Grain boundary partitioning of Ar and He

An experimental procedure has been developed that permits measurement of the partitioning of Ar and He between crystal interiors and the intergranular medium (ITM) that surrounds them in synthetic melt-free polycrystalline diopside aggregates. ^(37)Ar and ^(4)He are introduced into the samples via neutron irradiation. As samples are crystallized under sub-solidus conditions from a pure diopside glass in a piston cylinder apparatus, noble gases diffusively equilibrate between the evolving crystal and intergranular reservoirs. After equilibration, ITM Ar and He is distinguished from that incorporated within the crystals by means of step heating analysis. An apparent equilibrium state (i.e., constant partitioning) is reached after about 20 h in the 1450 °C experiments. Data for longer durations show a systematic trend of decreasing ITM Ar (and He) with decreasing grain boundary (GB) interfacial area as would be predicted for partitioning controlled by the network of planar grain boundaries (as opposed to ITM gases distributed in discrete micro-bubbles or melt). These data yield values of GB-area-normalized partitioning, K¯^(Ar)_(ITM), with units of (Ar/m^3 of solid)/(Ar/m^2 of GB) of 6.8 x 10^3 – 2.4 x 104 m^(-1). Combined petrographic microscope, SEM, and limited TEM observation showed no evidence that a residual glass phase or grain boundary micro-bubbles dominated the ITM, though they may represent minor components. If a nominal GB thickness (δ) is assumed, and if the density of crystals and the grain boundaries are assumed equal, then a true grain boundary partition coefficient (K^(Ar)_(GB) = X^(Ar)_(crystals)/X^(Ar)_(GB) may be determined. For reasonable values of δ, K^(Ar)_(GB) is at least an order of magnitude lower than the Ar partition coefficient between diopside and melt. Helium partitioning data provide a less robust constraint with K¯^(He)_(ITM) between 4 x 10^3 and 4 x 10^4 cm^(-1), similar to the Ar partitioning data. These data suggest that an ITM consisting of nominally melt free, bubble free, tight grain boundaries can constitute a significant but not infinite reservoir, and therefore bulk transport pathway, for noble gases in fine grained portions of the crust and mantle where aqueous or melt fluids are non-wetting and of very low abundance (i.e., <0.1% fluid). Heterogeneities in grain size within dry equilibrated systems will correspond to significant differences in bulk rock noble gas content

Hard squares with negative activity

We show that the hard-square lattice gas with activity z= -1 has a number of remarkable properties. We conjecture that all the eigenvalues of the transfer matrix are roots of unity. They fall into groups (``strings'') evenly spaced around the unit circle, which have interesting number-theoretic properties. For example, the partition function on an M by N lattice with periodic boundary condition is identically 1 when M and N are coprime. We provide evidence for these conjectures from analytical and numerical arguments.Comment: 8 page

MEME-LaB : motif analysis in clusters

Genome-wide expression analysis can result in large numbers of clusters of co-expressed genes. While there are tools for ab initio discovery of transcription factor binding sites, most do not provide a quick and easy way to study large numbers of clusters. To address this, we introduce a web-tool called MEME-LaB. The tool wraps MEME (an ab initio motif finder), providing an interface for users to input multiple gene clusters, retrieve promoter sequences, run motif finding, and then easily browse and condense the results, facilitating better interpretation of the results from large-scale datasets

Searching for velocity-dependent dark matter annihilation signals from extragalactic halos

We consider gamma-ray signals of dark matter annihilation in extragalactic halos in the case where dark matter annihilates from a $p$-wave or $d$-wave state. In these scenarios, signals from extragalactic halos are enhanced relative to other targets, such as the Galactic Center or dwarf spheroidal galaxies, because the typical relative speed of the dark matter is larger in extragalactic halos. We perform a mock data analysis of gamma rays produced by dark matter annihilation in halos detected by the Sloan Digital Sky Survey. We include a model for uncorrelated galactic and extragalactic gamma ray backgrounds, as well as a simple model for backgrounds due to astrophysical processes in the extragalactic halos detected by the survey. We find that, for models which are still allowed by other gamma ray searches, searches of extragalactic halos with the current Fermi exposure can produce evidence for dark matter annihilation, though it is difficult to distinguish the $p$-wave and $d$-wave scenarios. With a factor $10\times$ larger exposure, though, discrimination of the velocity-dependence is possible.Comment: 20 pages, 5 figures; replaced to match version accepted by JCA

Mississippian stratotypes

Working Group on the Mississippian of the U.S.A.Ope

Allosteric Conversation in the Androgen Receptor Ligand-Binding Domain Surfaces

Androgen receptor (AR) is a major therapeutic target that plays pivotal roles in prostate cancer (PCa) and androgen insensitivity syndromes. Wepreviously proposed that compounds recruited to ligand-binding domain (LBD) surfaces could regulate AR activity in hormone-refractory PCa and discovered several surface modulators of AR function. Surprisingly, the most effective compounds bound preferentially to a surface of unknown function [binding function 3 (BF-3)] instead of the coactivator-binding site [activation function 2 (AF-2)]. Different BF-3 mutations have been identified in PCa or androgen insensitivity syndrome patients, and they can strongly affect AR activity. Further, comparison of AR x-ray structures with and without bound ligands at BF-3 and AF-2 showed structural coupling between both pockets. Here, we combine experimental evidence and molecular dynamic simulations to investigate whether BF-3 mutations affect AR LBD function and dynamics possibly via allosteric conversation between surface sites. Our data indicate that AF-2 conformation is indeed closely coupled to BF-3 and provide mechanistic proof of their structural interconnection. BF-3 mutations may function as allosteric elicitors, probably shifting the AR LBD conformational ensemble toward conformations that alter AF-2 propensity to reorganize into subpockets that accommodate N-terminal domain and coactivator peptides. The induced conformation may result in either increased or decreased AR activity. Activating BF-3 mutations also favor the formation of another pocket (BF-4) in the vicinity of AF-2 and BF-3, which we also previously identified as a hot spot for a small compound. We discuss the possibility that BF-3 may be a protein-docking site that binds to the N-terminal domain and corepressors. AR surface sites are attractive pharmacological targets to develop allosteric modulators that might be alternative lead compounds for drug design. © 2012 by The Endocrie Society

Loop models and their critical points

Loop models have been widely studied in physics and mathematics, in problems ranging from polymers to topological quantum computation to Schramm-Loewner evolution. I present new loop models which have critical points described by conformal field theories. Examples include both fully-packed and dilute loop models with critical points described by the superconformal minimal models and the SU(2)_2 WZW models. The dilute loop models are generalized to include SU(2)_k models as well.Comment: 20 pages, 15 figure

Lattice Models with N=2 Supersymmetry

We introduce lattice models with explicit N=2 supersymmetry. In these interacting models, the supersymmetry generators Q^+ and Q^- yield the Hamiltonian H={Q^+,Q^-} on any graph. The degrees of freedom can be described as either fermions with hard cores, or as quantum dimers. The Hamiltonian of our simplest model contains a hopping term and a repulsive potential, as well as the hard-core repulsion. We discuss these models from a variety of perspectives: using a fundamental relation with conformal field theory, via the Bethe ansatz, and using cohomology methods. The simplest model provides a manifestly-supersymmetric lattice regulator for the supersymmetric point of the massless 1+1-dimensional Thirring (Luttinger) model. We discuss the ground-state structure of this same model on more complicated graphs, including a 2-leg ladder, and discuss some generalizations.Comment: 4 page