13,905 research outputs found
Constraints on scalar diffusion anomaly in three-dimensional flows having bounded velocity gradients
This study is concerned with the decay behaviour of a passive scalar
in three-dimensional flows having bounded velocity gradients. Given an
initially smooth scalar distribution, the decay rate of the
scalar variance is found to be bounded in terms of controlled
physical parameters. Furthermore, in the zero diffusivity limit, ,
this rate vanishes as if there exists an
independent of such that for
. This condition is satisfied if in the limit ,
the variance spectrum remains steeper than for large wave
numbers . When no such positive exists, the scalar field may be
said to become virtually singular. A plausible scenario consistent with
Batchelor's theory is that becomes increasingly shallower for
smaller , approaching the Batchelor scaling in the limit
. For this classical case, the decay rate also vanishes, albeit
more slowly -- like , where is the Prandtl or Schmidt
number. Hence, diffusion anomaly is ruled out for a broad range of scalar
distribution, including power-law spectra no shallower than . The
implication is that in order to have a -independent and non-vanishing
decay rate, the variance at small scales must necessarily be greater than that
allowed by the Batchelor spectrum. These results are discussed in the light of
existing literature on the asymptotic exponential decay , where is independent of .Comment: 6-7 journal pages, no figures. accepted for publication by Phys.
Fluid
Vibrational Tamm states at the edges of graphene nanoribbons
We study vibrational states localized at the edges of graphene nanoribbons.
Such surface oscillations can be considered as a phonon analog of Tamm states
well known in the electronic theory. We consider both armchair and zigzag
graphene stripes and demonstrate that surface modes correspond to phonons
localized at the edges of the graphene nanoribbon, and they can be classified
as in-plane and out-of-plane modes. In addition, in armchair nanoribbons
anharmonic edge modes can experience longitudinal localization in the form of
self-localized nonlinear modes, or surface breather solitons.Comment: 10 pages, 10 figure
The baboon endogenous virus genome. II. Provirus sequence variations in baboon cell DNA
Restriction analysis of the approximately 100 integrated baboon endogenous virus (BaEV) proviruses in baboon cells and tissues has revealed two major sequence variations, both in the gag gene region of the genome. One, a 150 nucleotide pair insert, is present in a small proportion of the proviral DNAs of some baboons, but is present in the majority of the proviral DNAs of other baboons. The second, a Bam HI recognition sequence located 2.25 kb from the proviral 5' end, is missing or modified in approximately one-half of the integrated genomes. We consider the possibility that accumulation of proviruses not containing the 0.15 kb insert is correlated with viral activation and expression since it is this form that is a replication intermediate in freshly infected permissive cells. It is evident from these initial studies that the organization of the multiple BaEV proviruses in baboon DNA has undergone modification during evolution
Calculation of nuclear-spin-dependent parity nonconservation in s-d transitions of Ba, Yb and Ra ions
We use correlation potential and many-body perturbation theory techniques to
calculate spin-independent and nuclear spin-dependent parts of the parity
nonconserving amplitudes of the transitions between the ground state
and the excited state of Ba and Yb and between the
ground state and the excited state of Ra. The results
are presented in a form convenient for extracting of the constants of
nuclear-spin-dependent interaction (such as, e.g., anapole moment) from the
measurements.Comment: 9 pages, 8 tables, no figure
The eta-photon transition form factor
The eta-photon transition form factor is evaluated in a formalism based on a
phenomenological description at low values of the photon virtuality, and a
QCD-based description at high photon virtualities, matching at a scale
. The high photon virtuality description makes use of a Distribution
Amplitude calculated in the Nambu-Jona-Lasinio model with Pauli-Villars
regularization at the matching scale , and QCD evolution from
to higher values of . A good description of the available
data is obtained. The analysis indicates that the recent data from the BaBar
collaboration on pion and eta transition form factor can be well reproduced, if
a small contribution of twist three at the matching scale is
included.Comment: 14 pages, 3 figures, revised version, minor corrections, references
added, conclusions unchanged. Accepted for publication in Phys. Rev.
Photoionization and Photoelectric Loading of Barium Ion Traps
Simple and effective techniques for loading barium ions into linear Paul
traps are demonstrated. Two-step photoionization of neutral barium is achieved
using a weak intercombination line (6s2 1S0 6s6p 3P1, 791 nm) followed by
excitation above the ionization threshold using a nitrogen gas laser (337 nm).
Isotopic selectivity is achieved by using a near Doppler-free geometry for
excitation of the triplet 6s6p 3P1 state. Additionally, we report a
particularly simple and efficient trap loading technique that employs an
in-expensive UV epoxy curing lamp to generate photoelectrons.Comment: 5 pages, Accepted to PRA 3/20/2007 -fixed typo -clarified figure 3
caption -added reference [15
Scotland, Catalonia and the ârightâ to self-determination: a comment suggested by Kathryn Crameriâs âDo Catalans Have the Right to Decide?
No abstract available
Nuclear collective motion with a coherent coupling interaction between quadrupole and octupole modes
A collective Hamiltonian for the rotation-vibration motion of nuclei is
considered, in which the axial quadrupole and octupole degrees of freedom are
coupled through the centrifugal interaction. The potential of the system
depends on the two deformation variables and . The system is
considered to oscillate between positive and negative -values, by
rounding an infinite potential core in the -plane with
. By assuming a coherent contribution of the quadrupole and octupole
oscillation modes in the collective motion, the energy spectrum is derived in
an explicit analytic form, providing specific parity shift effects. On this
basis several possible ways in the evolution of quadrupole-octupole
collectivity are outlined. A particular application of the model to the energy
levels and electric transition probabilities in alternating parity spectra of
the nuclei Nd, Sm, Gd and Dy is presented.Comment: 25 pages, 13 figures. Accepted in Phys. Rev.
Charged Higgs phenomenology in the flipped two Higgs doublet model
We study the phenomenology of the charged Higgs boson in the "flipped" two
Higgs doublet model, in which one doublet gives mass to up-type quarks and
charged leptons and the other gives mass to down-type quarks. We present the
charged Higgs branching ratios and summarize the indirect constraints. We
extrapolate existing LEP searches for H+H- and Tevatron searches for t tbar
with t --> H+ b into the flipped model and extract constraints on MH+ and the
parameter tan(beta). We finish by reviewing existing LHC charged Higgs searches
and suggest that the LHC reach in this model could be extended for charged
Higgs masses below the tb threshold by considering t tbar with t --> H+ b and
H+ --> q qbar, as has been used in Tevatron searches.Comment: 23 pages, 7 figures. V2: added refs on H+W- associated productio
Baboon endogenous virus genome. I. Restriction enzyme map of the unintegrated DNA genome of a primate retrovirus
A detailed restriction map was deduced for the genome of an endogenous retrovirus of a higher primate, that of baboon. The cleavage sites for 12 restriction enzymes were mapped. The unintegrated linear viral DNA intermediate that is produced by infection of permissive cells with baboon endogenous virus was isolated. Hybridization with a strong-stop complementary DNA probe demonstrated presence of a terminal repetition in the linear viral DNA. The positions of restriction sites for two particular enzymes, SmaI and XhoI, near each end were consistent with this result and indicated that the length of the repetition is 0.55 +/- 0.01 kilobase. The linear viral DNA had a unique restriction map indicating that it is not a set of random circular permutations of the RNA genome. From hybridization with a 3'-specific probe, the DNA restriction map was aligned relative to the 5'-to-3' orientation of the viral RNA. We observed a minor heterogeneity in a BamHI recognition site 1.95 kilobases from the right end of the linear map
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