33,268 research outputs found
Can a time-stratigraphic classification system be developed for Venus?
Magellan radar images reveal that Venus' exposed geologic record covers a relatively short and recent time span, as indicated by the low density of impact craters across the planet. Therefore, because impact cratering in itself will not be a useful tool to define geologic ages on Venus, it was questioned whether a useful stratigraphic scheme can be developed for the planet. We believe that a venusian stratigraphy is possible and that it can be based on the following: (1) an examination of the rationale and methods that have been used to develop such schemes for the other planets; and (2) what can be gleaned from Magellan and other datasets of Venus
Free energy of cluster formation and a new scaling relation for the nucleation rate
Recent very large molecular dynamics simulations of homogeneous nucleation
with Lennard-Jones atoms [Diemand et al. J. Chem. Phys. {\bf
139}, 074309 (2013)] allow us to accurately determine the formation free energy
of clusters over a wide range of cluster sizes. This is now possible because
such large simulations allow for very precise measurements of the cluster size
distribution in the steady state nucleation regime. The peaks of the free
energy curves give critical cluster sizes, which agree well with independent
estimates based on the nucleation theorem. Using these results, we derive an
analytical formula and a new scaling relation for nucleation rates: is scaled by , where the supersaturation ratio is ,
is the dimensionless surface energy, and is a dimensionless
nucleation rate. This relation can be derived using the free energy of cluster
formation at equilibrium which corresponds to the surface energy required to
form the vapor-liquid interface. At low temperatures (below the triple point),
we find that the surface energy divided by that of the classical nucleation
theory does not depend on temperature, which leads to the scaling relation and
implies a constant, positive Tolman length equal to half of the mean
inter-particle separation in the liquid phase.Comment: 7 figure
Laboratory Experiment of Checkerboard Pupil Mask Coronagraph
We present the results of the first laboratory experiment of checkerboard
shaped pupil binary mask coronagraphs using visible light, in the context of
the R&D activities for future mid-infrared space missions such as the 3.5 m
SPICA telescope. The primary aim of this work is to demonstrate the
coronagraphic performance of checkerboard masks down to a
peak-to-peak contrast, which is required to detect self-luminous extra-solar
planets in the mid-infrared region. Two masks, consisting of aluminum films on
a glass substrates, were manufactured using nano-fabrication techniques with
electron beam lithography: one mask was optimized for a pupil with a 30%
central obstruction and the other was for a pupil without obstruction. The
theoretical contrast for both masks was and no adaptive optics system
was employed. For both masks, the observed point spread functions were quite
consistent with the theoretical ones. The average contrast measured within the
dark regions was and . The
coronagraphic performance significantly outperformed the requirement
and almost reached the theoretical limit determined by the mask designs. We
discuss the potential application of checkerboard masks for mid-infrared
coronagraphy, and conclude that binary masks are promising for future
high-contrast space telescopes.Comment: 6 pages, 6 figure
Detection of high k turbulence using two dimensional phase contrast imaging on LHD
High k turbulence, up to 30 cm(-1), can be measured using the two dimensional CO₂ laser phase contrast imaging system on LHD. Recent hardware improvements and experimental results are presented. Precise control over the lens positions in the detection system is necessary because of the short depth of focus for high k modes. Remote controllable motors to move optical elements were installed, which, combined with measurements of the response to ultrasound injection, allowed experimental verification and shot-to-shot adjustment of the object plane. Strong high k signals are observed within the first 100-200 ms after the initial electron cyclotron heating (ECH) breakdown, in agreement with gyrotron scattering. During later times in the discharge, the entire k spectrum shifts to lower values (although the total amplitude does not change significantly), and the weaker high k signals are obscured by leakage of low k components at low frequency, and detector noise, at high frequency
Non-Universal Critical Behaviour of Two-Dimensional Ising Systems
Two conditions are derived for Ising models to show non-universal critical
behaviour, namely conditions concerning 1) logarithmic singularity of the
specific heat and 2) degeneracy of the ground state. These conditions are
satisfied with the eight-vertex model, the Ashkin-Teller model, some Ising
models with short- or long-range interactions and even Ising systems without
the translational or the rotational invariance.Comment: 17 page
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