Spin Anisotropy and Slow Dynamics in Spin Glasses

We report on an extensive study of the influence of spin anisotropy on spin glass aging dynamics. New temperature cycle experiments allow us to compare quantitatively the memory effect in four Heisenberg spin glasses with various degrees of random anisotropy and one Ising spin glass. The sharpness of the memory effect appears to decrease continuously with the spin anisotropy. Besides, the spin glass coherence length is determined by magnetic field change experiments for the first time in the Ising sample. For three representative samples, from Heisenberg to Ising spin glasses, we can consistently account for both sets of experiments (temperature cycle and magnetic field change) using a single expression for the growth of the coherence length with time.Comment: 4 pages and 4 figures - Service de Physique de l'Etat Condense CNRS URA 2464), DSM/DRECAM, CEA Saclay, Franc

Spin glass behavior in an interacting gamma-Fe2O3 nanoparticle system

In this paper we investigate the superspin glass behavior of a concentrated assembly of interacting maghemite nanoparticles and compare it to that of canonical atomic spin glass systems. ac versus temperature and frequency measurements show evidence of a superspin glass transition taking place at low temperature. In order to fully characterize the superspin glass phase, the aging behavior of both the thermo-remanent magnetization (TRM) and ac susceptibility has been investigated. It is shown that the scaling laws obeyed by superspin glasses and atomic spin glasses are essentially the same, after subtraction of a superparamagnetic contribution from the superspin glass response functions. Finally, we discuss a possible origin of this superparamagnetic contribution in terms of dilute spin glass models

The relative influences of disorder and of frustration on the glassy dynamics in magnetic systems

The magnetisation relaxations of three different types of geometrically frustrated magnetic systems have been studied with the same experimental procedures as previously used in spin glasses. The materials investigated are Y$_2$Mo$_2$O$_7$ (pyrochlore system), SrCr$_{8.6}$Ga$_{3.4}$O$_{19}$ (piled pairs of Kagom\'e layers) and (H$_3$O)Fe$_3$(SO$_4$)$_2$(OH)$_6$ (jarosite compound). Despite a very small amount of disorder, all the samples exhibit many characteristic features of spin glass dynamics below a freezing temperature $T_g$, much smaller than their Curie-Weiss temperature $\theta$. The ageing properties of their thermoremanent magnetization can be well accounted for by the same scaling law as in spin glasses, and the values of the scaling exponents are very close. The effects of temperature variations during ageing have been specifically investigated. In the pyrochlore and the bi-Kagom\'e compounds, a decrease of temperature after some waiting period at a certain temperature $T_p$ re-initializes ageing and the evolution at the new temperature is the same as if the system were just quenched from above $T_g$. However, as the temperature is raised back to $T_p$, the sample recovers the state it had previously reached at that temperature. These features are known in spin glasses as rejuvenation and memory effects. They are clear signatures of the spin glass dynamics. In the Kagom\'e compound, there is also some rejuvenation and memory, but much larger temperature changes are needed to observe the effects. In that sense, the behaviour of this compound is quantitatively different from that of spin glasses.Comment: latex VersionCorrigee4.tex, 4 files, 3 figures, 5 pages (Proceedings of the International Conference on Highly Frustrated Magnetism (HFM2003), August 26-30, 2003, Institut Laue Langevin (ILL), Grenoble, France

An Example of Quantum Anomaly in the Physics of Ultra-Cold Gases

In this article, we propose an experimental scheme for observation of a quantum anomaly---quantum-mechanical symmetry breaking---in a two-dimensional harmonically trapped Bose gas. The anomaly manifests itself in a shift of the monopole excitation frequency away from the value dictated by the Pitaevskii-Rosch dynamical symmetry [L. P. Pitaevskii and A. Rosch, Phys. Rev. A, 55, R853 (1997)]. While the corresponding classical Gross-Pitaevskii equation and the derived from it hydrodynamic equations do exhibit this symmetry, it is---as we show in our paper---violated under quantization. The resulting frequency shift is of the order of 1% of the carrier, well in reach for modern experimental techniques. We propose using the dipole oscillations as a frequency gauge.Comment: Misprints corrected, a discussion on damping added, text is polished and shortened. 5 pages, 1 figur

Production, regulation and role of nitric oxide in glial cells.

In neuropathological conditions such as Alzheimer's disease, Parkinson's disease, AIDS dementia complex and multiple selerosis, activation of microglial cells and astroglial cells is evident. Under these neuropathological conditions cellular damage in the brain is considered to arise indirectly from cytotoxic substances produced by activated glial cells. One of these toxins is NO which has been demonstrated to be produced during several neuropathological conditions. High NO levels are produced by glial cells and exert neurotoxic effects. Astroglial cells and microglial cells communicate in various ways to reduce NO production by microglial cells which is essential to maintain homeostasis in the brain. The production of TGFβ by glial cells and its activation by astrocyte-derived tPA represents one mechanism by which astroglia limit NO production in the brain

A Study on the Modeling and Simulation of Bio-inspired Hedgehog Spines Structures for More Efficient Use Digital Manufacturing Processes

Direct digital manufacture and additive manufacture have allowed designers the ability to design components without the design limitations witnessed in subtractive manufacturing process routes. In particular, designers can now design parts that fully utilize material usage resulting in a more sustainable and environmentally friendly application of manufacturing technology. Within this context, designing and manufacturing bio-inspired components have the potential to increase both component functionality and optimize material usage. One such area of biomimicry with advantageous strength-to-weight ratio can be found in hedgehog spines. Within this study, hedgehog spines were redesigned to facilitate production through additive manufacture. In addition, with the use of finite element analysis to quantify the resulting compressive characteristics, the optimal internal geometry and septa spacing were determined. Also, a design of experiments study was conducted to determine which design features have the greatest influence on the resulting stress in the spine. The analysis concluded that the combination of longitudinal stiffeners and equally spaced septa give the spine its superior compressive strength

Scaling Invariance in a Time-Dependent Elliptical Billiard

We study some dynamical properties of a classical time-dependent elliptical billiard. We consider periodically moving boundary and collisions between the particle and the boundary are assumed to be elastic. Our results confirm that although the static elliptical billiard is an integrable system, after to introduce time-dependent perturbation on the boundary the unlimited energy growth is observed. The behaviour of the average velocity is described using scaling arguments

Smart-GFEM for welding simulation

In this paper, a new and efficient method for welding simulation is proposed. This method, known as Smart-GFEM introduces in a GFEM framework an enrichment function which is capable of adapting the approximation space optimally. The enrichment function can be seen as a computational vademecum which depends on technological and material parameters of the process and it is completely computed off-line. In this work we present how to perform the thermal analysis of the welding and a general strategy to address the mechanical analysis