A quantum mechanical description of low-concentration hydrogen diffusion is presented. Using the Kubo formula for the diffusion coefficent, harmonic wave functions for the hydrogen atoms, and polaronic wave functions for the lattice, an exact expression for the diffusion coefficent can be found. Two interesting predictions of the theory are (i) a decrease in the activation energy with mass for fcc metals, in agreement with experiment, and (ii) an activated form for diffusion between localized states even in the purely tunneling regime.

Superconducting weak links are studied in which the weak region is created by locally driving the superconductor out of equilibrium using an independent source of quasiparticles. The characteristics of these weak links can be varied electrically or optically at any temperature below $T_c$. A simple model which describes the dependence of weak-link critical current on quasiparticle-injection rate is presented.

We report magnetic-form-factor measurements for Ir in the cubic antiferromagnet ${\mathrm{K}}_2$Ir${\mathrm{Cl}}_6$ ($T_N=3.05\mathrm{}$ K). Even though the Ir ions lie on an fcc lattice and are surrounded by octahedra of Cl, the magnetization density deviates strongly from the cubic symmetry that the charge density shows. This unusual asymmetry is dramatically magnified by the large amount of covalent bonding of the single $5d\left(t_{2g}\right)$ magnetic hole to the ${\mathrm{Cl}}_6$ complex.

The first measurements of magneto-optical effects in metals at energies above the quartz transmission limit are reported. The discussion focuses on the results for Gd which exhibits a $4f$ transition threshold that confirms the result of recent optical studies. Details of the magneto-optical spectrum suggest that this technique provides a sensitive probe of core-state splittings as well as other features of optically excited states in ferromagnetic metals.

Use of a computer-assisted normalization procedure and a resonant reabsorption technique has permitted frequency resolution of two central peaks in the quasielastic-light-scattering spectrum of KTa${\mathrm{O}}_3$ in the 0 to 3 ${\mathrm{cm}}^{-1\mathrm{}}$ range. The intensities, and the polarization, temperature, electric field, and angular dependences of the linewidths indicate that the narrow component (2.3±0.3 GHz at 300 K in right-angle scattering) is due to entropy fluctuations. A tentative identification of the broader component with two-phonon processes is made.

The spectral response from shallow negative donor ions in silicon has been studied over a range of donor concentrations between 1×${10}^{14}$ and 1×${10}^{17}$ ${\mathrm{cm}}^{-3\mathrm{}}$. At low donor concentrations the binding energy of the negative-ion state is well described by the effectivemass theory. The formation of the upper Hubbard band out of these states is observed at a donor concentration of 3×${10}^{15}$ ${\mathrm{cm}}^{-3\mathrm{}}$—about three orders of magnitude lower than the concentration at which the metal-insulator transition occurs in this material.

Fully self-consistent and relativistically exact solutions for the equilibrium of a nonneutral, cold, relativistic, electron beam in a magnetized conducting pipe reveal that the beam is neither of uniform density nor a rigid rotor when total momenta and energy are uniform. The profiles are simple rational functions of the radius. Voltage-current-magnetization characteristics are obtained, showing a maximum current and a cutoff in magnetization for fixed injection voltage.

Magnetic field perturbations accompanying the tearing instability allow rapid heat transport across regions enclosing mode-rational equilibrium flux surfaces. The electron-energy transport equation for this process is derived, under the assumptions of cylindrical geometry, incompressible flow, and relatively weak density gradients. For the thermoelectric instability predicted by kinetic theory, a quasilinear description of local flattening of the temperatures profile is obtained. The self-consistent field perturbation is shown to be quite weak, $\frac{\delta B}{B_0}\sim {10}^{-4\mathrm{}}$; yet it yields a localized heat flux far exceeding pseudoclassical estimates.

The specific heat of bcc ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ has been measured along three isochores for temperatures $T$ between 50 mK and the melting curve. The data for $T<\mathrm{}0.3\mathrm{}$ K can be described well by the function $C_V=\alpha T^{-2\mathrm{}}+\gamma T^3$ and are inconsistent with the existence of the low-temperature specific-heat anomaly reported previously by others.

It is shown that for sufficiently strong repulsive electron-electron interactions a quasi one-dimensional conductor has a phonon anomaly with wave vector component $4k_{\mathrm{F}}$ and an associated phase transition to a new kind of correlated charge-density wave state and lattice distortion, which can account for recent x-ray experiments on tetrathiafulvalene-tetracyanoquinodimethane. In the ordered state, the charge-density excitations are solitons and there is a gap in their spectrum.

High-resolution x-ray-photoemitted valence-electron spectra from a gold single crystal are found to exhibit significant differences for electron emission along various low-index directions, with $〈100〉$ and $〈111〉$ showing the most pronounced dissimilarities. Direct-transition-model calculations predict that such spectra result from initial wave vectors in different well-defined regions of the Brillouin zone. Densities of states in these regions agree very well with the qualitative differences observed between $〈100〉$ and $〈111〉$ spectra.

Exponential instability of waves on the cubically nonlinear, one-dimensional lattice is considered. The lattice has been modeled by the modified Korteweg-de Vries (mKdV) equation. It is shown that some solutions of the mKdV equation are unstable, and that these mKdV instabilities correspond to the instabilities observed on the lattice. Such instabilities may be important in determining whether or not a system behaves stochastically.

Direct evidence is presented showing a universal decrease of charge exchange $\left|\Delta Q\right|>\mathrm{}1\mathrm{}$ as a function of rapidity-gap length, independent of multiplicity and rapidity position within an event. The region over which charge exchange does occur (the "charge-mobility length") is 1.3 rapidity units.

We report on a study of pion charge-exchange scattering carried out at Fermilab in the energy range 20 to 200 GeV. The results can be described remarkably well by a simple Regge-pole model. The charge-exchange cross sections in the forward direction lead to a prediction for the difference in total cross sections of ${\pi }^{-}p$ and ${\pi }^{+}p$ which is in satisfactory agreement with direct measurements of this difference in another experiment at Fermilab.

Measurements on the reaction ${\pi }^{-}p\to \eta n$ have been carried out at Fermilab with beam energies from 20 to 200 GeV in the same experiment in which pion charge-exchange scattering was studied. The differential cross sections have a pronounced dip in the forward direction. The data can be described well by a simple Regge-pole model but the resulting $A_2$ trajectory is not degenerate with the $\rho$ trajectory extracted from the charge-exchange data.

Polarization in ${\pi }^{-}p$ elastic scattering, with emphasis in the region around the secondary dip and also ${\theta }_{\mathrm{c}.\mathrm{m}.\mathrm{}}=90\mathrm{}°$, has been measured at 2.93 and 3.25 GeV/c. We observe an interesting sign change in this angular region.

We present cross-section upper limits for the production of narrow states produced in 15.5-GeV ${\pi }^{+}N$ collisions, in coincidence with single muons. These limits are derived from an invariant-mass search involving all charged-hadron tracks in the final state, as observed in a large streamer chamber. We also give upper limits for the production of muon-associated ${\Lambda }^0$ and $K^0$ particles.

Transient magnetic fields have been observed from a phase shift of the time-differential perturbed angular correlation of ${}_{}{}^{19}{}_{}{}^{}\mathrm{F}$ nuclei recoiling into a nickel host. The Larmor precession associated with the phase shift grossly exceeds the Lindhard and Winther prediction. A spin-dependent electron-promotion mechanism via molecular orbitals is suggested which results in an intermittent $K$-shell spin polarization.

The velocity dependence of the total fine-structure-changing cross section has been measured for collisions of $\mathrm{Na}\mathrm{}\left(3p\right)\mathrm{}$ with Ar and Xe using a new velocity-selection technique based on the Doppler shift. The velocity dependence agrees with recent theoretical predictions of Pascale and Olson, but the magnitudes of the cross sections do not.

Time-resolved trajectories of the critical and quarter-critical densities were obtained through observation of plasma emissions at second and three-halves harmonics of the laser frequency. Characteristic behavior of the critical-density region is noted. Intensity histories of the plasma emissions were measured and compared to the incident pulse. The measurements were made during laser-fusion experiments where glass microballoon targets were irradiated with the 0.2-TW peak power output of a four-beam Nd: glass laser.