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 $SU(3)_c\otimes SU(4)_L\otimes U(1)_X$ 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 $M_{Z_2}$ and $M_{Z_3}$ 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 $M_{Z_2}$ and $M_{Z_3}$ 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