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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 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 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.

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.

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.