Spatial Density Fluctuations Research Articles

Kinetics of 'cluster eating'

The authors introduce and study the kinetics of 'cluster eating', in which a cluster of mass i and a cluster of mass j react to form a lighter cluster of mass mod i-j mod . They write the rate equations for this process, which describe the kinetics in the mean-field limit, where spatial fluctuations in cluster density and in cluster shape are neglected. An asymptotic solution to these equations is derived for the particular case in which the reaction rate is independent of the masses of the reacting clusters. At long times, they find that the density of clusters of mass k, ck(t) decays as Ak/( tau logk-1 tau ), where tau is proportional to the time, Ak=(N-1)!/(N-k)! and N is the largest cluster mass in the initial state. This very unusual behaviour is checked by numerical simulations. A more general situation where the reaction matrix depends on the parities of the masses of the two incident clusters is also discussed briefly and a wide variety of possible kinetic behaviours is delineated. Finally, the authors study cluster eating below the upper critical dimension, where fluctuations in cluster density give rise to a non-classical kinetic behaviour.

Critical effects in the infrared spectrum of hydrogen

Measurements of absorption in the fundamental (4000 to 5000 cm−1) vibrational band of H2 have revealed new features in the collision-induced spectrum which attain their maximum intensity under near-critical conditions (Tc=32.98 K for para-H2). The new features are relatively sharp ‘‘spikes’’ (FWHM≊5 cm−1) which occur at the frequencies of the Q1(0) and/or Q1(1) transitions, and the effect is most prominent in pure para-H2. The behavior of these features as a function of temperature and of ortho-/para-H2 ratio has been investigated, they have also been detected in the spectrum of D2 near the critical point. An interpretation is proposed on the basis that long-lived spatial fluctuations in density give rise to a spoiling of the normally active mechanism of destructive intercollisional interference.

Spatial density fluctuations in the radiation era of the Universe

It is shown that in the radiation era of the Universe spatial temperature fluctuations (ΔT/T)<10−5 in the cosmic plasma lead to huge changes of the density up to (Δϱ/ϱ)≃104. This effect results from the fact that the cosmic plasma in the radiation era can be considered as a general relativistic Boltzmann gas which is found in the very vincinity of equilibrium.

Observations oi fine scale structure in the mesosphere and lower thermosphere

An electrostatic probe designed to measure ion density with high time resolution and accuracy was flown on a NIKE-APACHE rocket from Andøya Rocket Range on 1 March 1978. From the measurements, spectra of the spatial density fluctuations were derived in one kilometer height intervals from 65 to 127 km. Below 95 km the power spectra had a slope of about −5 3 , as expected for isotropic turbulence. The relation between the observed fluctuations in ion density and the corresponding fluctuations in neutral gas density is discussed. Above 95 km the fluctuations were stronger and showed a “white noise” power spectrum. These fluctuations are most likely due to plasma instabilities.

Phase diagram for cell cytoplasm from the calf lens

We have determined the phase diagram for cytoplasmic homogenate isolated from the cells of calf lenses. In the intact lens, this material is associated with the reversible opacification known as cold cataract. The coexistence curve we find resembles that found in concentrated high polymer solutions. Two coexisting phases are found below the transition temperature. Both these phases have similar protein compositions but different net protein concentrations. The constituent proteins have the following molecular weights: 17,000, 20,000, 22,000, 26,000, 28,000, 31,000, 36,000, 52,000 and 55,000 daltons. Scanning electron microscopy shows that the phase separation in the cytoplasmic homogenate is accompanied by a subtle structural reorganization which consists of increased spatial density fluctuations of dimensions comparable to the wavelength of light.

Scattering of phonons by density fluctuations and thermal conductivity of amorphous solids

AbstractPhonon relaxation time due to the scattering by spatial density fluctuations is studied in amorphous solids. The relaxation time τf is found to vary with frequency ω as ω−4 in the case of phonons with frequencies small compared with ω0, which is a parameter characteristic of the density fluctuations. When ω is large in comparison with ω0, τf is found to vary as ω−2, and thus it undergoes a transition from a strong to a comparatively weak frequency dependence as ω increases and crosses ω0. It is discussed that the thermal conductivity of a typical amorphous solid can be explained in terms of the scattering by low lying tunnelling states and scattering by fluctuations in density and elasticity. The plateau in conductivity is interpreted as defining a temperature region where the former scattering mechanism with its comparatively weaker frequency dependence starts losing dominance over the latter. Finally the conductivity of vitreous silica is analysed, obtaining a good agreement between theoretical and experimental results.

Low‐Energy Gap Semiconductor Mobilities in Presence of Spatial Fluctuations of Carrier Density

AbstractIt is shown that the ionized impurity mobility is reduced by a factor close to [1 +(Δn/n0)]−l when spatial fluctuations of the carrier density are present in the crystal. This result is valid for degenerated and non degenerated semiconductors. It is also shown, that acoustical or optical scattering modes are not affected by these fluctuations.

The magnetospheric cleft-Ionospheric effects

The fluxes of electrons that precipitate through the day side clefts of the magnetosphere are intense enough and deposit their energy at a high enough altitude to heat and ionize the F layer and top side ionosphere substantially. Relevant synoptic results from bottom side and top side sounder measurements are reviewed, and new data from the Isis satellites are presented. The effects of precipitation through the cleft can be seen in bottom side ionograms, but the diurnal variations that appear in routine monthly median plots are probably due primarily to other effects. As seen from above the F layer by the top side sounders on Alouette and Isis spacecraft, the most obvious effects are an expansion (increase in scale height) of the top side ionosphere due to heating and the presence of small-scale spatial fluctuations in density. The electron density at the F layer peak is often, but not always, increased. The increased electron density at satellite altitudes associated with top side expansion is often seen at a slightly higher latitude than the precipitation, and this displacement in latitude (of ∼1°) is interpreted in terms of an antisunward drift due to magnetospheric convection.

Spatial fluctuations of carrier density in n type GaxIn1− xSb determined by SH-DH effect

Variation of carrier concentration through n-type Ga x In 1− x Sb single crystals, interpreted as due to vacancies behaving as acceptors, is deduced from Shubnikov-de Haas measurements. A quantitative agreement between calculated and experimental values of the mobility at 4.2°K is obtained with this model as well as some information about the influence of growing conditions on the quality of the crystal.

The scattering of electrons by surface oxide charges and by lattice vibrations at the silicon-silicon dioxide interface

The electron conductivity mobility on weakly inverted Si surfaces covered with thermally grown oxide is reported at temperatures from 30 to 300 °K as a function of the surface oxide charge density. The scattering of electrons by surface oxide charges and by ionized impurities is reformulated by treating the random spatial fluctuations of charge density as the perturbation. The theory is applied to a simple two-dimensional model with a random Poisson distribution of oxide charges, giving μ I ∝ T/ N I, in agreement with experiment at low temperatures. Surface phonon scattering mobility is extracted from the experimental data and the two-dimensional ion scattering theory using 1/μ L = 1/μexp − 1/μ I. The theory of two-dimensional surface acoustical phonon scattering is extended to include surface optical or intervalley phonons. Comparison with experimental values of μ L indicates that for T > 150°K the effects of optical or intervalley phonons must be included.

The Meaning of the OH-H_{2};O Maser Maps

view Abstract Citations (48) References (24) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Meaning of the OH-H_{2};O Maser Maps Litvak, M. M. Abstract The approximate conditions of density, diameter, and temperature of the OH and H2O maser regions are deduced from the very long baseline interferometry, from the presence of microwave saturation from the energy requirements, from the absence of excessive pressure or thermal broadening, from the microwave optical depths, and from the selection of hyperfine components. The amplifier surrounding each emission point has a diameter comparable to the distance between neighboring points and a mass close to 1 M0. Calculations indicate that the nonlinear effect of self-focusing is probably not occurring, that stimulated Raman scattering is yielding only the lowest hyperfine component in H2O with a lineshape asymmetry toward lower frequency, and that for both OH and 1120 the interferometers are observing "hot spots" of unsaturated amplification. Ordinary scattering from spatial fluctuations in electron density probably accounts for the observed angular sizes, especially for 18-cm OH signals. The region appears to consist of condensations each of about 1 Al0 on the edges of two parallel, slowly corotating disks each about 100 Alo. A stable, large-scale rotational discontinuity (of velocity and magnetic field) may be propagating between the two disks. Publication: The Astrophysical Journal Pub Date: November 1971 DOI: 10.1086/151189 Bibcode: 1971ApJ...170...71L full text sources ADS |

Noise Analysis for a Silicon Particle Detector with Internal Multiplication

Internal carrier multiplication in a particle detector makes possible a reduction in the noise contribution of the following amplifier. The carrier multiplication, however, introduces additional noise from two sources. First, the statistical nature of the multiplication process gives rise to variations in the magnitude of the carrier multiplications. Second, as pointed out by Shockley, l the inherent statistical spatial fluctuations of impurity density within the space charge layer of a p-n junction lead to corresponding local fluctuations of the avalanche breakdown voltage Vb. At a bias below Vb this corresponds to local fluctuations of the multiplication factor M. The significance of both noise sources is evaluated. It is found that in general multiplication will improve the energy resolution only for the detection of low energy particles and for moderate values of M. The findings are illustrated by detailed calculations for a silicon junction having a breakdown voltage of 30 volts with a corresponding space charge layer thickness of 1.3 microns, appropriate for low energy particle detection.

Radar exploration of space using incoherent scattering

Theoretical and experimental results of incoherent scattering are discussed within the context of the phenomenon of radio-wave scattering by spatial fluctuations in electron density. Emphasis is placed on the concepts involved in measuring properties of the upper ionosphere, such as electron density, temperature, electron drifts, and the strength and direction of the magnetic field

A new method of space exploration by radar, using incoherent scattering

Incoherent scattering is discussed in the context of the phenomenon of scattering of radio waves by spatial fluctuations in electron density. A review is given of both theoretical and experimental results. Emphasis is placed on the ideas involved in measuring properties of the upper ionosphere, such as the electron density, temperature, electron drifts, and the strength and direction of the magnetic field.