Nonlinear effects of electrolyte diodes and transistors in a polymer gel medium

The polarization curve of an acid-base interface in a hydrogel medium has a diode characteristic. Two of each such electrolyte diodes can be combined to give an electrolyte transistor. When a salt is added to the alkaline or to the acidic part of a reverse biased electrolyte diode, the current response is highly nonlinear. If the salt is added to the acidic side, even bistability can be observed. This bistability can generate complex oscillations in a base-acid-base electrolyte transistor. These nonlinear effects are studied experimentally and theoretically. While the nonlinear salt effect can be explained with the Nernst-Planck equations, to understand the bistable behavior further investigations are necessary. (c) 1999 American Institute of Physics.     

Tailoring magnetism in artificially structured materials: the new frontier

The current standard of electronic devices and data storage media has reached a level such that magnetic materials have to be fabricated on a nanometer scale. In particular, the emerging concept of spintronics, which is based on fact that current carriers have not only charge but also spin, requires the assembling of nanometer-sized magnetic structures with desired magnetic properties. It is this background that motivates scientists and engineers to attempt to grow and characterize magnetic objects at smaller and smaller length scales, from 2D films and multilayers to 1D wires and eventually to 0D dots. In this article, some of the most significant progress in recent years in the effort of growing artificially structured magnetic materials are reviewed. The new structural and magnetic properties of these materials are discussed, with an emphasis on the correlation between structure and magnetism, which also serves as guidance for improving their magnetic properties. The emerging emphasis is on converting the existing knowledge into growing and studying low-dimensional complex materials, which promise to have considerably higher “tuning” ability for desired properties.     

On the influence of backscattered electrons on the lateral resolution in scanning auger microscopy

A definition of edge resolution is proposed, which is adapted to the pecularities of scanning Auger microscopy. Based on recent monte-Carlo computer simulations for scanning electron microscopy, the influence of backscattered electrons on the edge resolution is estimated for low-Z (Al) and high-Z materials (Au). The resolution is found to be of the order of 100nm and to be nearly independent of the atomic number of the sample.     

Mapping out electron-electron interactions at surfaces

Using a high resolution coincidence technique, we measured for the first time the angular and energy correlation of an electron pair emitted from the valence band of a single crystal upon the impact of an electron with a specified momentum. We observe a hole in the measured two-particle correlation function when the two excited electrons have comparable momentum vectors, a fact traced back to exchange and repulsion among the electrons. We find the hole is not isotropic, has a finite extension, and is strongly suppressed when decoherence is operating.     

Absence of zero-bias anomaly in spin-polarized vacuum tunneling in Co(0001)

In a joint experimental and theoretical study, we investigate the bias-voltage dependence of the tunnel magnetoresistance (TMR) through a vacuum barrier. The TMR observed by spin-polarized scanning tunneling microscopy between an amorphous magnetic tip and a Co(0001) sample is almost independent of the bias voltage at large tip-sample separations. Whereas qualitative understanding is achieved by means of the electronic surface structure of Co, the experimental findings are compared quantitatively with bias-voltage dependent first-principles calculations for ballistic tunneling. At small tip-sample separations, a pronounced minimum in the experimental TMR was found at +200 mV bias.     

A miniaturized detector for high-resolution SEMPA

The basics of the scanning electron microscope with polarization analysis are briefly reviewed, emphasizing the achievable magnetic resolution and image contrast. The design of an optimized spin-polarization detector based on the well-established LEED scattering principle is presented. Results of first tests are reported. Received: 2 September 2002 / Accepted: 2 September 2002 / Published online: 5 March 2003 RID="*" ID="*"Corresponding author. Fax: +49-40/42838-6368, E-mail: RFroemte@PHYSnet.Uni-Hamburg.de     

Quantum mechanical study of the nonbonded forces in water-methanol complexes

The water-methanol dimer can adopt two possible configurations (WdM or MdW) depending on whether the water or the methanol acts as the hydrogen bond donor. The relative stability between the two configurations is less than 1 kcal/mol, and as a result, this dimer has been a challenging system to investigate using either theoretical or experimental techniques. In this paper, we present a systematic study of the dependence of the geometries, interaction energies, and harmonic frequencies on basis sets and treatment of electron correlation for the two configurations. At the highest theory level, MP2/aug-cc-pVQZ//MP2/aug-cc-pVTZ, interaction energies of -5.72 and -4.95 kcal/mol were determined for the WdM and MdW configurations, respectively, after correcting for basis set superposition error using the Boys-Bernardi counterpoise scheme. Extrapolating to the complete basis set limit resulted in interaction energies of -5.87 for WdM and -5.16 kcal/mol for MdW. The energy difference between the two configurations is larger than the majority of previously reported values, confirming that the WdM complex is preferred, in agreement with experimental observations. The effects that electron correlation have on the geometry were investigated by performing optimization at the MP2(full), MP4, and CCSD levels of theory. The approach trajectories for the formation of each dimer configuration are presented and the importance of these trajectories in the development of parameters for use in classical force fields is discussed.     

Direct evidence for mesoscopic relaxations in cobalt nanoislands on Cu(001)

Surface x-ray diffraction in combination with scanning tunneling microscopy and molecular dynamics calculations provide first quantitative evidence for unusually large relaxations in nanometer-sized Co islands deposited on Cu(001) at 170 K. These lead to sharply reduced interatomic Co distances as low as 2.36 A as compared to bulk Co (2.51 A) involving low symmetry Co adsorption sites. Our results prove the validity of the concept of mesoscopic mismatch which governs the strain relaxation of nanosized islands in general.