4,591 research outputs found
X-Ray Tomography To Measure Size Of Fragments From Penetration Of High-Velocity Tungsten Rods
Behind-armor debris that results from tungsten rods penetrating armor steel at 2 km/s was studied by analysis of recovered fragments. Fragment recovery was by means of particle board. Individual fragments were analyzed by x-ray tomography, which provides information for fragment identification, mass, shape, and penetration down to masses of a few milligrams. The experiments were complemented by AUTODYN and EPIC calculations. Fragments were steel or tungsten generated from the channel or from the breakout through the target rear surface. Channel fragment motions were well described by Tate theory. Breakout fragments had velocities from the projectile remnant to the channel velocity, apparently depending on where in the projectile a fragment originated. The fragment size distribution was extremely broad and did not correlate well with simple uniform-fragment-size models.Mechanical Engineerin
Levinson's theorem for the Schr\"{o}dinger equation in two dimensions
Levinson's theorem for the Schr\"{o}dinger equation with a cylindrically
symmetric potential in two dimensions is re-established by the Sturm-Liouville
theorem. The critical case, where the Schr\"{o}dinger equation has a finite
zero-energy solution, is analyzed in detail. It is shown that, in comparison
with Levinson's theorem in non-critical case, the half bound state for
wave, in which the wave function for the zero-energy solution does not decay
fast enough at infinity to be square integrable, will cause the phase shift of
wave at zero energy to increase an additional .Comment: Latex 11 pages, no figure and accepted by P.R.A (in August); Email:
[email protected], [email protected]
The Relativistic Levinson Theorem in Two Dimensions
In the light of the generalized Sturm-Liouville theorem, the Levinson theorem
for the Dirac equation in two dimensions is established as a relation between
the total number of the bound states and the sum of the phase shifts
of the scattering states with the angular momentum :
\noindent The critical case, where the Dirac equation has a finite
zero-momentum solution, is analyzed in detail. A zero-momentum solution is
called a half bound state if its wave function is finite but does not decay
fast enough at infinity to be square integrable.Comment: Latex 14 pages, no figure, submitted to Phys.Rev.A; Email:
[email protected], [email protected]
Levinson's Theorem for the Klein-Gordon Equation in Two Dimensions
The two-dimensional Levinson theorem for the Klein-Gordon equation with a
cylindrically symmetric potential is established. It is shown that
, where denotes
the difference between the number of bound states of the particle
and the ones of antiparticle with a fixed angular momentum , and
the is named phase shifts. The constants and
are introduced to symbol the critical cases where the half bound
states occur at .Comment: Revtex file 14 pages, submitted to Phys. Rev.
Quantum corrections in the Boltzmann conductivity of graphene and their sensitivity to the choice of formalism
Semiclassical spin-coherent kinetic equations can be derived from quantum
theory with many different approaches (Liouville equation based approaches,
nonequilibrium Green's functions techniques, etc.). The collision integrals
turn out to be formally different, but coincide in textbook examples as well as
for systems where the spin-orbit coupling is only a small part of the kinetic
energy like in related studies on the spin Hall effect. In Dirac cone physics
(graphene, surface states of topological insulators like Bi_{1-x}Sb_x, Bi_2Te_3
etc.), where this coupling constitutes the entire kinetic energy, the
difference manifests itself in the precise value of the electron-hole coherence
originated quantum correction to the Drude conductivity . The leading correction is derived analytically for single and multilayer
graphene with general scalar impurities. The often neglected principal value
terms in the collision integral are important. Neglecting them yields a leading
correction of order , whereas including them can give a
correction of order . The latter opens up a counterintuitive
scenario with finite electron-hole coherent effects at Fermi energies
arbitrarily far above the neutrality point regime, for example in the form of a
shift that only depends on the dielectric constant. This residual
conductivity, possibly related to the one observed in recent experiments,
depends crucially on the approach and could offer a setting for experimentally
singling out one of the candidates. Concerning the different formalisms we
notice that the discrepancy between a density matrix approach and a Green's
function approach is removed if the Generalized Kadanoff-Baym Ansatz in the
latter is replaced by an anti-ordered version.Comment: 31 pages, 1 figure. An important sign error has been rectified in the
principal value terms in equation (52) in the vN & NSO expression. It has no
implications for the results on the leading quantum correction studied in
this paper. However, for the higher quantum corrections studied in
arXiv:1304.3929 (see comment in the latter) the implications are crucia
Beyond locutionary denotations: exploring trust between practitioners and policy
This study reports the findings of a research on the trust relationship between practitioners in the Skills for Life (SfL) area and the policy that informs their practice. The exploration of this relationship was premised on an extended notion of trust relationship which draws from the Speech Act theory of Austin (1962; Searle 1969; Kissine 2008), leading to the claim that the existence of different layers of imports in textual analysis makes it possible for a trust relationship to exist between the human/physical and the non human/non physical. The study found that the majority of practitioners in the SfL field trust policy to deliver its inherent policy only to a limited extent. Amongst others, the study identified the impact of the perlocutionary import of policy text on practitioners as a viable reason for this limited level of trust. Such perlocutionary imports, it also found, have adverse impact on practitioners who are considered to have drawn from previous experience to mediate the import of contemporary policies
Comments on gluon scattering amplitudes via AdS/CFT
In this article we consider n gluon color ordered, planar amplitudes in N=4
super Yang Mills at strong 't Hooft coupling. These amplitudes are approximated
by classical surfaces in AdS_5 space. We compute the value of the amplitude for
a particular kinematic configuration for a large number of gluons and find that
the result disagrees with a recent guess for the exact value of the amplitude.
Our results are still compatible with a possible relation between amplitudes
and Wilson loops.
In addition, we also give a prescription for computing processes involving
local operators and asymptotic states with a fixed number of gluons. As a
byproduct, we also obtain a string theory prescription for computing the dual
of the ordinary Wilson loop, Tr P exp[ i\oint A ], with no couplings to the
scalars. We also evaluate the quark-antiquark potential at two loops.Comment: 27 pages, 9 figures,v3:minor correction
Short--range impurity in the vicinity of a saddle point and the levitation of the 2D delocalized states in a magnetic field
The effect of a short--range impurity on the transmission through a
saddle--point potential for an electron, moving in a strong magnetic field, is
studied. It is demonstrated that for a random position of an impurity and
random sign of its potential the impurity--induced mixing of the Landau levels
diminishes {\em on average} the transmission coefficient. This results in an
upward shift (levitation) of the energy position of the delocalized state in a
smooth potential. The magnitude of the shift is estimated. It increases with
decreasing magnetic field as .Comment: LaTeX, 20 page
External shock model for the large-scale, relativistic X-ray jets from the microquasar XTE J1550-564
Large-scale, decelerating, relativistic X-ray jets due to material ejected
from the black-hole candidate X-ray transient and microquasar XTE J1550-564 has
been recently discovered with Chandra by Corbel et al. (2002). We find that the
dynamical evolution of the eastern jet at the late time is consistent with the
well-known Sedov evolutionary phase. A trans-relativistic external shock
dynamic model by analogy with the evolution of gamma-ray burst remnants, is
shown to be able to fit the observation data reasonably well. The inferred
interstellar medium density around the source is well below the canonical value
. We find that the emission from the continuously shocked
interstellar medium (forward shock region) decays too slowly to be a viable
mechanism for the eastern X-ray jet. However, the rapidly fading X-ray emission
can be interpreted as synchrotron radiation from the non-thermal electrons in
the adiabatically expanding ejecta. These electrons were accelerated by the
reverse shock (moving back into the ejecta) which becomes important when the
inertia of the swept external matter leads to an appreciable slowing down of
the original ejecta. To ensure the dominance of the emission from the shocked
ejecta over that from the forward shock region during the period of the
observations, the magnetic field and electron energy fractions in the forward
shock region must be far below equipartition. Future continuous, follow-up
multi-wavelength observations of new ejection events from microquasars up to
the significant deceleration phase should provide more valuable insight into
the nature of the interaction between the jets and external medium.Comment: Accepted for publication in ApJ, minor changes; 11 pages, including 5
figure
Reorientation transition of ultrathin ferromagnetic films
We demonstrate that the reorientation transition from out-of-plane to
in-plane magnetization with decreasing temperature as observed experimentally
in Ni-films on Cu(001) can be explained on a microscopic basis. Using a
combination of mean field theory and perturbation theory, we derive an analytic
expression for the temperature dependent anisotropy. The reduced magnetization
in the film surface at finite temperatures plays a crucial role for this
transition as with increasing temperature the influence of the uniaxial
anisotropies is reduced at the surface and is enhanced inside the film.Comment: 4 pages(RevTeX), 3 figures (EPS
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