A free-exciton gas is studied as a function of generated particle density in ${\mathrm{Cu}}_2$O at 1.5 K. A line-shape analysis of the subsequent recombination shows a gradual evolution, from a classical regime at low density, to a highly quantum-statistical one with chemical potential $\mu \simeq 0$.

A study is reported of the magnetic properties of the random alloy ${\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x{\mathrm{Cl}}_2$, which represents an archetypal example of a system with competing orthogonal spin anisotropies. Behavior similar to previous experiments and theoretical predictions is found, but with important qualitative and quantitative differences; in particular the phase transition in one variable is drastically altered by the existence of long-range order in the other variable. It is hypothesized that this is due to microscopic random-field effects.

Introduction of a correlation length into the tunneling theory of depinning of charge-density waves by an electric field leads to a threshold field and to an expression for non-Ohmic conductivity in excellent agreement with experiments on the linear-chain compounds Nb${\mathrm{Se}}_3$ and Ta${\mathrm{S}}_3$. An extension of the theory leads to results for the frequency dependence of the ac conductivity, also in agreement with experiment.

If neutrinos dominate the mass density of the Universe, they play a critical role in the gravitational instability theory of galaxy formation. For neutrinos of mass $30m_{30}$ eV, a maximum Jeans mass $M_{\nu m}\approx 4\times {10}^{15}{m_{30}}^{-2\mathrm{}}{M}_{\odot }$ is derived. On smaller scales, neutrino fluctuations are damped, and the growth of baryon fluctuations is greatly inhibited. Structures on scales $>M_{\nu m}$ may collapse, forming galaxies as in Zel'dovich's "pancake" theory.

From an analysis of the reaction ${\nu }_{\mu }+d\to {\mu }^{-}+X$, obtained from an exposure of the Fermilab 15-ft deuterium-filled bubble chamber, the ratios of charged-current cross sections for $E_{\nu }>10\mathrm{}$ GeV are determined to be $\frac{\sigma \left(\nu n\right)}{\sigma \left(\nu p\right)}=2.03\mathrm{}\pm 0.28$ for $〈E_{\nu }〉=50\mathrm{}$ GeV, and $\frac{\sigma \left(\overline{\nu }n\right)}{\sigma \left(\overline{\nu }p\right)}=0.51\mathrm{}\pm 0.16$ for $〈E_{\overline{\nu }}〉=40\mathrm{}$ GeV. Bjorken-$x$ distributions for neutron and proton targets show that the majority quark in the nucleon has a broader momentum distribution in the infinite momentum frame than does the minority quark.

A parity-nonconservation asymmetry has been measured in the total cross section for 6-GeV/c polarized protons on a water target. The asymmetry $A_L=\frac{({\sigma }_{+}-{\sigma }_{-})}{({\sigma }_{+}+{\sigma }_{-})}$, defined as the fractional difference of total cross sections for positive- and negative- helicity protons on an unpolarized target, is $A_L=(2.65\mathrm{}\pm 0.60)\times {10}^{-6\mathrm{}}$. The quoted error includes both statistical and systematic contributions.

A search was made for spontaneously fissioning superheavy elements in damped collisions of two uranium nuclei. Different techniques were applied covering the elements 108 to 118 and ≃ 126, and a half-life range from 1 ms to more than 1 yr. No evidence for superheavy elements was found at upper cross-section limits of ${10}^{-32\mathrm{}}$, ${10}^{-33\mathrm{}}$, and ${10}^{-35\mathrm{}}$ ${\mathrm{cm}}^2$ for half-lives from 1 to 100 ms, 100 ms to 1 d, and 1 d to 1 yr, respectively.

Improved agreement between calculation and ${\pi }^{\pm }$ elastic-scattering data across the (3,3) resonance is obtained by assuming the $\pi$-nucleon collision energy, $\omega$, to be ≃ 30 MeV less than the incident pion energy. This effect has been observed for nuclei ranging from ${}_{}{}^9{}_{}{}^{}\mathrm{Be}$ to ${}_{}{}^{208}{}_{}{}^{}\mathrm{Pb}$.

Yrast states with spins up to about $22\hslash$ have been identified in ${}_{}{}^{176}{}_{}{}^{}\mathrm{O}$S, ${}_{}{}^{178}{}_{}{}^{}\mathrm{O}$S, ${}_{}{}^{180}{}_{}{}^{}\mathrm{O}$S, and ${}_{}{}^{182}{}_{}{}^{}\mathrm{O}$S. In each case anomalies are observed in the yrast sequence. The yrare extensions of the ground-state and Stockholm bands in ${}_{}{}^{178}{}_{}{}^{}\mathrm{O}$S and ${}_{}{}^{180}{}_{}{}^{}\mathrm{O}$S are also observed and the magnitude of the yrast-yrare interaction matrix elements extracted and discussed.

Ground-band states up to $16\hslash$ in ${}_{}{}^{160}{}_{}{}^{}\mathrm{Dy}$ and ${}_{}{}^{170,174}{}_{}{}^{}\mathrm{Yb}$ have been Coulomb excited by 350-MeV ${}_{}{}^{86}{}_{}{}^{}\mathrm{Kr}$ ions. The excited nuclei recoiled through magnetized iron where they experienced transient magnetic fields as high as ∼ 3800 T. The nuclear precessions, determined from $\gamma$-ray time-integral perturbed angular correlations, suggest a small decrease of the $g$ factors with increasing spin for all three nuclei.

The ($p$,$n$) reaction at 45 MeV is used to study two broad peaks found previously with the target ${}_{}{}^{90}{}_{}{}^{}\mathrm{Zr}$. They have now been observed with all but one of seventeen targets from ${}_{}{}^{90}{}_{}{}^{}\mathrm{Zr}$ to ${}_{}{}^{208}{}_{}{}^{}\mathrm{Pb}$. Energy systematics favor the conclusion that these peaks are antianalogs of the giant $M1\mathrm{}$ and $E1$ resonances in the target nucleus. The first experimental determinations of $T$, $T-1\mathrm{}$ splittings of the giant $E1$ resonance are reported. Their low values in comparison to $T$, $T+1\mathrm{}$ splittings observed previously can be interpreted as due to a tensor part of the effective isospin potential.

Gas-phase Raman spectra of the diatoms ${}_{}{}^3{}_{}{}^{}\mathrm{He}_2^{}{}_{}{}^{}$ and ${}_{}{}^4{}_{}{}^{}\mathrm{He}_2^{}{}_{}{}^{}$ are measured, the former for the first time. The depolarized spectra of the collisional pairs are in near agreement with computations based on a recent configuration-interaction calculation of the helium-diatom polarizability (anisotropic part). The polarized spectra, however, require a somewhat more substantial trace than the configuration-interaction prediction. Measurements of the second virial dielectric coefficient support this conclusion.

The nature of the final $f$-wave functions, i.e., spatially collapsed versus uncollapsed, involved in $4d\to 4f$, $\epsilon f$ giant absorption resonances in ${\mathrm{I}}^{-}$, Xe, and ${\mathrm{Cs}}^{+}$ is revealed by studying the post-collision interaction (PCI) in the $N_{4,5}{O}_{2,3}{O}_{2,3}$ Auger decay of the $4d$ hole. While small PCI Auger shifts ≲0.2 eV are observed for ${\mathrm{I}}^{-}$ and Xe with uncollapsed $f$-wave functions, a giant PCI Auger shift of ∼ 1 eV is observed for ${\mathrm{Cs}}^{+}$ with collapsed $f$-wave functions.

Pinhole bremsstrahlung photography was used to measure the angular distribution of electrons incident on the anode in the pinch region of relativistic-electron-beam diodes. It was found that the incidence angles of most of the electrons are limited to 30° with the diode axis and that the average electron direction at each point within the pinch is nearly parallel to this axis. These results may imply quite strong electric fields in the vicinity of the pinch.

A new mechanism is presented for ion heating in a plasma penetrated by an electron beam. Oscillating potentials comparable to the electron-beam energy are observed below the threshold of the stable double-layer formation. These fluctuations (formation and disruption of double layers) accelerate, trap, and detrap thermal ions, resulting in highly energetic ion bursts, which form a tail on the ion-energy distribution function.

The adsorption process of a mixture of nitrogen and oxygen on tungsten has been studied by field-emission microscopy. The observations of work function and field-emission-microscopy patterns suggest that when the tungsten surface at 300 K is exposed to this mixture the preferential adsorption of nitrogen, the replacement of adsorbed nitrogen by oxygen, and the adsorption of oxygen take place sequentially.

A parameter-free theory is shown to provide an explanation for both melting in alkali halides, and superionicity in alkaline-earth halides, in terms of different types of instabilities of the perfect lattice.

By Mössbauer spectroscopy, the anisotropy of recoilless $\gamma$ radiation emitted from a jumping atom is observed. The set of interstitial sites is determined for localized diffusion of Fe in an interstitial cage in Al. From the temperature dependence of the broadening of the Mössbauer resonance 18 (± 1) meV is obtained for the activation energy of this process.

The group velocity of high-frequency sound for $T\ll T_c$, and the group velocity and attenuation of sound at higher temperatures, have been measured and analyzed to give the frequencies and coupling strengths of two order-parameter collective modes in ${}_{}{}^3{}_{}{}^{}\mathrm{He}$-$B$. For the squashing mode at 13.1 bars and $T\to 0$, $\frac{{\omega }_{\mathrm{sq}}}{{\Delta }_{\mathrm{BCS}}}=1.501\mathrm{}$ and the coupling constant ${\lambda }_{\mathrm{sq}}=0.010\mathrm{}\pm 0.001$. The recently discovered collective mode at $\frac{{\omega }_{\nu }}{{\Delta }_{\mathrm{BCS}}}=1.075\mathrm{}$ ($T\to 0$) has a coupling constant determined from the group velocity to be ${\lambda }_{\nu }=5.8\mathrm{}\times {10}^{-6\mathrm{}}$.

The photon-stimulated desorption (PSD) of ${\mathrm{O}}^{+}$ ions from the $\alpha$-oxygen phase on Mo(100) has been monitored through and above the Mo $L_1\mathrm{}\left(2s\right)\mathrm{}$ absorption edge (2866 eV) and is compared to the absorption spectrum of clean Mo(100). The PSD ion yield is shown to reveal the extended-x-ray-absorption fine structure of Mo surface atoms which were bonded to oxygen atoms prior to their desorption. Studies of the present kind provide a new powerful tool for surface structure investigations.