891,447 research outputs found
Quark-quark Double Scattering and Modified (Anti-)Quark Fragmentation Functions in Nuclei
Quark-quark double scattering in eA DIS and its contribution to quark and
anti-quark fragmentation functions are investigated with the generalized
factorization of the relevant twist-four processes in pQCD. It is shown that
the resulting modifications to quark and anti-quark fragmentation functions are
different. While the numerical size of these effects cannot be determined from
pQCD, the structure of our result leads to a number of qualitative predictions
for the relative size of the effect for different hadrons. These qualitative
predictions agree with the multiplicity ratios for positive and negative
hadrons as observed by HERMES.Comment: 4 pages, 1 figure, talk at "Hard Probes 2006", Asilomar, California,
USA, June 9-16, 200
Observation of Floquet-Bloch states on the surface of a topological insulator
The unique electronic properties of the surface electrons in a topological
insulator are protected by time-reversal symmetry. Circularly polarized light
naturally breaks time-reversal symmetry, which may lead to an exotic surface
quantum Hall state. Using time- and angle-resolved photoemission spectroscopy,
we show that an intense ultrashort mid-infrared pulse with energy below the
bulk band gap hybridizes with the surface Dirac fermions of a topological
insulator to form Floquet-Bloch bands. These photon dressed surface bands
exhibit polarization-dependent band gaps at avoided crossings. Circularly
polarized photons induce an additional gap at the Dirac point, which is a
signature of broken time-reversal symmetry on the surface. These observations
establish the Floquet-Bloch bands in solids and pave the way for optical
manipulation of topological quantum states of matter.Comment: 23 pages including supplementary material
The LBFGS Quasi-Newtonian Method for Molecular Modeling Prion AGAAAAGA Amyloid Fibrils
Experimental X-ray crystallography, NMR (Nuclear Magnetic Resonance)
spectroscopy, dual polarization interferometry, etc are indeed very powerful
tools to determine the 3-Dimensional structure of a protein (including the
membrane protein); theoretical mathematical and physical computational
approaches can also allow us to obtain a description of the protein 3D
structure at a submicroscopic level for some unstable, noncrystalline and
insoluble proteins. X-ray crystallography finds the X-ray final structure of a
protein, which usually need refinements using theoretical protocols in order to
produce a better structure. This means theoretical methods are also important
in determinations of protein structures. Optimization is always needed in the
computer-aided drug design, structure-based drug design, molecular dynamics,
and quantum and molecular mechanics. This paper introduces some optimization
algorithms used in these research fields and presents a new theoretical
computational method - an improved LBFGS Quasi-Newtonian mathematical
optimization method - to produce 3D structures of Prion AGAAAAGA amyloid
fibrils (which are unstable, noncrystalline and insoluble), from the potential
energy minimization point of view. Because the NMR or X-ray structure of the
hydrophobic region AGAAAAGA of prion proteins has not yet been determined, the
model constructed by this paper can be used as a reference for experimental
studies on this region, and may be useful in furthering the goals of medicinal
chemistry in this field
Flexible and Creative Chinese Poetry Generation Using Neural Memory
It has been shown that Chinese poems can be successfully generated by
sequence-to-sequence neural models, particularly with the attention mechanism.
A potential problem of this approach, however, is that neural models can only
learn abstract rules, while poem generation is a highly creative process that
involves not only rules but also innovations for which pure statistical models
are not appropriate in principle. This work proposes a memory-augmented neural
model for Chinese poem generation, where the neural model and the augmented
memory work together to balance the requirements of linguistic accordance and
aesthetic innovation, leading to innovative generations that are still
rule-compliant. In addition, it is found that the memory mechanism provides
interesting flexibility that can be used to generate poems with different
styles
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