258 research outputs found
Field emission in ordered arrays of ZnO nanowires prepared by nanosphere lithography and extended Fowler-Nordheim analyses
A multistage chemical method based on nanosphere lithography was used to produce hexagonally patterned arrays of ZnO vertical nanowires, with 1 lm interspacing and aspect ratio 20, with a view to study the effects of emitter uniformity on the current emitted upon application of a dc voltage across a 250 lm vacuum gap. A new treatment, based on the use of analytical expressions for the image-potential correction functions, was applied to the linear region below 2000 V of the Fowler-Nordheim (FN) plot and showed the most suitable value of the work function / in the range 3.3–4.5 eV (conduction band emission) with a Schottky lowering parameter y ~ 0.72 and a field enhancement factor c in the 700–1100 range. A modeled c value of 200 was calculated for an emitter shape of a prolate ellipsoid of revolution and also including the effect of nanowire screening, in fair agreement with the experimental value. The Fowler-Nordheim current densities
and effective emission areas were derived as 1011 Am2 and 1017 m2, respectively, showing that field emission likely takes place in an area of atomic dimensions at the tip of the emitter. Possible causes for the observed departure from linear FN plot behavior above 2000 V were discussed
Diffusive Spreading of Chainlike Molecules on Surfaces
We study the diffusion and submonolayer spreading of chainlike molecules on
surfaces. Using the fluctuating bond model we extract the collective and tracer
diffusion coefficients D_c and D_t with a variety of methods. We show that
D_c(theta) has unusual behavior as a function of the coverage theta. It first
increases but after a maximum goes to zero as theta go to one. We show that the
increase is due to entropic repulsion that leads to steep density profiles for
spreading droplets seen in experiments. We also develop an analytic model for
D_c(theta) which agrees well with the simulations.Comment: 3 pages, RevTeX, 4 postscript figures, to appear in Phys. Rev.
Letters (1996
Low temperature properties of a quantum particle coupled to dissipative environments
We study the dynamics of a quantum particle coupled to dissipative (ohmic)
environments, such as an electron liquid. For some choices of couplings, the
properties of the particle can be described in terms of an effective mass. A
particular case is the three dimensional dirty electron liquid. In other
environments, like the one described by the Caldeira-Leggett model, the
effective mass diverges at low temperatures, and quantum effects are strongly
suppressed. For interactions within this class, arbitrarily weak potentials
lead to localized solutions. Particles bound to external potentials, or moving
in closed orbits, can show a first order transition, between strongly and
weakly localized regimes.Comment: 10 page
Force-velocity relation and density profiles for biased diffusion in an adsorbed monolayer
In this paper, which completes our earlier short publication [Phys. Rev.
Lett. 84, 511 (2000)], we study dynamics of a hard-core tracer particle (TP)
performing a biased random walk in an adsorbed monolayer, composed of mobile
hard-core particles undergoing continuous exchanges with a vapor phase. In
terms of an approximate approach, based on the decoupling of the third-order
correlation functions, we obtain the density profiles of the monolayer
particles around the TP and derive the force-velocity relation, determining the
TP terminal velocity, V_{tr}, as the function of the magnitude of external bias
and other system's parameters. Asymptotic forms of the monolayer particles
density profiles at large separations from the TP, and behavior of V_{tr} in
the limit of small external bias are found explicitly.Comment: Latex, 31 pages, 3 figure
Solitonic-exchange mechanism of surface~diffusion
We study surface diffusion in the framework of a generalized
Frenkel-Kontorova model with a nonconvex transverse degree of freedom. The
model describes a lattice of atoms with a given concentration interacting by
Morse-type forces, the lattice being subjected to a two-dimensional substrate
potential which is periodic in one direction and nonconvex (Morse) in the
transverse direction. The results are used to describe the complicated
exchange-mediated diffusion mechanism recently observed in MD simulations [J.E.
Black and Zeng-Ju Tian, Phys. Rev. Lett. {\bf 71}, 2445-2448(1993)].Comment: 22 Revtex pages, 9 figures to appear in Phys. Rev.
Dynamic correlations in an ordered c(22) lattice gas
We obtain the dynamic correlation function of two-dimensional lattice gas
with nearest-neighbor repulsion in ordered c(22) phase
(antiferromagnetic ordering) under the condition of low concentration of
structural defects. It is shown that displacements of defects of the ordered
state are responsible for the particle number fluctuations in the probe area.
The corresponding set of kinetic equations is derived and solved in linear
approximation on the defect concentration. Three types of strongly correlated
complex jumps are considered and their contribution to fluctuations is
analysed. These are jumps of excess particles, vacancies and flip-flop jumps.
The kinetic approach is more general than the one based on diffusion-like
equations used in our previous papers. Thus, it becomes possible to adequately
describe correlations of fluctuations at small times, where our previous theory
fails to give correct results. Our new analytical results for fluctuations of
particle number in the probe area agree well with those obtained by Monte Carlo
simulations.Comment: 10 pages, 7 figure
Diffusion processes and growth on stepped metal surfaces
We study the dynamics of adatoms in a model of vicinal (11m) fcc metal
surfaces. We examine the role of different diffusion mechanisms and their
implications to surface growth. In particular, we study the effect of steps and
kinks on adatom dynamics. We show that the existence of kinks is crucially
important for adatom motion along and across steps. Our results are in
agreement with recent experiments on Cu(100) and Cu(1,1,19) surfaces. The
results also suggest that for some metals exotic diffusion mechanisms may be
important for mass transport across the steps.Comment: 3 pages, revtex, complete file available from
ftp://rock.helsinki.fi/pub/preprints/tft/ or at
http://www.physics.helsinki.fi/tft/tft_preprints.html (to appear in Phys.
Rev. B Rapid Comm.
P300 and uncertainty reduction in a concept identification task.
The relationship between the amplitude of P300, the mean amplitude of the Slow Wave, and uncertainty reduction after (dis)confirmation of hypotheses was studied in a Concept-Identification task. The subjects had to categorize stimuli according to a conceptual rule (joint denial or exclusion) and to rate the confidence that their classification was correct. Three types of feedback were distinguished: confirming (subject's categorization was correct), disconfirming (subject's categorization was incorrect), and non-informative feedback. The EEG was averaged separately according to the three types of feedback and the two confidence ratings (low, high).
The data showed the predicted interaction between type of feedback and confidence level. A larger P300 amplitude turned up after confirming feedback when the subject was less confident, than when he was more confident. The reverse was found after disconfirming feedback. The P300 amplitude after non-informative feedback was not influenced by confidence. The mean amplitude of the Slow Wave showed approximately the same interaction pattern.
The results were interpreted in terms of changes in the probability of hypotheses which subjects use to categorize stimuli in a Concept-Identification task
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