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2013-10-01 | Ionized Plasma and Neutral Gas Coupling in the Sun's Chromosphere and Earth's Ionosphere/Thermosphere | We review our understanding of ionized plasma and neutral gas coupling in the
weakly ionized, stratified, electromagnetically-permeated regions of the Sun's
chromosphere and Earth's ionosphere/thermosphere. Using representative models
for each environment we derive fundamental descriptions of the coupling of the
constituent parts to each other and to the electric and magnetic fields, and we
examine the variation in magnetization of the ionized component. Using these
descriptions we compare related phenomena in the two environments, and discuss
electric currents, energy transfer and dissipation. We present a coupled
theoretical and numerical study of plasma instabilities in the two environments
that serves as an example of how the chromospheric and ionospheric communities
can further collaborate. We also suggest future collaborative studies that will
help improve our understanding of these two different atmospheres which share
many similarities, but have large disparities in key quantities. | 1310.0405v4 |
2013-11-13 | Excitation of radial collective modes in a quantum dot: Beyond linear response | We compare the response of five different models of two interacting electrons
in a quantum dot to an external short lived radial excitation that is strong
enough to excite the system well beyond the linear response regime. The models
considered describe the Coulomb interaction between the electrons in different
ways ranging from mean-field approaches to configuration interaction (CI)
models, where the two-electron Hamiltonian is diagonalized in a large truncated
Fock space. The radially symmetric excitation is selected in order to severely
put to test the different approaches to describe the interaction and
correlations of an electron system in a nonequilibrium state. As can be
expected for the case of only two electrons none of the mean-field models can
in full details reproduce the results obtained by the CI model. Nonetheless,
some linear and nonlinear characteristics are reproduced reasonably well. All
the models show activation of an increasing number of collective modes as the
strength of the excitation is increased. By varying slightly the confinement
potential of the dot we observe how sensitive the properties of the excitation
spectrum are to the Coulomb interaction and its correlation effects. In order
to approach closer the question of nonlinearity we solve one of the mean-field
models directly in a nonlinear fashion without resorting to iterations. | 1311.3252v2 |
2013-11-20 | Neutron-induced dpa, transmutations, gas production, and helium embrittlement of fusion materials | In a fusion reactor materials will be subjected to significant fluxes of
high-energy neutrons. As well as causing radiation damage, the neutrons also
initiate nuclear reactions leading to changes in the chemical composition of
materials (transmutation). Many of these reactions produce gases, particularly
helium, which cause additional swelling and embrittlement of materials. This
paper investigates, using a combination of neutron-transport and inventory
calculations, the variation in displacements per atom (dpa) and helium
production levels as a function of position within the high flux regions of a
recent conceptual model for the "next-step" fusion device DEMO. Subsequently,
the gas production rates are used to provide revised estimates, based on new
density-functional-theory results, for the critical component lifetimes
associated with the helium-induced grain-boundary embrittlement of materials.
The revised estimates give more optimistic projections for the lifetimes of
materials in a fusion power plant compared to a previous study, while at the
same time indicating that helium embrittlement remains one of the most
significant factors controlling the structural integrity of fusion power plant
components. | 1311.5079v1 |
2013-12-19 | The availability of research data declines rapidly with article age | Policies ensuring that research data are available on public archives are
increasingly being implemented at the government [1], funding agency [2-4], and
journal [5,6] level. These policies are predicated on the idea that authors are
poor stewards of their data, particularly over the long term [7], and indeed
many studies have found that authors are often unable or unwilling to share
their data [8-11]. However, there are no systematic estimates of how the
availability of research data changes with time since publication. We therefore
requested datasets from a relatively homogenous set of 516 articles published
between 2 and 22 years ago, and found that availability of the data was
strongly affected by article age. For papers where the authors gave the status
of their data, the odds of a dataset being extant fell by 17% per year. In
addition, the odds that we could find a working email address for the first,
last or corresponding author fell by 7% per year. Our results reinforce the
notion that, in the long term, research data cannot be reliably preserved by
individual researchers, and further demonstrate the urgent need for policies
mandating data sharing via public archives. | 1312.5670v1 |
2014-01-13 | On List-decodability of Random Rank Metric Codes | In the present paper, we consider list decoding for both random rank metric
codes and random linear rank metric codes. Firstly, we show that, for arbitrary
$0<R<1$ and $\epsilon>0$ ($\epsilon$ and $R$ are independent), if
$0<\frac{n}{m}\leq \epsilon$, then with high probability a random rank metric
code in $F_{q}^{m\times n}$ of rate $R$ can be list-decoded up to a fraction
$(1-R-\epsilon)$ of rank errors with constant list size $L$ satisfying $L\leq
O(1/\epsilon)$. Moreover, if $\frac{n}{m}\geq\Theta_R(\epsilon)$, any rank
metric code in $F_{q}^{m\times n}$ with rate $R$ and decoding radius
$\rho=1-R-\epsilon$ can not be list decoded in ${\rm poly}(n)$ time. Secondly,
we show that if $\frac{n}{m}$ tends to a constant $b\leq 1$, then every
$F_q$-linear rank metric code in $F_{q}^{m\times n}$ with rate $R$ and list
decoding radius $\rho$ satisfies the Gilbert-Varsharmov bound, i.e., $R\leq
(1-\rho)(1-b\rho)$. Furthermore, for arbitrary $\epsilon>0$ and any $0<\rho<1$,
with high probability a random $F_q$-linear rank metric codes with rate
$R=(1-\rho)(1-b\rho)-\epsilon$ can be list decoded up to a fraction $\rho$ of
rank errors with constant list size $L$ satisfying $L\leq O(\exp(1/\epsilon))$. | 1401.2693v2 |
2014-01-30 | Quantitative Decoding of Interactions in Tunable Nanomagnet Arrays Using First Order Reversal Curves | To develop a full understanding of interactions in nanomagnet arrays is a
persistent challenge, critically impacting their technological acceptance. This
paper reports the experimental, numerical and analytical investigation of
interactions in arrays of Co nanoellipses using the first-order reversal curve
(FORC) technique. A mean-field analysis has revealed the physical mechanisms
giving rise to all of the observed features: a shift of the non-interacting
FORC-ridge at the low-H$_c$ end off the local coercivity H$_c$ axis; a stretch
of the FORC-ridge at the high-H$_c$ end without shifting it off the H$_c$ axis;
and a formation of a tilted edge connected to the ridge at the low-H$_c$ end.
Changing from flat to Gaussian coercivity distribution produces a negative
feature, bends the ridge, and broadens the edge. Finally, nearest neighbor
interactions segment the FORC-ridge. These results demonstrate that the FORC
approach provides a comprehensive framework to qualitatively and quantitatively
decode interactions in nanomagnet arrays. | 1401.7749v1 |
2014-02-07 | For-all Sparse Recovery in Near-Optimal Time | An approximate sparse recovery system in $\ell_1$ norm consists of parameters
$k$, $\epsilon$, $N$, an $m$-by-$N$ measurement $\Phi$, and a recovery
algorithm, $\mathcal{R}$. Given a vector, $\mathbf{x}$, the system approximates
$x$ by $\widehat{\mathbf{x}} = \mathcal{R}(\Phi\mathbf{x})$, which must satisfy
$\|\widehat{\mathbf{x}}-\mathbf{x}\|_1 \leq
(1+\epsilon)\|\mathbf{x}-\mathbf{x}_k\|_1$. We consider the 'for all' model, in
which a single matrix $\Phi$, possibly 'constructed' non-explicitly using the
probabilistic method, is used for all signals $\mathbf{x}$. The best existing
sublinear algorithm by Porat and Strauss (SODA'12) uses $O(\epsilon^{-3}
k\log(N/k))$ measurements and runs in time $O(k^{1-\alpha}N^\alpha)$ for any
constant $\alpha > 0$.
In this paper, we improve the number of measurements to $O(\epsilon^{-2} k
\log(N/k))$, matching the best existing upper bound (attained by super-linear
algorithms), and the runtime to $O(k^{1+\beta}\textrm{poly}(\log
N,1/\epsilon))$, with a modest restriction that $\epsilon \leq (\log k/\log
N)^{\gamma}$, for any constants $\beta,\gamma > 0$. When $k\leq \log^c N$ for
some $c>0$, the runtime is reduced to $O(k\textrm{poly}(N,1/\epsilon))$. With
no restrictions on $\epsilon$, we have an approximation recovery system with $m
= O(k/\epsilon \log(N/k)((\log N/\log k)^\gamma + 1/\epsilon))$ measurements. | 1402.1726v2 |
2014-03-11 | Measuring the power spectrum of dark matter substructure using strong gravitational lensing | In recent years, it has become possible to detect individual dark matter
subhalos near images of strongly lensed extended background galaxies.
Typically, only the most massive subhalos in the strong lensing region may be
detected this way. In this work, we show that strong lenses may also be used to
constrain the much more numerous population of lower mass subhalos that are too
small to be detected individually. In particular, we show that the power
spectrum of projected density fluctuations in galaxy halos can be measured
using strong gravitational lensing. We develop the mathematical framework of
power spectrum estimation, and test our method on mock observations. We use our
results to determine the types of observations required to measure the
substructure power spectrum with high significance. We predict that deep
observations ($\sim10$ hours on a single target) with current facilities can
measure this power spectrum at the $3\sigma$ level, with no apparent degeneracy
with unknown clumpiness in the background source structure or fluctuations from
detector noise. Upcoming ALMA measurements of strong lenses are capable of
placing strong constraints on the abundance of dark matter subhalos and the
underlying particle nature of dark matter. | 1403.2720v3 |
2014-03-14 | Cosmological Spectral Deconvolution | One of the main goals of modern observational cosmology is to map the large
scale structure of the Universe. A potentially powerful approach for doing this
would be to exploit three-dimensional spectral maps, i.e. the specific
intensity of extragalactic light as a function of wavelength and direction on
the sky, to measure spatial variations in the total extragalactic light
emission and use these as a tracer of the clustering of matter. A main
challenge is that the observed intensity as a function of wavelength is a
convolution of the source luminosity density with the rest-frame spectral
energy distribution. In this paper, we introduce the method of spectral
deconvolution as a way to invert this convolution and extract the clustering
information. We show how one can use observations of the mean and angular
fluctuations of extragalactic light as a function of wavelength, assuming
statistical isotropy, to reconstruct jointly the rest-frame spectral energy
distribution of the sources and the source spatial density fluctuations. This
method is more general than the well known line mapping technique as it does
not rely on spectral lines in the emitted spectra. After introducing the
general formalism, we discuss its implementation and limitations. This formal
paper sets the stage for future more practical studies. | 1403.3727v1 |
2014-03-27 | On the Performance of Short Block Codes over Finite-State Channels in the Rare-Transition Regime | As the mobile application landscape expands, wireless networks are tasked
with supporting different connection profiles, including real-time traffic and
delay-sensitive communications. Among many ensuing engineering challenges is
the need to better understand the fundamental limits of forward error
correction in non-asymptotic regimes. This article characterizes the
performance of random block codes over finite-state channels and evaluates
their queueing performance under maximum-likelihood decoding. In particular,
classical results from information theory are revisited in the context of
channels with rare transitions, and bounds on the probabilities of decoding
failure are derived for random codes. This creates an analysis framework where
channel dependencies within and across codewords are preserved. Such results
are subsequently integrated into a queueing problem formulation. For instance,
it is shown that, for random coding on the Gilbert-Elliott channel, the
performance analysis based on upper bounds on error probability provides very
good estimates of system performance and optimum code parameters. Overall, this
study offers new insights about the impact of channel correlation on the
performance of delay-aware, point-to-point communication links. It also
provides novel guidelines on how to select code rates and block lengths for
real-time traffic over wireless communication infrastructures. | 1403.7232v1 |
2014-04-08 | Cost-oblivious storage reallocation | Databases need to allocate and free blocks of storage on disk. Freed blocks
introduce holes where no data is stored. Allocation systems attempt to reuse
such deallocated regions in order to minimize the footprint on disk. If
previously allocated blocks cannot be moved, the problem is called the memory
allocation problem, which is known to have a logarithmic overhead in the
footprint.
This paper defines the storage reallocation problem, where previously
allocated blocks can be moved, or reallocated, but at some cost. The algorithms
presented here are cost oblivious, in that they work for a broad and reasonable
class of cost functions, even when they do not know what the cost function is.
The objective is to minimize the storage footprint, that is, the largest
memory address containing an allocated object, while simultaneously minimizing
the reallocation costs. This paper gives asymptotically optimal algorithms for
storage reallocation, in which the storage footprint is at most (1+epsilon)
times optimal, and the reallocation cost is at most (1/epsilon) times the
original allocation cost, which is also optimal. The algorithms are cost
oblivious as long as the allocation/reallocation cost function is subadditive. | 1404.2019v3 |
2014-04-14 | Magneto-photonic phenomena at terahertz frequencies | Magneto-terahertz phenomena are the main focus of the thesis. This work
started as supporting research for the science of an X-ray laser (SwissFEL).
X-ray lasers have recently drawn great attention as an unprecedented tool for
scientific research on the ultrafast scale..... To answer this fundamental
question, we performed original numerical simulations using a coupled Landau-
Lifshitz-Gilbert Maxwell model. ... Those requirements were the motivations for
the experiments performed in the second part of the thesis. To shape the
terahertz pulses, .... Regarding the field intensities, we followed two
approaches. The first deals with field enhancement in nanoslits arrays. We
designed a subwavelength structure characterized by simultaneous high field
enhancement and high transmission at terahertz frequencies to suit nonlinear
sources. The second approach depended on up-scaling the generation from
laser-induced plasma by increasing the pump wavelengths. Numerical calculations
have also brought to our attention the importance of linear magnetoterahertz
effects. In particular, the simulations showed that the ultrafast dynamics
could lead to significant rotation of the polarization plane of the triggering
terahertz pulse. Motivated by this finding, we focused in the last part of the
thesis on the linear effects. We performed three original studies coming out
with first demonstrations of broadband non-reciprocal terahertz phase
retarders, terahertz magnetic modulators, and the non-reciprocal terahertz
isolators. In the first two experiments, we extended the unique properties of
the magnetic liquids (Ferrofluids) to the terahertz regime. In the latter
experiment, we used a permanent magnet (Ferrite) to experimentally show
complete isolation (unidirectional transmission) of the terahertz waves. | 1404.3764v1 |
2014-05-13 | The Star Formation Histories of Local Group Dwarf Galaxies II. Searching For Signatures of Reionization | We search for signatures of reionization in the star formation histories
(SFHs) of 38 Local Group dwarf galaxies (10$^4$ $<$ M$_{\star}$ $<$ 10$^9$
M$_{\odot}$). The SFHs are derived from color-magnitude diagrams using archival
Hubble Space Telescope/Wide Field Planetary Camera 2 imaging. Only five
quenched galaxies (And V, And VI, And XIII, Leo IV, Hercules) are consistent
with forming the bulk of their stars before reionization, when full
uncertainties are considered. Observations of 13 of the predicted `true
fossils' identified by Bovill & Ricotti show that only two (Hercules and Leo
IV) indicate star formation quenched by reionization. However, both are within
the virial radius of the Milky Way and evidence of tidal disturbance
complicates this interpretation. We argue that the late-time gas capture
scenario posited by Ricotti for the low mass, gas-rich, and star-forming fossil
candidate Leo T is observationally indistinguishable from simple gas retention.
Given the ambiguity between environmental effects and reionization, the best
reionization fossil candidates are quenched low mass field galaxies (e.g., KKR
25). | 1405.3281v1 |
2014-05-26 | Constraints for the Progenitor Masses of 17 Historic Core-Collapse Supernovae | Using resolved stellar photometry measured from archival HST imaging, we
generate color-magnitude diagrams of the stars within 50 pc of the locations of
historic core-collapse supernovae that took place in galaxies within 8 Mpc. We
fit these color-magnitude distributions with stellar evolution models to
determine the best-fit age distribution of the young population. We then
translate these age distributions into probability distributions for the
progenitor mass of each SNe. The measurements are anchored by the main-sequence
stars surrounding the event, making them less sensitive to assumptions about
binarity, post-main-sequence evolution, or circumstellar dust. We demonstrate
that, in cases where the literature contains masses that have been measured
from direct imaging, our measurements are consistent with (but less precise
than) these measurements. Using this technique, we constrain the progenitor
masses of 17 historic SNe, 11 of which have no previous estimates from direct
imaging. Our measurements still allow the possibility that all SNe progenitor
masses are <20 M_sun. However, the large uncertainties for the highest-mass
progenitors also allow the possibility of no upper-mass cutoff. | 1405.6626v1 |
2014-06-09 | Calibrating CHIME, A New Radio Interferometer to Probe Dark Energy | The Canadian Hydrogen Intensity Mapping Experiment (CHIME) is a transit
interferometer currently being built at the Dominion Radio Astrophysical
Observatory (DRAO) in Penticton, BC, Canada. We will use CHIME to map neutral
hydrogen in the frequency range 400 -- 800\,MHz over half of the sky, producing
a measurement of baryon acoustic oscillations (BAO) at redshifts between 0.8 --
2.5 to probe dark energy. We have deployed a pathfinder version of CHIME that
will yield constraints on the BAO power spectrum and provide a test-bed for our
calibration scheme. I will discuss the CHIME calibration requirements and
describe instrumentation we are developing to meet these requirements. | 1406.2267v1 |
2014-07-10 | Beating the Heat! Automated Characterization of Piezoelectric Tubes for Starbugs | The Australian Astronomical Observatory has extensively prototyped a new
robotic positioner to allow simultaneous positioning of optical fibers at the
focal plane called 'Starbugs'. The Starbug devices each consist of two
concentric piezoelectric tubes that 'walk' the optical fiber over the focal
plane to accuracy of several microns. Ongoing research has led to the
development of several Starbug prototypes, but lack of performance data has
hampered further progress in the design of the Starbug positioners and the
support equipment required to power and control them. Furthermore, Starbugs
have been selected for the TAIPAN instrument, a prototype for MANIFEST on the
GMT. A need now arises to measure and characterize 100's of piezoelectric tubes
before full scale production of Starbugs for TAIPAN. The manual measurements of
these piezoelectric tubes are a time consuming process taking several hours.
Therefore, a versatile automated system is needed to measure and characterize
these tubes in the laboratory before production of Starbugs. We have solved
this problem with the design of an automated LabVIEW application that
significantly reduces test times to several minutes. We present the various
design aspects of the automation system and provide analyses of example
piezoelectric tubes for Starbugs. | 1407.2681v1 |
2014-07-16 | TriAnd and its Siblings: Satellites of Satellites in the Milky Way Halo | We explore the Triangulum-Andromeda (TriAnd) overdensity in the SPLASH
(Spectroscopic and Photometric Landscape of Andromeda's Stellar Halo) and SEGUE
(the Sloan Extension for Galactic Understanding and Exploration) spectroscopic
surveys. Milky Way main sequence turn-off stars in the SPLASH survey reveal
that the TriAnd overdensity and the recently discovered PAndAS stream (Martin
et al. 2014) share a common heliocentric distance (D ~ 20 kpc), position on the
sky, and line-of-sight velocity (V_GSR ~ 50 km/s). Similarly, A-type, giant,
and main sequence turn-off stars selected from the SEGUE survey in the vicinity
of the Segue 2 satellite show that TriAnd is prevalent in these fields, with a
velocity and distance similar to Segue 2. The coincidence of the PAndAS stream
and Segue 2 satellite in positional and velocity space to TriAnd suggests that
these substructures are all associated, and may be a fossil record of
group-infall onto the Milky Way halo. In this scenario, the Segue 2 satellite
and PAndAS stream are "satellites of satellites", and the large, metal-rich
TriAnd overdensity is the remains of the group central. | 1407.4458v2 |
2014-07-17 | The Green's Function for the Hückel (Tight Binding) Model | Applications of the H\"uckel (tight binding) model are ubiquitous in quantum
chemistry and solid state physics. The matrix representation of this model is
isomorphic to an unoriented vertex adjacency matrix of a bipartite graph, which
is also the Laplacian matrix plus twice the identity. In this paper, we
analytically calculate the determinant and, when it exists, the inverse of this
matrix in connection with the Green's function, $\mathbf{G}$, of the $N\times
N$ H\"uckel matrix. A corollary is a closed form expression for a Harmonic sum
(Eq. 12). We then extend the results to $d-$dimensional lattices, whose linear
size is $N$. The existence of the inverse becomes a question of number theory.
We prove a new theorem in number theory pertaining to vanishing sums of cosines
and use it to prove that the inverse exists if and only if $N+1$ and $d$ are
odd and $d$ is smaller than the smallest divisor of $N+1$. We corroborate our
results by demonstrating the entry patterns of the Green's function and discuss
applications related to transport and conductivity. | 1407.4780v4 |
2014-07-29 | Formation of magnetic skyrmions with tunable properties in PdFe bilayer deposited on Ir(111) | We perform an extensive study of the spin-configurations in a PdFe bilayer on
Ir(111) in terms of ab initio and spin-model calculations. We use the
spin-cluster expansion technique to obtain spin model parameters, and solve the
Landau-Lifshitz-Gilbert equations at zero temperature. In particular, we focus
on effects of layer relaxations and the evolution of the magnetic ground state
in external magnetic field. In the absence of magnetic field, we find a
spin-spiral ground state, while applying external magnetic field skyrmions are
generated in the system. Based on energy calculations of frozen spin
configurations with varying magnetic field we obtain excellent agreement for
the phase boundaries with available experiments. We find that the wave length
of spin-spirals and the diameter of skyrmions decrease with increasing inward
Fe layer relaxation which is correlated with the increasing ratio of the
nearest-neighbor Dzyaloshinskii-Moriya interaction and the isotropic exchange
coupling, $D/J$. Our results also indicate that the applied field needed to
stabilize the skyrmion lattice increases when the diameter of individual
skyrmions decreases. Based on our observations, we suggest that the formation
of the skyrmion lattice can be tuned by small structural modification of the
thin film. | 1407.7718v2 |
2014-08-14 | Developing Structural, High-heat flux and Plasma Facing Materials for a near-term DEMO Fusion Power Plant: the EU Assessment | The findings of the EU 'Materials Assessment Group' (MAG), within the 2012 EU
Fusion Roadmap exercise, are discussed. MAG analysed the technological
readiness of structural, plasma facing and high heat flux materials for a DEMO
concept to be constructed in the early 2030s, proposing a coherent strategy for
R&D up to a DEMO construction decision. Technical consequences for the
materials required and the development, testing and modelling programmes, are
analysed using: a systems engineering approach, considering reactor operational
cycles, efficient maintenance and inspection requirements, and interaction with
functional materials/coolants; and a project-based risk analysis, with R&D to
mitigate risks from material shortcomings including development of specific
risk mitigation materials. | 1408.3546v1 |
2014-08-26 | Weedy adaptation in Setaria spp.: IX. Effects of salinity, temperature, light and seed dormancy on Setaria faberi seed germination | Life in salty habitats is a function of tolerance to those chemicals at all
critical phases of a plant's life history. The ability to withstand salt as an
established plant may require different mechanisms and plant traits than those
needed to germinate in salty soils. Seeds establishing themselves in high salt
content may respond differently depending on the light conditions and seed
germinability at the time of salty water imbibition. S. faberi seed (and S.
viridis and S. pumila) plants were discovered thriving along the seacoasts of
Southern Japan. These plants possess the ability to after-ripen, germinate,
emerge and establish themselves, grow and reproduce in the salty soils and
salt-laden atmospheres present in these windy habitats. The objectives of this
paper are to determine the effect of salt (NaCl) in water imbibed by S. faberi
seed during after-ripening and germination, as well temperature and light.
Observations made also provide insights on the possible relationship between
salt and drought tolerance. Seed germination of all phenotypes inhibited by two
percent or more of NaCl. The effects of lesser amounts of NaCl on each of the
three phenotypes was highly dependent on the specific temperature and light
conditions. The three test phenotypes provided a good range to detect responses
to salinity, allowing the observation of both stimulatory and inhibitory
responses. | 1408.6187v1 |
2014-09-02 | Accessing Different Spin-Disordered States using First Order Reversal Curves | Combined first order reversal curve (FORC) analyses of the magnetization
(M-FORC) and magnetoresistance (MR-FORC) have been employed to provide a
comprehensive study of the M-MR correlation in two canonical systems: a
NiFe/Cu/FePt pseudo spin-valve (PSV) and a [Co/Cu]8 multilayer. In the PSV, due
to the large difference in switching fields and minimal interactions between
the NiFe and FePt layers, the M and MR show a simple one-to-one relationship
during reversal. In the [Co/Cu]8 multilayer, the correlation between the
magnetization reversal and MR evolution is more complex. This is primarily due
to the similar switching fields of, and interactions between, the constituent
Co layers. The FORC protocol accesses states with much higher spin disorders
and larger MR than those found along the conventional major loop field-cycle.
Unlike the M-FORC measurements, which only probe changes in the macroscopic
magnetization, the MR-FORCs are more sensitive to the microscopic domain
configurations, as those are most important in determining the resultant MR
effect size. This approach is generally applicable to spintronic systems to
realize the maximum spin-disorder and the largest MR. | 1409.0825v1 |
2014-10-20 | Momentum space imaging of Cooper pairing in a half-Dirac-gas topological superconductor (a helical 2D topological superconductor) | Superconductivity in Dirac electrons has recently been proposed as a new
platform between novel concepts in high-energy and condensed matter physics. It
has been proposed that supersymmetry and exotic quasiparticles, both of which
remain elusive in particle physics, may be realized as emergent particles in
superconducting Dirac electron systems. Using artificially fabricated
topological insulator-superconductor heterostructures, we present direct
spectroscopic evidence for the existence of Cooper pairing in a half Dirac gas
2D topological superconductor. Our studies reveal that superconductivity in a
helical Dirac gas is distinctly different from that of in an ordinary
two-dimensional superconductor while considering the spin degrees of freedom of
electrons. We further show that the pairing of Dirac electrons can be
suppressed by time-reversal symmetry breaking impurities removing the
distinction. Our demonstration and momentum-space imaging of Cooper pairing in
a half Dirac gas and its magnetic behavior taken together serve as a critically
important 2D topological superconductor platform for future testing of novel
fundamental physics predictions such as emergent supersymmetry and quantum
criticality in topological systems. | 1410.5405v1 |
2014-12-23 | Dynamical symmetries of Markov processes with multiplicative white noise | We analyse various properties of stochastic Markov processes with
multiplicative white noise. We take a single-variable problem as a simple
example, and we later extend the analysis to the Landau-Lifshitz-Gilbert
equation for the stochastic dynamics of a magnetic moment. In particular, we
focus on the non-equilibrium transfer of angular momentum to the magnetization
from a spin-polarised current of electrons, a technique which is widely used in
the context of spintronics to manipulate magnetic moments. We unveil two hidden
dynamical symmetries of the generating functionals of these Markovian
multiplicative white-noise processes. One symmetry only holds in equilibrium
and we use it to prove generic relations such as the fluctuation-dissipation
theorems. Out of equilibrium, we take profit of the symmetry-breaking terms to
prove fluctuation theorems. The other symmetry yields strong dynamical
relations between correlation and response functions which can notably simplify
the numerical analysis of these problems. Our construction allows us to clarify
some misconceptions on multiplicative white-noise stochastic processes that can
be found in the literature. In particular, we show that a first-order
differential equation with multiplicative white noise can be transformed into
an additive-noise equation, but that the latter keeps a non-trivial memory of
the discretisation prescription used to define the former. | 1412.7564v2 |
2015-01-09 | Random Triangle Theory with Geometry and Applications | What is the probability that a random triangle is acute? We explore this old
question from a modern viewpoint, taking into account linear algebra, shape
theory, numerical analysis, random matrix theory, the Hopf fibration, and much
much more. One of the best distributions of random triangles takes all six
vertex coordinates as independent standard Gaussians. Six can be reduced to
four by translation of the center to $(0,0)$ or reformulation as a 2x2 matrix
problem.
In this note, we develop shape theory in its historical context for a wide
audience. We hope to encourage other to look again (and differently) at
triangles.
We provide a new constructive proof, using the geometry of parallelians, of a
central result of shape theory: Triangle shapes naturally fall on a hemisphere.
We give several proofs of the key random result: that triangles are uniformly
distributed when the normal distribution is transferred to the hemisphere. A
new proof connects to the distribution of random condition numbers.
Generalizing to higher dimensions, we obtain the "square root ellipticity
statistic" of random matrix theory.
Another proof connects the Hopf map to the SVD of 2 by 2 matrices. A new
theorem describes three similar triangles hidden in the hemisphere. Many
triangle properties are reformulated as matrix theorems, providing insight to
both. This paper argues for a shift of viewpoint to the modern approaches of
random matrix theory. As one example, we propose that the smallest singular
value is an effective test for uniformity. New software is developed and
applications are proposed. | 1501.03053v1 |
2015-02-03 | The use of covariates and random effects in evaluating predictive biomarkers under a potential outcome framework | Predictive or treatment selection biomarkers are usually evaluated in a
subgroup or regression analysis with focus on the treatment-by-marker
interaction. Under a potential outcome framework (Huang, Gilbert and Janes
[Biometrics 68 (2012) 687-696]), a predictive biomarker is considered a
predictor for a desirable treatment benefit (defined by comparing potential
outcomes for different treatments) and evaluated using familiar concepts in
prediction and classification. However, the desired treatment benefit is
unobservable because each patient can receive only one treatment in a typical
study. Huang et al. overcome this problem by assuming monotonicity of potential
outcomes, with one treatment dominating the other in all patients. Motivated by
an HIV example that appears to violate the monotonicity assumption, we propose
a different approach based on covariates and random effects for evaluating
predictive biomarkers under the potential outcome framework. Under the proposed
approach, the parameters of interest can be identified by assuming conditional
independence of potential outcomes given observed covariates, and a sensitivity
analysis can be performed by incorporating an unobserved random effect that
accounts for any residual dependence. Application of this approach to the
motivating example shows that baseline viral load and CD4 cell count are both
useful as predictive biomarkers for choosing antiretroviral drugs for
treatment-naive patients. | 1502.00757v1 |
2015-02-03 | The Blackhole-Dark Matter Halo Connection | We explore the connection between the central supermassive blackholes (SMBH)
in galaxies and the dark matter halo through the relation between the masses of
the SMBHs and the maximum circular velocities of the host galaxies, as well as
the relationship between stellar velocity dispersion of the spheroidal
component and the circular velocity. Our assumption here is that the circular
velocity is a proxy for the mass of the dark matter halo. We rely on a
heterogeneous sample containing galaxies of all types. The only requirement is
that the galaxy has a direct measurement of the mass of its SMBH and a direct
measurement of its circular velocity and its velocity dispersion. Previous
studies have analyzed the connection between the SMBH and dark matter halo
through the relationship between the circular velocity and the bulge velocity
dispersion, with the assumption that the bulge velocity dispersion stands in
for the mass of the SMBH, via the well{}-established SMBH mass{}-bulge velocity
dispersion relation. Using intermediate relations may be misleading when one is
studying them to decipher the active ingredients of galaxy formation and
evolution. We believe that our approach will provide a more direct probe of the
SMBH and the dark matter halo connection. We find that the correlation between
the mass of supermassive blackholes and the circular velocities of the host
galaxies is extremely weak, leading us to state the dark matter halo may not
play a major role in regulating the blackhole growth in the present Universe. | 1502.00775v1 |
2015-03-01 | Computing in continuous space with self-assembling polygonal tiles | In this paper we investigate the computational power of the polygonal tile
assembly model (polygonal TAM) at temperature 1, i.e. in non-cooperative
systems. The polygonal TAM is an extension of Winfree's abstract tile assembly
model (aTAM) which not only allows for square tiles (as in the aTAM) but also
allows for tile shapes that are polygons. Although a number of self-assembly
results have shown computational universality at temperature 1, these are the
first results to do so by fundamentally relying on tile placements in
continuous, rather than discrete, space. With the square tiles of the aTAM, it
is conjectured that the class of temperature 1 systems is not computationally
universal. Here we show that the class of systems whose tiles are composed of a
regular polygon P with n > 6 sides is computationally universal. On the other
hand, we show that the class of systems whose tiles consist of a regular
polygon P with n <= 6 cannot compute using any known techniques. In addition,
we show a number of classes of systems whose tiles consist of a non-regular
polygon with n >= 3 sides are computationally universal. | 1503.00327v2 |
2015-03-05 | Nonparametric Bounds and Sensitivity Analysis of Treatment Effects | This paper considers conducting inference about the effect of a treatment (or
exposure) on an outcome of interest. In the ideal setting where treatment is
assigned randomly, under certain assumptions the treatment effect is
identifiable from the observable data and inference is straightforward.
However, in other settings such as observational studies or randomized trials
with noncompliance, the treatment effect is no longer identifiable without
relying on untestable assumptions. Nonetheless, the observable data often do
provide some information about the effect of treatment, that is, the parameter
of interest is partially identifiable. Two approaches are often employed in
this setting: (i) bounds are derived for the treatment effect under minimal
assumptions, or (ii) additional untestable assumptions are invoked that render
the treatment effect identifiable and then sensitivity analysis is conducted to
assess how inference about the treatment effect changes as the untestable
assumptions are varied. Approaches (i) and (ii) are considered in various
settings, including assessing principal strata effects, direct and indirect
effects and effects of time-varying exposures. Methods for drawing formal
inference about partially identified parameters are also discussed. | 1503.01598v1 |
2015-03-16 | Dynamics of Current and Field Driven Domain Wall Motion under the Influence of Transverse Magnetic Field | The dynamics of transverse Neel domain wall in a ferromagnetic nanostrip in
the presence of driving field, current and transverse magnetic field is
investigated by the Landau-Lifshitz-Gilbert(LLG) equation with the adiabatic
and non-adiabatic spin-transfer torques both analytically and numerically. The
analytical expressions for the velocity, width, excitation angle and
displacement for the domain wall are obtained by using small angle
approximation along with Walkers trial function. The results show that the
initial velocity of the domain wall can be controlled by the adiabatic
spin-transfer torque and the saturated velocity can be controlled by the
non-adiabatic spin-transfer torque and driving field. The large increase in the
saturated velocity of the domain wall driven by current and field due to the
transverse magnetic field is identified through the presence of driving field.
There is no impact in the saturated velocity of the domain wall driven by
current from the transverse magnetic field. For the domain wall driven by the
current in the presence of the transverse magnetic field, the saturated
velocity remains constant. The transverse magnetic field along with current and
driving field is more advantageous that the transverse magnetic field along
with current for increasing the saturated velocity of the domain wall. The
numerical results showed that the saturated velocity is increased by the
transverse magnetic field with the irrespective of the directions of the
driving field and current further it is higher and lower when the directions of
driving field and current are antiparallel and parallel respectively. The
obtained analytical solutions are closely coincided with the computed numerical
results. | 1503.04560v2 |
2015-03-17 | Single Hit Energy-resolved Laue Diffraction | In-situ white light Laue diffraction has been successfully used to
interrogate the structure of single crystal materials undergoing rapid
(nanosecond) dynamic compression up to megabar pressures. However, information
on strain state accessible via this technique is limited, reducing its
applicability for a range of applications. We present an extension to the
existing Laue diffraction platform in which we record the photon energy of a
subset of diffraction peaks. This allows for a measurement of the longitudinal
and transverse strains in-situ during compression. Consequently, we demonstrate
measurement of volumetric compression of the unit cell, in addition to the
limited aspect ratio information accessible in conventional white light Laue.
We present preliminary results for silicon, where only an elastic strain is
observed. VISAR measurements show the presence of a two wave structure and
measurements show that material downstream of the second wave does not
contribute to the observed diffraction peaks, supporting the idea that this
material may be highly disordered, or has undergone large scale rotation. | 1503.05131v2 |
2015-03-24 | No variations in transit times for Qatar-1 b | The transiting hot Jupiter planet Qatar-1 b was presented to exhibit
variations in transit times that could be of perturbative nature. A hot Jupiter
with a planetary companion on a nearby orbit would constitute an unprecedented
planetary configuration, important for theories of formation and evolution of
planetary systems. We performed a photometric follow-up campaign to confirm or
refute transit timing variations. We extend the baseline of transit
observations by acquiring 18 new transit light curves acquired with 0.6-2.0 m
telescopes. These photometric time series, together with data available in the
literature, were analyzed in a homogenous way to derive reliable transit
parameters and their uncertainties. We show that the dataset of transit times
is consistent with a linear ephemeris leaving no hint for any periodic
variations with a range of 1 min. We find no compelling evidence for the
existence of a close-in planetary companion to Qatar-1 b. This finding is in
line with a paradigm that hot Jupiters are not components of compact
multi-planetary systems. Based on dynamical simulations, we place tighter
constraints on a mass of any fictitious nearby planet in the system.
Furthermore, new transit light curves allowed us to redetermine system
parameters with the precision better than that reported in previous studies.
Our values generally agree with previous determinations. | 1503.07191v1 |
2015-03-25 | Rigorous numerical study of strong microwave photon-magnon coupling in all-dielectric magnetic multilayers | We demonstrate theoretically a strong local enhancement of the intensity of
the in-plane microwave magnetic field in multilayered structures made from a
magneto-insulating yttrium iron garnet (YIG) layer sandwiched between two
non-magnetic layers with a high dielectric constant matching that of YIG. The
enhancement is predicted for the excitation regime when the microwave magnetic
field is induced inside the multilayer by the transducer of a stripline
Broadband Ferromagnetic Resonance (BFMR) setup. By means of a rigorous
numerical solution of the Landau-Lifshitz-Gilbert equation consistently with
the Maxwell's equations, we investigate the magnetisation dynamics in the
multilayer. We reveal a strong photon-magnon coupling, which manifests itself
as anti-crossing of the ferromagnetic resonance (FMR) magnon mode supported by
the YIG layer and the electromagnetic resonance mode supported by the whole
multilayered structure. The frequency of the magnon mode depends on the
external static magnetic field, which in our case is applied tangentially to
the multilayer in the direction perpendicular to the microwave magnetic field
induced by the stripline of the BFMR setup. The frequency of the
electromagnetic mode is independent of the static magnetic field. Consequently,
the predicted photon-magnon coupling is sensitive to the applied magnetic field
and thus can be used in magnetically tuneable metamaterials based on
simultaneously negative permittivity and permeability achievable thanks to the
YIG layer. We also suggest that the predicted photon-magnon coupling may find
applications in microwave quantum information systems. | 1503.07282v1 |
2015-04-22 | Thermally Driven Ratchet Motion of Skyrmion Microcrystal and Topological Magnon Hall Effect | Spontaneously emergent chirality is an issue of fundamental importance across
the natural sciences. It has been argued that a unidirectional (chiral)
rotation of a mechanical ratchet is forbidden in thermal equilibrium, but
becomes possible in systems out of equilibrium. Here we report our finding that
a topologically nontrivial spin texture known as a skyrmion - a particle-like
object in which spins point in all directions to wrap a sphere - constitutes
such a ratchet. By means of Lorentz transmission electron microscopy we show
that micron-sized crystals of skyrmions in thin films of Cu2OSeO3 and MnSi
display a unidirectional rotation motion. Our numerical simulations based on a
stochastic Landau-Lifshitz-Gilbert equation suggest that this rotation is
driven solely by thermal fluctuations in the presence of a temperature
gradient, whereas in thermal equilibrium it is forbidden by the Bohr-van
Leeuwen theorem. We show that the rotational flow of magnons driven by the
effective magnetic field of skyrmions gives rise to the skyrmion rotation,
therefore suggesting that magnons can be used to control the motion of these
spin textures. | 1504.05860v1 |
2015-05-05 | Evaluating the Potential of a Dual Randomized Kaczmarz Solver for Laplacian Linear Systems | A new method for solving Laplacian linear systems proposed by Kelner et al.
involves the random sampling and update of fundamental cycles in a graph.
Kelner et al. proved asymptotic bounds on the complexity of this method but did
not report experimental results. We seek to both evaluate the performance of
this approach and to explore improvements to it in practice. We compare the
performance of this method to other Laplacian solvers on a variety of real
world graphs. We consider different ways to improve the performance of this
method by exploring different ways of choosing the set of cycles and the
sequence of updates, with the goal of providing more flexibility and potential
parallelism. We propose a parallel model of the Kelner et al. method, for
evaluating potential parallelism in terms of the span of edges updated at each
iteration. We provide experimental results comparing the potential parallelism
of the fundamental cycle basis and our extended cycle set. Our preliminary
experiments show that choosing a non-fundamental set of cycles can save
significant work compared to a fundamental cycle basis. | 1505.00875v3 |
2015-06-18 | Area-Delay-Energy Tradeoffs of Strain-Mediated Multiferroic Devices | Multiferroic devices hold profound promise for ultra-low energy computing in
beyond Moore's law era. The magnetization of a magnetostrictive
shape-anisotropic single-domain nanomagnet strain-coupled with a piezoelectric
layer in a multiferroic composite structure can be switched between its two
stable states (separated by an energy barrier) with a tiny amount of voltage
via converse magnetoelectric effect. With appropriate choice of materials, the
magnetization can be switched with a few tens of millivolts of voltages in
sub-nanosecond switching delay while spending a miniscule amount of energy of
~1 attojoule at room-temperature. Here, we analyze the area-delay-energy
trade-offs of these multiferroic devices by solving stochastic
Landau-Lifshitz-Gilbert equation in the presence of room-temperature thermal
fluctuations. We particularly put attention on scaling down the lateral area of
the magnetostrictive nanomagnet that can increase the device density on a chip.
We show that the vertical thickness of the nanomagnet can be increased while
scaling down the lateral area and keeping the assumption of single-domain limit
valid. This has important consequence since it helps to some extent preventing
the deterioration of the induced stress-anisotropy energy in the
magnetostrictive nanomagnet, which is proportional to the nanomagnet's volume.
The results show that if we scale down the lateral area, the switching delay
increases while energy dissipation decreases. Avenues available to decrease the
switching delay while still reducing the energy dissipation are discussed. | 1506.07859v1 |
2015-07-30 | Confirmation of the Planetary Microlensing Signal and Star and Planet Mass Determinations for Event OGLE-2005-BLG-169 | We present Hubble Space Telescope (HST) Wide Field Camera 3 (WFC3)
observations of the source and lens stars for planetary microlensing event
OGLE-2005-BLG-169, which confirm the relative proper motion prediction due to
the planetary light curve signal observed for this event. This (and the
companion Keck result) provide the first confirmation of a planetary
microlensing signal, for which the deviation was only 2%. The follow-up
observations determine the flux of the planetary host star in multiple
passbands and remove light curve model ambiguity caused by sparse sampling of
part of the light curve. This leads to a precise determination of the
properties of the OGLE-2005-BLG-169Lb planetary system. Combining the
constraints from the microlensing light curve with the photometry and
astrometry of the HST/WFC3 data, we find star and planet masses of M_* = 0.69+-
0.02 M_solar and m_p = 14.1 +- 0.9 M_earth. The planetary microlens system is
located toward the Galactic bulge at a distance of D_L = 4.1 +- 0.4 kpc, and
the projected star-planet separation is a_perp = 3.5 +- 0.3 AU, corresponding
to a semi-major axis of a = 4.0 (+2.2 -0.6) AU. | 1507.08661v1 |
2015-08-19 | Variations in solar wind fractionation as seen by ACE/SWICS over a solar cycle and the implications for Genesis Mission results | We use ACE/SWICS elemental composition data to compare the variations in
solar wind fractionation as measured by SWICS during the last solar maximum
(1999-2001), the solar minimum (2006-2009) and the period in which the Genesis
spacecraft was collecting solar wind (late 2001 - early 2004). We differentiate
our analysis in terms of solar wind regimes (i.e. originating from interstream
or coronal hole flows, or coronal mass ejecta). Abundances are normalized to
the low-FIP ion magnesium to uncover correlations that are not apparent when
normalizing to high-FIP ions. We find that relative to magnesium, the other
low-FIP elements are measurably fractionated, but the degree of fractionation
does not vary significantly over the solar cycle. For the high-FIP ions,
variation in fractionation over the solar cycle is significant: greatest for
Ne/Mg and C/Mg, less so for O/Mg, and the least for He/Mg. When abundance
ratios are examined as a function of solar wind speed, we find a strong
correlation, with the remarkable observation that the degree of fractionation
follows a mass-dependent trend. We discuss the implications for correcting the
Genesis sample return results to photospheric abundances. | 1508.04566v1 |
2015-08-28 | Cosmic variance in the nanohertz gravitational wave background | We use large N-body simulations and empirical scaling relations between dark
matter halos, galaxies, and supermassive black holes to estimate the formation
rates of supermassive black hole binaries and the resulting low-frequency
stochastic gravitational wave background (GWB). We find this GWB to be
relatively insensitive ($\lesssim10\%$) to cosmological parameters, with only
slight variation between WMAP5 and Planck cosmologies. We find that uncertainty
in the astrophysical scaling relations changes the amplitude of the GWB by a
factor of $\sim 2$. Current observational limits are already constraining this
predicted range of models. We investigate the Poisson variance in the amplitude
of the GWB for randomly-generated populations of supermassive black holes,
finding a scatter of order unity per frequency bin below 10 nHz, and increasing
to a factor of $\sim 10$ near 100 nHz. This variance is a result of the rarity
of the most massive binaries, which dominate the signal, and acts as a
fundamental uncertainty on the amplitude of the underlying power law spectrum.
This Poisson uncertainty dominates at $\gtrsim 20$ nHz, while at lower
frequencies the dominant uncertainty is related to our poor understanding of
the astrophysical scaling relations, although very low frequencies may be
dominated by uncertainties related to the final parsec problem and the
processes which drive binaries to the gravitational wave dominated regime.
Cosmological effects are negligible at all frequencies. | 1508.07336v2 |
2015-09-01 | Magnon-driven longitudinal spin Seebeck effect in F|N and N|F|N structures: role of asymmetric in-plane magnetic anisotropy | The influence of an asymmetric in-plane magnetic anisotropy on the thermally
activated spin current is studied theoretically for two different systems; (i)
the system consisting of a ferromagnetic insulator in a direct contact with a
nonmagnetic metal, and the sandwich structure consisting of a ferromagnetic
insulating part sandwiched between two nonmagnetic metals. It is shown that
when the difference between the temperatures of the two nonmagnetic metals in a
structure is not large, the spin pumping currents from the magnetic part to the
nonmagnetic ones are equal in amplitude and have opposite directions, so only
the spin torque current contributes to the total spin current. The spin current
flows then from the nonmagnetic metal with the higher temperature to the
nonmagnetic metal having a lower temperature. Its amplitude varies linearly
with the difference in temperatures. In addition, we have found that if the
magnetic anisotropy is in the layer plane, then the spin current increases with
the magnon temperature, while in the case of an out-of-plane magnetic
anisotropy the spin current decreases when the magnon temperature enhances.
Enlarging the difference between the temperatures of the nonmagnetic metals,
the linear response becomes important, as confirmed by analytical expressions
inferred from the Fokker-Planck approach and by the results obtained upon a
full numerical integration of the stochastic Landau-Lifshitz-Gilbert equation. | 1509.00376v1 |
2015-09-08 | Stellar Dynamics around a Massive Black Hole II: Resonant Relaxation | We present a first-principles theory of Resonant Relaxation (RR) of a low
mass stellar system orbiting a more massive black hole (MBH). We first extend
the kinetic theory of Gilbert (1968) to include the Keplerian field of a black
hole of mass $M_\bullet$. Specializing to a Keplerian stellar system of mass $M
\ll M_\bullet$, we use the orbit-averaging method of Sridhar & Touma (2015;
Paper I) to derive a kinetic equation for RR. This describes the collisional
evolution of a system of $N \gg 1$ Gaussian Rings in a reduced 5-dim space,
under the combined actions of self-gravity, 1 PN and 1.5 PN relativistic
effects of the MBH and an arbitrary external potential. In general geometries
RR is driven by both apsidal and nodal resonances, so the distinction between
scalar-RR and vector-RR disappears. The system passes through a sequence of
quasi-steady secular collisionless equilibria, driven by irreversible 2-Ring
correlations that accrue through gravitational interactions, both direct and
collective. This correlation function is related to a `wake function', which is
the linear response of the system to the perturbation of a chosen Ring. The
wake function is easier to appreciate, and satisfies a simpler equation, than
the correlation function. We discuss general implications for the interplay of
secular dynamics and non-equilibrium statistical mechanics in the evolution of
Keplerian stellar systems toward secular thermodynamic equilibria, and set the
stage for applications to the RR of axisymmetric discs in Paper III. | 1509.02401v2 |
2015-10-11 | End-to-End Error-Correcting Codes on Networks with Worst-Case Symbol Errors | The problem of coding for networks experiencing worst-case symbol errors is
considered. We argue that this is a reasonable model for highly dynamic
wireless network transmissions. We demonstrate that in this setup prior network
error-correcting schemes can be arbitrarily far from achieving the optimal
network throughput. A new transform metric for errors under the considered
model is proposed. Using this metric, we replicate many of the classical
results from coding theory. Specifically, we prove new Hamming-type,
Plotkin-type, and Elias-Bassalygo-type upper bounds on the network capacity. A
commensurate lower bound is shown based on Gilbert-Varshamov-type codes for
error-correction. The GV codes used to attain the lower bound can be
non-coherent, that is, they do not require prior knowledge of the network
topology. We also propose a computationally-efficient concatenation scheme. The
rate achieved by our concatenated codes is characterized by a Zyablov-type
lower bound. We provide a generalized minimum-distance decoding algorithm which
decodes up to half the minimum distance of the concatenated codes. The
end-to-end nature of our design enables our codes to be overlaid on the
classical distributed random linear network codes [1]. Furthermore, the
potentially intensive computation at internal nodes for the link-by-link
error-correction is un-necessary based on our design. | 1510.03060v1 |
2015-10-12 | Analysis of laser shock experiments on precompressed samples using a quartz reference and application to warm dense hydrogen and helium | Megabar (1 Mbar = 100 GPa) laser shocks on precompressed samples allow
reaching unprecedented high densities and moderately high 10000-100000K
temperatures. We describe here a complete analysis framework for the
velocimetry (VISAR) and pyrometry (SOP) data produced in these experiments.
Since the precompression increases the initial density of both the sample of
interest and the quartz reference for pressure-density, reflectivity and
temperature measurements, we describe analytical corrections based on available
experimental data on warm dense silica and density-functional-theory based
molecular dynamics computer simulations. Using our improved analysis framework
we report a re-analysis of previously published data on warm dense hydrogen and
helium, compare the newly inferred pressure, density and temperature data with
most advanced equation of state models and provide updated reflectivity values. | 1510.03301v1 |
2015-11-09 | Simulations of the Pairwise Kinematic Sunyaev-Zel'dovich Signal | The pairwise kinematic Sunyaev-Zel'dovich (kSZ) signal from galaxy clusters
is a probe of their line-of-sight momenta, and thus a potentially valuable
source of cosmological information. In addition to the momenta, the amplitude
of the measured signal depends on the properties of the intra-cluster gas and
observational limitations such as errors in determining cluster centers and
redshifts. In this work we simulate the pairwise kSZ signal of clusters at z<1,
using the output from a cosmological N-body simulation and including the
properties of the intra-cluster gas via a model that can be varied in
post-processing. We find that modifications to the gas profile due to star
formation and feedback reduce the pairwise kSZ amplitude of clusters by ~50%,
relative to the naive 'gas traces mass' assumption. We demonstrate that
mis-centering can reduce the overall amplitude of the pairwise kSZ signal by up
to 10%, while redshift errors can lead to an almost complete suppression of the
signal at small separations. We confirm that a high-significance detection is
expected from the combination of data from current-generation, high-resolution
CMB experiments, such as the South Pole Telescope, and cluster samples from
optical photometric surveys, such as the Dark Energy Survey. Furthermore, we
forecast that future experiments such as Advanced ACTPol in conjunction with
data from the Dark Energy Spectroscopic Instrument will yield detection
significances of at least 20{\sigma}, and up to 57{\sigma} in an optimistic
scenario. Our simulated maps are publicly available at:
http://www.hep.anl.gov/cosmology/ksz.html | 1511.02843v2 |
2015-11-11 | Magnetization switching by current and microwaves | We propose a theoretical model of magnetization switching in a ferromagnetic
multilayer by both electric current and microwaves. The electric current gives
a spin transfer torque on the magnetization, while the microwaves induce a
precession of the magnetization around the initial state. Based on numerical
simulation of the Landau-Lifshitz-Gilbert (LLG) equation, it is found that the
switching current is significantly reduced compared with the switching caused
solely by the spin transfer torque when the microwave frequency is in a certain
range. We develop a theory of switching from the LLG equation averaged over a
constant energy curve. It was found that the switching current should be
classified into four regions, depending on the values of the microwave
frequency. Based on the analysis, we derive an analytical formula of the
optimized frequency minimizing the switching current, which is smaller than the
ferromagnetic resonance frequency. We also derive an analytical formula of the
minimized switching current. Both the optimized frequency and the minimized
switching current decrease with increasing the amplitude of the microwave
field. The results will be useful to achieve high thermal stability and low
switching current in spin torque systems simultaneously. | 1511.03366v2 |
2015-11-13 | Time-domain numerical modeling of brass instruments including nonlinear wave propagation, viscothermal losses, and lips vibration | A time-domain numerical modeling of brass instruments is proposed. On one
hand, outgoing and incoming waves in the resonator are described by the
Menguy-Gilbert model, which incorporates three key issues: nonlinear wave
propagation, viscothermal losses, and a variable section. The non-linear
propagation is simulated by a TVD scheme well-suited to non-smooth waves. The
fractional derivatives induced by the viscothermal losses are replaced by a set
of local-in-time memory variables. A splitting strategy is followed to couple
optimally these dedicated methods. On the other hand, the exciter is described
by a one-mass model for the lips. The Newmark method is used to integrate the
nonlinear ordinary differential equation so-obtained. At each time step, a
coupling is performed between the pressure in the tube and the displacement of
the lips. Finally, an extensive set of validation tests is successfully
completed. In particular, self-sustained oscillations of the lips are simulated
by taking into account the nonlinear wave propagation in the tube. Simulations
clearly indicate that the nonlinear wave propagation has a major influence on
the timbre of the sound, as expected. Moreover, simulations also highlight an
influence on playing frequencies, time envelopes and on the playability of the
low frequencies in the case of a variable lips tension. | 1511.04247v1 |
2015-11-24 | Planetary Candidates from the First Year of the K2 Mission | The Kepler Space Telescope is currently searching for planets transiting
stars along the ecliptic plane as part of its extended K2 mission. We processed
the publicly released data from the first year of K2 observations (Campaigns 0,
1, 2, and 3) and searched for periodic eclipse signals consistent with
planetary transits. Out of 59,174 targets we searched, we detect 234 planetary
candidates around 208 stars. These candidates range in size from gas giants to
smaller than the Earth, and range in orbital periods from hours to over a
month. We conducted initial reconnaissance spectroscopy of 68 of the brighter
candidate host stars, and present high resolution optical spectra for these
stars. We make all of our data products, including light curves, spectra, and
vetting diagnostics available to users online. | 1511.07820v2 |
2015-11-25 | A Search for Water in the Atmosphere of HAT-P-26b Using LDSS-3C | The characterization of a physically-diverse set of transiting exoplanets is
an important and necessary step towards establishing the physical properties
linked to the production of obscuring clouds or hazes. It is those planets with
identifiable spectroscopic features that can most effectively enhance our
understanding of atmospheric chemistry and metallicity. The newly-commissioned
LDSS-3C instrument on Magellan provides enhanced sensitivity and suppressed
fringing in the red optical, thus advancing the search for the spectroscopic
signature of water in exoplanetary atmospheres from the ground. Using data
acquired by LDSS-3C and the Spitzer Space Telescope, we search for evidence of
water vapor in the transmission spectrum of the Neptune-mass planet HAT-P-26b.
Our measured spectrum is best explained by the presence of water vapor, a lack
of potassium, and either a high-metallicity, cloud-free atmosphere or a
solar-metallicity atmosphere with a cloud deck at ~10 mbar. The emergence of
multi-scale-height spectral features in our data suggests that future
observations at higher precision could break this degeneracy and reveal the
planet's atmospheric chemical abundances. We also update HAT-P-26b's transit
ephemeris, t_0 = 2455304.65218(25) BJD_TDB, and orbital period, p =
4.2345023(7) days. | 1511.08226v2 |
2015-12-22 | Induced voltage in an open wire | A puzzle arising from Faraday's law is considered and solved concerning the
question which voltage is induced in an open wire with a time-varying
homogeneous magnetic field. In contrast to closed wires where the voltage is
determined by the time variance of magnetic field and enclosed area, in an open
wire we have to integrate the electric field along the wire. It is found that
the longitudinal electric field contributes with 1/3 and the transverse field
with 2/3 to the induced voltage. In order to find the electric fields the
sources of the magnetic fields are necessary to know. The representation of a
homogeneous and time-varying magnetic field implies unavoidably a certain
symmetry point or symmetry line which depend on the geometry of the source. As
a consequence the induced voltage of an open wire is found to be the area
covered with respect to this symmetry line or point perpendicular to the
magnetic field. This in turn allows to find the symmetry points of a magnetic
field source by measuring the voltage of an open wire placed with different
angles in the magnetic field. We present exactly solvable models for a symmetry
point and for a symmetry line, respectively. The results are applicable to open
circuit problems like corrosion and for astrophysical applications. | 1512.07133v3 |
2015-12-22 | Charge transport and vector meson dissociation across the thermal phase transition in lattice QCD with two light quark flavors | We compute and analyze correlation functions in the isovector vector channel
at vanishing spatial momentum across the deconfinement phase transition in
lattice QCD. The simulations are carried out at temperatures $T/T_c=0.156, 0.8,
1.0, 1.25$ and $1.67$ with $T_c\simeq203$MeV for two flavors of Wilson-Clover
fermions with a zero-temperature pion mass of $\simeq270$MeV. Exploiting exact
sum rules and applying a phenomenologically motivated ansatz allows us to
determine the spectral function $\rho(\omega,T)$ via a fit to the lattice
correlation function data. From these results we estimate the electrical
conductivity across the deconfinement phase transition via a Kubo formula and
find evidence for the dissociation of the $\rho$ meson by resolving its
spectral weight at the available temperatures. We also apply the Backus-Gilbert
method as a model-independent approach to this problem. At any given frequency,
it yields a local weighted average of the true spectral function. We use this
method to compare kinetic theory predictions and previously published
phenomenological spectral functions to our lattice study. | 1512.07249v1 |
2016-01-18 | Search for transiting exoplanets and variable stars in the open cluster NGC 7243 | We report results of the first five observing campaigns for the open stellar
cluster NGC 7243 in the frame of project Young Exoplanet Transit Initiative
(YETI). The project focuses on the monitoring of young and nearby stellar
clusters, with the aim to detect young transiting exoplanets, and to study
other variability phenomena on time-scales from minutes to years. After five
observing campaigns and additional observations during 2013 and 2014, a clear
and repeating transit-like signal was detected in the light curve of
J221550.6+495611. Furthermore, we detected and analysed 37 new eclipsing binary
stars in the studied region. The best fit parameters and light curves of all
systems are given. Finally, we detected and analysed 26 new, presumably
pulsating variable stars in the studied region. The follow-up investigation of
these objects, including spectroscopic measurements of the exoplanet candidate,
is currently planned. | 1601.04562v1 |
2016-01-21 | Basker: A Threaded Sparse LU Factorization Utilizing Hierarchical Parallelism and Data Layouts | Scalable sparse LU factorization is critical for efficient numerical
simulation of circuits and electrical power grids. In this work, we present a
new scalable sparse direct solver called Basker. Basker introduces a new
algorithm to parallelize the Gilbert-Peierls algorithm for sparse LU
factorization. As architectures evolve, there exists a need for algorithms that
are hierarchical in nature to match the hierarchy in thread teams, individual
threads, and vector level parallelism. Basker is designed to map well to this
hierarchy in architectures. There is also a need for data layouts to match
multiple levels of hierarchy in memory. Basker uses a two-dimensional
hierarchical structure of sparse matrices that maps to the hierarchy in the
memory architectures and to the hierarchy in parallelism. We present
performance evaluations of Basker on the Intel SandyBridge and Xeon Phi
platforms using circuit and power grid matrices taken from the University of
Florida sparse matrix collection and from Xyce circuit simulations. Basker
achieves a geometric mean speedup of 5.91x on CPU (16 cores) and 7.4x on Xeon
Phi (32 cores) relative to KLU. Basker outperforms Intel MKL Pardiso (PMKL) by
as much as 53x on CPU (16 cores) and 13.3x on Xeon Phi (32 cores) for low
fill-in circuit matrices. Furthermore, Basker provides 5.4x speedup on a
challenging matrix sequence taken from an actual Xyce simulation. | 1601.05725v1 |
2016-02-16 | JSJ decompositions of groups | This is an account of the theory of JSJ decompositions of finitely generated
groups, as developed in the last twenty years or so.
We give a simple general definition of JSJ decompositions (or rather of their
Bass-Serre trees), as maximal universally elliptic trees. In general, there is
no preferred JSJ decomposition, and the right object to consider is the whole
set of JSJ decompositions, which forms a contractible space: the JSJ
deformation space (analogous to Outer Space).
We prove that JSJ decompositions exist for any finitely presented group,
without any assumption on edge groups. When edge groups are slender, we
describe flexible vertices of JSJ decompositions as quadratically hanging
extensions of 2-orbifold groups.
Similar results hold in the presence of acylindricity, in particular for
splittings of torsion-free CSA groups over abelian groups, and splittings of
relatively hyperbolic groups over virtually cyclic or parabolic subgroups.
Using trees of cylinders, we obtain canonical JSJ trees (which are invariant
under automorphisms).
We introduce a variant in which the property of being universally elliptic is
replaced by the more restrictive and rigid property of being universally
compatible. This yields a canonical compatibility JSJ tree, not just a
deformation space. We show that it exists for any finitely presented group.
We give many examples, and we work throughout with relative decompositions
(restricting to trees where certain subgroups are elliptic). | 1602.05139v2 |
2016-03-28 | Write error rate of spin-transfer-torque random access memory including micromagnetic effects using rare event enhancement | Spin-transfer-torque random access memory (STT-RAM) is a promising candidate
for the next-generation of random-access-memory due to improved scalability,
read-write speeds and endurance. However, the write pulse duration must be long
enough to ensure a low write error rate (WER), the probability that a bit will
remain unswitched after the write pulse is turned off, in the presence of
stochastic thermal effects. WERs on the scale of 10$^{-9}$ or lower are
desired. Within a macrospin approximation, WERs can be calculated analytically
using the Fokker-Planck method to this point and beyond. However, dynamic
micromagnetic effects within the bit can affect and lead to faster switching.
Such micromagnetic effects can be addressed via numerical solution of the
stochastic Landau-Lifshitz-Gilbert-Slonczewski (LLGS) equation. However,
determining WERs approaching 10$^{-9}$ would require well over 10$^{9}$ such
independent simulations, which is infeasible. In this work, we explore
calculation of WER using "rare event enhancement" (REE), an approach that has
been used for Monte Carlo simulation of other systems where rare events
nevertheless remain important. Using a prototype REE approach tailored to the
STT-RAM switching physics, we demonstrate reliable calculation of a WER to
10$^{-9}$ with sets of only approximately 10$^{3}$ ongoing stochastic LLGS
simulations, and the apparent ability to go further. | 1603.08512v2 |
2016-04-04 | Probing unconventional superconductivity in inversion symmetric doped Weyl semimetal | Unconventional superconductivity has been predicted to arise in the
topologically non-trivial Fermi surface of doped inversion symmetric Weyl
semimetals (WSM). In particular, Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) and
nodal BCS states are theoretically predicted to be possible superconductor
pairing states in inversion symmetric doped WSM. In an effort to resolve
preferred pairing state, we theoretically study two separate four terminal
quantum transport methods that each exhibit a unique electrical signature in
the presence of FFLO and nodal BCS states in doped WSMs. We first introduce a
Josephson junction that consists of a doped WSM and an s-wave superconductor in
which we show that the application of a transverse uniform current in s-wave
superconductor effectively cancels the momentum carried by FFLO states in doped
WSM. From our numerical analysis, we find a peak in Josephson current amplitude
at finite uniform current in s-wave superconductor that serves as an indicator
of FFLO states in doped WSMs. Furthermore, we show using a four terminal
measurement configuration that the nodal points may be shifted by an
application of transverse uniform current in doped WSM. We analyze the
topological phase transitions induced by nodal pair annihilation in
non-equilibrium by constructing the phase diagram and we find a characteristic
decrease in the density of states that serves as a signature of the quantum
critical point in the topological phase transition, thereby identifying nodal
BCS states in doped WSM. | 1604.01040v1 |
2016-04-22 | Opt: A Domain Specific Language for Non-linear Least Squares Optimization in Graphics and Imaging | Many graphics and vision problems can be expressed as non-linear least
squares optimizations of objective functions over visual data, such as images
and meshes. The mathematical descriptions of these functions are extremely
concise, but their implementation in real code is tedious, especially when
optimized for real-time performance on modern GPUs in interactive applications.
In this work, we propose a new language, Opt (available under
http://optlang.org), for writing these objective functions over image- or
graph-structured unknowns concisely and at a high level. Our compiler
automatically transforms these specifications into state-of-the-art GPU solvers
based on Gauss-Newton or Levenberg-Marquardt methods. Opt can generate
different variations of the solver, so users can easily explore tradeoffs in
numerical precision, matrix-free methods, and solver approaches. In our
results, we implement a variety of real-world graphics and vision applications.
Their energy functions are expressible in tens of lines of code, and produce
highly-optimized GPU solver implementations. These solver have performance
competitive with the best published hand-tuned, application-specific GPU
solvers, and orders of magnitude beyond a general-purpose auto-generated
solver. | 1604.06525v3 |
2016-05-06 | Spin orbit alignment for KELT-7b and HAT-P-56b via Doppler tomography with TRES | We present Doppler tomographic analyses for the spectroscopic transits of
KELT-7b and HAT-P-56b, two hot-Jupiters orbiting rapidly rotating F-dwarf host
stars. These include analyses of archival TRES observations for KELT-7b, and a
new TRES transit observation of HAT-P-56b. We report spin-orbit aligned
geometries for KELT-7b (2.7 +/- 0.6 deg) and HAT-P-56b (8 +/- 2 deg). The host
stars KELT-7 and HAT-P-56 are among some of the most rapidly rotating
planet-hosting stars known. We examine the tidal re-alignment model for the
evolution of the spin-orbit angle in the context of the spin rates of these
stars. We find no evidence that the rotation rates of KELT-7 and HAT-P-56 have
been modified by star-planet tidal interactions, suggesting that the spin-orbit
angle of systems around these hot stars may represent their primordial
configuration. In fact, KELT-7 and HAT-P-56 are two of three systems in
super-synchronous, spin-orbit aligned states, where the rotation periods of the
host stars are faster than the orbital periods of the planets. | 1605.01991v1 |
2016-06-18 | Mathematical Foundations of the GraphBLAS | The GraphBLAS standard (GraphBlas.org) is being developed to bring the
potential of matrix based graph algorithms to the broadest possible audience.
Mathematically the Graph- BLAS defines a core set of matrix-based graph
operations that can be used to implement a wide class of graph algorithms in a
wide range of programming environments. This paper provides an introduction to
the mathematics of the GraphBLAS. Graphs represent connections between vertices
with edges. Matrices can represent a wide range of graphs using adjacency
matrices or incidence matrices. Adjacency matrices are often easier to analyze
while incidence matrices are often better for representing data. Fortunately,
the two are easily connected by matrix mul- tiplication. A key feature of
matrix mathematics is that a very small number of matrix operations can be used
to manipulate a very wide range of graphs. This composability of small number
of operations is the foundation of the GraphBLAS. A standard such as the
GraphBLAS can only be effective if it has low performance overhead. Performance
measurements of prototype GraphBLAS implementations indicate that the overhead
is low. | 1606.05790v2 |
2016-07-15 | Influence of grain size and exchange interaction on the LLB modeling procedure | Reliably predicting bit-error rates in realistic heat-assisted magnetic
recording simulations is a challenging task. Integrating the
Landau-Lifshitz-Bloch (LLB) equation can reduce the computational effort to
determine the magnetization dynamics in the vicinity of the Curie temperature.
If one aims that these dynamics coincide with trajectories calculated from the
atomistic Landau-Lifshitz-Gilbert equation, one has to carefully model required
temperature dependent material functions such as the zero-field equilibrium
magnetization as well as the parallel and normal susceptibilities. We present
an extensive study on how these functions depend on grain size and exchange
interactions. We show that, if the size or the exchange constant of a reference
grain is modified, the material functions can be scaled, according to the
changed Curie temperature, yielding negligible errors. This is shown to be
valid for volume changes of up to $\pm 40$ % and variations of the exchange
constant of up to $\pm10$ %. Besides the temperature dependent material curves,
computed switching probabilities also agree well with probabilities separately
determined for each system. Our study suggest that there is no need to
recalculate the required LLB input functions for each particle. Within the
presented limits it is sufficient to scale them to the Curie temperature of the
altered system. | 1607.04480v1 |
2016-08-23 | Thermal stability and irreversibility of skyrmion-lattice phases in Cu$_2$OSeO$_3$ | Small angle neutron scattering measurements have been performed to study the
thermodynamic stability of skyrmion-lattice phases in Cu$_2$OSeO$_3$. We found
that the two distinct skyrmion-lattice phases [SkX(1) and SkX(2) phases] can be
stabilized through different thermal histories; by cooling from the
paramagnetic phase under finite magnetic field, the SkX(2) phase is selected.
On the other hand, the 30$^{\circ}$-rotated SkX(1) phase becomes dominant by
heating the sample from the ordered conical phase under finite field. This
difference in stabilization is surprisingly similar to the irreversibility
observed in spin glasses. The zero-field cooling results in the co-existence of
the two phases. It is further found that once one of the skyrmion-lattice
phases is formed, it is hardly destabilized. This indicates unusual thermal
stability of the two skyrmion-lattice phases originating from an unexpectedly
large energy barrier between them. | 1608.06359v2 |
2016-08-24 | Carbon Stars in the Satellites and Halo of M31 | We spectroscopically identify a sample of carbon stars in the satellites and
halo of M31 using moderate-resolution optical spectroscopy from the
Spectroscopic and Photometric Landscape of Andromeda's Stellar Halo survey. We
present the photometric properties of our sample of 41 stars, including their
brightness with respect to the tip of the red giant branch (TRGB) and their
distributions in various color-color spaces. This analysis reveals a bluer
population of carbon stars fainter than the TRGB and a redder population of
carbon stars brighter than the TRGB. We then apply principal component analysis
to determine the sample's eigenspectra and eigencoefficients. Correlating the
eigencoefficients with various observable properties reveals the spectral
features that trace effective temperature and metallicity. Putting the
spectroscopic and photometric information together, we find the carbon stars in
the satellites and halo of M31 to be minimally impacted by dust and internal
dynamics. We also find that while there is evidence to suggest that the
sub-TRGB stars are extrinsic in origin, it is also possible that they are are
particularly faint members of the asymptotic giant branch. | 1608.06714v1 |
2016-09-12 | Discovery and Precise Characterization by the MEarth Project of LP 661-13, an Eclipsing Binary Consisting of Two Fully Convective Low-mass Stars | We report the detection of stellar eclipses in the LP 661-13 system. We
present the discovery and characterization of this system, including high
resolution spectroscopic radial velocities and a photometric solution spanning
two observing seasons. LP 661-13 is a low mass binary system with an orbital
period of $4.7043512^{+0.0000013}_{-0.0000010}$ days at a distance of $24.9 \pm
1.3$ parsecs. LP 661-13A is a $0.30795 \pm 0.00084$ $M_\odot$ star while LP
661-13B is a $0.19400 \pm 0.00034$ $M_\odot$ star. The radius of each component
is $0.3226 \pm 0.0033$ $R_\odot$ and $0.2174 \pm 0.0023$ $R_\odot$,
respectively. We detect out of eclipse modulations at a period slightly shorter
than the orbital period, implying that at least one of the components is not
rotating synchronously. We find that each component is slightly inflated
compared to stellar models, and that this cannot be reconciled through age or
metallicity effects. As a nearby eclipsing binary system where both components
are near or below the full-convection limit, LP 661-13 will be a valuable test
of models for the structure of cool dwarf stars. | 1609.03591v1 |
2016-09-14 | Topological superconductivity in an ultrathin, magnetically-doped topological insulator proximity coupled to a conventional superconductor | As a promising candidate system to realize topological superconductivity, the
system of a 3D topological insulator (TI) grown on top of the s-wave
superconductor has been extensively studied. To access the topological
superconductivity experimentally, the 3D TI sample must be thin enough to allow
for Cooper pair tunneling to the exposed surface of TI. The use of magnetically
ordered dopants to break time-reversal symmetry may allow the surface of a TI
to host Majorana fermion, which are believed to be a signature of topological
superconductivity. In this work, we study a magnetically-doped thin film
TI-superconductor hybrid systems. Considering the proximity induced order
parameter in thin film of TI, we analyze the gap closing points of the
Hamiltonian and draw the phase diagram as a function of relevant parameters:
the hybridization gap, Zeeman energy, and chemical potential of the TI system.
Our findings provide a useful guide in choosing relevant parameters to
facilitate the observation of topological superconductivity in thin film
TI-superconductor hybrid systems. In addition, we further perform numerical
analysis on a TI proximity coupled to a s-wave superconductor and find that,
due to the spin-momentum locked nature of the surface states in TI, the induced
s-wave order parameter of the surface states persists even at large magnitude
of the Zeeman energy. | 1609.04129v2 |
2016-09-16 | Convex separation from convex optimization for large-scale problems | We present a scheme, based on Gilbert's algorithm for quadratic minimization
[SIAM J. Contrl., vol. 4, pp. 61-80, 1966], to prove separation between a point
and an arbitrary convex set $S\subset\mathbb{R}^{n}$ via calls to an oracle
able to perform linear optimizations over $S$. Compared to other methods, our
scheme has almost negligible memory requirements and the number of calls to the
optimization oracle does not depend on the dimensionality $n$ of the underlying
space. We study the speed of convergence of the scheme under different promises
on the shape of the set $S$ and/or the location of the point, validating the
accuracy of our theoretical bounds with numerical examples. Finally, we present
some applications of the scheme in quantum information theory. There we find
that our algorithm out-performs existing linear programming methods for certain
large scale problems, allowing us to certify nonlocality in bipartite scenarios
with upto $42$ measurement settings. We apply the algorithm to upper bound the
visibility of two-qubit Werner states, hence improving known lower bounds on
Grothendieck's constant $K_G(3)$. Similarly, we compute new upper bounds on the
visibility of GHZ states and on the steerability limit of Werner states for a
fixed number of measurement settings. | 1609.05011v2 |
2016-09-18 | Narrowly distributed crystal orientation in biomineral vaterite | Biominerals formed by animals provide skeletal support, and many other
functions. They were previously shown to grow by aggregation of amorphous
nanoparticles, but never to grow ion-by-ion from solution, which is a common
growth mechanism for abiotic crystals. We analyze vaterite CaCO3 multi
crystalline spicules from the solitary tunicate Herdmania momus, with
Polarization dependent Imaging Contrast PIC mapping, scanning and aberration
corrected transmission electron microscopies. The first fully quantitative PIC
mapping data, presented here, measured 0{\deg} 30{\deg} angle spreads between
immediately adjacent crystals. Such narrowly distributed crystal orientations
demonstrate that crystallinity does not propagate from one crystal to another
0{\deg} angle spreads, nor that new crystals with random orientation 90{\deg}
nucleate. There are no organic layers at the interface between crystals, hence
a new, unknown growth mechanism must be invoked, with crystal nucleation
constrained within 30{\deg}. Two observations are consistent with crystal
growth from solution: vaterite microcrystals express crystal faces, and are
smooth at the nanoscale after cryo fracture. The observation of 30{\deg} angle
spreads, lack of interfacial organic layers, and smooth fracture figures
broadens the range of known biomineralization mechanisms and may inspire novel
synthetic crystal growth strategies. Spherulitic growth from solution is one
possible mechanism consistent with all these observations. | 1609.05449v1 |
2016-09-29 | Multiscale Model Approach for Magnetization Dynamics Simulations | Simulations of magnetization dynamics in a multiscale environment enable
rapid evaluation of the Landau-Lifshitz-Gilbert equation in a mesoscopic sample
with nanoscopic accuracy in areas where such accuracy is required. We have
developed a multiscale magnetization dynamics simulation approach that can be
applied to large systems with spin structures that vary locally on small length
scales. To implement this, the conventional micromagnetic simulation framework
has been expanded to include a multiscale solving routine. The software
selectively simulates different regions of a ferromagnetic sample according to
the spin structures located within in order to employ a suitable discretization
and use either a micromagnetic or an atomistic model. To demonstrate the
validity of the multiscale approach, we simulate the spin wave transmission
across the regions simulated with the two different models and different
discretizations. We find that the interface between the regions is fully
transparent for spin waves with frequency lower than a certain threshold set by
the coarse scale micromagnetic model with no noticeable attenuation due to the
interface between the models. As a comparison to exact analytical theory, we
show that in a system with Dzyaloshinskii-Moriya interaction leading to spin
spiral, the simulated multiscale result is in good quantitative agreement with
the analytical calculation. | 1609.09295v1 |
2016-12-21 | Geometric generalised Lagrangian mean theories | Many fluctuation-driven phenomena in fluids can be analysed effectively using
the generalised Lagrangian mean (GLM) theory of Andrews & McIntyre (1978). This
theory relies on particle-following averaging to incorporate the constraints
imposed by the material conservations. It relies implicitly on an Euclidean
structure; as a result, it does not have a geometrically intrinsic
interpretation and suffers from undesirable features, including the divergence
of the Lagrangian-mean velocity for incompressible fluids. Motivated by this,
we develop a geometric generalisation of GLM that we formulate intrinsically.
The theory applies to arbitrary Riemannian manifolds; it also establishes a
clear distinction between results that stem directly from geometric consistency
and those that depend on particular choices. We show that the Lagrangian mean
momentum -- the average of the pull-back of the momentum one-form -- obeys a
simple equation which guarantees the conservation of Kelvin's circulation,
irrespective of the mean-flow definition. We discuss four possible definitions
of the mean flow: a direct extension of standard GLM, a definition based on
optimal transportation, a definition based on a geodesic distance in the group
of volume-preserving diffeomorphisms, and the glm definition proposed by Soward
& Roberts (2010). Assuming small-amplitude perturbations, we carry out
order-by-order calculations to obtain explicit expressions for the mean flow
and pseudomomentum at leading order. We also show how the wave-action
conservation of GLM extends to the geometric setting. To make the paper
self-contained, we introduce the tools of differential geometry and main ideas
of geometric fluid dynamics on which we rely. We mostly focus on the Euler
equations for incompressible inviscid fluids but sketch out extensions to the
rotating-stratified Boussinesq, compressible Euler and magnetohydrodynamic
equations. | 1612.07111v3 |
2017-01-13 | A Multi-Planet System Transiting the $V$ = 9 Rapidly Rotating F-Star HD 106315 | We report the discovery of a multi-planet system orbiting HD 106315, a
rapidly rotating mid F-type star, using data from the K2 mission. HD 106315
hosts a $2.51\pm0.12\,R_\oplus$ sub-Neptune in a 9.5 day orbit, and a
$4.31_{-0.27}^{+0.24}\,R_\oplus$ super-Neptune in a 21 day orbit. The projected
rotational velocity of HD 106315 (12.9 km s$^{-1}$) likely precludes precise
measurements of the planets' masses, but could enable a measurement of the
sky-projected spin-orbit obliquity for the outer planet via Doppler tomography.
The eccentricities of both planets were constrained to be consistent with 0,
following a global modeling of the system that includes a Gaia distance and
dynamical arguments. The HD 106315 system is one of few multi-planet systems
hosting a Neptune-sized planet for which orbital obliquity measurements are
possible, making it an excellent test-case for formation mechanisms of
warm-Neptunian systems. The brightness of the host star also makes HD 106315 c
a candidate for future transmission spectroscopic follow-up studies. | 1701.03807v2 |
2017-01-17 | The Landau-Lifshitz equation, the NLS, and the magnetic rogue wave as a by-product of two colliding regular "positons" | In this article we present a new method for construction of exact solutions
of the Landau-Lifshitz-Gilbert equation (LLG) for ferromagnetic nanowires. The
method is based on the established relationship between the LLG and the
nonlinear Schr\"odinger equation (NLS), and is aimed at resolving an old
problem: how to produce multiple-rogue wave solutions of NLS using just the
Darboux-type transformations. The solutions of this type - known as P-breathers
- have been proven to exist by Dubard and Matveev, but their technique heavily
relied on using the solutions of yet another nonlinear equation,
Kadomtsev-Petviashvili I equation (KP-I), and its relationship with NLS. We
have shown that in fact one doesn't have to use KP-I but can instead reach the
same results just with NLS solutions, but only if they are dressed via the
binary Darboux transformation. In particular, our approach allows to construct
all the Dubard-Matveev P-breathers. Furthermore, the new method can lead to
some completely new, previously unknown solutions. One particular solution that
we have constructed describes two positon-like waves, colliding with each other
and in the process producing a new, short-lived rogue wave. We called this
unusual solution (rogue wave begotten after the impact of two solitons) the
"impacton". | 1701.04903v3 |
2017-01-24 | Partial Bridging of Vaccine Efficacy to New Populations | Suppose one has data from one or more completed vaccine efficacy trials and
wishes to estimate the efficacy in a new setting. Often logistical or ethical
considerations make running another efficacy trial impossible. Fortunately, if
there is a biomarker that is the primary modifier of efficacy, then the
biomarker-conditional efficacy may be identical in the completed trials and the
new setting, or at least informative enough to meaningfully bound this
quantity. Given a sample of this biomarker from the new population, we might
hope we can bridge the results of the completed trials to estimate the vaccine
efficacy in this new population. Unfortunately, even knowing the true
conditional efficacy in the new population fails to identify the marginal
efficacy due to the unknown conditional unvaccinated risk. We define a curve
that partially identifies (lower bounds) the marginal efficacy in the new
population as a function of the population's marginal unvaccinated risk, under
the assumption that one can identify bounds on the conditional unvaccinated
risk in the new population. Interpreting the curve only requires identifying
plausible regions of the marginal unvaccinated risk in the new population. We
present a nonparametric estimator of this curve and develop valid lower
confidence bounds that concentrate at a parametric rate. We use vaccine
terminology throughout, but the results apply to general binary interventions
and bounded outcomes. | 1701.06739v1 |
2017-02-07 | Resonant spin transfer torque nano-oscillators | Spin transfer torque nano-oscillators are potential candidates for replacing
the traditional inductor based voltage controlled oscillators in modern
communication devices. Typical oscillator designs are based on trilayer
magnetic tunnel junctions which are disadvantaged by low power outputs and poor
conversion efficiencies. In this letter, we theoretically propose to use
resonant spin filtering in pentalayer magnetic tunnel junctions as a possible
route to alleviate these issues and present device designs geared toward a high
microwave output power and an efficient conversion of the d.c. input power. We
attribute these robust qualities to the resulting non-trivial spin current
profiles and the ultra high tunnel magnetoresistance, both arising from
resonant spin filtering. The device designs are based on the nonequilibrium
Green's function spin transport formalism self-consistently coupled with the
stochastic Landau-Lifshitz-Gilbert-Slonczewski's equation and the Poisson's
equation. We demonstrate that the proposed structures facilitate oscillator
designs featuring a large enhancement in microwave power of around $775\%$ and
an efficiency enhancement of over $1300\%$ in comparison with typical trilayer
designs. We also rationalize the optimum operating regions via an analysis of
the dynamic and static device resistances. This work sets stage for pentalyer
spin transfer torque nano-oscillator device designs that extenuate most of the
issues faced by the typical trilayer designs. | 1702.01869v1 |
2017-03-17 | Communication Primitives in Cognitive Radio Networks | Cognitive radio networks are a new type of multi-channel wireless network in
which different nodes can have access to different sets of channels. By
providing multiple channels, they improve the efficiency and reliability of
wireless communication. However, the heterogeneous nature of cognitive radio
networks also brings new challenges to the design and analysis of distributed
algorithms.
In this paper, we focus on two fundamental problems in cognitive radio
networks: neighbor discovery, and global broadcast. We consider a network
containing $n$ nodes, each of which has access to $c$ channels. We assume the
network has diameter $D$, and each pair of neighbors have at least $k\geq 1$,
and at most $k_{max}\leq c$, shared channels. We also assume each node has at
most $\Delta$ neighbors. For the neighbor discovery problem, we design a
randomized algorithm CSeek which has time complexity
$\tilde{O}((c^2/k)+(k_{max}/k)\cdot\Delta)$. CSeek is flexible and robust,
which allows us to use it as a generic "filter" to find "well-connected"
neighbors with an even shorter running time. We then move on to the global
broadcast problem, and propose CGCast, a randomized algorithm which takes
$\tilde{O}((c^2/k)+(k_{max}/k)\cdot\Delta+D\cdot\Delta)$ time. CGCast uses
CSeek to achieve communication among neighbors, and uses edge coloring to
establish an efficient schedule for fast message dissemination.
Towards the end of the paper, we give lower bounds for solving the two
problems. These lower bounds demonstrate that in many situations, CSeek and
CGCast are near optimal. | 1703.06130v1 |
2017-03-22 | Magnetization induced dynamics of a Josephson junction coupled to a nanomagnet | We study the superconducting current of a Josephson junction (JJ) coupled to
an external nanomagnet driven by a time dependent magnetic field both without
and in the presence of an external AC drive. We provide an analytic, albeit
perturbative, solution for the Landau-Lifshitz (LL) equations governing the
coupled JJ-nanomagnet system in the presence of a magnetic field with arbitrary
time-dependence oriented along the easy axis of the nanomagnet's magnetization
and in the limit of weak dimensionless coupling $\epsilon_0$ between the JJ and
the nanomagnet. We show the existence of Shapiro-like steps in the I-V
characteristics of the JJ subjected to a voltage bias for a constant or
periodically varying magnetic field and explore the effect of rotation of the
magnetic field and the presence of an external AC drive on these steps. We
support our analytic results with exact numerical solution of the LL equations.
We also extend our results to dissipative nanomagnets by providing a
perturbative solution to the Landau-Lifshitz-Gilbert (LLG) equations for weak
dissipation. We study the fate of magnetization-induced Shapiro steps in the
presence of dissipation both from our analytical results and via numerical
solution of the coupled LLG equations. We discuss experiments which can test
our theory. | 1703.07717v3 |
2017-04-19 | Integrating optimization with thermodynamics and plant physiology for crop ideotype design | A computational framework integrating optimization algorithms, parallel
computing and plant physiology was developed to explore crop ideotype design.
The backbone of the framework is a plant physiology model that accurately
tracks water use (i.e. a plant hydraulic model) coupled with mass transport
(CO2 exchange and transport), energy conversion (leaf temperature due to
radiation, convection and mass transfer) and photosynthetic biochemistry of an
adult maize plant. For a given trait configuration, soil parameters and hourly
weather data, the model computes water use and photosynthetic output over the
life of an adult maize plant. We coupled this validated model with a parallel,
meta-heuristic optimization algorithm, specifically a genetic algorithm (GA),
to identify trait sets (ideotypes) that resulted in desired water use behavior
of the adult maize plant. We detail features of the model as well as the
implementation details of the coupling with the optimization framework and
deployment on high performance computing platforms. We illustrate a
representative result of this framework by identifying maize ideotypes with
optimized photosynthetic yields using weather and soil conditions corresponding
to Davis, CA. Finally, we show how the framework can be used to identify broad
ideotype trends that can inform breeding efforts. The developed presented tool
has the potential to inform the development of future climate-resilient crops. | 1704.05885v1 |
2017-04-28 | From deep inelastic scattering to heavy-flavor semi-leptonic decays: Total rates into multi-hadron final states from lattice QCD | We present a new technique for extracting decay and transition rates into
final states with any number of hadrons. The approach is only sensitive to
total rates, in which all out-states with a given set of QCD quantum numbers
are included. For processes involving photons or leptons, differential rates
with respect to the non-hadronic kinematics may also be extracted. Our method
involves constructing a finite-volume Euclidean four-point function, whose
corresponding spectral function measures the decay and transition rates in the
infinite-volume limit. This requires solving the inverse problem of extracting
the spectral function from the correlator and also necessitates a smoothing
procedure so that a well-defined infinite-volume limit exists. Both of these
steps are accomplished by the Backus-Gilbert method and, as we show with a
numerical example, reasonable precision can be expected in cases with multiple
open decay channels. Potential applications include nucleon structure functions
and the onset of the deep inelastic scattering regime, as well as semi-leptonic
$D$ and $B$ decay rates. | 1704.08993v2 |
2017-05-01 | Measuring galaxy cluster masses with CMB lensing using a Maximum Likelihood estimator: Statistical and systematic error budgets for future experiments | We develop a Maximum Likelihood estimator (MLE) to measure the masses of
galaxy clusters through the impact of gravitational lensing on the temperature
and polarization anisotropies of the cosmic microwave background (CMB). We show
that, at low noise levels in temperature, this optimal estimator outperforms
the standard quadratic estimator by a factor of two. For polarization, we show
that the Stokes Q/U maps can be used instead of the traditional E- and B-mode
maps without losing information. We test and quantify the bias in the recovered
lensing mass for a comprehensive list of potential systematic errors. Using
realistic simulations, we examine the cluster mass uncertainties from
CMB-cluster lensing as a function of an experiment's beam size and noise level.
We predict the cluster mass uncertainties will be 3 - 6% for SPT-3G, AdvACT,
and Simons Array experiments with 10,000 clusters and less than 1% for the
CMB-S4 experiment with a sample containing 100,000 clusters. The mass
constraints from CMB polarization are very sensitive to the experimental beam
size and map noise level: for a factor of three reduction in either the beam
size or noise level, the lensing signal-to-noise improves by roughly a factor
of two. | 1705.00411v2 |
2017-05-03 | Current driven second harmonic domain wall resonance in ferromagnetic metal/ nonmagnetic metal bilayer: a field-free method for spin Hall angle measurements | We study the ac current-driven domain wall motion in bilayer ferromagnetic
metal (FM)/nonmagnetic metal (NM) nanowire. The solution of the modified
Landau-Lifshitz-Gilbert equation including all the spin transfer torques is
used to describe motion of the domain wall in presence of the spin Hall effect.
We show that the domain wall center has second harmonic frequency response in
addition to the known first harmonic excitation. In contrast to the
experimentally observed second harmonic response in harmonic Hall measurements
of spin-orbit torque in magnetic thin films, this second harmonic response
directly originates from spin-orbit torque driven domain wall dynamics. Based
on the spin current generated by domain wall dynamics, the longitudinal spin
motive force generated voltage across the length of the nanowire is determined.
The second harmonic response introduces additionally a new practical field-free
and all-electrical method to probe the effective spin Hall angle for FM/NM
bilayer structures that could be applied in experiments. Our results also
demonstrate the capability of utilizing FM/NM bilayer structure in domain wall
based spin torque signal generators and resonators. | 1705.01355v5 |
2017-05-20 | SVM via Saddle Point Optimization: New Bounds and Distributed Algorithms | We study two important SVM variants: hard-margin SVM (for linearly separable
cases) and $\nu$-SVM (for linearly non-separable cases). We propose new
algorithms from the perspective of saddle point optimization. Our algorithms
achieve $(1-\epsilon)$-approximations with running time $\tilde{O}(nd+n\sqrt{d
/ \epsilon})$ for both variants, where $n$ is the number of points and $d$ is
the dimensionality. To the best of our knowledge, the current best algorithm
for $\nu$-SVM is based on quadratic programming approach which requires
$\Omega(n^2 d)$ time in worst case~\cite{joachims1998making,platt199912}. In
the paper, we provide the first nearly linear time algorithm for $\nu$-SVM. The
current best algorithm for hard margin SVM achieved by Gilbert
algorithm~\cite{gartner2009coresets} requires $O(nd / \epsilon )$ time. Our
algorithm improves the running time by a factor of $\sqrt{d}/\sqrt{\epsilon}$.
Moreover, our algorithms can be implemented in the distributed settings
naturally. We prove that our algorithms require $\tilde{O}(k(d
+\sqrt{d/\epsilon}))$ communication cost, where $k$ is the number of clients,
which almost matches the theoretical lower bound. Numerical experiments support
our theory and show that our algorithms converge faster on high dimensional,
large and dense data sets, as compared to previous methods. | 1705.07252v4 |
2017-06-15 | On the Maximum Size of Block Codes Subject to a Distance Criterion | We establish a general formula for the maximum size of finite length block
codes with minimum pairwise distance no less than $d$. The achievability
argument involves an iterative construction of a set of radius-$d$ balls, each
centered at a codeword. We demonstrate that the number of such balls that cover
the entire code alphabet cannot exceed this maximum size. Our approach can be
applied to codes $i)$ with elements over arbitrary code alphabets, and $ii)$
under a broad class of distance measures, thereby ensuring the generality of
our formula. Our formula indicates that the maximum code size can be fully
characterized by the cumulative distribution function of the distance measure
evaluated at two independent and identically distributed random codewords. When
the two random codewords assume a uniform distribution over the entire code
alphabet, our formula recovers and obtains a natural generalization of the
Gilbert-Varshamov (GV) lower bound. We also establish a general formula for the
zero-error capacity of any sequence of channels. Finally, we extend our study
to the asymptotic setting, where we establish first- and second-order bounds on
the asymptotic code rate subject to a normalized minimum distance constraint. | 1706.04709v2 |
2017-06-19 | Capability of Detecting Ultra-Violet Counterparts of Gravitational Waves with GLUV | With the discovery of gravitational waves (GW), attention has turned towards
detecting counterparts to these sources. In discussions on counterpart
signatures and multi-messenger follow-up strategies to GW detections,
ultra-violet (UV) signatures have largely been neglected, due to UV facilities
being limited to SWIFT, which lacks high-cadence UV survey capabilities. In
this paper, we examine the UV signatures from merger models for the major GW
sources, highlighting the need for further modelling, while presenting
requirements and a design for an effective UV survey telescope. Using $u'$-band
models as an analogue, we find that a UV survey telescope requires a limiting
magnitude of m$_{u'}\rm (AB)\approx 24$ to fully complement the aLIGO range and
sky localisation. We show that a network of small, balloon-based UV telescopes
with a primary mirror diameter of 30~cm could be capable of covering the aLIGO
detection distance from $\sim$60--100\% for BNS events and $\sim$40\% for BHNS
events. The sensitivity of UV emission to initial conditions suggests that a UV
survey telescope would provide a unique dataset, that can act as an effective
diagnostic to discriminate between models. | 1706.06106v2 |
2017-07-27 | LCD codes over ${\mathbb F}_q $ are as good as linear codes for q at least four | The hull $H(C)$ of a linear code $C$ is defined by $H(C)=C \cap C^\perp$. A
linear code with a complementary dual (LCD) is a linear code with $H(C)=\{0\}$.
The dimension of the hull of a code is an invariant under permutation
equivalence. For binary and ternary codes the dimension of the hull is also
invariant under monomial equivalence and we show that this invariant is
determined by the extended weight enumerator of the code.\\ The hull of a code
is not invariant under monomial equivalence if $q\geq 4$. We show that every
${\mathbb F}_q $-linear code is monomial equivalent with an LCD code in case $q
\geq 4$. The proof uses techniques from Gr\"obner basis theory. We conclude
that if there exists an ${\mathbb F}_q $-linear code with parameters
$[n,k,d]_q$ and $q \geq 4$, then there exists also a LCD code with the same
parameters. Hence this holds for optimal and MDS codes. In particular there
exist LCD codes that are above the Gilbert-Varshamov bound if $q$ is a square
and $q\geq 49$ by the existence of such codes that are algebraic geometric.\\
Similar results are obtained with respect to Hermitian LCD codes. | 1707.08856v1 |
2017-08-04 | Energy release in the solar atmosphere from a stream of infalling prominence debris | Recent high-resolution and high-cadence EUV imaging has revealed a new
phenomenon, impacting prominence debris, where prominence material from failed
or partial eruptions can impact the lower atmosphere, releasing energy. We
report a clear example of energy release and EUV brightening due to infalling
prominence debris that occurred on 2011 September 7-8. The initial eruption of
material was associated with an X1.8-class flare from AR11283, occurring at
22:30 UT on 2011 September 7. Subsequently, a semi-continuous stream of this
material returned to the solar surface with a velocity v > 150 km/s, impacting
a region remote from the original active region between 00:20 - 00:40 UT on
2011 September 8. Using SDO/AIA, the differential emission measure of the
plasma was estimated throughout this brightening event. We found that the
radiated energy of the impacted plasma was L_rad ~10^27 ergs, while the thermal
energy peaked at ~10^28 ergs. From this we were able to determine the mass
content of the debris to be in the range 2x10^14 < m < 2x10^15 g. Given typical
promimence masses, the likely debris mass is towards the lower end of this
range. This clear example of a prominence debris event shows that significant
energy release takes place during these events, and that such impacts may be
used as a novel diagnostic tool for investigating prominence material
properties. | 1708.01555v2 |
2017-08-16 | Magneto Acoustic Spin Hall Oscillators | This paper introduces a novel oscillator that combines the tunability of spin
Hall-driven nano oscillators with the high quality factor (Q) of high overtone
bulk acoustic wave resonators (HBAR), integrating both reference and tunable
oscillators on the same chip with CMOS. In such magneto acoustic spin Hall
(MASH) oscillators, voltage oscillations across the magnetic tunnel junction
(MTJ) that arise from a spin-orbit torque (SOT) are shaped by the transmission
response of the HBAR that acts as a multiple peak-bandpass filter and a delay
element due to its large time constant, providing delayed feedback. The
filtered voltage oscillations can be fed back to the MTJ via a) strain, b)
current, or c) magnetic field. We develop a SPICE-based circuit model by
combining experimentally benchmarked models including the stochastic
Landau-Lifshitz-Gilbert (sLLG) equation for magnetization dynamics and the
Butterworth Van Dyke (BVD) circuit for the HBAR. Using the self-consistent
model, we project up to $\sim$ 50X enhancement in the oscillator linewidth with
Q reaching up to 52825 at 3 GHz, while preserving the tunability by locking the
STNO to the nearest high Q peak of the HBAR. We expect that our results will
inspire MEMS-based solutions to spintronic devices by combining attractive
features of both fields for a variety of applications. | 1708.04735v2 |
2017-09-01 | An order optimal policy for exploiting idle spectrum in cognitive radio networks | In this paper a spectrum sensing policy employing recency-based exploration
is proposed for cognitive radio networks. We formulate the problem of finding a
spectrum sensing policy for multi-band dynamic spectrum access as a stochastic
restless multi-armed bandit problem with stationary unknown reward
distributions. In cognitive radio networks the multi-armed bandit problem
arises when deciding where in the radio spectrum to look for idle frequencies
that could be efficiently exploited for data transmission. We consider two
models for the dynamics of the frequency bands: 1) the independent model where
the state of the band evolves randomly independently from the past and 2) the
Gilbert-Elliot model, where the states evolve according to a 2-state Markov
chain. It is shown that in these conditions the proposed sensing policy attains
asymptotically logarithmic weak regret. The policy proposed in this paper is an
index policy, in which the index of a frequency band is comprised of a sample
mean term and a recency-based exploration bonus term. The sample mean promotes
spectrum exploitation whereas the exploration bonus encourages for further
exploration for idle bands providing high data rates. The proposed recency
based approach readily allows constructing the exploration bonus such that it
will grow the time interval between consecutive sensing time instants of a
suboptimal band exponentially, which then leads to logarithmically increasing
weak regret. Simulation results confirming logarithmic weak regret are
presented and it is found that the proposed policy provides often improved
performance at low complexity over other state-of-the-art policies in the
literature. | 1709.00237v1 |
2017-09-08 | EndoSensorFusion: Particle Filtering-Based Multi-sensory Data Fusion with Switching State-Space Model for Endoscopic Capsule Robots | A reliable, real time multi-sensor fusion functionality is crucial for
localization of actively controlled capsule endoscopy robots, which are an
emerging, minimally invasive diagnostic and therapeutic technology for the
gastrointestinal (GI) tract. In this study, we propose a novel multi-sensor
fusion approach based on a particle filter that incorporates an online
estimation of sensor reliability and a non-linear kinematic model learned by a
recurrent neural network. Our method sequentially estimates the true robot pose
from noisy pose observations delivered by multiple sensors. We experimentally
test the method using 5 degree-of-freedom (5-DoF) absolute pose measurement by
a magnetic localization system and a 6-DoF relative pose measurement by visual
odometry. In addition, the proposed method is capable of detecting and handling
sensor failures by ignoring corrupted data, providing the robustness expected
of a medical device. Detailed analyses and evaluations are presented using
ex-vivo experiments on a porcine stomach model prove that our system achieves
high translational and rotational accuracies for different types of endoscopic
capsule robot trajectories. | 1709.03401v3 |
2017-09-12 | Distributed Scheduling in Time Dependent Environments: Algorithms and Analysis | Consider the problem of a multiple access channel in a time dependent
environment with a large number of users. In such a system, mostly due to
practical constraints (e.g., decoding complexity), not all users can be
scheduled together, and usually only one user may transmit at any given time.
Assuming a distributed, opportunistic scheduling algorithm, we analyse the
system's properties, such as delay, QoS and capacity scaling laws.
Specifically, we start with analyzing the performance while \emph{assuming the
users are not necessarily fully backlogged}, focusing on the queueing problem
and, especially, on the \emph{strong dependence between the queues}. We first
extend a known queueing model by Ephremides and Zhu, to give new results on the
convergence of the probability of collision to its average value (as the number
of users grows), and hence for the ensuing system performance metrics, such as
throughput and delay. This model, however, is limited in the number of users
one can analyze. We thus suggest a new model, which is much simpler yet can
accurately describes the system behaviour when the number of users is large.
We then proceed to the analysis of this system under the assumption of time
dependent channels. Specifically, we assume each user experiences a different
channel state sequence, expressing different channel fluctuations
(specifically, the Gilbert-Elliott model). The system performance under this
setting is analysed, along with the channel capacity scaling laws. | 1709.04361v1 |
2017-09-16 | A differential memristive synapse circuit for on-line learning in neuromorphic computing systems | Spike-based learning with memristive devices in neuromorphic computing
architectures typically uses learning circuits that require overlapping pulses
from pre- and post-synaptic nodes. This imposes severe constraints on the
length of the pulses transmitted in the network, and on the network's
throughput. Furthermore, most of these circuits do not decouple the currents
flowing through memristive devices from the one stimulating the target neuron.
This can be a problem when using devices with high conductance values, because
of the resulting large currents. In this paper we propose a novel circuit that
decouples the current produced by the memristive device from the one used to
stimulate the post-synaptic neuron, by using a novel differential scheme based
on the Gilbert normalizer circuit. We show how this circuit is useful for
reducing the effect of variability in the memristive devices, and how it is
ideally suited for spike-based learning mechanisms that do not require
overlapping pre- and post-synaptic pulses. We demonstrate the features of the
proposed synapse circuit with SPICE simulations, and validate its learning
properties with high-level behavioral network simulations which use a
stochastic gradient descent learning rule in two classification tasks. | 1709.05484v1 |
2017-09-21 | Impacts of Surface Depletion on the Plasmonic Properties of Doped Semiconductor Nanocrystals | Degenerately doped semiconductor nanocrystals (NCs) exhibit a localized
surface plasmon resonance (LSPR) in the infrared range of the electromagnetic
spectrum. Unlike metals, semiconductor NCs offer tunable LSPR characteristics
enabled by doping, or via electrochemical or photochemical charging. Tuning
plasmonic properties through carrier density modulation suggests potential
applications in smart optoelectronics, catalysis, and sensing. Here, we
elucidate fundamental aspects of LSPR modulation through dynamic carrier
density tuning in Sn-doped Indium Oxide NCs. Monodisperse Sn-doped Indium Oxide
NCs with various doping level and sizes were synthesized and assembled in
uniform films. NC films were then charged in an in situ electrochemical cell
and the LSPR modulation spectra were monitored. Based on spectral shifts and
intensity modulation of the LSPR, combined with optical modeling, it was found
that often-neglected semiconductor properties, specifically band structure
modification due to doping and surface states, strongly affect LSPR modulation.
Fermi level pinning by surface defect states creates a surface depletion layer
that alters the LSPR properties; it determines the extent of LSPR frequency
modulation, diminishes the expected near field enhancement, and strongly
reduces sensitivity of the LSPR to the surroundings. | 1709.07136v2 |
2017-10-05 | Transport theory for femtosecond laser-induced spin-transfer torques | Ultrafast demagnetization of magnetic layers pumped by a femtosecond laser
pulse is accompanied by a nonthermal spin-polarized current of hot electrons.
These spin currents are studied here theoretically in a spin valve with
noncollinear magnetizations. To this end, we introduce an extended model of
superdiffusive spin transport that enables to treat noncollinear magnetic
configurations, and apply it to the perpendicular spin valve geometry. We show
how spin-transfer torques arise due to this mechanism and calculate their
action on the magnetization present, as well as how the latter depends on the
thicknesses of the layers and other transport parameters. We demonstrate that
there exists a certain optimum thickness of the out-of-plane magnetized
spin-current polarizer such that the torque acting on the second magnetic layer
is maximal. Moreover, we study the magnetization dynamics excited by the
superdiffusive spin-transfer torque due to the flow of hot electrons employing
the Landau-Lifshitz-Gilbert equation. Thereby we show that a femtosecond laser
pulse applied to one magnetic layer can excite small-angle precessions of the
magnetization in the second magnetic layer. We compare our calculations with
recent experimental results. | 1710.02083v2 |
2017-10-12 | A critical comparison of methods for the determination of the ageing sensitivity in biomedical grade yttria stabilized zirconia | Since the recent failure events of two particular series of zirconia femoral
heads for total hip replacement prosthesis, a large decrease in the use of
zirconia ceramics for orthopaedic implants has been observed. In spite of the
biomedical success of this material during the last ten years, this was
required for safety reasons, until the cause of the failures is known. It has
been shown that these failures were related to the low temperature hydrothermal
degradation (also known as ageing). Thus it is crucial to better understand the
ageing behaviour, in order to be able to assess its importance and then control
it if required. In this paper, various techniques relevant to assess the
hydrothermal degradation sensitivity of biomedical grade yttria stabilized
zirconia are discussed and compared. The expected outputs of conventional
methods, i.e. X-Ray diffraction and scanning electron microscopy are examined.
More recent methods like optical interferometry and atomic force microscopy are
presented, with their respective benefits and drawbacks. An up to date
comparison of these different techniques is provided, and their use for
ensuring the long term reliability of a particular batch of zirconia in terms
of ageing degradation is demonstrated. | 1710.04449v1 |
2017-10-26 | Evaluation of Treatment Effect Modification by Biomarkers Measured Pre- and Post-randomization in the Presence of Non-monotone Missingness | In vaccine studies, investigators are often interested in studying effect
modifiers of clinical treatment efficacy by biomarker-based principal strata,
which is useful for selecting biomarker study endpoints for evaluating
treatments in new trials, exploring biological mechanisms of clinical treatment
efficacy, and studying mediators of clinical treatment efficacy. However, in
trials where participants may enter the study with prior exposure therefore
with variable baseline biomarker values, clinical treatment efficacy may depend
jointly on a biomarker measured at baseline and measured at a fixed time after
vaccination. Therefore, it is of interest to conduct a bivariate effect
modification analysis by biomarker-based principal strata and baseline
biomarker values. Previous methods allow this assessment if participants who
have the biomarker measured at the the fixed time point post randomization
would also have the biomarker measured at baseline. However, additional
complications in study design could happen in practice. For example, in the
Dengue correlates study, baseline biomarker values were only available from a
fraction of participants who have biomarkers measured post-randomization. How
to conduct the bivariate effect modification analysis in these studies remains
an open research question. In this article, we propose an estimated likelihood
method to utilize the sub-sampled baseline biomarker in the effect modification
analysis and illustrate our method with datasets from two dengue phase 3
vaccine efficacy trials. | 1710.09923v1 |
2017-10-29 | If it ain't broke, don't fix it: Sparse metric repair | Many modern data-intensive computational problems either require, or benefit
from distance or similarity data that adhere to a metric. The algorithms run
faster or have better performance guarantees. Unfortunately, in real
applications, the data are messy and values are noisy. The distances between
the data points are far from satisfying a metric. Indeed, there are a number of
different algorithms for finding the closest set of distances to the given ones
that also satisfy a metric (sometimes with the extra condition of being
Euclidean). These algorithms can have unintended consequences, they can change
a large number of the original data points, and alter many other features of
the data.
The goal of sparse metric repair is to make as few changes as possible to the
original data set or underlying distances so as to ensure the resulting
distances satisfy the properties of a metric. In other words, we seek to
minimize the sparsity (or the $\ell_0$ "norm") of the changes we make to the
distances subject to the new distances satisfying a metric. We give three
different combinatorial algorithms to repair a metric sparsely. In one setting
the algorithm is guaranteed to return the sparsest solution and in the other
settings, the algorithms repair the metric. Without prior information, the
algorithms run in time proportional to the cube of the number of input data
points and, with prior information we can reduce the running time considerably. | 1710.10655v1 |
2017-12-06 | Monitoring the orientation of rare-earth-doped nanorods for flow shear tomography | Rare-earth phosphors exhibit unique luminescence polarization features
originating from the anisotropic symmetry of the emitter ion's chemical
environment. However, to take advantage of this peculiar property, it is
necessary to control and measure the ensemble orientation of the host particles
with a high degree of precision. Here, we show a methodology to obtain the
photoluminescence polarization of Eu-doped LaPO4 nano rods assembled in an
electrically modulated liquid-crystalline phase. We measure Eu3+ emission
spectra for the three main optimal configurations ({\sigma}, {\pi} and
{\alpha}, depending on the direction of observation and the polarization axes)
and use them as a reference for the nano rod orientation analysis. Based on the
fact that flowing nano rods tend to orient along the shear strain profile, we
use this orientation analysis to measure the local shear rate in a flowing
liquid. The potential of this approach is then demonstrated through tomographic
imaging of the shear rate distribution in a microfluidic system. | 1712.02191v1 |
2017-12-08 | Shrewd Selection Speeds Surfing: Use Smart EXP3! | In this paper, we explore the use of multi-armed bandit online learning
techniques to solve distributed resource selection problems. As an example, we
focus on the problem of network selection. Mobile devices often have several
wireless networks at their disposal. While choosing the right network is vital
for good performance, a decentralized solution remains a challenge. The
impressive theoretical properties of multi-armed bandit algorithms, like EXP3,
suggest that it should work well for this type of problem. Yet, its real-word
performance lags far behind. The main reasons are the hidden cost of switching
networks and its slow rate of convergence. We propose Smart EXP3, a novel
bandit-style algorithm that (a) retains the good theoretical properties of
EXP3, (b) bounds the number of switches, and (c) yields significantly better
performance in practice. We evaluate Smart EXP3 using simulations, controlled
experiments, and real-world experiments. Results show that it stabilizes at the
optimal state, achieves fairness among devices and gracefully deals with
transient behaviors. In real world experiments, it can achieve 18% faster
download over alternate strategies. We conclude that multi-armed bandit
algorithms can play an important role in distributed resource selection
problems, when practical concerns, such as switching costs and convergence
time, are addressed. | 1712.03038v3 |
2017-12-08 | Qatar Exoplanet Survey: Qatar-6b -- a grazing transiting hot Jupiter | We report the discovery of Qatar-6b, a new transiting planet identified by
the Qatar Exoplanet Survey (QES). The planet orbits a relatively bright
(V=11.44), early-K main-sequence star at an orbital period of P~3.506 days. An
SED fit to available multi-band photometry, ranging from the near-UV to the
mid-IR, yields a distance of d = 101 +/- 6 pc to the system. From a global fit
to follow-up photometric and spectroscopic observations, we calculate the mass
and radius of the planet to be Mp = 0.67 +/- 0.07 Mjup and Rp = 1.06 +/- 0.07
Rjup, respectively. We use multi-color photometric light curves to show that
the transit is grazing, making Qatar-6b one of the few exoplanets known in a
grazing transit configuration. It adds to the short list of targets that offer
the best opportunity to look for additional bodies in the host planetary system
through variations in the transit impact factor and duration. | 1712.03216v1 |
2018-01-25 | Generating survival times using Cox proportional hazards models with cyclic time-varying covariates, with application to a multiple-dose monoclonal antibody clinical trial | In two harmonized efficacy studies to prevent HIV infection through multiple
infusions of the monoclonal antibody VRC01, a key objective is to evaluate
whether the serum concentration of VRC01, which changes cyclically over time
along with the infusion schedule, is associated with the rate of HIV infection.
Simulation studies are needed in the development of such survival models. In
this paper, we consider simulating event time data with a continuous
time-varying covariate whose values vary with time through multiple drug
administration cycles, and whose effect on survival changes differently before
and after a threshold within each cycle. The latter accommodates settings with
a zero-protection biomarker threshold above which the drug provides a varying
level of protection depending on the biomarker level, but below which the drug
provides no protection. We propose two simulation approaches: one based on
simulating survival data under a single-dose regimen first before data are
aggregated over multiple doses, and another based on simulating survival data
directly under a multiple-dose regimen. We generate time-to-event data
following a Cox proportional hazards model based on inverting the cumulative
hazard function and a log link function for relating the hazard function to the
covariates. The method's validity is assessed in two sets of simulation
experiments. The results indicate that the proposed procedures perform well in
producing data that conform to their cyclic nature and assumptions of the Cox
proportional hazards model. | 1801.08248v1 |
2018-01-29 | Band-pass superlattice magnetic tunnel junctions | Significant scientific and technological progress in the field of spintronics
is based on trilayer magnetic tunnel junction devices which principally rely on
the physics of single barrier tunneling. While technologically relevant devices
have been prototyped, the physics of single barrier tunneling poses ultimate
limitations on the performance of magnetic tunnel junction devices. Here, we
propose a fresh route toward high performance magnetic tunnel junctions by
making electronic analogs of optical phenomena such as anti-reflections and
Fabry-P\`erot resonances. The devices we propose feature anti-reflection
enabled superlattice heterostructures sandwiched between the fixed and the free
ferromagnets of the magnetic tunnel junction structure. Our predictions are
based on the non-equilibrium Green's function spin transport formalism coupled
self-consistently with the Landau-Lifshitz-Gilbert-Slonczewski equation. Owing
to the physics of bandpass spin filtering in the bandpass superlattice magnetic
tunnel junction device, we demonstrate an ultra-high boost in the tunnel
magneto-resistance (TMR$\approx5\times10^4\%$) and nearly 92% suppression of
spin transfer torque switching bias in comparison to a traditional trilayer
magnetic tunnel junction device. We rationalize improvised spin transfer torque
switching via analysis of the Slonczewski spin current transmission spectra.
The proof of concepts presented here can lead to next-generation spintronics
device design harvesting the rich physics of superlattice heterostructures and
exploiting spintronic analogs of optical phenomena. | 1801.09409v2 |
2018-01-29 | Theory of AC quantum transport with fully electrodynamic coupling | With the continued scaling of microelectronic devices along with the growing
demand of high-speed wireless telecommunications technologies, there is
increasing need for high-frequency device modeling techniques that accurately
capture the quantum mechanical nature of charge transport in nanoscale devices
along with the dynamic fields that are generated. In an effort to fill this
gap, we develop a simulation methodology that self-consistently couples AC
non-equilibrium Green functions (NEGF) with the full solution of Maxwell's
equations in the frequency domain. We apply this technique to simulate
radiation from a quantum-confined, quarter-wave, monopole antenna where the
length $L$ is equal to one quarter of the wavelength, $\lambda_0$. Classically,
such an antenna would have a narrower, more directed radiation pattern compared
to one with $L \ll \lambda_0$, but we find that a quantum quarter-wave antenna
has no directivity gain compared to the classical solution. We observe that the
quantized wave function within the antenna significantly alter the charge and
current density distribution along the length of the wire, which in turn
modifies the far-field radiation pattern from the antenna. These results show
that high-frequency radiation from quantum systems can be markedly different
from classical expectations. Our method, therefore, will enable accurate
modeling of the next generation of high-speed nanoscale electronic devices. | 1801.09611v1 |
2018-02-17 | Design and Implementation of the Andromeda Proof Assistant | Andromeda is an LCF-style proof assistant where the user builds derivable
judgments by writing code in a meta-level programming language AML. The only
trusted component of Andromeda is a minimalist nucleus (an implementation of
the inference rules of an object-level type theory), which controls
construction and decomposition of type-theoretic judgments.
Since the nucleus does not perform complex tasks like equality checking
beyond syntactic equality, this responsibility is delegated to the user, who
implements one or more equality checking procedures in the meta-language. The
AML interpreter requests witnesses of equality from user code using the
mechanism of algebraic operations and handlers. Dynamic checks in the nucleus
guarantee that no invalid object-level derivations can be constructed. %even if
the AML code (or interpreter) is untrusted.
To demonstrate the flexibility of this system structure, we implemented a
nucleus consisting of dependent type theory with equality reflection. Equality
reflection provides a very high level of expressiveness, as it allows the user
to add new judgmental equalities, but it also destroys desirable meta-theoretic
properties of type theory (such as decidability and strong normalization).
The power of effects and handlers in AML is demonstrated by a standard
library that provides default algorithms for equality checking, computation of
normal forms, and implicit argument filling. Users can extend these new
algorithms by providing local "hints" or by completely replacing these
algorithms for particular developments. We demonstrate the resulting system by
showing how to axiomatize and compute with natural numbers, by axiomatizing the
untyped $\lambda$-calculus, and by implementing a simple automated system for
managing a universe of types. | 1802.06217v1 |
2018-03-02 | Broadband spectroscopy of thermodynamic magnetization fluctuations through a ferromagnetic spin-reorientation transition | We use scanning optical magnetometry to study the broadband frequency spectra
of spontaneous magnetization fluctuations, or "magnetization noise", in an
archetypal ferromagnetic film that can be smoothly tuned through a spin
reorientation transition (SRT). The SRT is achieved by laterally varying the
magnetic anisotropy across an ultrathin Pt/Co/Pt trilayer, from the
perpendicular to in-plane direction, via graded Ar$^+$ irradiation. In regions
exhibiting perpendicular anisotropy, the power spectrum of the magnetization
noise, $S(\nu)$, exhibits a remarkably robust $\nu^{-3/2}$ power law over
frequencies $\nu$ from 1~kHz to 1~MHz. As the SRT region is traversed, however,
$S(\nu)$ spectra develop a steadily-increasing critical frequency, $\nu_0$,
below which the noise power is spectrally flat, indicating an evolving
low-frequency cutoff for magnetization fluctuations. The magnetization noise
depends strongly on applied in- and out-of-plane magnetic fields, revealing
local anisotropies and also a field-induced emergence of fluctuations in
otherwise stable ferromagnetic films. Finally, we demonstrate that higher-order
correlators can be computed from the noise. These results highlight broadband
spectroscopy of thermodynamic fluctuations as a powerful tool to characterize
the interplay between thermal and magnetic energy scales, and as a means of
characterizing phase transitions in ferromagnets. | 1803.00962v1 |
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