18,268 research outputs found
A More Precise Extraction of |V_{cb}| in HQEFT of QCD
The more precise extraction for the CKM matrix element |V_{cb}| in the heavy
quark effective field theory (HQEFT) of QCD is studied from both exclusive and
inclusive semileptonic B decays. The values of relevant nonperturbative
parameters up to order 1/m^2_Q are estimated consistently in HQEFT of QCD.
Using the most recent experimental data for B decay rates, |V_{cb}| is updated
to be |V_{cb}| = 0.0395 \pm 0.0011_{exp} \pm 0.0019_{th} from B\to D^{\ast} l
\nu decay and |V_{cb}| = 0.0434 \pm 0.0041_{exp} \pm 0.0020_{th} from B\to D l
\nu decay as well as |V_{cb}| = 0.0394 \pm 0.0010_{exp} \pm 0.0014_{th} from
inclusive B\to X_c l \nu decay.Comment: 7 pages, revtex, 4 figure
Study of axial strain induced torsion of single wall carbon nanotubes by 2D continuum anharmonic anisotropic elastic model
Recent molecular dynamic simulations have found chiral single wall carbon
nanotubes (SWNTs) twist during stretching, which is similar to the motion of a
screw. Obviously this phenomenon, as a type of curvature-chirality effect, can
not be explained by usual isotropic elastic theory of SWNT. More interestingly,
with larger axial strains (before buckling), the axial strain induced torsion
(a-SIT) shows asymmetric behaviors for axial tensile and compressing strains,
which suggests anharmonic elasticity of SWNTs plays an important role in real
a-SIT responses. In order to study the a-SIT of chiral SWNTs with actual sizes,
and avoid possible deviations of computer simulation results due to the
finite-size effect, we propose a 2D analytical continuum model which can be
used to describe the the SWNTs of arbitrary chiralities, curvatures, and
lengths, with the concerning of anisotropic and anharmonic elasticity of SWNTs.
This elastic energy of present model comes from the continuum limit of lattice
energy based on Second Generation Reactive Empirical Bond Order potential
(REBO-II), a well-established empirical potential for solid carbons. Our model
has no adjustable parameters, except for those presented in REBO-II, and all
the coefficients in the model can be calculated analytically. Using our method,
we obtain a-SIT responses of chiral SWNTs with arbitrary radius, chiralities
and lengthes. Our results are in reasonable agreement with recent molecular
dynamic simulations. [Liang {\it et. al}, Phys. Rev. Lett, , 165501
(2006).] Our approach can also be used to calculate other curvature-chirality
dependent anharmonic mechanic responses of SWNTs.Comment: 14 pages, 2 figure
In situ imaging of field emission from individual carbon nanotubes and their structural damage
©2002 American Institute of Physics. The electronic version of this article is the complete one and can be found online at: http://link.aip.org/link/?APPLAB/80/856/1DOI:10.1063/1.1446994Field emission of individual carbon nanotubes was observed by in situ
transmission electron microscopy. A fluctuation in emission current was due to a
variation in distance between the nanotube tip and the counter electrode owing
to a "head-shaking" effect of the nanotube during field emission. Strong
field-induced structural damage of a nanotube occurs in two ways: a
piece-by-piece and segment-by-segment pilling process of the graphitic layers,
and a concentrical layer-by-layer stripping process. The former is believed
owing to a strong electrostatic force, and the latter is likely due to heating
produced by emission current that flowed through the most outer graphitic
layers
Exclusive B-meson Rare Decays and General Relations of Form Factors in Effective Field Theory of Heavy Quarks
B meson rare decays ( and ) are
analyzed in the framework of effective field theory of heavy quarks. The
semileptonic and penguin type form factors for these decays are calculated by
using the light cone sum rules method at the leading order of
expansion. Four exact relations between the two types of form factors are
obtained at the leading order of expansion. Of particular, the
relations are found to hold for whole momentum transfer region. We also
investigate the validity of the relations resulted from the large energy
effective theory based on the general relations obtained in the present
approach. The branching ratios of the rare decays are presented and their
potential importance for extracting the CKM matrix elements and probing new
physics is emphasized.Comment: 23 pages, Revtex, 32 figures, published version with the errors of
numerical results caused by the computer program are correcte
Optimization of robustness of scale-free network to random and targeted attacks
The scale-fee networks, having connectivity distribution (where is the site connectivity), is very resilient to random
failures but fragile to intentional attack. The purpose of this paper is to
find the network design guideline which can make the robustness of the network
to both random failures and intentional attack maximum while keeping the
average connectivity per node constant. We find that when $=3$ the
robustness of the scale-free networks reach its maximum value if the minimal
connectivity $m=1$, but when is larger than four, the networks will
become more robust to random failures and targeted attacks as the minimal
connectivity gets larger
Phase Reversal Diffraction in incoherent light
Phase reversal occurs in the propagation of an electromagnetic wave in a
negatively refracting medium or a phase-conjugate interface. Here we report the
experimental observation of phase reversal diffraction without the above
devices. Our experimental results and theoretical analysis demonstrate that
phase reversal diffraction can be formed through the first-order field
correlation of chaotic light. The experimental realization is similar to phase
reversal behavior in negatively refracting media.Comment: 8 pages, 5 figure
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