It is possible for a plasma containing finite-amplitude ion-density fluctuations and drifting electrons to exhibit a negative resistance for high-frequency electric fields. Under such circumstances both longitudinal plasma oscillations and electromagnetic waves can be amplified or even undergo unstable growth. We have investigated this possibility for longitudinal waves by theoretical analysis and numerical simulation of a one-dimensional plasma model. Substantial growth rates and saturation levels are found.

A Green's-function formalism is presented to study in a unified manner the propagation of mass-spin-fluctuation waves in solid helium with isotopic impurities. Based on this, consistent physical mechanisms are put forward for certain relaxation phenomena in impure solid helium.

The attenuation and velocity of an ultrasonic wave in superfluid helium-4 have been calculated for temperatures below 0.6°K. The calculation is based on the kinetic equations given by Khalatnikov. The phonon Boltzmann equation is solved numerically without making simplifying approximations about the form of the collision integral. The theory predicts that the velocity of sound increases with frequency for very low frequencies, passes through a maximum, decreases to a minimum, and finally increases towards a constant value at high frequencies. A simple explanation for this peculiar behavior is given. The theory is in good agreement with the experimental results of Abrahams et al., and Waters et al.

The radial distribution function and static structure factor of liquid sodium at temperatures of 100 and 200°C have been calculated by a Monte Carlo method using a realistic ion-ion potential. The calculated structure factor is in excellent agreement with recent x-ray-diffraction measurements.

Covariant constitutive relations are developed for an arbitrary, anisotropic, dispersive, dissipative medium, thus allowing for relaxation phenomena, and the covariant wave-dispersion relation is derived. The susceptibility tensor of order 4 is given explicitly for a Vlasov plasma with arbitrary velocity distribution and nonzero dc electric field. It is then shown how to determine the constitutive relation explicitly from the moment of the single-particle current, without using the Vlasov equation. The familiar "dielectric tensor" is obtained as a special case.

Our previous evaluation of the specific heat of an ideal Bose gas confined to a thin-film geometry is extended to second order in ($\frac{\overline{l}}{D}$), where $\overline{l}$ is the mean interparticle distance. Explicit expressions, involving a dimensionless parameter $y$, are obtained both for the specific heat of the system and for the temperature at which the specific heat is maximum. Occurrence of the parameter $y$, which is a measure of the chemical potential of the system in terms of the spacing of the single-particle energy levels, enables us to cast our expression for the specific heat of the system into a form consistent with the scaling theory of Fisher and Barber. Our results for the specific-heat maximum, as a function of ($\frac{D}{\overline{l}}$), are a considerable improvement over the first-order ones reported earlier, and have come remarkably close to the ones obtained numerically by Goble and Trainor.

We investigate the photocurrent power spectrum induced by laser light scattered to a detector from a suspension of homogeneous charged particles in the presence of a square-wave electric field. Constraints associated with optimal heat dissipation require that a time-dependent field be used in order to obtain good resolution in measurements of electrophoretic mobilities. A calculation of the power spectrum yields an expression that depends on both the diffusion constant and electrophoretic mobility of the particle. This expression is used to analyze experimental data in order to obtain values for those parameters.

An explicit thermodynamic equation of state is proposed for the region near the line of liquid-vapor critical points in mixtures of ${\mathrm{He}}^3$ and ${\mathrm{He}}^4$. The equation (of the scaling form) embodies nonclassical exponents and uses chemical potentials or activities as independent variables in place of composition and volume. The properties of the mixtures are shown to result, in the main, from an interpolation between the critical properties of the pure components, in a suitable set of variables. Failure of experiments to reveal a weak divergence in the isothermal compressibility (at fixed composition), in contrast to previous phenomenological theories, is explained through the small size of the region near the critical line where one might expect to see the "true" asymptotic behavior.

Correlation effects in parametric photon-pair production are studied within the framework of a physically realistic model. The analysis, which is fully quantum mechanical, takes into account the finite sizes of the target and the beam cross section, and allows for dispersion and anisotropy in the linear susceptibility. The correlations in position and time at which the two members of a parametrically generated pair may be detected are carefully evaluated. These correlations, which have been measured experimentally, are intrinsically quantum mechanical; i.e., they can be explained by no theory in which the subharmonic fields are described purely by $c$-number functions. A complete solution, from which field correlation functions of arbitrarily high order may be evaluated, is obtained by a method which at the same time allows for an arbitrary degree of parametric gain. The solution is expressed entirely in terms of a particular two-point field-correlation-function, as evaluated in lowest order in the incident field strength, at points distant from the target. The function in question is found by directly examining the fluctuating currents in the material medium, rather than by eliminating the matter variables at the outset through the introduction of a nonlinear electomagnetic susceptibility.

This paper discusses the case of a Doppler-broadened gas laser with a saturable intracavity absorber gas. The semiclassical equations governing the field intensities and the atomic density-matrix elements are set up. The latter are written as Fourier series, the components of which are shown to obey coupled difference equations. In this paper the exact steady-state solution in single-mode operation is obtained. The lowest approximation (rate-equation approximation) is used to compute detailed characteristics of the system. In particular, the influence of the pumping rates in the two cells are investigated and displayed in the figures. The regions of bistable operation are determined and conditions for the occurrence of an inverted Lamb dip are given. The effect of a slow modulation of the pumping rates is discussed. The range of validity of the lowest approximation is investigated both analytically and numerically. Finally, there is a brief discussion of the influence of pressure on the parameters of the theory.

This paper determines the stability of single-mode operation of a laser with an intracavity absorber. The strong-signal theory for one mode is taken from the previous paper, and the linear response at a different cavity mode is calculated. When its gain exceeds the losses, the chosen operating point is unstable. This determines the regions of stability. The influence of the intensity and position of the strong mode is determined in the rate-equation approximation (REA). The REA is found to follow the exact results rather poorly and an improved approximation (IREA) is given, which at least qualitatively reproduces the structure of the exact gain functions.

Reiss's zeroth-order term is obtained directly from the ordinary perturbation theory in terms of the interaction Hamiltonian $H^{\prime }=-\left(\frac{e}{\mathrm{mc}}\right)\overrightarrow{\mathrm{A}}·\overrightarrow{\mathrm{p}}+\frac{e^2{A}^2}{2m{c}^2}$.

Recently we have shown that an anistropic Anderson-Morel state may be stabilized by feedback effects in the effective interaction arising from introduction of a gap in the spin-fluctuation spectrum. In this paper we determine the general form of the fourth-order term in the free energy owing to the above mechanism and use this result to make predictions regarding the phase diagram in the presence of a magnetic field and as a function of pressure.

The Bogoliubov-Zubarev method of collective coordinates is used to derive the energy spectrum of elementary excitations in a weakly interacting many-boson system through second-lowest order in a weak coupling expansion. The obtained result of the spectrum is shown to be equivalent to that evaluated by means of the variation-perturbation procedure based on the method of correlated basis functions in the uniform limit. Comparison of the excitation spectrum is also made briefly with two other results calculated using the Bogoliubov-Zubarev Hamiltonian in somewhat related but different approaches.

By using the Woo—Tan—Massey theory of ${\mathrm{He}}^3$-${\mathrm{He}}^4$ solutions, an expression is derived for $V_0$, the long-wavelength limit of the effective ${\mathrm{He}}^3$ quasiparticle interaction in dilute ${\mathrm{He}}^3$-${\mathrm{He}}^4$ solutions. The expression supports the calculation of Eckstein, Eckstein, Kuper, and Ron, and indicates that the Baym, Bardeen, Pines result for $V_0$ is incorrect. Arguments are also given to suggest that $V_0$ may be positive definite at all pressures.

Values of the inertial mass of ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ in superfluid ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ calculated from experimental measurements of the velocity of second sound and osmotic pressure, and depending only on thermodynamic and hydrodynamic arguments and not on any microscopic model of ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ quasiparticle interactions, are presented. These values cover the range of ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ concentrations $1<X<\mathrm{}6\%$ and temperatures $0.03<T<\mathrm{}0.6\mathrm{}$ K at saturated vapor pressure and at hydrostatic pressures of 10 and 20 atm.

Some objections concerning the validity of a new nonperturbative treatment of multiphoton processes ("momentum-translation approximation") are presented and discussed. In the present state of the question, it seems that the multiphoton amplitudes given by this theory cannot be considered as reliable.

A misinterpretation by Wang and Herman of the earlier treatment by Batra, Enns, and Pohl of the electrocaloric effect and STS-I (stimulated thermal scattering-I) is discussed.

The model proposed by Manalis to account for electron—ion recombination is claimed to be incorrect.

The matching of leading singular contributions to the different mean-field or critical behaviors of a system with long-but-finite-ranged spatially anisotropic interactions leads to "scaling laws" connecting criticial exponents for different dimensions. The results are exact for mean-field theory (MFT) and spherical model exponents and predict MFT exponents for $d=4\mathrm{}$. From the known $d=2\mathrm{}$ Ising-model exponents, the scaling laws give $\alpha =0\mathrm{}$, $\beta =\frac{4}{11}$, $\gamma =\frac{14}{11}$, $\nu =\frac{2}{3}$ for $d=3\mathrm{}$. Agreement with previous results for the $d=2,3\mathrm{}$ excluded volume problem is also quite good. For the $X-Y$ and Heisenberg models the results predict $\gamma \approx 2.0 \mathrm{and} 2.4$, respectively, for $d=2\mathrm{}$.