Radiation from 56-MeV positrons ($\gamma =111\mathrm{}$) channeled between the major planes or along the $〈110〉$ axis of an 18-μm-thick silicon crystal has been observed. The energies of the measured planar-channeling spectral peaks agree well with theoretical predictions.

I propose kinetically determined topological models of lightly doped ($x\lesssim 0.05$) amorphous ${(\mathrm{G}\mathrm{e},\mathrm{S}\mathrm{i})}_{1-x}{Y}_x$ alloys ($Y=\mathrm{H},\mathrm{F},\mathrm{B},\mathrm{P},\dots$). Without adjustable parameters the model relates the lowest concentration ${x_{\mathrm{H}}}^0$ of hydrogen atoms required to quench paramagnetic dangling bonds to the concentration $c_0$ of unpaired spins at $x=0\mathrm{}$. Also discussed are the structural differences between freshly evaporated $\alpha$-(Ge, Si) and various stages of ion-bombarded ("amorphized") crystalline material.

We have measured the velocities of strong laser-driven shocks in aluminum. Pressures of 0.6 and 1.8 TPa were produced by incident laser intensities of 8×${10}^{13}$ and 3×${10}^{14}$ W/${\mathrm{cm}}^2$. This is the first time that pressures this high have been inferred in a laboratory experiment.

We have observed the longitudinal NMR in ${}_{}{}^3{}_{}{}^{}\mathrm{He}$-$B$ down to $0.3T_c$ at 0.4, 10.0, 20.0, and 29.3 bars. From the measured ringing decay time $T_R$, we deduce the strong-coupling correction to the gap parameter and the quasiparticle-spin scattering cross section.

A calculation of the frequency spectrum of the band modes of hydrogen located at a tetrahedral interstitial site shows a strong resonantlike peak with very little isotope effect. This mode was observed by neutron scattering in Nb${\mathrm{H}}_{0.05}$ at 3.8 THz. The band modes provide a mechanism for classical diffusion with low activation energies.

The possible existence of a Kosterlitz-Thouless vortex-antivortex dissociation transition in thin superconducting films is discussed. It is found that in practice the situation should be closely analogous to that predicted for superfluid-helium films. A simple relationship is found between the Kosterlitz-Thouless transition temperature and the sheet resistance. This relationship is compared with the observed broadening of the resistive transition of superconducting films with high sheet resistance.

We propose a method to calculate the unbinding of a gas of thermally activated vortex pairs above a critical temperature $T_{2\mathrm{D}}$ lying below the bulk transition temperature $T_{\mathrm{B}\mathrm{}\mathrm{CS}}$ of thin superconducting films, as a function of temperature and applied magnetic field. We hence determine the phase diagram of the films and their resistivity as a function of field and temperature.

The $M_{5,4}$ soft-x-ray appearance-potential spectrum of ${\mathrm{Yb}}_2$${\mathrm{O}}_3$ does not show the intense structure characteristic of the lighter rare-earth elements even though a single $4f$ vacancy exists in ${\mathrm{Yb}}_2$${\mathrm{O}}_3$. This indicates that the strong $M_{5,4}$ spectral response of each rare-earth element previously measured using the soft-x-ray appearance potential technique is determined primarily by the availability of vacant $4f$ atomic states to accept both the excited $d$ electron and the incident electron.

The electron-impurity scattering rate and resulting Dingle temperatures $X_{\mathrm{D}}$ are calculated for dilute $\mathrm{Al}\mathrm{Li}$. The multiple-plane-wave model of Sorbello is used, but unlike previous approaches the scattering of individual plane waves is treated self-consistently, using the Kohn-Sham density-functional method (and including the main effect of the lattice potential). Electron-phonon renormalization of $X_{\mathrm{D}}$ is included. We discuss the implications of the local approximation for the exchange and correlation.

The thermopower of highly conducting quarter-filled-band TCNQ (tetracyanoquinodimethane) salts corresponds to an entropy per carrier of $k\ln 2$. We have measured the change of the thermoelectric power upon application of a magnetic field for quinolinium ditetracyanoquinodimethane. We find an isotropic shift which agrees with the entropy change of a free-spin system. Coupled with the observation of a negligibly small magnetoresistance, this provides definite evidence that the conducting electrons are correlated as in the strong-coupling Hubbard model.

Measurement of the de Haas-van Alphen effect at 1.1 K in the linear-chain mercury compound ${\mathrm{Hg}}_{3-\delta }\mathrm{As}{\mathrm{F}}_6$ shows that the Fermi surface consists of a set of straight or nearly straight cylinders with axes along the $\overrightarrow{\mathrm{c}}$ direction. The cylinders are formed from one-dimensional Fermi-surface sheets by the interaction of mutually perpendicular mercury chains and by translation by superlattice vectors resulting from the periodicity of the incommensurate mercury chains.

It is shown that the same two features that determine the structure of ordinary gravity and supergravity theories also determine the structure of conformal gravity. The relevant local gauge symmetry turns out to be the inhomogeneous Weyl group. The transformation laws of the fields follow from geometry. Some consequences of breaking the scale symmetry are briefly discussed.

We report on 39 trimuon events observed in high-energy neutrino interactions at Fermilab. The observed trimuon rate relative to single-muon production is (1.1±0.5)×${10}^{-4\mathrm{}}$ for $E_{\nu }>100\mathrm{}$ GeV. The properties of the trimuons are consistent with those for electromagnetic and direct production of muon pairs, although the production of charm-anticharm pairs may account for approximately 20% of the observed rate. New heavy leptons or heavy quarks do not contribute significantly.

We have measured two-particle quantum-number correlations in 400-GeV/c proton-nucleus collisions. A double-arm spectrometer was used to detect two charged particles produced near rapidity $y_{\mathrm{c}.\mathrm{m}.\mathrm{}}=-0.4\mathrm{}$ in the proton-nucleon center-of-mass system. In proton-beryllium collisions we observe positive $p\overline{p}$ and $K^{+}{K}^{-}$ correlations which are independent of the transverse momenta of the particles over the range $1<~p_{\perp }<~1.8$ GeV/c. These correlations are weaker in proton-lead interactions.

We have measured the nucleon-number ($A$) dependence of hadron-pair production in 400-GeV/c proton-nucleus collisions, using Pb and Be targets. Charged-hadron pairs were observed near rapidity $y_{\mathrm{c}.\mathrm{m}.\mathrm{}}=-0.4\mathrm{}$ with $\Delta \varphi \cong 180°$. The $A$-dependence exponent rises from 1.1 to 1.2 in the range $2.0<~\left|p_{{\perp }_1}\mathrm{}\right|+\left|\mathrm{}{p}_{{\perp }_2}\right|<~4.5$ GeV/c. The dihadron $p_{\perp }$ correlation function is significantly smaller for Pb than for Be.

The possible occurrence of precritical phenomena in finite nuclei due to the relative proximity of the pion condensation threshold is investigated. We point out that such precritical behavior may occur in inelastic scattering differential cross sections for the excitation of unnatural-parity states at high momentum transfers [$q\simeq \mathrm{}(2-3)\mathrm{}{m}_{\pi }$]. As an example, we discuss inelastic proton scattering to $1^{+}$ states in ${}_{}{}^{208}{}_{}{}^{}\mathrm{Pb}$.

We have measured the hyperfine frequency and wall shift of hydrogen atoms at 4.2 K stored in a bulb coated with solid ${\mathrm{H}}_2$. The phase shift per wall collision is - 0.29(1) rad. The adsorption energy of H on ${\mathrm{H}}_2$ is 9(2) K, and the adsorption time at 4.2 K is approximately 30 nsec. Transverse and longitudinal relaxation times have been measured, and atomic densities greater than ${10}^{14}$/${\mathrm{cm}}^3$ have been observed.

Magnetic resonance studies are reported for a low-density gas of hydrogen atoms for $4.2<T<\mathrm{}77\mathrm{}$ K. The resonance observed is between the two lowest hyperfine levels in a field of 6.5 kG, where this splitting has its minimum value (765 MHz). Information is obtained about translational diffusion through ${\mathrm{He}}^4$ and ${\mathrm{H}}_2$ buffer gases, frequency shifts and broadening due to wall collisions, and the effects of spin-exchange broadening.

Intermodulated optogalvanic spectroscopy, a new method of Doppler-free saturation spectroscopy, is described. Preliminary values of 725±4 MHz for the ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ $3{}_{}{}^3{}_{}{}^{}D_2^{}{}_{}{}^{}(F=\frac{3}{2})-3\mathrm{}{}_{}{}^3{}_{}{}^{}D_1^{}{}_{}{}^{}(F=\frac{1}{2})$ hyperfine splitting, and of 5830±6 MHz for the $3{}_{}{}^3{}_{}{}^{}D_1^{}{}_{}{}^{}(F=\frac{1}{2})-3\mathrm{}{}_{}{}^3{}_{}{}^{}D_1^{}{}_{}{}^{}(F=\frac{3}{2})$ hyperfine splitting obtained using this method are reported. Intermodulated optogalvanic spectroscopy is limited in sensitivity only by shot noise in the direct current sustaining the discharge, and compares favorably in sensitivity with other Doppler-free methods.

A nonperturbative formula is given for the $\mathcal{r}$-photon-resonant, ($\mathcal{r}+p$)-photon ionization rate of an atom and from it is derived an expression for the exponential index.