We report the first APOGEE metallicities and alpha-element abundances measured for 3600 red giant stars spanning a large radial range of both the Large (LMC) and Small Magellanic Clouds, the largest Milky Way (MW) dwarf galaxies. Our sample is an order of magnitude larger than that of previous studies and extends to much larger radial distances. These are the first results presented that make use of the newly installed southern APOGEE instrument on the du Pont telescope at Las Campanas Observatory. Our unbiased sample of the LMC spans a large range in metallicity, from [Fe/H] = -0.2 to very metal-poor stars with [Fe/H] -2.5, the most metal-poor Magellanic Cloud (MC) stars detected to date. The LMC [alpha/Fe]-[Fe/H] distribution is very flat over a large metallicity range but rises by similar to 0.1 dex at -1.0 < [Fe/H] less than or similar to -0.5. We interpret this as a sign of the known recent increase in MC star formation activity and are able to reproduce the pattern with a chemical evolution model that includes a recent "starburst." At the metal-poor end, we capture the increase of [alpha/Fe] with decreasing [Fe/H] and constrain the "alpha-knee" to [Fe/H] less than or similar to -2.2 in both MCs, implying a low star formation efficiency of similar to 0.01 Gyr(-1). The MC knees are more metal-poor than those of less massive MW dwarf galaxies such as Fornax, Sculptor, or Sagittarius. One possible interpretation is that the MCs formed in a lower-density environment than the MW, a hypothesis that is consistent with the paradigm that the MCs fell into the MW's gravitational potential only recently.

Controlling the solid-state polymerization of organic molecules to form crystalline materials remains a challenge for the synthetic chemist. In an effort to control reaction pathways through topochemistry, we have compressed the 1:1 naphthalene-octafluoronaphthalene cocrystal, C10H8 center dot C10F8. This starting material displays a unique structure wherein the molecules are aligned in a nearly sandwich-like pi-pi stacking arrangement because of the inverse polarities of naphthalene and its perfluoronated derivative. This stacking arrangement and the use of fluorine as an sp(3)-templating functional group creates favorable interactions between the molecules along the crystallographic a axis, providing topological control over the reaction pathway at high pressure. Reaction of C10H8 center dot C10F8 along the molecular stacking axis to form polymerized sp(3) rods with single-crystalline order was confirmed through in situ single-crystal X-ray diffraction and infrared spectroscopy, as well as GC-MS analysis of the recovered polymerized material, and supported by computational models. Polymerization occurs at room temperature under rapid compression without uniaxial stress indicating enhanced control through the topology of the molecular precursor.

Nanothreads are one-dimensional sp(3) hydrocarbons that pack within pseudohexagonal crystalline lattices. They are believed to lack long-range order along the thread axis and also lack interthread registry. Here we investigate the phase behavior of thiophene up to 35 GPa and establish a pressure-induced phase transition sequence that mirrors previous observations in low-temperature studies. Slow compression to 35 GPa results in the formation of a recoverable saturated product with a 2D monoclinic diffraction pattern along (0001) that agrees closely with atomistic simulations for single crystals of thiophene-derived nanothreads. Paradoxically, this lower-symmetry packing signals a higher degree of structural order since it must arise from constituents with a consistent azimuthal orientation about their shared axis. The simplicity of thiophene reaction pathways (with only four carbon atoms per ring) apparently yields the first nanothreads with orientational order, a striking outcome considering that a single point defect in a 1D system can disrupt long-range structural order.

We present stellar age distributions of the Milky Way bulge region using ages for similar to 6000 high-luminosity (log (g), metal-rich ([Fe/H] >= -0.5) bulge stars observed by the Apache Point Observatory Galactic Evolution Experiment. Ages are derived using The Cannon label-transfer method, trained on a sample of nearby luminous giants with precise parallaxes for which we obtain ages using a Bayesian isochrone-matching technique. We find that the metal-rich bulge is predominantly composed of old stars (>8 Gyr). We find evidence that the planar region of the bulge (vertical bar Z(GC)vertical bar <= 0.25 kpc) is enriched in metallicity, Z, at a faster rate (dZ/dt similar to 0.0034 Gyr(-1)) than regions farther from the plane (dZ/dt similar to 0.0013 Gyr(-1) at vertical bar Z(GC)vertical bar > 1.00 kpc). We identify a nonnegligible fraction of younger stars (age similar to 2-5 Gyr) at metallicities of +0.2 < [Fe/H] < +0.4. These stars are preferentially found in the plane (vertical bar Z(GC)vertical bar <= 0.25 kpc) and at R-cy approximate to 2-3 kpc, with kinematics that are more consistent with rotation than are the kinematics of older stars at the same metallicities. We do not measure a significant age difference between stars found inside and outside the bar. These findings show that the bulge experienced an initial starburst that was more intense close to the plane than far from the plane. Then, star formation continued at supersolar metallicities in a thin disk at 2 kpc less than or similar to R-cy less than or similar to 3 kpc until similar to 2 Gyr ago.

The interior of the Earth is an important reservoir for elements that are chemically bound in minerals, melts, and gases. Analyses of the proportions of redox-sensitive elements in ancient and contemporary natural rocks provide information on the temporal redox evolution of our planet. Natural inclusions trapped in diamonds, xenoliths, and erupted magmas provide unique windows into the redox conditions of the deep Earth, and reveal evidence for heterogeneities in the mantle's oxidation state. By examining the natural rock record, we assess how redox boundaries in the deep Earth have controlled elemental cycling and what effects these boundaries have had on the temporal and chemical evolution of oxygen fugacity in the Earth's interior and atmosphere.

Solid-state topochemical polymerization (SSTP) requires well-defined geometries and space symmetries between the starting monomers and resulting polymer, and diacetylenes are excellent precursors, reacting through a 1,4-addition mechanism. The hydrocarbon molecule 1,4-diphenyl-1,3-butadiyne (DPB) has a four-carbon chain with alternating triple/single bonds, capped on each end with a phenyl group, i.e. centrosymmetric with unsaturated pi-bonding characteristics. To fully realize its potential for photocatalytic applications, improved control over the assembly process is desirable to form well-ordered poly(diphenylbutadiyne) (PDPB). Here, it is shown that with increasing pressure, DPB undergoes a series of solid-state chemical reactions while maintaining crystalline order related to the starting monomeric structure. Quenchable PDPB compounds begin forming at ca. 5 GPa, which exhibit optically-tunable absorbance and photoluminescence that is controllable through the extent of compression. Above ca. 15 GPa, the system transforms into a nonhexagonally-packed crystalline array with mixed sp(2)/sp(3) character. These stepwise changes with compression are irreversible in nature, as observed by in situ diffraction and spectroscopic methods. For the first time, the simple SSTP synthesis route allows well-aligned DPB molecules to directly transform into a PDPB material via self-assembly solely through pressure generation within a diamond anvil cell without the traditional use of catalysts, temperature, radiation, templates, or solvents.

Recent shear wave splitting measurements from the fore-arc region of the Ryukyu subduction system show large magnitude (0.3-1.6 s) trenchparallel splitting in both local and teleseismic phases. The similarity of splitting parameters associated with shallow local-S and teleseismic phases suggests that the source of anisotropy is located in the fore-arc mantle. One explanation for this pattern of shear wave splitting involves a transition from commonly observed high-temperature olivine fabrics with flow-parallel seismically fast directions to a flow-normal B-type olivine fabric in the cold fore-arc mantle of the Ryukyu wedge. We test the B-type fabric hypothesis by comparing observed splitting parameters to those predicted from geodynamic models that incorporate olivine fabric development. The distribution of olivine fabric is calculated with high-resolution thermomechanical models of the Ryukyu subduction zone that include realistic slab geometry and an experimentally based wet olivine rheology. We conclude that B-type fabric can explain the magnitude and trench-parallel orientation of deep local-S phases that sample the core of the foreare mantle. However, our calculations show that B-type fabric alone cannot account for large magnitude trench-parallel splitting associated with teleseismic phases that sample the shallow tip of the fore-arc mantle. Alternative models for trench-parallel teleseismic splitting in the shallow tip of the fore-arc mantle involve the addition of crustal or slab anisotropy and highly anisotropic foliated antigorite serpentinite. (c) 2008 Elsevier B.V. All rights reserved.

Quantum Monte Carlo (QMC) methods are useful for studies of strongly correlated materials because they are many body in nature and use the physical Hamiltonian. Typical calculations assume as a starting point a wave function constructed from single-particle orbitals obtained from one-body methods, e.g., density functional theory. However, mean-field-derived wave functions can sometimes lead to systematic QMC biases if the meanfield result poorly describes the true ground state. Here, we study the accuracy and flexibility of QMC trial wave functions using variational and fixed-node diffusion QMC estimates of the total spin density and lattice distortion of antiferromagnetic iron oxide (FeO) in the ground state B1 crystal structure. We found that for relatively simple wave functions the predicted lattice distortion was controlled by the choice of single-particle orbitals used to construct the wave function, rather than by subsequent wave function optimization techniques within QMC. By optimizing the orbitals with QMC, we then demonstrate starting-point independence of the trial wave function with respect to the method by which the orbitals were constructed by demonstrating convergence of the energy, spin density, and predicted lattice distortion for two qualitatively different sets of orbitals. The results suggest that orbital optimization is a promising method for accurate many-body calculations of strongly correlated condensed phases.

We measure the electrical resistivity of hcp iron up to similar to 170 GPa and similar to 3000 K using a four-probe van der Pauw method coupled with homogeneous flattop laser heating in a DAC, and compute its electrical and thermal conductivity by first-principles molecular dynamics including electron-phonon and electron-electron scattering. We find that the measured resistivity of hcp iron increases almost linearly with temperature, and is consistent with our computations. The results constrain the resistivity and thermal conductivity of hcp iron to similar to 80 +/- 5 mu Omega cm and similar to 100 +/- 10 W m(-1) K-1, respectively, at conditions near the core-mantle boundary. Our results indicate an adiabatic heat flow of similar to 10 +/- 1 TW out of the core, supporting a present-day geodynamo driven by thermal and compositional convection.