12,225 research outputs found
Phosphorus in land-water systems
Analyses were made to obtain information on distribution of different forms of phosphate in different environmental media, including soils, eroding material, and bottom sediment. Major emphasis was placed on determining several forms of inorganic phosphate in each media. Results show that eroding material can transport significant quantities of phosphates from soils
Analysis of quantum conductance of carbon nanotube junctions by the effective mass approximation
The electron transport through the nanotube junctions which connect the
different metallic nanotubes by a pair of a pentagonal defect and a heptagonal
defect is investigated by Landauer's formula and the effective mass
approximation. From our previous calculations based on the tight binding model,
it has been known that the conductance is determined almost only by two
parameters,i.e., the energy in the unit of the onset energy of more than two
channels and the ratio of the radii of the two nanotubes. The conductance is
calculated again by the effective mass theory in this paper and a simple
analytical form of the conductance is obtained considering a special boundary
conditions of the envelop wavefunctions. The two scaling parameters appear
naturally in this treatment. The results by this formula coincide fairly well
with those of the tight binding model.
The physical origin of the scaling law is clarified by this approach.Comment: RevTe
Tunable Kondo screening in a quantum dot device
We consider electron transport along a single-mode channel which is in
contact, via tunnel junctions in its walls, with two quantum dots. Electron
tunneling to and from the dots contributes to the electron backscattering, and
thus modifies the channel conductance. If the dots carry spin, the channel
conductance becomes temperature dependent due to the Kondo effect. The two-dot
device geometry allows for a formation of S=1 localized spin due to the
indirect exchange interaction, called Ruderman-Kittel-Kasuya-Yosida
interaction. This device offers a possibility to study the crossover between
fully screened and under-screened Kondo impurity. We investigate the
manifestation of such crossover in the channel conductance
Quantum Phonon Optics: Coherent and Squeezed Atomic Displacements
In this paper we investigate coherent and squeezed quantum states of phonons.
The latter allow the possibility of modulating the quantum fluctuations of
atomic displacements below the zero-point quantum noise level of coherent
states. The expectation values and quantum fluctuations of both the atomic
displacement and the lattice amplitude operators are calculated in these
states---in some cases analytically. We also study the possibility of squeezing
quantum noise in the atomic displacement using a polariton-based approach.Comment: 6 pages, RevTe
Band structures of periodic carbon nanotube junctions and their symmetries analyzed by the effective mass approximation
The band structures of the periodic nanotube junctions are investigated by
the effective mass theory and the tight binding model.
The periodic junctions are constructed by introducing pairs of a pentagonal
defect and a heptagonal defect periodically in the carbon nanotube.
We treat the periodic junctions whose unit cell is composed by two kinds of
metallic nanotubes with almost same radii, the ratio of which is between 0.7
and 1 .
The discussed energy region is near the undoped Fermi level where the channel
number is kept to two, so there are two bands.
The energy bands are expressed with closed analytical forms by the effective
mass theory with some assumptions, and they coincide well with the numerical
results by the tight binding model. Differences between the two methods are
also discussed. Origin of correspondence between the band structures and the
phason pattern discussed in Phys. Rev. B {\bf 53}, 2114, is clarified. The
width of the gap and the band are in inverse proportion to the length of the
unit cell, which is the sum of the lengths measured along the tube axis in each
tube part and along 'radial' direction in the junction part. The degeneracy and
repulsion between the two bands are determined only from symmetries.Comment: RevTeX, gif fil
Influence of the scattering potential model on low energy electron diffraction from Cu(001)−c(2 × 2)-Pb
A dynamical LEED intensity analysis is reported for Cu(001)−c(2 × 2)-Pb. The adsorbate layer distance from the substrate is determined as 2.29 Å, and the topmost interlayer spacing for the substrate is the same as in bulk Cu, in contrast to a contraction for clean Cu(001). This structural result is, within the accuracy reached, insensitive to changes in the assumed scattering potential models. The r-factors suggest a weak preference for an energy-dependent exchange correlation and a moderate one for adding a localized adsorption part inside the muffin-tin spheres. The sensitivity of spectra and r-factors to changes in the assumed isotropic Debye temperature for Pb suggests that vibrational anisotropy should be taken into account in order to improve the accuracy of the analysis. Calculated spin polarization spectra are very sensitive to the exchange approximation, the localized absorption and the Debye temperature. Together with experimental data, they should be useful in particular for determining the vibrational anisotropy
Cepheids and other short-period variables near the Galactic Centre
We report the result of our near-infrared survey of short-period variable
stars (P<60d) in a field-of-view of 20'x30' towards the Galactic Centre.
Forty-five variables are discovered and we classify the variables based on
their light curve shapes and other evidence. In addition to 3 classical
Cepheids reported previously, we find 16 type II Cepheids, 24 eclipsing
binaries, one pulsating star with P=0.265d (RR Lyr or delta Sct) and one
Cepheid-like variable whose nature is uncertain. Eclipsing binaries are
separated into the foreground objects and those significantly obscured by
interstellar extinction. One of the reddened binaries contains an O-type
supergiant and its light curve indicates an eccentric orbit. We discuss the
nature and distribution of type II Cepheids as well as the distance to the
Galactic Centre based on these Cepheids and other distance indicators. The
estimates of R0(GC) we obtained based on photometric data agree with previous
results obtained with kinematics of objects around the GC. Furthermore, our
result gives a support to the reddening law obtained by Nishiyama and
collaborators, A(Ks)/E(H-Ks)=1.44, because a different reddening law would
result in a rather different distance estimate.Comment: 13 pages, 10 figures, 7tables, accepted for publication in MNRA
Active Galaxies and Cluster Gas
Two lines of evidence indicate that active galaxies, principally radio
galaxies, have heated the diffuse hot gas in clusters. The first is the general
need for additional heating to explain the steepness of the X-ray
luminosity--temperature relation in clusters, the second is to solve the
cooling flow problem in cluster cores. The inner core of many clusters is
radiating energy as X-rays on a timescale much shorter than its likely age.
Although the temperature in this region drops by a factor of about 3 from that
of the surrounding gas, little evidence is found for gas much cooler than that.
Some form of heating appears to be taking place, probably by energy transported
outward from the central accreting black hole or radio source. How that energy
heats the gas depends on poorly understood transport properties (conductivity
and viscosity) of the intracluster medium. Viscous heating is discussed as a
possibility. Such heating processes have consequences for the truncation of the
luminosity function of massive galaxies.Comment: 14 pages, 16 fig, Feb 2004 talk for Phil Trans Roy So
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