Spatial transformations of diffusion tensor magnetic resonance images

The authors address the problem of applying spatial transformations (or “image warps”) to diffusion tensor magnetic resonance images. The orientational information that these images contain must be handled appropriately when they are transformed spatially during image registration. The authors present solutions for global transformations of three-dimensional images up to 12-parameter affine complexity and indicate how their methods can be extended for higher order transformations. Several approaches are presented and tested using synthetic data. One method, the preservation of principal direction algorithm, which takes into account shearing, stretching and rigid rotation, is shown to be the most effective. Additional registration experiments are performed on human brain data obtained from a single subject, whose head was imaged in three different orientations within the scanner. All of the authors' methods improve the consistency between registered and target images over naive warping algorithms

Anderson-Yuval approach to the multichannel Kondo problem

We analyze the structure of the perturbation expansion of the general multichannel Kondo model with channel anisotropic exchange couplings and in the presence of an external magnetic field, generalizing to this case the Anderson-Yuval technique. For two channels, we are able to map the Kondo model onto a generalized resonant level model. Limiting cases in which the equivalent resonant level model is solvable are identified. The solution correctly captures the properties of the two channel Kondo model, and also allows an analytic description of the cross-over from the non Fermi liquid to the Fermi liquid behavior caused by the channel anisotropy.Comment: 23 pages, ReVTeX, 4 figures av. on reques

Soil-type influences human selenium status and underlies widespread selenium deficiency risks in Malawi

Selenium (Se) is an essential human micronutrient with critical roles in immune functioning and antioxidant defence. Estimates of dietary Se intakes and status are scarce for Africa although crop surveys indicate deficiency is probably widespread in Malawi. Here we show that Se deficiency is likely endemic in Malawi based on the Se status of adults consuming food from contrasting soil types. These data are consistent with food balance sheets and composition tables revealing that >80% of the Malawi population is at risk of dietary Se inadequacy. Risk of dietary Se inadequacy is >60% in seven other countries in Southern Africa, and 22% across Africa as a whole. Given that most Malawi soils cannot supply sufficient Se to crops for adequate human nutrition, the cost and benefits of interventions to alleviate Se deficiency should be determined; for example, Se-enriched nitrogen fertilisers could be adopted as in Finland

Scaling Regimes, Crossovers, and Lattice Corrections in 2D Heisenberg Antiferromagnets

We study scaling behavior in 2D, S=1/2 and S=1 Heisenberg antiferromagnets using the data on full q-dependences of the equal time structure factor and the static susceptibility, calculated through high temperature expansions. We also carry out comparisons with a model of two coupled S=1/2 planes with the interlayer coupling tuned to the T=0 critical point. We separately determine the spin-wave velocity c and mass $m=c/\xi$, in addition to the correlation length, $\xi$, and find that c is temperature dependent; only for T\alt JS, it approaches its known T=0 value $c_0$. Despite this temperature dependent spin-wave velocity, full q- and $\omega$-dependences of the dynamical susceptibility $\chi(\bf q,\omega)$ agree with the universal scaling functions computable for the $\sigma$-model, for temperatures upto $T_0 \sim 0.6c_0/a$. Detailed comparisons show that below $T_0$ the S=1 model is in the renormalized classical (RC) regime, the two plane model is in the quantum critical (QC) regime, and the S=1/2 model exhibits a RC-QC crossover, centered at T=0.55J. In particular, for the S=1/2 model above this crossover and for the two-plane model at all T, the spin-wave mass is in excellent agreement with the universal QC prediction, $m\simeq 1.04\,T$. In contrast, for the S=1/2 model below the RC-QC crossover, and for the S=1 model at all T, the behavior agrees with the known RC expression. For all models nonuniversal behavior occurs above $T\sim 0.6c_0/a$. Our results strongly support the conjecture of Chubukov and Sachdev that the S=1/2 model is close to the T=0 critical point to exhibit QC behavior.Comment: 13 pages, REVTeX with attached PostScript (see file for addl info

Interfaces with a single growth inhomogeneity and anchored boundaries

The dynamics of a one dimensional growth model involving attachment and detachment of particles is studied in the presence of a localized growth inhomogeneity along with anchored boundary conditions. At large times, the latter enforce an equilibrium stationary regime which allows for an exact calculation of roughening exponents. The stochastic evolution is related to a spin Hamiltonian whose spectrum gap embodies the dynamic scaling exponent of late stages. For vanishing gaps the interface can exhibit a slow morphological transition followed by a change of scaling regimes which are studied numerically. Instead, a faceting dynamics arises for gapful situations.Comment: REVTeX, 11 pages, 9 Postscript figure

Los métodos de diagnóstico de la sarna sarcóptica en cerdos

Poster apresentado no II Congreso Ibérico de Epidemiologia Veterinária, que decorreu em Barcelona, na FVUAB de 2 a 5 de Fevereiro de 2010.El ácaro astigmatídeo Sarcoptes scabiei (Figura 1), que causa la sarna, es una especie adaptada a diferentes hospedadores y con especial importancia en el cerdo. La sarna es una enfermedad parasitaria de la piel comunes en los animales estabulados o explotados en virtud de las malas condiciones de higiene y por lo general se produce a finales de invierno o principios de primavera. La importancia económica de la enfermedad se asocia con disminución en la producción, con la devaluación de los canales en el matadero y el uso continuo de acaricidas en los animales infectados (Damriyasa et al., 2004)

Localization and fluctuations of local spectral density on tree-like structures with large connectivity: Application to the quasiparticle line shape in quantum dots

We study fluctuations of the local density of states (LDOS) on a tree-like lattice with large branching number $m$. The average form of the local spectral function (at given value of the random potential in the observation point) shows a crossover from the Lorentzian to semicircular form at $\alpha\sim 1/m$, where $\alpha= (V/W)^2$, $V$ is the typical value of the hopping matrix element, and $W$ is the width of the distribution of random site energies. For $\alpha>1/m^2$ the LDOS fluctuations (with respect to this average form) are weak. In the opposite case, $\alpha<1/m^2$, the fluctuations get strong and the average LDOS ceases to be representative, which is related to the existence of the Anderson transition at $\alpha_c\sim 1/(m^2\log^2m)$. On the localized side of the transition the spectrum is discrete, and LDOS is given by a set of $\delta$-like peaks. The effective number of components in this regime is given by $1/P$, with $P$ being the inverse participation ratio. It is shown that $P$ has in the transition point a limiting value $P_c$ close to unity, $1-P_c\sim 1/\log m$, so that the system undergoes a transition directly from the deeply localized to extended phase. On the side of delocalized states, the peaks in LDOS get broadened, with a width $\sim\exp\{-{const}\log m[(\alpha-\alpha_c)/\alpha_c]^{-1/2}\}$ being exponentially small near the transition point. We discuss application of our results to the problem of the quasiparticle line shape in a finite Fermi system, as suggested recently by Altshuler, Gefen, Kamenev, and Levitov.Comment: 12 pages, 1 figure. Misprints in eqs.(21) and (28) corrected, section VII added. Accepted for publication in Phys. Rev.

Interacting one dimensional electron gas with open boundaries

We discuss the properties of interacting electrons on a finite chain with open boundary conditions. We extend the Haldane Luttinger liquid description to these systems and study how the presence of the boundaries modifies various correlation functions. In view of possible experimental applications to quantum wires, we analyse how tunneling measurements can reveal the underlying Luttinger liquid properties. The two terminal conductance is calculated. We also point out possible applications to quasi one dimensional materials and study the effects of magnetic impurities.Comment: 38 pages, ReVTeX, 7 figures (available upon request

Thermodynamic Properties of the Dimerised and Frustrated S=1/2 Chain

By high temperature series expansion, exact diagonalisation and temperature density-matrix renormalisation the magnetic susceptibility $\chi(T)$ and the specific heat $C(T)$ of dimerised and frustrated $S=1/2$ chains are computed. All three methods yield reliable results, in particular for not too small temperatures or not too small gaps. The series expansion results are provided in the form of polynomials allowing very fast and convenient fits in data analysis using algebraic programmes. We discuss the difficulty to extract more than two coupling constants from the temperature dependence of $\chi(T)$.Comment: 14 pages, 13 figures, 4 table

The long-time dynamics of two hydrodynamically-coupled swimming cells

Swimming micro-organisms such as bacteria or spermatozoa are typically found in dense suspensions, and exhibit collective modes of locomotion qualitatively different from that displayed by isolated cells. In the dilute limit where fluid-mediated interactions can be treated rigorously, the long-time hydrodynamics of a collection of cells result from interactions with many other cells, and as such typically eludes an analytical approach. Here we consider the only case where such problem can be treated rigorously analytically, namely when the cells have spatially confined trajectories, such as the spermatozoa of some marine invertebrates. We consider two spherical cells swimming, when isolated, with arbitrary circular trajectories, and derive the long-time kinematics of their relative locomotion. We show that in the dilute limit where the cells are much further away than their size, and the size of their circular motion, a separation of time scale occurs between a fast (intrinsic) swimming time, and a slow time where hydrodynamic interactions lead to change in the relative position and orientation of the swimmers. We perform a multiple-scale analysis and derive the effective dynamical system - of dimension two - describing the long-time behavior of the pair of cells. We show that the system displays one type of equilibrium, and two types of rotational equilibrium, all of which are found to be unstable. A detailed mathematical analysis of the dynamical systems further allows us to show that only two cell-cell behaviors are possible in the limit of $t\to\infty$, either the cells are attracted to each other (possibly monotonically), or they are repelled (possibly monotonically as well), which we confirm with numerical computations