11,279 research outputs found
Effects of Vacancies on Properties of Relaxor Ferroelectrics: a First-Principles Study
A first-principles-based model is developed to investigate the influence of
lead vacancies on the properties of relaxor ferroelectric Pb(Sc1/2Nb1/2)O3
(PSN). Lead vacancies generate large, inhomogeneous, electric fields that
reduce barriers between energy minima for different polarization directions.
This naturally explains why relaxors with significant lead vacancy
concentrations have broadened dielectric peaks at lower temperatures, and why
lead vacancies smear properties in the neighborhood of the ferroelectric
transition in PSN. We also reconsider the conventional wisdom that lead
vacancies reduce the magnitude of dielectric response.Comment: 11 pages, 1 figur
Effects of quark family nonuniversality in SU(3)_c X SU(4)_L X U(1)_x models
Flavour changing neutral currents arise in the extension of the standard model because anomaly cancellation among the
fermion families requires one generation of quarks to transform differently
from the other two under the gauge group. In the weak basis the distinction
between quark families is meaningless. However, in the mass eigenstates basis,
the Cabibbo-Kobayashi-Maskawa mixing matrix motivates us to classify
left-handed quarks in families. In this sense there are, in principle, three
different assignments of quark weak eigenstates into mass eigenstates. In this
work, by using measurements at the Z-pole, atomic parity violation data and
experimental input from neutral meson mixing, we examine two different models
without exotic electric charges based on the 3-4-1 symmetry, and address the
effects of quark family nonuniversality on the bounds on the mixing angle
between two of the neutral currents present in the models and on the mass
scales and of the new neutral gauge bosons predicted by the
theory. The heaviest family of quarks must transform differently in order to
keep lower bounds on and as low as possible without
violating experimental constraints.Comment: 27 pages, 10 tables, 2 figures. Equation (19) and typos corrected.
Matches version to appear in Phys. Rev.
Internal transitions of negatively charged magnetoexcitons in quantum dots
We report calculations of oscillator strengths for the far infrared
absorption of light by the excitonic complexes Xn- (the excess charge, n,
ranging from one to four) confined in quantum dots. The magnetic field is
varied in an interval which corresponds to ``filling factors'' between 2 and
3/5. Electron-hole interaction effects are seen in the deviations of the peak
positions from the Kohn lines, and in the spreading of the oscillator strengths
over a few final states. Transition densities are used as an additional tool to
characterize the absorption peaks.Comment: Presented as a poster in the Third Stig Lundqvist Conference on
Advancing Frontiers of Condensed Matter Physics: Fundamental Interactions and
Excitations in Confined Systems, Trieste, August 11 - 1
General Relativistic Radiant Shock Waves in the Post-Quasistatic Approximation
An evolution of radiant shock wave front is considered in the framework of a
recently presented method to study self-gravitating relativistic spheres, whose
rationale becomes intelligible and finds full justification within the context
of a suitable definition of the post-quasistatic approximation. The spherical
matter configuration is divided into two regions by the shock and each side of
the interface having a different equation of state and anisotropic phase. In
order to simulate dissipation effects due to the transfer of photons and/or
neutrinos within the matter configuration, we introduce the flux factor, the
variable Eddington factor and a closure relation between them. As we expected
the strength of the shock increases the speed of the fluid to relativistic
values and for some critical ones is larger than light speed. In addition, we
find that energy conditions are very sensible to the anisotropy, specially the
strong one. As a special feature of the model, we find that the contribution of
the matter and radiation to the radial pressure are the same order of magnitude
as in the mant as in the core, moreover, in the core radiation pressure is
larger than matter pressure.Comment: To appear in Journal of Physics:Conference Series:"XXIX Spanish
Relativity Meeting (ERE 2006): Einstein's Legacy: From the Theoretical
Paradise to Astrophysical Observations
Transversely projective foliations on surfaces: existence of normal forms and prescription of the monodromy
We introduce a notion of normal form for transversely projective structures
of singular foliations on complex manifolds. Our first main result says that
this normal form exists and is unique when ambient space is two-dimensional.
From this result one obtains a natural way to produce invariants for
transversely projective foliations on surfaces. Our second main result says
that on projective surfaces one can construct singular transversely projective
foliations with prescribed monodromy
Extended Loops: A New Arena for Nonperturbative Quantum Gravity
We propose a new representation for gauge theories and quantum gravity. It
can be viewed as a generalization of the loop representation. We make use of a
recently introduced extension of the group of loops into a Lie Group. This
extension allows the use of functional methods to solve the constraint
equations. It puts in a precise framework the regularization problems of the
loop representation. It has practical advantages in the search for quantum
states. We present new solutions to the Wheeler-DeWitt equation that reinforce
the conjecture that the Jones Polynomial is a state of nonperturbative quantum
gravity.Comment: 12pp, Revtex, no figures, CGPG-93/12-
Spontaneous superconducting islands and Hall voltage in clean superconductors
We study a clean superconductor in the Hall configuration, in the framework
of a purely dissipative time-dependent Ginzburg--Landau theory. We find
situations in which the order parameter differs significantly from zero in a
set of islands that appear to form a periodic structure. When the pattern of
islands becomes irregular, it moves in or against the direction of the current
and a Hall voltage is found. Tiny differences in the initial state may reverse
the sign of the Hall voltage. When the average Hall voltage vanishes, the local
Hall voltage does not necessarily vanish. We examine the influence that several
boundary conditions at the electrodes have on these effects.Comment: 6 pages, Includes additional cases and more detailed result
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