Colloidal motility and pattern formation under rectified diffusiophoresis

In this letter, we characterize experimentally the diffusiophoretic motion of colloids and lambda- DNA toward higher concentration of solutes, using microfluidic technology to build spatially- and temporally-controlled concentration gradients. We then demonstrate that segregation and spatial patterning of the particles can be achieved from temporal variations of the solute concentration profile. This segregation takes the form of a strong trapping potential, stemming from an osmotically induced rectification mechanism of the solute time-dependent variations. Depending on the spatial and temporal symmetry of the solute signal, localization patterns with various shapes can be achieved. These results highlight the role of solute contrasts in out-of-equilibrium processes occuring in soft matter

Correlation between structure and properties in multiferroic La0.7_{0.7}Ca0.3_{0.3}MnO3_3/BaTiO3_3 superlattices

Superlattices composed of ferromagnetics, namely La$_{0.7}$Ca$_{0.3}$MnO$_3$ (LCMO), and ferroelectrics, namely, BaTiO$_3$(BTO) were grown on SrTiO$_3$ at 720$^o$C by pulsed laser deposition process. While the out-of-plane lattice parameters of the superlattices, as extracted from the X-ray diffraction studies, were found to be dependent on the BTO layer thickness, the in-plane lattice parameter is almost constant. The evolution of the strains, their nature, and their distribution in the samples, were examined by the conventional sin$^2\psi$ method. The effects of structural variation on the physical properties, as well as the possible role of the strain on inducing the multiferroism in the superlattices, have also been discussed.Comment: To be published in Journal of Applied Physic

Are gravitational waves from giant magnetar flares observable?

Are giant flares in magnetars viable sources of gravitational radiation? Few theoretical studies have been concerned with this problem, with the small number using either highly idealized models or assuming a magnetic field orders of magnitude beyond what is supported by observations. We perform nonlinear general-relativistic magnetohydrodynamics simulations of large-scale hydromagnetic instabilities in magnetar models. We utilise these models to find gravitational wave emissions over a wide range of energies, from 10^40 to 10^47 erg. This allows us to derive a systematic relationship between the surface field strength and the gravitational wave strain, which we find to be highly nonlinear. In particular, for typical magnetar fields of a few times 10^15 G, we conclude that a direct observation of f-modes excited by global magnetic field reconfigurations is unlikely with present or near-future gravitational wave observatories, though we also discuss the possibility that modes in a low-frequency band up to 100 Hz could be sufficiently excited to be relevant for observation.Comment: 4 pages, 3 figures. Further information can be found at http://www.physik.uni-tuebingen.de/institute/astronomie-astrophysik/institut/theoretische-astrophysik/forschung.htm

Microscopic Derivation of Non-Markovian Thermalization of a Brownian Particle

In this paper, the first microscopic approach to the Brownian motion is developed in the case where the mass density of the suspending bath is of the same order of magnitude as that of the Brownian (B) particle. Starting from an extended Boltzmann equation, which describes correctly the interaction with the fluid, we derive systematicaly via the multiple time-scale analysis a reduced equation controlling the thermalization of the B particle, i.e. the relaxation towards the Maxwell distribution in velocity space. In contradistinction to the Fokker-Planck equation, the derived new evolution equation is non-local both in time and in velocity space, owing to correlated recollision events between the fluid and particle B. In the long-time limit, it describes a non-markovian generalized Ornstein-Uhlenbeck process. However, in spite of this complex dynamical behaviour, the Stokes-Einstein law relating the friction and diffusion coefficients is shown to remain valid. A microscopic expression for the friction coefficient is derived, which acquires the form of the Stokes law in the limit where the mean-free in the gas is small compared to the radius of particle B.Comment: 28 pages, no figure, submitted to Journal of Statistical Physic

Magnetically Accreting Isolated Old Neutron Stars

Previous work on the emission from isolated old neutron stars (IONS) accreting the inter-stellar medium (ISM) focussed on gravitational capture - Bondi accretion. We propose a new class of sources which accrete via magnetic interaction with the ISM. While for the Bondi mechanism, the accretion rate decreases with increasing NS velocity, in magnetic accretors (MAGACs="magics") the accretion rate increases with increasing NS velocity. MAGACs will be produced among high velocity (~> 100 km s-1) high magnetic field (B> 1e14 G) radio pulsars - the ``magnetars'' - after they have evolved first through magnetic dipole spin-down, followed by a ``propeller'' phase (when the object sheds angular momentum on a timescale ~< 1e10 yr). The properties of MAGACS may be summarized thus: dipole magnetic fields of B~>1e14 G; minimum velocities relative to the ISM of >25-100 km s-1, depending on B, well below the median in the observed radio-pulsar population; spin-periods of >days to years; accretion luminosities of 1e28- 1e31 ergs s-1 ; and effective temperatures kT=0.3 - 2.5 keV if they accrete onto the magnetic polar cap. We find no examples of MAGACs among previously observed source classes (anomalous X-ray pulsars, soft-gamma-ray repeaters or known IONS). However, MAGACs may be more prevelant in flux-limited X-ray catalogs than their gravitationally accreting counterparts.Comment: ApJ, accepte

Finite-temperature perturbation theory for quasi-one-dimensional spin-1/2 Heisenberg antiferromagnets

We develop a finite-temperature perturbation theory for quasi-one-dimensional quantum spin systems, in the manner suggested by H.J. Schulz (1996) and use this formalism to study their dynamical response. The corrections to the random-phase approximation formula for the dynamical magnetic susceptibility obtained with this method involve multi-point correlation functions of the one-dimensional theory on which the random-phase approximation expansion is built. This ``anisotropic'' perturbation theory takes the form of a systematic high-temperature expansion. This formalism is first applied to the estimation of the N\'eel temperature of S=1/2 cubic lattice Heisenberg antiferromagnets. It is then applied to the compound Cs$_2$CuCl$_4$, a frustrated S=1/2 antiferromagnet with a Dzyaloshinskii-Moriya anisotropy. Using the next leading order to the random-phase approximation, we determine the improved values for the critical temperature and incommensurability. Despite the non-universal character of these quantities, the calculated values are different by less than a few percent from the experimental values for both compounds.Comment: 11 pages, 6 figure

The Diffusion of Humans and Cultures in the Course of the Spread of Farming

The most profound change in the relationship between humans and their environment was the introduction of agriculture and pastoralism. [....] For an understanding of the expansion process, it appears appropriate to apply a diffusive model. Broadly, these numerical modeling approaches can be catego- rized in correlative, continuous and discrete. Common to all approaches is the comparison to collections of radiocarbon data that show the apparent wave of advance of the transition to farming. However, these data sets differ in entry density and data quality. Often they disregard local and regional specifics and research gaps, or dating uncertainties. Thus, most of these data bases may only be used on a very general, broad scale. One of the pitfalls of using irregularly spaced or irregularly documented radiocarbon data becomes evident from the map generated by Fort (this volume, Chapter 16): while the general east-west and south-north trends become evident, some areas appear as having undergone anomalously early transitions to farming. This may be due to faulty entries into the data base or regional problems with radiocarbon dating, if not unnoticed or undocumented laboratory mistakes.Comment: 20 pages, 5 figures, submitted to Diffusive Spreading in Nature, Technology and Society, edited by Armin Bunde, J\"urgen Caro, J\"org K\"arger, Gero Vogl, Chapter 1

Spin-dynamics of the low-dimensional magnet (CH3)2NH2CuCl3

Dimethylammonium copper (II) chloride (also known as DMACuCl3 or MCCL) is a low dimensional S=1/2 quantum spin system proposed to be an alternating ferro-antiferromagnetic chain with similar magnitude ferromagnetic (FM) and antiferromagnetic (AFM) exchange interactions. Subsequently, it was shown that the existing bulk measurements could be adequately modeled by considering DMACuCl3 as independent AFM and FM dimer spin pairs. We present here new inelastic neutron scattering measurements of the spin-excitations in single crystals of DMACuCl3. These results show significant quasi-one-dimensional coupling, however the magnetic excitations do not propagate along the expected direction. We observe a band of excitations with a gap of 0.95 meV and a bandwidth of 0.82 meV.Comment: 3 pages, 2 figures included in text, submitted to proceedings of International Conference on Neutron Scattering, December 200

Covariant calculation of mesonic baryon decays

We present covariant predictions for pi and eta decay modes of N and Delta resonances from relativistic constituent-quark models based on one-gluon-exchange and Goldstone-boson-exchange dynamics. The results are calculated within the point-form approach to Poincare-invariant relativistic quantum mechanics applying a spectator-model decay operator. The direct predictions of the constituent-quark models for covariant pi and eta decay widths show a behaviour completely different from previous ones calculated in nonrelativistic or so-called semirelativistic approaches. It is found that the present theoretical results agree with experiment only in a few cases but otherwise always remain smaller than the experimental data (as compiled by the Particle Data Group). Possible reasons for this behaviour are discussed with regard to the quality of both the quark-model wave functions and the mesonic decay operator.Comment: 10 pages, 2 figures, accepted for publication in Phys. Rev.