Domain wall network evolution in (N+1)-dimensional FRW universes

We develop a velocity-dependent one-scale model for the evolution of domain wall networks in flat expanding or collapsing homogeneous and isotropic universes with an arbitrary number of spatial dimensions, finding the corresponding scaling laws in frictionless and friction dominated regimes. We also determine the allowed range of values of the curvature parameter and the expansion exponent for which a linear scaling solution is possible in the frictionless regime.Comment: 5 pages, 2 figure

Scaling laws for weakly interacting cosmic (super)string and p-brane networks

In this paper we find new scaling laws for the evolution of $p$-brane networks in $N+1$-dimensional Friedmann-Robertson-Walker universes in the weakly-interacting limit, giving particular emphasis to the case of cosmic superstrings ($p=1$) living in a universe with three spatial dimensions (N=3). In particular, we show that, during the radiation era, the root-mean-square velocity is ${\bar v} =1/{\sqrt 2}$ and the characteristic length of non-interacting cosmic string networks scales as $L \propto a^{3/2}$ ($a$ is the scale factor), thus leading to string domination even when gravitational backreaction is taken into account. We demonstrate, however, that a small non-vanishing constant loop chopping efficiency parameter $\tilde c$ leads to a linear scaling solution with constant $L H \ll 1$ ($H$ is the Hubble parameter) and ${\bar v} \sim 1/{\sqrt 2}$ in the radiation era, which may allow for a cosmologically relevant cosmic string role even in the case of light strings. We also determine the impact that the radiation-matter transition has on the dynamics of weakly interacting cosmic superstring networks.Comment: 5 pages, 2 figure

Evolution of domain wall networks: the PRS algorithm

The Press-Ryden-Spergel (PRS) algorithm is a modification to the field theory equations of motion, parametrized by two parameters ($\alpha$ and $\beta$), implemented in numerical simulations of cosmological domain wall networks, in order to ensure a fixed comoving resolution. In this paper we explicitly demonstrate that the PRS algorithm provides the correct domain wall dynamics in $N+1$-dimensional Friedmann-Robertson-Walker (FRW) universes if $\alpha+\beta/2=N$, fully validating its use in numerical studies of cosmic domain evolution. We further show that this result is valid for generic thin featureless domain walls, independently of the Lagrangian of the model.Comment: 4 page

Brane bounce-type configurations in a string-like scenario

Brane world six dimensional scenarios with string like metric has been proposed to alleviate the problem of field localization. However, these models have been suffering from some drawbacks related with energy conditions as well as from difficulties to find analytical solutions. In this work, we propose a model where a brane is made of a scalar field with bounce-type configurations and embedded in a bulk with a string-like metric. This model produces a sound AdS scenario where none of the important physical quantities is infinite. Among these quantities are the components of the energy momentum tensor, which have its positivity ensured by a suitable choice of the bounce configurations. Another advantage of this model is that the warp factor can be obtained analytically from the equations of motion for the scalar field, obtaining as a result a thick brane configuration, in a six dimensional context. Moreover, the study of the scalar field localization in these scenario is done.Comment: 15 pages, 5 figures. To appear in Physics Letters

Respective influence of in-plane and out-of-plane spin-transfer torques in magnetization switching of perpendicular magnetic tunnel junctions

The relative contributions of in-plane (damping-like) and out-of-plane (field-like) spin-transfer-torques in the magnetization switching of out-of-plane magnetized magnetic tunnel junctions (pMTJ) has been theoretically analyzed using the transformed Landau-Lifshitz (LL) equation with the STT terms. It is demonstrated that in a pMTJ structure obeying macrospin dynamics, the out-of-plane torque influences the precession frequency but it does not contribute significantly to the STT switching process (in particular to the switching time and switching current density), which is mostly determined by the in-plane STT contribution. This conclusion is confirmed by finite temperature and finite writing pulse macrospin simulations of the current-field switching diagrams. It contrasts with the case of STT-switching in in-plane magnetized MTJ in which the field-like term also influences the switching critical current. This theoretical analysis was successfully applied to the interpretation of voltage-field STT switching diagrams experimentally measured on perpendicular MTJ pillars 36 nm in diameter, which exhibit macrospin-like behavior. The physical nonequivalence of Landau and Gilbert dissipation terms in presence of STT-induced dynamics is also discussed

Restoration of UA_A(1) symmetry and meson spectrum in hot or dense matter

We explore the effects of breaking and restoration of chiral and axial symmetries using an extended three-flavor Nambu-Jona-Lasinio model that incorporates explicitly the axial anomaly through the 't Hooft interaction. We implement a temperature (density) dependence of the anomaly coefficient motivated by lattice results for the topological susceptibility. The spectrum of scalar and pseudoscalar mesons is analyzed bearing in mind the identification of chiral partners and the study of its convergence. We also concentrate on the behavior of the mixing angles that give us relevant information on the issue under discussion. The results suggest that the axial part of the symmetry is restored before the possible restoration of the full U(3)$\otimes$U(3) chiral symmetry might occur.Comment: 9 pages, 5 figures. Talk given at Joint Meeting Heidelberg-Liege-Paris-Rostock (HLPR 2004), Spa, Belgium, 16-18 Dec 200