We report on a first-principles demonstration of the superfluorescence effect in a large system. Under proper conditions, a system of two-level atoms, confined to a pencil-shaped volume, and initially prepared in an uncorrelated excited state, goes through a nonexponential cooperative decay, giving rise to a superfluorescence burst. Radiation is all confined in two diffraction patterns and the central frequency presents a time-dependent shift (chirping).

Wilson's incomplete-integration renormalization-group equations have been solved in $4-\epsilon$ dimensions for an arbitrary cutoff function. The two relevent exponents are computed to order $\epsilon$ and the exponent $\eta$ is computed to order ${\epsilon }^2$. To order $\epsilon$ the exponents agree with the sharp-cutoff renormalization group. In order ${\epsilon }^2$, however, $\eta$ appears to depend on the choice of the cutoff function.

A new method has been developed for analyzing small-angle x-ray scattering data from fluids in the critical region without assuming the form of the angular distribution of the scattered intensity. By use of data obtained previously, the scaling function for the scattered intensity has been evaluated for argon. This function expresses the angular distribution of the scattering in terms of $q\xi$, where $\xi$ is the long-range correlation length, $q=4\mathrm{}\pi {\lambda }^{-1\mathrm{}}sin\left(\frac{a}{2}\right), \lambda$ is the x-ray wavelength, and $a$ is the scattering angle.

It is found that the static dielectric constant and mean polarizability of a simple nonpolar fluid share the singularity of the internal energy at the critical point. This singularity can also be expected in the index of refraction at nonzero frequencies along with a more singular term that vanishes as the frequency goes to zero.

The confinement of $Z=2\mathrm{}$ barium test ions in a $Z=1\mathrm{}$ plasma was found to be strongly dependent on density and background plasma ion mass. Most of the characteristics of the parametric dependence of the barium confinement could be related to the superposition of an anomalous outward and a classical inward diffusion.

An intense, relativistic electron beam (50 nsec, 400 kV, 20 kA) has been successfully injected and confined in a toroidlike system. Up to 50% of the total beam energy was injected into a 3.8-kG toroidal field. The beam was observed to have been trapped for at least 300 nsec which corresponded to five revolutions. Magnetic-probe and hard-x-ray data are presented to confirm trapping, and some general trends are noted from these preliminary results.

Extremely high-power microwave emission is generated when a pulsed relativistic electron beam propagates down the axis of a corrugated-wall wave guide. The radiation occurs at a frequency such that the phase velocities of the negative-energy space charge wave on the beam and a low-order TM mode in the wave guide are equal. Power levels of 500 MW are generated. The conversion efficiency of electron energy into electromagnetic energy radiation is 17%.

We report the first observation of pressure-enhanced lattice transformation in the high-temperature superconductor ${\mathrm{Nb}}_3$Sn. The opposite pressure effects on the lattice transformation and the superconducting transition in A15 compounds can thus be understood on the basis of the Weger-Labbé-Friedel model by taking into account the pressure-induced interband charge transfer.

A microscopic explanation is given for the variation of the superconducting transition temperature $T_c$ with the electron-per-atom ratio in transition metals and their alloys.

A new method of taking uv photoemission data, employing synchrotron radiation, is described. The method has significant advantages for investigating final-state properties of interband transitions and for investigating many-body effects. Applying it to KCl, strong peaks in the conduction-band density of states are located at 9.7 and 12.6 eV. The data also suggest a probable interpretation of an optical-reflectance peak ($h\nu \simeq 13$ eV) whose origin has been a matter of doubt.

Measurements are reported of the kinetic energy distributions of field-ionized noble gases on both clean and nitrogen-adsorbate-covered surfaces of tungsten. The structure observed is attributed to the energy distribution of unoccupied electronic states at the metal surface. Correspondences are found with recent calculations of one-dimensional densities of states. A nitrogen-adsorption peak is observed at 0.6 eV above the Fermi energy.

The frequency-wave-vector dispersion relations of a single crystal of Pd${\mathrm{D}}_{0.63}$ have been measured by coherent neutron inelastic scattering at 78, 150, 220, and 295 K. The acoustic modes are considerably lower in frequency than the corresponding modes for pure Pd, in accord with the observed lattice expansion. The optic modes, in which D motions predominate, show considerable dispersion. In particular, the shape of the [100] LO branch shows conclusively that second-like-neighbor D-D interactions are comparable to the first-like-neighbor D-D interactions.

The ground-state magnetic resonance of ${\mathrm{Mn}}^{2+}$ in Ca${\mathrm{F}}_2$: Ce, Mn can be observed by monitoring the change in either the ${\mathrm{Mn}}^{2+}$ or ${\mathrm{Ce}}^{3+}$ luminescent intensity. This spin-dependent luminescence is evidence for an exchange-coupled energy-transfer process between the cerium sensitizer and the manganese activator.

The compressible Ising-like model of Larkin and Pikin is equivalent to a constrained Ising model, as was recently shown by Sak. I introduce finite pressure effects into this system and find that the Ising transition is first order at low pressures and second order "renormalized" at high pressures. The tricritical pressure is equal to the rigidity modulus for the model, but will be lower for more realistic systems. The spherical tricritical point and Gaussian tetracritical point are also observable in principle as the pressure is varied.

We report a result of (1±4)×${10}^{-7\mathrm{}}$ for the parity-nonconserving component in the $p-p$ nuclear cross section at 15 MeV. Our experiment uses rapid spin reversal of a longitudinally polarized proton beam and an unpolarized ${\mathrm{H}}_2$ target. Sources of systematic error are discussed and found to be <${10}^{-7\mathrm{}}$.

We analyze the spectral profile for the resonant enhancement of four-wave parametric generation in the vicinity of autoionizing transitions. A comparison between theory and experiment is presented for an autoionizing line of Sr i seen at 1867 Å.

Low-temperature techniques were applied to the study of a hydrodynamic instability. High-precision results for the Nusselt number $N$ as a function of the Rayleigh number $R$ for liquid and gaseous helium revealed no singularities in $N\left(R\right)\mathrm{}$, except at the convective threshold $R_c$. For $R>\mathrm{}2.19\mathrm{}{R}_c$, a new turbulent state was found and characterized by measuring the frequency spectrum and the amplitude of $N\left(R\right)\mathrm{}$.

The approach to and development of turbulence of low-Prandtl-number fluids in the Rayleigh-Bénard geometry is studied with truncated Boussinesq equations. Semiquantitative agreement is found with experimental results on helium for the turbulent threshold, and for the intensity and frequency spectrum of fluctuations at slightly higher Rayleigh numbers. It is argued that this transition conforms with abstract mathematical proposals of Ruelle and Takens, but that other transitions violate their picture.

The anisotropy of the translational self-diffusion tensor in the smectic-$A$ ($\frac{〈D_{\parallel }〉}{〈D_{\perp }〉}=0.3\mathrm{}$) and smectic-$C$ phases of terephthal-bis-4-$n$-butylaniline has been determined by multipulse NMR. The experimental results are interpreted in terms of a pseudolattice model with anisotropic potential barriers which seems to provide a better description of the physical situation than the "two-dimensional-liquid" model.

The dispersion and attenuation of superthermal phonons ($h\omega \gg \mathrm{kT}$) in He II at 0.1 K are remeasured and reinterpreted as a function of pressure. We find evidence for the existence of the three-phonon process and an upper energy cutoff to this process implying anomalous dispersion. Our earlier conclusion on the absence of anomalous dispersion at saturated vapor pressure is shown to be incorrect.