We suitably modify the scheme of Singwi et al. for calculating the density response function of interacting electron gas to take into account the fact that the pair correlation function vanishes in the highly repulsive hard-core region of the potential for classical liquids. This modified self-consistent iterative scheme is applied to calculate the structure factor for liquid sodium and rubidium. Both equilibrium and dynamical properties of classical liquids can now be calculated from a single theory.

Mössbauer spectroscopy was used to study the dynamics of Al self-interstitials trapped at ${}_{}{}^{57}{}_{}{}^{}\mathrm{Co}$ impurities. When a single interstitial atom is trapped at the ${}_{}{}^{57}{}_{}{}^{}\mathrm{Co}$ atom, the Debye-Waller factor decreases rapidly by more than a factor 4 in the temperature interval between 13 and 20 K. We try to explain this anomaly by the following model: The ${}_{}{}^{57}{}_{}{}^{}\mathrm{Co}$ atom jumps between six different positions, thereby forming mixed Co-Al dumbbells with changing Al partners.

We give numerical evidence that in a two-dimensional lattice-dynamical $\mathrm{XY}$ model with quartic anisotropy a conventional continuous phase transition occurs. The formation of clusters in the angular displacement field separated by 90° walls is also demonstrated. Moreover, we give evidence that the critical dynamics of the order-parameter fluctuations is dominated by a central peak which splits at finite wave vectors. This phenomenon is traced back to damped traveling 90° walls.

We succeeded in substituting the paramagnetic ${\mathrm{Cr}}^{5+}$ ($S=\frac{1}{2}$) ion with the same size as ${\mathrm{As}}^{5+}$ in K${\mathrm{H}}_2$As${\mathrm{O}}_4$, for ${\mathrm{P}}^{5+}$ in K${\mathrm{H}}_2$P${\mathrm{O}}_4$, and in their deuterated isomorphs. In the crystals the high-temperature resonance spectrum showing a local tetragonal symmetry is broken on the time scale of the EPR experiment below $T^{*}>T_C$, so that orthorhombic spectra appear in the paraelectric tetragonal phase. $T^{*}$ shifts proportionally to the Curie temperature $T_C$ upon deuteration. We deduce the existence of ferroelectric clusters with lifetimes $\tau$ longer than 1.5×${10}^{-8\mathrm{}}$ sec below $T^{*}$.

The quenched random Ising chain is treated by a decimation transformation. An exact recursion relation is derived for the distribution of nearest-neighbor couplings and magnetic fields. Fixed distributions associated with Griffiths singularities and with the spin-glass state are explicitly exhibited.

We show that classical sine-Gordon solitons maintain their integrity to a high degree in the presence of external perturbations. Two examples, of particular importance in condensed matter, are described in detail: (i) A model impurity is found to bind low-velocity solitons but merely phase shift those with high velocities, and (ii) external static driving terms with damping accelerate the soliton to a terminal velocity. The importance of a translation mode is emphasized and it is concluded that the soliton behaves as a classical particle in all essential respects.

We report preliminary results on the production of electron-positron pairs in the mass range 2.5 to 4 GeV in 400-GeV $p$-Be interactions. Production cross sections of the $\psi \mathrm{}\left(3100\right)\mathrm{}$ near $x=0\mathrm{}$ as a function of $p_t$, $x$, and the decay angle are presented and implications of these new data for single direct leptons are discussed. A ${\psi }^{\prime }\mathrm{}\left(3700\right)\mathrm{}$ signal is observed at a level corresponding to $\frac{\sigma \left({\psi }^{\prime }\right)}{\sigma \left(\psi \right)}=(10\mathrm{}\pm 3)\%$.

Scaling violation in deep inelastic lepton-proton processes is shown to be related to the apparent ${p_T}^{-8\mathrm{}}$ behavior of the large-$p_T$ pion inclusive distribution at $\sqrt{s}>23.5\mathrm{}$ GeV, within the framework of the quark-quark hard-collision model. Power-law scale-breaking fits of the structure functions as determined by the $\mathrm{ep}$ and $\mu p$ data are used as inputs in our calculations. Excellent agreement with the high-energy data on large-$p_T$ pions is obtained in both the energy and $x_T$ dependences. No adjustable parameters are used except for one overall normalization.

We have studied muon-produced hadrons from a deuterium target. The structure functions and the charge ratios are reported for neutrons; the transverse momentum and azimuthal distributions are reported for deuterons. The structure function for the neutron is similar to that of the proton. The charge ratio of produced hadrons follows the expectation of a simple spin-½ quark model. Transverse-momentum results agree with those at lower energy and are similar to those from hadron-hadron interactions. No azimuthal anisotropy is seen.

We show that the coefficient of the logarithmic lepton-mass singularity in the radiative corrections of order $\alpha$ to the total ${\pi }_{l2\mathrm{}}$ decay probability is not affected by the strong interactions and can therefore be rigorously computed. The phenomenological implications of this result and its connection with electron-muon universality are briefly discussed.

We have studied $\omega$ photoproduction using 7.5- to 10.5-GeV tagged photons. Cross sections from hydrogen lie 22% below the bubble chamber results of Ballam et al., but have a similar slope. Density matrices indicate approximate $s$-channel helicity conservation. The proton-neutron cross-section difference is midway between zero and the value suggested by the $\gamma p-\gamma n$ total-cross-section difference. Fits to the cross sections for D, Be, and Cu and those for Be, C, Al, Cu, and Pb from a previous experiment yield ${\sigma }_{\omega N}=25.4\mathrm{}\pm 2.7$ mb and $\frac{{{\gamma }_{\omega }}^2}{4\pi }=7.6\mathrm{}\pm 1.2$.

${\rho }^{\pm }$ photoproduction from hydrogen and deuterium at 9.6 GeV is studied. The reactions $\gamma p\to {\rho }^{+}n$ and $\gamma p\to {\rho }^{-}{\Delta }^{++}\mathrm{}\left(1236\right)\mathrm{}$ have cross sections much larger than expected from pion exchange alone. $t$ distributions do not show the sharp forward peak characteristic of one-pion exchange contributions. Density matrices indicate an isotropic decay distribution. The ratio of ${\rho }^{+}$ to ${\rho }^{-}$ production on deuterium differs from unity and by the amount expected from interference between $\rho$ and $A_2$ exchanges. We conclude that $\pi$ exchange is unimportant for ${\rho }^{\pm }$ photoproduction, and infer that $\rho$ exchange dominates.

The energetic protons seen at 180° in 600- and 800-MeV proton-nucleus collisions by Frankel et al. are accounted for by a single-scattering mechanism that incorporates a new phenomenological form for the momentum distribution in the nucleus.

The Faddeev differential equations in configuration space are solved numerically for the nucleon-nucleon interaction of de Tourreil and Sprung effective in the states ${}_{}{}^1{}_{}{}^{}S_0^{}{}_{}{}^{}$, ${}_{}{}^3{}_{}{}^{}D_1^{}{}_{}{}^{}$, ${}_{}{}^3{}_{}{}^{}D_1^{}{}_{}{}^{}$, ${}_{}{}^1{}_{}{}^{}P_1^{}{}_{}{}^{}$, and ${}_{}{}^3{}_{}{}^{}P_{0,1,2}^{}{}_{}{}^{}$. Differential cross section, polarizations $P_n$, $i{T}_{11}$, $Q$, and $R$, and spin correlation coefficients $C_{\mathrm{xx}}$ and $C_{\mathrm{yy}}$ are computed here. Contributions from other interaction terms are evaluated perturbatively. Agreement with experimental data is found, except for the vector polarization $i{T}_{11}$, and $C_{\mathrm{xx}}$.

The ${}_{}{}^{18}{}_{}{}^{}\mathrm{O}(\gamma , p){}_{}{}^{17}{}_{}{}^{}\mathrm{N}$ cross section has been measured with monoenergetic photons from threshold to 30 MeV. The results show striking similarities and differences with the ${}_{}{}^{18}{}_{}{}^{}\mathrm{O}(\gamma , p){}_{}{}^{17}{}_{}{}^{}\mathrm{N}$ cross section.

It has recently been predicted that Cooper-type minima of the photoionization cross section should also exist for $l\to l-1\mathrm{}$ transitions. This theoretical conclusion is supported by measurements of the spin polarization of photoelectrons that have been ejected from thallium atoms by circularly polarized light.

In the case of $e$-${\mathrm{H}}_2^{+}$ recombination the formation of Rydberg states of the molecule is an important channel through which recombination occurs. Through the energy interval 0.01 to ∼4 eV, the direct H + H pair-production process is in direct competition with the indirect one which proceeds through an intermediate bound Rydberg state ${\mathrm{H}}_2^{\mathrm{R}}$. In the experiment one can see clear evidence of autoionization competing with predissociation and ${\mathrm{H}}^{+}$-${\mathrm{H}}^{-}$ pair production, particularly when vibrational levels $v>\mathrm{}2\mathrm{}$ are suppressed in the ${\mathrm{H}}_2^{+}$ beam.

The impact-parameter dependence of the noncharacteristic radiation emitted in 35-MeV Cl-Cl collisions has been measured. The coincidence spectra are very similar in shape to the "singles" x-ray spectrum, and show almost no dependence on the impact parameter. In the quasimolecular-radiation picture, the quasistatic approximation is in clear disagreement with the coincidence results.

The observation of two-photon optical nutation and free-induction decay is reported. The experimental result, on a two-photon transition in N${\mathrm{H}}_3$ with all relevant parameters known, is shown to be in quantitative agreement with the prediction of the two-photon vector model.

We have observed ion pulses of ${10}^{13}$ protons by passing hollow relativistic electron beams through a magnetic cusp. Ion pulses are observed with drift-chamber fill pressures from 75 to 600 mTorr of ${\mathrm{H}}_2$. Magnetic fields of 0.8 kG suppress the mechanism responsible for acceleration without magnetic field. A different mechanism appears to turn on and peaks as the cusp threshold is approached. More than ${10}^{11}$ protons with energies greater than 2 MeV are observed.