2,790 research outputs found
Mach-Number Measurement with Laser and Pressure Probes in Humid Supersonic Flow
Mach-number measurements using a nonintrusive optical technique, laser-induced thermal acoustics (LITA), are compared to pressure probes in humid supersonic airflow. The two techniques agree well in dry flow (-35 C dew point), but LITA measurements show about five times larger fractional change in Mach number than that of the pressure-probe when water is purposefully introduced into the flow. Possible reasons for this discrepancy are discussed
The unique chemical reactivity of a graphene nanoribbon's zigzag edge
The zigzag edge of a graphene nanoribbon possesses a unique electronic state
that is near the Fermi level and localized at the edge carbon atoms. We
investigate the chemical reactivity of these zigzag edge sites by examining
their reaction energetics with common radicals from first principles. A
"partial radical" concept for the edge carbon atoms is introduced to
characterize their chemical reactivity, and the validity of this concept is
verified by comparing the dissociation energies of edge-radical bonds with
similar bonds in molecules. In addition, the uniqueness of the zigzag-edged
graphene nanoribbon is further demonstrated by comparing it with other forms of
sp2 carbons, including a graphene sheet, nanotubes, and an armchair-edged
graphene nanoribbon.Comment: 24 pages, 9 figure
Focal-Plane Imaging of Crossed Beams in Nonlinear Optics Experiments
An application of focal-plane imaging that can be used as a real time diagnostic of beam crossing in various optical techniques is reported. We discuss two specific versions and demonstrate the capability of maximizing system performance with an example in a combined dual-pump coherent anti-Stokes Raman scattering interferometric Rayleigh scattering experiment (CARS-IRS). We find that this imaging diagnostic significantly reduces beam alignment time and loss of CARS-IRS signals due to inadvertent misalignments
Magnetic anisotropy and spin-spiral wave in V, Cr and Mn atomic chains on Cu(001) surface: First principles calculations
Recent ab intio studies of the magnetic properties of all 3d transition
metal(TM) freestanding atomic chains predicted that these nanowires could have
a giant magnetic anisotropy energy (MAE) and might support a spin-spiral
structure, thereby suggesting that these nanowires would have technological
applicationsin, e.g., high density magnetic data storages. In order to
investigate how the substrates may affect the magnetic properties of the
nanowires, here we systematically study the V, Cr and Mn linear atomic chains
on the Cu(001) surface based on the density functional theory with the
generalized gradient approximation. We find that V, Cr, and Mn linear chains on
the Cu(001) surface still have a stable or metastable ferromagnetic state.
However, the ferromagnetic state is unstable against formation of a
noncollinear spin-spiral structure in the Mn linear chains and also the V
linear chain on the atop sites on the Cu(001) surface, due to the frustrated
magnetic interactions in these systems. Nonetheless, the presence of the
Cu(001) substrate does destabilize the spin-spiral state already present in the
freestanding V linear chain and stabilizes the ferromagnetic state in the V
linear chain on the hollow sites on Cu(001). When spin-orbit coupling (SOC) is
included, the spin magnetic moments remain almost unchanged, due to the
weakness of SOC in 3d TM chains. Furthermore, both the orbital magnetic moments
and MAEs for the V, Cr and Mn are small, in comparison with both the
corresponding freestanding nanowires and also the Fe, Co and Ni linear chains
on the Cu (001) surface.Comment: Accepted for publication in J. Phys. D: Applied Physic
Designing a Regulatory and Supervisory Framework for Integrated Financial Markets
The financial crisis that started in 2007 casts doubt about the ability of national laws and
competent authorities to manage the stability of the financial system and to protect investors.
This is due to the relevant evolving features of financial intermediation, like the cross-border
strategies in banking, with many M&A undertaken, especially in Europe, and more in general
the globalization of finance, also through the many recent operations among exchanges. The
associated regulatory and supervisory challenges have proved to be difficult to tackle.
An international perspective is needed on single banking regulatory instruments, even if it is
impossible at this stage to imagine unique rules and single international authorities managing
capital ratios, deposit insurance, reserve requirements and lending of last resort, as well as other
tools for providing financial markets stability. However, some common principles on regulation
and the structure of supervision may be stated both in US and in Europe: we suggest a “four
peak” approach to the matter.The financial crisis that started in 2007 casts doubt about the ability of national laws and
competent authorities to manage the stability of the financial system and to protect investors.
This is due to the relevant evolving features of financial intermediation, like the cross-border
strategies in banking, with many M&A undertaken, especially in Europe, and more in general
the globalization of finance, also through the many recent operations among exchanges. The
associated regulatory and supervisory challenges have proved to be difficult to tackle.
An international perspective is needed on single banking regulatory instruments, even if it is
impossible at this stage to imagine unique rules and single international authorities managing
capital ratios, deposit insurance, reserve requirements and lending of last resort, as well as other
tools for providing financial markets stability. However, some common principles on regulation
and the structure of supervision may be stated both in US and in Europe: we suggest a “four
peak” approach to the matter.Refereed Working Papers / of international relevanc
Shock-Strength Determination With Seeded and Seedless Laser Methods
Two nonintrusive laser diagnostics were independently used to demonstrate the measurement of time-averaged and spatially-resolved pressure change across a twodimensional (2-D) shock wave. The first method is Doppler global velocimetry (DGV) which uses water seeding and generates 2-D maps of 3-orthogonal components of velocity. A DGV-measured change in flow direction behind an oblique shock provides an indirect determination of pressure jump across the shock, when used with the known incoming Mach number and ideal shock relations (or Prandtl-Meyer flow equations for an expansion fan). This approach was demonstrated at Mach 2 on 2-D shocks and expansions generated from a flat plate at angles-of-attack approx. equals -2.4deg and +0.6deg, respectively. This technique also works for temperature jump (as well as pressure) and for normal shocks (as well as oblique). The second method, laser-induced thermal acoustics (LITA), is a seedless approach that was used to generate 1-D spatial profiles of streamwise Mach number, sound speed, pressure, and temperature across the same shock waves. Excellent agreement was obtained between the DGV and LITA methods, suggesting that either technique is viable for noninvasive shock-strength measurements
Non-collinear coupling between magnetic adatoms in carbon nanotubes
The long range character of the exchange coupling between localized magnetic
moments indirectly mediated by the conduction electrons of metallic hosts often
plays a significant role in determining the magnetic order of low-dimensional
structures. In addition to this indirect coupling, here we show that the direct
exchange interaction that arises when the moments are not too far apart may
induce a non-collinear magnetic order that cannot be characterized by a
Heisenberg-like interaction between the magnetic moments. We argue that this
effect can be manipulated to control the magnetization alignment of magnetic
dimers adsorbed to the walls of carbon nanotubes.Comment: 13 pages, 5 figures, submitted to PR
Demonstration of Imaging Flow Diagnostics Using Rayleigh Scattering in Langley 0.3-Meter Transonic Cryogenic Tunnel
The feasibility of using the Rayleigh scattering technique for molecular density imaging of the free-stream flow field in the Langley 0.3-Meter Transonic Cryogenic Tunnel has been experimentally demonstrated. The Rayleigh scattering was viewed with a near-backward geometry with a frequency-doubled output from a diode-pumped CW Nd:YAG laser and an intensified charge-coupled device camera. Measurements performed in the range of free-stream densities from 3 x 10(exp 25) to 24 x 10(exp 25) molecules/cu m indicate that the observed relative Rayleigh signal levels are approximately linear with flow field density. The absolute signal levels agree (within approx. 30 percent) with the expected signal levels computed based on the well-known quantities of flow field density, Rayleigh scattering cross section for N2, solid angle of collection, transmission of the optics, and the independently calibrated camera sensitivity. These results show that the flow field in this facility is primarily molecular (i.e., not contaminated by clusters) and that Rayleigh scattering is a viable technique for quantitative nonintrusive diagnostics in this facility
On the Geometry of Surface Stress
We present a fully general derivation of the Laplace--Young formula and
discuss the interplay between the intrinsic surface geometry and the extrinsic
one ensuing from the immersion of the surface in the ordinary euclidean
three-dimensional space. We prove that the (reversible) work done in a general
surface deformation can be expressed in terms of the surface stress tensor and
the variation of the intrinsic surface metric
Dewetting of an ultrathin solid film on a lattice-matched or amorphous substrate
An evolution partial differential equation for the surface of a non-wetting
single-crystal film in an attractive substrate potential is derived and used to
study the dynamics of a pinhole for the varying initial depth of a pinhole and
the strengths of the potential and the surface energy anisotropy. The results
of the simulations demonstrate how the corresponding parameters may lead to
complete or partial dewetting of the film. Anisotropy of the surface energy,
through faceting of the pinhole walls, is found to most drastically affect the
time to film rupture. In particular, the similations support the conjecture
that the strong anisotropy is capable of the complete suppression of dewetting
even when the attractive substrate potential is strong.Comment: Submitted to PR
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