M-flation and its spectators

M-flation is an implementation of assisted inflation, in which the inflaton fields are three N_c x N_c non-abelian hermitean matrices. The model can be consistently truncated to an effectively single field inflation model, with all ``spectator'' fields fixed at the origin. We show that starting with random initial conditions for all fields the truncated sector is not a late-time attractor, but instead the system evolves towards quadratic assisted inflation with all fields mass degenerate. Demanding the energy density during inflation to be below the effective quantum gravity scale, we find that the number of fields, and thus the assisted effect, is bounded N_c < 10^2.Comment: 26 pages, published versio

Structure formation in binary colloids

A theoretical study of the structure formation observed very recently [Phys. Rev. Lett. 90, 128303 (2003)] in binary colloids is presented. In our model solely the dipole-dipole interaction of the particles is considered, electrohidrodynamic effects are excluded. Based on molecular dynamics simulations and analytic calculations we show that the total concentration of the particles, the relative concentration and the relative dipole moment of the components determine the structure of the colloid. At low concentrations the kinetic aggregation of particles results in fractal structures which show a crossover behavior when increasing the concentration. At high concentration various lattice structures are obtained in a good agreement with experiments.Comment: revtex, 4 pages, figures available from authors due to size problem

Low Mach number effect in simulation of high Mach number flow

In this note, we relate the two well-known difficulties of Godunov schemes: the carbuncle phenomena in simulating high Mach number flow, and the inaccurate pressure profile in simulating low Mach number flow. We introduced two simple low-Mach-number modifications for the classical Roe flux to decrease the difference between the acoustic and advection contributions of the numerical dissipation. While the first modification increases the local numerical dissipation, the second decreases it. The numerical tests on the double-Mach reflection problem show that both modifications eliminate the kinked Mach stem suffered by the original flux. These results suggest that, other than insufficient numerical dissipation near the shock front, the carbuncle phenomena is strongly relevant to the non-comparable acoustic and advection contributions of the numerical dissipation produced by Godunov schemes due to the low Mach number effect.Comment: 9 pages, 1 figur

Preheating After Modular Inflation

We study (p)reheating in modular (closed string) inflationary scenarios, with a special emphasis on Kahler moduli/Roulette models. It is usually assumed that reheating in such models occurs through perturbative decays. However, we find that there are very strong non-perturbative preheating decay channels related to the particular shape of the inflaton potential (which is highly nonlinear and has a very steep minimum). Preheating after modular inflation, proceeding through a combination of tachyonic instability and broad-band parametric resonance, is perhaps the most violent example of preheating after inflation known in the literature. Further, we consider the subsequent transfer of energy to the standard model sector in scenarios where the standard model particles are confined to a D7-brane wrapping the inflationary blow-up cycle of the compactification manifold or, more interestingly, a non-inflationary blow up cycle. We explicitly identify the decay channels of the inflaton in these two scenarios. We also consider the case where the inflationary cycle shrinks to the string scale at the end of inflation; here a field theoretical treatment of reheating is insufficient and one must turn instead to a stringy description. We estimate the decay rate of the inflaton and the reheat temperature for various scenarios.Comment: 34 pages, 10 figures. Accepted for publication in JCA

Atomic Bose Gas with Negative Scattering Length

We derive the equation of state of a dilute atomic Bose gas with an interatomic interaction that has a negative scattering length and argue that two continuous phase transitions, occuring in the gas due to quantum degeneracy effects, are preempted by a first-order gas-liquid or gas-solid transition depending on the details of the interaction potential. We also discuss the consequences of this result for future experiments with magnetically trapped spin-polarized atomic gasses such as lithium and cesium.Comment: 16 PAGES, REVTEX 3.0, ACCEPTED FOR PUBLICATION IN PHYS. REV.

Phosphinecarboxamide based InZnP QDs – an air tolerant route to luminescent III–V semiconductors

We describe a new synthetic methodology for the preparation of high quality, emission tuneable InP-based quantum dots (QDs) using a solid, air- and moisture-tolerant primary phosphine as a group-V precursor. This presents a significantly simpler synthetic pathway compared to the state-of-the-art precursors currently employed in phosphide quantum dot synthesis which are volatile, dangerous and air-sensitive, e.g. P(Si(CH3)3)3

Molecular dynamics approach: from chaotic to statistical properties of compound nuclei

Statistical aspects of the dynamics of chaotic scattering in the classical model of $\alpha$-cluster nuclei are studied. It is found that the dynamics governed by hyperbolic instabilities which results in an exponential decay of the survival probability evolves to a limiting energy distribution whose density develops the Boltzmann form. The angular distribution of the corresponding decay products shows symmetry with respect to $\pi/2$ angle. Time estimated for the compound nucleus formation ranges within the order of $10^{-21}$s.Comment: 11 pages, LaTeX, non

Hot String Soup

Above the Hagedorn energy density closed fundamental strings form a long string phase. The dynamics of weakly interacting long strings is described by a simple Boltzmann equation which can be solved explicitly for equilibrium distributions. The average total number of long strings grows logarithmically with total energy in the microcanonical ensemble. This is consistent with calculations of the free single string density of states provided the thermodynamic limit is carefully defined. If the theory contains open strings the long string phase is suppressed.Comment: 13 pages, no figures, uses LaTex, some errors in equations have been corrected, NSF-ITP-94-83, UCSBTH-94-3

On the construction of a geometric invariant measuring the deviation from Kerr data

This article contains a detailed and rigorous proof of the construction of a geometric invariant for initial data sets for the Einstein vacuum field equations. This geometric invariant vanishes if and only if the initial data set corresponds to data for the Kerr spacetime, and thus, it characterises this type of data. The construction presented is valid for boosted and non-boosted initial data sets which are, in a sense, asymptotically Schwarzschildean. As a preliminary step to the construction of the geometric invariant, an analysis of a characterisation of the Kerr spacetime in terms of Killing spinors is carried out. A space spinor split of the (spacetime) Killing spinor equation is performed, to obtain a set of three conditions ensuring the existence of a Killing spinor of the development of the initial data set. In order to construct the geometric invariant, we introduce the notion of approximate Killing spinors. These spinors are symmetric valence 2 spinors intrinsic to the initial hypersurface and satisfy a certain second order elliptic equation ---the approximate Killing spinor equation. This equation arises as the Euler-Lagrange equation of a non-negative integral functional. This functional constitutes part of our geometric invariant ---however, the whole functional does not come from a variational principle. The asymptotic behaviour of solutions to the approximate Killing spinor equation is studied and an existence theorem is presented.Comment: 36 pages. Updated references. Technical details correcte

Quantum-well states in ultrathin Ag(111) films deposited onto H-passivated Si(111)-(1x1) surfaces

Ag(111) films were deposited at room temperature onto H-passivated Si(111)-(1x1) substrates, and subsequently annealed at 300 C. An abrupt non-reactive Ag/Si interface is formed, and very uniform non-strained Ag(111) films of 6-12 monolayers have been grown. Angle resolved photoemission spectroscopy has been used to study the valence band electronic properties of these films. Well-defined Ag sp quantum-well states (QWS) have been observed at discrete energies between 0.5-2eV below the Fermi level, and their dispersions have been measured along the GammaK, GammaMM'and GammaL symmetry directions. QWS show a parabolic bidimensional dispersion, with in-plane effective mass of 0.38-0.50mo, along the GammaK and GammaMM' directions, whereas no dispersion has been found along the GammaL direction, indicating the low-dimensional electronic character of these states. The binding energy dependence of the QWS as a function of Ag film thickness has been analyzed in the framework of the phase accumulation model. According to this model, a reflectivity of 70% has been estimated for the Ag-sp states at the Ag/H/Si(111)-(1x1) interface.Comment: 6 pages, 6 figures, submitted to Phys. Rev.