16,936 research outputs found
Fractional Flux Periodicity in Doped Carbon Nanotubes
An anomalous magnetic flux periodicity of the ground state is predicted in
two-dimensional cylindrical surface composed of square and honeycomb lattice.
The ground state and persistent currents exhibit an approximate fractional
period of the flux quantum for a specific Fermi energy. The period depends on
the aspect ratio of the cylinder and on the lattice structure around the axis.
We discuss possibility of this nontrivial periodicity in a heavily doped
armchair carbon nanotube.Comment: 5 pages, 4 figure
Stabilization mechanism of edge states in graphene
It has been known that edge states of a graphite ribbon are zero-energy,
localized eigen-states. We show that next nearest-neighbor hopping process
decreases the energy of the edge states at zigzag edge with respect to the
Fermi energy. The energy reduction of the edge states is calculated
analytically by first-order perturbation theory and numerically. The resultant
model is consistent with the peak of recent scanning tunneling spectroscopy
measurements.Comment: 4 pages, 2 figures, final version to appear in Applied Physics
Letter
The Reionization History and Early Metal Enrichment inferred from the Gamma-Ray Burst Rate
Based on the gamma-ray burst (GRB) event rate at redshifts of , which is assessed by the spectral peak energy-to-luminosity relation
recently found by Yonetoku et al., we observationally derive the star formation
rate (SFR) for Pop III stars in a high redshift universe. As a result, we find
that Pop III stars could form continuously at . Using the
derived Pop III SFR, we attempt to estimate the ultraviolet (UV) photon
emission rate at in which redshift range no observational
information has been hitherto obtained on ionizing radiation intensity. We find
that the UV emissivity at can make a noticeable contribution
to the early reionization. The maximal emissivity is higher than the level
required to keep ionizing the intergalactic matter at .
However, if the escape fraction of ionizing photons from Pop III objects is
smaller than 10%, then the IGM can be neutralized at some redshift, which may
lead to the double reionization. As for the enrichment, the ejection of all
metals synthesized in Pop III objects is marginally consistent with the IGM
metallicity, although the confinement of metals in Pop III objects can reduce
the enrichment significantly.Comment: 12 pages, 2 figures, ApJL accepte
Comparative Morphology of the Penis and Clitoris in Four Species of Moles (Talpidae).
The penile and clitoral anatomy of four species of Talpid moles (broad-footed, star-nosed, hairy-tailed, and Japanese shrew moles) were investigated to define penile and clitoral anatomy and to examine the relationship of the clitoral anatomy with the presence or absence of ovotestes. The ovotestis contains ovarian tissue and glandular tissue resembling fetal testicular tissue and can produce androgens. The ovotestis is present in star-nosed and hairy-tailed moles, but not in broad-footed and Japanese shrew moles. Using histology, three-dimensional reconstruction, and morphometric analysis, sexual dimorphism was examined with regard to a nine feature masculine trait score that included perineal appendage length (prepuce), anogenital distance, and presence/absence of bone. The presence/absence of ovotestes was discordant in all four mole species for sex differentiation features. For many sex differentiation features, discordance with ovotestes was observed in at least one mole species. The degree of concordance with ovotestes was highest for hairy-tailed moles and lowest for broad-footed moles. In relationship to phylogenetic clade, sex differentiation features also did not correlate with the similarity/divergence of the features and presence/absence of ovotestes. Hairy-tailed and Japanese shrew moles reside in separated clades, but they exhibit a high degree of congruence. Broad-footed and hairy-tailed moles reside within the same clade but had one of the lowest correlations in features and presence/absence of ovotestes. Thus, phylogenetic affinity and the presence/absence of ovotestes are poor predictors for most sex differentiation features within mole external genitalia
Decoherence in Phase Space
Much of the discussion of decoherence has been in terms of a particle moving
in one dimension that is placed in an initial superposition state (a
Schr\"{o}dinger "cat" state) corresponding to two widely separated wave
packets. Decoherence refers to the destruction of the interference term in the
quantum probability function. Here, we stress that a quantitative measure of
decoherence depends not only on the specific system being studied but also on
whether one is considering coordinate, momentum or phase space. We show that
this is best illustrated by considering Wigner phase space where the measure is
again different. Analytic results for the time development of the Wigner
distribution function for a two-Gaussian Schrodinger "cat" state have been
obtained in the high-temperature limit (where decoherence can occur even for
negligible dissipation) which facilitates a simple demonstration of our
remarks.Comment: in press in Laser Phys.13(2003
Existence of an upper limit on the density of excitons in carbon nanotubes by diffusion-limited exciton-exciton annihilation: Experiment and theory
Through an investigation of photoemission properties of highly-photoexcited
single-walled carbon nanotubes, we demonstrate that there is an upper limit on
the achievable excitonic density. As the intensity of optical excitation
increases, all photoluminescence emission peaks arising from different
chirality single-walled carbon nanotubes showed clear saturation in intensity.
Each peak exhibited a saturation value that was independent of the excitation
wavelength, indicating that there is an upper limit on the excitonic density
for each nanotube species. We propose that this saturation behavior is a result
of efficient exciton-exciton annihilation through which excitons decay
non-radiatively. In order to explain the experimental results and obtain
excitonic densities in the saturation regime, we have developed a model, taking
into account the generation, diffusion-limited exciton-exciton annihilation,
and spontaneous decays of one-dimensional excitons. Using the model, we were
able to reproduce the experimentally obtained saturation curves under certain
approximations, from which the excitonic densities were estimated. The validity
of the model was confirmed through comparison with Monte Carlo simulations.
Finally, we show that the conventional rate equation for exciton-exciton
annihilation without taking into account exciton diffusion fails to fit the
experimentally observed saturation behavior, especially at high excitonic
densities.Comment: 5 figures, 1 tabl
pH-triggered phase inversion and separation of hydrophobised bacterial cellulose stabilised Pickering emulsions
The pH-triggered transitional phase behaviour of Pickering emulsions stabilised by hydrophobised bacterial cellulose (BC) is reported in this work. Neat BC was esterified with acetic (C2–), hexanoic (C6–) and dodecanoic (C12–) acids, respectively. We observed that C6– and C12–BC stabilised emulsions exhibited a pH-triggered reversible transitional phase separation. Water-in-toluene emulsions containing of 60 vol.% dispersed phase stabilised by C6– and C12–BC were produced at pH 5. Lowering the pH of the aqueous phase to 1 did not affect the emulsion type. Increasing the pH to 14, however, caused the emulsions to phase separate. This phase separation was caused by electrostatic repulsion between modified BC due to dissociable acidic surface groups at high pH, which lowered the surface coverage of the water droplets by modified BC. When the pH was re-adjusted to 1 again, w/o emulsions re-formed for C6– and C12–BC stabilised emulsions. C2–BC stabilised emulsions, on the other hand, underwent an irreversible pH-triggered transitional phase separation and inversion. This difference in phase behaviour between C2–BC and C6–/C12–BC was attributed to the hydrolysis of the ester bonds of C2–BC at high pH. This hypothesis is in good agreement with the measured degree of surface substitution (DSS) of modified BC after the pH-triggered experiments. The DSS of C2–BC decreased by 20% whilst the DSS remained constant for C6– and C12–BC
Controlling edge states of zigzag carbon nanotubes by the Aharonov-Bohm flux
It has been known theoretically that localized states exist around zigzag
edges of a graphite ribbon and of a carbon nanotube, whose energy eigenvalues
are located between conduction and valence bands. We found that in metallic
single-walled zigzag carbon nanotubes two of the localized states become
critical, and that their localization length is sensitive to the mean curvature
of a tube and can be controlled by the Aharonov-Bohm flux. The curvature
induced mini-gap closes by the relatively weak magnetic field. Conductance
measurement in the presence of the Aharonov-Bohm flux can give information
about the curvature effect and the critical states.Comment: 5 pages, 4 figure
Spin Hall effects in diffusive normal metals
We consider spin and charge flow in normal metals. We employ the Keldysh
formalism to find transport equations in the presence of spin-orbit
interaction, interaction with magnetic impurities, and non-magnetic impurity
scattering. Using the quasiclassical approximation, we derive diffusion
equations which include contributions from skew scattering, side-jump
scattering and the anomalous spin-orbit induced velocity. We compute the
magnitude of various spin Hall effects in experimental relevant geometries and
discuss when the different scattering mechanisms are important.Comment: 10 pages, 4 figure
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