25,787 research outputs found
Sputtered gold mask for deep chemical etching of silicon
Sputtered mask resists chemical attack from acid and has adherence to withstand prolonged submergence in etch solution without lifting from silicon surface. Even under prolonged etch conditions with significant undercutting, gold mask maintained excellent adhesion to silicon surface and imperviousness to acid
Confinement in the 3-dimensional Gross-Neveu model
We consider the -components 3-dimensional massive Gross-Neveu model
compactified in one spatial direction, the system being constrained to a slab
of thickness . We derive a closed formula for the effective renormalized
-dependent coupling constant in the large-N limit, using bag-model boundary
conditions. For values of the fixed coupling constant in absence of boundaries
, we obtain ultra-violet asymptotic
freedom (for ) and confinement for a length such that , being the fermionic mass. Taking
for an average of the masses of the quarks composing the proton, we obtain
a confining legth which is comparable with an estimated proton
diameter.Comment: Latex, 4 pages, 2 figures (one new), some changes in tex
The stability of the spectator, Dirac, and Salpeter equations for mesons
Mesons are made of quark-antiquark pairs held together by the strong force.
The one channel spectator, Dirac, and Salpeter equations can each be used to
model this pairing. We look at cases where the relativistic kernel of these
equations corresponds to a time-like vector exchange, a scalar exchange, or a
linear combination of the two. Since the model used in this paper describes
mesons which cannot decay physically, the equations must describe stable
states. We find that this requirement is not always satisfied, and give a
complete discussion of the conditions under which the various equations give
unphysical, unstable solutions
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The effect of treatment on pathogen virulence.
The optimal virulence of a pathogen is determined by a trade-off between maximizing the rate of transmission and maximizing the duration of infectivity. Treatment measures such as curative therapy and case isolation exert selective pressure by reducing the duration of infectivity, reducing the value of duration-increasing strategies to the pathogen and favoring pathogen strategies that maximize the rate of transmission. We extend the trade-off models of previous authors, and represents the reproduction number of the pathogen as a function of the transmissibility, host contact rate, disease-induced mortality, recovery rate, and treatment rate, each of which may be influenced by the virulence. We find that when virulence is subject to a transmissibility-mortality trade-off, treatment can lead to an increase in optimal virulence, but that in other scenarios (such as the activity-recovery trade-off) treatment decreases the optimal virulence. Paradoxically, when levels of treatment rise with pathogen virulence, increasing control efforts may raise predicted levels of optimal virulence. Thus we show that conflict can arise between the epidemiological benefits of treatment and the evolutionary risks of heightened virulence
Mid-IR continuous-wave fiber-laser-pumped optical parametric oscillators
We review recent developments in continuous-wave mid-infrared optical parametric oscillators pumped by fiber lasers. Such devices are potentially valuable spectroscopic sources providing high output powers and rapid, wide-range tuning in the mid-infrared molecular fingerprint region
Detection of the tagged or untagged photons in acousto-optic imaging of thick highly scattering media by photorefractive adaptive holography
We propose an original adaptive wavefront holographic setup based on the
photorefractive effect (PR), to make real-time measurements of acousto-optic
signals in thick scattering media, with a high flux collection at high rates
for breast tumor detection. We describe here our present state of art and
understanding on the problem of breast imaging with PR detection of the
acousto-optic signal
Vortices in Bose-Einstein-Condensed Atomic Clouds
The properties of vortex states in a Bose-Einstein condensed cloud of atoms
are considered at zero temperature. Using both analytical and numerical methods
we solve the time-dependent Gross-Pitaevskii equation for the case when a cloud
of atoms containing a vortex is released from a trap. In two dimensions we find
the simple result that the time dependence of the cloud radius is given by
, where is the trap frequency. We calculate and
compare the expansion of the vortex core and the cloud radius for different
numbers of particles and interaction strengths, in both two and three
dimensions, and discuss the circumstances under which vortex states may be
observed experimentally.Comment: Revtex, 11 pages including 5 eps figures, submitted to Phys. Rev. A;
new reference added, remark added in Sec. IIIB, axis label added in Fig.
Calculating the Rest Tension for a Polymer of String Bits
We explore the application of approximation schemes from many body physics,
including the Hartree-Fock method and random phase approximation (RPA), to the
problem of analyzing the low energy excitations of a polymer chain made up of
bosonic string bits. We accordingly obtain an expression for the rest tension
of the bosonic relativistic string in terms of the parameters
characterizing the microscopic string bit dynamics. We first derive an exact
connection between the string tension and a certain correlation function of the
many-body string bit system. This connection is made for an arbitrary
interaction potential between string bits and relies on an exact dipole sum
rule. We then review an earlier calculation by Goldstone of the low energy
excitations of a polymer chain using RPA. We assess the accuracy of the RPA by
calculating the first order corrections. For this purpose we specialize to the
unique scale invariant potential, namely an attractive delta function potential
in two (transverse) dimensions. We find that the corrections are large, and
discuss a method for summing the large terms. The corrections to this improved
RPA are roughly 15\%.Comment: 44 pages, phyzzx, psfig required, Univ. of Florida preprint,
UFIFT-HEP-94
Macroscopic Quantum Tunneling of a Bose-Einstein Condensate with Attractive Interaction
A Bose-Einstein condensate with attractive interaction can be metastable if
it is spatially confined and if the number of condensate bosons is below
a certain critical value . By applying a variational method and the
instanton techinique to the Gross-Pitaevskii energy functional, we find
analytically the frequency of the collective excitation and the rate of
macroscopic quantum tunneling (MQT). We show that near the critical point the
tunneling exponent vanishes according to and that MQT
can be a dominant decay mechanism of the condensate for very close to
.Comment: RevTex 4 pages with 1 postscript figure. Accepted for publication in
Physical Review Letter
Time-dependent Gross-Pitaevskii equation for composite bosons as the strong-coupling limit of the fermionic BCS-RPA approximation
The linear response to a space- and time-dependent external disturbance of a
system of dilute condensed composite bosons at zero temperature, as obtained
from the linearized version of the time-dependent Gross-Pitaevskii equation, is
shown to result also from the strong-coupling limit of the time-dependent BCS
(or broken-symmetry RPA) approximation for the constituent fermions subject to
the same external disturbance. In this way, it is possible to connect
excited-state properties of the bosonic and fermionic systems by placing the
Gross-Pitaevskii equation in perspective with the corresponding fermionic
approximationsComment: 4 pages, 1 figur
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