18,937 research outputs found
Robust Convergence of Power Flow using Tx Stepping Method with Equivalent Circuit Formulation
Robust solving of critical large power flow cases (with 50k or greater buses)
forms the backbone of planning and operation of any large connected power grid.
At present, reliable convergence with applications of existing power flow tools
to large power systems is contingent upon a good initial guess for the system
state. To enable robust convergence for large scale systems starting with an
arbitrary initial guess, we extend our equivalent circuit formulation for power
flow analysis to include a novel continuation method based on transmission line
(Tx) stepping. While various continuation methods have been proposed for use
with the traditional PQV power flow formulation, these methods have either
failed to completely solve the problem or have resulted in convergence to a low
voltage solution. The proposed Tx Stepping method in this paper demonstrates
robust convergence to the high voltage solution from an arbitrary initial
guess. Example systems, including 75k+ bus test cases representing different
loading and operating conditions for Eastern Interconnection of the U.S. power
grid, are solved from arbitrary initial guesses.Interconnection of the U.S.
power grid, are solved from arbitrary initial guesses
Search for Gamma-ray Emission from Dark Matter Annihilation in the Large Magellanic Cloud with the Fermi Large Area Telescope
At a distance of 50 kpc and with a dark matter mass of
M, the Large Magellanic Cloud (LMC) is a natural target for indirect
dark matter searches. We use five years of data from the Fermi Large Area
Telescope (LAT) and updated models of the gamma-ray emission from standard
astrophysical components to search for a dark matter annihilation signal from
the LMC. We perform a rotation curve analysis to determine the dark matter
distribution, setting a robust minimum on the amount of dark matter in the LMC,
which we use to set conservative bounds on the annihilation cross section. The
LMC emission is generally very well described by the standard astrophysical
sources, with at most a excess identified near the kinematic center
of the LMC once systematic uncertainties are taken into account. We place
competitive bounds on the dark matter annihilation cross section as a function
of dark matter particle mass and annihilation channel.Comment: 33 pages, 22 figures Version 2: minor corrections and clarifications
after journal peer review proces
Structural and optical investigation of non-polar (1-100) GaN grown by the ammonothermal method
This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. This article appeared in Journal of Applied Physics 113, 203513 (2013) and may be found at https://doi.org/10.1063/1.4807581.We studied the structural and optical properties of state-of-the-art non-polar bulk GaN grown by the ammonothermal method. The investigated samples have an extremely low dislocation density (DD) of less than 5 × 104 cm−2, which results in very narrow high-resolution x-ray rocking curves. The a and c lattice parameters of these stress-free GaN samples were precisely determined by using an x-ray diffraction technique based on the modified Bond method. The obtained values are compared to the lattice parameters of free-standing GaN from different methods and sources. The observed differences are discussed in terms of free-electron concentrations, point defects, and DD. Micro Raman spectroscopy revealed a very narrow phonon linewidth and negligible built-in strain in accordance with the high-resolution x-ray diffraction data. The optical transitions were investigated by cathodoluminescence measurements. The analysis of the experimental data clearly demonstrates the excellent crystalline perfection of ammonothermal GaN material and its potential for fabrication of non-polar substrates for homoepitaxial growth of GaN based device structures
Search for Gamma-ray Emission from Dark Matter Annihilation in the Small Magellanic Cloud with the Fermi Large Area Telescope
The Small Magellanic Cloud (SMC) is the second-largest satellite galaxy of
the Milky Way and is only 60 kpc away. As a nearby, massive, and dense object
with relatively low astrophysical backgrounds, it is a natural target for dark
matter indirect detection searches. In this work, we use six years of Pass 8
data from the Fermi Large Area Telescope to search for gamma-ray signals of
dark matter annihilation in the SMC. Using data-driven fits to the gamma-ray
backgrounds, and a combination of N-body simulations and direct measurements of
rotation curves to estimate the SMC DM density profile, we found that the SMC
was well described by standard astrophysical sources, and no signal from dark
matter annihilation was detected. We set conservative upper limits on the dark
matter annihilation cross section. These constraints are in agreement with
stronger constraints set by searches in the Large Magellanic Cloud and approach
the canonical thermal relic cross section at dark matter masses lower than 10
GeV in the and channels.Comment: 17 pages, 11 figures. Accepted by PR
Suppression of the quantum-confined Stark effect in polar nitride heterostructures
Recently, we suggested an unconventional approach (the so-called Internal-Field-Guarded-Active-Region Design “IFGARD”) for the elimination of the quantum-confined Stark effect in polar semiconductor heterostructures. The IFGARD-based suppression of the Stark redshift on the order of electronvolt and spatial charge carrier separation is independent of the specific polar semiconductor material or the related growth procedures. In this work, we demonstrate by means of micro-photoluminescence techniques the successful tuning as well as the elimination of the quantum-confined Stark effect in strongly polar [000-1] wurtzite GaN/AlN nanodiscs as evidenced by a reduction of the exciton lifetimes by up to four orders of magnitude. Furthermore, the tapered geometry of the utilized nanowires (which embed the investigated IFGARD nanodiscs) facilitates the experimental differentiation between quantum confinement and Stark emission energy shifts. Due to the IFGARD, both effects become independently adaptable.DFG, 43659573, SFB 787: Halbleiter - Nanophotonik: Materialien, Modelle, Bauelement
One-dimensional Continuum Electronic Structure with the Density Matrix Renormalization Group and Its Implications For Density Functional Theory
We extend the density matrix renormalization group to compute exact ground
states of continuum many-electron systems in one dimension with long-range
interactions. We find the exact ground state of a chain of 100 strongly
correlated artificial hydrogen atoms. The method can be used to simulate 1d
cold atom systems and to study density functional theory in an exact setting.
To illustrate, we find an interacting, extended system which is an insulator
but whose Kohn-Sham system is metallic.Comment: 5 pages, 4 figures. Published version, including new section on exact
Kohn-Sham gap of a Mott insulato
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