1,902 research outputs found

    Top Quark Decays into Heavy Quark Mesons

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    For top quark decays into heavy quark mesons Υ\Upsilon and Bˉc\bar{B}_c^* , a complete calculation to the leading order both in QCD coupling constant αs\alpha_s and in vv, the typical velocity of the heavy quarks inside the mesons, is performed. Relatons between the top quark mass and the decay branching ratios are studied. Comparion with the results which are obtained by using the quark frangmentation functions is also discussed. The branching ratios are consistent (within a factor of 232\sim 3 ) with that obtained using fragmentation functions at mt150m_t\sim 150 GeV.Comment: 15 pages in LaTex form, 4 figures include

    Electronic structure, phase stability and chemical bonding in Th2_2Al and Th2_2AlH4_4

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    We present the results of theoretical investigation on the electronic structure, bonding nature and ground state properties of Th2_2Al and Th2_2AlH4_4 using generalized-gradient-corrected first-principles full-potential density-functional calculations. Th2_2AlH4_4 has been reported to violate the "2 \AA rule" of H-H separation in hydrides. From our total energy as well as force-minimization calculations, we found a shortest H-H separation of 1.95 {\AA} in accordance with recent high resolution powder neutron diffraction experiments. When the Th2_2Al matrix is hydrogenated, the volume expansion is highly anisotropic, which is quite opposite to other hydrides having the same crystal structure. The bonding nature of these materials are analyzed from the density of states, crystal-orbital Hamiltonian population and valence-charge-density analyses. Our calculation predicts different nature of bonding for the H atoms along aa and cc. The strongest bonding in Th2_2AlH4_4 is between Th and H along cc which form dumb-bell shaped H-Th-H subunits. Due to this strong covalent interaction there is very small amount of electrons present between H atoms along cc which makes repulsive interaction between the H atoms smaller and this is the precise reason why the 2 {\AA} rule is violated. The large difference in the interatomic distances between the interstitial region where one can accommodate H in the acac and abab planes along with the strong covalent interaction between Th and H are the main reasons for highly anisotropic volume expansion on hydrogenation of Th2_2Al.Comment: 14 pages, 9 figure

    Electric field and strain induced Rashba effect in hybrid halide perovskites

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    Using first principles density functional theory calculations, we show how Rashba-type energy band splitting in the hybrid organic-inorganic halide perovskites APbX3_3 (A=CH3_3NH3+_3^+, CH(NH2_2)2+_2^+, Cs+^+ and X=I, Br) can be tuned and enhanced with electric fields and anisotropic strain. In particular, we demonstrate that the magnitude of the Rashba splitting of tetragonal (CH3_3NH3_3)PbI3_3 grows with increasing macroscopic alignment of the organic cations and electric polarization, indicating appreciable tunability with experimentally-feasible applied fields, even at room temperature. Further, we quantify the degree to which this effect can be tuned via chemical substitution at the A and X sites, which alters amplitudes of different polar distortion patterns of the inorganic PbX3_3 cage that directly impact Rashba splitting. In addition, we predict that polar phases of CsPbI3_3 and (CH3_3NH3_3)PbI3_3 with R3cR3c symmetry possessing considerable Rashba splitting might be accessible at room temperature via anisotropic strain induced by epitaxy, even in the absence of electric fields

    L-space surgery and twisting operation

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    A knot in the 3-sphere is called an L-space knot if it admits a nontrivial Dehn surgery yielding an L-space, i.e. a rational homology 3-sphere with the smallest possible Heegaard Floer homology. Given a knot K, take an unknotted circle c and twist K n times along c to obtain a twist family { K_n }. We give a sufficient condition for { K_n } to contain infinitely many L-space knots. As an application we show that for each torus knot and each hyperbolic Berge knot K, we can take c so that the twist family { K_n } contains infinitely many hyperbolic L-space knots. We also demonstrate that there is a twist family of hyperbolic L-space knots each member of which has tunnel number greater than one.Comment: The final version, accepted for publication by Algebr. Geom. Topo

    Dynamical mean-field theory of photoemission spectra of actinide compounds

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    A model of photoemission spectra of actinide compounds is presented. The complete multiplet spectrum of a single ion is calculated by exact diagonalization of the two-body Hamiltonian of the f^n shell. A coupling to auxiliary fermion states models the interaction with a conduction sea. The ensuing self-energy function is combined with a band Hamiltonian of the compound, calculated in the local-density approximation, to produce a solid state Green's function. The theory is applied to PuSe and elemental Am. For PuSe a sharp resonance at the Fermi level arises from mixed valent behavior, while several features at larger binding energies can be identified with quantum numbers of the atomic system. For Am the ground state is dominated by the |f^6;J=0> singlet but the strong coupling to the conduction electrons mixes in a significant amount of f^7 character.Comment: Solid State Communications, in press; 4 pages 4 figure

    Mini-chromosome maintenance complexes form a filament to remodel DNA structure and topology.

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    Deregulation of mini-chromosome maintenance (MCM) proteins is associated with genomic instability and cancer. MCM complexes are recruited to replication origins for genome duplication. Paradoxically, MCM proteins are in excess than the number of origins and are associated with chromatin regions away from the origins during G1 and S phases. Here, we report an unusually wide left-handed filament structure for an archaeal MCM, as determined by X-ray and electron microscopy. The crystal structure reveals that an α-helix bundle formed between two neighboring subunits plays a critical role in filament formation. The filament has a remarkably strong electro-positive surface spiraling along the inner filament channel for DNA binding. We show that this MCM filament binding to DNA causes dramatic DNA topology change. This newly identified function of MCM to change DNA topology may imply a wider functional role for MCM in DNA metabolisms beyond helicase function. Finally, using yeast genetics, we show that the inter-subunit interactions, important for MCM filament formation, play a role for cell growth and survival