508 research outputs found
Crystal Growth in the Presence of Surface Melting: Novel Behavior of the Principal Facets of Ice
We present measurements of the growth rates of the principal facet surfaces
of ice from water vapor as a function of supersaturation over the temperature
range -2 C > T > -40 C. Our data are well described by a dislocation-free
layer-nucleation model, parameterized by the attachment coefficient as a
function of supersaturation \alpha(\sigma) = Aexp(-\sigma_0/\sigma). The
measured parameters A(T) and \sigma_0(T) for the basal and prism facets exhibit
a complex behavior that likely originates from structural changes in the ice
surface with temperature, in particular the onset and development of surface
melting for T > -15 C. From \sigma_0(T) we extract the terrace step energy
\beta(T) as a function of temperature for both facet surfaces. As a basic
property of the equilibrium ice surface, the step energy \beta(T) may be
amenable to calculation using molecular dynamics simulations, potentially
yielding new insights into the enigmatic surface structure of ice near the
triple point
Measurements of Growth Rates of (0001) Ice Crystal Surfaces
We present measurements of growth rates of the (0001) facet surface of ice as
a function of water vapor supersaturation over the temperature range C. From these data we infer the temperature dependence of premelting
on the basal surface and the effects of premelting on the ice growth dynamics.
Over this entire temperature range the growth was consistent with a simple 2D
nucleation model, allowing a measurement of the critical supersaturation
as a function of temperature. We find that the 2D nucleation
barrier is substantially diminished when the premelted layer is partially
developed, as indicated by a reduced while the barrier is higher
both when the premelted layer is fully absent or fully developed
A basic lock-in amplifier experiment for the undergraduate laboratory
We describe a basic experiment for the undergraduate laboratory that demonstrates aspects of both, the science and the art of precision electronic measurements. The essence of the experiment is to measure the resistance of a small length of brass-wire to high accuracy using a simple voltage divider and a lock-in amplifier. By performing the measurement at different frequencies and different drive currents, one observes various random noise sources and systematic measurement effects
Macroscopic coherence effects in a mesoscopic system: Weak localization of thin silver films in an undergraduate lab
We present an undergraduate lab that investigates weak localization in thin
silver films. The films prepared in our lab have thickness, , between 60-200
\AA, a mesoscopic length scale. At low temperatures, the inelastic dephasing
length for electrons, , exceeds the thickness of the film (), and the films are then quasi-2D in nature. In this situation, theory
predicts specific corrections to the Drude conductivity due to coherent
interference between conducting electrons' wavefunctions, a macroscopically
observable effect known as weak localization. This correction can be destroyed
with the application of a magnetic field, and the resulting magnetoresistance
curve provides information about electron transport in the film. This lab is
suitable for Junior or Senior level students in an advanced undergraduate lab
course.Comment: 16 pages, 9 figures. Replaces earlier version of paper rejected by
Am. J. Phys. because of too much content on vacuum systems. New version deals
with the undergraduate experiment on weak localization onl
Mechanisms of Dendrites Occurrence during Crystallization: Features of the Ice Crystals Formation
Dendrites formation in the course of crystallization presents very general
phenomenon, which is analyzed in details via the example of ice crystals growth
in deionized water. Neutral molecules of water on the surface are combined into
the double electric layer (DEL) of oriented dipoles; its field reorients
approaching dipoles with observable radio-emission in the range of 150 kHz. The
predominant attraction of oriented dipoles to points of gradients of this field
induces dendrites growth from them, e.g. formation of characteristic form of
snowflakes at free movement of clusters through saturated vapor in atmosphere.
The constant electric field strengthens DELs' field and the growth of
dendrites. Described phenomena should appear at crystallization of various
substances with dipole molecules, features of radio-emission can allow the
monitoring of certain processes in atmosphere and in technological processes.
Crystallization of particles without constant moments can be stimulated by DELs
of another nature with attraction of virtual moments of particles to gradients
of fields and corresponding dendrites formation.Comment: 6 page
Measurement of Thermal Noise in Multilayer Coatings with Optimized Layer Thickness
A standard quarter-wavelength multilayer optical coating will produce the
highest reflectivity for a given number of coating layers, but in general it
will not yield the lowest thermal noise for a prescribed reflectivity. Coatings
with the layer thicknesses optimized to minimize thermal noise could be useful
in future generation interferometric gravitational wave detectors where coating
thermal noise is expected to limit the sensitivity of the instrument. We
present the results of direct measurements of the thermal noise of a standard
quarter-wavelength coating and a low noise optimized coating. The measurements
indicate a reduction in thermal noise in line with modeling predictions.Comment: 8 pages, 14 figure
Numerical computations of facetted pattern formation in snow crystal growth
Facetted growth of snow crystals leads to a rich diversity of forms, and
exhibits a remarkable sixfold symmetry. Snow crystal structures result from
diffusion limited crystal growth in the presence of anisotropic surface energy
and anisotropic attachment kinetics. It is by now well understood that the
morphological stability of ice crystals strongly depends on supersaturation,
crystal size and temperature. Until very recently it was very difficult to
perform numerical simulations of this highly anisotropic crystal growth. In
particular, obtaining facet growth in combination with dendritic branching is a
challenging task. We present numerical simulations of snow crystal growth in
two and three space dimensions using a new computational method recently
introduced by the authors. We present both qualitative and quantitative
computations. In particular, a linear relationship between tip velocity and
supersaturation is observed. The computations also suggest that surface energy
effects, although small, have a larger effect on crystal growth than previously
expected. We compute solid plates, solid prisms, hollow columns, needles,
dendrites, capped columns and scrolls on plates. Although all these forms
appear in nature, most of these forms are computed here for the first time in
numerical simulations for a continuum model.Comment: 12 pages, 28 figure
Coulomb Distortion of Pion Spectra from Heavy-Ion Collisions
The effects of final-state π-nucleus electromagnetic interactions in heavy-ion collisions are investigated in a covariant classical formulation. Experimentally observed mid-rapidity peaks in π+ spectra are reproduced by a simple model and are shown to be sensitive to the gross features of the time-dependent nuclear charge distribution
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