366 research outputs found
Process mapping of laser surface modification of AISI 316L stainless steel for biomedical applications
A 1.5-kW CO2 laser in pulsed mode at 3 kHz was used to investigate the effects of varied laser process parameters and resulting morphology of AISI 316L stainless steel. Irradiance and residence time were varied between 7.9 to 23.6 MW/cm2 and 50 to 167 ”s respectively. A strong correlation between irradiance, residence time, depth of processing and roughness of processed steel was established. The high depth of altered microstructure and increased roughness were linked to higher levels of both irradiance and residence times. Energy fluence and surface temperature models were used to predict levels of melting occurring on the surface through the analysis of roughness and depth of the region processed. Microstructural images captured by the SEM revealed significant grain structure changes at higher irradiances, but due to increased residence times, limited to the laser in use, the hardness values were not improved
Persistence in higher dimensions : a finite size scaling study
We show that the persistence probability , in a coarsening system of
linear size at a time , has the finite size scaling form where is the persistence exponent and
is the coarsening exponent. The scaling function for
and is constant for large . The scaling form implies a fractal
distribution of persistent sites with power-law spatial correlations. We study
the scaling numerically for Glauber-Ising model at dimension to 4 and
extend the study to the diffusion problem. Our finite size scaling ansatz is
satisfied in all these cases providing a good estimate of the exponent
.Comment: 4 pages in RevTeX with 6 figures. To appear in Phys. Rev.
Slow Relaxation in a Constrained Ising Spin Chain: a Toy Model for Granular Compaction
We present detailed analytical studies on the zero temperature coarsening
dynamics in an Ising spin chain in presence of a dynamically induced field that
favors locally the `-' phase compared to the `+' phase. We show that the
presence of such a local kinetic bias drives the system into a late time state
with average magnetization m=-1. However the magnetization relaxes into this
final value extremely slowly in an inverse logarithmic fashion. We further map
this spin model exactly onto a simple lattice model of granular compaction that
includes the minimal microscopic moves needed for compaction. This toy model
then predicts analytically an inverse logarithmic law for the growth of density
of granular particles, as seen in recent experiments and thereby provides a new
mechanism for the inverse logarithmic relaxation. Our analysis utilizes an
independent interval approximation for the particle and the hole clusters and
is argued to be exact at late times (supported also by numerical simulations).Comment: 9 pages RevTeX, 1 figures (.eps
Universal Extra Dimensions and Kaluza Klein Bound States
We study the bound states of the Kaluza-Klein (KK) excitations of quarks in
certain models of Universal Extra Dimensions. Such bound states may be detected
at future lepton colliders in the cross section for the pair production of
KK-quarks near threshold. For typical values of model parameters, we find that
"KK-quarkonia" have widths in the 10 - 100 MeV range, and production cross
sections of order a few picobarns for the lightest resonances. Two body decays
of the constituent KK-quarks lead to distinctive experimental signatures. We
point out that such KK resonances may be discovered before any of the higher KK
modes.Comment: 21 pages LaTeX, 9 eps figure
Algebraic approach to quantum black holes: logarithmic corrections to black hole entropy
The algebraic approach to black hole quantization requires the horizon area
eigenvalues to be equally spaced. As shown previously, for a neutral
non-rotating black hole, such eigenvalues must be -fold degenerate if
one constructs the black hole stationary states by means of a pair of creation
operators subject to a specific algebra. We show that the algebra of these two
building blocks exhibits symmetry, where the area
operator generates the U(1) symmetry. The three generators of the SU(2)
symmetry represent a {\it global} quantum number (hyperspin) of the black hole,
and we show that this hyperspin must be zero. As a result, the degeneracy of
the -th area eigenvalue is reduced to for large , and
therefore, the logarithmic correction term should be added to the
Bekenstein-Hawking entropy. We also provide a heuristic approach explaining
this result, and an evidence for the existence of {\it two} building blocks.Comment: 15 pages, Revtex, to appear in Phys. Rev.
Cosmic Super-Strings and Kaluza-Klein Modes
Cosmic super-strings interact generically with a tower of relatively light
and/or strongly coupled Kaluza-Klein (KK) modes associated with the geometry of
the internal space. In this paper, we study the production of spin-2 KK
particles by cusps on loops of cosmic F- and D-strings. We consider cosmic
super-strings localized either at the bottom of a warped throat or in a flat
internal space with large volume. The total energy emitted by cusps in KK modes
is comparable in both cases, although the number of produced KK modes may
differ significantly. We then show that KK emission is constrained by the
photo-dissociation of light elements and by observations of the diffuse gamma
ray background. We show that this rules out regions of the parameter space of
cosmic super-strings that are complementary to the regions that can be probed
by current and upcoming gravitational wave experiments. KK modes are also
expected to play an important role in the friction-dominated epoch of cosmic
super-string evolution.Comment: 35pp, 5 figs, v2: minor modifications and Refs. added, matches
published versio
Focusing and the Holographic Hypothesis
The ``screen mapping" introduced by Susskind to implement 't Hooft's
holographic hypothesis is studied. For a single screen time, there are an
infinite number of images of a black hole event horizon, almost all of which
have smaller area on the screen than the horizon area. This is consistent with
the focusing equation because of the existence of focal points. However, the
{\it boundary} of the past (or future) of the screen obeys the area theorem,
and so always gives an expanding map to the screen, as required by the
holographic hypothesis. These considerations are illustrated with several
axisymmetric static black hole spacetimes.Comment: 8 pages, plain latex, 5 figures included using psfi
Supersymmetry of the 2+1 black holes
The supersymmetry properties of the asymptotically anti-de Sitter black holes
of Einstein theory in 2+1 dimensions are investigated. It is shown that (i) the
zero mass black hole has two exact super- symmetries; (ii) extreme
black holes with have only one; and (iii) generic black holes do
not have any. It is also argued that the zero mass hole is the ground state of
(1,1)-adS supergravity with periodic (``Ramond") boundary conditions on the
spinor fields.Comment: 9 pages LaTeX file, ULB-PMIF-93/0
Brans-Dicke Theory and primordial black holes in Early Matter-Dominated Era
We show that primordial black holes can be formed in the matter-dominated era
with gravity described by the Brans-Dicke theory. Considering an early
matter-dominated era between inflation and reheating, we found that the
primordial black holes formed during that era evaporate at a quicker than those
of early radiation-dominated era. Thus, in comparison with latter case, less
number of primordial black holes could exist today. Again the constraints on
primordial black hole formation tend towards the larger value than their
radiation-dominated era counterparts indicating a significant enhancement in
the formation of primordial black holes during the matter-dominaed era.Comment: 9 page
Critical properties of 1-D spin 1/2 antiferromagnetic Heisenberg model
We discuss numerical results for the 1-D spin 1/2 antiferromagnetic
Heisenberg model with next-to-nearest neighbour coupling and in the presence of
an uniform magnetic field. The model develops zero frequency excitations at
field dependent soft mode momenta. We compute critical quantities from finite
size dependence of static structure factors.Comment: talk given by H. Kr{\"o}ger at Heraeus Seminar Theory of Spin
Lattices and Lattice Gauge Models, Bad Honnef (1996), 20 pages, LaTeX + 18
figures, P
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