56,355 research outputs found
Noise-assisted Mound Coarsening in Epitaxial Growth
We propose deposition noise to be an important factor in unstable epitaxial
growth of thin films. Our analysis yields a geometrical relation H=(RWL)^2
between the typical mound height W, mound size L, and the film thickness H.
Simulations of realistic systems show that the parameter R is a characteristic
of the growth conditions, and generally lies in the range 0.2-0.7. The
constancy of R in late-stage coarsening yields a scaling relation between the
coarsening exponent 1/z and the mound height exponent \beta which, in the case
of saturated mound slope, gives \beta = 1/z = 1/4.Comment: 4 pages, RevTex Macros, 3 eps figure
Slow Atomic Motion in Zr-Ti-Cu-Ni-Be Metallic Glasses Studied by NMR
Nuclear magnetic resonance is used for the first time to detect slow atomic motion in metallic glasses, specifically, Be motion in Zr-Ti-Cu-Ni-Be bulk metallic glasses. The observations are not consistent with the vacancy-assisted and interstitial diffusion mechanisms and favor the spread-out free volume fluctuation mechanism for Be diffusion. Comparison with the results of Be diffusion measured by elastic backscattering the NMR results also indicates that the energy barriers for short- and long-range Be motion are the same
Structure of excited vortices with higher angular momentum in Bose-Einstein condensates
The structure of vortices in Bose-Einstein condensed atomic gases is studied
taking into account many-body correlation effects. It is shown that for excited
vortices the particle density in the vortex core increases as the angular
momentum of the system increases. The core density can increase by several
times with only a few percent change in the angular momentum. This result
provides an explanation for the observations in which the measured angular
momentum is higher than the estimation based on counting the number of
vortices, and the visibility of the vortex cores is simultaneously reduced. The
calculated density profiles for the excited vortices are in good agreement with
experimental measurements.Comment: 4 pages, 1 figur
Coulomb interaction and transient charging of excited states in open nanosystems
We obtain and analyze the effect of electron-electron Coulomb interaction on
the time dependent current flowing through a mesoscopic system connected to
biased semi-infinite leads. We assume the contact is gradually switched on in
time and we calculate the time dependent reduced density operator of the sample
using the generalized master equation. The many-electron states (MES) of the
isolated sample are derived with the exact diagonalization method. The chemical
potentials of the two leads create a bias window which determines which MES are
relevant to the charging and discharging of the sample and to the currents,
during the transient or steady states. We discuss the contribution of the MES
with fixed number of electrons N and we find that in the transient regime there
are excited states more active than the ground state even for N=1. This is a
dynamical signature of the Coulomb blockade phenomenon. We discuss numerical
results for three sample models: short 1D chain, 2D lattice, and 2D parabolic
quantum wire.Comment: 12 pages, 12 figure
Novel Precursors for Boron Nanotubes: The Competition of Two-Center and Three-Center Bonding in Boron Sheets
We present a new class of boron sheets, composed of triangular and hexagonal
motifs, that are more stable than structures considered to date and thus are
likely to be the precursors of boron nanotubes. We describe a simple and clear
picture of electronic bonding in boron sheets and highlight the importance of
three-center bonding and its competition with two-center bonding, which can
also explain the stability of recently discovered boron fullerenes. Our
findings call for reconsideration of the literature on boron sheets, nanotubes,
and clusters.Comment: 4 pages, 4 figures, 1 tabl
Dynamic communicability predicts infectiousness
Using real, time-dependent social interaction data, we look at correlations between some recently proposed dynamic centrality measures and summaries from large-scale epidemic simulations. The evolving network arises from email exchanges. The centrality measures, which are relatively inexpensive to compute, assign rankings to individual nodes based on their ability to broadcast information over the dynamic topology. We compare these with node rankings based on infectiousness that arise when a full stochastic SI simulation is performed over the dynamic network. More precisely, we look at the proportion of the network that a node is able to infect over a fixed time period, and the length of time that it takes for a node to infect half the network.We find that the dynamic centrality measures are an excellent, and inexpensive, proxy for the full simulation-based measures
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