Frequency-dependent effective permeability tensor of unsaturated polycrystalline ferrites

Frequency-dependent permeability tensor for unsaturated polycrystalline ferrites is derived through an effective medium approximation that combines both domain-wall motion and rotation of domains in a single consistent scattering framework. Thus derived permeability tensor is averaged on a distribution function of the free energy that encodes paramagnetic states for anhysteretic loops. The initial permeability is computed and frequency spectra are given by varying macroscopic remanent field.Comment: 24 pages, 3 figure

Enhanced tunneling across nanometer-scale metal-semiconductor interfaces

We have measured electrical transport across epitaxial, nanometer-sized metal-semiconductor interfaces by contacting CoSi2-islands grown on Si(111) with an STM-tip. The conductance per unit area was found to increase with decreasing diode area. Indeed, the zero-bias conductance was found to be about 10^4 times larger than expected from downscaling a conventional diode. These observations are explained by a model, which predicts a narrower barrier for small diodes and therefore a greatly increased contribution of tunneling to the electrical transport.Comment: 3 pages, 2 EPS-figures; accepted for publication in Appl. Phys. Let

A Real Options Perspective On R&D Portfolio Diversification

This paper shows that the conditionality of investment decisions in R&D has a critical impact on portfolio risk, and implies that traditional diversification strategies should be reevaluated when a portfolio is constructed. Real option theory argues that research projects have conditional or option-like risk and return properties, and are different from unconditional projects. Although the risk of a portfolio always depends on the correlation between projects, a portfolio of conditional R&D projects with real option characteristics has a fundamentally different risk than a portfolio of unconditional projects. When conditional R&D projects are negatively correlated, diversification only slightly reduces portfolio risk. When projects are positively correlated, however, diversification proves more effective than conventional tools predict.real options;portfolio analysis;research & development

Scaling of nano-Schottky-diodes

A generally applicable model is presented to describe the potential barrier shape in ultra small Schottky diodes. It is shown that for diodes smaller than a characteristic length $l_c$ (associated with the semiconductor doping level) the conventional description no longer holds. For such small diodes the Schottky barrier thickness decreases with decreasing diode size. As a consequence, the resistance of the diode is strongly reduced, due to enhanced tunneling. Without the necessity of assuming a reduced (non-bulk) Schottky barrier height, this effect provides an explanation for several experimental observations of enhanced conduction in small Schottky diodes.Comment: 4 pages, 4 figures, accepted for publication in Appl. Phys. Lett., some minor additions and correction

Complex, Dynamic Combination of Physical, Chemical and Nutritional Variables Controls Spatio-Temporal Variation of Sandy Beach Community Structure

Sandy beach ecological theory states that physical features of the beach control macrobenthic community structure on all but the most dissipative beaches. However, few studies have simultaneously evaluated the relative importance of physical, chemical and biological factors as potential explanatory variables for meso-scale spatio-temporal patterns of intertidal community structure in these systems. Here, we investigate macroinfaunal community structure of a micro-tidal sandy beach that is located on an oligotrophic subtropical coast and is influenced by seasonal estuarine input. We repeatedly sampled biological and environmental variables at a series of beach transects arranged at increasing distances from the estuary mouth. Sampling took place over a period of five months, corresponding with the transition between the dry and wet season. This allowed assessment of biological-physical relationships across chemical and nutritional gradients associated with a range of estuarine inputs. Physical, chemical, and biological response variables, as well as measures of community structure, showed significant spatio-temporal patterns. In general, bivariate relationships between biological and environmental variables were rare and weak. However, multivariate correlation approaches identified a variety of environmental variables (i.e., sampling session, the C:N ratio of particulate organic matter, dissolved inorganic nutrient concentrations, various size fractions of photopigment concentrations, salinity and, to a lesser extent, beach width and sediment kurtosis) that either alone or combined provided significant explanatory power for spatio-temporal patterns of macroinfaunal community structure. Overall, these results showed that the macrobenthic community on Mtunzini Beach was not structured primarily by physical factors, but instead by a complex and dynamic blend of nutritional, chemical and physical drivers. This emphasises the need to recognise ocean-exposed sandy beaches as functional ecosystems in their own right

Emergent electronic phases in cuprate strange metals

In this thesis, the electronic structure of (Pb,Bi)²Sr²−xLaxCuO6+δ (or Bi2201), is investigated using high resolution Angle Resolved Photoemission Spectroscopy (ARPES). The phase diagram, showing the electronic phases of Bi2201 as function of doping and temperature is brought in line with those of other major cuprates, by determining the dependence of superconductivity and the pseudogap state on holedoping in a combined magnetotransport and ARPES study. Next, the spectral function in the nodal direction of the Brillouin zone is studied as function of temperature and doping. The electronic interactions are found to be excellently described by a model containing powerlaws with a momentum dependent scaling exponent. Such a mathematical description follows naturally from the holographic duality, where strongly interacting systems such as the cuprates can be modelled by a dual, gravitational problem in the framework of general relativity. This work constitutes the first time that real ARPES data is compared to holographic calculations. In the last part of the thesis, the Fermi surface of Bi2201 is investigated more closely. Density functional theory calculations using the crystal parameters as determined by X-ray diffraction are found to accurately describe the Fermi surface, and underpin a scenario where self-energy in the two nodal directions is anisotropic. In the overdoped region the states around the antinode are found to get increasingly coherent, something expressed in the width of the spectral function for k = kF . These findings can possibly account for anomalous missing carriers, as has been observed in transport experiments

Anomalous Hall effect as a probe of the chiral order in spin glasses

Anomalous Hall effect arising from the noncoplanar spin configuration (chirality) is discussed as a probe of the chiral order in spin glasses. It is shown that the Hall coefficient yields direct information about the linear and nonlinear chiral susceptibilities of the spin sector, which has been hard to obtain experimentally from the standard magnetic measurements. Based on the chirality scenario of spin-glass transition, predictions are given on the behavior of the Hall resistivity of canonical spin glasses.Comment: Order estimate of the effect given, one reference added. To appear in Phys. Rev. Letter