cem: Software for Coarsened Exact Matching

This program is designed to improve causal inference via a method of matching that is widely applicable in observational data and easy to understand and use (if you understand how to draw a histogram, you will understand this method). The program implements the coarsened exact matching (CEM) algorithm, described below. CEM may be used alone or in combination with any existing matching method. This algorithm, and its statistical properties, are described in Iacus, King, and Porro (2008)

Direct measurement of the singlet generation yield in polymer light-emitting diodes

In this study, the singlet and triplet exciton generation yields of a representative blue-emitting conjugated polymer are directly compared using simultaneous optical and electrical excitation measurements. After carefully accounting for bimolecular triplet annihilation and knowing the independently measured solid state inter-system-crossing yield of the polymer, a singlet generation yield of 44% is obtained, in the working device, which is clearly in excess of the simple quantum statistical 25% limit

A study of the porosity of nuclear graphite using small-angle neutron scattering

Small angle neutron scattering (SANS) measures porosity in nuclear graphites, including both open pores, caused by escaping decomposition gases, and internal cracks (in coke particles) generated by anisotropic thermal contraction along the c-direction (Mrozowski Cracks). Porosity changes on the length scale observable by SANS must control the development of internal stresses and hence of cracking in AGR graphite due to irradiation (both fast neutron displacements of carbon atoms and radiolytic corrosion by CO2). Such cracking may cause premature reactor shutdown. SANS measurements show that porosity is fractal on a length scale between ~0.2-300 nm, presumably due to Mrozowski cracks – because the fractal index of the SANS signal depends only on the porosity of the graphitic filler. We report here two novel uses of the SANS technique as applied to reactor graphite – contrast matching with D-toluene (to measure the fraction of the porosity open to the surface) and the temperature dependence of the scattering (to measure pore width changes up to 2000 °C). These results provide important new information on AGR graphite porosity and its evolution during irradiation

Charged lepton contributions to the solar neutrino mixing and theta_13

A charged lepton contribution to the solar neutrino mixing induces a contribution to theta_13, barring cancellations/correlations, which is independent of the model building options in the neutrino sector. We illustrate two robust arguments for that contribution to be within the expected sensitivity of high intensity neutrino beam experiments. We find that the case in which the neutrino sector gives rise to a maximal solar angle (the natural situation if the hierarchy is inverse) leads to a theta_13 close to or exceeding the experimental bound depending on the precise values of theta_12, theta_23, an unknown phase and possible additional contributions. We finally discuss the possibility that the solar angle originates predominantly in the charged lepton sector. We find that the construction of a model of this sort is more complicated. We comment on a recent example of natural model of this type.Comment: 10 pages, 1 figur

Nanotube Piezoelectricity

We combine ab initio, tight-binding methods and analytical theory to study piezoelectric effect of boron nitride nanotubes. We find that piezoelectricity of a heteropolar nanotube depends on its chirality and diameter and can be understood starting from the piezoelectric response of an isolated planar sheet, along with a structure specific mapping from the sheet onto the tube surface. We demonstrate that coupling between the uniaxial and shear deformation are only allowed in the nanotubes with lower chiral symmetry. Our study shows that piezoelectricity of nanotubes is fundamentally different from its counterpart in three dimensional (3D) bulk materials.Comment: 4 pages, with 3 postscript figures embedded. Uses REVTEX4 macros. Also available at http://www.physics.upenn.edu/~nsai/preprints/bn_piezo/index.htm

The use of small angle neutron scattering with contrast matching and variable adsorbate partial pressures in the study of porosity in activated carbons

The porosity of a typical activated carbon is investigated with small angle neutron scattering (SANS), using the contrast matching technique, by changing the hydrogen/deuterium content of the absorbed liquid (toluene) to extract the carbon density at different scattering vector (Q) values and by measuring the p/p0 dependence of the SANS, using fully deuterated toluene. The contrast matching data shows that the apparent density is Q-dependent, either because of pores opening near the carbon surface during the activation processor or changes in D-toluene density in nanoscale pores. For each p/p0 value, evaluation of the Porod Invariant yields the fraction of empty pores. Hence, comparison with the adsorption isotherm shows that the fully dry powder undergoes densification when liquid is added. An algebraic function is developed to fit the SANS signal at each p/p0 value hence yielding the effective Kelvin radii of the liquid surfaces as a function of p/p0. These values, when compared with the Kelvin Equation, show that the resultant surface tension value is accurate for the larger pores but tends to increase for small (nanoscale) pores. The resultant pore size distribution is less model-dependent than for the traditional methods of analyzing the adsorption isotherms

Testing the spectrum hypothesis of problematic online behaviors: A network analysis approach.

The validity of the constructs of problematic Internet or smartphone use and Internet or smartphone addiction has been extensively debated. The spectrum hypothesis posits that problematic online behaviors (POBs) may be conceptualized within a spectrum of related yet distinct entities. To date, the hypothesis has received preliminary support, and further robust empirical studies are still needed. The present study tested the spectrum hypothesis of POBs in an Australian community sample (n = 1,617) using a network analysis approach. Psychometrically validated self-report instruments were used to assess six types of POBs: problematic online gaming, cyberchondria, problematic cybersex, problematic online shopping, problematic use of social networking sites, and problematic online gambling. A tetrachoric correlation matrix was computed to explore relationships between online activities and a network analysis was used to analyze relationships between POBs. Correlations between online activities were positive and significant, but of small magnitude (0.051 ≤ r ≤ 0.236). The community detection analysis identified six distinct communities, corresponding to each POB, with strong relationships between items within each POB and weaker relationships between POBs. These findings provide further empirical support for the spectrum hypothesis, suggesting that POBs occur as distinct entities and with little overlap

Binaphthyl-1,2,3-triazole peptidomimetics with activity against Clostridium difficile and other pathogenic bacteria

Clostridium difficile (C. difficile) is a problematic Gram positive bacterial pathogen causing moderate to severe gastrointestinal infections. Based on a lead binaphthyl-tripeptide dicationic antimicrobial, novel mono-, di- and tri-peptidomimetic analogues targeting C. difficile were designed and synthesized incorporating one, two or three d-configured cationic amino acid residues, with a common 1,2,3-triazole ester isostere at the C-terminus. Copper- and ruthenium-click chemistry facilitated the generation of a 46 compound library for in vitro bioactivity assays, with structure-activity trends over the largest compound subset revealing a clear advantage to triazole-substitution with a linear or branched hydrophobic group. The most active compounds were dicationic-dipeptides where the triazole was substituted with a 4- or 5-cyclohexylmethyl or 4,5-diphenyl moiety, providing MICs of 4 μg mL-1 against three human isolates of C. difficile. Further biological screening revealed significant antimicrobial activity for several compounds against other common bacterial pathogens, both Gram positive and negative, including S. aureus (MICs ≥2 μg mL-1), S. pneumoniae (MICs ≥1 μg mL-1), E. coli (MICs ≥4 μg mL-1), A. baumannii (MICs ≥4 μg mL-1) and vancomycin-resistant E. faecalis (MICs ≥4 μg mL-1)

Constraints on Natural MNS Parameters from |U_e3|

The MNS matrix structure emerging as a result of recent neutrino measurements strongly suggests two large mixing angles (solar and atmospheric) and one small angle (|U_e3| << 1). Especially when combined with the neutrino mass hierarchy, these values turn out to impose rather stringent constraints on possible flavor models connecting the three active fermion generations. Specifically, we show that an extremely small value of |U_e3| would require fine tuning of Majorana mass matrix parameters, particularly in the context of seesaw models.Comment: 21 pages, ReVTeX, 2 .eps figure files, updated references and acknowledgment