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2016-06-27
|
Five Planets Transiting a Ninth Magnitude Star
|
The Kepler mission has revealed a great diversity of planetary systems and
architectures, but most of the planets discovered by Kepler orbit faint stars.
Using new data from the K2 mission, we present the discovery of a five planet
system transiting a bright (V = 8.9, K = 7.7) star called HIP 41378. HIP 41378
is a slightly metal-poor late F-type star with moderate rotation (v sin(i) = 7
km/s) and lies at a distance of 116 +/- 18 from Earth. We find that HIP 41378
hosts two sub-Neptune sized planets orbiting 3.5% outside a 2:1 period
commensurability in 15.6 and 31.7 day orbits. In addition, we detect three
planets which each transit once during the 75 days spanned by K2 observations.
One planet is Neptune sized in a likely ~160 day orbit, one is sub-Saturn sized
likely in a ~130 day orbit, and one is a Jupiter sized planet in a likely ~1
year orbit. We show that these estimates for the orbital periods can be made
more precise by taking into account dynamical stability considerations. We also
calculate the distribution of stellar reflex velocities expected for this
system, and show that it provides a good target for future radial velocity
observations. If a precise orbital period can be determined for the outer
Jovian planet through future observations, it will be an excellent candidate
for follow-up transit observations to study its atmosphere and measure its
oblateness.
|
1606.08441v1
|
2016-07-06
|
Holographic Beam Mapping of the CHIME Pathfinder Array
|
The Canadian Hydrogen Intensity Mapping Experiment (CHIME) Pathfinder radio
telescope is currently surveying the northern hemisphere between 400 and 800
MHz. By mapping the large scale structure of neutral hydrogen through its
redshifted 21 cm line emission between $z \sim 0.8-2.5$ CHIME will contribute
to our understanding of Dark Energy. Bright astrophysical foregrounds must be
separated from the neutral hydrogen signal, a task which requires precise
characterization of the polarized telescope beams. Using the DRAO John A. Galt
26 m telescope, we have developed a holography instrument and technique for
mapping the CHIME Pathfinder beams. We report the status of the instrument and
initial results of this effort.
|
1607.01473v1
|
2016-07-21
|
Long-Term Photometry of IC 348 with the YETI Network
|
We present long-term photometric observations of the young open cluster IC
348 with a baseline time-scale of 2.4 yr. Our study was conducted with several
telescopes from the Young Exoplanet Transit Initiative (YETI) network in the
Bessel $R$ band to find periodic variability of young stars. We identified 87
stars in IC 348 to be periodically variable; 33 of them were unreported before.
Additionally, we detected 61 periodic non-members of which 41 are new
discoveries. Our wide field of view was the key to those numerous newly found
variable stars. The distribution of rotation periods in IC 348 has always been
of special interest. We investigate it further with our newly detected periods
but we cannot find a statistically significant bimodality. We also report the
detection of a close eclipsing binary in IC 348 composed of a low-mass stellar
component ($M \gtrsim 0.09\,\mathrm{M}_{\odot}$) and a K0 pre-main sequence
star ($M \approx 2.7\,\mathrm{M}_{\odot}$). Furthermore, we discovered three
detached binaries among the background stars in our field of view and confirmed
the period of a fourth one.
|
1607.06322v1
|
2016-08-02
|
Atomic-scale observation and manipulation of plaquette antiferromagnetic order in iron-based superconductor
|
The symmetry requirement and the origin of magnetic orders coexisting with
superconductivity have been strongly debated issues of iron-based
superconductors (FeSCs). Observation of C$_4$-symmetric antiferromagnetism in
violation of the inter-band nesting condition of spin-density waves in
superconducting ground state will require significant change in our
understanding of the mechanism of FeSC. The superconducting material
Sr$_2$VO$_3$FeAs, a bulk version of monolayer FeSC in contact with a perovskite
layer with its magnetism (T$_N$ ~ 50 K) and superconductivity (T$_c$ ~ 37 K)
coexisting at parent state, has no reported structural orthorhombic distortion
and thus makes a perfect system to look for theoretically expected C$_4$
magnetisms. Based on variable temperature spin-polarized scanning tunneling
microscopy (SPSTM) with newly discovered imaging mechanism that removes the
static surface reconstruction (SR) pattern by fluctuating it rapidly with
spin-polarized tunneling current, we could visualize underlying C$_4$ symmetric
(2$\times$2) magnetic domains and its phase domain walls. We find that this
magnetic order is perfectly consistent with the plaquette antiferromagnetic
order in tetragonal Fe spin lattice expected from theories based on the
Heisenberg exchange interaction of local Fe moments and the quantum order by
disorder. The inconsistency of its modulation Q vectors from the nesting
condition also implies that the nesting-based C$_2$ symmetric magnetism is not
a unique prerequisite of high-T$_c$ FeSC. Furthermore, the plaquette
antiferromagnetic domain wall dynamics under the influence of small spin torque
effect of spin-polarized tunneling current are shown to be consistent with
theoretical simulation based on the extended Landau-Lifshitz-Gilbert equation.
|
1608.00884v3
|
2016-08-10
|
POLARBEAR-2: an instrument for CMB polarization measurements
|
POLARBEAR-2 (PB-2) is a cosmic microwave background (CMB) polarization
experiment that will be located in the Atacama highland in Chile at an altitude
of 5200 m. Its science goals are to measure the CMB polarization signals
originating from both primordial gravitational waves and weak lensing. PB-2 is
designed to measure the tensor to scalar ratio, r, with precision {\sigma}(r) <
0.01, and the sum of neutrino masses, {\Sigma}m{\nu}, with
{\sigma}({\Sigma}m{\nu}) < 90 meV. To achieve these goals, PB-2 will employ
7588 transition-edge sensor bolometers at 95 GHz and 150 GHz, which will be
operated at the base temperature of 250 mK. Science observations will begin in
2017.
|
1608.03025v1
|
2016-08-15
|
ICE-based Custom Full-Mesh Network for the CHIME High Bandwidth Radio Astronomy Correlator
|
New generation radio interferometers encode signals from thousands of antenna
feeds across large bandwidth. Channelizing and correlating this data requires
networking capabilities that can handle unprecedented data rates with
reasonable cost. The Canadian Hydrogen Intensity Mapping Experiment (CHIME)
correlator processes 8-bits from N=2048 digitizer inputs across 400~MHz of
bandwidth. Measured in $N^2~\times $ bandwidth, it is the largest radio
correlator that has been built. Its digital back-end must exchange and
reorganize the 6.6~terabit/s produced by its 128 digitizing and channelizing
nodes, and feed it to the 256-node spatial correlator in a way that each node
obtains data from all digitizer inputs but across a small fraction of the
bandwidth (i.e. `corner-turn'). In order to maximize performance and
reliability of the corner-turn system while minimizing cost, a custom
networking solution has been implemented. The system makes use of Field
Programmable Gate Array (FPGA) transceivers to implement direct, passive,
full-mesh, high speed serial connections between sixteen circuit boards in a
crate, to exchange data between crates, and to offload the data to a cluster of
256 graphics processing unit (GPU) nodes using standard 10~Gbit/s Ethernet
links. The GPU nodes complete the corner-turn by combining data from all crates
and then computing visibilities. Eye diagrams and frame error counters confirm
error-free operation of the corner-turn network in both the currently operating
CHIME Pathfinder telescope (a prototype for the full CHIME telescope) and a
representative fraction of the full CHIME hardware providing an end-to-end
system validation.
An analysis of an equivalent corner-turn system built with Ethernet switches
instead of custom passive data links is provided.
|
1608.04347v1
|
2016-08-22
|
ICE: a scalable, low-cost FPGA-based telescope signal processing and networking system
|
We present an overview of the 'ICE' hardware and software framework that
implements large arrays of interconnected FPGA-based data acquisition, signal
processing and networking nodes economically. The system was conceived for
application to radio, millimeter and sub-millimeter telescope readout systems
that have requirements beyond typical off-the-shelf processing systems, such as
careful control of interference signals produced by the digital electronics,
and clocking of all elements in the system from a single precise
observatory-derived oscillator. A new generation of telescopes operating at
these frequency bands and designed with a vastly increased emphasis on digital
signal processing to support their detector multiplexing technology or
high-bandwidth correlators---data rates exceeding a terabyte per second---are
becoming common. The ICE system is built around a custom FPGA motherboard that
makes use of an Xilinx Kintex-7 FPGA and ARM-based co-processor. The system is
specialized for specific applications through software, firmware, and custom
mezzanine daughter boards that interface to the FPGA through the
industry-standard FMC specifications. For high density applications, the
motherboards are packaged in 16-slot crates with ICE backplanes that implement
a low-cost passive full-mesh network between the motherboards in a crate, allow
high bandwidth interconnection between crates, and enable data offload to a
computer cluster. A Python-based control software library automatically detects
and operates the hardware in the array. Examples of specific telescope
applications of the ICE framework are presented, namely the
frequency-multiplexed bolometer readout systems used for the SPT and Simons
Array and the digitizer, F-engine, and networking engine for the CHIME and
HIRAX radio interferometers.
|
1608.06262v1
|
2016-09-14
|
Revised Stellar Properties of Kepler Targets for the Q1-17 (DR25) Transit Detection Run
|
The determination of exoplanet properties and occurrence rates using Kepler
data critically depends on our knowledge of the fundamental properties (such as
temperature, radius and mass) of the observed stars. We present revised stellar
properties for 197,096 Kepler targets observed between Quarters 1-17 (Q1-17),
which were used for the final transiting planet search run by the Kepler
Mission (Data Release 25, DR25). Similar to the Q1--16 catalog by Huber et al.
the classifications are based on conditioning published atmospheric parameters
on a grid of Dartmouth isochrones, with significant improvements in the adopted
methodology and over 29,000 new sources for temperatures, surface gravities or
metallicities. In addition to fundamental stellar properties the new catalog
also includes distances and extinctions, and we provide posterior samples for
each stellar parameter of each star. Typical uncertainties are ~27% in radius,
~17% in mass, and ~51% in density, which is somewhat smaller than previous
catalogs due to the larger number of improved logg constraints and the
inclusion of isochrone weighting when deriving stellar posterior distributions.
On average, the catalog includes a significantly larger number of evolved
solar-type stars, with an increase of 43.5% in the number of subgiants. We
discuss the overall changes of radii and masses of Kepler targets as a function
of spectral type, with particular focus on exoplanet host stars.
|
1609.04128v2
|
2016-11-16
|
Low Barrier Nanomagnets as p-bits for Spin Logic
|
It has recently been shown that a suitably interconnected network of tunable
telegraphic noise generators or "p-bits" can be used to perform even precise
arithmetic functions like a 32-bit adder. In this paper we use simulations
based on the stochastic Landau-Lifshitz-Gilbert (sLLG) equation to demonstrate
that similar impressive functions can be performed using unstable nanomagnets
with energy barriers as low as a fraction of a kT. This is surprising since the
magnetization of low barrier nanomagnets is not telegraphic with discrete
values of +1 and -1. Rather it fluctuates randomly among all values between -1
and +1, and the output magnets are read with a thresholding device that
translates all positive values to 1 and all negative values to zero. We present
sLLG-based simulations demonstrating the operation of a 32-bit adder with a
network of several hundred nanomagnets, exhibiting a remarkably precise
correlation: The input magnets {A} and {B} as well as the output magnets {S}
all fluctuate randomly and yet the quantity A+B-S is sharply peaked around
zero! If we fix {A} and {B}, the sum magnets {S} rapidly converge to a unique
state with S=A+B so that the system acts as an adder. But unlike standard
adders, the operation is invertible. If we fix {S} and {B}, the remaining
magnets {A} converge to the difference A=S-B. These examples suggest a new
direction for the field of nanomagnetics away from stable high barrier magnets
towards stochastic low barrier magnets which not only operate with lower
currents, but are also more promising for continued downscaling.
|
1611.05477v3
|
2016-11-19
|
Slow links, fast links, and the cost of gossip
|
Consider the classical problem of information dissemination: one (or more)
nodes in a network have some information that they want to distribute to the
remainder of the network. In this paper, we study the cost of information
dissemination in networks where edges have latencies, i.e., sending a message
from one node to another takes some amount of time. We first generalize the
idea of conductance to weighted graphs by defining $\phi_*$ to be the "critical
conductance" and $\ell_*$ to be the "critical latency". One goal of this paper
is to argue that $\phi_*$ characterizes the connectivity of a weighted graph
with latencies in much the same way that conductance characterizes the
connectivity of unweighted graphs.
We give near tight lower and upper bounds on the problem of information
dissemination, up to polylogarithmic factors. Specifically, we show that in a
graph with (weighted) diameter $D$ (with latencies as weights) and maximum
degree $\Delta$, any information dissemination algorithm requires at least
$\Omega(\min(D+\Delta, \ell_*/\phi_*))$ time % in the worst case. We show
several variants of the lower bound (e.g., for graphs with small diameter,
graphs with small max-degree, etc.) by reduction to a simple combinatorial
game.
We then give nearly matching algorithms, showing that information
dissemination can be solved in $O(\min((D+\Delta)\log^3{n}, (\ell_*/\phi_*)\log
n)$ time. This is achieved by combining two cases. We show that the classical
push-pull algorithm is (near) optimal when the diameter or the maximum degree
is large. For the case where the diameter and the maximum degree are small, we
give an alternative strategy in which we first discover the latencies and then
use an algorithm for known latencies based on a weighted spanner construction.
(Our algorithms are within polylogarithmic factors of being tight both for
known and unknown latencies.)
|
1611.06343v4
|
2016-12-22
|
Eroding dipoles and vorticity growth for Euler flows in $ \scriptstyle{\mathbb{R}}^3$: The hairpin geometry as a model for finite-time blowup
|
A theory of an eroding "hairpin" vortex dipole structure in three dimensions
is developed, extending our previous study of an axisymmetric eroding dipole
without swirl. The hairpin is here similarly proposed as a model to produce
large "self-stretching" of vorticity, with the possibility of finite-time
blow-up. We derive a system of partial differential equations of "generalized"
form, involving contour averaging of a locally two-dimensional Euler flow. We
do not attempt here to solve the system exactly, but point out that
non-existence of physically acceptable solutions would most probably be a
result of the axial flow. Because of the axial flow the vorticity distribution
within the dipole eddies is no longer of the simple Sadovskii type (vorticity
constant over a cross-section) obtained in the axisymmetric problem. Thus the
solution of the system depends upon the existence of a larger class of
propagating two-dimensional dipoles.
The hairpin model is obtained by formal asymptotic analysis. As in the
axisymmetric problem a local transformation to "shrinking" coordinates is
introduced, but now in a self-similar form appropriate to the study of a
possible finite-time singularity. We discuss some properties of the model,
including a study of the helicity and a first step in iterating toward a
solution from the Sadovskii structure. We also present examples of
two-dimensional propagating dipoles not previously studied, which have a
vorticity profile consistent with our model. Although no rigorous results can
be given, and analysis of the system is only partial, the formal calculations
are consistent with the possibility of a finite time blowup of vorticity at a
point of vanishing circulation of the dipole eddies, but depending upon the
existence of the necessary two-dimensional propagating dipole.
|
1612.07709v2
|
2017-06-22
|
Efficiency of magnetic hyperthermia in the presence of rotating and static fields
|
Single-domain ferromagnetic nanoparticle systems can be used to transfer
energy from a time-dependent magnetic field into their environment. This local
heat generation, i.e., magnetic hyperthermia, receives applications in cancer
therapy which requires the enhancement of the energy loss. A possible way to
improve the efficiency is to chose a proper type of applied field, e.g., a
rotating instead of an oscillating one. The latter case is very well studied
and there is an increasing interest in the literature to investigate the former
although it is still unclear under which circumstances the rotating applied
field can be more favourable than the oscillating one. The goal of this work is
to incorporate the presence of a static field and to perform a systematic study
of the non-linear dynamics of the magnetisation in the framework of the
deterministic Landau-Lifshitz-Gilbert equation in order to calculate energy
losses. Two cases are considered: the static field is either assumed to be
perpendicular to the plane of rotation or situated in the plane of rotation. In
the latter case a significant increase in the energy loss/cycle is observed if
the magnitudes of the static and the rotating fields have a certain ratio (e.g.
it should be one for isotropic nanoparticles). It can be used to
"super-localise" the heat transfer: in case of an inhomogeneous applied static
field, tissues are heated up only where the magnitudes of the static and
rotating fields reach the required ratio.
|
1706.07426v4
|
2017-07-05
|
Measuring Reionization, Neutrino Mass, and Cosmic Inflation with BFORE
|
BFORE is a NASA high-altitude ultra-long-duration balloon mission proposed to
measure the cosmic microwave background (CMB) across half the sky during a
28-day mid-latitude flight launched from Wanaka, New Zealand. With the unique
access to large angular scales and high frequencies provided by the balloon
platform, BFORE will significantly improve measurements of the optical depth to
reionization tau, breaking parameter degeneracies needed for a measurement of
neutrino mass with the CMB. The large angular scale data will enable BFORE to
hunt for the large-scale gravitational wave B-mode signal, as well as the
degree-scale signal, each at the r~0.01 level. The balloon platform allows
BFORE to map Galactic dust foregrounds at frequencies where they dominate, in
order to robustly separate them from CMB signals measured by BFORE, in addition
to complementing data from ground-based telescopes. The combination of
frequencies will also lead to velocity measurements for thousands of galaxy
clusters, as well as probing how star-forming galaxies populate dark matter
halos. The mission will be the first near-space use of TES multichroic
detectors (150/217 GHz and 280/353 GHz bands) using highly-multiplexed mSQUID
microwave readout, raising the technical readiness level of both technologies.
|
1707.01488v1
|
2017-08-28
|
Three statistically validated K2 transiting warm Jupiter exoplanets confirmed as low-mass stars
|
We have identified three K2 transiting star-planet systems, K2-51 (EPIC
202900527), K2-67 (EPIC 206155547), and K2-76 (EPIC 206432863), as stellar
binaries with low-mass stellar secondaries. The three systems were
statistically validated as transiting planets, and through measuring their
orbits by radial velocity monitoring we have derived the companion masses to be
$0.1459^{+0.0029}_{-0.0032}$ $M_{Sun}$ (EPIC 202900527 B),
$0.1612^{+0.0072}_{-0.0067}$ $M_{Sun}$ (EPIC 206155547 B), and $0.0942 \pm
0.0019$ $M_{Sun}$ (EPIC 206432863 B). Therefore they are not planets but small
stars, part of the small sample of low-mass stars with measured radius and
mass. The three systems are at an orbital period range of $12-24$ days, and the
secondaries have a radius within $0.9-1.9$ $R_J$, not inconsistent with the
properties of warm Jupiter planets. These systems illustrate some of the
existing challenges in the statistical validation approach. We point out a few
possible origins for the initial misclassification of these objects, including
poor characterization of the host star, the difficulty in detecting a secondary
eclipse in systems on an eccentric orbit, and the difficulty in distinguishing
between the smallest stars and gas giant planets as the two populations have
indistinguishable radius distributions. Our work emphasizes the need for
obtaining medium-precision radial velocity measurements to distinguish between
companions that are small stars, brown dwarfs, and gas giant planets.
|
1708.08455v2
|
2017-09-29
|
Zodiacal Exoplanets in Time (ZEIT) VI: a three-planet system in the Hyades cluster including an Earth-sized planet
|
Planets in young clusters are powerful probes of the evolution of planetary
systems. Here we report the discovery of three planets transiting K2-136 (EPIC
247589423), a late K dwarf in the Hyades (~800 Myr) cluster, and robust
detection limits for additional planets in the system. The planets were
identified from their K2 light curves, as part of our survey of young clusters
and star forming regions. The smallest planet has a radius comparable to Earth
(0.99 +/- 0.05 Earth radii), making it one of the few Earth-sized planets with
a known, young age. The two larger planets are likely a mini-Neptune and a
super-Earth, with radii of 2.91+/-0.11 and 1.45+/-0.10 Earth radii,
respectively. The predicted radial velocity signals from these planets are
between 0.4 and 2 m/s, achievable with modern precision RV spectrographs.
Because the target star is bright (V=11.2) and has relatively low-amplitude
stellar variability for a young star (2-6 mmag), EPIC 247589423 hosts the best
planets known in a young open cluster for precise radial velocity follow-up,
enabling a robust test of earlier claims that young planets are less dense than
their older counterparts.
|
1709.10328v2
|
2017-12-11
|
Stellar Stream and Halo Structure in the Andromeda Galaxy From a Subaru/Hyper Suprime-Cam Survey
|
We present wide and deep photometry of the northwest part of the halo of the
Andromeda galaxy (M31) using Hyper Suprime-Cam on the Subaru Telescope. The
survey covers 9.2 deg$^{2}$ field in the $g$, $i$, and $NB515$ bands and shows
a clear red giant branch (RGB) of M31's halo stars and a pronounced red clump
(RC) feature. The spatial distribution of RC stars shows a prominent stream
feature, the North Western (NW) Stream, and a diffuse substructure in the south
part of our survey field. We estimate the distances based on the RC method and
obtain $(m-M)$ = 24.63$\pm 0.191$(random)$\pm0.057$(systematic) and 24.29$\pm
0.211$(random)$\pm0.057$(systematic) mag for the NW stream and diffuse
substructure, respectively, implying that the NW Stream is located behind M31,
whereas the diffuse substructure is located in front. We also estimate
line-of-sight distances along the NW Stream and find that the south part of the
stream is $\sim$20 kpc closer to us relative to the north part. The distance to
the NW Stream inferred from the isochrone fitting to the color-magnitude
diagram favors the RC-based distance, but the TRGB-based distance estimated for
$NB515$-selected RGB stars does not agree with it. The surface number density
distribution of RC stars across the NW Stream is found to be approximately
Gaussian with a FWHM of $\sim$25 arcmin (5.7 kpc), with a slight skew to the
south-west side. That along the NW Stream shows a complicated structure
including variations in number density and a significant gap in the stream.
|
1712.03654v1
|
2018-02-15
|
Spin and charge pumping by steady or pulse current-driven magnetic domain wall: A self-consistent multiscale time-dependent-quantum/time-dependent-classical approach
|
We introduce a multiscale framework which combines time-dependent
nonequilibrium Green function (TD-NEGF) algorithms, scaling linearly in the
number of time steps and describing quantum-mechanically conduction electrons
in the presence of time-dependent fields of arbitrary strength or frequency,
with classical time evolution of localized magnetic moments described by the
Landau-Lifshitz-Gilbert (LLG) equation. The TD-NEGF+LLG framework can be
applied to a variety of problems where current-driven spin torque induces
dynamics of magnetic moments as the key resource for next generation
spintronics. Using magnetic domain wall (DW) as an example, we predict that its
motion will pump time-dependent spin and charge currents (on the top of
unpolarized DC charge current injected through normal metal leads to drive the
DW motion). The conversion of AC components of spin current, whose amplitude
increases (decreases) as the DW approaches (distances from) the normal metal
lead, into AC voltage via the inverse spin Hall effect offers a tool to
precisely track the DW position along magnetic nanowire. We also quantify the
DW transient inertial displacement due to its acceleration and deceleration by
pulse current and the entailed spin and charge pumping. Finally, TD-NEGF+LLG as
a nonperturbative (i.e., numerically exact) framework allows us to establish
the limits of validity of the so-called spin-motive force (SMF) theory for
pumped charge current by time-dependent magnetic textures---the perturbative
analytical formula of SMF theory becomes inapplicable for large frequencies
(but unrealistic in magnetic system) and, more importantly, for increasing
noncollinearity when the angles between neighboring magnetic moments exceed
$\simeq 10^\circ$.
|
1802.05682v3
|
2018-03-21
|
Constraints for the Progenitor Masses of Historic Core-Collapse Supernovae
|
We age-date the stellar populations associated with 12 historic nearby
core-collapse supernovae (CCSNe) and 2 supernova impostors, and from these
ages, we infer their initial masses and associated uncertainties. To do this,
we have obtained new HST imaging covering these CCSNe. Using these images, we
measure resolved stellar photometry for the stars surrounding the locations of
the SNe. We then fit the color-magnitude distributions of this photometry with
stellar evolution models to determine the ages of any young existing
populations present. From these age distributions, we infer the most likely
progenitor mass for all of the SNe in our sample. We find ages between 4 and 50
Myr, corresponding to masses from 7.5 to 59 solar masses. There were no SNe
that lacked a young population within 50~pc. Our sample contains 4 type Ib/c
SNe; their masses have a wide range of values, suggesting that the progenitors
of stripped-envelope SNe are binary systems. Both impostors have masses
constrained to be $\lesssim$7.5 solar masses. In cases with precursor imaging
measurements, we find that age-dating and precursor imaging give consistent
progenitor masses. This consistency implies that, although the uncertainties
for each technique are significantly different, the results of both are
reliable to the measured uncertainties. We combine these new measurements with
those from our previous work and find that the distribution of 25 core-collapse
SNe progenitor masses is consistent with a standard Salpeter power-law mass
function, no upper mass cutoff, and an assumed minimum mass for core-collapse
of 7.5~M$_{\odot}$.
|
1803.08112v1
|
2018-03-29
|
The CHIME Fast Radio Burst Project: System Overview
|
The Canadian Hydrogen Intensity Mapping Experiment (CHIME) is a novel transit
radio telescope operating across the 400-800-MHz band. CHIME is comprised of
four 20-m x 100-m semi-cylindrical paraboloid reflectors, each of which has 256
dual-polarization feeds suspended along its axis, giving it a >200 square
degree field-of-view. This, combined with wide bandwidth, high sensitivity, and
a powerful correlator makes CHIME an excellent instrument for the detection of
Fast Radio Bursts (FRBs). The CHIME Fast Radio Burst Project (CHIME/FRB) will
search beam-formed, high time-and frequency-resolution data in real time for
FRBs in the CHIME field-of-view. Here we describe the CHIME/FRB backend,
including the real-time FRB search and detection software pipeline as well as
the planned offline analyses. We estimate a CHIME/FRB detection rate of 2-42
FRBs/sky/day normalizing to the rate estimated at 1.4-GHz by Vander Wiel et al.
(2016). Likely science outcomes of CHIME/FRB are also discussed. CHIME/FRB is
currently operational in a commissioning phase, with science operations
expected to commence in the latter half of 2018.
|
1803.11235v1
|
2018-05-15
|
Parallel Working-Set Search Structures
|
In this paper we present two versions of a parallel working-set map on p
processors that supports searches, insertions and deletions. In both versions,
the total work of all operations when the map has size at least p is bounded by
the working-set bound, i.e., the cost of an item depends on how recently it was
accessed (for some linearization): accessing an item in the map with recency r
takes O(1+log r) work. In the simpler version each map operation has O((log
p)^2+log n) span (where n is the maximum size of the map). In the pipelined
version each map operation on an item with recency r has O((log p)^2+log r)
span. (Operations in parallel may have overlapping span; span is additive only
for operations in sequence.)
Both data structures are designed to be used by a dynamic multithreading
parallel program that at each step executes a unit-time instruction or makes a
data structure call. To achieve the stated bounds, the pipelined data structure
requires a weak-priority scheduler, which supports a limited form of 2-level
prioritization. At the end we explain how the results translate to practical
implementations using work-stealing schedulers.
To the best of our knowledge, this is the first parallel implementation of a
self-adjusting search structure where the cost of an operation adapts to the
access sequence. A corollary of the working-set bound is that it achieves work
static optimality: the total work is bounded by the access costs in an optimal
static search tree.
|
1805.05787v3
|
2018-05-28
|
Zodiacal Exoplanets in Time (ZEIT) VII: A Temperate Candidate Super-Earth in the Hyades Cluster
|
Transiting exoplanets in young open clusters present opportunities to study
how exoplanets evolve over their lifetimes. Recently, significant progress
detecting transiting planets in young open clusters has been made with the K2
mission, but so far all of these transiting cluster planets orbit close to
their host stars, so planet evolution can only be studied in a high-irradiation
regime. Here, we report the discovery of a long-period planet candidate, called
HD 283869 b, orbiting a member of the Hyades cluster. Using data from the K2
mission, we detected a single transit of a super-Earth-sized (1.96 +/- 0.12
R_earth) planet candidate orbiting the K-dwarf HD 283869 with a period longer
than 72 days. Since we only detected a single transit event, we cannot validate
HD 283869 b with high confidence, but our analysis of the K2 images, archival
data, and follow-up observations suggests that the source of the event is
indeed a transiting planet. We estimated the candidate's orbital parameters and
find that if real, it has a period P~100 days and receives approximately
Earth-like incident flux, giving the candidate a 71% chance of falling within
the circumstellar habitable zone. If confirmed, HD 283869 b would have the
longest orbital period, lowest incident flux, and brightest host star of any
known transiting planet in an open cluster, making it uniquely important to
future studies of how stellar irradiation affects planetary evolution.
|
1805.11117v1
|
2018-06-07
|
Discovery of a Candidate Black Hole - Giant Star Binary System in the Galactic Field
|
We report the discovery of the first likely black hole in a non-interacting
binary system with a field red giant. By combining radial velocity measurements
from the Apache Point Observatory Galactic Evolution Experiment (APOGEE) with
photometric variability data from the All-Sky Automated Survey for Supernovae
(ASAS-SN), we identified the bright rapidly-rotating giant 2MASS
J05215658+4359220 as a binary system with a massive unseen companion.
Subsequent radial velocity measurements reveal a system with an orbital period
of 83 days and near-zero eccentricity. The photometric variability period of
the giant is consistent with the orbital period, indicative of star spots and
tidal synchronization. Constraints on the giant's mass and radius from its
luminosity, surface gravity, and temperature imply an unseen companion with
mass of $3.3^{+2.8}_{-0.7}$ M$_\odot$, indicating a low-mass black hole or an
exceedingly massive neutron star. Measurement of the astrometric binary motion
by {\it Gaia} will further characterize the system. This discovery demonstrates
the potential of massive spectroscopic surveys like APOGEE and all-sky,
high-cadence photometric surveys like ASAS-SN to revolutionize our
understanding of the compact object mass function, and to test theories of
binary star evolution and the supernova mechanism.
|
1806.02751v2
|
2018-06-08
|
A TESS Dress Rehearsal: Planetary Candidates and Variables from K2 Campaign 17
|
We produce light curves for all ~34,000 targets observed with K2 in Campaign
17 (C17), identifying 34 planet candidates, 184 eclipsing binaries, and 222
other periodic variables. The location of the C17 field means follow-up can
begin immediately now that the campaign has concluded and interesting targets
have been identified. The C17 field has a large overlap with C6, so this latest
campaign also offers a rare opportunity to study a large number of targets
already observed in a previous K2 campaign. The timing of the C17 data release,
shortly before science operations begin with the Transiting Exoplanet Survey
Satellite (TESS), also lets us exercise some of the tools and methods developed
for identification and dissemination of planet candidates from TESS. We find
excellent agreement between these results and those identified using only
K2-based tools. Among our planet candidates are several planet candidates with
sizes < 4 R_E and orbiting stars with KepMag < 10 (indicating good RV targets
of the sort TESS hopes to find) and a Jupiter-sized single-transit event around
a star already hosting a 6 d planet candidate.
|
1806.03127v1
|
2018-08-31
|
Design and characterization of the SPT-3G receiver
|
The SPT-3G receiver was commissioned in early 2017 on the 10-meter South Pole
Telescope (SPT) to map anisotropies in the cosmic microwave background (CMB).
New optics, detector, and readout technologies have yielded a multichroic,
high-resolution, low-noise camera with impressive throughput and sensitivity,
offering the potential to improve our understanding of inflationary physics,
astroparticle physics, and growth of structure. We highlight several key
features and design principles of the new receiver, and summarize its
performance to date.
|
1809.00032v1
|
2018-10-02
|
Project AMIGA: Distance and Metallicity Gradients Along Andromeda's Giant Southern Stream from the Red Clump
|
The Giant Southern Stream (GSS) of M31, a keystone signature of a major
accretion event, yields crucial constraints on M31 formation and evolution
models. Currently, our understanding of the GSS, in terms of both its geometry
and its chemistry, results from either wide-field imaging probing only a few
magnitudes below the red giant branch tip, or deep imaging or spectroscopy of
isolated regions. Here, we take an alternative approach, using Hubble Space
Telescope (HST) imaging to characterize the horizontal branch red clump (RC)
using unbinned maximum likelihood fits to luminosity functions (LFs) from
observed color-magnitude diagrams (CMDs). Comparing the RC mean magnitude
across three fields at projected distances of 21, 52 and 80 kpc from M31, we
find a line of sight distance gradient identical to recent literature
measurements in fields along the core. We also find tentative evidence that the
line of sight distance dispersion increases with projected distance from M31.
Meanwhile, the metallicity in the 52 kpc field westward of the GSS core is at
least as high as that in the 21 kpc GSS core field, and the peak colors of the
RC in these two fields imply identical metallicities to within 0.2 dex. We
discuss implications for distance and metallicity gradients both along and
perpendicular to the GSS in the context of recent ground-based photometric and
spectroscopic results, including evidence for a dropoff in metallicity moving
westward from the GSS, as well as prospects for further constraining stellar
populations in the vicinity of the GSS.
|
1810.01525v1
|
2018-10-30
|
MAMMO: A Deep Learning Solution for Facilitating Radiologist-Machine Collaboration in Breast Cancer Diagnosis
|
With an aging and growing population, the number of women requiring either
screening or symptomatic mammograms is increasing. To reduce the number of
mammograms that need to be read by a radiologist while keeping the diagnostic
accuracy the same or better than current clinical practice, we develop Man and
Machine Mammography Oracle (MAMMO) - a clinical decision support system capable
of triaging mammograms into those that can be confidently classified by a
machine and those that cannot be, thus requiring the reading of a radiologist.
The first component of MAMMO is a novel multi-view convolutional neural network
(CNN) with multi-task learning (MTL). MTL enables the CNN to learn the
radiological assessments known to be associated with cancer, such as breast
density, conspicuity, suspicion, etc., in addition to learning the primary task
of cancer diagnosis. We show that MTL has two advantages: 1) learning refined
feature representations associated with cancer improves the classification
performance of the diagnosis task and 2) issuing radiological assessments
provides an additional layer of model interpretability that a radiologist can
use to debug and scrutinize the diagnoses provided by the CNN. The second
component of MAMMO is a triage network, which takes as input the radiological
assessment and diagnostic predictions of the first network's MTL outputs and
determines which mammograms can be correctly and confidently diagnosed by the
CNN and which mammograms cannot, thus needing to be read by a radiologist.
Results obtained on a private dataset of 8,162 patients show that MAMMO reduced
the number of radiologist readings by 42.8% while improving the overall
diagnostic accuracy in comparison to readings done by radiologists alone. We
analyze the triage of patients decided by MAMMO to gain a better understanding
of what unique mammogram characteristics require radiologists' expertise.
|
1811.02661v1
|
2018-11-22
|
Elemental Abundances in M31: Alpha and Iron Element Abundances from Low-Resolution Resolved Stellar Spectroscopy in the Stellar Halo
|
Measurements of [Fe/H] and [$\alpha$/Fe] can probe the minor merging history
of a galaxy, providing a direct way to test the hierarchical assembly paradigm.
While measurements of [$\alpha$/Fe] have been made in the stellar halo of the
Milky Way, little is known about detailed chemical abundances in the stellar
halo of M31. To make progress with existing telescopes, we apply spectral
synthesis to low-resolution DEIMOS spectroscopy (R $\sim$ 2500 at 7000
Angstroms) across a wide spectral range (4500 Angstroms $<$ $\lambda$ $<$ 9100
Angstroms). By applying our technique to low-resolution spectra of 170 giant
stars in 5 MW globular clusters, we demonstrate that our technique reproduces
previous measurements from higher resolution spectroscopy. Based on the
intrinsic dispersion in [Fe/H] and [$\alpha$/Fe] of individual stars in our
combined cluster sample, we estimate systematic uncertainties of $\sim$0.11 dex
and $\sim$0.09 dex in [Fe/H] and [$\alpha$/Fe], respectively. We apply our
method to deep, low-resolution spectra of 11 red giant branch stars in the
smooth halo of M31, resulting in higher signal-to-noise per spectral resolution
element compared to DEIMOS medium-resolution spectroscopy, given the same
exposure time and conditions. We find $\langle$[$\alpha$/Fe]$\rangle$ = 0.49
$\pm$ 0.29 dex and $\langle$[Fe/H]$\rangle$ = 1.59 $\pm$ 0.56 dex for our
sample. This implies that---much like the Milky Way---the smooth halo of M31 is
likely composed of disrupted dwarf galaxies with truncated star formation
histories that were accreted early in the halo's formation.
|
1811.09279v2
|
2018-12-14
|
$ε^\prime/ε$-2018: A Christmas Story
|
I was supposed to review the status of $\epsilon^\prime/\epsilon$ both at the
CKM Workshop in September in Heidelberg and recently at the Discrete 2018
Conference in Vienna. Unfortunately I had to cancel both talks for family
reasons. My main goal in these talks was to congratulate NA48 and KTeV
collaborations for the discovery of new sources of CP violation through their
heroic efforts to measure the ratio $\epsilon^\prime/\epsilon$ in the 1980s and
1990s with final results presented roughly 16 years ago. As I will not attend
any other conferences this year I will reach this goal in this writing. In this
context I will give arguments, why I am convinced about the presence of new
physics in $\epsilon^\prime/\epsilon$ on the basis of my work with Jean-Marc
Gerard within the context of the Dual QCD (DQCD) approach and why RBC-UKQCD
collaboration and in particular Chiral Perturbation Theory practitioners are
still unable to reach this conclusion. I will demonstrate that even in the
presence of pion loops, as large as advocated recently by Gilbert and Pich, the
value of $\epsilon^\prime/\epsilon$ is significantly below the data, when the
main non-factorizable QCD dynamics at long distance scales, represented in DQCD
by {the meson evolution}, is taken into account. As appriopriate for a
Christmas story, I will prophesy the final value of $\epsilon^\prime/\epsilon$
within the SM, which should include in addition to the correct matching between
long and short distance contributions, isospin breaking effects, NNLO QCD
corrections to both QCD penguin and electroweak penguin contributions and final
state interactions. Such final SM result will probably be known from lattice
QCD only in the middle of 2020s, but already in 2019 we should be able to see
some signs of NP in the next result on $\epsilon^\prime/\epsilon$ from
RBC-UKQCD.
|
1812.06102v2
|
2018-12-20
|
Single-spin qubits in isotopically enriched silicon at low magnetic field
|
Single-electron spin qubits employ magnetic fields on the order of 1 Tesla or
above to enable quantum state readout via spin-dependent-tunnelling. This
requires demanding microwave engineering for coherent spin resonance control
and significant on-chip real estate for electron reservoirs, both of which
limit the prospects for large scale multi-qubit systems. Alternatively,
singlet-triplet (ST) readout enables high-fidelity spin-state measurements in
much lower magnetic fields, without the need for reservoirs. Here, we
demonstrate low-field operation of metal-oxide-silicon (MOS) quantum dot qubits
by combining coherent single-spin control with high-fidelity, single-shot,
Pauli-spin-blockade-based ST readout. We discover that the qubits decohere
faster at low magnetic fields with $T_{2}^{Rabi}=18.6$~$\mu$s and
$T_2^*=1.4$~$\mu$s at 150~mT. Their coherence is limited by spin flips of
residual $^{29}$Si nuclei in the isotopically enriched $^{28}$Si host material,
which occur more frequently at lower fields. Our finding indicates that new
trade-offs will be required to ensure the frequency stabilization of spin
qubits and highlights the importance of isotopic enrichment of device
substrates for the realization of a scalable silicon-based quantum processor.
|
1812.08347v5
|
2019-01-17
|
Lithium-loaded Liquid Scintillator Production for the PROSPECT experiment
|
This work reports the production and characterization of lithium-loaded
liquid scintillator (LiLS) for the Precision Reactor Oscillation and Spectrum
Experiment (PROSPECT). Fifty-nine 90 liter batches of LiLS (${}^6{\rm Li}$ mass
fraction 0.082%$\pm$0.001%) were produced and samples from all batches were
characterized by measuring their optical absorbance relative to air, light
yield relative to a pure liquid scintillator reference, and pulse shape
discrimination capability. Fifty-seven batches passed the quality assurance
criteria and were used for the PROSPECT experiment.
|
1901.05569v2
|
2019-01-31
|
Spirit: Multifunctional Framework for Atomistic Spin Simulations
|
The \textit{Spirit} framework is designed for atomic scale spin simulations
of magnetic systems of arbitrary geometry and magnetic structure, providing a
graphical user interface with powerful visualizations and an easy to use
scripting interface. An extended Heisenberg type spin-lattice Hamiltonian
including competing exchange interactions between neighbors at arbitrary
distance, higher-order exchange, Dzyaloshinskii-Moriya and dipole-dipole
interactions is used to describe the energetics of a system of classical spins
localised at atom positions. A variety of common simulations methods are
implemented including Monte Carlo and various time evolution algorithms based
on the Landau-Lifshitz-Gilbert equation of motion, which can be used to
determine static ground state and metastable spin configurations, sample
equilibrium and finite temperature thermodynamical properties of magnetic
materials and nanostructures or calculate dynamical trajectories including spin
torques induced by stochastic temperature or electric current. Methods for
finding the mechanism and rate of thermally assisted transitions include the
geodesic nudged elastic band method, which can be applied when both initial and
final states are specified, and the minimum mode following method when only the
initial state is given. The lifetime of magnetic states and rate of transitions
can be evaluated within the harmonic approximation of transition-state theory.
The framework offers performant CPU and GPU parallelizations. All methods are
verified and applications to several systems, such as vortices, domain walls,
skyrmions and bobbers are described.
|
1901.11350v1
|
2019-02-24
|
Latency, Capacity, and Distributed MST
|
We study the cost of distributed MST construction in the setting where each
edge has a latency and a capacity, along with the weight. Edge latencies
capture the delay on the links of the communication network, while capacity
captures their throughput (in this case, the rate at which messages can be
sent). Depending on how the edge latencies relate to the edge weights, we
provide several tight bounds on the time and messages required to construct an
MST.
When edge weights exactly correspond with the latencies, we show that,
perhaps interestingly, the bottleneck parameter in determining the running time
of an algorithm is the total weight $W$ of the MST (rather than the total
number of nodes $n$, as in the standard CONGEST model). That is, we show a
tight bound of $\tilde{\Theta}(D + \sqrt{W/c})$ rounds, where $D$ refers to the
latency diameter of the graph, $W$ refers to the total weight of the
constructed MST and edges have capacity $c$. The proposed algorithm sends
$\tilde{O}(m+W)$ messages, where $m$, the total number of edges in the network
graph under consideration, is a known lower bound on message complexity for MST
construction. We also show that $\Omega(W)$ is a lower bound for fast MST
constructions.
When the edge latencies and the corresponding edge weights are unrelated, and
either can take arbitrary values, we show that (unlike the sub-linear time
algorithms in the standard CONGEST model, on small diameter graphs), the best
time complexity that can be achieved is $\tilde{\Theta}(D+n/c)$. However, if we
restrict all edges to have equal latency $\ell$ and capacity $c$ while having
possibly different weights (weights could deviate arbitrarily from $\ell$), we
give an algorithm that constructs an MST in $\tilde{O}(D + \sqrt{n\ell/c})$
time. In each case, we provide nearly matching upper and lower bounds.
|
1902.08979v2
|
2019-03-20
|
Qatar Exoplanet Survey: Qatar-8b, 9b and 10b --- A Hot Saturn and Two Hot Jupiters
|
In this paper we present three new extrasolar planets from the Qatar
Exoplanet Survey (QES). Qatar-8b is a hot Saturn, with Mpl = 0.37 Mjup and Rpl
= 1.3 Rjup, orbiting a solar-like star every Porb = 3.7 days. Qatar-9b is a hot
Jupiter with a mass of Mpl = 1.2 Mjup and a radius of Rpl = 1 Rjup, in a Porb =
1.5 days orbit around a low mass, Mstar = 0.7 Msun, mid-K main-sequence star.
Finally, Qatar-10b is a hot, Teq ~ 2000 K, sub-Jupiter mass planet, Mpl = 0.7
Mjup, with a radius of Rpl = 1.54 Rjup and an orbital period of Porb = 1.6
days, placing it on the edge of the sub-Jupiter desert.
|
1903.09258v4
|
2019-03-25
|
Identifying Exoplanets with Deep Learning II: Two New Super-Earths Uncovered by a Neural Network in K2 Data
|
For years, scientists have used data from NASA's Kepler Space Telescope to
look for and discover thousands of transiting exoplanets. In its extended K2
mission, Kepler observed stars in various regions of sky all across the
ecliptic plane, and therefore in different galactic environments. Astronomers
want to learn how the population of exoplanets are different in these different
environments. However, this requires an automatic and unbiased way to identify
the exoplanets in these regions and rule out false positive signals that mimic
transiting planet signals. We present a method for classifying these exoplanet
signals using deep learning, a class of machine learning algorithms that have
become popular in fields ranging from medical science to linguistics. We
modified a neural network previously used to identify exoplanets in the Kepler
field to be able to identify exoplanets in different K2 campaigns, which range
in galactic environments. We train a convolutional neural network, called
AstroNet-K2, to predict whether a given possible exoplanet signal is really
caused by an exoplanet or a false positive. AstroNet-K2 is highly successful at
classifying exoplanets and false positives, with accuracy of 98% on our test
set. It is especially efficient at identifying and culling false positives, but
for now, still needs human supervision to create a complete and reliable planet
candidate sample. We use AstroNet-K2 to identify and validate two previously
unknown exoplanets. Our method is a step towards automatically identifying new
exoplanets in K2 data and learning how exoplanet populations depend on their
galactic birthplace.
|
1903.10507v1
|
2019-04-23
|
Spin injection and pumping generated by a direct current flowing through a magnetic tunnel junction
|
A charge flow through a magnetic tunnel junction (MTJ) leads to the
generation of a spin-polarized current which exerts a spin-transfer torque
(STT) on the magnetization. When the density of applied direct current exceeds
some critical value, the STT excites high-frequency magnetization precession in
the "free" electrode of MTJ. Such precession gives rise to microwave output
voltage and, furthermore, can be employed for spin pumping into adjacent normal
metal or semiconductor. Here we describe theoretically the spin dynamics and
charge transport in the CoFeB/MgO/CoFeB/Au tunneling heterostructure connected
to a constant-current source. The magnetization dynamics in the free CoFeB
layer with weak perpendicular anisotropy is calculated by numerical integration
of the Landau-Lifshitz-Gilbert-Slonczewski equation accounting for both STT and
voltage controlled magnetic anisotropy associated with the CoFeB|MgO interface.
It is shown that a large-angle magnetization precession, resulting from
electrically induced dynamic spin reorientation transition, can be generated in
a certain range of relatively low current densities. An oscillating spin
current, which is pumped into the Au overlayer owing to such precession, is
then evaluated together with the injected spin current. Considering both the
driving spin-polarized charge current and the pumped spin current, we also
describe the charge transport in the CoFeB/Au bilayer with the account of
anomalous and inverse spin Hall effects. An electric potential difference
between the lateral sides of the CoFeB/Au bilayer is calculated as a function
of distance from the CoFeB|MgO interface. It is found that this transverse
voltage signal in Au is large enough for experimental detection, which
indicates significant efficiency of the proposed current-driven spin injector.
|
1904.10361v1
|
2019-04-26
|
TOI-216b and TOI-216c: Two warm, large exoplanets in or slightly wide of the 2:1 orbital resonance
|
Warm, large exoplanets with 10-100 day orbital periods pose a major challenge
to our understanding of how planetary systems form and evolve. Although high
eccentricity tidal migration has been invoked to explain their proximity to
their host stars, a handful reside in or near orbital resonance with nearby
planets, suggesting a gentler history of in situ formation or disk migration.
Here we confirm and characterize a pair of warm, large exoplanets discovered by
the TESS Mission orbiting K-dwarf TOI-216. Our analysis includes additional
transits and transit exclusion windows observed via ground-based follow-up. We
find two families of solutions, one corresponding to a sub-Saturn-mass planet
accompanied by a Neptune-mass planet and the other to a Jupiter in resonance
with a sub-Saturn-mass planet. We prefer the second solution based on the
orbital period ratio, the planet radii, the lower free eccentricities, and
libration of the 2:1 resonant argument, but cannot rule out the first. The free
eccentricities and mutual inclination are compatible with stirring by other,
undetected planets in the system, particularly for the second solution. We
discuss prospects for better constraints on the planets' properties and orbits
through follow-up, including transits observed from the ground.
|
1904.11852v1
|
2019-04-28
|
Periodic Bandits and Wireless Network Selection
|
Bandit-style algorithms have been studied extensively in stochastic and
adversarial settings. Such algorithms have been shown to be useful in
multiplayer settings, e.g. to solve the wireless network selection problem,
which can be formulated as an adversarial bandit problem. A leading bandit
algorithm for the adversarial setting is EXP3. However, network behavior is
often repetitive, where user density and network behavior follow regular
patterns. Bandit algorithms, like EXP3, fail to provide good guarantees for
periodic behaviors. A major reason is that these algorithms compete against
fixed-action policies, which is ineffective in a periodic setting.
In this paper, we define a periodic bandit setting, and periodic regret as a
better performance measure for this type of setting. Instead of comparing an
algorithm's performance to fixed-action policies, we aim to be competitive with
policies that play arms under some set of possible periodic patterns $F$ (for
example, all possible periodic functions with periods $1,2,\cdots,P$). We
propose Periodic EXP4, a computationally efficient variant of the EXP4
algorithm for periodic settings. With $K$ arms, $T$ time steps, and where each
periodic pattern in $F$ is of length at most $P$, we show that the periodic
regret obtained by Periodic EXP4 is at most $O\big(\sqrt{PKT \log K + KT \log
|F|}\big)$. We also prove a lower bound of $\Omega\big(\sqrt{PKT + KT
\frac{\log |F|}{\log K}} \big)$ for the periodic setting, showing that this is
optimal within log-factors. As an example, we focus on the wireless network
selection problem. Through simulation, we show that Periodic EXP4 learns the
periodic pattern over time, adapts to changes in a dynamic environment, and far
outperforms EXP3.
|
1904.12355v1
|
2019-04-30
|
An Inpainting Approach to Tackle the Kinematic and Thermal SZ Induced Biases in CMB-Cluster Lensing Estimators
|
A galaxy cluster's own Sunyaev-Zel{'}dovich (SZ) signal is known to be a
major contaminant when reconstructing the cluster's underlying lensing
potential using cosmic microwave background (CMB) temperature maps. In this
work, we develop a modified quadratic estimator (QE) that is designed to
mitigate the lensing biases due to the kinematic and thermal SZ effects. The
idea behind the approach is to use inpainting to eliminate the cluster's own
emission from the large-scale CMB gradient map. In this inpainted gradient map,
we fill the pixel values at the cluster location using a constrained Gaussian
realization based on the information from surrounding regions. We show that the
noise induced due to inpainting process is small compared to other noise
sources for upcoming surveys and has minimal impact on the final lensing
signal-to-noise. Without any foreground cleaning, we find a stacked mass
uncertainty of 6.5% for the CMB-S4 experiment on a cluster sample containing
5000 clusters with $M_{200c} = 2 \times 10^{14}\ M_{\odot}$ at z = 0.7. In
addition to the SZ-induced lensing biases, we also quantify the low mass bias
arising due to the contamination of the CMB gradient by the cluster
convergence. For the fiducial cluster sample considered in this work, we find
that bias is negligible compared to the statistical uncertainties for both the
standard and the modified QE even when modes up to $\sim 2700$ are used for the
gradient estimation. With more gradient modes, we demonstrate that the
sensitivity can be increased by 14% compared to the fiducial result above with
gradient modes up to $2000$
|
1904.13392v2
|
2019-05-31
|
Balancing spreads of influence in a social network
|
The personalization of our news consumption on social media has a tendency to
reinforce our pre-existing beliefs instead of balancing our opinions. This
finding is a concern for the health of our democracies which rely on an access
to information providing diverse viewpoints. To tackle this issue from a
computational perspective, Garimella et al. (NIPS'17) modeled the spread of
these viewpoints, also called campaigns, using the well-known independent
cascade model and studied an optimization problem that aims at balancing
information exposure in a social network when two opposing campaigns propagate
in the network. The objective in their $NP$-hard optimization problem is to
maximize the number of people that are exposed to either both or none of the
viewpoints. For two different settings, one corresponding to a model where
campaigns spread in a correlated manner, and a second one, where the two
campaigns spread in a heterogeneous manner, they provide constant ratio
approximation algorithms. In this paper, we investigate a more general
formulation of this problem. That is, we assume that $\mu$ different campaigns
propagate in a social network and we aim to maximize the number of people that
are exposed to either $\nu$ or none of the campaigns, where $\mu\ge\nu\ge2$. We
provide dedicated approximation algorithms for both the correlated and
heterogeneous settings. Interestingly, for the heterogeneous setting with
$\nu\ge 3$, we give a reduction leading to several approximation hardness
results. Maybe most importantly, we obtain that the problem cannot be
approximated within a factor of $n^{-g(n)}$ for any $g(n)=o(1)$ assuming
Gap-ETH, denoting with $n$ the number of nodes in the social network. For $\nu
\ge 4$, there is no $n^{-\epsilon}$-approximation algorithm if a certain class
of one-way functions exists, where $\epsilon > 0$ is a given constant which
depends on $\nu$.
|
1906.00074v1
|
2019-06-17
|
The Radioactive Source Calibration System of the PROSPECT Reactor Antineutrino Detector
|
The Precision Reactor Oscillation and Spectrum (PROSPECT) Experiment is a
reactor neutrino experiment designed to search for sterile neutrinos with a
mass on the order of 1 eV/c$^2$ and to measure the spectrum of electron
antineutrinos from a highly-enriched $^{235}$U nuclear reactor. The PROSPECT
detector consists of an 11 by 14 array of optical segments in $^{6}$Li-loaded
liquid scintillator at the High Flux Isotope Reactor in Oak Ridge National
Laboratory. Antineutrino events are identified via inverse beta decay and read
out by photomultiplier tubes located at the ends of each segment. The detector
response is characterized using a radioactive source calibration system. This
paper describes the design, operation, and performance of the PROSPECT source
calibration system.
|
1906.07244v2
|
2019-07-17
|
Topical Review on Skyrmions and Hall Transport
|
We review recent progresses towards an understanding of the Skyrmion Hall
transport in insulating as well as conducting materials. First, we consider a
theoretical breakthrough based on the quantum field theory Ward identity, a
first principle analysis, relying on symmetries and conservation laws. Broken
parity (inversion) symmetry plays a crucial role in Skyrmion Hall transport. In
addition to the well known thermal and electric Hall conductivities, our
analysis has led us to the discovery of a new and unforeseen physical quantity,
Hall viscosity - an anti-symmetric part of the viscosity tensor. We propose a
simple way to confirm the existence of Hall viscosity in the measurements of
Hall conductivity as a function of momentum. We provide various background
materials to assist the readers to understand the quantum field theory Ward
identity.
In the second part, we review recent theoretical and experimental
advancements of the Skyrmion Hall effects and the topological (Magnon) Hall
effects for conducting (insulting) magnets. For this purpose, we consider two
enveloping themes: spin torque and thermo-electromagnetic effect. First, we
overview various spin torques, such as spin transfer torque, spin-orbit torque,
and spin Hall torque, and generalized Landau-Lifshitz-Gilbert equations and
Thiele equations using a phenomenological approach. Second, we consider
irreversible thermodynamics to survey possible thermo-electromagnetic effects,
such as Seebeck, Peltier and Thompson effects in the presence of the electric
currents, along with the Hall effects in the presence of a background magnetic
field. Recently developed spin Seebeck effects are also a significant part of
the survey. We also accommodate extensive background materials to make this
review self-contained. Finally, we revisit the Skyrmion Hall transport from the
Ward identity view point.
|
1907.07696v1
|
2019-07-29
|
Research and Development for HI Intensity Mapping
|
Development of the hardware, data analysis, and simulation techniques for
large compact radio arrays dedicated to mapping the 21 cm line of neutral
hydrogen gas has proven to be more difficult than imagined twenty years ago
when such telescopes were first proposed. Despite tremendous technical and
methodological advances, there are several outstanding questions on how to
optimally calibrate and analyze such data. On the positive side, it has become
clear that the outstanding issues are purely technical in nature and can be
solved with sufficient development activity. Such activity will enable science
across redshifts, from early galaxy evolution in the pre-reionization era to
dark energy evolution at low redshift.
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1907.13090v1
|
2019-07-03
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Deep Learning Based Energy Disaggregation and On/Off Detection of Household Appliances
|
Energy disaggregation, a.k.a. Non-Intrusive Load Monitoring, aims to separate
the energy consumption of individual appliances from the readings of a mains
power meter measuring the total energy consumption of, e.g. a whole house.
Energy consumption of individual appliances can be useful in many applications,
e.g., providing appliance-level feedback to the end users to help them
understand their energy consumption and ultimately save energy. Recently, with
the availability of large-scale energy consumption datasets, various neural
network models such as convolutional neural networks and recurrent neural
networks have been investigated to solve the energy disaggregation problem.
Neural network models can learn complex patterns from large amounts of data and
have been shown to outperform the traditional machine learning methods such as
variants of hidden Markov models. However, current neural network methods for
energy disaggregation are either computational expensive or are not capable of
handling long-term dependencies. In this paper, we investigate the application
of the recently developed WaveNet models for the task of energy disaggregation.
Based on a real-world energy dataset collected from 20 households over two
years, we show that WaveNet models outperforms the state-of-the-art deep
learning methods proposed in the literature for energy disaggregation in terms
of both error measures and computational cost. On the basis of energy
disaggregation, we then investigate the performance of two deep-learning based
frameworks for the task of on/off detection which aims at estimating whether an
appliance is in operation or not. Based on the same dataset, we show that for
the task of on/off detection the second framework, i.e., directly training a
binary classifier, achieves better performance in terms of F1 score.
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1908.00941v2
|
2019-08-07
|
Parallel Finger Search Structures
|
In this paper we present two versions of a parallel finger structure FS on p
processors that supports searches, insertions and deletions, and has a finger
at each end. This is to our knowledge the first implementation of a parallel
search structure that is work-optimal with respect to the finger bound and yet
has very good parallelism (within a factor of O( (log p)^2 ) of optimal). We
utilize an extended implicit batching framework that transparently facilitates
the use of FS by any parallel program P that is modelled by a dynamically
generated DAG D where each node is either a unit-time instruction or a call to
FS.
The total work done by either version of FS is bounded by the finger bound
F[L] (for some linearization L of D ), i.e. each operation on an item with
finger distance r takes O( log r + 1 ) amortized work; it is cheaper for items
closer to a finger. Running P using the simpler version takes O( ( T[1] + F[L]
) / p + T[inf] + d * ( (log p)^2 + log n ) ) time on a greedy scheduler, where
T[1],T[inf] are the size and span of D respectively, and n is the maximum
number of items in FS, and d is the maximum number of calls to FS along any
path in D. Using the faster version, this is reduced to O( ( T[1] + F[L] ) / p
+ T[inf] + d * (log p)^2 + s[L] ) time, where s[L] is the weighted span of D
where each call to FS is weighted by its cost according to F[L]. We also sketch
how to extend FS to support a fixed number of movable fingers.
The data structures in our paper fit into the dynamic multithreading
paradigm, and their performance bounds are directly composable with other data
structures given in the same paradigm. Also, the results can be translated to
practical implementations using work-stealing schedulers.
|
1908.02741v4
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2019-08-12
|
Percolation phase transitions for the SIR model with random powers
|
This thesis considers three models which describe a multihop ad-hoc
telecommunication system. These systems consist of users sending messages,
which can jump to other users to reach the target user. The first two models
have already been examined extensively, whereas it is the first time the third
model is studied. In all these models our goal is to understand under which
conditions users situated far away from each other can communicate. This is the
fundamental question of continuum percolation, which was introduced by Gilbert
(1961), who established the Boolean model for Poisson point processes. In the
first part of this thesis we introduce this model and prove the existence of a
phase transition between a subcritical phase, where no infinite connected
component of users exists, and a supercritical phase, where such a component
exists. We also consider the case of random connection radii, following Meester
and Roy (1996). In the second part we study the SINR model for Cox point
processes in two or higher dimensions, following T\'obi\'as (2019). He proved
that there exists a phase transition under certain stabilization and
connectedness conditions on the intensity measure. The SINR model for
homogeneous Poisson point processes in $\mathbb R^2$ was introduced by Dousse
et al. (2005). In the third part we study percolation in the SINR model with
random powers for Poisson point processes in $\mathbb R^d$, $d \geq 2$. This
model was studied by Kong and Yeh (2007) for $d=2$ under strong boundedness
conditions on the random power. In this thesis we weaken these conditions and
only assume that the random power is nonnegative, integrable and not a.s. zero.
We show that there exists a subcritical phase if the path-loss function decays
fast enough. Further, we prove that a supercritical phase exists if random
power has a large enough essential supremum and some exponential moments.
|
1908.07375v1
|
2019-08-30
|
Elemental Abundances in M31: A Comparative Analysis of Iron and Alpha Element Abundances in the Outer Disk, Giant Stellar Stream, and Inner Halo of M31
|
We measured [Fe/H] and [$\alpha$/Fe] using spectral synthesis of
low-resolution stellar spectroscopy for 70 individual red giant branch stars
across four fields spanning the outer disk, Giant Stellar Stream (GSS), and
inner halo of M31. Fields at M31-centric projected distances of 23 kpc in the
halo, 12 kpc in the halo, 22 kpc in the GSS, and 26 kpc in the outer disk are
$\alpha$-enhanced, with $\langle$[$\alpha$/Fe]$\rangle$ = 0.43, 0.50, 0.41, and
0.58, respectively. The 23 kpc and 12 kpc halo fields are relatively
metal-poor, with $\langle$[Fe/H]$\rangle$ = $-$1.54 and $-$1.30, whereas the 22
kpc GSS and 26 kpc outer disk fields are relatively metal-rich with
$\langle$[Fe/H]$\rangle$ = $-$0.84 and $-$0.92, respectively. For fields with
substructure, we separated the stellar populations into kinematically hot
stellar halo components and kinematically cold components. We did not find any
evidence of an [$\alpha$/Fe] gradient along the high surface brightness core of
the GSS between $\sim$17$-$22 kpc. However, we found tentative suggestions of a
negative [$\alpha$/Fe] gradient in the stellar halo, which may indicate that
different progenitor(s) or formation mechanisms contributed to the build up of
the inner versus outer halo. Additionally, the [$\alpha$/Fe] distribution of
the metal-rich ([Fe/H] $>$ $-$1.5), smooth inner stellar halo (r$_{\rm{proj}}$
$\lesssim$ 26 kpc) is inconsistent with having formed from the disruption of
progenitor(s) similar to present-day M31 satellite galaxies. The 26 kpc outer
disk is most likely associated with the extended disk of M31, where its high
$\alpha$-enhancement provides support for an episode of rapid star formation in
M31's disk, possibly induced by a major merger.
|
1909.00006v2
|
2019-09-25
|
Deep Learning for RF Signal Classification in Unknown and Dynamic Spectrum Environments
|
Dynamic spectrum access (DSA) benefits from detection and classification of
interference sources including in-network users, out-network users, and jammers
that may all coexist in a wireless network. We present a deep learning based
signal (modulation) classification solution in a realistic wireless network
setting, where 1) signal types may change over time; 2) some signal types may
be unknown for which there is no training data; 3) signals may be spoofed such
as the smart jammers replaying other signal types; and 4) different signal
types may be superimposed due to the interference from concurrent
transmissions. For case 1, we apply continual learning and train a
Convolutional Neural Network (CNN) using an Elastic Weight Consolidation (EWC)
based loss. For case 2, we detect unknown signals via outlier detection applied
to the outputs of convolutional layers using Minimum Covariance Determinant
(MCD) and k-means clustering methods. For case 3, we extend the CNN structure
to capture phase shifts due to radio hardware effects to identify the spoofing
signal sources. For case 4, we apply blind source separation using Independent
Component Analysis (ICA) to separate interfering signals. We utilize the signal
classification results in a distributed scheduling protocol, where in-network
(secondary) users employ signal classification scores to make channel access
decisions and share the spectrum with each other while avoiding interference
with out-network (primary) users and jammers. Compared with benchmark
TDMA-based schemes, we show that distributed scheduling constructed upon signal
classification results provides major improvements to in-network user
throughput and out-network user success ratio.
|
1909.11800v1
|
2019-09-30
|
First-principles theory of proximity spin-orbit torque on a two-dimensional magnet: Current-driven antiferromagnet-to-ferromagnet reversible transition in bilayer CrI$_3$
|
The recently discovered two-dimensional (2D) magnetic insulator CrI$_3$ is an
intriguing case for basic research and spintronic applications since it is a
ferromagnet in the bulk, but an antiferromagnet in bilayer form, with its
magnetic ordering amenable to external manipulations. Using first-principles
quantum transport approach, we predict that injecting unpolarized charge
current parallel to the interface of bilayer-CrI$_3$/monolayer-TaSe$_2$ van der
Waals heterostructure will induce spin-orbit torque (SOT) and thereby driven
dynamics of magnetization on the first monolayer of CrI$_3$ in direct contact
with TaSe$_2$. By combining calculated complex angular dependence of SOT with
the Landau-Lifshitz-Gilbert equation for classical dynamics of magnetization,
we demonstrate that current pulses can switch the direction of magnetization on
the first monolayer to become parallel to that of the second monolayer, thereby
converting CrI$_3$ from antiferromagnet to ferromagnet while not requiring any
external magnetic field. We explain the mechanism of this reversible
current-driven nonequilibrium phase transition by showing that first monolayer
of CrI$_3$ carries current due to evanescent wavefunctions injected by metallic
transition metal dichalcogenide TaSe$_2$, while concurrently acquiring strong
spin-orbit coupling (SOC) via such proximity effect, whereas the second
monolayer of CrI$_3$ remains insulating. The transition can be detected by
passing vertical read current through the vdW heterostructure, encapsulated by
bilayer of hexagonal boron nitride and sandwiched between graphite electrodes,
where we find tunneling magnetoresistance of $\simeq 240$%.
|
1909.13876v2
|
2019-10-14
|
An Extreme-mass Ratio, Short-period Eclipsing Binary Consisting of a B Dwarf Primary and a Pre-main Sequence M Star Companion Discovered by KELT
|
We present the discovery of \thisstar\ (HD 58730), a very low mass ratio ($q
\equiv M_2/M_1 \approx 0.07$) eclipsing binary (EB) identified by the
Kilodegree Extremely Little Telescope (KELT) survey. We present the discovery
light curve and perform a global analysis of four high-precision ground-based
light curves, the Transiting Exoplanets Survey Satellite (TESS) light curve,
radial velocity (RV) measurements, Doppler Tomography (DT) measurements, and
the broad-band spectral energy distribution (SED). Results from the global
analysis are consistent with a fully convective ($M_2 = 0.22 \pm 0.02\
M_{\odot})$ M star transiting a late-B primary ($M_1 = 3.34^{+0.07}_{-0.09}\
M_{\odot};\ T_{\rm eff,1} = 11960^{+430}_{-520}\ {\rm K}$). We infer that the
primary star is $183_{-30}^{+33}$ Myr old and that the companion star's radius
is inflated by $26 \pm 8\%$ relative to the predicted value from a low-mass
isochrone of similar age. We separately and analytically fit for the
variability in the out-of-eclipse TESS phase curve, finding good agreement
between the resulting stellar parameters and those from the global fit. Such
systems are valuable for testing theories of binary star formation and
understanding how the environment of a star in a close-but-detached binary
affects its physical properties. In particular, we examine how a star's
properties in such a binary might differ from the properties it would have in
isolation.
|
1910.06212v2
|
2019-10-28
|
A detailed study of Andromeda XIX, an extreme local analogue of ultra diffuse galaxies
|
With a central surface brightness of $\mu_0=29.3$ mag. per sq. arcsec, and
half-light radius of $r_{\rm half}=3.1^{+0.9}_{-1.1}$~kpc, Andromeda XIX (And
XIX) is an extremely diffuse satellite of Andromeda. We present spectra for
$\sim100$ red giant branch stars in this galaxy, plus 16 stars in a nearby
stellar stream. With this exquisite dataset, we re-derive the properties of And
XIX, measuring a systemic velocity of $v_r=-109.0\pm1.6$ km/s and a velocity
dispersion of $\sigma_v = 7.8^{+1.7}_{-1.5}$ km/s (higher than derived in our
previous work). We marginally detect a velocity gradient along the major axis
of ${\rm d}v/{\rm d}r = -2.1\pm1.8$ km/s kpc$^{-1}$. We find its mass-to-light
ratio is higher than galaxies of comparable stellar mass ($[M/L]_{\rm half} =
278^{+146}_{-198}M_\odot/L_\odot$), but its dynamics place it in a halo with a
similar total mass to these galaxies. This could suggest that And XIX is a
"puffed up" dwarf galaxy, whose properties have been altered by tidal
processes, similar to its Milky Way counterpart, Antlia II. For the nearby
stream, we measure $v_r=-279.2\pm3.7$ km/s, and $\sigma_v=13.8^{+3.5}_{-2.6}$
km/s. We measure its metallicity, and find it to be more metal rich than And
XIX, implying that the two features are unrelated. Finally, And XIX's dynamical
and structural properties imply it is a local analogue to ultra diffuse
galaxies (UDGs). Its complex dynamics suggest that the masses of distant UDGs
measured from velocity dispersions alone should be carefully interpreted
|
1910.12879v2
|
2019-10-28
|
Added Value of Intraoperative Data for Predicting Postoperative Complications: Development and Validation of a MySurgeryRisk Extension
|
To test the hypothesis that accuracy, discrimination, and precision in
predicting postoperative complications improve when using both preoperative and
intraoperative data input features versus preoperative data alone. Models that
predict postoperative complications often ignore important intraoperative
physiological changes. Incorporation of intraoperative physiological data may
improve model performance. This retrospective cohort analysis included 52,529
inpatient surgeries at a single institution during a 5 year period. Random
forest machine learning models in the validated MySurgeryRisk platform made
patient-level predictions for three postoperative complications and mortality
during hospital admission using electronic health record data and patient
neighborhood characteristics. For each outcome, one model trained with
preoperative data alone and one model trained with both preoperative and
intraoperative data. Models were compared by accuracy, discrimination
(expressed as AUROC), precision (expressed as AUPRC), and reclassification
indices (NRI). Machine learning models incorporating both preoperative and
intraoperative data had greater accuracy, discrimination, and precision than
models using preoperative data alone for predicting all three postoperative
complications (intensive care unit length of stay >48 hours, mechanical
ventilation >48 hours, and neurological complications including delirium) and
in-hospital mortality (accuracy: 88% vs. 77%, AUROC: 0.93 vs. 0.87, AUPRC: 0.21
vs. 0.15). Overall reclassification improvement was 2.9-10.0% for complications
and 11.2% for in-hospital mortality. Incorporating both preoperative and
intraoperative data significantly increased accuracy, discrimination, and
precision for machine learning models predicting postoperative complications.
|
1910.12895v2
|
2019-12-04
|
Elemental Abundances in M31: The Kinematics and Chemical Evolution of Dwarf Spheroidal Satellite Galaxies
|
We present deep spectroscopy from Keck/DEIMOS of Andromeda I, III, V, VII,
and X, all of which are dwarf spheroidal satellites of M31. The sample includes
256 spectroscopic members across all five dSphs. We confirm previous
measurements of the velocity dispersions and dynamical masses, and we provide
upper limits on bulk rotation. Our measurements confirm that M31 satellites
obey the same relation between stellar mass and stellar metallicity as Milky
Way (MW) satellites and other dwarf galaxies in the Local Group. The
metallicity distributions show similar trends with stellar mass as MW
satellites, including evidence in massive satellites for external influence,
like pre-enrichment or gas accretion. We present the first measurements of
individual element ratios, like [Si/Fe], in the M31 system, as well as
measurements of the average [alpha/Fe] ratio. The trends of [alpha/Fe] with
[Fe/H] also follow the same galaxy mass-dependent patterns as MW satellites.
Less massive galaxies have more steeply declining slopes of [alpha/Fe] that
begin at lower [Fe/H]. Finally, we compare the chemical evolution of M31
satellites to M31's Giant Stellar Stream and smooth halo. The properties of the
M31 system support the theoretical prediction that the inner halo is composed
primarily of massive galaxies that were accreted early. As a result, the inner
halo exhibits higher [Fe/H] and [alpha/Fe] than surviving satellite galaxies.
|
1912.02186v1
|
2020-01-03
|
The First Habitable Zone Earth-sized Planet from TESS. III: Climate States and Characterization Prospects for TOI-700 d
|
We present self-consistent three-dimensional climate simulations of possible
habitable states for the newly discovered Habitable Zone Earth-sized planet,
TOI-700 d. We explore a variety of atmospheric compositions, pressures, and
rotation states for both ocean-covered and completely desiccated planets in
order to assess the planet's potential for habitability. For all 20 of our
simulated cases, we use our climate model outputs to synthesize transmission
spectra, combined-light spectra, and integrated broadband phase curves. These
climatologically-informed observables will help the community assess the
technological capabilities necessary for future characterization of this planet
- as well as similar transiting planets discovered in the future - and will
provide a guide for distinguishing possible climate states if one day we do
obtain sensitive spectral observations of a habitable planet around a M-star.
We find that TOI-700 d is a strong candidate for a habitable world and can
potentially maintain temperate surface conditions under a wide variety of
atmospheric compositions. Unfortunately, the spectral feature depths from the
resulting transmission spectra and the peak flux and variations from our
synthesized phase curves for TOI-700 d do not exceed 10 ppm. This will likely
prohibit the James Webb Space Telescope (JWST) from characterizing its
atmosphere; however, this motivates the community to invest in future
instrumentation that perhaps can one day reveal the true nature of TOI-700 d,
and to continue to search for similar planets around less distant stars.
|
2001.00955v2
|
2020-01-07
|
X-ray Observations of the Peculiar Cepheid V473 Lyr Identify A Low-Mass Companion
|
V473 Lyr is a classical Cepheid which is unique in having substantial
amplitude variations with a period of approximately 3.3 years, thought to be
similar to the Blazhko variations in RR Lyrae stars. We obtained an {\it
XMM-Newton} observation of this star to followup a previous detection in
X-rays. Rather than the X-ray burst and rapid decline near maximum radius seen
in $\delta$ Cephei itself, the X-ray flux in V473 Lyr remained constant for a
third of the pulsation cycle covered by the observation. Thus the X-rays are
most probably not produced by the changes around the pulsation cycle. The X-ray
spectrum is soft (kT = 0.6 keV), with
X-ray properties which are consistent with a young low mass companion.
Previously there was no evidence of a companion in radial velocities or in {\it
Gaia} and {\it Hipparcos} proper motions. While this rules out companions which
are very close or very distant, a binary companion at a separation between 30
and 300 AU is possible. This is an example of an X-ray observation revealing
evidence of a low mass companion, which is important in completing the mass
ratio statistics of binary Cepheids. Furthermore, the detection of a young
X-ray bright companion is a further indication that the Cepheid (primary) is a
Population I star, even though its pulsation behavior differs from other
classical Cepheids.
|
2001.02253v1
|
2020-02-11
|
Spintronics meets density matrix renormalization group: Quantum spin torque driven nonclassical magnetization reversal and dynamical buildup of long-range entanglement
|
We introduce time-dependent density matrix renormalization group (tDMRG) as a
solution to long standing problem in spintronics -- how to describe
spin-transfer torque (STT) between flowing spins of conduction electrons and
localized spins within a magnetic material by treating the dynamics of both
spin species fully quantum-mechanically. In contrast to conventional
Slonczewski-Berger STT, where the localized spins are viewed as classical
vectors obeying the Landau-Lifshitz-Gilbert equation and where their STT-driven
dynamics is initiated only when the spin-polarization of flowing electrons and
localized spins are noncollinear, quantum STT can occur when these vectors are
collinear but antiparallel. Using tDMRG, we simulate the time evolution of a
many-body quantum state of electrons and localized spins, where the former are
injected as a spin-polarized current pulse while the latter comprise a quantum
Heisenberg ferromagnetic metallic (FM) spin-$\frac{1}{2}$ XXZ chain initially
in the ground state with spin-polarization antiparallel to that of injected
electrons. The quantum STT reverses the direction of localized spins, but
without rotation from the initial orientation, when the number of injected
electrons exceeds the number of localized spins. Such nonclassical reversal,
which is absent from LLG dynamics, is strikingly inhomogeneous across the FM
chain and it can be accompanied by reduction of the magnetization associated
with localized spins, even to zero at specific locations. This is because
quantum STT generates a highly entangled nonequilibrium many-body state of all
flowing and localized spins, despite starting from the initially unentangled
ground state of a mundane FM. Furthermore, the mutual information between
localized spins at the FM edges remains nonzero even at infinite separation as
the signature of dynamical buildup of long-range entanglement.
|
2002.04655v4
|
2020-02-27
|
Les Houches 2019 Physics at TeV Colliders: New Physics Working Group Report
|
This report presents the activities of the `New Physics' working group for
the `Physics at TeV Colliders' workshop (Les Houches, France, 10--28 June,
2019). These activities include studies of direct searches for new physics,
approaches to exploit published data to constrain new physics, as well as the
development of tools to further facilitate these investigations. Benefits of
machine learning for both the search for new physics and the interpretation of
these searches are also presented.
|
2002.12220v1
|
2020-03-19
|
Realizing an Isotropically Coercive Magnetic Layer for Memristive Applications by Analogy to Dry Friction
|
We investigate the possibility of realizing a spintronic memristive device
based on the dependence of the tunnel conductance on the relative angle between
the magnetization of the two magnetic electrodes in in-plane magnetized tunnel
junctions. For this, it is necessary to design a free layer whose magnetization
can be stabilized along several or even any in-plane direction between the
parallel and the antiparallel magnetic configurations. We experimentally show
that this can be achieved by exploiting antiferromagnet-ferromagnet exchange
interactions in a regime where the antiferromagnet is thin enough to induce
enhanced coercivity and no exchange bias. The frustration of exchange
interactions at the interfaces due to competing ferro- and antiferromagnetic
interactions is at the origin of an isotropic dissipation mechanism yielding
isotropic coercivity. From a modeling point of view, it is shown that this
isotropic dissipation can be described by a dry friction term in the
Landau-Lifshitz-Gilbert equation. The influence of this dry friction term on
the magnetization dynamics of an in-plane magnetized layer submitted to a
rotating in-plane field is investigated both analytically and numerically. The
possibility to control the free layer magnetization orientation in an in-plane
magnetized magnetic tunnel junction by using the spin transfer torque from an
additional perpendicular polarizer is also investigated through macrospin
simulation. It is shown that the memristor function can be achieved by the
injection of current pulses through the stack in the presence of an in-plane
static field transverse to the reference layer magnetization, the aim of which
is to limit the magnetization rotation between 0{\deg} and 180{\deg}.
|
2003.08850v1
|
2020-04-07
|
Elemental abundances in M31: [Fe/H] and [α/Fe] in M31 Dwarf Galaxies Using Coadded Spectra
|
We present chemical abundances of red giant branch (RGB) stars in the dwarf
spheroidal (dSph) satellite system of Andromeda (M31), using spectral synthesis
of medium resolution (R $\sim 6000$) spectra obtained with the Keck II
telescope and DEIMOS spectrograph via the Spectroscopic and Photometric
Landscape of Andromeda's Stellar Halo (SPLASH) survey. We coadd stars according
to their similarity in photometric metallicity or effective temperature to
obtain a signal-to-noise ratio (S/N) high enough to measure average [Fe/H] and
[$\alpha$/Fe] abundances. We validate our method using high S/N spectra of RGB
stars in Milky Way globular clusters as well as deep observations for a subset
of the M31 dSphs in our sample. For this set of validation coadds, we compare
the weighted average abundance of the individual stars with the abundance
determined from the coadd. We present individual and coadded measurements of
[Fe/H] and [$\alpha$/Fe] for stars in ten M31 dSphs, including the first
[$\alpha$/Fe] measurements for And IX, XIV, XV, and XVIII. These fainter, less
massive dSphs show declining [$\alpha$/Fe] relative to [Fe/H], implying an
extended star formation history. In addition, these dSphs also follow the same
mass-metallicity relation found in other Local Group satellites. The
conclusions we infer from coadded spectra agree with those from previous
measurements in brighter M31 dSphs with individual abundance measurements, as
well as conclusions from photometric studies. These abundances greatly increase
the number of spectroscopic measurements of the chemical composition of M31's
less massive dwarf satellites, which are crucial to understanding their star
formation history and interaction with the M31 system.
|
2004.03425v1
|
2020-04-15
|
Toward Trustworthy AI Development: Mechanisms for Supporting Verifiable Claims
|
With the recent wave of progress in artificial intelligence (AI) has come a
growing awareness of the large-scale impacts of AI systems, and recognition
that existing regulations and norms in industry and academia are insufficient
to ensure responsible AI development. In order for AI developers to earn trust
from system users, customers, civil society, governments, and other
stakeholders that they are building AI responsibly, they will need to make
verifiable claims to which they can be held accountable. Those outside of a
given organization also need effective means of scrutinizing such claims. This
report suggests various steps that different stakeholders can take to improve
the verifiability of claims made about AI systems and their associated
development processes, with a focus on providing evidence about the safety,
security, fairness, and privacy protection of AI systems. We analyze ten
mechanisms for this purpose--spanning institutions, software, and hardware--and
make recommendations aimed at implementing, exploring, or improving those
mechanisms.
|
2004.07213v2
|
2020-04-22
|
Impurity-dependent gyrotropic motion, deflection and pinning of current-driven ultrasmall skyrmions in PdFe/Ir(111) surface
|
Resting on multi-scale modelling simulations, we explore dynamical aspects
characterizing skyrmions driven by spin-transfer-torque towards repulsive and
pinning 3d and 4d single atomic defects embedded in a Pd layer deposited on the
Fe/Ir(111) surface. The latter is known to host sub-10 nm skyrmions which are
of great interest in information technology. The Landau-Lifshitz-Gilbert
equation is parametrized with magnetic exchange interactions extracted from
first-principles. Depending on the nature of the defect and the magnitude of
the applied magnetic field, the skyrmion deforms by either shrinking or
increasing in size, experiencing thereby elliptical distortions. After applying
a magnetic field of 10 Tesla, ultrasmall skyrmions are driven along a straight
line towards the various defects which permits a simple analysis of the impact
of the impurities. Independently from the nature of the skyrmion-defect complex
interaction a gyrotropic motion is observed. A repulsive force leads to a
skyrmion trajectory similar to the one induced by an attractive one. We unveil
that the circular motion is clockwise around pinning impurities but counter
clockwise around the repulsive ones, which can be used to identify the
interaction nature of the defects by observing the skyrmions trajectories.
Moreover, and as expected, the skyrmion always escapes the repulsive defects in
contrast to the pinning defects, which require a minimal depinning current to
observe impurity avoidance. This unveils the richness of the motion regimes of
skyrmions. We discuss the results of the simulations in terms of the Thiele
equation, which provides a reasonable qualitative description of the observed
phenomena. Finally, we show an example of a double track made of pinning
impurities, where the engineering of their mutual distance allows to control
the skyrmion motion with enhanced velocity.
|
2004.10509v1
|
2020-04-23
|
Spectroscopic Orbits of Eleven Nearby, Mid-to-Late M Dwarf Binaries
|
We present the spectroscopic orbits of eleven nearby, mid-to-late M dwarf
binary systems in a variety of configurations: two single-lined binaries
(SB1s), seven double-lined binaries (SB2s), one double-lined triple (ST2), and
one triple-lined triple (ST3). Eight of these orbits are the first published
for these systems, while five are newly identified multiples. We obtained
multi-epoch, high-resolution spectra with the TRES instrument on the 1.5m
Tillinghast Reflector at the Fred Lawrence Whipple Observatory located on Mt.
Hopkins in AZ. Using the TiO molecular bands at 7065 -- 7165 Angstroms, we
calculated radial velocities for these systems, from which we derived their
orbits. We find LHS 1817 to have in a 7-hour period a companion that is likely
a white dwarf, due to the ellipsoidal modulation we see in our MEarth-North
light curve data. We find G 123-45 and LTT 11586 to host companions with
minimum masses of 41 M_Jup and 44 M_Jup with orbital periods of 35 and 15 days,
respectively. We find 2MA 0930+0227 to have a rapidly rotating stellar
companion in a 917-day orbital period. GJ 268, GJ 1029, LP 734-34, GJ 1182, G
258-17, and LTT 7077 are SB2s with stellar companions with orbital periods of
10, 96, 34, 154, 5, and 84 days; LP 655-43 is an ST3 with one companion in an
18-day orbital period and an outer component in a longer undetermined period.
In addition, we present radial velocities for both components of L 870-44AB and
for the outer components of LTT 11586 and LP 655-43.
|
2004.11225v2
|
2020-05-25
|
Recovering the city street lighting fraction from skyglow measurements in a large-scale municipal dimming experiment
|
Anthropogenic skyglow dominates views of the natural night sky in most urban
settings, and the associated emission of artificial light at night (ALAN) into
the environment of cities involves a number of known and suspected negative
externalities. One approach to lowering consumption of ALAN in cities is
dimming or extinguishing publicly owned outdoor lighting during overnight
hours; however, there are few reports in the literature about the efficacy of
these programs. Here we report the results of one of the largest municipal
lighting dimming experiments to date, involving $\sim$20,000 roadway luminaires
owned and operated by the City of Tucson, Arizona, U.S. We analyzed both
single-channel and spatially resolved ground-based measurements of broadband
night sky radiance obtained during the tests, determining that the zenith sky
brightness during the tests decreased by ($-5.4\pm0.9$)% near the city center
and ($-3.6\pm0.9$)% at an adjacent suburban location on nights when the output
of the street lighting system was dimmed from 90% of its full power draw to 30%
after local midnight. Modeling these changes with a radiative transfer code
yields results suggesting that street lights account for about ($14\pm1$)% of
light emissions resulting in skyglow seen over the city. A separate derivation
from first principles implies that street lighting contributes only 2-3% of
light seen at the zenith over Tucson. We discuss this inconsistency and suggest
routes for future work.
|
2005.12357v1
|
2020-05-31
|
Ontology-based systematic classification and analysis of coronaviruses, hosts, and host-coronavirus interactions towards deep understanding of COVID-19
|
Given the existing COVID-19 pandemic worldwide, it is critical to
systematically study the interactions between hosts and coronaviruses including
SARS-Cov, MERS-Cov, and SARS-CoV-2 (cause of COVID-19). We first created four
host-pathogen interaction (HPI)-Outcome postulates, and generated a HPI-Outcome
model as the basis for understanding host-coronavirus interactions (HCI) and
their relations with the disease outcomes. We hypothesized that ontology can be
used as an integrative platform to classify and analyze HCI and disease
outcomes. Accordingly, we annotated and categorized different coronaviruses,
hosts, and phenotypes using ontologies and identified their relations. Various
COVID-19 phenotypes are hypothesized to be caused by the backend HCI
mechanisms. To further identify the causal HCI-outcome relations, we collected
35 experimentally-verified HCI protein-protein interactions (PPIs), and applied
literature mining to identify additional host PPIs in response to coronavirus
infections. The results were formulated in a logical ontology representation
for integrative HCI-outcome understanding. Using known PPIs as baits, we also
developed and applied a domain-inferred prediction method to predict new PPIs
and identified their pathological targets on multiple organs. Overall, our
proposed ontology-based integrative framework combined with computational
predictions can be used to support fundamental understanding of the intricate
interactions between human patients and coronaviruses (including SARS-CoV-2)
and their association with various disease outcomes.
|
2006.00639v1
|
2020-06-23
|
Simultaneous compression and opacity data from time-series radiography with a Lagrangian marker
|
Time-resolved radiography can be used to obtain absolute shock Hugoniot
states by simultaneously measuring at least two mechanical parameters of the
shock, and this technique is particularly suitable for one-dimensional
converging shocks where a single experiment probes a range of pressures as the
converging shock strengthens. However, at sufficiently high pressures, the
shocked material becomes hot enough that the x-ray opacity falls significantly.
If the system includes a Lagrangian marker, such that the mass within the
marker is known, this additional information can be used to constrain the
opacity as well as the Hugoniot state. In the limit that the opacity changes
only on shock heating, and not significantly on subsequent isentropic
compression, the opacity of shocked material can be determined uniquely. More
generally, it is necessary to assume the form of the variation of opacity with
isentropic compression, or to introduce multiple marker layers. Alternatively,
assuming either the equation of state or the opacity, the presence of a marker
layer in such experiments enables the non-assumed property to be deduced more
accurately than from the radiographic density reconstruction alone. An example
analysis is shown for measurements of a converging shock wave in polystyrene,
at the National Ignition Facility.
|
2006.13342v3
|
2020-06-30
|
EndoSLAM Dataset and An Unsupervised Monocular Visual Odometry and Depth Estimation Approach for Endoscopic Videos: Endo-SfMLearner
|
Deep learning techniques hold promise to develop dense topography
reconstruction and pose estimation methods for endoscopic videos. However,
currently available datasets do not support effective quantitative
benchmarking. In this paper, we introduce a comprehensive endoscopic SLAM
dataset consisting of 3D point cloud data for six porcine organs, capsule and
standard endoscopy recordings as well as synthetically generated data. A Panda
robotic arm, two commercially available capsule endoscopes, two conventional
endoscopes with different camera properties, and two high precision 3D scanners
were employed to collect data from 8 ex-vivo porcine gastrointestinal
(GI)-tract organs. In total, 35 sub-datasets are provided with 6D pose ground
truth for the ex-vivo part: 18 sub-dataset for colon, 12 sub-datasets for
stomach and 5 sub-datasets for small intestine, while four of these contain
polyp-mimicking elevations carried out by an expert gastroenterologist.
Synthetic capsule endoscopy frames from GI-tract with both depth and pose
annotations are included to facilitate the study of simulation-to-real transfer
learning algorithms. Additionally, we propound Endo-SfMLearner, an unsupervised
monocular depth and pose estimation method that combines residual networks with
spatial attention module in order to dictate the network to focus on
distinguishable and highly textured tissue regions. The proposed approach makes
use of a brightness-aware photometric loss to improve the robustness under fast
frame-to-frame illumination changes. To exemplify the use-case of the EndoSLAM
dataset, the performance of Endo-SfMLearner is extensively compared with the
state-of-the-art. The codes and the link for the dataset are publicly available
at https://github.com/CapsuleEndoscope/EndoSLAM. A video demonstrating the
experimental setup and procedure is accessible through
https://www.youtube.com/watch?v=G_LCe0aWWdQ.
|
2006.16670v3
|
2020-08-06
|
Magnon versus electron mediated spin-transfer torque exerted by spin currents across antiferromagnetic insulator to switch magnetization of adjacent ferromagnetic metal
|
The recent experiment [Y. Wang et al., Science 366, 1125 (2019)] on
magnon-mediated spin-transfer torque (MSTT) was interpreted in terms of a
picture where magnons are excited within an antiferromagnetic insulator (AFI),
by applying nonequilibrium electronic spin density at one of its surfaces, so
that their propagation across AFI deprived of conduction electrons eventually
leads to reversal of magnetization of a ferromagnetic metal (FM) attached to
the opposite surface of AFI. We employ a recently developed time-dependent
nonequilibrium Green functions combined with the Landau-Lifshitz-Gilbert
equation (TDNEGF+LLG) formalism to evolve conduction electrons
quantum-mechanically while they interact via self-consistent back-action with
localized magnetic moments described classically by atomistic spin dynamics
solving a system of LLG equations. Upon injection of square current pulse as
the initial condition, TDNEGF+LLG simulations of FM-polarizer/AFI/FM-analyzer
junctions show that reversal of localized magnetic moments within FM-analyzer
is less efficient, in the sense of requiring larger pulse height and its longer
duration, than conventional electron-mediated STT (ESTT) driving magnetization
switching in standard FM-polarizer/normal-metal/FM-analyzer spin valve. Since
both electronic, generated by spin pumping from AFI, and magnonic, generated by
direct transmission from AFI, spin currents are injected into the FM-analyzer,
its localized magnetic moments will experience combined MSTT and ESTT.
Nevertheless, by artificially turning off ESTT we demonstrate that MSTT plays a
dominant role whose understanding, therefore, paves the way for
all-magnon-driven magnetization switching devices with no electronic parts.
|
2008.02794v3
|
2020-08-29
|
VR-Caps: A Virtual Environment for Capsule Endoscopy
|
Current capsule endoscopes and next-generation robotic capsules for diagnosis
and treatment of gastrointestinal diseases are complex cyber-physical platforms
that must orchestrate complex software and hardware functions. The desired
tasks for these systems include visual localization, depth estimation, 3D
mapping, disease detection and segmentation, automated navigation, active
control, path realization and optional therapeutic modules such as targeted
drug delivery and biopsy sampling. Data-driven algorithms promise to enable
many advanced functionalities for capsule endoscopes, but real-world data is
challenging to obtain. Physically-realistic simulations providing synthetic
data have emerged as a solution to the development of data-driven algorithms.
In this work, we present a comprehensive simulation platform for capsule
endoscopy operations and introduce VR-Caps, a virtual active capsule
environment that simulates a range of normal and abnormal tissue conditions
(e.g., inflated, dry, wet etc.) and varied organ types, capsule endoscope
designs (e.g., mono, stereo, dual and 360{\deg}camera), and the type, number,
strength, and placement of internal and external magnetic sources that enable
active locomotion. VR-Caps makes it possible to both independently or jointly
develop, optimize, and test medical imaging and analysis software for the
current and next-generation endoscopic capsule systems. To validate this
approach, we train state-of-the-art deep neural networks to accomplish various
medical image analysis tasks using simulated data from VR-Caps and evaluate the
performance of these models on real medical data. Results demonstrate the
usefulness and effectiveness of the proposed virtual platform in developing
algorithms that quantify fractional coverage, camera trajectory, 3D map
reconstruction, and disease classification.
|
2008.12949v2
|
2020-09-01
|
Elemental Abundances in M31: Properties of the Inner Stellar Halo
|
We present measurements of [Fe/H] and [$\alpha$/Fe] for 128 individual red
giant branch stars (RGB) in the stellar halo of M31, including its Giant
Stellar Stream (GSS), obtained using spectral synthesis of low- and
medium-resolution Keck/DEIMOS spectroscopy ($R \sim 3000$ and 6000,
respectively). We observed four fields in M31's stellar halo (at projected
radii of 9, 18, 23, and 31 kpc), as well as two fields in the GSS (at 33 kpc).
In combination with existing literature measurements, we have increased the
sample size of [Fe/H] and [$\alpha$/Fe] measurements from 101 to a total of 229
individual M31 RGB stars. From this sample, we investigate the chemical
abundance properties of M31's inner halo, finding $\langle$[Fe/H]$\rangle$ =
$-$1.08 $\pm$ 0.04 and $\langle$[$\alpha$/Fe]$\rangle$ = 0.40 $\pm$ 0.03.
Between 8--34 kpc, the inner halo has a steep [Fe/H] gradient ($-$0.025 $\pm$
0.002 dex kpc$^{-1}$) and negligible [$\alpha$/Fe] gradient, where substructure
in the inner halo is systematically more metal-rich than the smooth component
of the halo at a given projected distance. Although the chemical abundances of
the inner stellar halo are largely inconsistent with that of present-day dwarf
spheroidal (dSph) satellite galaxies of M31, we identified 22 RGB stars
kinematically associated with the smooth component of the stellar halo that
have chemical abundance patterns similar to M31 dSphs. We discuss formation
scenarios for M31's halo, concluding that these dSph-like stars may have been
accreted from galaxies of similar stellar mass and star formation history, or
of higher stellar mass and similar star formation efficiency.
|
2009.00529v1
|
2020-09-02
|
The Solar Orbiter mission -- Science overview
|
Solar Orbiter, the first mission of ESA's Cosmic Vision 2015-2025 programme
and a mission of international collaboration between ESA and NASA, will explore
the Sun and heliosphere from close up and out of the ecliptic plane. It was
launched on 10 February 2020 04:03 UTC from Cape Canaveral and aims to address
key questions of solar and heliospheric physics pertaining to how the Sun
creates and controls the Heliosphere, and why solar activity changes with time.
To answer these, the mission carries six remote-sensing instruments to observe
the Sun and the solar corona, and four in-situ instruments to measure the solar
wind, energetic particles, and electromagnetic fields. In this paper, we
describe the science objectives of the mission, and how these will be addressed
by the joint observations of the instruments onboard. The paper first
summarises the mission-level science objectives, followed by an overview of the
spacecraft and payload. We report the observables and performance figures of
each instrument, as well as the trajectory design. This is followed by a
summary of the science operations concept. The paper concludes with a more
detailed description of the science objectives. Solar Orbiter will combine
in-situ measurements in the heliosphere with high-resolution remote-sensing
observations of the Sun to address fundamental questions of solar and
heliospheric physics. The performance of the Solar Orbiter payload meets the
requirements derived from the mission's science objectives. Its science return
will be augmented further by coordinated observations with other space missions
and ground-based observatories.
|
2009.00861v1
|
2020-09-03
|
Mapping the Escape Fraction of Ionizing Photons Using Resolved Stars: A Much Higher Escape Fraction for NGC 4214
|
We demonstrate a new method for measuring the escape fraction of ionizing
photons using Hubble Space Telescope imaging of resolved stars in NGC 4214, a
local analog of high-redshift starburst galaxies that are thought to be
responsible for cosmic reionization. Specifically, we forward model the UV
through near-IR spectral energy distributions of $\sim$83,000 resolved stars to
infer their individual ionizing flux outputs. We constrain the local escape
fraction by comparing the number of ionizing photons produced by stars to the
number that are either absorbed by dust or consumed by ionizing the surrounding
neutral hydrogen in individual star-forming regions. We find substantial
spatial variation in the escape fraction (0-40%). Integrating over the entire
galaxy yields a global escape fraction of 25% (+16%/-15%). This value is much
higher than previous escape fractions of zero reported for this galaxy. We
discuss sources of this apparent tension, and demonstrate that the viewing
angle and the 3D ISM geometric effects are the cause. If we assume the NGC 4214
has no internal dust, like many high-redshift galaxies, we find an escape
fraction of 59% (an upper-limit for NGC 4214). This is the first non-zero
escape fraction measurement for UV-faint (M$_{\rm FUV}$ = -15.9) galaxies at
any redshift, and supports the idea that starburst UV-faint dwarf galaxies can
provide a sufficient amount of ionizing photons to the intergalactic medium.
|
2009.01844v1
|
2020-09-16
|
Reflection-mode virtual histology using photoacoustic remote sensing microscopy
|
Histological visualizations are critical to clinical disease management and
are fundamental to biological understanding. However, current approaches that
rely on bright-field microscopy require extensive tissue preparation prior to
imaging. These processes are labor intensive and contribute to delays in
clinical feedback that can extend to two to three weeks for standard
paraffin-embedded tissue preparation and interpretation. Here, we present a
label-free reflection-mode imaging modality that reveals cellular-scale
morphology by detecting intrinsic endogenous contrast. We accomplish this with
the novel photoacoustic remote sensing (PARS) detection system that permits
non-contact optical absorption contrast to be extracted from thick and opaque
biological targets with optical resolution. PARS was examined both as a rapid
assessment tool that is capable of managing large samples (>1 cm2) in under 10
minutes, and as a high contrast imaging modality capable of extracting specific
biological contrast to simulate conventional histological stains such as
hematoxylin and eosin (H&E). The capabilities of the proposed method are
demonstrated in a variety of human tissue preparations including formalin-fixed
paraffin-embedded tissue blocks and unstained slides sectioned from these
blocks, including normal and neoplastic human brain, and breast epithelium
involved with breast cancer. Similarly, PARS images of human skin prepared by
frozen section clearly demonstrated basal cell carcinoma and normal human skin
tissue. Finally, we imaged unprocessed murine kidney and achieved
histologically relevant subcellular morphology in fresh tissue. This represents
a vital step towards an effective real-time clinical microscope that overcomes
the limitations of standard histopathologic tissue preparations and enables
real-time pathology assessment.
|
2009.10010v1
|
2020-09-24
|
Quantum spin torque driven transmutation of antiferromagnetic Mott insulator
|
The standard model of spin-transfer torque (STT) in antiferromagnetic
spintronics considers exchange of angular momentum between quantum spins of
flowing electrons and noncollinear-to-them localized spins treated as classical
vectors. These vectors are assumed to realize N\'{e}el order in equilibrium,
$\uparrow \downarrow \ldots \uparrow \downarrow$, and their STT-driven dynamics
is described by the Landau-Lifshitz-Gilbert (LLG) equation. However, many
experimentally employed materials (such as archetypal NiO) are strongly
electron-correlated antiferromagnetic Mott insulators (AFMI) where localized
spins form a ground state quite different from the unentangled N\'{e}el state
$|\!\! \uparrow \downarrow \ldots \uparrow \downarrow \rangle$. The true ground
state is entangled by quantum spin fluctuations, leading to expectation value
of all localized spins being zero, so that LLG dynamics of classical vectors of
fixed length rotating due to STT cannot even be initiated. Instead, a fully
quantum treatment of both conduction electrons and localized spins is necessary
to capture exchange of spin angular momentum between them, denoted as quantum
STT. We use a recently developed time-dependent density matrix renormalization
group approach to quantum STT to predict how injection of a spin-polarized
current pulse into a normal metal layer coupled to AFMI overlayer via exchange
interaction and possibly small interlayer hopping -- which mimics, e.g.,
topological-insulator/NiO bilayer employed experimentally -- will induce
nonzero expectation value of AFMI localized spins. This new nonequilibrium
phase is a spatially inhomogeneous ferromagnet with zigzag profile of localized
spins. The total spin absorbed by AFMI increases with electron-electron
repulsion in AFMI, as well as when the two layers do not exchange any charge.
|
2009.11833v3
|
2020-09-29
|
The Terahertz Intensity Mapper (TIM): a Next-Generation Experiment for Galaxy Evolution Studies
|
Understanding the formation and evolution of galaxies over cosmic time is one
of the foremost goals of astrophysics and cosmology today. The cosmic star
formation rate has undergone a dramatic evolution over the course of the last
14 billion years, and dust obscured star forming galaxies (DSFGs) are a crucial
component of this evolution. A variety of important, bright, and unextincted
diagnostic lines are present in the far-infrared (FIR) which can provide
crucial insight into the physical conditions of galaxy evolution, including the
instantaneous star formation rate, the effect of AGN feedback on star
formation, the mass function of the stars, metallicities, and the spectrum of
their ionizing radiation. FIR spectroscopy is technically difficult but
scientifically crucial. Stratospheric balloons offer a platform which can
outperform current instrument sensitivities and are the only way to provide
large-area, wide bandwidth spatial/spectral mapping at FIR wavelengths. NASA
recently selected TIM, the Terahertz Intensity Mapper, with the goal of
demonstrating the key technical milestones necessary for FIR spectroscopy. The
TIM instrument consists of an integral-field spectrometer from 240-420 microns
with 3600 kinetic-inductance detectors (KIDs) coupled to a 2-meter
low-emissivity carbon fiber telescope. In this paper, we will summarize plans
for the TIM experiment's development, test and deployment for a planned flight
from Antarctica.
|
2009.14340v1
|
2020-10-07
|
Fairness in Influence Maximization through Randomization
|
The influence maximization paradigm has been used by researchers in various
fields in order to study how information spreads in social networks. While
previously the attention was mostly on efficiency, more recently fairness
issues have been taken into account in this scope. In this paper, we propose to
use randomization as a mean for achieving fairness. Similar to previous works
like Fish et al. (WWW '19) and Tsang et al. (IJCAI '19), we study the maximin
criterion for (group) fairness. In contrast to their work however, we model the
problem in such a way that, when choosing the seed sets, probabilistic
strategies are possible rather than only deterministic ones. We introduce two
different variants of this probabilistic problem, one that entails
probabilistic strategies over nodes (node-based problem) and a second one that
entails probabilistic strategies over sets of nodes (set-based problem). While
the original deterministic problem involving the maximin criterion has been
shown to be inapproximable, interestingly, we show that both probabilistic
variants permit approximation algorithms that achieve a constant multiplicative
factor of 1-1/e plus an additive arbitrarily small error that is due to the
simulation of the information spread. For an experimental study, we provide
implementations of multiplicative-weight routines for both problems and compare
the achieved fairness values to existing methods. Maybe non-surprisingly, we
show that the ex-ante values of the computed probabilistic strategies are
significantly larger than the (ex-post) fairness values of previous methods.
This indicates that studying fairness via randomization is a worthwhile path to
follow. Interestingly and maybe more surprisingly, we observe that even the
ex-post fairness values computed by our routines, dominate over the fairness
achieved by previous methods on most of the instances tested.
|
2010.03438v4
|
2020-10-10
|
Multi-path Neural Networks for On-device Multi-domain Visual Classification
|
Learning multiple domains/tasks with a single model is important for
improving data efficiency and lowering inference cost for numerous vision
tasks, especially on resource-constrained mobile devices. However,
hand-crafting a multi-domain/task model can be both tedious and challenging.
This paper proposes a novel approach to automatically learn a multi-path
network for multi-domain visual classification on mobile devices. The proposed
multi-path network is learned from neural architecture search by applying one
reinforcement learning controller for each domain to select the best path in
the super-network created from a MobileNetV3-like search space. An adaptive
balanced domain prioritization algorithm is proposed to balance optimizing the
joint model on multiple domains simultaneously. The determined multi-path model
selectively shares parameters across domains in shared nodes while keeping
domain-specific parameters within non-shared nodes in individual domain paths.
This approach effectively reduces the total number of parameters and FLOPS,
encouraging positive knowledge transfer while mitigating negative interference
across domains. Extensive evaluations on the Visual Decathlon dataset
demonstrate that the proposed multi-path model achieves state-of-the-art
performance in terms of accuracy, model size, and FLOPS against other
approaches using MobileNetV3-like architectures. Furthermore, the proposed
method improves average accuracy over learning single-domain models
individually, and reduces the total number of parameters and FLOPS by 78% and
32% respectively, compared to the approach that simply bundles single-domain
models for multi-domain learning.
|
2010.04904v2
|
2020-10-29
|
Group-Harmonic and Group-Closeness Maximization -- Approximation and Engineering
|
Centrality measures characterize important nodes in networks. Efficiently
computing such nodes has received a lot of attention. When considering the
generalization of computing central groups of nodes, challenging optimization
problems occur. In this work, we study two such problems, group-harmonic
maximization and group-closeness maximization both from a theoretical and from
an algorithm engineering perspective.
On the theoretical side, we obtain the following results. For group-harmonic
maximization, unless $P=NP$, there is no polynomial-time algorithm that
achieves an approximation factor better than $1-1/e$ (directed) and $1-1/(4e)$
(undirected), even for unweighted graphs. On the positive side, we show that a
greedy algorithm achieves an approximation factor of $\lambda(1-2/e)$
(directed) and $\lambda(1-1/e)/2$ (undirected), where $\lambda$ is the ratio of
minimal and maximal edge weights. For group-closeness maximization, the
undirected case is $NP$-hard to be approximated to within a factor better than
$1-1/(e+1)$ and a constant approximation factor is achieved by a local-search
algorithm. For the directed case, however, we show that, for any
$\epsilon<1/2$, the problem is $NP$-hard to be approximated within a factor of
$4|V|^{-\epsilon}$.
From the algorithm engineering perspective, we provide efficient
implementations of the above greedy and local search algorithms. In our
experimental study we show that, on small instances where an optimum solution
can be computed in reasonable time, the quality of both the greedy and the
local search algorithms come very close to the optimum. On larger instances,
our local search algorithms yield results with superior quality compared to
existing greedy and local search solutions, at the cost of additional running
time. We thus advocate local search for scenarios where solution quality is of
highest concern.
|
2010.15435v1
|
2020-11-19
|
A novel in vitro device to deliver induced electromagnetic fields to cell and tissue cultures
|
We have developed a novel in vitro instrument that can deliver intermediate
frequency (100 - 400 kHz), moderate intensity (up to and exceeding 6.5 V/cm
pk-pk) electric fields (EFs) to cell and tissue cultures generated using
induced electromagnetic fields (EMFs) in a solenoid coil. A major application
of these EFs is as an emerging cancer treatment modality. In vitro studies by
Novocure Ltd. reported that intermediate frequency (100 - 300 kHz), low
amplitude (1 - 3 V/cm) EFs, which they called "Tumor Treating Fields"
(TTFields), had an anti-mitotic effect on glioblastoma multiforme (GBM) cells.
The effect was found to increase with increasing EF amplitude. Despite
continued theoretical, preclinical, and clinical study, the mechanism of action
remains incompletely understood. Previous in vitro studies of "TTFields" have
used attached, capacitively coupled electrodes to deliver alternating EFs to
cell and tissue cultures. This contacting delivery method suffers from a poorly
characterized EF profile and conductive heating that limits the duration and
amplitude of the applied EFs. In contrast, our device delivers EFs with a
well-characterized radial profile in a non-contacting manner, eliminating
conductive heating and enabling thermally regulated EF delivery. To test and
demonstrate our system, we generated continuous 200 kHz EMF with an EF
amplitude profile spanning 0 - 6.5 V/cm pk-pk and applied them to human thyroid
cell cultures for 72 hours. We observed moderate reduction in cell density (<
10%) at low EF amplitudes (< 4 V/cm) and a greater reduction in cell density of
up to 25% at higher amplitudes (4 - 6.5 V/cm). Our device can be extended to
other EF frequency and amplitude regimes. Future studies with this device
should contribute to the ongoing debate about the efficacy and mechanism(s) of
action of "TTFields" by better isolating the effects of EFs.
|
2011.09698v1
|
2020-11-23
|
Revealing defect-induced spin disorder in nanocrystalline Ni
|
We combine magnetometry and magnetic small-angle neutron scattering to study
the influence of the microstructure on the macroscopic magnetic properties of a
nanocrystalline Ni bulk sample, which was prepared by straining via
high-pressure torsion. As seen by magnetometry, the mechanical deformation
leads to a significant increase of the coercivity compared to nondeformed
polycrystalline Ni. The neutron data reveal a significant spin-misalignment
scattering caused by the high density of crystal defects inside the sample,
which were created by the severe plastic deformation during the sample
preparation. The corresponding magnetic correlation length, which characterizes
the spatial magnetization fluctuations in real space, indicates an average
defect size of 11 nm, which is smaller than the average crystallite size of 60
nm. In the remanent state, the strain fields around the defects cause spin
disorder in the surrounding ferromagnetic bulk, with a penetration depth of
around 22 nm. The range and amplitude of the disorder is systematically
suppressed by an increasing external magnetic field. Our findings are supported
and illustrated by micromagnetic simulations, which, for the particular case of
nonmagnetic defects (holes) embedded in a ferromagnetic Ni phase, further
highlight the role of localized spin perturbations for the magnetic
microstructure of defect-rich magnets such as high-pressure torsion materials.
|
2011.11318v4
|
2021-01-05
|
The Panchromatic Hubble Andromeda Treasury: Triangulum Extended Region (PHATTER) I. Ultraviolet to Infrared Photometry of 22 Million Stars in M33
|
We present panchromatic resolved stellar photometry for 22 million stars in
the Local Group dwarf spiral Triangulum (M33), derived from Hubble Space
Telescope (HST) observations with the Advanced Camera for Surveys (ACS) in the
optical (F475W, F814W), and the Wide Field Camera 3 (WFC3) in the near
ultraviolet (F275W, F336W) and near-infrared (F110W, F160W) bands. The large,
contiguous survey area covers $\sim$14 square kpc and extends to 3.5 kpc (14
arcmin, or 1.5-2 scale lengths) from the center of M33. The PHATTER observing
strategy and photometry technique closely mimic those of the Panchromatic
Hubble Andromeda Treasury (PHAT), but with updated photometry techniques that
take full advantage of all overlapping pointings (aligned to within $<$5-10
milliarcseconds) and improved treatment of spatially-varying point spread
functions. The photometry reaches a completeness-limited depth of
F475W$\sim$28.5 in the lowest surface density regions observed in M33 and
F475W$\sim$26.5 in the most crowded regions found near the center of M33. We
find the young populations trace several relatively tight arms, while the old
populations show a clear, looser two-armed structure. We present extensive
analysis of the data quality including artificial star tests to quantify
completeness, photometric uncertainties, and flux biases. This stellar catalog
is the largest ever produced for M33, and is publicly available for download by
the community.
|
2101.01293v1
|
2021-01-20
|
Beyond Fine-tuning: Classifying High Resolution Mammograms using Function-Preserving Transformations
|
The task of classifying mammograms is very challenging because the lesion is
usually small in the high resolution image. The current state-of-the-art
approaches for medical image classification rely on using the de-facto method
for ConvNets - fine-tuning. However, there are fundamental differences between
natural images and medical images, which based on existing evidence from the
literature, limits the overall performance gain when designed with algorithmic
approaches. In this paper, we propose to go beyond fine-tuning by introducing a
novel framework called MorphHR, in which we highlight a new transfer learning
scheme. The idea behind the proposed framework is to integrate
function-preserving transformations, for any continuous non-linear activation
neurons, to internally regularise the network for improving mammograms
classification. The proposed solution offers two major advantages over the
existing techniques. Firstly and unlike fine-tuning, the proposed approach
allows for modifying not only the last few layers but also several of the first
ones on a deep ConvNet. By doing this, we can design the network front to be
suitable for learning domain specific features. Secondly, the proposed scheme
is scalable to hardware. Therefore, one can fit high resolution images on
standard GPU memory. We show that by using high resolution images, one prevents
losing relevant information. We demonstrate, through numerical and visual
experiments, that the proposed approach yields to a significant improvement in
the classification performance over state-of-the-art techniques, and is indeed
on a par with radiology experts. Moreover and for generalisation purposes, we
show the effectiveness of the proposed learning scheme on another large
dataset, the ChestX-ray14, surpassing current state-of-the-art techniques.
|
2101.07945v1
|
2021-01-21
|
A decade of radial-velocity monitoring of Vega and new limits on the presence of planets
|
We present an analysis of 1524 spectra of Vega spanning 10 years, in which we
search for periodic radial velocity variations. A signal with a periodicity of
0.676 days and a semi-amplitude of ~10 m/s is consistent with the rotation
period measured over much shorter time spans by previous spectroscopic and
spectropolarimetric studies, confirming the presence of surface features on
this A0 star. The timescale of evolution of these features can provide insight
into the mechanism that sustains the weak magnetic fields in normal A type
stars. Modeling the radial velocities with a Gaussian process using a
quasi-periodic kernel suggests that the characteristic spot evolution timescale
is ~180 days, though we cannot exclude the possibility that it is much longer.
Such long timescales may indicate the presence of failed fossil magnetic fields
on Vega. TESS data reveal Vega's photometric rotational modulation for the
first time, with a total amplitude of only 10 ppm, and a comparison of the
spectroscopic and photometric amplitudes suggest the surface features may be
dominated by bright plages rather than dark spots. For the shortest orbital
periods, transit and radial velocity injection recovery tests exclude the
presence of transiting planets larger than 2 Earth radii and most
non-transiting giant planets. At long periods, we combine our radial velocities
with direct imaging from the literature to produce detection limits for Vegan
planets and brown dwarfs out to distances of 15 au. Finally, we detect a
candidate radial velocity signal with a period of 2.43 days and a
semi-amplitude of 6 m/s. If caused by an orbiting companion, its minimum mass
would be ~20 Earth masses; because of Vega's pole-on orientation, this would
correspond to a Jovian planet if the orbit is aligned with the stellar spin. We
discuss the prospects for confirmation of this candidate planet.
|
2101.08801v1
|
2021-02-04
|
Magnon-spinon dichotomy in the Kitaev hyperhoneycomb $β$-Li$_2$IrO$_3$
|
The family of edge-sharing tri-coordinated iridates and ruthenates has
emerged in recent years as a major platform for Kitaev spin liquid physics,
where spins fractionalize into emergent magnetic fluxes and Majorana fermions
with Dirac-like dispersions. While such exotic states are usually pre-empted by
long-range magnetic order at low temperatures, signatures of Majorana fermions
with long coherent times have been predicted to manifest at intermediate and
higher energy scales, similar to the observation of spinons in quasi-1D spin
chains. Here we present a Resonant Inelastic X-ray Scattering study of the
magnetic excitations of the hyperhoneycomb iridate $\beta$-Li$_2$IrO$_3$ under
a magnetic field with a record-high-resolution spectrometer. At
low-temperatures, dispersing spin waves can be resolved around the predicted
intertwined incommensurate spiral and field-induced zigzag orders, whose
excitation energy reaches a maximum of 16meV. A 2T magnetic field softens the
dispersion around ${\bf Q}=0$. The behavior of the spin waves under magnetic
field is consistent with our semiclassical calculations for the ground state
and the dynamical spin structure factor, which further predicts that the ensued
intertwined uniform states remain robust up to very high fields (100 T). Most
saliently, the low-energy magnon-like mode is superimposed by a broad continuum
of excitations, centered around 35meV and extending up to 100meV. This
high-energy continuum survives up to at least 300K -- well above the ordering
temperature of 38K -- and gives evidence for pairs of long-lived Majorana
fermions of the proximate Kitaev spin liquid.
|
2102.02714v2
|
2008-04-04
|
Inhomogeneous Gilbert damping from impurities and electron-electron interactions
|
We present a unified theory of magnetic damping in itinerant electron
ferromagnets at order $q^2$ including electron-electron interactions and
disorder scattering. We show that the Gilbert damping coefficient can be
expressed in terms of the spin conductivity, leading to a Matthiessen-type
formula in which disorder and interaction contributions are additive. In a weak
ferromagnet regime, electron-electron interactions lead to a strong enhancement
of the Gilbert damping.
|
0804.0820v2
|
2006-12-01
|
Gilbert damping and spin Coulomb drag in a magnetized electron liquid with spin-orbit interaction
|
We present a microscopic calculation of the Gilbert damping constant for the
magnetization of a two-dimensional spin-polarized electron liquid in the
presence of intrinsic spin-orbit interaction. First we show that the Gilbert
constant can be expressed in terms of the auto-correlation function of the
spin-orbit induced torque. Then we specialize to the case of the Rashba
spin-orbit interaction and we show that the Gilbert constant in this model is
related to the spin-channel conductivity. This allows us to study the Gilbert
damping constant in different physical regimes, characterized by different
orderings of the relevant energy scales -- spin-orbit coupling, Zeeman
coupling, momentum relaxation rate, spin-momentum relaxation rate, spin
precession frequency -- and to discuss its behavior in various limits.
Particular attention is paid to electron-electron interaction effects,which
enter the spin conductivity and hence the Gilbert damping constant via the spin
Coulomb drag coefficient.
|
0612015v1
|
2011-09-22
|
Hole spin relaxation and coefficients in Landau-Lifshitz-Gilbert equation in ferromagnetic GaMnAs
|
We investigate the temperature dependence of the coefficients in the
Landau-Lifshitz-Gilbert equation in ferromagnetic GaMnAs by employing the Zener
model. We first calculate the hole spin relaxation time based on the
microscopic kinetic equation. We find that the hole spin relaxation time is
typically several tens femtoseconds and can present a nonmonotonic temperature
dependence due to the variation of the interband spin mixing, influenced by the
temperature related Zeeman splitting. With the hole spin relaxation time, we
are able to calculate the coefficients in the Landau-Lifshitz-Gilbert equation,
such as the Gilbert damping, nonadiabatic spin torque, spin stiffness and
vertical spin stiffness coefficients. We find that the nonadiabatic spin torque
coefficient $\beta$ is around $0.1\sim 0.3$ at low temperature, which is
consistent with the experiment [Adam {\em et al.}, Phys. Rev. B {\bf 80},
193204 (2009)]. As the temperature increases, $\beta$ monotonically increases
and can exceed one in the vicinity of the Curie temperature. In the low
temperature regime with $\beta<1$, the Gilbert damping coefficient $\alpha$
increases with temperature, showing good agreement with the experiments [Sinova
{\em et al.}, Phys. Rev. B {\bf 69}, 085209 (2004); Khazen {\em et al.}, {\em
ibid.} {\bf 78}, 195210 (2008)]. Furthermore, we predict that $\alpha$
decreases with increasing temperature once $\beta>1$ near the Curie
temperature. We also find that the spin stiffness decreases with increasing
temperature, especially near the Curie temperature due to the modification of
the finite $\beta$. Similar to the Gilbert damping, the vertical spin stiffness
coefficient is also found to be nonmonotonically dependent on the temperature.
|
1109.4964v1
|
2002-02-11
|
Radiation Induced Landau-Lifshitz-Gilbert Damping in Ferromagnets
|
The Landau-Lifshitz-Gilbert damping coefficient employed in the analysis of
spin wave ferromagnetic resonance is related to the electrical conductivity of
the sample. The changing magnetization (with time) radiates electromagnetic
fields. The electromagnetic energy is then absorbed by the sample and the
resulting heating effect describes magnetic dissipative damping. The
ferromagnetic resonance relaxation rate theoretically depends on the geometry
(shape and size) of the sample as well as temperature in agreement with
experiment.
|
0202181v1
|
2008-08-09
|
Gilbert Damping in Conducting Ferromagnets I: Kohn-Sham Theory and Atomic-Scale Inhomogeneity
|
We derive an approximate expression for the Gilbert damping coefficient
\alpha_G of itinerant electron ferromagnets which is based on their description
in terms of spin-density-functional-theory (SDFT) and Kohn-Sham quasiparticle
orbitals. We argue for an expression in which the coupling of magnetization
fluctuations to particle-hole transitions is weighted by the spin-dependent
part of the theory's exchange-correlation potential, a quantity which has large
spatial variations on an atomic length scale. Our SDFT result for \alpha_G is
closely related to the previously proposed spin-torque correlation-function
expression.
|
0808.1373v1
|
2003-09-29
|
Damping rates of the atomic velocity in Sisyphus cooling
|
We present a theoretical and experimental study of the damping process of the
atomic velocity in Sisyphus cooling. The relaxation rates of the atomic kinetic
temperature are determined for a 3D lin$\perp$lin optical lattice. We find that
the damping rates of the atomic temperature depend linearly on the optical
pumping rate, for a given depth of the potential wells. This is at variance
with the behavior of the friction coefficient as calculated from the spatial
diffusion coefficients within a model of Brownian motion. The origin of this
different behavior is identified by distinguishing the role of the trapped and
traveling atoms.
|
0309209v1
|
2012-11-15
|
Spin transport and tunable Gilbert damping in a single-molecule magnet junction
|
We study time-dependent electronic and spin transport through an electronic
level connected to two leads and coupled with a single-molecule magnet via
exchange interaction. The molecular spin is treated as a classical variable and
precesses around an external magnetic field. We derive expressions for charge
and spin currents by means of the Keldysh non-equilibrium Green's functions
technique in linear order with respect to the time-dependent magnetic field
created by this precession. The coupling between the electronic spins and the
magnetization dynamics of the molecule creates inelastic tunneling processes
which contribute to the spin currents. The inelastic spin currents, in turn,
generate a spin-transfer torque acting on the molecular spin. This back-action
includes a contribution to the Gilbert damping and a modification of the
precession frequency. The Gilbert damping coefficient can be controlled by the
bias and gate voltages or via the external magnetic field and has a
non-monotonic dependence on the tunneling rates.
|
1211.3611v2
|
2021-05-08
|
A second-order numerical method for Landau-Lifshitz-Gilbert equation with large damping parameters
|
A second order accurate numerical scheme is proposed and implemented for the
Landau-Lifshitz-Gilbert equation, which models magnetization dynamics in
ferromagnetic materials, with large damping parameters. The main advantages of
this method are associated with the following features: (1) It only solves
linear systems of equations with constant coefficients where fast solvers are
available, so that the numerical efficiency has been greatly improved, in
comparison with the existing Gauss-Seidel project method. (2) The second-order
accuracy in time is achieved, and it is unconditionally stable for large
damping parameters. Moreover, both the second-order accuracy and the great
efficiency improvement will be verified by several numerical examples in the 1D
and 3D simulations. In the presence of large damping parameters, it is observed
that this method is unconditionally stable and finds physically reasonable
structures while many existing methods have failed. For the domain wall
dynamics, the linear dependence of wall velocity with respect to the damping
parameter and the external magnetic field will be obtained through the reported
simulations.
|
2105.03576v1
|
2017-09-29
|
Non-local Gilbert damping tensor within the torque-torque correlation model
|
An essential property of magnetic devices is the relaxation rate in magnetic
switching which depends strongly on the damping in the magnetisation dynamics.
It was recently measured that damping depends on the magnetic texture and,
consequently, is a non-local quantity. The damping enters the
Landau-Lifshitz-Gilbert equation as the phenomenological Gilbert damping
parameter $\alpha$, that does not, in a straight forward formulation, account
for non-locality. Efforts were spent recently to obtain Gilbert damping from
first principles for magnons of wave vector $\mathbf{q}$. However, to the best
of our knowledge, there is no report about real space non-local Gilbert damping
$\alpha_{ij}$. Here, a torque-torque correlation model based on a tight binding
approach is applied to the bulk elemental itinerant magnets and it predicts
significant off-site Gilbert damping contributions, that could be also
negative. Supported by atomistic magnetisation dynamics simulations we reveal
the importance of the non-local Gilbert damping in atomistic magnetisation
dynamics. This study gives a deeper understanding of the dynamics of the
magnetic moments and dissipation processes in real magnetic materials. Ways of
manipulating non-local damping are explored, either by temperature, material's
doping or strain.
|
1709.10365v1
|
2008-07-18
|
Current-induced dynamics of spiral magnet
|
We study the dynamics of the spiral magnet under the charge current by
solving the Landau-Lifshitz-Gilbert equation numerically. In the steady state,
the current ${\vec j}$ induces (i) the parallel shift of the spiral pattern
with velocity $v=(\beta/\alpha)j$ ($\alpha$, $\beta$: the Gilbert damping
coefficients), (ii) the uniform magnetization $M$ parallel or anti-parallel to
the current depending on the chirality of the spiral and the ratio $\beta /
\alpha $, and (iii) the change in the wavenumber $k$ of the spiral. These are
analyzed by the continuum effective theory using the scaling argument, and the
various nonequilibrium phenomena such as the chaotic behavior and
current-induced annealing are also discussed.
|
0807.2901v1
|
2010-03-19
|
Dynamics of magnetization on the topological surface
|
We investigate theoretically the dynamics of magnetization coupled to the
surface Dirac fermions of a three dimensional topological insulator, by
deriving the Landau-Lifshitz-Gilbert (LLG) equation in the presence of charge
current. Both the inverse spin-Galvanic effect and the Gilbert damping
coefficient $\alpha$ are related to the two-dimensional diagonal conductivity
$\sigma_{xx}$ of the Dirac fermion, while the Berry phase of the ferromagnetic
moment to the Hall conductivity $\sigma_{xy}$. The spin transfer torque and the
so-called $\beta$-terms are shown to be negligibly small. Anomalous behaviors
in various phenomena including the ferromagnetic resonance are predicted in
terms of this LLG equation.
|
1003.3769v1
|
2016-02-23
|
Experimental Investigation of Temperature-Dependent Gilbert Damping in Permalloy Thin Films
|
The Gilbert damping of ferromagnetic materials is arguably the most important
but least understood phenomenological parameter that dictates real-time
magnetization dynamics. Understanding the physical origin of the Gilbert
damping is highly relevant to developing future fast switching spintronics
devices such as magnetic sensors and magnetic random access memory. Here, we
report an experimental study of temperature-dependent Gilbert damping in
permalloy (Py) thin films of varying thicknesses by ferromagnetic resonance.
From the thickness dependence, two independent contributions to the Gilbert
damping are identified, namely bulk damping and surface damping. Of particular
interest, bulk damping decreases monotonically as the temperature decreases,
while surface damping shows an enhancement peak at the temperature of ~50 K.
These results provide an important insight to the physical origin of the
Gilbert damping in ultrathin magnetic films.
|
1602.07325v1
|
2019-02-22
|
Strongly Enhanced Gilbert Damping in 3d Transition Metal Ferromagnet Monolayers in Contact with Topological Insulator Bi2Se3
|
Engineering Gilbert damping of ferromagnetic metal films is of great
importance to exploit and design spintronic devices that are operated with an
ultrahigh speed. Based on scattering theory of Gilbert damping, we extend the
torque method originally used in studies of magnetocrystalline anisotropy to
theoretically determine Gilbert dampings of ferromagnetic metals. This method
is utilized to investigate Gilbert dampings of 3d transition metal ferromagnet
iron, cobalt and nickel monolayers that are contacted by the prototypical
topological insulator Bi2Se3. Amazingly, we find that their Gilbert dampings
are strongly enhanced by about one order in magnitude, compared with dampings
of their bulks and free-standing monolayers, owing to the strong spin-orbit
coupling of Bi2Se3. Our work provides an attractive route to tailoring Gilbert
damping of ferromagnetic metallic films by putting them in contact with
topological insulators.
|
1902.08700v1
|
2018-08-20
|
Gilbert damping of [Co/Pd]n/Py multilayer thin films
|
Understanding the Gilbert damping in exchange-coupled multilayer materials is
particularly important to develop future fast switching spintronics devices.
Here, we report an experimental investigation of temperature-dependent Gilbert
damping in [Co/Pd]n/Py multilayer films of varying the number of Co/Pd
repetitions by ferromagnetic resonance. The results demonstrate that three
independent contributions to the Gilbert damping are identified, namely the
intrinsic Gilbert damping, the inhomogeneous linewidth broadening and the
two-magnon scattering contribution. Of particular interest, the two-magnon
scattering intensity increases as the enlargement of number repetitions of
Co/Pd due to the larger pinning effect at the interface between Py and the
Co/Pd layers. The Gilbert damping increases monotonically as the temperature
decreases from 300K to 50K. Our findings open the door to comprehend the
physical origin of the Gilbert damping in ultrathin exchange-coupled multilayer
films.
|
1808.06515v2
|
2006-10-10
|
Spin-transfer in an open ferromagnetic layer: from negative damping to effective temperature
|
Spin-transfer is a typical spintronics effect that allows a ferromagnetic
layer to be switched by spin-injection. Most of the experimental results about
spin transfer are described on the basis of the Landau-Lifshitz-Gilbert
equation of the magnetization, in which additional current-dependent damping
factors are added, and can be positive or negative. The origin of the damping
can be investigated further by performing stochastic experiments, like one shot
relaxation experiments under spin-injection in the activation regime of the
magnetization. In this regime, the N\'eel-Brown activation law is observed
which leads to the introduction of a current-dependent effective temperature.
In order to justify the introduction of these counterintuitive parameters
(effective temperature and negative damping), a detailed thermokinetic analysis
of the different sub-systems involved is performed. We propose a thermokinetic
description of the different forms of energy exchanged between the electric and
the ferromagnetic sub-systems at a Normal/Ferromagnetic junction. The
corresponding Fokker Planck equations, including relaxations, are derived. The
damping coefficients are studied in terms of Onsager-Casimir transport
coefficients, with the help of the reciprocity relations. The effective
temperature is deduced in the activation regime.
|
0610264v1
|
2016-07-18
|
Magnetic Skyrmion Transport in a Nanotrack With Spatially Varying Damping and Non-adiabatic Torque
|
Reliable transport of magnetic skyrmions is required for any future
skyrmion-based information processing devices. Here we present a micromagnetic
study of the in-plane current-driven motion of a skyrmion in a ferromagnetic
nanotrack with spatially sinusoidally varying Gilbert damping and/or
non-adiabatic spin-transfer torque coefficients. It is found that the skyrmion
moves in a sinusoidal pattern as a result of the spatially varying Gilbert
damping and/or non-adiabatic spin-transfer torque in the nanotrack, which could
prevent the destruction of the skyrmion caused by the skyrmion Hall effect. The
results provide a guide for designing and developing the skyrmion transport
channel in skyrmion-based spintronic applications.
|
1607.04983v3
|
2003-08-19
|
Magnetization relaxation in (Ga,Mn)As ferromagnetic semiconductors
|
We describe a theory of Mn local-moment magnetization relaxation due to p-d
kinetic-exchange coupling with the itinerant-spin subsystem in the
ferromagnetic semiconductor (Ga,Mn)As alloy. The theoretical Gilbert damping
coefficient implied by this mechanism is calculated as a function of Mn moment
density, hole concentration, and quasiparticle lifetime. Comparison with
experimental ferromagnetic resonance data suggests that in annealed strongly
metallic samples, p-d coupling contributes significantly to the damping rate of
the magnetization precession at low temperatures. By combining the theoretical
Gilbert coefficient with the values of the magnetic anisotropy energy, we
estimate that the typical critical current for spin-transfer magnetization
switching in all-semiconductor trilayer devices can be as low as $\sim 10^{5}
{\rm A cm}^{-2}$.
|
0308386v3
|
2007-08-24
|
Enhancement of the Gilbert damping constant due to spin pumping in noncollinear ferromagnet/nonmagnet/ferromagnet trilayer systems
|
We analyzed the enhancement of the Gilbert damping constant due to spin
pumping in non-collinear ferromagnet / non-magnet / ferromagnet trilayer
systems. We show that the Gilbert damping constant depends both on the
precession angle of the magnetization of the free layer and on the direction of
the magntization of the fixed layer. We find the condition to be satisfied to
realize strong enhancement of the Gilbert damping constant.
|
0708.3323v1
|
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