Modeling of light scattering and haze in semicrystalline polymers

This article reports a new model approach for the description of light scattering in semicrystalline polymers, to describe more precisely the influence of supermolecular structure on the optical properties. This is the first study in which light scattering of polymer films has been modeled using exact Mie scattering theory of radially anisotropic spheres. As a model material a well‐known polymer, isotactic polypropylene (iPP) was used. Samples were prepared with different sample thicknesses and crystalline structures in order to identify the key parameters of light scattering in polycrystalline polymeric systems. Validation haze measurements were carried out with a spectrophotometer equipped with a 150 mm snap‐in integrating sphere. It was found that the optical properties of the polycrystalline sample can be described using multiple light scattering on these scattering centers. Good agreement was found between the simulated and experimentally measured haze values which proves the reliability and applicability of our new approach

Controlling magnetic order and quantum disorder in molecule-based magnets

We investigate the structural and magnetic properties of two molecule-based magnets synthesized from the same starting components. Their different structural motifs promote contrasting exchange pathways and consequently lead to markedly different magnetic ground states. Through examination of their structural and magnetic properties we show that [Cu(pyz)(H2O)(gly)2](ClO4)2 may be considered a quasi-one-dimensional quantum Heisenberg antiferromagnet whereas the related compound [Cu(pyz)(gly)](ClO4), which is formed from dimers of antiferromagnetically interacting Cu2+ spins, remains disordered down to at least 0.03 K in zero field but shows a field-temperature phase diagram reminiscent of that seen in materials showing a Bose-Einstein condensation of magnons

SDPD-SX: combining a single crystal X-ray diffraction setup with advanced powder data structure determination for use in early stage drug discovery

We report a method for routine crystal structure determination on very small (typically 0.1 mg or less) amounts of crystalline material using powder X-ray diffraction data from a laboratory-based single-crystal diffractometer. The solved structures span a wide range of molecular and crystallographic complexity

A Chandra HETG Observation of the Quasar H 1821+643 and Its Surrounding Cluster

We present the high-resolution X-ray spectrum of the low-redshift quasar H 1821+643 and its surrounding hot cluster observed with the Chandra High Energy Transmission Grating Spectrometer (HETGS). An iron emission line attributed to the quasar at ~6.43 keV (rest frame) is clearly resolved, with an equivalent width of ~100 eV. Although we cannot rule out contributions to the line from a putative torus, the diskline model provides an acceptable fit to this iron line. We also detect a weak emission feature at ~6.9 keV (rest frame). We suggest that both lines could originate in an accretion disk comprised of a highly ionized optically thin atmosphere sitting atop a mostly neutral disk. We search for absorption features from a warm/hot component of the intergalactic medium along the ~1.5Gpc/h line of sight to the quasar. No absorption features are detected at or above the 3 sigma level while a total of six OVI intervening absorption systems have been detected with HST and FUSE. Based on the lack of OVII and OVIII absorption lines and by assuming collisionally ionization, we constrain the gas temperature of a typical OVI absorber to 10^5 < T < 10^6 K, which is consistent with the results from hydrodynamic simulations of the intergalactic medium. The zeroth order image reveals the extended emission from the surrounding cluster. We have been able to separate the moderate CCD X-ray spectrum of the surrounding cluster from the central quasar and find that this is a hot cluster with a temperature of ~10 keV and a metal abundance of ~0.3 Zo. We also independently obtain the redshift of the cluster, which is consistent with the optical results. We estimate that the cluster makes negligible contributions to the 6.9 keV iron K line flux.Comment: 33 pages, 9 figures (2 color). Accepted for publication in the Astrophysical Journa

Magnetoresistance and magnetic breakdown in the quasi-two-dimensional conductors (BEDT-TTF)2_2MHg(SCN)4_4[M=K,Rb,Tl]

The magnetic field dependence of the resistance of (BEDT-TTF)$_2$MHg(SCN)$_4$[M=K,Rb,Tl] in the density-wave phase is explained in terms of a simple model involving magnetic breakdown and a reconstructed Fermi surface. The theory is compared to measurements in pulsed magnetic fields up to 51 T. The value implied for the scattering time is consistent with independent determinations. The energy gap associated with the density-wave phase is deduced from the magnetic breakdown field. Our results have important implications for the phase diagram.Comment: 5 pages, RevTeX + epsf, 3 figures. To appear in Physical Review B, Rapid Communications, September 15, 199

Bimetallic MOFs (H3O)x[Cu(MF6)(pyrazine)2]·(4 − x)H2O (M = V4+, x = 0; M = Ga3+, x = 1): co-existence of ordered and disordered quantum spins in the V4+ system

The title compounds are bimetallic MOFs containing [Cu(pyz)2]2+ square lattices linked by MF6n− octahedra. In each, only the Cu2+ spins exhibit long-range magnetic order below 3.5 K (M = V4+) and 2.6 K (M = Ga3+). The V4+ spins remain disordered down to 0.5 K

Purification, crystallization and preliminary X-ray analysis of adenylosuccinate synthetase from the fungal pathogen Cryptococcus neoformans

With increasingly large immunocompromised populations around the world, opportunistic fungal pathogens such as Cryptococcus neoformans are a growing cause of morbidity and mortality. To combat the paucity of antifungal compounds, new drug targets must be investigated. Adenylosuccinate synthetase is a crucial enzyme in the ATP de novo biosynthetic pathway, catalyzing the formation of adenylosuccinate from inosine monophosphate and aspartate. Although the enzyme is ubiquitous and well characterized in other kingdoms, no crystallographic studies on the fungal protein have been performed. Presented here are the expression, purification, crystallization and initial crystallographic analyses of cryptococcal adenylosuccinate synthetase. The crystals had the symmetry of space group P2(1)2(1)2(1) and diffracted to 2.2 angstrom resolution

Comparison of coherent and weakly incoherent transport models for the interlayer magnetoresistance of layered Fermi liquids

The interlayer magnetoresistance of layered metals in a tilted magnetic field is calculated for two distinct models for the interlayer transport. The first model involves coherent interlayer transport and makes use of results of semi-classical or Bloch-Boltzmann transport theory. The second model involves weakly incoherent interlayer transport where the electron is scattered many times within a layer before tunneling into the next layer. The results are relevant to the interpretation of experiments on angular-dependent magnetoresistance oscillations (AMRO) in quasi-one- and quasi-two-dimensional metals. We find that the dependence of the magnetoresistance on the direction of the magnetic field is identical for both models except when the field is almost parallel to the layers. An important implication of this result is that a three-dimensional Fermi surface is not necessary for the observation of the Yamaji and Danner oscillations seen in quasi-two- and quasi-one-dimensional metals, respectively. A universal expression is given for the dependence of the resistance at AMRO maxima and minima on the magnetic field and scattering time (and thus the temperature). We point out three distinctive features of coherent interlayer transport: (i) a beat frequency in the magnetic oscillations of quasi-two-dimensional systems, (ii) a peak in the angular-dependent magnetoresistance when the field is sufficiently large and parallel to the layers, and (iii) a crossover from a linear to a quadratic field dependence for the magnetoresistance when the field is parallel to the layers. Properties (i) and (ii) are compared with published experimental data for a range of quasi-two-dimensional organic metals and for Sr2RuO4.Comment: 21 pages, RevTeX + epsf, 4 figures. Published version. Subsection added. References update

Polymer Crystallization in 25 nm Spheres

Crystallization within the discrete spheres of a block copolymer mesophase was studied by time-resolved x-ray scattering. The cubic packing of microdomains, established by self-assembly in the melt, is preserved throughout crystallization by strong interblock segregation even though the amorphous matrix block is well above its glass transition temperature. Homogeneous nucleation within each sphere yields isothermal crystallizations which follow first-order kinetics, contrasting with the sigmoidal kinetics normally exhibited in the quiescent crystallization of bulk polymers.Comment: accepted for publication in Physical Review Letters, 2/28/2000; scheduled for 5/1/2000 issu