The H1 Forward Track Detector at HERA II

In order to maintain efficient tracking in the forward region of H1 after the luminosity upgrade of the HERA machine, the H1 Forward Track Detector was also upgraded. While much of the original software and techniques used for the HERA I phase could be reused, the software for pattern recognition was completely rewritten. This, along with several other improvements in hit finding and high-level track reconstruction, are described in detail together with a summary of the performance of the detector.Comment: Minor revision requested by journal (JINST) edito

Mechanisms limiting the coherence time of spontaneous magnetic oscillations driven by DC spin-polarized currents

The spin-transfer torque from a DC spin-polarized current can generate highly-coherent magnetic precession in nanoscale magnetic-multilayer devices. By measuring linewidths of spectra from the resulting resistance oscillations, we argue that the coherence time can be limited at low temperature by thermal deflections about the equilibrium magnetic trajectory, and at high temperature by thermally-activated transitions between dynamical modes. Surprisingly, the coherence time can be longer than predicted by simple macrospin simulations.Comment: 12 pages, 4 figure

A new multi-center approach to the exchange-correlation interactions in ab initio tight-binding methods

A new approximate method to calculate exchange-correlation contributions in the framework of first-principles tight-binding molecular dynamics methods has been developed. In the proposed scheme on-site (off-site) exchange-correlation matrix elements are expressed as a one-center (two-center) term plus a {\it correction} due to the rest of the atoms. The one-center (two-center) term is evaluated directly, while the {\it correction} is calculated using a variation of the Sankey-Niklewski \cite{Sankey89} approach generalized for arbitrary atomic-like basis sets. The proposed scheme for exchange-correlation part permits the accurate and computationally efficient calculation of corresponding tight-binding matrices and atomic forces for complex systems. We calculate bulk properties of selected transition (W,Pd), noble (Au) or simple (Al) metals, a semiconductor (Si) and the transition metal oxide Ti$O_2$ with the new method to demonstrate its flexibility and good accuracy.Comment: 17 pages, 5 figure

First-Principles Studies of Hydrogenated Si(111)--7×\times7

The relaxed geometries and electronic properties of the hydrogenated phases of the Si(111)-7$\times$7 surface are studied using first-principles molecular dynamics. A monohydride phase, with one H per dangling bond adsorbed on the bare surface is found to be energetically favorable. Another phase where 43 hydrogens saturate the dangling bonds created by the removal of the adatoms from the clean surface is found to be nearly equivalent energetically. Experimental STM and differential reflectance characteristics of the hydrogenated surfaces agree well with the calculated features.Comment: REVTEX manuscript with 3 postscript figures, all included in uu file. Also available at http://www.phy.ohiou.edu/~ulloa/ulloa.htm

Evaluation of Exchange-Correlation Energy, Potential, and Stress

We describe a method for calculating the exchange and correlation (XC) contributions to the total energy, effective potential, and stress tensor in the generalized gradient approximation. We avoid using the analytical expressions for the functional derivatives of E_xc*rho, which depend on discontinuous second-order derivatives of the electron density rho. Instead, we first approximate E_xc by its integral in a real space grid, and then we evaluate its partial derivatives with respect to the density at the grid points. This ensures the exact consistency between the calculated total energy, potential, and stress, and it avoids the need of second-order derivatives. We show a few applications of the method, which requires only the value of the (spin) electron density in a grid (possibly nonuniform) and returns a conventional (local) XC potential.Comment: 7 pages, 3 figure

Tunable linear and quadratic optomechanical coupling for a tilted membrane within an optical cavity: theory and experiment

We present an experimental study of an optomechanical system formed by a vibrating thin semi-transparent membrane within a high-finesse optical cavity. We show that the coupling between the optical cavity modes and the vibrational modes of the membrane can be tuned by varying the membrane position and orientation. In particular we demonstrate a large quadratic dispersive optomechanical coupling in correspondence with avoided crossings between optical cavity modes weakly coupled by scattering at the membrane surface. The experimental results are well explained by a first order perturbation treatment of the cavity eigenmodes.Comment: 10 pages, 6 figure

Numerical atomic orbitals for linear scaling

The performance of basis sets made of numerical atomic orbitals is explored in density-functional calculations of solids and molecules. With the aim of optimizing basis quality while maintaining strict localization of the orbitals, as needed for linear-scaling calculations, several schemes have been tried. The best performance is obtained for the basis sets generated according to a new scheme presented here, a flexibilization of previous proposals. The basis sets are tested versus converged plane-wave calculations on a significant variety of systems, including covalent, ionic and metallic. Satisfactory convergence (deviations significantly smaller than the accuracy of the underlying theory) is obtained for reasonably small basis sizes, with a clear improvement over previous schemes. The transferability of the obtained basis sets is tested in several cases and it is found to be satisfactory as well.Comment: 9 pages with 2 encapsulated postscript figures, submitted to Phys. Rev.

Strain Relaxation Mechanisms and Local Structural Changes in Si_{1-x}$Ge_{x} Alloys

In this work, we address issues pertinent to the understanding of the structural and electronic properties of Si_{1-x} Ge_{x}alloys, namely, (i) how does the lattice constant mismatch between bulk Si and bulk Ge manifests itself in the alloy system? and (ii) what are the relevant strain release mechanisms? To provide answers to these questions, we have carried out an in-depth study of the changes in the local geometric and electronic structures arising from the strain relaxation in Si_{1-x} Ge_{x} alloys using an ab initio molecular dynamics scheme. The optimized lattice constant, while exhibiting a general trend of linear dependence on the composition (Vegard's law), shows a negative deviation from Vegard's law in the vicinity of x=0.5. We delineate the mechanisms responsible for each one of the above features. We show that the radial-strain relaxation through bond stretching is responsible for the overall trend of linear dependence of the lattice constant on the composition. On the other hand, the negative deviation from Vegard's law is shown to arise from the angular-strain relaxation.Comment: 21 pages, 7 figure

Fast algorithm for calculating two-photon absorption spectra

We report a numerical calculation of the two-photon absorption coefficient of electrons in a binding potential using the real-time real-space higher-order difference method. By introducing random vector averaging for the intermediate state, the task of evaluating the two-dimensional time integral is reduced to calculating two one-dimensional integrals. This allows the reduction of the computation load down to the same order as that for the linear response function. The relative advantage of the method compared to the straightforward multi-dimensional time integration is greater for the calculation of non-linear response functions of higher order at higher energy resolution.Comment: 4 pages, 2 figures. It will be published in Phys. Rev. E on 1, March, 199