10,361 research outputs found
Beta-Product Poisson-Dirichlet Processes
Time series data may exhibit clustering over time and, in a multiple time
series context, the clustering behavior may differ across the series. This
paper is motivated by the Bayesian non--parametric modeling of the dependence
between the clustering structures and the distributions of different time
series. We follow a Dirichlet process mixture approach and introduce a new
class of multivariate dependent Dirichlet processes (DDP). The proposed DDP are
represented in terms of vector of stick-breaking processes with dependent
weights. The weights are beta random vectors that determine different and
dependent clustering effects along the dimension of the DDP vector. We discuss
some theoretical properties and provide an efficient Monte Carlo Markov Chain
algorithm for posterior computation. The effectiveness of the method is
illustrated with a simulation study and an application to the United States and
the European Union industrial production indexes
Perpendicular Reading of Single Confined Magnetic Skyrmions
Thin-film sub-5 nm magnetic skyrmions constitute an ultimate scaling
alternative for future digital data storage. Skyrmions are robust non-collinear
spin-textures that can be moved and manipulated by small electrical currents.
We show here an innovative technique to detect isolated nanoskyrmions with a
current-perpendicular-to-plane geometry, which has immediate implications for
device concepts. We explore the physics behind such a mechanism by studying the
atomistic electronic structure of the magnetic quasiparticles. We investigate
how the isolated skyrmion local-density-of-states which tunnels into the
vacuum, when compared to the ferromagnetic background, is modified by the
site-dependent spin-mixing of electronic states with different relative canting
angles. Local transport properties are sensitive to this effect, as we report
an atomistic conductance anisotropy of over 20% for magnetic skyrmions in
Pd/Fe/Ir(111) thin-films. In single skyrmions, engineering this spin-mixing
magnetoresistance possibly could be incorporated in future magnetic storage
technologies
Thermodynamics of protein folding: a random matrix formulation
The process of protein folding from an unfolded state to a biologically
active, folded conformation is governed by many parameters e.g the sequence of
amino acids, intermolecular interactions, the solvent, temperature and chaperon
molecules. Our study, based on random matrix modeling of the interactions,
shows however that the evolution of the statistical measures e.g Gibbs free
energy, heat capacity, entropy is single parametric. The information can
explain the selection of specific folding pathways from an infinite number of
possible ways as well as other folding characteristics observed in computer
simulation studies.Comment: 21 Pages, no figure
Multiple scattering of light by atoms with internal degeneracy
An analytical microscopic theory for the resonant multiple scattering of
light by cold atoms with arbitrary internal degeneracy is presented. It permits
to calculate the average amplitude and the average intensity for one-photon
states of the full transverse electromagnetic field in a dilute medium of
unpolarized atoms. Special emphasis is laid upon an analysis in terms of
irreducible representations of the rotation group. It allows to sum explicitly
the ladder and maximally crossed diagrams, giving the average intensity in the
Boltzmann approximation and the interference corrections responsible for weak
localization and coherent backscattering. The exact decomposition into field
modes shows that the atomic internal degeneracy contributes to the
depolarization of the average intensity and suppresses the interference
corrections. Static as well as dynamic quantities like the transport velocity,
diffusion constants and relaxation times for all field modes and all atomic
transitions are derived.Comment: Corrected minor errors. Slightly extended version of the article
appeared in prin
Long-range correlation energy calculated from coupled atomic response functions
An accurate determination of the electron correlation energy is essential for
describing the structure, stability, and function in a wide variety of systems,
ranging from gas-phase molecular assemblies to condensed matter and
organic/inorganic interfaces. Even small errors in the correlation energy can
have a large impact on the description of chemical and physical properties in
the systems of interest. In this context, the development of efficient
approaches for the accurate calculation of the long-range correlation energy
(and hence dispersion) is the main challenge. In the last years a number of
methods have been developed to augment density functional approximations via
dispersion energy corrections, but most of these approaches ignore the
intrinsic many-body nature of correlation effects, leading to inconsistent and
sometimes even qualitatively incorrect predictions. Here we build upon the
recent many-body dispersion (MBD) framework, which is intimately linked to the
random-phase approximation for the correlation energy. We separate the
correlation energy into short-range contributions that are modeled by
semi-local functionals and long-range contributions that are calculated by
mapping the complex all-electron problem onto a set of atomic response
functions coupled in the dipole approximation. We propose an effective
range-separation of the coupling between the atomic response functions that
extends the already broad applicability of the MBD method to non-metallic
materials with highly anisotropic responses, such as layered nanostructures.
Application to a variety of high-quality benchmark datasets illustrates the
accuracy and applicability of the improved MBD approach, which offers the
prospect of first-principles modeling of large structurally complex systems
with an accurate description of the long-range correlation energy.Comment: 15 pages, 3 figure
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