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

Mean square convergence rates for maximum quasi-likelihood estimators

In this note we study the behavior of maximum quasilikelihood estimators (MQLEs) for a class of statistical models, in which only knowledge about the first two moments of the response variable is assumed. This class includes, but is not restricted to, generalized linear models with general link function. Our main results are related to guarantees on existence, strong consistency and mean square convergence rates of MQLEs. The rates are obtained from first principles and are stronger than known a.s. rates. Our results find important application in sequential decision problems with parametric uncertainty arising in dynamic pricing

Salpivory by colonial reef corals at Curaçao, Southern Caribbean

A salp swarm was observed in Director’s Bay, Curaçao in July 2021, where salps were caught and consumed by three scleractinian colonial reef corals: Madracis auretenra, Locke, Weil & Coates, 2017; Meandrina meandrites (Linnaeus, 1758), and Montastraea cavernosa (Linnaeus, 1767). The first two scleractinians are newly recorded salpivores. Since the coral polyps were collaborating, predation was not restricted by polyp size. This is the first detailed report on salpivorous corals in the Caribbean

Internal-wave radiation by a horizontally oscillating body in a uniformly stratified fluid

International audienceIn this experimental-theoretical study we consider the waves emitted by a horizontally oscillating sphere in a linearly stratified fluid. In contrast to former investigations, the thus generated wave pattern is a-symmetric and three-dimensional. We consider large and small amplitude horizontal oscillations for different size spheres. The spatial structure of internal waves has a non-trivial dependence on the body geometry, direction and frequency of oscillations. The flowfield is measured quantitatively, using an alternative version of the synthetic schlieren technique. In addition we exploit the technique to visualise internal waves with fluorescein dye planes used by Hopfinger et al (Exp. in Fluids, 11, 1991) to measure the displacement field of the internal waves. For the theory a uniformly stratified viscous Boussinesq fluid of infinite extent is considered, with small viscosity and the boundary layer on the body surface neglected. For small amplitude oscillations, the comparison with the theory is good, with the near-field theory being in very good agreement with the experimental results and the far field theory slightly overestimating the wave amplitude

Internal wave structure emitted by a horizontally oscillating sphere

International audienceAn oscillating body in a stratified fluid generates a double cone-shaped internal-wave pattern, the 3D analogue of the classic St.Andrew-cross. For sufficiently low frequency and large amplitude oscillations, higher-order wave harmonics may be generated along with the fundamental one. We present an experimental study of the 3D structure of first- and second-order wave fields emitted by a horizontally oscillating sphere. In contrast to the axisymmetric wave pattern found for a vertically oscillating sphere, for horizontal oscillations, the first- and higher-order-harmonic waves have different distributions of wave amplitudes in the azimuthal direction. The amplitude of the first-order waves is shown to follow the cosine dependence on the azimuthal angle, in accordance with theoretical predictions. The azimuthal distribution of the amplitude of the second-order waves gives evidence of a quadrupolar distribution, with four preferential directions of wave radiation in a horizontal plane, along the direction of oscillation and normal to it. Noteworthy is that the amplitudes of these second-order waves may exceed the amplitude of first-order waves

First and second harmonic internal waves from a horizontally oscillating sphere

International audienceA horizontally oscillating sphere in a density-stratified fluid is studied experimentally and theoretically, as a paradigm of the generation of three-dimensional internal tides by supercritical topography. The experiments implement a novel technique for the measurement of the spatial structure of internal wave fields, based on horizontal fluorescent dye planes and a mobile vertical laser sheet; they are compared with an original linear theory. Spectral analysis reveals the presence of two harmonics, namely a first harmonics at the fundamental frequency and a second harmonics at twice this frequency. The first harmonics has a dipolar structure, an amplitude varying linearly with the amplitude of oscillation, and is quantitatively described by the theory. The second harmonics is present at amplitudes of oscillation higher than one tenth of the sphere radius and has a quadrupolar structure. Its amplitude varies quadratically with the amplitude of oscillation, and may exceed the amplitude of the first harmonics. At frequencies smaller than half the buoyancy frequency, the second harmonics is evanescent and confined to the vicinity of the sphere; at frequencies larger than half the buoyancy frequency, it propagates away

First-passage time asymptotics over moving boundaries for random walk bridges

We study the asymptotic tail behavior of the first-passage time over a moving boundary for a random walk conditioned to return to zero, where the increments of the random walk have finite variance. Typically, the asymptotic tail behavior may be described through a regularly varying function with exponent -1/2, where the impact of the boundary is captured by the slowly varying function. Yet, the moving boundary may have a stronger effect when the tail is considered at a time close to the return point of the random walk bridge, leading to a possible phase transition depending on the order of the distance between zero and the moving boundary