Spin waves in paramagnetic bcc iron: spin dynamics simulations

Large scale computer simulations are used to elucidate a long-standing controversy regarding the existence, or otherwise, of spin waves in paramagnetic bcc iron. Spin dynamics simulations of the dynamic structure factor of a Heisenberg model of Fe with first principles interactions reveal that well defined peaks persist far above Curie temperature Tc. At large wave vectors these peaks can be ascribed to propagating spin waves; at small wave vectors the peaks correspond to overdamped spin waves. Paradoxically, spin wave excitations exist despite only limited magnetic short-range order at and above Tc.     

Advances in nanomagnetism via X-ray techniques

This report examines the current status and the future directions of the field of nanomagnetism and assesses the ability of hard X-ray synchrotron facilities to provide new capabilities for making advances in this field. The report first identifies major research challenges that lie ahead in three broadly defined subfields of nanomagnetism: confined systems, clusters and complex oxides. It then examines the relevant experimental capabilities that are currently available at hard X-ray synchrotron light sources, using the Advanced Photon Source (APS) at Argonne as an example. Finally, recommendations are made for future development in X-ray facilities that will enhance the study of nanomagnetism, including new experimental directions, modifications that would enable in situ sample preparation, and measurements at high magnetic fields and/or low temperatures. In particular, in situ sample preparation is of high priority in many experiments, especially those in the area of surface magnetism.     

First observation of photoconductivity in the semiconducting phase of VO2

We report in this paper the first successful measurements of photoconductivity in the semiconducting phase of VO₂ single crystals. At low temperatures the frequency dependence of the photosensitivity (ps) exhibits an edge which gradually flattens at higher temperatures. The resulting mobility gap at 4.2 K amounts to 0.95 eV and thus differs from the optically determined band gap (0.65–0.75 eV).¹ These measurements, together with other known data suggest a “quasi-amorphous” electronic behavior of crystalline and semiconducting VO₂.     

Wang-Landau algorithm for continuous models and joint density of states

We present a modified Wang-Landau algorithm for models with continuous degrees of freedom. We demonstrate this algorithm with the calculation of the joint density of states of ferromagnet Heisenberg models and a model polymer chain. The joint density of states contains more information than the density of states of a single variable-energy, but is also much more time consuming to calculate. We present strategies to significantly speed up this calculation for large systems over a large range of energy and order parameter.     

Experimental calibration and field investigation of the oxygen isotopic fractionation between biogenic aragonite and water

Marine molluscs have long been recognised as potential records of palaeoclimate change using the patterns and differences in the stable isotopic composition of the carbonate shells. The aim of this study is to improve the robustness of this approach for aragonitic molluscs by completing the first experimental calibration of the fractionation between water and biogenic aragonite. Fractionation factors were calibrated by growing specimens of the freshwater mollusc Lymnaea peregra under controlled conditions of water temperature and isotopic composition. Fifteen populations of L. peregra were maintained at constant temperature and isotopic conditions for five months (at five different temperatures and using three different water compositions). Water samples and temperature measurements were taken regularly throughout the experiment. The temperature dependence of the fractionation factor, between 8 and 24 degrees C, is given by: 1000 ln alpha=16.74x(1000T(-1))-26.39 (T in Kelvin) and the relationship between temperature (T), delta(18)O(carb) and delta(18)O(wat) is given by: T=21.36-4.83xdelta(+ degrees )O(carb)-delta(+ degrees )O(wat) (T is in degrees C, delta(18)O(carb) is with respect to Vienna Pee Dee Belemnite (PDB), the International Atomic Energy Agency (IAEA) replacement standard for PDB, and delta(18)O(wat) is with respect to Vienna standard mean ocean water (VSMOW)) The outcome of the controlled experiment is compared with previous studies on synthetic, and biogenic, calcite and aragonite from field and laboratory investigations. These comparisons suggest that although a vital offset exists between the fractionation of isotopes in synthetic and biogenic aragonite for molluscs in general, there is no vital effect that is specific either to freshwater, or to individual, genera. Therefore, the calibrated relationship may be used for any freshwater or marine mollusc to derive palaeotemperatures providing the isotopic composition of the environmental water can be reliably constrained. Copyright 1999 John Wiley & Sons, Ltd.     

Surface composition of Ni-10at%A1(111) in thermodynamic equilibrium

Ion-scattering spectroscopy using 1000 eV He ions was applied to determine the equilibrium concentration at the (111) surface of Ni-10at%Al. Special care was taken to eliminate the falsification of the results by sulfur segregation as well as by the effects of sputtering. A technique for rapid determination of the surface concentrations is presented. It was found that the surface has to be annealed at 800°C or higher to reach the equilibrium with 25 at% Al. The surface composition remains unchanged if the temperature is subsequently varied between 700°C and 1000°C.     

A Lipophilic Derivative of Vitamin B12 as Selective Carrier for Anions

Incorporation of the lipophilic Co(III)-cobyrinate octadecyl-cobester 1 and of its ionic aqua-cyano perchlorate derivative 2 into poly(vinyl chloride)/bis(1-butylpentyl) adipate liquid membranes induces a selectivity, measured potentiometrically, of about 10³ for SCN^? an NO with respect to CI^?, but only of about 4 for ClO vs. CI^?. This is in contrast to classical anion-exchanger membranes, which exhibit a selectivity sequence ClO > SCN^? ? NO > Cl^? in accordance with the Hofmeister, series. The Co(III)-corrins 1 and 2, when components in solvent polymeric membranes, undergo exchange of axial ligands an behave as highly selective carriers fof SCN^? and NO.     

Effect of three-body forces on the lattice dynamics of noble metals

A simple method to generate an effective electron-ion interaction pseudopotential from the energy wave number characteristic obtained by first principles calculations has been suggested. This effective potential has been used, in third order perturbation, to study the effect of three-body forces on the lattice dynamics of noble metals. It is found that three-body forces, in these metals, do play an important role. The inclusion of such three-body forces appreciably improves the agreement between the experimental and theoretical phonon dispersion curves.     

Selective Transport Membranes and their Applicability for Novel Sensors

Selective transport of given substrates through artificial membranes can be achieved by incorporation of electrically neutral or charged carriers. Such mediated transport described for alkali and alkaline-earth metal cations is reported here for the first time for certain anions using cobyrinate-type ionophores, and it is also conceivable to be realized for nonionic substrates (e.g. glucose). The fundamental requirements for corresponding co-transport and counter-transport membrane systems are set forth in view of a design of novel substrate-selective sensors for analytical-chemical applications.