Effects of interdot dipole coupling in mesoscopic epitaxial Fe(100) dot arrays

The domain structure and the coercivity of epitaxial Fe(100) circular dot arrays of different diameters and separations have been studied using magnetic force microscopy (MFM) and focused magneto-optical Kerr effect (MOKE). The MFM images of the 1 µm diameter single domain dot arrays show direct evidence of strong interdot dipole coupling when the separation is reduced down to 0.1 µm. The coercivity of the dots is also found to be dependent on the separation, indicating the effect of the interdot dipole coupling on the magnetization reversal process

Granular size segregation in underwater sand ripples

We report an experimental study of a binary sand bed under an oscillating water flow. The formation and evolution of ripples is observed. The appearance of a granular segregation is shown to strongly depend on the sand bed preparation. The initial wavelength of the mixture is measured. In the final steady state, a segregation in volume is observed instead of a segregation at the surface as reported before. The correlation between this phenomenon and the fluid flow is emphasised. Finally, different ``exotic'' patterns and their geophysical implications are presented.Comment: 8 page

Tibialis posterior transfer in central palsy of foot levators: A propos of 17 cases

Tibialis posterior transfer in central palsy of foot levators: a propos of 17cases.AimTo evaluate, in spastic patients with a lack of tibialis anterior spontaneous contraction, the efficiency of the tibialis posterior transfer and the occurrence of adverse effects on the static foot posture.Patients and methodsSeventeen patients were evaluated retrospectively, on average 69months after intervention (9–108). Mean age was 47years (26–61). Seven patients presented stroke, 4cranial trauma, 3medullar trauma, 3patients suffered respectively from cerebral palsy, cerebral tumor and cervical myelopathy. The tibialis posterior was transferred on the tibialis anterior in 9cases, on the peroneus brevis in 5cases, on the calcaneocuboid capsule once and on both tibialis anterior and calcaneocuboid capsule once. Three isolated talo-navicular arthrodesis and one triple arthrodesis were associated.ResultsWe found the need of orthosis decreased (P=0,021), 9patients no longer needed their orthosis. The walking distance was significantly increased (P=0,031) in 9patients. The average satisfaction score was 2.71/4 (0–4).On average, the maximum active dorsiflexion reached the neutral position (−20 to 20) with knee extended and 6° (−10–20) with knee flexed; the arc of movement averaged 9° (0–40) knee extended and 16,2° (0–40) knee flexed during analytic testing and 2,8° (0–10) when walking. Only half of the patients presented a tenodesis effect when walking. Dorsiflexion strength averaged 1,5 (0–5). Six patients had a normal plantar footprint, 8 a cavus foot and 2 a flatfoot, without any worsening compared to preoperative status. The Djian angle averaged 119,5° (105–138) and the hindfoot alignment angle was 7,7° valgus. There was no significant difference with the non-operated foot.DiscussionThe tibialis posterior transfer is effective in foot-drop in half of the patients, with a tenodesis effect that is not systematic in spastic patients. A flat valgus foot does not appear to be a long-term complication of this procedure

Horizon effects with surface waves on moving water

Surface waves on a stationary flow of water are considered, in a linear model that includes the surface tension of the fluid. The resulting gravity-capillary waves experience a rich array of horizon effects when propagating against the flow. In some cases three horizons (points where the group velocity of the wave reverses) exist for waves with a single laboratory frequency. Some of these effects are familiar in fluid mechanics under the name of wave blocking, but other aspects, in particular waves with negative co-moving frequency and the Hawking effect, were overlooked until surface waves were investigated as examples of analogue gravity [Sch\"utzhold R and Unruh W G 2002 Phys. Rev. D 66 044019]. A comprehensive presentation of the various horizon effects for gravity-capillary waves is given, with emphasis on the deep water/short wavelength case kh>>1 where many analytical results can be derived. A similarity of the state space of the waves to that of a thermodynamic system is pointed out.Comment: 30 pages, 15 figures. Minor change

On the electrodynamics of moving bodies at low velocities

We discuss the seminal article in which Le Bellac and Levy-Leblond have identified two Galilean limits of electromagnetism, and its modern implications. We use their results to point out some confusion in the literature and in the teaching of special relativity and electromagnetism. For instance, it is not widely recognized that there exist two well defined non-relativistic limits, so that researchers and teachers are likely to utilize an incoherent mixture of both. Recent works have shed a new light on the choice of gauge conditions in classical electromagnetism. We retrieve Le Bellac-Levy-Leblond's results by examining orders of magnitudes, and then with a Lorentz-like manifestly covariant approach to Galilean covariance based on a 5-dimensional Minkowski manifold. We emphasize the Riemann-Lorenz approach based on the vector and scalar potentials as opposed to the Heaviside-Hertz formulation in terms of electromagnetic fields. We discuss various applications and experiments, such as in magnetohydrodynamics and electrohydrodynamics, quantum mechanics, superconductivity, continuous media, etc. Much of the current technology where waves are not taken into account, is actually based on Galilean electromagnetism

Acoustic Black Holes from Abelian Higgs Model with Lorentz Symmetry Breaking

In this paper we derive acoustic black hole metrics in the (3+1) and (2+1)-dimensional Abelian Higgs model with Lorentz symmetry breaking. In this set up the sound waves lose the Lorentz boost invariance and suffer a 'birefringence' effect. We have found acoustic black holes and respective Hawking temperatures depending on the Lorentz violating parameter. Furthermore, we obtain an acoustic Kerr-like black hole metric with the Lorentz violating term affecting its rate of loss of mass. We also have shown that for suitable values of the Lorentz violating parameter a wider spectrum of particle wave function can be scattered with increased amplitude by the acoustic black hole.Comment: 12 pages, Latex, no figures, version accepted to Phys. Lett.

The Basics of Water Waves Theory for Analogue Gravity

This chapter gives an introduction to the connection between the physics of water waves and analogue gravity. Only a basic knowledge of fluid mechanics is assumed as a prerequisite.Comment: 36 pages. Lecture Notes for the IX SIGRAV School on "Analogue Gravity", Como (Italy), May 201