11,603 research outputs found
An {\it ab initio} study of the magnetic and electronic properties of Fe, Co, and Ni nanowires on Cu(001) surface
Magnetism at the nanoscale has been a very active research area in the past
decades, because of its novel fundamental physics and exciting potential
applications. We have recently performed an {\it ab intio} study of the
structural, electronic and magnetic properties of all 3 transition metal
(TM) freestanding atomic chains and found that Fe and Ni nanowires have a giant
magnetic anisotropy energy (MAE), indicating that these nanowires would have
applications in high density magnetic data storages. In this paper, we perform
density functional calculations for the Fe, Co and Ni linear atomic chains on
Cu(001) surface within the generalized gradient approximation, in order to
investigate how the substrates would affect the magnetic properties of the
nanowires. We find that Fe, Co and Ni linear chains on Cu(001) surface still
have a stable or metastable ferromagnetic state. When spin-orbit coupling (SOC)
is included, the spin magnetic moments remain almost unchanged, due to the
weakness of SOC in 3 TM chains, whilst significant orbital magnetic moments
appear and also are direction-dependent. Finally, we find that the MAE for Fe,
and Co remains large, i.e., being not much affected by the presence of Cu
substrate.Comment: 4 pages, 2 figure
Analysis of the Flow About Delta Wings with Leading Edge Separation at Supersonic Speeds
A research program was conducted to develop an improved theoretical flow model for the flow about sharp edge delta wings with leading-edge separation at supersonic speeds. The flow model incorporates a representation of the secondary separation region which occurs just inboard of the leading edge on such wings and is based on a slender-wing theory whereby the full three-dimensional problem is reduced to a quasi two-dimensional problem in the cross-flow plane. The secondary separation region was modeled by a surface distribution of singularities or a linearized type of cavity representation. The primary vortex and separation were modeled by a concentrated vortex and cut in the cross-flow potential which represents its feeding sheet. The cross-flow solutions for the cavity model were obtained, but these solutions have physical significance only in a very restricted range of angle of attack. The reasons for the failure of the flow model are discussed. The analysis is presented so that other interested researchers may critically review the work
Heavy Quark Mass Effects in Deep Inelastic Scattering and Global QCD Analysis
A new implementation of the general PQCD formalism of Collins, including
heavy quark mass effects, is described. Important features that contribute to
the accuracy and efficiency of the calculation of both neutral current (NC) and
charged current (CC) processess are explicitly discussed. This new
implementation is applied to the global analysis of the full HERA I data sets
on NC and CC cross sections, with correlated systematic errors, in conjunction
with the usual fixed-target and hadron collider data sets. By using a variety
of parametrizations to explore the parton parameter space, robust new parton
distribution function (PDF) sets (CTEQ6.5) are obtained. The new quark
distributions are consistently higher in the region x ~ 10^{-3} than previous
ones, with important implications on hadron collider phenomenology, especially
at the LHC. The uncertainties of the parton distributions are reassessed and
are compared to the previous ones. A new set of CTEQ6.5 eigenvector PDFs that
encapsulates these uncertainties is also presented.Comment: 32 pages, 12 figures; updated, Publication Versio
Magnetic moment and magnetic anisotropy of linear and zigzag 4{\it d} and 5{\it d} transition metal nanowires: First-principles calculations
An extensive {\it ab initio} study of the physical properties of both linear
and zigzag atomic chains of all 4 and 5 transition metals (TM) within the
GGA by using the accurate PAW method, has been carried out. All the TM linear
chains are found to be unstable against the corresponding zigzag structures.
All the TM chains, except Nb, Ag and La, have a stable (or metastable) magnetic
state in either the linear or zigzag or both structures. Magnetic states appear
also in the sufficiently stretched Nb and La linear chains and in the largely
compressed Y and La chains. The spin magnetic moments in the Mo, Tc, Ru, Rh, W,
Re chains could be large (1.0 /atom). Structural transformation
from the linear to zigzag chains could suppress the magnetism already in the
linear chain, induce the magnetism in the zigzag structure, and also cause a
change of the magnetic state (ferromagnetic to antiferroamgetic or vice verse).
The calculations including the spin-orbit coupling reveal that the orbital
moments in the Zr, Tc, Ru, Rh, Pd, Hf, Ta, W, Re, Os, Ir and Pt chains could be
rather large (0.1 /atom). Importantly, large magnetic anisotropy
energy (1.0 meV/atom) is found in most of the magnetic TM chains,
suggesting that these nanowires could have fascinating applications in
ultrahigh density magnetic memories and hard disks. In particular, giant
magnetic anisotropy energy (10.0 meV/atom) could appear in the Ru, Re,
Rh, and Ir chains. Furthermore, the magnetic anisotropy energy in several
elongated linear chains could be as large as 40.0 meV/atom. A
spin-reorientation transition occurs in the Ru, Ir, Ta, Zr, La and Zr, Ru, La,
Ta and Ir linear chains when they are elongated. Remarkably, all the 5 as
well as Tc and Pd chains show the colossal magnetic anisotropy (i.e., it is
impossible to rotate magnetization into certain directions). Finally, the
electronic band structure and density of states of the nanowires have also been
calculated in order to understand the electronic origin of the large magnetic
anisotropy and orbital magnetic moment as well as to estimate the conduction
electron spin polarization.Comment: To appear in Phys. Rev.
A method to define a minimum-phase transfer function within the bounded region of phase-gain specifications
Method to define minimum phase transfer function within bounded region of phase gain specifications at several discrete frequencie
Quasi-Local Energy Flux of Spacetime Perturbation
A general expression for quasi-local energy flux for spacetime perturbation
is derived from covariant Hamiltonian formulation using functional
differentiability and symplectic structure invariance, which is independent of
the choice of the canonical variables and the possible boundary terms one
initially puts into the Lagrangian in the diffeomorphism invariant theories.
The energy flux expression depends on a displacement vector field and the
2-surface under consideration. We apply and test the expression in Vaidya
spacetime. At null infinity the expression leads to the Bondi type energy flux
obtained by Lindquist, Schwartz and Misner. On dynamical horizons with a
particular choice of the displacement vector, it gives the area balance law
obtained by Ashtekar and Krishnan.Comment: 8 pages, added appendix, version to appear in Phys. Rev.
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