553 research outputs found
Casimir interaction between two concentric cylinders: exact versus semiclassical results
The Casimir interaction between two perfectly conducting, infinite,
concentric cylinders is computed using a semiclassical approximation that takes
into account families of classical periodic orbits that reflect off both
cylinders. It is then compared with the exact result obtained by the
mode-by-mode summation technique. We analyze the validity of the semiclassical
approximation and show that it improves the results obtained through the
proximity theorem.Comment: 28 pages, 5 figures include
Phase transitions in BaTiO from first principles
We develop a first-principles scheme to study ferroelectric phase transitions
for perovskite compounds. We obtain an effective Hamiltonian which is fully
specified by first-principles ultra-soft pseudopotential calculations. This
approach is applied to BaTiO, and the resulting Hamiltonian is studied
using Monte Carlo simulations. The calculated phase sequence, transition
temperatures, latent heats, and spontaneous polarizations are all in good
agreement with experiment. The order-disorder vs.\ displacive character of the
transitions and the roles played by different interactions are discussed.Comment: 13 page
Low-Energy Photon-Photon Collisions to Two-Loop Order
We evaluate the amplitude for to two
loops in chiral perturbation theory. The three new counterterms which enter at
this order in the low-energy expansion are estimated with resonance saturation.
We find that the cross section agrees rather well with the available data and
with dispersion theoretic calculations even substantially above threshold.
Numerical results for the Compton cross section and for the neutral pion
polarizabilities are also given to two-loop accuracy.Comment: 48 pages, LaTex, 11 figs. (figures not included; available upon
request from [email protected]),BUTP-93/18,LNF-93/077(P),PSI-PR-93-1
Existence of Weak Solutions for a Diffuse Interface Model for Two-Phase Flows of Incompressible Fluids with Different Densities
We prove existence of weak solutions for a diffuse interface model for the
flow of two viscous incompressible Newtonian fluids in a bounded domain in two
and three space dimensions. In contrast to previous works, we study a new model
recently developed by Abels, Garcke, and Gr\"un for fluids with different
densities, which leads to a solenoidal velocity field. The model is given by a
non-homogeneous Navier-Stokes system with a modified convective term coupled to
a Cahn-Hilliard system. The density of the mixture depends on an order
parameter.Comment: 33 page
Normal and Lateral Casimir Forces between Deformed Plates
The Casimir force between macroscopic bodies depends strongly on their shape
and orientation. To study this geometry dependence in the case of two deformed
metal plates, we use a path integral quantization of the electromagnetic field
which properly treats the many-body nature of the interaction, going beyond the
commonly used pairwise summation (PWS) of van der Waals forces. For arbitrary
deformations we provide an analytical result for the deformation induced change
in Casimir energy, which is exact to second order in the deformation amplitude.
For the specific case of sinusoidally corrugated plates, we calculate both the
normal and the lateral Casimir forces. The deformation induced change in the
Casimir interaction of a flat and a corrugated plate shows an interesting
crossover as a function of the ratio of the mean platedistance H to the
corrugation length \lambda: For \lambda \ll H we find a slower decay \sim
H^{-4}, compared to the H^{-5} behavior predicted by PWS which we show to be
valid only for \lambda \gg H. The amplitude of the lateral force between two
corrugated plates which are out of registry is shown to have a maximum at an
optimal wavelength of \lambda \approx 2.5 H. With increasing H/\lambda \gtrsim
0.3 the PWS approach becomes a progressively worse description of the lateral
force due to many-body effects. These results may be of relevance for the
design and operation of novel microelectromechanical systems (MEMS) and other
nanoscale devices.Comment: 20 pages, 5 figure
Superization of Homogeneous Spin Manifolds and Geometry of Homogeneous Supermanifolds
Let M_0=G_0/H be a (pseudo)-Riemannian homogeneous spin manifold, with
reductive decomposition g_0=h+m and let S(M_0) be the spin bundle defined by
the spin representation Ad:H->\GL_R(S) of the stabilizer H. This article
studies the superizations of M_0, i.e. its extensions to a homogeneous
supermanifold M=G/H whose sheaf of superfunctions is isomorphic to
Lambda(S^*(M_0)). Here G is the Lie supergroup associated with a certain
extension of the Lie algebra of symmetry g_0 to an algebra of supersymmetry
g=g_0+g_1=g_0+S via the Kostant-Koszul construction. Each algebra of
supersymmetry naturally determines a flat connection nabla^{S} in the spin
bundle S(M_0). Killing vectors together with generalized Killing spinors (i.e.
nabla^{S}-parallel spinors) are interpreted as the values of appropriate
geometric symmetries of M, namely even and odd Killing fields. An explicit
formula for the Killing representation of the algebra of supersymmetry is
obtained, generalizing some results of Koszul. The generalized spin connection
nabla^{S} defines a superconnection on M, via the super-version of a theorem of
Wang.Comment: 50 page
Anisotropic flow of charged hadrons, pions and (anti-)protons measured at high transverse momentum in Pb-Pb collisions at TeV
The elliptic, , triangular, , and quadrangular, , azimuthal
anisotropic flow coefficients are measured for unidentified charged particles,
pions and (anti-)protons in Pb-Pb collisions at TeV
with the ALICE detector at the Large Hadron Collider. Results obtained with the
event plane and four-particle cumulant methods are reported for the
pseudo-rapidity range at different collision centralities and as a
function of transverse momentum, , out to GeV/.
The observed non-zero elliptic and triangular flow depends only weakly on
transverse momentum for GeV/. The small dependence
of the difference between elliptic flow results obtained from the event plane
and four-particle cumulant methods suggests a common origin of flow
fluctuations up to GeV/. The magnitude of the (anti-)proton
elliptic and triangular flow is larger than that of pions out to at least
GeV/ indicating that the particle type dependence persists out
to high .Comment: 16 pages, 5 captioned figures, authors from page 11, published
version, figures at http://aliceinfo.cern.ch/ArtSubmission/node/186
Centrality dependence of charged particle production at large transverse momentum in Pb-Pb collisions at TeV
The inclusive transverse momentum () distributions of primary
charged particles are measured in the pseudo-rapidity range as a
function of event centrality in Pb-Pb collisions at
TeV with ALICE at the LHC. The data are presented in the range
GeV/ for nine centrality intervals from 70-80% to 0-5%.
The Pb-Pb spectra are presented in terms of the nuclear modification factor
using a pp reference spectrum measured at the same collision
energy. We observe that the suppression of high- particles strongly
depends on event centrality. In central collisions (0-5%) the yield is most
suppressed with at -7 GeV/. Above
GeV/, there is a significant rise in the nuclear modification
factor, which reaches for GeV/. In
peripheral collisions (70-80%), the suppression is weaker with almost independently of . The measured nuclear
modification factors are compared to other measurements and model calculations.Comment: 17 pages, 4 captioned figures, 2 tables, authors from page 12,
published version, figures at
http://aliceinfo.cern.ch/ArtSubmission/node/284
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