NASTRAN as a resource in code development

A case history is presented in which the NASTRAN system provided both guidelines and working software for use in the development of a discrete element program, PATCHES-111. To avoid duplication and to take advantage of the wide spread user familiarity with NASTRAN, the PATCHES-111 system uses NASTRAN bulk data syntax, NASTRAN matrix utilities, and the NASTRAN linkage editor. Problems in developing the program are discussed along with details on the architecture of the PATCHES-111 parametric cubic modeling system. The system includes model construction procedures, checkpoint/restart strategies, and other features

Monopolelike probes for quantitative magnetic force microscopy: calibration and application

A local magnetization measurement was performed with a Magnetic Force Microscope (MFM) to determine magnetization in domains of an exchange coupled [Co/Pt]/Co/Ru multilayer with predominant perpendicular anisotropy. The quantitative MFM measurements were conducted with an iron filled carbon nanotube tip, which is shown to behave like a monopole. As a result we determined an additional in-plane magnetization component of the multilayer, which is explained by estimating the effective permeability of the sample within the \mu*-method.Comment: 3 pages, 3 figure

Domain structure of epitaxial Co films with perpendicular anisotropy

Epitaxial hcp Cobalt films with pronounced c-axis texture have been prepared by pulsed lased deposition (PLD) either directly onto Al2O3 (0001) single crystal substrates or with an intermediate Ruthenium buffer layer. The crystal structure and epitaxial growth relation was studied by XRD, pole figure measurements and reciprocal space mapping. Detailed VSM analysis shows that the perpendicular anisotropy of these highly textured Co films reaches the magnetocrystalline anisotropy of hcp-Co single crystal material. Films were prepared with thickness t of 20 nm < t < 100 nm to study the crossover from in-plane magnetization to out-of-plane magnetization in detail. The analysis of the periodic domain pattern observed by magnetic force microscopy allows to determine the critical minimum thickness below which the domains adopt a pure in-plane orientation. Above the critical thickness the width of the stripe domains is evaluated as a function of the film thickness and compared with domain theory. Especially the discrepancies at smallest film thicknesses show that the system is in an intermediate state between in-plane and out-of-plane domains, which is not described by existing analytical domain models

Tunneling dynamics of side chains and defects in proteins, polymer glasses, and OH-doped network glasses

Simulations on a Lennard-Jones computer glass are performed to study effects arising from defects in glasses at low temperatures. The numerical analysis reveals that already a low concentration of defects may dramatically change the low temperature properties by giving rise to extrinsic double-well potentials (DWP's). The main characteristics of these extrinsic DWP's are (i) high barrier heights, (ii) high probability that a defect is indeed connected with an extrinsic DWP, (iii) highly localized dynamics around this defect, and (iv) smaller deformation potential coupling to phonons. Designing an extension of the Standard Tunneling Model (STM) which parametrizes this picture and comparing with ultrasound experiments on the wet network glass $a$-B$_2$O$_3$ shows that effects of OH-impurities are accurately accounted for. This model is then applied to organic polymer glasses and proteins. It is suggested that side groups may act similarly like doped impurities inasmuch as extrinsic DWP's are induced, which possess a distribution of barriers peaked around a high barrier height. This compares with the structurlessly distributed barrier heights of the intrinsic DWP's, which are associated with the backbone dynamics. It is shown that this picture is consistent with elastic measurements on polymers, and can explain anomalous nonlogarithmic line broadening recently observed in hole burning experiments in PMMA.Comment: 34 pages, Revtex, 9 eps-figures, accepted for publication in J. Chem. Phy

Rigorous mean field model for CPA: Anderson model with free random variables

A model of a randomly disordered system with site-diagonal random energy fluctuations is introduced. It is an extension of Wegner's $n$-orbital model to arbitrary eigenvalue distribution in the electronic level space. The new feature is that the random energy values are not assumed to be independent at different sites but free. Freeness of random variables is an analogue of the concept of independence for non-commuting random operators. A possible realization is the ensemble of at different lattice-sites randomly rotated matrices. The one- and two-particle Green functions of the proposed hamiltonian are calculated exactly. The eigenstates are extended and the conductivity is nonvanishing everywhere inside the band. The long-range behaviour and the zero-frequency limit of the two-particle Green function are universal with respect to the eigenvalue distribution in the electronic level space. The solutions solve the CPA-equation for the one- and two-particle Green function of the corresponding Anderson model. Thus our (multi-site) model is a rigorous mean field model for the (single-site) CPA. We show how the Llyod model is included in our model and treat various kinds of noises.Comment: 24 pages, 2 diagrams, Rev-Tex. Diagrams are available from the authors upon reques

Random matrix theory for CPA: Generalization of Wegner's nn--orbital model

We introduce a generalization of Wegner's $n$-orbital model for the description of randomly disordered systems by replacing his ensemble of Gaussian random matrices by an ensemble of randomly rotated matrices. We calculate the one- and two-particle Green's functions and the conductivity exactly in the limit $n\to\infty$. Our solution solves the CPA-equation of the $(n=1)$-Anderson model for arbitrarily distributed disorder. We show how the Lloyd model is included in our model.Comment: 3 pages, Rev-Te

Direct observation of superconducting vortex clusters pinned by a periodic array of magnetic dots in ferromagnetic/superconducting hybrid structures

Strong pinning of superconducting flux quanta by a square array of 1 $\mu$m-sized ferromagnetic dots in a magnetic-vortex state was visualized by low-temperature magnetic force microscopy (LT-MFM). A direct correlation of the superconducting flux lines with the positions of the dots was derived. The force that the MFM tip exerts on the individual vortex in the depinning process was used to estimate the spatial modulation of the pinning potential. It was found, that the superconducting vortices which are preferably located on top of the Py dots experience about 15 times stronger pinning forces as compared to the pinning force in the pure Nb film. The strong pinning exceeds the repulsive interaction between the superconducting vortices and allows the vortex clusters to be located at each dot. Our microscopic studies are consistent with global magnetoresistace measurements on these hybrid structures.Comment: 4 pages, 4 figure

Quantitative assessment of pinning forces and the superconducting gap in NbN thin films from complementary magnetic force microscopy and transport measurements

Epitaxial niobium-nitride thin films with a critical temperature of Tc=16K and a thickness of 100nm were fabricated on MgO(100) substrates by pulsed laser deposition. Low-temperature magnetic force microscopy (MFM) images of the supercurrent vortices were measured after field cooling in a magnetic field of 3mT at various temperatures. Temperature dependence of the penetration depth has been evaluated by a two-dimensional fitting of the vortex profiles in the monopole-monopole model. Its subsequent fit to a single s-wave gap function results in the superconducting gap amplitude Delta(0) = 2.9 meV = 2.1*kB*Tc, in perfect agreement with previous reports. The pinning force has been independently estimated from local depinning of individual vortices by lateral forces exerted by the MFM tip and from transport measurements. A good quantitative agreement between the two techniques shows that for low fields, B << Hc2, MFM is a powerful and reliable technique to probe the local variations of the pinning landscape. We also demonstrate that the monopole model can be successfully applied even for thin films with a thickness comparable to the penetration depth.Comment: 6 pages, 6 figures, 2 table