1,269 research outputs found
Distinguishing step relaxation mechanisms via pair correlation functions
Theoretical predictions of coupled step motion are tested by direct STM
measurement of the fluctuations of near-neighbor pairs of steps on
Si(111)-root3 x root3 R30 - Al at 970K. The average magnitude of the
pair-correlation function is within one standard deviation of zero, consistent
with uncorrelated near-neighbor step fluctuations. The time dependence of the
pair-correlation function shows no statistically significant agreement with the
predicted t^1/2 growth of pair correlations via rate-limiting atomic diffusion
between adjacent steps. The physical considerations governing uncorrelated step
fluctuations occurring via random attachment/detachment events at the step edge
are discussed.Comment: 17 pages, 4 figure
Absence of non-trivial asymptotic scaling in the Kashchiev model of polynuclear growth
In this brief comment we show that, contrary to previous claims [Bartelt M C
and Evans J W 1993 {\it J.\ Phys.\ A} 2743], the asymptotic
behaviour of the Kashchiev model of polynuclear growth is trivial in all
spatial dimensions, and therefore lies outside the Kardar-Parisi-Zhang
universality class.Comment: 3 pages, 4 postscript figures, uses eps
Fluctuations, line tensions, and correlation times of nanoscale islands on surfaces
We analyze in detail the fluctuations and correlations of the (spatial)
Fourier modes of nano-scale single-layer islands on (111) fcc crystal surfaces.
We analytically show that the Fourier modes of the fluctuations couple due to
the anisotropy of the crystal, changing the power spectrum of the fluctuations,
and that the actual eigenmodes of the fluctuations are the appropriate linear
combinations of the Fourier modes. Using kinetic Monte Carlo simulations with
bond-counting parameters that best match realistic energy barriers for hopping
rates, we deduce absolute line tensions as a function of azimuthal orientation
from the analyses of the fluctuation of each individual mode. The
autocorrelation functions of these modes give the scaling of the correlation
times with wavelength, providing us with the rate-limiting kinetics driving the
fluctuations, here step-edge diffusion. The results for the energetic
parameters are in reasonable agreement with available experimental data for
Pb(111) surfaces, and we compare the correlation times of island-edge
fluctuations to relaxation times of quenched Pb crystallites.Comment: 11 pages, 8 figures; to appear in PRB 70, xxx (15 Dec 2004), changes
in MC and its implication
Multi-wavelength reflection spectra from an acousto-optic modulated fiber Bragg grating in a highly birefringent suspended core fiber
The interaction of a fiber Bragg grating and longitudinal acoustic waves in a highly birefringent suspended-core fiber is investigated for the realization of a multi-wavelength reflection property. The modulated grating couples power from the fast and slow polarization modes to shifted superposed modes supported by the grating. The grating reflectivity of the superposed modes are tuned by the voltage of an electrical signal. Up to five different wavelength reflection peaks have been generated indicating new possibilities for compact and fast fiber-integrated multi-wavelength dynamic filters
Electrically Tunable Multiwavelength Bragg Grating Filter Acoustically Induced in a Highly Birefringent Suspended Core Fiber
Multiwavelength reflection spectra induced by an acoustically modulated fiber Bragg grating (FBG) in a highly birefringent suspended core fiber are experimentally investigated. Longitudinal acoustic waves interacting with a grating generate side lobes in the reflectivity spectrum and produce a superposed reflection band. The reflectivity of up to five wavelength peaks can be actively tuned by the voltage of the electrical signal inducing the acoustic waves. This indicates new possibilities for compact and fast multiwavelength dynamic and fiber-integrated reflection filters
Anomalous Dimension and Spatial Correlations in a Point-Island Model
We examine the island size distribution function and spatial correlation
function of a model for island growth in the submonolayer regime in both 1 and
2 dimensions. In our model the islands do not grow in shape, and a fixed number
of adatoms are added, nucleate, and are trapped at islands as they diffuse.
We study the cases of various critical island sizes for nucleation as a
function of initial coverage. We found anomalous scaling of the island size
distribution for large . Using scaling, random walk theory, a version of
mean-field theory we obtain a closed form for the spatial correlation function.
Our analytic results are verified by Monte Carlo simulations
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Nanoscopic tip sensors fabricated by gas phase etching of optical glass fibers
Silica-based fiber tips are used in a variety of spectroscopic, micro- or nano-scopic optical sensor applications and photonic micro-devices. The miniaturization of optical sensor systems and the technical implementation using optical fibers can provide new sensor designs with improved properties and functionality for new applications. The selective-etching of specifically doped silica fibers is a promising method in order to form complex photonic micro structures at the end or within fibers such as tips and cavities in various shapes useful for the all-fiber sensor and imaging applications. In the present study, we investigated the preparation of geometrically predefined, nanoscaled fiber tips by taking advantage of the dopant concentration profiles of highly doped step-index fibers. For this purpose, a gas phase etching process using hydrofluoric acid (HF) vapor was applied. The shaping of the fiber tips was based on very different etching rates as a result of the doping characteristics of specific optical fibers. Technological studies on the influence of the etching gas atmosphere on the temporal tip shaping and the final geometry were performed using undoped and doped silica fibers. The influence of the doping characteristics was investigated in phosphorus-, germanium-, fluorine- and boron-doped glass fibers. Narrow exposed as well as protected internal fiber tips in various shapes and tip radiuses down to less than 15 nm were achieved and characterized geometrically and topologically. For investigations into surface plasmon resonance effects, the fiber tips were coated with nanometer-sized silver layers by means of vapour deposition and finally subjected to an annealing treatment
Reflectivity and Bandwidth Modulation of Fiber Bragg Gratings in a Suspended Core Fiber by Tunable Acoustic Waves
The acousto-optic modulation of fiber Bragg gratings in a four-hole suspended core fiber is experimentally demonstrated. Strong modulations with a reflectivity amplitude decrease by up to 67% and a 57% bandwidth increase in the Bragg resonance are obtained for gratings of 0.26- and 1-nm 3-dB bandwidths, respectively. The reduction of the required acoustic power for achieving the acousto-optic modulation compared to conventional solid-core single-mode fibers points to more efficient modulator devices in suspended core fibers
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Soudan nucleon-decay programme: a progress report
The Soudan nucleon decay programme consists of Soudan I, a 30-ton tracking ionization calorimeter at a 1700 mwe depth, a 100 to 400 MeV charged particle test beam at Argonne National Laboratory, and Soudan II, a 1000 ton tracking ionization calorimeter at a 1800 mwe depth. Soudan I has been fully operational for 26 days giving a lower limit on the nucleon lifetime of > 2 x 10/sup 29/ years and an upper limit on monopole flux of < 1.8 x 10/sup -3//m/sup 2/ Sr d. The test beam data gives a Soudan I energy resolution of about 25% for electrons and muons in the appropriate energy range. Soudan II has been proposed and awaits funding
A lattice gas model of II-VI(001) semiconductor surfaces
We introduce an anisotropic two-dimensional lattice gas model of metal
terminated II-IV(001) seminconductor surfaces. Important properties of this
class of materials are represented by effective NN and NNN interactions, which
result in the competition of two vacancy structures on the surface. We
demonstrate that the experimentally observed c(2x2)-(2x1) transition of the
CdTe(001) surface can be understood as a phase transition in thermal
equilbrium. The model is studied by means of transfer matrix and Monte Carlo
techniques. The analysis shows that the small energy difference of the
competing reconstructions determines to a large extent the nature of the
different phases. Possible implications for further experimental research are
discussed.Comment: 7 pages, 2 figure
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