Stochastic Properties of Static Friction

The onset of frictional motion is mediated by rupture-like slip fronts, which nucleate locally and propagate eventually along the entire interface causing global sliding. The static friction coefficient is a macroscopic measure of the applied force at this particular instant when the frictional interface loses stability. However, experimental studies are known to present important scatter in the measurement of static friction; the origin of which remains unexplained. Here, we study the nucleation of local slip at interfaces with slip-weakening friction of random strength and analyze the resulting variability in the measured global strength. Using numerical simulations that solve the elastodynamic equations, we observe that multiple slip patches nucleate simultaneously, many of which are stable and grow only slowly, but one reaches a critical length and starts propagating dynamically. We show that a theoretical criterion based on a static equilibrium solution predicts quantitatively well the onset of frictional sliding. We develop a Monte-Carlo model by adapting the theoretical criterion and pre-computing modal convolution terms, which enables us to run efficiently a large number of samples and to study variability in global strength distribution caused by the stochastic properties of local frictional strength. The results demonstrate that an increasing spatial correlation length on the interface, representing geometric imperfections and roughness, causes lower global static friction. Conversely, smaller correlation length increases the macroscopic strength while its variability decreases. We further show that randomness in local friction properties is insufficient for the existence of systematic precursory slip events. Random or systematic non-uniformity in the driving force, such as potential energy or stress drop, is required for arrested slip fronts. Our model and observations..

Reversible inhibition of mammalian tubulin assembly in vitro and effects in Saccharomyces cerevisiae D61.M by mitomycin C

Gaulden reported a novel and unexpected mitomycin C (MMC) effect, namely a pronounced retardation of very late prophase and loss of chromosome orientation in neuroblasts of the grasshopper Chortophaga viridifasciate. Because this effect may be due to interactions of MMC with non-DNA targets, MMC was tested for its interaction with porcine brain tubulin assembly in vitro and for the induction of chromosomal malsegregation in the diploid yeast Saccharomyces cerevisiae strain D61.M. A reversible dose-dependent inhibition of tubulin assembly was observed. Since no biological activation system was present in the incubation mixture this inhibition seems to result from an interaction of unactivated MMC with the assembly process. The possible chemical activation of MMC by reduction with 1, 4-dithioerythritol (DTE) was investigated by omission of this compound during isolation and polymerization of tubulin. The absence of DTE resulted in a strong reduction of the net tubulin assembly. Also under these conditions MMC led to a dose-dependent inhibition of the assembly, indicating that the effect of MMC on tubulin assembly is independent of a reductive chemical modification. In S.cerevisiae D61.M, MMC did not induce chromosome loss, but induced other genetic events (possibly mutations, deletions or mitotic recombination) as was detected by an increase of the total number and of the frequency of cycloheximide-resistant colonies. This effect could be observed with and without the addition of rat liver S9 as an exogenous activation syste

Analyticity and Integrabiity in the Chiral Potts Model

We study the perturbation theory for the general non-integrable chiral Potts model depending on two chiral angles and a strength parameter and show how the analyticity of the ground state energy and correlation functions dramatically increases when the angles and the strength parameter satisfy the integrability condition. We further specialize to the superintegrable case and verify that a sum rule is obeyed.Comment: 31 pages in harvmac including 9 tables, several misprints eliminate

Parallelism for Quantum Computation with Qudits

Robust quantum computation with d-level quantum systems (qudits) poses two requirements: fast, parallel quantum gates and high fidelity two-qudit gates. We first describe how to implement parallel single qudit operations. It is by now well known that any single-qudit unitary can be decomposed into a sequence of Givens rotations on two-dimensional subspaces of the qudit state space. Using a coupling graph to represent physically allowed couplings between pairs of qudit states, we then show that the logical depth of the parallel gate sequence is equal to the height of an associated tree. The implementation of a given unitary can then optimize the tradeoff between gate time and resources used. These ideas are illustrated for qudits encoded in the ground hyperfine states of the atomic alkalies $^{87}$Rb and $^{133}$Cs. Second, we provide a protocol for implementing parallelized non-local two-qudit gates using the assistance of entangled qubit pairs. Because the entangled qubits can be prepared non-deterministically, this offers the possibility of high fidelity two-qudit gates.Comment: 9 pages, 3 figure

Ferromagnetic resonance study of Fe/FePt coupled films with perpendicular anisotropy

Exchange spring magnets with perpendicular magnetic anisotropy represent new magnetic properties with respect to their constituent components. These systems typically consist of a hard magnetic layer and a soft magnetic layer which are strongly coupled. The modification of their bulk magnetic properties arises from this strong ferromagnetic exchange coupling, interfacial effects and competing magnetic anisotropies of the two magnetic layers. We have studied the magnetic bilayer system which consists of an Fe (soft) film exchange coupled to an FePt (hard) layer which has an easy axis aligned along the direction perpendicular to the film plane. The entire structure has the form: MgO/FePt (10 nm)/Fe (2nm or 3.5nm)/ Ag (2nm), where the Ag overlayer acts as protection against oxidation. The epitaxial FePt layers were deposit- ed on MgO (100) substrates using the RF sputtering technique at a substrate temperature of about 390 C. The epitaxy of this layer was studied using x-ray and electron diffraction techniques. Layer morphologies were further studied using atomic force microscopy (AFM), these studies reveal a granular morphology with grain sizes of the order of 40 - 50 nm. We have made detailed angular measurements using the ferromagnetic resonance (FMR) at room temperature. This angular FMR study, which includes the orientations of in-plane and out-of-plane, was performed in order to study the magnetic anisotropies as well as the exchange coupling between the magnetic layers and interfacial effects. In particular, we have chosen to study two samples with 2 nm and 3.5 nm of Fe, which effectively constitute the rigid magnet (RM) and exchange spring (ES) regimes, respectively. The RM and ES regimes depend implicitly on the magnetic anositropies and properties of the two coupled layers [1]. In figure 1 (a) we show an example of an FMR spectrum for the RM (2 nm Fe) sample. Of the various resonances observed, only the three low field lines are due to the Fe layer. It will be noted that the FePt does not have any FMR signature in the field range studied due to it very high magnetocrystalline anisotropy. The other resonance features evident in the spectrum arise from the MgO substrate and show no significant angular variations. As such the only FMR signals observed in our samples will arise from the Fe layer. In figure 1 (b) we show the angular variation of the resonance field of the three Fe resonance lines. Of these, two resonances display a uniaxial anisotropy with the easy axes aligned along the direction perpendicular to the film plane and will be directly related to the exchange coupling with the hard (FePt) layer. The third resonance, while also manifesting a uniaxial anisotropy, displays an easy axis direction which is canted by about 50 degrees from the film normal. While the origin of this resonance is not entirely clear, we suspect it may arise from the interfacial region between the FePt and Fe layers. In figure 2 we show the corresponding FMR results for the ES (3.5 nm Fe) sample. It will be noted that in addition to the resonances observed in the RM sample, there are a further two resonance, whose angular dependences are illustrated in figure 2 (b). These also display a uniaxial like behaviour with easy axes close to the film normal. In all spectra lines were fit using a home made programme which allows multiple peak fitting of Lorentzian and Gaussian lines. We develop a model of FMR based on the magnetic free energy of the coupled layers which is required to interpret the angular dependences of the resonance fields [1]. Existing models fall short of a full explanation of all the resonance lines and we are working to bridge this gap by considering the effects of boundary conditions and spin wave modes. [1] G. Asti et al., Phys. Rev. B, 73, 094406 (2006

The Q-operator and Functional Relations of the Eight-vertex Model at Root-of-unity η=2mKN\eta = \frac{2m K}{N} for odd N

Following Baxter's method of producing Q_{72}-operator, we construct the Q-operator of the root-of-unity eight-vertex model for the crossing parameter $\eta = \frac{2m K}{N}$ with odd $N$ where Q_{72} does not exist. We use this new Q-operator to study the functional relations in the Fabricius-McCoy comparison between the root-of-unity eight-vertex model and the superintegrable N-state chiral Potts model. By the compatibility of the constructed Q-operator with the structure of Baxter's eight-vertex (solid-on-solid) SOS model, we verify the set of functional relations of the root-of-unity eight-vertex model using the explicit form of the Q-operator and fusion weights of SOS model.Comment: Latex 28 page; Typos corrected, minor changes in presentation, References added and updated-Journal versio

Risk factors of overweight and obesity among preschool children with different ethnic background

partially_open5noIn this study, we evaluated the risk factors associated with overweight and obesity in 2,640 preschool children in Italy taking into account the ethnic background of the parents. Height and weight were measured and BMI was calculated. Personal and lifestyle data for the children (birth weight, type of breastfeeding, sleep duration, skipping breakfast, snacking, physical activity) and parents (ethnicity, educational level, occupation, weight, height) were collected by means of a questionnaire. Italian and Other European children generally showed the highest percentage of normal weight, while the other ethnic groups presented a greater imbalance. Overweight and obesity were high in African males, who also presented high birth weight. Breastfeeding was most common, although formula feeding was significantly higher in Italians than in immigrants. Immigrants, particularly males, tended to skip breakfast more than Italians. Physical activity was significantly higher in Italians than in immigrants. In the parents, underweight was particularly high in Italian and Other mothers. African parents had high rates of overweight and obesity and a low educational level. The most common profession was worker for the fathers and housewife for the mothers, with the exception of Italians in which clerical work prevailed. Multiple linear regression analysis showed that the BMI of children was closely related to the BMI of the parents and the birth weight. Hence, these are the most informative parameters in preventing obesity.openToselli S; Zaccagni L; Celenza F; Albertini A; Gualdi-Russo EToselli, S; Zaccagni, Luciana; Celenza, F; Albertini, A; Gualdi, Emanuel

Completeness of the Bethe Ansatz solution of the open XXZ chain with nondiagonal boundary terms

A Bethe Ansatz solution of the open spin-1/2 XXZ quantum spin chain with nondiagonal boundary terms has recently been proposed. Using a numerical procedure developed by McCoy et al., we find significant evidence that this solution can yield the complete set of eigenvalues for generic values of the bulk and boundary parameters satisfying one linear relation. Moreover, our results suggest that this solution is practical for investigating the ground state of this model in the thermodynamic limit.Comment: 15 pages, LaTeX; amssymb, amsmath, no figures, 5 tables; v2 contains an additional footnote and a "Note Added"; v3 contains an Addendu