Subextensive Scaling in the Athermal, Quasistatic Limit of Amorphous Matter in Plastic Shear Flow

We present the results of numerical simulations of an atomistic system undergoing plastic shear flow in the athermal, quasistatic limit. The system is shown to undergo cascades of local rearrangements, associated with quadrupolar energy fluctuations, which induce system-spanning events organized into lines of slip oriented along the Bravais axes of the simulation cell. A finite size scaling analysis reveals subextensive scaling of the energy drops and participation numbers, linear in the length of the simulation cell, in good agreement with the observed real-space structure of the plastic events.Comment: 4 pages, 6 figure

Editorial note to "The beginning of the world from the point of view of quantum theory"

This is an editorial note to accompany reprinting as a Golden Oldie in the Journal of General Relativity and Gravitation of the famous note by Georges Lemaitre on the quantum birth of the universe, published in Nature in 1931. We explain why this short (457 words) article can be considered to be the true "Charter" of the modern Big Bang theory.Comment: This is an editorial comment to accompany reprinting of a classical paper in the Journal of General Relativity and Gravitation. 16 pages, 2 figure

Can a charged dust ball be sent through the Reissner--Nordstr\"{o}m wormhole?

In a previous paper we formulated a set of necessary conditions for the spherically symmetric weakly charged dust to avoid Big Bang/Big Crunch, shell crossing and permanent central singularities. However, we did not discuss the properties of the energy density, some of which are surprising and seem not to have been known up to now. A singularity of infinite energy density does exist -- it is a point singularity situated on the world line of the center of symmetry. The condition that no mass shell collapses to $R = 0$ if it had $R > 0$ initially thus turns out to be still insufficient for avoiding a singularity. Moreover, at the singularity the energy density $\epsilon$ is direction-dependent: $\epsilon \to - \infty$ when we approach the singular point along a $t =$ const hypersurface and $\epsilon \to + \infty$ when we approach that point along the center of symmetry. The appearance of negative-energy-density regions turns out to be inevitable. We discuss various aspects of this property of our configuration. We also show that a permanently pulsating configuration, with the period of pulsation independent of mass, is possible only if there exists a permanent central singularity.Comment: 30 pages, 21 figures; several corrections after referee's comments, 4 figures modifie

Comment on ``Deterministic equations of motion and phase ordering dynamics''

Zheng [Phys. Rev. E {\bf 61}, 153 (2000), cond-mat/9909324] claims that phase ordering dynamics in the microcanonical $\phi^4$ model displays unusual scaling laws. We show here, performing more careful numerical investigations, that Zheng only observed transient dynamics mostly due to the corrections to scaling introduced by lattice effects, and that Ising-like (model A) phase ordering actually takes place at late times. Moreover, we argue that energy conservation manifests itself in different corrections to scaling.Comment: 5 pages, 4 figure

Optical alignment and polarization conversion of neutral exciton spin in individual InAs/GaAs quantum dots

We investigate exciton spin memory in individual InAs/GaAs self-assembled quantum dots via optical alignment and conversion of exciton polarization in a magnetic field. Quasiresonant phonon-assisted excitation is successfully employed to define the initial spin polarization of neutral excitons. The conservation of the linear polarization generated along the bright exciton eigenaxes of up to 90% and the conversion from circular- to linear polarization of up to 47% both demonstrate a very long spin relaxation time with respect to the radiative lifetime. Results are quantitatively compared with a model of pseudo-spin 1/2 including heavy-to-light hole mixing.Comment: 5 pages, 3 figure

The Breakdown of Kinetic Theory in Granular Shear Flows

We examine two basic assumptions of kinetic theory-- binary collisions and molecular chaos-- using numerical simulations of sheared granular materials. We investigate a wide range of densities and restitution coefficients and demonstrate that kinetic theory breaks down at large density and small restitution coefficients. In the regimes where kinetic theory fails, there is an associated emergence of clusters of spatially correlated grains

Spin precession and inverted Hanle effect in a semiconductor near a finite-roughness ferromagnetic interface

Although the creation of spin polarization in various non-magnetic media via electrical spin injection from a ferromagnetic tunnel contact has been demonstrated, much of the basic behavior is heavily debated. It is reported here for semiconductor/Al2O3/ferromagnet tunnel structures based on Si or GaAs that local magnetostatic fields arising from interface roughness dramatically alter and even dominate the accumulation and dynamics of spins in the semiconductor. Spin precession in the inhomogeneous magnetic fields is shown to reduce the spin accumulation up to tenfold, and causes it to be inhomogeneous and non-collinear with the injector magnetization. The inverted Hanle effect serves as experimental signature. This interaction needs to be taken into account in the analysis of experimental data, particularly in extracting the spin lifetime and its variation with different parameters (temperature, doping concentration). It produces a broadening of the standard Hanle curve and thereby an apparent reduction of the spin lifetime. For heavily doped n-type Si at room temperature it is shown that the spin lifetime is larger than previously determined, and a new lower bound of 0.29 ns is obtained. The results are expected to be general and occur for spins near a magnetic interface not only in semiconductors but also in metals, organic and carbon-based materials including graphene, and in various spintronic device structures.Comment: Final version, with text restructured and appendices added (25 pages, 9 figures). To appear in Phys. Rev.

Field-induced domain wall propagation: beyond the one-dimensional model

We have investigated numerically the field-driven propagation of perpendicularly magnetized ferromagnetic layers. It was then compared to the historical one-dimensional domain wall (DW) propagation model widely used in spintronics studies of magnetic nanostructures. In the particular regime of layer thickness (h) of the order of the exchange length, anomalous velocity peaks appear in the precessional regime, their shape and position shifting with h. This has also been observed experimentally. Analyses of the simulations show a distinct correlation between the curvature of the DW and the twist of the magnetization vector within it, and the velocity peak. Associating a phenomenological description of this twist with a four-coordinate DW propagation model, we reproduce very well these kinks and show that they result from the torque exerted by the stray field created by the domains on the twisted magnetization. The position of the peaks is well predicted from the DW's first flexural mode frequency, and depends strongly on the layer thickness. Comparison of the proposed model to DW propagation data obtained on dilute semiconductor ferromagnets GaMnAs and GaMnAsP sheds light on the origin of the measured peaks

Double Distribution of Dark Matter Halos with respect to Mass and Local Overdensity

We present a double distribution function of dark matter halos, with respect to both object mass and local over- (or under-) density. This analytical tool provides a statistical treatment of the properties of matter surrounding collapsed objects, and can be used to study environmental effects on hierarchical structure formation. The size of the "local environment" of a collapsed object is defined to depend on the mass of the object. The Press-Schechter mass function is recovered by integration of our double distribution over the density contrast. We also present a detailed treatment of the evolution of overdensities and underdensities in Einstein-deSitter and flat LCDM universes, according to the spherical evolution model. We explicitly distinguish between true and linearly extrapolated overdensities and provide conversion relations between the two quantities.Comment: 25 pages, 10 figures, comments welcom