14,781 research outputs found
Static dielectric response and Born effective charge of BN nanotubes from {\it ab initio} finite electric field calculations
{\it Ab initio} investigations of the full static dielectric response and
Born effective charge of BN nanotubes (BN-NTs) have been performed for the
first time using finite electric field method. It is found that the ionic
contribution to the static dielectric response of BN-NTs is substantial and
also that a pronounced chirality-dependent oscillation is superimposed on the
otherwise linear relation between the longitudinal electric polarizability and
the tube diameter (), as for a thin dielectric cylinderical shell. In
contrast, the transverse dielectric response of the BN-NTs resemble the
behavior of a thin (non-ideal) conducting cylindrical shell of a diameter of
\AA, with a screening factor of 2 for the inner electric field. The
medium principal component of the Born effective charge corresponding
to the transverse atomic displacement tangential to the BN-NT surface, has a
pronounced -dependence (but independent of chirality), while the large
longitudinal component exhibits a clear chirality dependence (but
nearly -independent), suggesting a powerful way to characterize the diameter
and chirality of a BN-NT.Comment: submitted to PR
Lattice Thermal Conductance in Nanowires at Low Temperatures: Breakdown and Recovery of Quantization
The quantization of lattice thermal conductance g normalized by g0=Ļ2k2BT/3h (the universal quantum of thermal conductance) was recently predicted theoretically to take an integer value over a finite range of temperature and then observed experimentally in nanowires with catenoidal contacts. The prediction of this quantization by Rego and Kirczenow [Phys. Rev. Lett. 81, 232 (1998)] relies on a study of only dilatational (longitudinal) vibrational mode in the wires. We study the thermal conductance in catenoidal wires by explicitly calculating the transmission rates of the six distinct vibrational modes (four acoustic and two low-lying optical modes) and applying the Landauer formula for the one-dimensional thermal transport in the ballistic regime. In a temperature range similar to the one predicted by Rego and Kirczenow, we find the presence of a plateau in gāg0. However, below this temperature range gāg0 is modifiedāthat is, the quantization is brokenādue to imperfect transmission of the acoustic modes of vibration. Our new observation is that, as temperature goes down further, the recovery of the quantization of gāg0 should occur. These results are found assuming GaAs as a wire material, but we also make similar calculations for silicon nitride wires used experimentally
Phonon Universal Transmission Fluctuations and Localization in Semiconductor Superlattices with a Controlled Degree of Order
We study both analytically and numerically phonon transmission fluctuations
and localization in partially ordered superlattices with correlations among
neighboring layers. In order to generate a sequence of layers with a varying
degree of order we employ a model proposed by Hendricks and Teller as well as
partially ordered versions of deterministic aperiodic superlattices. By
changing a parameter measuring the correlation among adjacent layers, the
Hendricks- Teller superlattice exhibits a transition from periodic ordering,
with alterna- ting layers, to the phase separated opposite limit; including
many intermediate arrangements and the completely random case. In the partially
ordered versions of deterministic superlattices, there is short-range order
(among any conse- cutive layers) and long range disorder, as in the N-state
Markov chains. The average and fluctuations in the transmission, the
backscattering rate, and the localization length in these multilayered systems
are calculated based on the superlattice structure factors we derive
analytically. The standard deviation of the transmission versus the average
transmission lies on a {\it universal\/} curve irrespective of the specific
type of disorder of the SL. We illustrate these general results by applying
them to several GaAs-AlAs superlattices for the proposed experimental
observation of phonon universal transmission fluctuations.Comment: 16-pages, Revte
Iron Abundance Profiles of 12 Clusters of Galaxies Observed With BeppoSAX
We have derived azimuthally-averaged radial iron abundance profiles of the
X-ray gas contained within 12 clusters of galaxies with redshift 0.03 < z < 0.2
observed with BeppoSAX. We find evidence for a negative metal abundance
gradient in most of the clusters, particularly significant in clusters that
possess cooling flows. The composite profile from the 12 clusters resembles
that of cluster simulations of Metzler & Evrard (1997). This abundance gradient
could be the result of the spatial distribution of gas-losing galaxies within
the cluster being more centrally condensed than the primordial hot gas. Both
inside and outside the core region, we find a higher abundance in cooling flow
clusters than in non-cooling flow clusters. Outside of the cooling region this
difference cannot be the result of more efficient sputtering of metals into the
gaseous phase in cooling flow clusters, but might be the result of the mixing
of low metallicity gas from the outer regions of the cluster during a merger.Comment: 8 pages, 2 embedded Postscript figures, accepted by Astrophysical
Journa
Adhesion and Endocytosis of Calcium Oxalate Crystals on Renal Tubular Cells
The present investigation was designed to study interactions between Madin-Darby canine kidney (MOCK) cells and calcium oxalate monohydrate (COM) crystals and to clarify the significance of these crystal-cell interactions in stone pathogenesis.
MOCK cells cultured in the presence of COM crystals showed a time-dependent uptake of crystals; this was specific for COM crystals. In the dynamic model system designed to study these phenomena under more physiological conditions, COM crystals adhered to the cell surface and were subsequently internalized. In this endocytotic process, the microvilli of the cell appeared to play an important role. The observation by scanning electron microscopy of complexes consisting of aggregated COM crystals and cell debris led us to speculate that adhesion and endocytosis of crystals might provide the calculus nidus for aggregation and retention of crystals in the renal tubule. Furthermore, glycosaminoglycans and the macromolecular fraction of human urine were shown to have the ability to inhibit the cellular uptake of crystals.
Evidence that similar processes may also occur in vivo was obtained using an experimental stone model in rats. Our experiments revealed that most of the COM crystals adhered to the tubular cells and some crystals were endocytosed by the cell. Thus, these crystal-cell interactions might be one of the earliest processes in the formation of kidney stones. Further elucidation of the mechanism and the regulatory factors involved in this process may provide new insight into stone pathogenesis
Optical to Near-IR Spectrum of a Massive Evolved Galaxy at z = 1.26
We present the optical to near-infrared (IR) spectrum of the galaxy TSPS
J1329-0957, a red and bright member of the class of extremely red objects
(EROs) at z = 1.26. This galaxy was found in the course of the Tokyo-Stromlo
Photometry Survey (TSPS) which we are conducting in the southern sky. The
spectroscopic observations were carried out with the Gemini Multi-Object
Spectrograph (GMOS) and the Gemini Near Infra-Red Spectrograph (GNIRS) mounted
on the Gemini-South telescope. The wide wavelength coverage of 0.6 - 2.3 um
provides useful clues as to the nature of EROs while most published spectra are
limited to a narrower spectral range which is dictated by the need for
efficient redshift determination in a large survey. We compare our spectrum
with several optical composite spectra obtained in recent large surveys, and
with stellar population synthesis models. The effectiveness of using near-IR
broad-band data, instead of the spectral data, in deriving the galaxy
properties are also investigated. We find that TSPS J1329-0957 formed when the
universe was 2 - 3 Gyr old, and subsequently evolved passively to become one of
the most massive galaxies found in the z = 1 - 2 universe. Its early type and
estimated stellar mass of M* = 10^{11.5} Msun clearly point to this galaxy
being a direct ancestor of the brightest elliptical and spheroidal galaxies in
the local universe.Comment: 18 pages, 4 figures. Accepted for publication in Ap
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