33,618 research outputs found
Cluster expansion for dimerized spin systems
We have studied dimerized spin systems by realizing the cluster expansion to
high order. We have extended our previous dimer expansion for one-dimensional
systems to cover weakly interacting chains for a quantitative description of
three dimensional materials like PHCC and KCuCl_3. By comparison with recent
inelastic neutron scattering data we are able to determine the exchange
energies between individual spins. We have further investigated the
incommensurate region of zigzag chains with isotropic exchange coupling
constants near the disorder-line where the dispersion curve exhibits a minimum
at a finite wavevector. Our approach clearly shows the gradual transition
between the minimum of the dispersion at wavevector 0 and wavevector Pi within
this region. The extent of the incommensurate regime is given analytically in
an expansion in the coupling constants.Comment: 3 pages, 3 figures; contribution to ICNS2001; uses svjour.clo,
svglobal.clo (included
A Modified "Bottom-up" Thermalization in Heavy Ion Collisions
In the initial stage of the bottom-up picture of thermalization in heavy ion
collisions, the gluon distribution is highly anisotropic which can give rise to
plasma instability. This has not been taken account in the original paper. It
is shown that in the presence of instability there are scaling solutions, which
depend on one parameter, that match smoothly onto the late stage of bottom-up
when thermalization takes place.Comment: 8 pages and 1 embedded figure, talk presented at the Workshop on
"Quark-Gluon Plasma Thermalization", Vienna, Austria, 10-12 August 200
The Aoki phase for N_f=2 Wilson fermions revisited
We report on a numerical reinvestigation of the Aoki phase in full lattice
QCD with two flavors of unimproved Wilson fermions. For zero temperature the
Aoki phase can be confirmed at inverse coupling and ,
but not at and . At non-zero temperature the Aoki phase
was found to exist also at .Comment: 3 pages LaTex, 5 figures. Parallel talk at Lattice2003(Theory
Non-Gaussian Correlations in the McLerran-Venugopalan Model
We argue that the statistical weight function W[rho] appearing in the
McLerran-Venugopalan model of a large nucleus is intrinsically non-Gaussian,
even if we neglect quantum corrections. Based on the picture where the nucleus
of radius R consists of a collection of color-neutral nucleons, each of radius
a<<R, we show that to leading order in alpha_s and a/R only the Gaussian part
of W[rho] enters into the final expression for the gluon number density. Thus,
the existing results in the literature which assume a Gaussian weight remain
valid.Comment: 21 pages with 4 figures (revtex
N/P GaAs concentrator solar cells with an improved grid and bushbar contact design
The major requirements for a solar cell used in space applications are high efficiency at AMO irradiance and resistance to high energy radiation. Gallium arsenide, with a band gap of 1.43 eV, is one of the most efficient sunlight to electricity converters (25%) when the the simple diode model is used to calculate efficiencies at AMO irradiance, GaAs solar cells are more radiation resistant than silicon solar cells and the N/P GaAs device has been reported to be more radiation resistant than similar P/N solar cells. This higher resistance is probably due to the fact that only 37% of the current is generated in the top N layer of the N/P cell compared to 69% in the top layer of a P/N solar cell. This top layer of the cell is most affected by radiation. It has also been theoretically calculated that the optimized N/P device will prove to have a higher efficiency than a similar P/N device. The use of a GaP window layer on a GaAs solar cell will avoid many of the inherent problems normally associated with a GaAlAs window while still proving good passivation of the GaAs surface. An optimized circular grid design for solar cell concentrators has been shown which incorporates a multi-layer metallization scheme. This multi-layer design allows for a greater current carrying capacity for a unit area of shading, which results in a better output efficiency
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