Exhaustion of Nucleation in a Closed System

We determine the distribution of cluster sizes that emerges from an initial phase of homogeneous aggregation with conserved total particle density. The physical ingredients behind the predictions are essentially classical: Super-critical nuclei are created at the Zeldovich rate, and before the depletion of monomers is significant, the characteristic cluster size is so large that the clusters undergo diffusion limited growth. Mathematically, the distribution of cluster sizes satisfies an advection PDE in "size-space". During this creation phase, clusters are nucleated and then grow to a size much larger than the critical size, so nucleation of super-critical clusters at the Zeldovich rate is represented by an effective boundary condition at zero size. The advection PDE subject to the effective boundary condition constitutes a "creation signaling problem" for the evolving distribution of cluster sizes during the creation era. Dominant balance arguments applied to the advection signaling problem show that the characteristic time and cluster size of the creation era are exponentially large in the initial free-energy barrier against nucleation, G_*. Specifically, the characteristic time is proportional to exp(2 G_*/ 5 k_B T) and the characteristic number of monomers in a cluster is proportional to exp(3G_*/5 k_B T). The exponentially large characteristic time and cluster size give a-posteriori validation of the mathematical signaling problem. In a short note, Marchenko obtained these exponentials and the numerical pre-factors, 2/5 and 3/5. Our work adds the actual solution of the kinetic model implied by these scalings, and the basis for connection to subsequent stages of the aggregation process after the creation era.Comment: Greatly shortened paper. Section on growth model removed. Added a section analyzing the error in the solution of the integral equation. Added reference

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

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

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

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

Cardiovascular impairment in Shiga-toxin-2-induced experimental hemolytic-uremic syndrome: a pilot study.

INTRODUCTION: Hemolytic-uremic syndrome (HUS) can occur as a systemic complication of infection with Shiga toxin (Stx)-producing Escherichia coli (STEC). Most well-known aspects of the pathophysiology are secondary to microthrombotic kidney disease including hemolytic anemia and thrombocytopenia. However, extrarenal manifestations, such as cardiac impairment, have also been reported. We have investigated whether these cardiac abnormalities can be reproduced in a murine animal model, in which administration of Stx, the main virulence factor of STEC, is used to induce HUS. METHODS: Mice received either one high or multiple low doses of Stx to simulate the (clinically well-known) different disease courses. Cardiac function was evaluated by echocardiography and analyses of biomarkers in the plasma (troponin I and brain natriuretic peptide). RESULTS: All Stx-challenged mice showed reduced cardiac output and depletion of intravascular volume indicated by a reduced end-diastolic volume and a higher hematocrit. Some mice exhibited myocardial injury (measured as increases in cTNI levels). A subset of mice challenged with either dosage regimen showed hyperkalemia with typical electrocardiographic abnormalities. DISCUSSION: Myocardial injury, intravascular volume depletion, reduced cardiac output, and arrhythmias as a consequence of hyperkalemia may be prognosis-relevant disease manifestations of HUS, the significance of which should be further investigated in future preclinical and clinical studies

Laboratory astrophysics on ASDEX Upgrade: Measurements and analysis of K-shell O, F, and Ne spectra in the 9 - 20 A region

High-resolution measurements of K-shell emission from O, F, and Ne have been performed at the ASDEX Upgrade tokamak in Garching, Germany. Independently measured temperature and density profiles of the plasma provide a unique test bed for model validation. We present comparisons of measured spectra with calculations based on transport and collisional-radiative models and discuss the reliability of commonly used diagnostic line ratios

Engineering chromium related single photon emitters in single crystal diamond

Color centers in diamond as single photon emitters, are leading candidates for future quantum devices due to their room temperature operation and photostability. The recently discovered chromium related centers are particularly attractive since they possess narrow bandwidth emission and a very short lifetime. In this paper we investigate the fabrication methodologies to engineer these centers in monolithic diamond. We show that the emitters can be successfully fabricated by ion implantation of chromium in conjunction with oxygen or sulfur. Furthermore, our results indicate that the background nitrogen concentration is an important parameter, which governs the probability of success to generate these centers.Comment: 14 pages, 5 figure