776 research outputs found
Realistic many-body models for Manganese Monoxide under pressure
In materials like transition metals oxides where electronic Coulomb
correlations impede a description in terms of standard band-theories, the
application of genuine many-body techniques is inevitable. Interfacing the
realism of density-functional based methods with the virtues of Hubbard-like
Hamiltonians, requires the joint ab initio construction of transfer integrals
and interaction matrix elements (like the Hubbard U) in a localized basis set.
In this work, we employ the scheme of maximally localized Wannier functions and
the constrained random phase approximation to create effective low-energy
models for Manganese monoxide, and track their evolution under external
pressure. We find that in the low pressure antiferromagnetic phase, the
compression results in an increase of the bare Coulomb interaction for specific
orbitals. As we rationalized in recent model considerations [Phys. Rev. B 79,
235133 (2009)], this seemingly counter-intuitive behavior is a consequence of
the delocalization of the respective Wannier functions. The change of screening
processes does not alter this tendency, and thus, the screened on-site
component of the interaction - the Hubbard U of the effective low-energy system
- increases with pressure as well. The orbital anisotropy of the effects
originates from the orientation of the orbitals vis-a-vis the deformation of
the unit-cell. Within the high pressure paramagnetic phase, on the other hand,
we find the significant increase of the Hubbard U is insensitive to the orbital
orientation and almost exclusively owing to a substantial weakening of
screening channels upon compression.Comment: 13 pages, 6 figure
Investigation of quasi-periodic varaiations in hard X-rays of solar flares
The aim of the present paper is to use quasi-periodic oscillations in hard
X-rays (HXRs) of solar flares as a diagnostic tool for investigation of
impulsive electron acceleration. We have selected a number of flares which
showed quasi-periodic oscillations in hard X-rays and their loop-top sources
could be easily recognized in HXR images. We have considered MHD standing waves
to explain the observed HXR oscillations. We interpret these HXR oscillations
as being due to oscillations of magnetic traps within cusp-like magnetic
structures. This is confirmed by a good correlation between periods of the
oscillations and the sizes of the loop-top sources. We argue that a model of
oscillating magnetic traps is adequate to explain the observations. During the
compressions of a trap particles are accelerated, but during its expansions
plasma, coming from chromospheric evaporation, fills the trap, which explains
the large number of electrons being accelerated during a sequence of strong
impulses. The advantage of our model of oscillating magnetic traps is that it
can explain both the impulses of electron acceleration and quasi-periodicity of
their distribution in time.Comment: 21 pages, 11 figures, 3 tables, submitted to Solar Physic
Learning Arbitrary Statistical Mixtures of Discrete Distributions
We study the problem of learning from unlabeled samples very general
statistical mixture models on large finite sets. Specifically, the model to be
learned, , is a probability distribution over probability
distributions , where each such is a probability distribution over . When we sample from , we do not observe
directly, but only indirectly and in very noisy fashion, by sampling from
repeatedly, independently times from the distribution . The problem is
to infer to high accuracy in transportation (earthmover) distance.
We give the first efficient algorithms for learning this mixture model
without making any restricting assumptions on the structure of the distribution
. We bound the quality of the solution as a function of the size of
the samples and the number of samples used. Our model and results have
applications to a variety of unsupervised learning scenarios, including
learning topic models and collaborative filtering.Comment: 23 pages. Preliminary version in the Proceeding of the 47th ACM
Symposium on the Theory of Computing (STOC15
Investigation of quasi-periodic variations in hard X-rays of solar flares. II. Further investigation of oscillating magnetic traps
In our recent paper (Solar Physics 261, 233) we investigated quasi-periodic
oscillations of hard X-rays during impulsive phase of solar flares. We have
come to conclusion that they are caused by magnetosonic oscillations of
magnetic traps within the volume of hard-X-ray (HXR) loop-top sources. In the
present paper we investigate four flares which show clear quasi-periodic
sequences of HXR pulses. We also describe our phenomenological model of
oscillating magnetic traps to show that it can explain observed properties of
HXR oscillations. Main results are the following: 1. We have found that
low-amplitude quasi-periodic oscillations occur before impulsive phase of some
flares. 2. We have found that quasi-period of the oscillations can change in
some flares. We interpret this as being due to changes of the length of
oscillating magnetic traps. 3. During impulsive phase a significant part of the
energy of accelerated (non-thermal) electrons is deposited within the HXR
loop-top source. 4. Our analysis suggests that quick development of impulsive
phase is due to feedback between pulses of the pressure of accelerated
electrons and the amplitude of magnetic-trap oscillation. 5. We have also
determined electron number density and magnetic filed strength for HXR loop-top
sources of several flares. The values fall within the limits of cm, gauss.Comment: 18 pages, 14 figures, submitted to Solar Physic
The Outstanding Decisions of the United States Supreme Court in 1954
We perform a kinematic and morphological analysis of 44 star-forming galaxies at z ̃ 2 in the COSMOS legacy field using near-infrared spectroscopy from Keck/MOSFIRE and F160W imaging from CANDELS/3D-HST as part of the ZFIRE survey. Our sample consists of cluster and field galaxies from 2.0 < z < 2.5 with K-band multi-object slit spectroscopic measurements of their Hα emission lines. Hα rotational velocities and gas velocity dispersions are measured using the Heidelberg Emission Line Algorithm (HELA), which compares directly to simulated 3D data cubes. Using a suite of simulated emission lines, we determine that HELA reliably recovers input S 0.5 and angular momentum at small offsets, but V 2.2/σ g values are offset and highly scattered. We examine the role of regular and irregular morphology in the stellar mass kinematic scaling relations, deriving the kinematic measurement S 0.5, and finding {log}({S}0.5)=(0.38+/- 0.07){log}(M/{M}☉ -10)+(2.04+/- 0.03) with no significant offset between morphological populations and similar levels of scatter (̃0.16 dex). Additionally, we identify a correlation between M ⋆ and V 2.2/σ g for the total sample, showing an increasing level of rotation dominance with increasing M ⋆, and a high level of scatter for both regular and irregular galaxies. We estimate the specific angular momenta (j disk) of these galaxies and find a slope of 0.36 ± 0.12, shallower than predicted without mass-dependent disk growth, but this result is possibly due to measurement uncertainty at M ⋆ < 9.5 However, through a Kolmogorov-Smirnov test we find irregular galaxies to have marginally higher j disk values than regular galaxies, and high scatter at low masses in both populations
Structured Random Matrices
Random matrix theory is a well-developed area of probability theory that has
numerous connections with other areas of mathematics and its applications. Much
of the literature in this area is concerned with matrices that possess many
exact or approximate symmetries, such as matrices with i.i.d. entries, for
which precise analytic results and limit theorems are available. Much less well
understood are matrices that are endowed with an arbitrary structure, such as
sparse Wigner matrices or matrices whose entries possess a given variance
pattern. The challenge in investigating such structured random matrices is to
understand how the given structure of the matrix is reflected in its spectral
properties. This chapter reviews a number of recent results, methods, and open
problems in this direction, with a particular emphasis on sharp spectral norm
inequalities for Gaussian random matrices.Comment: 46 pages; to appear in IMA Volume "Discrete Structures: Analysis and
Applications" (Springer
Downfolded Self-Energy of Many-Electron Systems
Starting from the full many-body Hamiltonian of interacting electrons the
effective self-energy acting on electrons residing in a subspace of the full
Hilbert space is derived. This subspace may correspond to, for example,
partially filled narrow bands, which often characterize strongly correlated
materials. The formalism delivers naturally the frequency-dependent effective
interaction (the Hubbard U) and provides a general framework for constructing
theoretical models based on the Green function language. It also furnishes a
general scheme for first-principles calculations of complex systems in which
the main correlation effects are concentrated on a small subspace of the full
Hilbert space.Comment: 5 page
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