2,495 research outputs found
Antiferromagnetic MnNi tips for spin-polarized scanning probe microscopy
Spin-polarized scanning tunneling microscopy (SP-STM) measures tunnel
magnetoresistance (TMR) with atomic resolution. While various methods for
achieving SP probes have been developed, each is limited with respect to
fabrication, performance, and allowed operating conditions. In this study, we
present the fabrication and use of SP-STM tips made from commercially available
antiferromagnetic foil. The tips are intrinsically SP,
which is attractive for exploring magnetic phenomena in the zero field limit.
The tip material is relatively ductile and straightforward to etch. We
benchmark the conventional STM and spectroscopic performance of our tips and
demonstrate their spin sensitivity by measuring the two-state switching of
holmium single atom magnets on MgO/Ag(100)
The structure of borders in a small world
Geographic borders are not only essential for the effective functioning of
government, the distribution of administrative responsibilities and the
allocation of public resources, they also influence the interregional flow of
information, cross-border trade operations, the diffusion of innovation and
technology, and the spatial spread of infectious diseases. However, as growing
interactions and mobility across long distances, cultural, and political
borders continue to amplify the small world effect and effectively decrease the
relative importance of local interactions, it is difficult to assess the
location and structure of effective borders that may play the most significant
role in mobility-driven processes. The paradigm of spatially coherent
communities may no longer be a plausible one, and it is unclear what structures
emerge from the interplay of interactions and activities across spatial scales.
Here we analyse a multi-scale proxy network for human mobility that
incorporates travel across a few to a few thousand kilometres. We determine an
effective system of geographically continuous borders implicitly encoded in
multi-scale mobility patterns. We find that effective large scale boundaries
define spatially coherent subdivisions and only partially coincide with
administrative borders. We find that spatial coherence is partially lost if
only long range traffic is taken into account and show that prevalent models
for multi-scale mobility networks cannot account for the observed patterns.
These results will allow for new types of quantitative, comparative analyses of
multi-scale interaction networks in general and may provide insight into a
multitude of spatiotemporal phenomena generated by human activity.Comment: 9 page
Composing Scalable Nonlinear Algebraic Solvers
Most efficient linear solvers use composable algorithmic components, with the
most common model being the combination of a Krylov accelerator and one or more
preconditioners. A similar set of concepts may be used for nonlinear algebraic
systems, where nonlinear composition of different nonlinear solvers may
significantly improve the time to solution. We describe the basic concepts of
nonlinear composition and preconditioning and present a number of solvers
applicable to nonlinear partial differential equations. We have developed a
software framework in order to easily explore the possible combinations of
solvers. We show that the performance gains from using composed solvers can be
substantial compared with gains from standard Newton-Krylov methods.Comment: 29 pages, 14 figures, 13 table
Modeling one-dimensional island growth with mass-dependent detachment rates
We study one-dimensional models of particle diffusion and
attachment/detachment from islands where the detachment rates gamma(m) of
particles at the cluster edges increase with cluster mass m. They are expected
to mimic the effects of lattice mismatch with the substrate and/or long-range
repulsive interactions that work against the formation of long islands.
Short-range attraction is represented by an overall factor epsilon<<1 in the
detachment rates relatively to isolated particle hopping rates [epsilon ~
exp(-E/T), with binding energy E and temperature T]. We consider various
gamma(m), from rapidly increasing forms such as gamma(m) ~ m to slowly
increasing ones, such as gamma(m) ~ [m/(m+1)]^b. A mapping onto a column
problem shows that these systems are zero-range processes, whose steady states
properties are exactly calculated under the assumption of independent column
heights in the Master equation. Simulation provides island size distributions
which confirm analytic reductions and are useful whenever the analytical tools
cannot provide results in closed form. The shape of island size distributions
can be changed from monomodal to monotonically decreasing by tuning the
temperature or changing the particle density rho. Small values of the scaling
variable X=epsilon^{-1}rho/(1-rho) favour the monotonically decreasing ones.
However, for large X, rapidly increasing gamma(m) lead to distributions with
peaks very close to and rapidly decreasing tails, while slowly increasing
gamma(m) provide peaks close to /2$ and fat right tails.Comment: 16 pages, 6 figure
Spin excitations in a 4f-3d heterodimer on MgO
We report on the magnetic properties of HoCo dimers as a model system for the
smallest intermetallic transition metal-lanthanide compound. The dimers are
adsorbed on ultrathin MgO(100) films grown on Ag(100). New for elements,
we detect inelastic excitations with scanning tunneling microscopy and prove by
their behaviour in applied magnetic field that they are spin-excitations. In
combination with density functional theory and spin Hamiltonian analysis we
determine the magnetic level distribution, as well as sign and magnitude of the
exchange interaction between the two atoms. In contrast to typical bulk
compounds, we find ferromagnetic coupling in the dimer
A Three-Dimensional Solution of Flows over Wings with Leading-Edge Vortex Separation. Part 1: Engineering Document
A method of predicting forces, moments, and detailed surface pressures on thin, sharp-edged wings with leading-edge vortex separation in incompressible flow is presented. The method employs an inviscid flow model in which the wing and the rolled-up vortex sheets are represented by piecewise, continuous quadratic doublet sheet distributions. The Kutta condition is imposed on all wing edges. Computed results are compared with experimental data and with the predictions of the leading-edge suction analogy for a selected number of wing planforms over a wide range of angle of attack. These comparisons show the method to be very promising, capable of producing not only force predictions, but also accurate predictions of detailed surface pressure distributions, loads, and moments
Developments in Longwall Ventilation
Rapid development in longwall mining technology has brought significant changes in panel layout and geometry. These changes require adaptations in the ventilation system to provide sufficient air quantities in longwall face and bleeder areas. At CONSOL, various longwall bleeder systems in the Pittsburgh No. 8 Seam have been studied with detailed ventilation surveys. Computer model network simulations were conducted from these surveys to study the effects of different bleeder configurations and ventilation adjustments. This paper examines the relationships between the longwall face air quantity and the convergence in the tailgate-to-bleeder entries, number of development entries, bleeder fan pressure and the tailgate ventilation scheme. It shows that, using conventional ventilation patterns, the face air quantity may be limited if the gob caves tightly. In such cases, modification of the ventilation pattern to an internal bleeder system, combined with appropriate tailgate ventilation and higher bleeder fan pressure may be required. Experience in CONSOL\u27s operations has proven this method successful especially in mines that changed from four-entry to three-entry longwall development
High-resolution spatial mapping of a superconducting NbN wire using single-electron detection
Superconducting NbN wires have recently received attention as detectors for
visible and infrared photons. We present experiments in which we use a NbN wire
for high-efficiency (40 %) detection of single electrons with keV energy. We
use the beam of a scanning electron microscope as a focussed, stable, and
calibrated electron source. Scanning the beam over the surface of the wire
provides a map of the detection efficiency. This map shows features as small as
150 nm, revealing wire inhomogeneities. The intrinsic resolution of this
mapping method, superior to optical methods, provides the basis of a
characterization tool relevant for photon detectors.Comment: 2009 IEEE Toronto International Conference, Science and Technology
for Humanity (TIC-STH
Multiparticle Quantum Superposition and Stimulated Entanglement by Parity Selective Amplification of Entangled States
A multiparticle quantum superposition state has been generated by a novel
phase-selective parametric amplifier of an entangled two-photon state. This
realization is expected to open a new field of investigations on the
persistence of the validity of the standard quantum theory for systems of
increasing complexity, in a quasi decoherence-free environment. Because of its
nonlocal structure the new system is expected to play a relevant role in the
modern endeavor on quantum information and in the basic physics of
entanglement.Comment: 13 pages and 3 figure
Impact of killer-immunoglobulin-like receptor and human leukocyte antigen genotypes on the efficacy of immunotherapy in acute myeloid leukemia
Interactions between killer-immunoglobulin-like receptors (KIRs) and their HLA class I ligands are instrumental in natural killer (NK) cell regulation and protect normal tissue from NK cell attack. Human KIR haplotypes comprise genes encoding mainly inhibitory receptors (KIR A) or activating and inhibitory receptors (KIR B). A substantial fraction of humans lack ligands for inhibitory KIRs (iKIRs), that is, a 'missing ligand' genotype. KIR B/x and missing ligand genotypes may thus give rise to potentially autoreactive, unlicensed NK cells. Little is known regarding the impact of such genotypes in untransplanted acute myeloid leukemia (AML). For this study, NK cell phenotypes and KIR/HLA genotypes were determined in 81 AML patients who received immunotherapy with histamine dihydrochloride and low-dose IL-2 for relapse prevention (NCT01347996). We observed that presence of unlicensed NK cells impacted favorably on clinical outcome, in particular among patients harboring functional NK cells reflected by high expression of the natural cytotoxicity receptor (NCR) NKp46. Genotype analyses suggested that the clinical benefit of high NCR expression was restricted to patients with a missing ligand genotype and/or a KIR B/x genotype. These data imply that functional NK cells are significant anti-leukemic effector cells in patients with KIR/HLA genotypes that favor NK cell autoreactivity
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