gains. Second, we
analyze robustness of the controller to parameters uncertainty for a specific
class of power converters, by establishing precise stability margins. Third, we
propose a modification of the controller by introducing a leakage, in order to
overcome some of the intrinsic performance and robustness limitations.
Interestingly, such controller can be interpreted at steady-state as a droop
between the input and the passive output, similar to traditional primary
controllers. Fourth, we robustify the design against saturation of the control
input via an appropriate monotone transformation of the controller. The
obtained results are thoroughly discussed and validated by simulations on two
relevant power applications: a dc/dc boost converter and an HVDC grid-connected
voltage source converter. | 2101.05047v2 |
2021-02-15 | Recent Developments in Blockchain Technology and their Impact on Energy Consumption | The enormous power consumption of Bitcoin has led to undifferentiated
discussions in science and practice about the sustainability of blockchain and
distributed ledger technology in general. However, blockchain technology is far
from homogeneous - not only with regard to its applications, which now go far
beyond cryptocurrencies and have reached businesses and the public sector, but
also with regard to its technical characteristics and, in particular, its power
consumption. This paper summarizes the status quo of the power consumption of
various implementations of blockchain technology, with special emphasis on the
recent 'Bitcoin Halving' and so-called 'zk-rollups'. We argue that although
Bitcoin and other proof-of-work blockchains do indeed consume a lot of power,
alternative blockchain solutions with significantly lower power consumption are
already available today, and new promising concepts are being tested that could
further reduce in particular the power consumption of large blockchain networks
in the near future. From this we conclude that although the criticism of
Bitcoin's power consumption is legitimate, it should not be used to derive an
energy problem of blockchain technology in general. In many cases in which
processes can be digitised or improved with the help of more energy-efficient
blockchain variants, one can even expect net energy savings. | 2102.07886v1 |
2021-03-11 | Toward the Next Generation of News Recommender Systems | This paper proposes a vision and research agenda for the next generation of
news recommender systems (RS), called the table d'hote approach. A table d'hote
(translates as host's table) meal is a sequence of courses that create a
balanced and enjoyable dining experience for a guest. Likewise, we believe news
RS should strive to create a similar experience for the users by satisfying the
news-diet needs of a user. While extant news RS considers criteria such as
diversity and serendipity, and RS bundles have been studied for other contexts
such as tourism, table d'hote goes further by ensuring the recommended articles
satisfy a diverse set of user needs in the right proportions and in a specific
order. In table d'hote, available articles need to be stratified based on the
different ways that news can create value for the reader, building from
theories and empirical research in journalism and user engagement. Using
theories and empirical research from communication on the uses and
gratifications (U&G) consumers derive from media, we define two main strata in
a table d'hote news RS, each with its own substrata: 1) surveillance, which
consists of information the user needs to know, and 2) serendipity, which are
the articles offering unexpected surprises. The diversity of the articles
according to the defined strata and the order of the articles within the list
of recommendations are also two important aspects of the table d'hote in order
to give the users the most effective reading experience. We propose our vision,
link it to the existing concepts in the RS literature, and identify challenges
for future research. | 2103.06909v1 |
2021-03-16 | Machine learning methods for the prediction of micromagnetic magnetization dynamics | Machine learning (ML) entered the field of computational micromagnetics only
recently. The main objective of these new approaches is the automatization of
solutions of parameter-dependent problems in micromagnetism such as fast
response curve estimation modeled by the Landau-Lifschitz-Gilbert (LLG)
equation. Data-driven models for the solution of time- and parameter-dependent
partial differential equations require high dimensional training
data-structures. ML in this case is by no means a straight-forward trivial
task, it needs algorithmic and mathematical innovation. Our work introduces
theoretical and computational conceptions of certain kernel and neural network
based dimensionality reduction approaches for efficient prediction of solutions
via the notion of low-dimensional feature space integration. We introduce
efficient treatment of kernel ridge regression and kernel principal component
analysis via low-rank approximation. A second line follows neural network (NN)
autoencoders as nonlinear data-dependent dimensional reduction for the training
data with focus on accurate latent space variable description suitable for a
feature space integration scheme. We verify and compare numerically by means of
a NIST standard problem. The low-rank kernel method approach is fast and
surprisingly accurate, while the NN scheme can even exceed this level of
accuracy at the expense of significantly higher costs. | 2103.09079v2 |
2021-03-18 | Bounding the detection efficiency threshold in Bell tests using multiple copies of the maximally entangled two-qubit state carried by a single pair of particles | In this paper, we investigate the critical efficiency of detectors to observe
Bell nonlocality using multiple copies of the maximally entangled two-qubit
state carried by a single pair of particles, such as hyperentangled states, and
the product of Pauli measurements. It is known that in a
Clauser-Horne-Shimony-Holt (CHSH) Bell test the symmetric detection efficiency
of $82.84\%$ can be tolerated for the two-qubit maximally entangled state. We
beat this enigmatic threshold by entangling two particles with multiple degrees
of freedom. The obtained upper bounds of the symmetric detection efficiency
thresholds are $80.86\%$, $73.99\%$ and $69.29\%$ for two, three and four
copies of the two-qubit maximally entangled state, respectively. The number of
measurements and outcomes in the respective cases are 4, 8 and 16. To find the
improved thresholds, we use large-scale convex optimization tools, which allows
us to significantly go beyond state-of-the-art results. The proof is exact up
to three copies, while for four copies it is due to reliable numerical
computations. Specifically, we used linear programming to obtain the two-copy
threshold and the corresponding Bell inequality, and convex optimization based
on Gilbert's algorithm for three and four copies of the two-qubit state. We
show analytically that the symmetric detection efficiency threshold decays
exponentially with the number of copies of the two-qubit state. Our techniques
can also be applied to more general Bell nonlocality scenarios with more than
two parties. | 2103.10413v2 |
2021-04-05 | When Can Liquid Democracy Unveil the Truth? | In this paper, we investigate the so-called ODP-problem that has been
formulated by Caragiannis and Micha [10]. Here, we are in a setting with two
election alternatives out of which one is assumed to be correct. In ODP, the
goal is to organise the delegations in the social network in order to maximize
the probability that the correct alternative, referred to as ground truth, is
elected. While the problem is known to be computationally hard, we strengthen
existing hardness results by providing a novel strong approximation hardness
result: For any positive constant $C$, we prove that, unless $P=NP$, there is
no polynomial-time algorithm for ODP that achieves an approximation guarantee
of $\alpha \ge (\ln n)^{-C}$, where $n$ is the number of voters. The reduction
designed for this result uses poorly connected social networks in which some
voters suffer from misinformation. Interestingly, under some hypothesis on
either the accuracies of voters or the connectivity of the network, we obtain a
polynomial-time $1/2$-approximation algorithm. This observation proves formally
that the connectivity of the social network is a key feature for the efficiency
of the liquid democracy paradigm. Lastly, we run extensive simulations and
observe that simple algorithms (working either in a centralized or
decentralized way) outperform direct democracy on a large class of instances.
Overall, our contributions yield new insights on the question in which
situations liquid democracy can be beneficial. | 2104.01828v1 |
2021-04-05 | Floquet prethermalization with lifetime exceeding 90s in a bulk hyperpolarized solid | We report the observation of long-lived Floquet prethermal states in a bulk
solid composed of dipolar-coupled $^{13}$C nuclei in diamond at room
temperature. For precessing nuclear spins prepared in an initial transverse
state, we demonstrate pulsed spin-lock Floquet control that prevents their
decay over multiple-minute long periods. We observe Floquet prethermal
lifetimes $T_2'\approx$90.9s, extended >60,000-fold over the nuclear free
induction decay times. The spins themselves are continuously interrogated for
$\sim$10min, corresponding to the application of $\approx$5.8M control pulses.
The $^{13}$C nuclei are optically hyperpolarized by lattice Nitrogen Vacancy
(NV) centers; the combination of hyperpolarization and continuous spin readout
yields significant signal-to-noise in the measurements. This allows probing the
Floquet thermalization dynamics with unprecedented clarity. We identify four
characteristic regimes of the thermalization process, discerning short-time
transient processes leading to the prethermal plateau, and long-time system
heating towards infinite temperature. This work points to new opportunities
possible via Floquet control in networks of dilute, randomly distributed,
low-sensitivity nuclei. In particular, the combination of minutes-long
prethermal lifetimes and continuous spin interrogation opens avenues for
quantum sensors constructed from hyperpolarized Floquet prethermal nuclei. | 2104.01988v2 |
2021-04-14 | Generalized Simple Streaming Codes from MDS Codes | Streaming codes represent a packet-level FEC scheme for achieving reliable,
low-latency communication. In the literature on streaming codes, the
commonly-assumed Gilbert-Elliott channel model, is replaced by a more
tractable, delay-constrained, sliding-window (DCSW) channel model that can
introduce either random or burst erasures. The known streaming codes that are
rate optimal over the DCSW channel model are constructed by diagonally
embedding a scalar block code across successive packets. These code
constructions have field size that is quadratic in the delay parameter $\tau$
and have a somewhat complex structure with an involved decoding procedure. This
led to the introduction of simple streaming (SS) codes in which diagonal
embedding is replaced by staggered-diagonal embedding (SDE). The SDE approach
reduces the impact of a burst of erasures and makes it possible to construct
near-rate-optimal streaming codes using Maximum Distance Separable (MDS) code
having linear field size. The present paper takes this development one step
further, by retaining the staggered-diagonal feature, but permitting the
placement of more than one code symbol from a given scalar codeword within each
packet. These generalized, simple streaming codes allow us to improve upon the
rate of SS codes, while retaining the simplicity of working with MDS codes. We
characterize the maximum code rate of streaming codes under a constraint on the
number of contiguous packets over which symbols of the underlying scalar code
are dispersed. Such a constraint leads to simplified code construction and
reduced-complexity decoding. | 2104.07005v1 |
2021-04-22 | COVID-19 and Big Data: Multi-faceted Analysis for Spatio-temporal Understanding of the Pandemic with Social Media Conversations | COVID-19 has been devastating the world since the end of 2019 and has
continued to play a significant role in major national and worldwide events,
and consequently, the news. In its wake, it has left no life unaffected. Having
earned the world's attention, social media platforms have served as a vehicle
for the global conversation about COVID-19. In particular, many people have
used these sites in order to express their feelings, experiences, and
observations about the pandemic. We provide a multi-faceted analysis of
critical properties exhibited by these conversations on social media regarding
the novel coronavirus pandemic. We present a framework for analysis, mining,
and tracking the critical content and characteristics of social media
conversations around the pandemic. Focusing on Twitter and Reddit, we have
gathered a large-scale dataset on COVID-19 social media conversations. Our
analyses cover tracking potential reports on virus acquisition, symptoms,
conversation topics, and language complexity measures through time and by
region across the United States. We also present a BERT-based model for
recognizing instances of hateful tweets in COVID-19 conversations, which
achieves a lower error-rate than the state-of-the-art performance. Our results
provide empirical validation for the effectiveness of our proposed framework
and further demonstrate that social media data can be efficiently leveraged to
provide public health experts with inexpensive but thorough insight over the
course of an outbreak. | 2104.10807v1 |
2021-05-05 | exoplanet: Gradient-based probabilistic inference for exoplanet data & other astronomical time series | "exoplanet" is a toolkit for probabilistic modeling of astronomical time
series data, with a focus on observations of exoplanets, using PyMC3 (Salvatier
et al., 2016). PyMC3 is a flexible and high-performance model-building language
and inference engine that scales well to problems with a large number of
parameters. "exoplanet" extends PyMC3's modeling language to support many of
the custom functions and probability distributions required when fitting
exoplanet datasets or other astronomical time series. While it has been used
for other applications, such as the study of stellar variability, the primary
purpose of "exoplanet" is the characterization of exoplanets or multiple star
systems using time-series photometry, astrometry, and/or radial velocity. In
particular, the typical use case would be to use one or more of these datasets
to place constraints on the physical and orbital parameters of the system, such
as planet mass or orbital period, while simultaneously taking into account the
effects of stellar variability. | 2105.01994v2 |
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