1,587 research outputs found
Constraints on Hf and Zr mobility in high-sulfidation epithermal systems: formation of kosnarite, KZr2(PO4)3, in the Chaquicocha gold deposit, Yanacocha district, Peru
We report the first occurrence of Hf-rich kosnarite [K(Hf,Zr)2(PO4)3], space group R-3c, Z = 6, in the giant Chaquicocha high-sulfidation epithermal gold deposit in the Yanacocha mining district, Peru. Kosnarite crystals are small (<100 μm) and occur in 2–3-mm-thick veins that cut intensively silicified rocks. The paragenesis includes a first stage of As-free pyrite and quartz (plus gratonite and rutile), followed by trace metal-rich pyrite [(Fe,As,Pb,Au)S2] and secondary Fe sulfates. Kosnarite is associated with quartz and is clearly late within the paragenetic sequence. Electron microprobe analyses (EMPA) of kosnarite show relatively high concentrations of HfO2 and Rb2O (7.61 and 1.05 wt.%, respectively). The re-calculated chemical formulas of kosnarite vary from KΣ1.00(Zr1.93Na0.01Hf0.01Mn0.01)Σ1.96(P3.04O4)Σ3 to (K0.92Rb0.05Na0.03)Σ1.00(Zr1.81Hf0.19)Σ2.00 [(P2.98Si0.02As0.01)Σ3.01O4]Σ3, where Hf and Rb are most likely incorporated according to a coupled substitution of Hf4+ + Rb+ ⇔ Zr4+ + K+. Back-scattered electron (BSE) images and elemental mapping of kosnarite reveal that Hf and Rb are enriched in 2–10-μm-wide oscillatory and/or sector zones. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) observations of such zones reveal a pattern of alternating, 5–50-nm-thick, Hf-rich and Zr-rich nanozones. These high-resolution observations indicate that the incorporation of Hf does not appear to cause significant distortion in the kosnarite structure. Semiquantitative TEM-energy-dispersive X-ray spectrometry (EDS) analyses of the nano-layers show up to 22 wt.% of HfO2, which corresponds to 31 mol% of the hypothetical, KHf2(PO4)3, end-member. The presence of kosnarite in the advanced argillic alteration zone at Yanacocha is indicative of Hf and Zr mobility under highly acidic conditions and points towards an unforeseen role of phosphates as sinks of Zr and Hf in high-sulfidation epithermal environments. Finally, potentially new geochronological applications of highly insoluble vein kosnarite, including Rb-Sr dating, may provide further age constraints in pervasively altered areas where other isotopic systems might have been reset
Model analysis of the world data on the pion transition form factor
We discuss the impact of recent Belle data on our description of the pion
transition form factor based on the assumption that a perturbative formalism
and a nonperturbative one can be matched in a physically acceptable manner at a
certain hadronic scale . We discuss the implications of the different
parameters of the model in comparing with world data and conclude that within
experimental errors our description remains valid. Thus we can assert that the
low nonperturbative description together with an additional term
at the matching scale have a strong influence on the behavior up to very
high values of .Comment: 6 pages and 3 figures. Contains a comparison with other models and
additional reference
Efficient Recursion Method for Inverting Overlap Matrix
A new O(N) algorithm based on a recursion method, in which the computational
effort is proportional to the number of atoms N, is presented for calculating
the inverse of an overlap matrix which is needed in electronic structure
calculations with the the non-orthogonal localized basis set. This efficient
inverting method can be incorporated in several O(N) methods for
diagonalization of a generalized secular equation. By studying convergence
properties of the 1-norm of an error matrix for diamond and fcc Al, this method
is compared to three other O(N) methods (the divide method, Taylor expansion
method, and Hotelling's method) with regard to computational accuracy and
efficiency within the density functional theory. The test calculations show
that the new method is about one-hundred times faster than the divide method in
computational time to achieve the same convergence for both diamond and fcc Al,
while the Taylor expansion method and Hotelling's method suffer from numerical
instabilities in most cases.Comment: 17 pages and 4 figure
Systematic generation of finite-range atomic basis sets for linear-scaling calculations
Basis sets of atomic orbitals are very efficient for density functional
calculations but lack a systematic variational convergence.
We present a variational method to optimize numerical atomic orbitals using a
single parameter to control their range.
The efficiency of the basis generation scheme is tested and compared with
other schemes for multiple zeta basis sets.
The scheme shows to be comparable in quality to other widely used schemes
albeit offering better performance for linear-scaling computations
EXTRA SPINDLE POLES (Separase) controls anisotropic cell expansion in Norway spruce (Picea abies) embryos independently of its role in anaphase progression
The caspase-related protease separase (EXTRA SPINDLE POLES, ESP) plays a major role in chromatid disjunction and cell expansion in Arabidopsis thaliana. Whether the expansion phenotypes are linked to defects in cell division in Arabidopsis ESP mutants remains elusive. Here we present the identification, cloning and characterization of the gymnosperm Norway spruce (Picea abies, Pa) ESP. We used the P. abies somatic embryo system and a combination of reverse genetics and microscopy to explore the roles of Pa ESP during embryogenesis. Pa ESP was expressed in the proliferating embryonal mass, while it was absent in the suspensor cells. Pa ESP associated with kinetochore microtubules in metaphase and then with anaphase spindle midzone. During cytokinesis, it localized on the phragmoplast microtubules and on the cell plate. Pa ESP deficiency perturbed anisotropic expansion and reduced mitotic divisions in cotyledonary embryos. Furthermore, whilst Pa ESP can rescue the chromatid nondisjunction phenotype of Arabidopsis ESP mutants, it cannot rescue anisotropic cell expansion. Our data demonstrate that the roles of ESP in daughter chromatid separation and cell expansion are conserved between gymnosperms and angiosperms. However, the mechanisms of ESP-mediated regulation of cell expansion seem to be lineage-specific
Pion-Photon Transition Form Factor and Pion Distribution Amplitude in QCD: Facing the Enigmatic Behavior of the BaBar Data
We present an extended analysis of the data for the pion-photon transition
form factor from different experiments, CELLO, CLEO, and BaBar, and discuss
various theoretical approaches which try to reason from them. We focus on the
divergent behavior of the BaBar data for the pion and those for the
pseudoscalar mesons and comment on recently proposed explanations
for this discrepancy. We argue that it is not possible at present to
accommodate these data within the standard QCD framework self-consistently.Comment: 5 pages, 2 figures, talk presented by the second author at the
International Workshop on e^+e^- collisions from phi to psi (PHIPSI11),
Novosibirsk (Russia), Sept. 19--22, 201
Fabrication of crystals from single metal atoms
YesMetal nanocrystals offer new concepts for the design of nanodevices with a range of potential applications. Currently the formation of metal nanocrystals cannot be controlled at the level of individual atoms. Here we describe a new general method for the fabrication of multi-heteroatom-doped graphitic matrices decorated with very small, ångström-sized, three-dimensional (3D)-metal crystals of defined size. We irradiate boron-rich precious-metal-encapsulated self-spreading polymer micelles with electrons and produce, in real time, a doped graphitic support on which individual osmium atoms hop and migrate to form 3D-nanocrystals, as small as 15 Å in diameter, within 1 h. Crystal growth can be observed, quantified and controlled in real time. We also synthesize the first examples of mixed ruthenium–osmium 3D-nanocrystals. This technology not only allows the production of ångström-sized homo- and hetero-crystals, but also provides new experimental insight into the dynamics of nanocrystals and pathways for their assembly from single atoms.We thank the Leverhulme Trust (Early Career Fellowship No. ECF-2013-414 to NPEB), the University of Warwick (Grant No. RDF 2013-14 to NPEB), the Swiss National Science Foundation (Grant No. PA00P2_145308 to NPEB and PBNEP2_142949 to APB), the ERC (Grant No. 247450 to PJS), EPSRC (EP/G004897/1 to RKOR, and EP/F034210/1 to PJS) and Science City (AWM/ERDF) for support. We thank the Wellcome Trust (Grant No. 055663/Z/98/Z) for funding the Electron Microscopy Facility, School of Life Sciences, University of Warwick. We also thank COST Action CM1105 for stimulating discussions, Thomas Wilks for supplying the micelle image for Figure 1, and the Australian Synchrotron and the University of Monash for allocation of time on the SAXS/ WAXS beamline and funding. The 2000FX Gatan Orius digital TEM camera used in this research was funded by Science City: Creating and Characterizing Next Generation Advanced Materials, with support from Advantage West Midlands and part funded by the European Regional Development Fund
Entropy and Spin Susceptibility of s-wave Type-II Superconductors near
A theoretical study is performed on the entropy and the spin
susceptibility near the upper critical field of s-wave
type-II superconductors with arbitrary impurity concentrations. The changes of
these quantities through may be expressed as , for example, where is the average flux density
and denotes entropy in the normal state. It is found that the
slopes and at T=0 are identical, connected
directly with the zero-energy density of states, and vary from 1.72 in the
dirty limit to in the clean limit. This mean-free-path dependence
of and at T=0 is quantitatively the same as that
of the slope for the flux-flow resistivity studied
previously. The result suggests that and near
T=0 are convex downward (upward) in the dirty (clean) limit, deviating
substantially from the linear behavior . The specific-heat
jump at also shows fairly large mean-free-path dependence.Comment: 8 pages, 5 figure
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