1,902 research outputs found

### Top Quark Decays into Heavy Quark Mesons

For top quark decays into heavy quark mesons $\Upsilon$ and $\bar{B}_c^*$ , a
complete calculation to the leading order both in QCD coupling constant
$\alpha_s$ and in $v$, 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 $2\sim 3$) with that obtained using
fragmentation functions at $m_t\sim 150$ GeV.Comment: 15 pages in LaTex form, 4 figures include

### Electronic structure, phase stability and chemical bonding in Th$_2$Al and Th$_2$AlH$_4$

We present the results of theoretical investigation on the electronic
structure, bonding nature and ground state properties of Th$_2$Al and
Th$_2$AlH$_4$ using generalized-gradient-corrected first-principles
full-potential density-functional calculations. Th$_2$AlH$_4$ 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 Th$_2$Al 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 $a$ and $c$. The strongest
bonding in Th$_2$AlH$_4$ is between Th and H along $c$ 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 $c$ 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 $ac$ and $ab$ planes along with the strong covalent interaction
between Th and H are the main reasons for highly anisotropic volume expansion
on hydrogenation of Th$_2$Al.Comment: 14 pages, 9 figure

### Electric field and strain induced Rashba effect in hybrid halide perovskites

Using first principles density functional theory calculations, we show how
Rashba-type energy band splitting in the hybrid organic-inorganic halide
perovskites APbX$_3$ (A=CH$_3$NH$_3^+$, CH(NH$_2$)$_2^+$, 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 (CH$_3$NH$_3$)PbI$_3$ 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 PbX$_3$ cage that directly
impact Rashba splitting. In addition, we predict that polar phases of CsPbI$_3$
and (CH$_3$NH$_3$)PbI$_3$ with $R3c$ 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

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

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.

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 Î±-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

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