18,811 research outputs found
Airborne Fraunhofer Line Discriminator
Airborne Fraunhofer Line Discriminator enables prospecting for fluorescent materials, hydrography with fluorescent dyes, and plant studies based on fluorescence of chlorophyll. Optical unit design is the coincidence of Fraunhofer lines in the solar spectrum occurring at the characteristic wavelengths of some fluorescent materials
Fermi arcs and the hidden zeros of the Green's function in the pseudogap state
We investigate the low energy properties of a correlated metal in the
proximity of a Mott insulator within the Hubbard model in two dimensions. We
introduce a new version of the Cellular Dynamical Mean Field Theory using
cumulants as the basic irreducible objects. These are used for re-constructing
the lattice quantities from their cluster counterparts. The zero temperature
one particle Green's function is characterized by the appearance of lines of
zeros, in addition to a Fermi surface which changes topology as a function of
doping. We show that these features are intimately connected to the opening of
a pseudogap in the one particle spectrum and provide a simple picture for the
appearance of Fermi arcs.Comment: revised version; 5 pages, 3 figure
Derived Categories of Coherent Sheaves and Triangulated Categories of Singularities
In this paper we establish an equivalence between the category of graded
D-branes of type B in Landau-Ginzburg models with homogeneous superpotential W
and the triangulated category of singularities of the fiber of W over zero. The
main result is a theorem that shows that the graded triangulated category of
singularities of the cone over a projective variety is connected via a fully
faithful functor to the bounded derived category of coherent sheaves on the
base of the cone. This implies that the category of graded D-branes of type B
in Landau-Ginzburg models with homogeneous superpotential W is connected via a
fully faithful functor to the derived category of coherent sheaves on the
projective variety defined by the equation W=0.Comment: 26 pp., LaTe
Micromagnetic Simulations of Ferromagnetic Rings
Thin nanomagnetic rings have generated interest for fundamental studies of
magnetization reversal and also for their potential in various applications,
particularly as magnetic memories. They are a rare example of a geometry in
which an analytical solution for the rate of thermally induced magnetic
reversal has been determined, in an approximation whose errors can be estimated
and bounded. In this work, numerical simulations of soft ferromagnetic rings
are used to explore aspects of the analytical solution. The evolution of the
energy near the transition states confirms that, consistent with analytical
predictions, thermally induced magnetization reversal can have one of two
intermediate states: either constant or soliton-like saddle configurations,
depending on ring size and externally applied magnetic field. The results
confirm analytical predictions of a transition in thermally activated reversal
behavior as magnetic field is varied at constant ring size. Simulations also
show that the analytic one dimensional model continues to hold even for wide
rings
Thermal Stability of the Magnetization in Perpendicularly Magnetized Thin Film Nanomagnets
Understanding the stability of thin film nanomagnets with perpendicular
magnetic anisotropy (PMA) against thermally induced magnetization reversal is
important when designing perpendicularly magnetized patterned media and
magnetic random access memories. The leading-order dependence of magnetization
reversal rates are governed by the energy barrier the system needs to surmount
in order for reversal to proceed. In this paper we study the reversal dynamics
of these systems and compute the relevant barriers using the string method of
E, Vanden-Eijnden, and Ren. We find the reversal to be often spatially
incoherent; that is, rather than the magnetization flipping as a rigid unit,
reversal proceeds instead through a soliton-like domain wall sweeping through
the system. We show that for square nanomagnetic elements the energy barrier
increases with element size up to a critical length scale, beyond which the
energy barrier is constant. For circular elements the energy barrier continues
to increase indefinitely, albeit more slowly beyond a critical size. In both
cases the energy barriers are smaller than those expected for coherent
magnetization reversal.Comment: 5 pages, 4 Figure
Strong Coupling Theory for Interacting Lattice Models
We develop a strong coupling approach for a general lattice problem. We argue
that this strong coupling perspective represents the natural framework for a
generalization of the dynamical mean field theory (DMFT). The main result of
this analysis is twofold: 1) It provides the tools for a unified treatment of
any non-local contribution to the Hamiltonian. Within our scheme, non-local
terms such as hopping terms, spin-spin interactions, or non-local Coulomb
interactions are treated on equal footing. 2) By performing a detailed
strong-coupling analysis of a generalized lattice problem, we establish the
basis for possible clean and systematic extensions beyond DMFT. To this end, we
study the problem using three different perspectives. First, we develop a
generalized expansion around the atomic limit in terms of the coupling
constants for the non-local contributions to the Hamiltonian. By analyzing the
diagrammatics associated with this expansion, we establish the equations for a
generalized dynamical mean-field theory (G-DMFT). Second, we formulate the
theory in terms of a generalized strong coupling version of the Baym-Kadanoff
functional. Third, following Pairault, Senechal, and Tremblay, we present our
scheme in the language of a perturbation theory for canonical fermionic and
bosonic fields and we establish the interpretation of various strong coupling
quantities within a standard perturbative picture.Comment: Revised Version, 17 pages, 5 figure
Experimental study of acoustic displays of flight parameters in a simulated aerospace vehicle
Evaluating acoustic displays of target location in target detection and of flight parameters in simulated aerospace vehicle
Validating Predictions of Unobserved Quantities
The ultimate purpose of most computational models is to make predictions,
commonly in support of some decision-making process (e.g., for design or
operation of some system). The quantities that need to be predicted (the
quantities of interest or QoIs) are generally not experimentally observable
before the prediction, since otherwise no prediction would be needed. Assessing
the validity of such extrapolative predictions, which is critical to informed
decision-making, is challenging. In classical approaches to validation, model
outputs for observed quantities are compared to observations to determine if
they are consistent. By itself, this consistency only ensures that the model
can predict the observed quantities under the conditions of the observations.
This limitation dramatically reduces the utility of the validation effort for
decision making because it implies nothing about predictions of unobserved QoIs
or for scenarios outside of the range of observations. However, there is no
agreement in the scientific community today regarding best practices for
validation of extrapolative predictions made using computational models. The
purpose of this paper is to propose and explore a validation and predictive
assessment process that supports extrapolative predictions for models with
known sources of error. The process includes stochastic modeling, calibration,
validation, and predictive assessment phases where representations of known
sources of uncertainty and error are built, informed, and tested. The proposed
methodology is applied to an illustrative extrapolation problem involving a
misspecified nonlinear oscillator
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