4,249 research outputs found
On the miscible Rayleigh-Taylor instability: two and three dimensions
We investigate the miscible Rayleigh-Taylor (RT) instability in both 2 and 3
dimensions using direct numerical simulations, where the working fluid is
assumed incompressible under the Boussinesq approximation. We first consider
the case of randomly perturbed interfaces. With a variety of diagnostics, we
develop a physical picture for the detailed temporal development of the mixed
layer: We identify three distinct evolutionary phases in the development of the
mixed layer, which can be related to detailed variations in the growth of the
mixing zone. Our analysis provides an explanation for the observed differences
between two and three-dimensional RT instability; the analysis also leads us to
concentrate on the RT models which (1) work equally well for both laminar and
turbulent flows, and (2) do not depend on turbulent scaling within the mixing
layer between fluids. These candidate RT models are based on point sources
within bubbles (or plumes) and interaction with each other (or the background
flow). With this motivation, we examine the evolution of single plumes, and
relate our numerical results (of single plumes) to a simple analytical model
for plume evolution.Comment: 31 pages, 27 figures, to appear in November issue of JFM, 2001. For
better figures: http://astro.uchicago.edu/~young/ps/jfmtry08.ps.
Non-factorizable effects in top quark production
The production of top-antitop pairs at the Fermilab Tevatron shows a
forward-backward asymmetry in which the top quark tends to follow the proton
direction, while the antitop tends to follow the antiproton direction. The
effect grows with increasing effective mass of the top-antitop
pair, and with increasing rapidity difference between the top and antitop. The
observed effect is about three times as large as predicted by
next-to-leading-order QCD, but with the same sign. An estimate of
non-factorizable effects based on a QCD string picture finds they are
negligible, but that small distortions of the spectrum are possible.
Tests for such effects, both at and above the level of this estimate, are
suggested.Comment: 7 pages, 1 figure. Added text and references. Submitted to Phys. Rev.
Magnetic pyroxenes LiCrGe2O6 and LiCrSi2O6: dimensionality crossover in a non-frustrated S=3/2 Heisenberg model
The magnetism of magnetoelectric = 3/2 pyroxenes LiCrSiO and
LiCrGeO is studied by density functional theory (DFT) calculations,
quantum Monte Carlo (QMC) simulations, neutron diffraction, as well as
low-field and high-field magnetization measurements. In contrast with earlier
reports, we find that the two compounds feature remarkably different, albeit
non-frustrated magnetic models. In LiCrSiO, two relevant exchange
integrals, 9 K along the structural chains and
2 K between the chains, form a 2D anisotropic honeycomb lattice. In
contrast, the spin model of LiCrGeO is constituted of three different
exchange couplings. Surprisingly, the leading exchange
2.3 K operates between the chains, while 1.2 K is about
two times smaller. The additional interlayer coupling
renders this model 3D. QMC simulations reveal excellent agreement between
our magnetic models and the available experimental data. Underlying mechanisms
of the exchange couplings, magnetostructural correlations, as well as
implications for other pyroxene systems are discussed.Comment: 11 pages, 8 figures, 3 tables + Supplementary informatio
Vibrational Modes in LiBC: Theory Compared with Experiment
The search for other superconductors in the MgB2 class currently is focussed
on Li{1-x}BC, which when hole-doped (concentration x) should be a metal with
the potential to be a better superconductor than MgB2. Here we present the
calculated phonon spectrum of the parent semiconductor LiBC. The calculated
Raman-active modes are in excellent agreement with a recent observation, and
comparison of calculated IR-active modes with a recent report provides a
prediction of the LO--TO splitting for these four modes, which is small for the
B-C bond stretching mode at ~1200 cm^{-1}, but large for clearly resolved modes
at 540 cm^{-1} and 620 cm^{-1}.Comment: 4 pages, two embedded figures. Physica B (in press
Determining the Quark Mixing Matrix From CP-Violating Asymmetries
If the Standard Model explanation of CP violation is correct, then
measurements of CP-violating asymmetries in meson decays can in principle
determine the entire quark mixing matrix.Comment: 8 pages (plain TeX), 1 figure (postscript file appended), DAPNIA/SPP
94-06, NSF-PT-94-2, UdeM-LPN-TH-94-18
Role of C in MgC_xNi_3 investigated from first principles
The influence of vacancies in the sub-lattice of , on its
structural, electronic and magnetic properties are studied by means of the
density-functional based Korringa-Kohn-Rostoker Green's function method
formulated in the atomic sphere approximation. Disorder is taken into account
by means of coherent-potential approximation. Characterizations representing
the change in the lattice properties include the variation in the equilibrium
lattice constants, bulk modulus and pressure derivative of the bulk modulus,
and that of electronic structure include the changes in the, total, partial and
-resolved density of states. The incipient magnetic properties are
studied by means of fixed-spin moment method of alloy theory, together in
conjunction with the phenomenological Ginzburg-Landau equation for magnetic
phase transition. The first-principles calculations reveal that due to the
breaking of the - bonds, some of the 3d states, which were lowered
in energy due to strong hybridization, are transfered back to higher energies
thereby increasing the itinerant character in the material. The Bloch spectral
densities evaluated at the high symmetry points however reveal that the charge
redistribution is not uniform over the cubic Brillouin zone, as new states are
seen to be created at the point, while a shift in the states on the
energy scale are seen at other high symmetry points
Low-Mass Baryon-Antibaryon Enhancements in B Decays
The nature of low-mass baryon-antibaryon enhancements seen in B decays is
explored. Three possibilities include (i) states near threshold as found in a
model by Nambu and Jona-Lasinio, (ii) isoscalar states with coupled to a pair of gluons, and (iii) low-mass enhancements favored by the
fragmentation process. Ways of distinguishing these mechanisms using angular
distributions and flavor symmetry are proposed.Comment: 8 pages, LaTeX, no figures, to be submitted to Phys. Rev. D. One
reference adde
Signatures of Coronal Heating Mechanisms
Alfven waves created by sub-photospheric motions or by magnetic reconnection
in the low solar atmosphere seem good candidates for coronal heating. However,
the corona is also likely to be heated more directly by magnetic reconnection,
with dissipation taking place in current sheets. Distinguishing observationally
between these two heating mechanisms is an extremely difficult task. We perform
1.5-dimensional MHD simulations of a coronal loop subject to each type of
heating and derive observational quantities that may allow these to be
differentiated.Comment: To appear in "Magnetic Coupling between the Interior and the
Atmosphere of the Sun", eds. S.S. Hasan and R.J. Rutten, Astrophysics and
Space Science Proceedings, Springer-Verlag, Heidelberg, Berlin, 200
Fermi Surfaces of Diborides: MgB2 and ZrB2
We provide a comparison of accurate full potential band calculations of the
Fermi surfaces areas and masses of MgB2 and ZrB2 with the de Haas-van Alphen
date of Yelland et al. and Tanaka et al., respectively. The discrepancies in
areas in MgB2 can be removed by a shift of sigma-bands downward with respect to
pi-bands by 0.24 eV. Comparison of effective masses lead to orbit averaged
electron-phonon coupling constants lambda(sigma)=1.3 (both orbits),
lambda(pi)=0.5. The required band shifts, which we interpret as an exchange
attraction for sigma states beyond local density band theory, reduces the
number of holes from 0.15 to 0.11 holes per cell. This makes the occurrence of
superconductivity in MgB2 a somewhat closer call than previously recognized,
and increases the likelihood that additional holes can lead to an increased Tc.Comment: 7 pages including 4 figure
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