Phonon Dispersion Effects and the Thermal Conductivity Reduction in GaAs/AlAs Superlattices

The experimentally observed order-of-magnitude reduction in the thermal conductivity along the growth axis of (GaAs)_n/(AlAs)_n (or n x n) superlattices is investigated theoretically for (2x2), (3x3) and (6x6) structures using an accurate model of the lattice dynamics. The modification of the phonon dispersion relation due to the superlattice geometry leads to flattening of the phonon branches and hence to lower phonon velocities. This effect is shown to account for a factor-of-three reduction in the thermal conductivity with respect to bulk GaAs along the growth direction; the remainder is attributable to a reduction in the phonon lifetime. The dispersion-related reduction is relatively insensitive to temperature (100 < T < 300K) and n. The phonon lifetime reduction is largest for the (2x2) structures and consistent with greater interface scattering. The thermal conductivity reduction is shown to be appreciably more sensitive to GaAs/AlAs force constant differences than to those associated with molecular masses.Comment: 5 figure

High Condition Male Rhinoceros Beetles Transfer More Nutrients to Females During Mating, Contributing to Female Preference for Body Condition Rather than Body or Weapon Size

Males in many insect species transfer nutrients along with sperm in an enclosed package known as a spermatophore. Females utilize these spermatophores to increase their fecundity. However, not all males offer the same size spermatophore. This can drive female preference toward males who are able to provide the largest spermatophores. The largest males typically produce the largest spermatophore sizes, but this is not always the case. Adult insects with rigid exoskeletons may suffer drastic reductions in body condition that are not visible on the outside. In these taxa, females may select males in excellent physiological condition rather than simply choosing males with the largest body size. Male rhinoceros beetles use their iconic “pitchfork” horns in battles with rival males over feeding territories. Once a male beetle has won a territory he will court females using stridulatory song and trembling dances. For years females were assumed to be passive in the courtship process because males who are able to win territories usually have the largest body size and the longest horns. However, it is now clear that females routinely reject males, including large males with long horns. We suspect that hours of repeated battles can exhaust males, depleting their energy reserves and thus their body condition. If poor condition males transfer smaller spermatophores, then females may be using courtship to select mating partners based on their body condition. Here I tested a critical prediction of this hypothesis. I restricted food intake for a subset of males and compared the spermatophore sizes of 25 nutrient-stressed and 25 well-fed males. I observed and filmed male courtships and then froze females immediately after mating for dissection. Spermatophore sizes significantly smaller in starved males, confirming that male short-term body condition does affect the amount of nutrients a male transfers to a female during mating. Female insects continue to amaze us with the sophistication of their mate choices, and this study reminds us that it isn’t always all about body or weapon size

Predictions for Impurity-Induced Tc Suppression in the High-Temperature Superconductors

We address the question of whether anisotropic superconductivity is compatible with the evidently weak sensitivity of the critical temperature Tc to sample quality in the high-Tc copper oxides. We examine this issue quantitatively by solving the strong-coupling Eliashberg equations numerically as well as analytically for s-wave impurity scattering within the second Born approximation. For pairing interactions with a characteristically low energy scale, we find an approximately universal dependence of the d-wave superconducting transition temperature on the planar residual resistivity which is independent of the details of the microscopic pairing. These results, in conjunction with future systematic experiments, should help elucidate the symmetry of the order parameter in the cuprates.Comment: 13 pages, 4 figures upon request, revtex version

Passivrauchen in der Schweizer Bevölkerung 2009

Die Berichte zu diesem Thema geben Auskunft über die Passivrauchexposition der Schweizer Wohnbevölkerung und deren subjektiv empfundene Belästigung an verschiedenen Orten (Arbeitsort, Gaststätten, Veranstaltungsorte usw.). Des Weiteren wurden Fragen zur Akzeptanz von Massnahmen zum Schutz der Nichtrauchenden in öffentlichen Räumen erhoben. Ebenfalls wurde das Wissen bezüglich der Schädlichkeit des Passivrauchens erfrag

Antiferromagnetic phase of the gapless semiconductor V3Al

Discovering new antiferromagnetic compounds is at the forefront of developing future spintronic devices without fringing magnetic fields. The antiferromagnetic gapless semiconducting D03 phase of V3Al was successfully synthesized via arc-melting and annealing. The antiferromagnetic properties were established through synchrotron measurements of the atom-specific magnetic moments, where the magnetic dichroism reveals large and oppositely-oriented moments on individual V atoms. Density functional theory calculations confirmed the stability of a type G antiferromagnetism involving only two-third of the V atoms, while the remaining V atoms are nonmagnetic. Magnetization, x-ray diffraction and transport measurements also support the antiferromagnetism. This archetypal gapless semiconductor may be considered as a cornerstone for future spintronic devices containing antiferromagnetic elements.Comment: Accepted to Physics Review B on 02/23/1

Sensitivity of European Temperature to Albedo Parameterization in the Regional Climate Model COSMO-CLM Linked to Extreme Land Use Changes

Previous studies based on observations and models are uncertain about the biophysical impact of af- and deforestation in the northern hemisphere mid-latitude summers, and show either a cooling or warming. The magnitude and direction is still uncertain. In this study, the effect of three different albedo parameterizations in the regional climate model COSMO-CLM (v5.09) is examined performing afforestation experiments at 0.44° horizontal resolution across the EURO-CORDEX domain during 1986-2015. Idealized de- and af-forestation simulations are compared to a simulation with no land cover change. Emphasis is put on the impact of changes in radiation and turbulent fluxes. A clear latitudinal pattern is found, which results partly due to the strong land cover conversion from forest- to grassland in the high latitudes and open land to forest conversion in mid-latitudes. Afforestation warms the climate in winter, and strongest in mid-latitudes. Results are indifferent in summer owing to opposing albedo and evapotranspiration effects of comparable size but different sign. Thus, the net effect is small for summer. Depending on the albedo parameterization in the model, the temperature effect can turn from cooling to warming in mid-latitude summers. The summer warming due to deforestation to grassland is up to 3°C higher than due to afforestation. The cooling by grass or warming by forest is in magnitude comparable and small in winter. The strength of the described near-surface temperature changes depends on the magnitude of the individual biophysical changes in the specific background climate conditions of the region. Thus, the albedo parameterization need to account for different vegetation types. Furthermore, we found that, depending on the region, the land use change effect is more important than the model uncertainty due to albedo parameterization. This is important information for model development