Thermodynamic Properties of Kagome Antiferromagnets with different Perturbations

We discuss the results of several small perturbations to the thermodynamic properties of Kagome Lattice Heisenberg Model (KLHM) at high and intermediate temperatures, including Curie impurities, dilution, in-plane and out of plane Dzyaloshinski-Moria (DM) anisotropies and exchange anisotropy. We examine the combined role of Curie impurities and dilution in the behavior of uniform susceptibility. We also study the changes in specific heat and entropy with various anisotropies. Their relevance to newly discovered materials ZnCu3(OH)6Cl2 is explored. We find that the magnetic susceptibility is well described by about 6 percent impurity and dilution. We also find that the entropy difference between the material and KLHM is well described by the DM parameter D_z/J~0.1.Comment: 6 pages, 3 figures, proceedings of the HFM 2008 Conferenc

Asteroseismic effects in close binary stars

Turbulent processes in the convective envelopes of the sun and stars have been shown to be a source of internal acoustic excitations. In single stars, acoustic waves having frequencies below a certain cutoff frequency propagate nearly adiabatically and are effectively trapped below the photosphere where they are internally reflected. This reflection essentially occurs where the local wavelength becomes comparable to the pressure scale height. In close binary stars, the sound speed is a constant on equipotentials, while the pressure scale height, which depends on the local effective gravity, varies on equipotentials and may be much greater near the inner Lagrangian point (L_1). As a result, waves reaching the vicinity of L_1 may propagate unimpeded into low density regions, where they tend to dissipate quickly due to non-linear and radiative effects. We study the three dimensional propagation and enhanced damping of such waves inside a set of close binary stellar models using a WKB approximation of the acoustic field. We find that these waves can have much higher damping rates in close binaries, compared to their non-binary counterparts. We also find that the relative distribution of acoustic energy density at the visible surface of close binaries develops a ring-like feature at specific acoustic frequencies and binary separations

Searching for a trail of evidence in a maze

Consider a graph with a set of vertices and oriented edges connecting pairs of vertices. Each vertex is associated with a random variable and these are assumed to be independent. In this setting, suppose we wish to solve the following hypothesis testing problem: under the null, the random variables have common distribution N(0,1) while under the alternative, there is an unknown path along which random variables have distribution $N(\mu,1)$, $\mu> 0$, and distribution N(0,1) away from it. For which values of the mean shift $\mu$ can one reliably detect and for which values is this impossible? Consider, for example, the usual regular lattice with vertices of the form $$\{(i,j):0\le i,-i\le j\le i and j has the parity of i\}$$ and oriented edges $(i,j)\to (i+1,j+s)$, where $s=\pm1$. We show that for paths of length $m$ starting at the origin, the hypotheses become distinguishable (in a minimax sense) if $\mu_m\gg1/\sqrt{\log m}$, while they are not if $\mu_m\ll1/\log m$. We derive equivalent results in a Bayesian setting where one assumes that all paths are equally likely; there, the asymptotic threshold is $\mu_m\approx m^{-1/4}$. We obtain corresponding results for trees (where the threshold is of order 1 and independent of the size of the tree), for distributions other than the Gaussian and for other graphs. The concept of the predictability profile, first introduced by Benjamini, Pemantle and Peres, plays a crucial role in our analysis.Comment: Published in at http://dx.doi.org/10.1214/07-AOS526 the Annals of Statistics (http://www.imstat.org/aos/) by the Institute of Mathematical Statistics (http://www.imstat.org

Searching for Large Scale Structure in Deep Radio Surveys

(Abridged Abstract) We calculate the expected amplitude of the dipole and higher spherical harmonics in the angular distribution of radio galaxies. The median redshift of radio sources in existing catalogues is z=1, which allows us to study large scale structure on scales between those accessible to present optical and infrared surveys, and that of the Cosmic Microwave Background (CMB). The dipole is due to 2 effects which turn out to be of comparable magnitude: (i) our motion with respect to the CMB, and (ii) large scale structure, parameterised here by a family of Cold Dark Matter power-spectra. We make specific predictions for the Green Bank (87GB) and Parkes-MIT-NRAO (PMN) catalogues. For these relatively sparse catalogues both the motion and large scale structure dipole effects are expected to be smaller than the Poisson shot-noise. However, we detect dipole and higher harmonics in the combined 87GB-PMN catalogue which are far larger than expected. We attribute this to a 2 % flux mismatch between the two catalogues. We also investigate the existence and extent of the Supergalactic Plane in the above catalogues. In a strip of +- 10 deg of the standard Supergalactic equator, we find a 3-sigma detection in PMN, but only 1-sigma in 87GB.Comment: 15 pages, 5 ps figures, Latex, Submitted to MNRA

Holographic Duals of Long Open Strings

We study the holographic map between long open strings, which stretch between D-branes separated in the bulk space-time, and operators in the dual boundary theory. We focus on a generalization of the Sakai-Sugimoto holographic model of QCD, where the simplest chiral condensate involves an operator of this type. Its expectation value is dominated by a semi-classical string worldsheet, as for Wilson loops. We also discuss the deformation of the model by this operator, and in particular its effect on the meson spectrum. This deformation can be thought of as a generalization of a quark mass term to strong coupling. It leads to the first top-down holographic model of QCD with a non-Abelian chiral symmetry which is both spontaneously and explicitly broken, as in QCD. Other examples we study include half-supersymmetric open Wilson lines, and systems of D-branes ending on NS5-branes, which can be analyzed using worldsheet methods.Comment: 35 pages, 4 figures, harvmac. v2: added reference