Assessment of mismatched epitaxial layers by X-ray rocking curve measurements and simulations

In the design of heteroepitaxial systems the knowledge of composition, strain and thickness of the various layers involved is of great importance. The determination of these structural parameters is among the applications most common in high-resolution X-ray diffraction. Numerical simulation of rocking curves has emerged as an important aid in the determination of the relevant structural parameters. Based upon the dynamical theory of X-ray diffraction a FORTRAN program has been written to simulate and refine structural parameters from rocking curve data. X-ray rocking curve analysis has been used to develop and optimize MOCVD-growth of Ga_xIn_1-xP/GaAs heteroepitaxial systems. We observed superlattice peaks in samples of low misfit. With the aid of computer simulations we attribute these to unintentional periodic compositional fluctuations (Ga/In) of the order of 1%.     

E-FiRST: Electric field responsive shear thickening fluids

A novel electrorheological (ER) effect is presented where the application of an electric field, orthogonal to the vorticity-flow plane, increases the critical hydrodynamic stress required to shear thicken concentrated, colloidal dispersions (the E-FiRST effect). The shear thickening behavior of a Brownian charge stabilized dispersion (226 nm silica in 4-methylcyclohexanol at 53, 50. and 41 vol.%) is studied in the presence of an electric field as a function of the field strength and coupling parameter ( ß) where the latter is a function of a.c. field frequency due to diffusion limitations on the polarization of the particles' double layer. A mechanism is proposed whereby the applied electric field suppresses the formation of the self-organized hydrocluster microstructure responsible for shear thickening, thus delaying the onset of shear thickening to higher applied shear stresses. A Mason-number type scaling law is found to scale the effect, which supports the proposed mechanism.     

Microscopic calculations of beta decay far from stability

Beta decay properties of nuclei far from stability are of decisive importance for the understanding of the element synthesis in the universe and the determination of the age of the universe by means of cosmochronometers. A new large scale microscopic calculation ofβ ^?-decay half-lives up to the neutron drip line is presented. This new approach uses the proton-neutron quasiparticle random phase approximation with a Gamow-Teller residual interaction.     

Next-Generation Personal Memory Aids

Personal memory assistance is a natural application of ubiquitous computing. Portable computers are decreasing in size, increasing in capability, and the barriers constraining earlier computer-based memory aids are rapidly diminishing. With each engineering advance, a new generation of 'personal memory aids' is enabled. This paper presents iRemember, our prototype wearable 'memory prosthesis'. It allows the wearer to capture and accrue daily experiences (primarily audio) and attempts to remedy a limited set of common memory problems by providing tools to find memory triggers within such collections. This is an early step in an area ripe for growth and controversy. The social and legal implications of ubiquitous recording are discussed and additional memory aids designed to address other memory problems are proposed.     

Lattice fermions at non-zero temperature and chemical potential

We study the free fermion gas at finite temperature and chemical potential in the lattice regularized version proposed by Hasenfratz and Karsch and by Kogut et al. Special emphasis is placed on the identification of the particle and antiparticle contributions to the partition function. In the case of naive fermions we show that the partition function no longer separates into particle-antiparticle contributions in the way familiar from the continuum formulation. The use of Wilson fermions, on the other hand, eliminates this unpleasant feature, and leads, after subtracting the vacuum contributions, to the familiar expressions for the average energy and charge densities.     

Affective Learning — A Manifesto

The use of the computer as a model, metaphor, and modelling tool has tended to privilege the 'cognitive' over the 'affective' by engendering theories in which thinking and learning are viewed as information processing and affect is ignored or marginalised. In the last decade there has been an accelerated flow of findings in multiple disciplines supporting a view of affect as complexly intertwined with cognition in guiding rational behaviour, memory retrieval, decision-making, creativity, and more. It is time to redress the imbalance by developing theories and technologies in which affect and cognition are appropriately integrated with one another. This paper describes work in that direction at the MIT Media Lab and projects a large perspective of new research in which computer technology is used to redress the imbalance that was caused (or, at least, accentuated) by the computer itself.     

Failure of the naïve loop expansion for the effective potential in φ4 field theory when there is “broken symmetry”

We determine the effective potential V[φ] in zero dimensional gφ⁴ field theory when there is broken symmetry classically $\text{[V}{}_{\mathrm{<mtext>cl</mtext>}}\text{=}\text{1}\text{4}\text{g(φ}{}^2\text{? a}{}^2\text{)}{}^2\text{]}$ as a power series in 1/g. We find that the usual loop expansion ignores a secondary minimum of the path integral and is only valid for |φ| ? a.When |ja| ? 1, where j is the external source, there is a secondary minimum of equal importance to the absolute minimum. In that region a kink solution to the differential equation for φ[j] exists, φ[j] = a tanh(ja) + O(ja²/ga⁴), which interpolates between the minima at φ = ?a and φ = +a. We obtain the 1/g expansion for V[φ] for φ < a and φ ? a. For all g, a mean-field approximation gives an expression for V[φ] which interpolates between these two regimes. In higher dimensions the kink solution becomes $\text{φ[j] = a}\text{tanh}\text{(jaΩ)}$, where Ω is the volume of space-time, suggesting a discontinuity in φ[j] in the infinite-volume limit.