Dissociation of O2 at Al(111): The Role of Spin Selection Rules

A most basic and puzzling enigma in surface science is the description of the dissociative adsorption of O2 at the (111) surface of Al. Already for the sticking curve alone, the disagreement between experiment and results of state-of-the-art first-principles calculations can hardly be more dramatic. In this paper we show that this is caused by hitherto unaccounted spin selection rules, which give rise to a highly non-adiabatic behavior in the O2/Al(111) interaction. We also discuss problems caused by the insufficient accuracy of present-day exchange-correlation functionals.Comment: 4 pages including 3 figures; related publications can be found at http://www.fhi-berlin.mpg.de/th/th.htm

The ASDEX Upgrade UTDC and DIO cards - A family of PCI/cPCI devices for real-time DAQ under Solaris

A Universal Time to Digital Converter (UTDC) and a Digital I/O (DIO) card have been built to purpose for the new ASDEX Upgrade control system, the Thomson Scattering diagnostic, a renovation of the Magnetic Measurement, and a couple of other diagnostics requiring renewed timing or real-time features. The salient features of these cards will be presented, and we will show how synergy in hardware and software development could be achieved. Examples of real-life applications in diagnostics will be given. Measurements of the real-time behaviour of those diagnostics show a low jitter of the Solaris operating system and highly reliable function under moderate real-time conditions. Future extension of the usability of this family of cards will be described as an outlook. Topic: Diagnostic Control and Data Acquisitio

Non-adiabatic Effects in the Dissociation of Oxygen Molecules at the Al(111) Surface

The measured low initial sticking probability of oxygen molecules at the Al(111) surface that had puzzled the field for many years was recently explained in a non-adiabatic picture invoking spin-selection rules [J. Behler et al., Phys. Rev. Lett. 94, 036104 (2005)]. These selection rules tend to conserve the initial spin-triplet character of the free O2 molecule during the molecule's approach to the surface. A new locally-constrained density-functional theory approach gave access to the corresponding potential-energy surface (PES) seen by such an impinging spin-triplet molecule and indicated barriers to dissociation which reduce the sticking probability. Here, we further substantiate this non-adiabatic picture by providing a detailed account of the employed approach. Building on the previous work, we focus in particular on inaccuracies in present-day exchange-correlation functionals. Our analysis shows that small quantitative differences in the spin-triplet constrained PES obtained with different gradient-corrected functionals have a noticeable effect on the lowest kinetic energy part of the resulting sticking curve.Comment: 17 pages including 11 figures; related publications can be found at http://www.fhi-berlin.mpg.de/th/th.htm

Non-Adiabatic Potential-Energy Surfaces by Constrained Density-Functional Theory

Non-adiabatic effects play an important role in many chemical processes. In order to study the underlying non-adiabatic potential-energy surfaces (PESs), we present a locally-constrained density-functional theory approach, which enables us to confine electrons to sub-spaces of the Hilbert space, e.g. to selected atoms or groups of atoms. This allows to calculate non-adiabatic PESs for defined charge and spin states of the chosen subsystems. The capability of the method is demonstrated by calculating non-adiabatic PESs for the scattering of a sodium and a chlorine atom, for the interaction of a chlorine molecule with a small metal cluster, and for the dissociation of an oxygen molecule at the Al(111) surface.Comment: 11 pages including 7 figures; related publications can be found at http://www.fhi-berlin.mpg.de/th/th.htm

Coulomb Gap and Correlated Vortex Pinning in Superconductors

The positions of columnar pins and magnetic flux lines determined from a decoration experiment on BSCCO were used to calculate the single--particle density of states at low temperatures in the Bose glass phase. A wide Coulomb gap is found, with gap exponent $s \approx 1.2$, as a result of the long--range interaction between the vortices. As a consequence, the variable--range hopping transport of flux lines is considerably reduced with respect to the non--interacting case, the effective Mott exponent being enhanced from $p_0 = 1/3$ to $p_{\rm eff} \approx 0.5$ for this specific experiment.Comment: 10 pages, Revtex, 4 figures appended as uu-encoded postscript files, also available as hardcopies from [email protected]

Comparison of Aquatic-Insect Habitat and Diversity Above and Below Road Crossings in Low-Order Streams

The effects of road crossings on fish communities have been extensively studied; yet little attention has been given to macroinvertebrate communities. This study evaluated physical stream characteristics, water quality, and aquatic-insect richness from above and below road crossings of low-order streams in the Ouachita National Forest in Arkansas. Fifteen road crossings were sampled during October and November 2005. Erosion was significantly higher below road crossings than above. Sites downstream of road crossings had significantly lower pH and significantly higher turbidity than sites upstream of road crossings. Despite differences in water quality and habitat, there was no apparent difference in aquatic-insect richness from above and below road crossings based on the EPT index, suggesting that road crossings did not act as barriers to insect movement. The water-quality differences observed were well within acceptable limits and likely not biologically important

Performance and Cost Assessment of Machine Learning Interatomic Potentials.

Machine learning of the quantitative relationship between local environment descriptors and the potential energy surface of a system of atoms has emerged as a new frontier in the development of interatomic potentials (IAPs). Here, we present a comprehensive evaluation of machine learning IAPs (ML-IAPs) based on four local environment descriptors-atom-centered symmetry functions (ACSF), smooth overlap of atomic positions (SOAP), the spectral neighbor analysis potential (SNAP) bispectrum components, and moment tensors-using a diverse data set generated using high-throughput density functional theory (DFT) calculations. The data set comprising bcc (Li, Mo) and fcc (Cu, Ni) metals and diamond group IV semiconductors (Si, Ge) is chosen to span a range of crystal structures and bonding. All descriptors studied show excellent performance in predicting energies and forces far surpassing that of classical IAPs, as well as predicting properties such as elastic constants and phonon dispersion curves. We observe a general trade-off between accuracy and the degrees of freedom of each model and, consequently, computational cost. We will discuss these trade-offs in the context of model selection for molecular dynamics and other applications

Interactions, Distribution of Pinning Energies, and Transport in the Bose Glass Phase of Vortices in Superconductors

We study the ground state and low energy excitations of vortices pinned to columnar defects in superconductors, taking into account the long--range interaction between the fluxons. We consider the ``underfilled'' situation in the Bose glass phase, where each flux line is attached to one of the defects, while some pins remain unoccupied. By exploiting an analogy with disordered semiconductors, we calculate the spatial configurations in the ground state, as well as the distribution of pinning energies, using a zero--temperature Monte Carlo algorithm minimizing the total energy with respect to all possible one--vortex transfers. Intervortex repulsion leads to strong correlations whenever the London penetration depth exceeds the fluxon spacing. A pronounced peak appears in the static structure factor $S(q)$ for low filling fractions $f \leq 0.3$. Interactions lead to a broad Coulomb gap in the distribution of pinning energies $g(\epsilon)$ near the chemical potential $\mu$, separating the occupied and empty pins. The vanishing of $g(\epsilon)$ at $\mu$ leads to a considerable reduction of variable--range hopping vortex transport by correlated flux line pinning.Comment: 16 pages (twocolumn), revtex, 16 figures not appended, please contact [email protected]