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Is there an overriding principle of nature, hitherto overlooked, that governs all population behavior? A single principle that drives all the regimes observed in nature - exponential-like growth, saturated growth, population decline, population extinction, oscillatory behavior? In current orthodox population theory, this diverse range of population behaviors is described by many different equations - each with its own specific justification. The signature of an overriding principle would be a differential equation which, in a single statement, embraces all the panoply of regimes. A candidate such governing equation is proposed. The principle from which the equation is derived is this: The effect on the environment of a population's success is to alter that environment in a way that opposes the success., Comment: Revised equation-numbering to correspond to published version

Wigner found unreasonable the "effectiveness of mathematics in the natural sciences". But if the mathematics we use to describe nature is simply a carefully coded expression of our experience then its effectiveness is quite reasonable. Its effectiveness is built into its design. We consider group theory, the logic of symmetry. We examine the premise that symmetry is identity; that group theory encodes our experience of identification. To decide whether group theory describes the world in such an elemental way we catalogue the detailed correspondence between elements of the physical world and elements of the formalism. Providing an unequivocal match between concept and mathematical statement completes the case. It makes effectiveness appear reasonable. The case that symmetry is identity is a strong one but it is not complete. The further validation required suggests that unexpected entities might be describable by the irreducible representations of group theory.

We report the utilization of a high T/sub c/ symmetric two-hole RF-SQUID (superconducting quantum interference device) operating at liquid nitrogen temperature as a gradiometer in nondestructive testing (NDT). This technology offers a possibility of solving simultaneously four requirements: sensitivity, penetration depth, resolution, and portability of the device. We used the simple dipole model, and showed that the signal attenuation in the near zone agreed with our experimental results and is less than that of a plane wave. In our experiment we used a low frequency current of 30 Hz and demonstrated that the amplitude of the signal was reduced only by 20% after the insertion of 1" (2.54 cm) of aluminum alloy plates between the mock crack and the SQUID in agreement with the theoretical calculations. >

This paper reports the utilization of a high temperature superconducting (HTS) RF SQUID gradiometer in non-destructive testing (NDT). A two-hole HTS RF SQUID, capable of working in a noisy environment without any magnetic shielding, is employed as a gradiometer sensor without using flux transformer coils. This gradiometer, maintained at LN 2 temperature, is sensitive enough to detect small mock cracks in an aluminium alloy slab covered by another 25.4mm thick solid aluminium alloy slab. Such an NDT device should have a significant impact on metal fatigue detection technology and demonstrates an important practical usage of high temperature superconductors.

Proposed here is that an overriding principle of nature governs all population behavior; that a single tenet drives the many regimes observed in nature-exponential--like growth, saturated growth, population decline, population extinction, and oscillatory behavior. The signature of such an all embracing principle is a differential equation which, in a single statement, embraces the entire panoply of observations. In current orthodox theory, this diverse range of population behaviors is described by many different equations-each with its own specific justification. Here, a single equation governing all the regimes is proposed together with the principle from which it derives. The principle is: The effect on the environment of a population's success is to alter that environment in a way that opposes the success. Experiments are suggested which could validate or refute the theory. Predictions are made about population behaviors.

A superfluid characteristic length has consistently emerged from a variety of independent experiments on helium thin films. Both the magnitude and the temperature dependence of this quantity are now quite well defined within relatively narrow limits. This length appears to consist of two parts: the thickness of solid blanketing the substrate underneath the liquid film plus a pseudolength deriving from supplementary excitations in the system. The first contribution is directly calculated, and the results appear to fit experiment quite well. For the remaining pseudolength the parameters necessary for a surface-roton-excitation hypothesis to fit experiment are estimated. It is found, using the analysis presented, that the quantitative features of a surface- excitation spectrum may be deduced with considerable precision from presently available experimental data. (AIP)

The results of our experiments on superfluidity in thin films of $^{3}\mathrm{He}$-$^{4}\mathrm{He}$ mixtures are reported. The measurements span the temperature range from 1.3 to 2.1 K. All concentrations of $^{3}\mathrm{He}$ were investigated for films ranging up to 100 \AA{} in thickness. We present detailed curves showing the superfluid content of mixture films as a function of the total amount of film present. For the first time, the dependence of these curves upon $^{3}\mathrm{He}$ concentration is exhibited. Characteristic of the superfluid onset process we observed are distinctive distances which have an interpretation as measuring the "healing length." These are presented as functions of temperature and $^{3}\mathrm{He}$ concentration.

Expectations are presented with regard to the concentration profile and the extent of superfluidity in thin films of /sup 3/He-/sup 4/He in a Van der Waals field. A thermodynamic method of analysis is outlined which, although treating a somewhat idealized configuration, is amenable to experimental confirmation. The connection between this analysis and experiment is discussed in detail. (AIP)

The results of a systematic study of helium thin-film adsorption are presented. Measurements are reported, obtained with a 24-MHz crystal adsorptometer, on the adsorbed mass per unit area as a function of the three independent variables temperature, concentration, and pressure of the gas of 3He-4He atoms generating the adsorbed film. The investigations span concentrations from zero to unity, all pressures up to the dew point for each mixture, and temperatures from 1.3 to 2.1 K. From the experimental results we display the detailed form of the van der Waals attractive potential holding the helium atoms to the surface. The expected dependence on the cube of the distance from the substrate is confirmed over the range of distances explored. The strength of the van der Waals potential is measured and reported. This attractive potential is demonstrated to be of the same form and of the same strength both for the 3He atoms and for the 4He atoms in the mixture at all concentrations. We further deduce, directly from our measurements, the detailed relationship between Σ, the mass per unit area of film absorbed, and d, the thickness of the film.

The experimental data which we have gathered on the superfluid mass content in films of /sup 3/He-/sup 4/He mixtures yield a characteristic length which varies markedly both with temperature and /sup 3/He concentration. The data are analyzed to present their implications on the energy gap and on other parameters characterizing the two-dimensional helium-boundary-interface excitation spectrum.

A configurational emf refers to an electrical manifestation in the electron gas of the analogue of the Bernoulli effect for fluid flow. A theoretical account of this phenomenon is given. The object of the theory is to present a description of the effect from first principles which in this instance is taken as the Boltzmann transport equation. The transport equation is solved for a two band (holes and electrons) Fermi particle system. The solutions are carried beyond first order so as to exhibit the effect. It is shown that the form of the emf is, indeed, extractable from the Boltzmann equation. And its magnitude is obtained in terms of a coefficient C for which a formula is given in terms of familiar current carrier parameters. (Author)

The resonant frequency of a quartz crystal oscillating in the thickness-shear mode changes by virtue of the mass loading due to any type of film laid down on its surfaces. For a helium film only the normal fraction remains rigidly coupled to the substrate motion. Adsorption isotherms of He-4 measured with this technique show clear departures from conventional nonsuperfluid behavior. We attribute this effect to the onset and presence of superfluidity in the film.

Experiments are reported on the dissipative steady-state flow regime of liquid helium II. These were done with a view to examining the theoretical assertion that the chemical potential gradient $\ensuremath{\nabla}\ensuremath{\mu}$ is related to the superfluid-flow speed ${v}_{s}$, by the equation $\ensuremath{-}\ensuremath{\nabla}\ensuremath{\mu}=G\mathrm{exp} (\frac{1\ensuremath{-}{v}_{c}(T)}{{v}_{s}}]$. Our experiments showed good qualitative agreement with this equation. Other assertions about the magnitude of the parameters in this equation and their dependence upon the superfluid fraction $\frac{{\ensuremath{\rho}}_{s}}{\ensuremath{\rho}}$ and temperature $T$ were also explored experimentally.

Experiments are reported in a double-bucket system with a view to examining the application to helium film flow of recent predictions regarding the exponential relationship between the superflow speed and the chemical potential gradient driving force. The theory is found to be generally commensurate with our experimental results with a notable exception. One of the parameters, which is predicted to be constant, is experimentally found to have a definite temperature dependence which we give in detail. We report also our evidence with regard to the height dependence of film thickness in the dissipative regime. We find that this thickness appears to be determined by the mean distance to the two bath levels.

A thermodynamic analysis is given for an energy conversion process which employs the phase transition in a superconductor. The conversion process is a cyclic one. An expression for the conversion efficiency is obtained. Quantitative estimates are calculated from the formula for several superconducting materials. These indicate that conversion efficiencies as high as 44% may be obtainable under ideal conditions.

At high flow rates superfluid helium exhibits dissipation in that a chemical potential gradient is necessary to drive the flow. On the basis of a specific model for the cause of this behavior a detailed relationship between the chemical potential gradient and the resultant flow is derived. Some implications of the theory with regard to recent experiments are discussed.

The effects of an externally applied electric field on the absorption of light by silicon have been measured under a wide variety of experimental conditions. The measurements on 52 samples of both $n$ and $p$ type and a variety of doping species show conclusively that the electro-absorption spectrum is associated with the pure silicon lattice and not with any impurity or imperfection. The spectral characteristics of the electro-absorption peaks associated with phonon-assisted processes are strongly affected by the magnitude of the applied electric field over the measured range of ${10}^{4}$ to ${10}^{6}$ V/cm. The electro-absorption spectrum is unchanged, however, in relative magnitude and peak position by a change in temperature, and the absolute spectral positions follow the band-gap dependence on temperature. Concurrent measurements of the maximum-electro-absorption-peak position and high-resolution, field-free optical absorption establish the final state in the transition process as an exciton state. An interpretation of these measurements is given, based on an optically assisted tunneling process to a final exciton state.

A critical frequency for thermal fluctuations is calculated above which heat transport proceeds by wave propagation rather than by diffusion. This phenomenon should occur in some dielectric solids. It is the analog of second sound in helium II. A macroscopic point of view is used which relies upon a modification in the Fourier heat equation. Some quantitative results are obtained on the magnitude of this modification.

There is a pressure drop associated with an increase in the flow rate of a moving fluid. The venturi meter and the aspirator are familiar devices which derive from this phenomenon. This paper concerns itself with the detection of an analogous effect in the electron gas in a metal.

The resonant frequency of a thickness-shear mode oscillating quartz crystal is lowered by virtue of the mass loading of any film laid down on its surfaces.1,2 For a uniformly deposited, thin solid film with thickness less than 1% or so of the crystal thickness this change Δf, is given by the formula $$ - \Delta f = 2(2{f^2}/c{\rho _q})\sigma $$ (1) Here σ represents that part of the mass per unit area which remains rigidly coupled to the substrate motion, f is the resonant frequency, c is the shear wave velocity in the crystal, and ρ q is the density of the quartz.

Using as a thermal detector a granular carbon film with an area of about 25 μ on a side we have looked for a possible diffraction pattern caused by liquid helium II flowing through a wire grating. At a detection distance of about 1 4 in. from the grating we observed no effect to within about 10 microdegrees.

The precipitous onset step of Kosterlitz, Thouless, and Nelson has been investigated in $^{3}\mathrm{He}$-$^{4}\mathrm{He}$ mixture films over the temperature range 1.3 K to ${T}_{\ensuremath{\lambda}}$. The constant relating the minimum value of superfluid density to the absolute temperature is found experimentally to be universal up to 30% $^{3}\mathrm{He}$. Comparison is made with recent mixture theories. Measurements were made at 24 MHz with a quartz microbalance adsorptometer.