Properties of nuclear matter are discussed within the framework of the relativistic Hartree-Fock approximation. Particular attention is paid to the π (pseudoscalar or pseudoscalar coupling) and ρ (vector and tensor couplings) degrees of freedom. The contributions of the various mesons to the different components of the baryon self-energy are discussed, as well as the implications expected for finite nuclei.

We show that a priori atomic parameters (valence-orbital radii) allow classifications of molecular shapes for a large number of sp-bonded ${\mathrm{AB}}_2$ compounds with the number of valence electrons N≤16, in parallel with classifications of crystal structures in the solid phase. Triatomic ${\mathrm{A}}_3$ clusters with N≤15 are structurally classified with the same scheme.

Energy-level patterns of ${\mathrm{He}}^{\mathrm{*}\mathrm{*}}$, Be, Mg, Ca, Sr, and Ba are compared to the spectrum of a linear rotating ABA molecule with bending vibrations. The spectra of the alkaline earths are found to agree with the rotor-vibrator model even better than the spectrum of ${\mathrm{He}}^{\mathrm{*}\mathrm{*}}$, the system for which the molecular model was originally proposed.

The spectral density of the Nyquist noise current in a tuned circuit coupled to a sample of nuclear spins has been measured at ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ temperatures with a dc SQUID used as a rf amplifier. When the sample is in thermal equilibrium, a dip is observed in the spectral density at the Larmor frequency. For zero spin polarization, on the other hand, a bump in the spectral density is observed. This bump is due to temperature-independent fluctuations in the transverse component of magnetization, and represents spontaneous emission from the spins into the circuit.

Near the onset of a period-doubling bifurcation a parametrically modulated NMR laser has been used as a detector of weak input signals with a strongly peaked response curve centered near half the modulation frequency. The maximum sensitivity increases the closer the bifurcation point. A Bloch-Kirchhoff model with realistic parameters has been used for computer simulations. Experiments and theory are in fair agreement. The results suggest that any parametrically modulated laser device could be used as a small-signal detector for selected frequencies.

Infinite pointwise stretching in a finite time for general initial conditions is found in a simulation of the Biot-Savart equation for a slender vortex tube in three dimensions. Viscosity is ineffective in limiting the divergence in the vorticity as long as it remains concentrated in tubes. Stability has not been shown.

An enhancement of ∼100 of stimulated emission over spontaneous emission of the C vi 182-Å line (one-pass gain ∼6.5) was measured in a recombining, magnetically confined plasma column by two independent techniques involving intensity calibrated extreme xuv monochromators. Additional confirmation that the enhancement was due to stimulated emission has been obtained with a soft x-ray mirror; with 12% measured effective reflectivity of the mirror, a 120% increase in intensity of the C vi 182-Å line in the axial direction was observed.

An enhancement of ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ nuclear magnetization has been observed in the experiment of ${}_{}{}^3{}_{}{}^{}\mathrm{-}_{}^{63}{}_{}{}^{}$Cu double nuclear resonance for ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ in contact with small copper particles, when the ${}_{}{}^{63}{}_{}{}^{}\mathrm{Cu}$ nuclear resonance was simultaneously excited. The enhancement is explained by a model which assumes that electron spins of surface cupric oxides mediate a magnetic coupling between ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ and ${}_{}{}^{63}{}_{}{}^{}\mathrm{Cu}$ nuclear spins.

We have investigated the electron mobility on the surface of ${\mathrm{H}}_2$ crystals coated with a thin He film. The mobility is found to be smaller than on the bare ${\mathrm{H}}_2$ or on the bulk He surfaces. The extra electron scattering is due to the He gas atoms, the ripplons, and the density fluctuations in the uppermost He monolayer. The scattering from the density fluctuations is a direct measure of ${\mathrm{K}}_2$, the two-dimensional compressibility of the He monolayer.

Silicon surface reconstructions are directly observed in profile by high-resolution transmission-electron microscopy. Low-energy surface facets are formed at edges by in situ annealing of a 〈110〉 thin specimen at an ambient pressure of ${10}^{\mathrm{-}9}$ Torr. As well as 〈111〉, 〈100〉, and 〈110〉 reconstructed surfaces, extensive areas of flat 〈113〉 surface are found. By inspection of high-resolution images from the 〈113〉 surface a model involving one dimer per surface (1×1) unit cell is proposed, suggesting that low-energy surfaces need not be confined to high-symmetry orientations.

We report the first realization of a quasiperiodic (incommensurate) superlattice. The sample, grown by molecular-beam epitaxy, consists of alternating layers of GaAs and AlAs to form a Fibonacci sequence in which the ratio of incommensurate periods is equal to the golden mean $\tau$. X-ray and Raman scattering measurements are presented that reveal some of the unique properties of these novel structures.

A Monte Carlo simulation of a lattice model of the nematic–to–smectic-A transition in three dimensions shows a continuous phase transition with the specific-heat exponent α≃0. The order-parameter correlation functions show anisotropic critical behavior in the nematic phase, where the behavior of the correlation functions in the superconducting gauge strongly indicates an isotropic critical point. Several features observed in experiments are qualitatively reproduced in the simulation.

The first observaton of Raman scattering is reported for an amorphous metal. An interference-enhancing trilayer configuration is used. Measurements on transition-metal– metalloid alloys of a-${\mathrm{Ni}}_{0.67}$${\mathrm{B}}_{0.33}$ provide information on the form of the weighted phonon density of states of Ni-Ni and Ni-B derived bands and Raman-coupling-parameter variations.

Quasi one-dimensional magnetic compounds have given good examples of soliton excitations. We present new results on soliton dynamics obtained by the neutron-spin-echo method. This technique allowed a unique accurate line-shape analysis of the antiferromagnetic mode (centered at q=π/a in reciprocal space) in the time domain 0.02–1.7 nsec. Thus we could directly probe the antiferromagnetic short-range order in the chains. The analysis of the relaxation line shape of this order in (${\mathrm{CD}}_3$${)}_4$${\mathrm{NMnCl}}_3$ doped with copper reveals an exp (- √t ) time decay—a signature of diffusive soliton dynamics—and does not show the conventional exp(-t) decay expected for ballistic solitons.

Molecular-dynamics simulations have been used to identify two cooperative atom-ejection mechanisms which increase sputtering yield. Their effects are analogous to the ‘‘thermal spike,’’ ‘‘shock wave,’’ and ‘‘reduced binding energy’’ sputtering mechanisms. They are examples of nonlinear, many-body, effects in cascades which go beyond collisions between randomly moving particles.

The density of states for ideal (100) cation- and anion-terminated surfaces of ${\mathrm{Hg}}_{1\mathrm{-}\mathrm{x}}$${\mathrm{Cd}}_{\mathrm{x}}$Te (x=0.5) have been determined with a site-dependent coherent-potential approximation for the alloy. Alloy effects are greatly enhanced at a cation surface. The cation-surface density of states exhibits HgTe- and CdTe-like surface-state bands with no virtual-crystal–related surface-state features. This bimodal character does not appear in the bulk or anion surface density of states. Similarly, the self-energy at a cation surface site is very different from those in the bulk or near-anion surfaces.

The strong inequivalence of spin-up and spin-down subbands in a ferromagnet causes a coupling between the charge and spin transport across the interface of a ferromagnetic and a contiguous paramagnetic metal. This allows the use of sensitive electronic measurements to probe spin transport. Application of small static magnetic fields results in a Hanle effect which permits determination of the spin-relaxation time ${\mathrm{T}}_2$. The unique features of the method should make it applicable to a wide range of studies.

In low-field nuclear magnetic resonance in solids, transitions are not driven directly by the oscillating field, but by the dipole-dipole interactions modulated by the applied oscillating field. This results in strong Δm=2 transitions due to oscillating internal field gradients. Tunnel sidebands, symmetry forbidden in ordinary NMR, become strongly allowed. Tunnel frequencies of rotating molecular groups are measured in a previously inaccessible frequency range.

We present calculations, using the properly normalized couplings and masses for Dine-Fischler-Srednicki axions, of power rates and signal temperatures for axion-photon conversion in microwave cavities. The importance of the galactic-halo axion line shape is emphasized. We mention spin-coupled detection as an alternative to magnetic-field–coupled detection.

The two-point spatial correlation function for cosmic string loops in an expanding universe is determined and shown to closely match the observed two-point correlation function for clusters of galaxies. Further implications for the string theory of galaxy formation are discussed.