7,002 research outputs found
Solitons in Trapped Bose-Einstein condensates in one-dimensional optical lattices
We use Quantum Monte Carlo simulations to show the presence and study the
properties of solitons in the one dimensional soft-core bosonic Hubbard model
with near neighbor interaction in traps. We show that when the half-filled
Charge Density Wave (CDW) phase is doped, solitons are produced and quasi long
range order established. We discuss the implications of these results for the
presence and robustness of this solitonic phase in Bose-Einstein Condensates
(BEC) on one dimensional optical lattices in traps and study the associated
excitation spectrum. The density profile exhibits the coexistence of Mott
insulator, CDW, and superfluid regions.Comment: 5 pages, Latex with figure
Coaxial prime focus feeds for paraboloidal reflectors
A TE11 - TM11 dual mode coaxial feed for use in prime focus paraboloidal antenna systems is investigated. The scattering matrix parameters of the internal bifurcation junction was determined by the residue calculus technique. The scattering parameters and radiation fields of the aperture were found from the Weinstein solution. The optimum modeing ratio for minimum cross-polarization was determined along with the corresponding optimum feed dimensions. A peak cross-polarization level of -58 dB is predicted. The frequency characteristics were also investigated and a bandwidth of 5% is predicted over which the cross-polarization remains below -30 dB, the input VSWR is below 1.15, and the phase error is less than 10 deg. Theoretical radiation patterns and efficiency curves for a paraboloidal reflector illuminated by this feed were computed. The predicted sidelobe level is below -30 dB and aperture efficiencies greater than 70% are possible. Experimental results are also presented that substantiates the theoretical results. In addition, experimental results for a 'short-cup' coaxial feed are given. The report includes extensive design data for the dual-mode feed along with performance curves showing cross-polarization as a function of feed parameters. The feed is useful for low-cost ground based receiving antennas for use in direct television satellite broadcasting service
Magnetic and Transport Properties of a Coupled Hubbard Bilayer with Electron and Hole Doping
The single band, two dimensional Hubbard Hamiltonian has been extensively
studied as a model for high temperature superconductivity. While Quantum Monte
Carlo simulations within the dynamic cluster approximation are now providing
considerable evidence for a d-wave superconducting state at low temperature,
such a transition remains well out of reach of finite lattice simulations
because of the "sign problem". We show here that a bilayer Hubbard model, in
which one layer is electron doped and one layer is hole doped, can be studied
to lower temperatures and exhibits an interesting signal of d-wave pairing. The
results of our simulations bear resemblance to a recent report on the magnetic
and superconducting properties of BaCaCuOF which contains
both electron and hole doped CuO planes. We also explore the phase diagram
of bilayer models in which each sheet is at half-filling
A note on the convexity number for complementary prisms
In the geodetic convexity, a set of vertices of a graph is
if all vertices belonging to any shortest path between two
vertices of lie in . The cardinality of a maximum proper convex
set of is the of . The
of a graph arises from the
disjoint union of the graph and by adding the edges of a
perfect matching between the corresponding vertices of and .
In this work, we we prove that the decision problem related to the convexity
number is NP-complete even restricted to complementary prisms, we determine
when is disconnected or is a cograph, and we
present a lower bound when .Comment: 10 pages, 2 figure
A Calculated Risk: The Effects of Nicolae Ceauşescu’s Denunciation of the 1968 Warsaw Pact Invasion of Czechoslovakia on US-Romanian Relations
Abstract
For most of the Cold War, the United States attempted to maintain friendly relations with the Communist nations comprising the Eastern Bloc, but with no other Soviet satellite was the relationship as close as it was with Romania. No other member nation of the Warsaw Pact took to the United States’ overtures so eagerly. Diplomatic relations between the United States and the Romanian Communist government were established relatively early, almost immediately following the end of the Second World War. However, it was not until 1968, when Romanian dictator Nicolae Ceauşescu denounced the Warsaw Pact invasion of Czechoslovakia, that the Romanians finally gained the Americans’ trust. Ceauşescu’s 1968 speech attacking the Warsaw Pact invasion of Czechoslovakia, and the diplomatic maneuverings surrounding it, was the pivotal moment in the relationship between the two nations, fostering an amicable relationship that would last well into the 1980s
Quantum Monte Carlo Study of an Interaction-Driven Band Insulator to Metal Transition
We study the transitions from band insulator to metal to Mott insulator in
the ionic Hubbard model on a two dimensional square lattice using determinant
Quantum Monte Carlo. Evaluation of the temperature dependence of the
conductivity demonstrates that the metallic region extends for a finite range
of interaction values. The Mott phase at strong coupling is accompanied by
antiferromagnetic (AF) order. Inclusion of these intersite correlations changes
the phase diagram qualitatively compared to dynamical mean field theory.Comment: 4 pages, 6 figure
The Impacts of Using Augmented Reality to Support Aircraft Maintenance
The United States Air Force (USAF) expends significant resources to address the rise in aviation mishaps derived from an overworked, understaffed maintenance community, and high operational environment. Currently, paper-based technical orders (T.O.) are utilized by maintainers to accomplish aircraft inspections, servicing, and maintenance tasks. As technology advances, many civilian agencies have begun to leverage augmented reality (AR) to improve organizational proficiency. This research seeks to identify if the inclusion of AR within aircraft maintenance will positively or negatively affect maintenance task accuracy and completion time. A single variable randomized complete block design (RCBD), within-subject design of experiment (DOE) asses the differences between a treatment group (AR-enabled T.O.) contrary to the control group (paper-based T.O.). Results conclude AR-enabled T.O.s designed from the AF perspective will reduce simple task errors, but will not impact total task completion time. Differentiation from prior findings, application specificity, will impact AR effectiveness and utilization within the organization employed. Additionally, experimental research revealed the need to address current AF infrastructure barriers before implementation of the technology within the organization
Effect of Impurities on Interfacial Void Formation in Aluminum
The effect of impurities on formation of interfacial metallic voids, during uniform dissolution of aluminum in 1 M NaOH, was investigated. These voids are thought to act as initiation sites for pitting corrosion, and were previously shown to be formed by NaOH dissolution. Samples of three different bulk purities were compared: 99.98, 99.997, and 99.9995%. Positron annihilation spectroscopy and atomic force microscopy revealed that nanometer-scale voids were formed by dissolution in each foil. For each sample, the void volume fraction interpreted from these measurements increased to a maximum during dissolution, and then declined. As the purity increased, more extensive dissolution was required to produce voids. Accumulation of near-surface Cu and Fe impurities during dissolution was characterized using Rutherford backscattering spectrometry. The results suggested a possible general correlation of void volume fraction with copper surface concentration. Processes involving near-surface copper impurities may then at least partly control the formation of voids. © 2004 The Electrochemical Society. All rights reserved
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