6,968 research outputs found
Casimir Densities for a Massive Fermionic Quantum Field in a Global Monopole Background with Spherical Boundary
We investigate the vacuum expectation value of the energy-momentum tensor
associated with a massive fermionic field obeying the MIT bag boundary
condition on a spherical shell in the global monopole spacetime. The asymptotic
behavior of the vacuum densities is investigated near the sphere center and
surface, and at large distances from the sphere. In the limit of strong
gravitational field corresponding to small values of the parameter describing
the solid angle deficit in global monopole geometry, the sphere-induced
expectation values are exponentially suppressed.Comment: 8 pages, 4 figures, 6th Alexander Friedmann International Seminar on
Gravitation and Cosmolog
Texture transitions in the liquid crystalline alkyloxybenzoic acid 6OBAC
The 4,n-alkyloxybenzoic acid 6OBAC has a very rich variety of crystalline
structures and two nematic sub-phases, characterised by different textures. It
is a material belonging to a family of liquid crystals formed by hydrogen
bonded molecules, the 4,n-alkyloxybenzoic acids indicates the homologue
number). The homologues with n ranging from 7 to 13 display both smectic C and
N phases. In spite of the absence of a smectic phase, 6OBAC exhibits two
sub-phases with different textures, as it happens in other materials of the
homologue series which possess the smectic phase. This is the first material
that exhibits a texture transition in a nematic phase directly originated from
a crystal phase. Here we present the results of an image processing assisted
optical investigation to characterise the textures and the transitions between
textures. This processing is necessary to discriminate between crystal
modifications and nematic sub-phases.Comment: 12 pages, 10 figure
Electrostatic in Reissner-Nordstrom space-time with a conical defect
We calculate the electrostatic potential generated by a point charge in the
space-time of Reissner-Nordstrom with a conical defect. An expression for the
self-energy is also presented.Comment: 7 pages, LATEX fil
SAMplus: adaptive optics at optical wavelengths for SOAR
Adaptive Optics (AO) is an innovative technique that substantially improves
the optical performance of ground-based telescopes. The SOAR Adaptive Module
(SAM) is a laser-assisted AO instrument, designed to compensate ground-layer
atmospheric turbulence in near-IR and visible wavelengths over a large Field of
View. Here we detail our proposal to upgrade SAM, dubbed SAMplus, that is
focused on enhancing its performance in visible wavelengths and increasing the
instrument reliability. As an illustration, for a seeing of 0.62 arcsec at 500
nm and a typical turbulence profile, current SAM improves the PSF FWHM to 0.40
arcsec, and with the upgrade we expect to deliver images with a FWHM of
arcsec -- up to 0.23 arcsec FWHM PSF under good seeing
conditions. Such capabilities will be fully integrated with the latest SAM
instruments, putting SOAR in an unique position as observatory facility.Comment: To appear in Proc. SPIE 10703 (Ground-based and Airborne
Instrumentation for Astronomy VII; SPIEastro18
Equivalence of Fokker-Planck approach and non-linear -model for disordered wires in the unitary symmetry class
The exact solution of the Dorokhov-Mello-Pereyra-Kumar-equation for quasi
one-dimensional disordered conductors in the unitary symmetry class is employed
to calculate all -point correlation functions by a generalization of the
method of orthogonal polynomials. We obtain closed expressions for the first
two conductance moments which are valid for the whole range of length scales
from the metallic regime () to the insulating regime () and
for arbitrary channel number. In the limit (with )
our expressions agree exactly with those of the non-linear -model
derived from microscopic Hamiltonians.Comment: 9 pages, Revtex, one postscript figur
Electronic Phase Separation Transition as the Origin of the Superconductivity and the Pseudogap Phase of Cuprates
We propose a new phase of matter, an electronic phase separation transition
that starts near the upper pseudogap and segregates the holes into high and low
density domains. The Cahn-Hilliard approach is used to follow quantitatively
this second order transition. The resulting grain boundary potential confines
the charge in domains and favors the development of intragrain superconducting
amplitudes. The zero resistivity transition arises only when the intergrain
Josephson coupling is of the order of the thermal energy and phase
locking among the superconducting grains takes place. We show that this
approach explains the pseudogap and superconducting phases in a natural way and
reproduces some recent scanning tunneling microscopy dataComment: 4 pages and 5 eps fig
Intensity correlations in electronic wave propagation in a disordered medium: the influence of spin-orbit scattering
We obtain explicit expressions for the correlation functions of transmission
and reflection coefficients of coherent electronic waves propagating through a
disordered quasi-one-dimensional medium with purely elastic diffusive
scattering in the presence of spin-orbit interactions. We find in the metallic
regime both large local intensity fluctuations and long-range correlations
which ultimately lead to universal conductance fluctuations. We show that the
main effect of spin-orbit scattering is to suppress both local and long-range
intensity fluctuations by a universal symmetry factor 4. We use a scattering
approach based on random transfer matrices.Comment: 15 pages, written in plain TeX, Preprint OUTP-93-42S (University of
Oxford), to appear in Phys. Rev.
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