753 research outputs found
Density matrix renormalization group study of optical conductivity in the one-dimensional Mott insulator Sr_2CuO_3
Applying newly developed dynamical density matrix renormalization group
techniques at zero and finite temperatures to a Hubbard-Holstein model at
half-filling, we examine the optical conductivity of a typical one-dimensional
Mott insulator Sr_2CuO_3. We find a set of parameters in the Hubbard-Holstein
model, which can describe optical conductivity for both Mott-gap excitation in
the high-energy region and phonon-assisted spin excitation in the low-energy
region. We also find that electron-phonon interaction gives additional
broadening in the temperature dependence of the Mott-gap excitation.Comment: 5 pages, 3figure
Quantum annealing for systems of polynomial equations
Numerous scientific and engineering applications require numerically solving
systems of equations. Classically solving a general set of polynomial equations
requires iterative solvers, while linear equations may be solved either by
direct matrix inversion or iteratively with judicious preconditioning. However,
the convergence of iterative algorithms is highly variable and depends, in
part, on the condition number. We present a direct method for solving general
systems of polynomial equations based on quantum annealing, and we validate
this method using a system of second-order polynomial equations solved on a
commercially available quantum annealer. We then demonstrate applications for
linear regression, and discuss in more detail the scaling behavior for general
systems of linear equations with respect to problem size, condition number, and
search precision. Finally, we define an iterative annealing process and
demonstrate its efficacy in solving a linear system to a tolerance of
.Comment: 11 pages, 4 figures. Added example for a system of quadratic
equations. Supporting code is available at
https://github.com/cchang5/quantum_poly_solver . This is a post-peer-review,
pre-copyedit version of an article published in Scientific Reports. The final
authenticated version is available online at:
https://www.nature.com/articles/s41598-019-46729-
The little-studied cluster Berkeley 90. II. The foreground ISM
Context: Nearly one century after their discovery, the carrier(s) of Diffuse
Interstellar Bands is/are still unknown and there are few sightlines studied in
detail for a large number of DIBs. Aims: We want to study the ISM sightlines
towards LS III +46 11 and LS III +46 12, two early-O-type stellar systems, and
LS III +46 11 B, a mid-B-type star. The three targets are located in the
stellar cluster Berkeley 90 and have a high extinction. Methods: We use the
multi-epoch high-S/N optical spectra presented in paper I (Ma\'iz Apell\'aniz
et al. 2015), the extinction results derived there, and additional spectra.
Results: We have measured equivalent widths, velocities, and FWHMs for a large
number of absorption lines in the rich ISM spectrum in front of Berkeley 90.
The absorbing ISM has at least two clouds at different velocities, one with a
lower column density (thinner) in the K I lines located away from Berkeley 90
and another one with a higher column density (thicker) associated with the
cluster. The first cloud has similar properties for both O-star sightlines but
the second one is thicker for LS III +46 11. The comparison between species
indicate that the cloud with a higher column density has a denser core,
allowing us to classify the DIBs in a sigma-zeta scale, some of them for the
first time. The LS III +46 12 sightline also has a high-velocity redshifted
component.Comment: Accepted for publication in A&
Lucky Spectroscopy, an equivalent technique to Lucky Imaging. Spatially resolved spectroscopy of massive close visual binaries using the William Herschel Telescope
CONTEXT: Many massive stars have nearby companions whose presence hamper
their characterization through spectroscopy. AIMS: We want to obtain spatially
resolved spectroscopy of close massive visual binaries to derive their spectral
types. METHODS: We obtain a large number of short long-slit spectroscopic
exposures of five close binaries under good seeing conditions, select those
with the best characteristics, extract the spectra using multiple-profile
fitting, and combine the results to derive spatially separated spectra.
RESULTS: We demonstrate the usefulness of Lucky Spectroscopy by presenting the
spatially resolved spectra of the components of each system, in two cases with
separations of only ~0.3". Those are delta Ori Aa+Ab (resolved in the optical
for the first time) and sigma Ori AaAb+B (first time ever resolved). We also
spatially resolve 15 Mon AaAb+B, zeta Ori AaAb+B (both previously resolved with
GOSSS, the Galactic O-Star Spectroscopic Survey), and eta Ori AaAb+B, a system
with two spectroscopic B+B binaries and a fifth visual component. The systems
have in common that they are composed of an inner pair of slow rotators orbited
by one or more fast rotators, a characteristic that could have consequences for
the theories of massive star formation.Comment: Accepted for publication in A&A, 7 page
The AMIGA sample of isolated galaxies - II. Morphological refinement
We present a complete POSS II-based refinement of the optical morphologies
for galaxies in the Karatchenseva's Catalog of Isolated Galaxies that forms the
basis of the AMIGA project. Comparison with independent classifications made
for an SDSS overlap sample of more than 200 galaxies confirms the reliability
of the early vs. late-type discrimination and the accuracy of spiral subtypes
within DeltaT = 1-2. CCD images taken at the OSN were also used to solve
ambiguities. 193 galaxies are flagged for the presence of nearby companions or
signs of distortion likely due to interaction. This most isolated sample of
galaxies in the local Universe is dominated by 2 populations: 1) 82% spirals
(Sa-Sd) with the bulk being luminous systems with small bulges (63% between
types Sb-Sc) and 2) a significant population of early-type E-S0 galaxies (14%).
Most of the types later than Sd are low luminosity galaxies concentrated in the
local supercluster where isolation is difficult to evaluate. The late-type
spiral majority of the sample spans a luminosity range M_B-corr = -18 to -22
mag. Few of the E/S0 population are more luminous than -21.0 marking an absence
of, an often sought, super L* merger (eg fossil elliptical) population. The
rarity of high luminosity systems results in a fainter derived M* for this
population compared to the spiral optical luminosity function (OLF). The E-S0
population is from 0.2 to 0.6 mag fainter depending how the sample is defined.
This marks the AMIGA sample as almost unique among samples that compare early
and late-type OLFs separately. In other samples, which always involve galaxies
in higher density environments, M*(E/S0) is almost always 0.3-0.5 mag brighter
than M*(S), presumably reflecting a stronger correlation between M* and
environmental density for early-type galaxies.Comment: A&A accepted, 13 pages, 9 figures, 8 tables. Higher resolution Fig. 1
and full tables are available on the AMIGA (Analysis of the interstellar
Medium of Isolated GAlaxies) website at http://www.iaa.es/AMIGA.htm
A Stochastic Variance Reduced Nesterov's Accelerated Quasi-Newton Method
Recently algorithms incorporating second order curvature information have
become popular in training neural networks. The Nesterov's Accelerated
Quasi-Newton (NAQ) method has shown to effectively accelerate the BFGS
quasi-Newton method by incorporating the momentum term and Nesterov's
accelerated gradient vector. A stochastic version of NAQ method was proposed
for training of large-scale problems. However, this method incurs high
stochastic variance noise. This paper proposes a stochastic variance reduced
Nesterov's Accelerated Quasi-Newton method in full (SVR-NAQ) and limited
(SVRLNAQ) memory forms. The performance of the proposed method is evaluated in
Tensorflow on four benchmark problems - two regression and two classification
problems respectively. The results show improved performance compared to
conventional methods.Comment: Accepted in ICMLA 201
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