Antiferromagnetic spin fluctuations in superconducting ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{6+\mathrm{x}}$ have been studied using inelastic neutron scattering. In a crystal with x=0.5 and $T_c$=50 K, we have observed spin fluctuations at 12 K having a cross section which increases with increasing excitation energy. The data are consistent with highly overdamped spin waves and a very short spin-spin correlation length.

We have artificially varied the coupling between ultrathin ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ layers by interposing insulating planes of ${\mathrm{PrBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ in multilayers and studied the consequences of decoupling 12-Å ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ unit cells on the superconducting properties. We find that the ${\mathrm{T}}_{\mathrm{c}}$ (midpoint) of a 12-Å/12-Å multilayer is 55 K, twice that of the corresponding alloy. ${\mathrm{T}}_{\mathrm{c}}$ decreases linearly with further separation of the ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ unit cells, the transition width increases and shows evidence of 2D fluctuations, and the influence of a magnetic field parallel to the layers is markedly reduced.

The average kinetic-energy release is measured for the evaporation of CsI and (CsI${)}_2$ molecules from sputtered Cs(CsI${)}_{\mathrm{n}}^{+}$ clusters (n=2–17) in two different time windows. Size and temporal (temperature) dependence of the average kinetic-energy release (KER) is found to be in reasonable agreement with the predictions of the evaporative ensemble model, except for evaporation processes involving the cluster with magic number n=13. Agreement can be obtained for the latter if the assumption of the independence of the heat of evaporation on the cluster size in the model is removed.

We describe some novel effects generated by the gravitational Chern-Simons term similar to those induced by its electromagnetic counterpart. In particular, a structureless particle source generates a gravimagnetic field. Conversely, it causes a spinning source to contribute to the static Newtonian potential, thereby violating the equivalence principle; the potential can even become repulsive.

Using partial-wave unitarity and the observed density of the Universe, we show that a stable elementary particle which was once in thermal equilibrium cannot have a mass greater than 340 TeV. An extended object which was once in thermal equilibrium cannot have a radius less than 7.5×${10}^{\mathrm{-}7}$ fm. A lower limit to the relic abundance of such particles is also found.

A grand unified model based on SU(15) is described. Baryon-numbe-violating processes such as proton decay are absent in both the gauge and Higgs sectors. A number of new gauge bosons are predicted at Superconducting Super Collider energy, including doubly charged gauge bosons coupling only to leptons.

A direct experimental determination has been made of the ratio of the forward-angle positive-pion electroproduction cross section for a proton bound in the deuteron with that of a free proton for invariant masses of 1160 and 1232 MeV. A significant quenching of the reaction in the deuteron is observed.

Algebraic potentials from SO(3,1) and SO(3,2) representations of scattering functions are deduced by matching to scattering functions obtained by fitting ${}_{}{}^{12}{}_{}{}^{}\mathrm{-}_{}^{12}{}_{}{}^{}$C elastic-scattering differential cross sections. Their variations with energy suggest a simple mapping between algebriac and coordinate-space interactions.

Energy spectra of δ electrons emitted in coincidence with deep-inelastic heavy-ion collisions are calculated within a Langevin Monte Carlo description using the surface-friction model. We investigate the effects that wipe out the originally expected oscillatory pattern of the spectra. Emphasis is placed on the role of statistical fluctuations in extracting interaction time distributions of deep-inelastic collisions from the measured δ-electron spectra. The calculations are compared to data for U+Au at 8.65 MeV/nucleon.

We have used the electron-energy dependence of yields of heliumlike and lithiumlike argon ions from the Kansas State University electron-beam ions source (EBIS) to measure the ratio of the cross section for Δ n=1 dielectronic recombination of heliumlike argon to that for electron ionization of lithiumlike argon. By normalizing to the latter cross section we obtain absolute dielectronic recombination cross sections and find good agreement with theoretical calculations for the lower-energy resonances.

Molecular vibrations in a Morse potential driven by a high-frequency electromagnetic field are described by a return map for the energy change within a vibration period and the canonically conjugate phase angle. Quantizing this dynamical system we study the molecular excitation and dissociation for initial states in the classically regular and chaotic region.

Numerical simulations demonstrate that laminar breakdown in a boundary layer induced by the secondary instability of two-dimensional Tollmien-Schlichting waves to three-dimensional subharmonic disturbances need not take the conventional lambda vortex–high-shear layer path.

Stable and unstable diocotron modes varying as $e^{\mathrm{il}\theta }$, with l=1, are measured on a magnetized, partially hollow electron column. The unstable mode is observed to grow exponentially over several decades, in contradiction to present 2 D fluid theory. This l=1 instability can dominate the evolution well into the nonlinear regime, resulting in cross-field transport to a stable, monotonically decreasing density profile. To the extent this process is described by 2D E×B drifts, it is isomorphic to the Kelvin-Helmholtz shear-flow instability of inviscid fluids.

An axisymmetric, amgnetically confined electron column, in which the E×B rotation frequency is not a monotone function of radius, is linearly unstable to two-dimensional, electrostatic disturbances with azimuthal mode number l=1. The perturbation density is asymptotically proportional to √t and may be described as a shift of the core of the column. A particle-in-cell simulation indicates that harmonics grow rapidly and that there are secondary instabilities. An identical instability arises in hollow circular vortex columns in an inviscid, incompressible neutral fluid.

We show that the exact equilibrium crystal shape (ECS) for a wide class of two-dimensional Ising models can simply be read off from the analytical expression for the bulk free energy: The ECS is given as the locus of purely imaginary zeros of the ‘‘momentum’’-space lattice-path propagator. From these shapes, one may obtain numerically, to arbitrary accuracy, the high-temperature direction-dependent correlation lengths of the dual system. These exact ECS’s and corresponding correlation lengths were previously known only for the rectangular, triangular, and honeycomb lattices.

The kinetics of nucleation and growth in the first-order transition belonging to the one-dimensional nonconserved-order-parameter system has been studied by time-resolved synchrotron x-ray diffraction measurements of the stacking transition in stage 1 rubidium-graphite intercalation compounds. The volume of growing phase can be scaled by the theoretical function of Avrami. The line shape is scaled by a squared Lorentzian which is consistent with a recent theory. The nuclear size has a nearly logarithmic time dependence which is in contrast to the linear time dependence in the usual theories.

We present a new approach to first-principles simulations of the statics and dynamics of ionic molecular crystals. It is shown that the new method gives very realistic simulations of the phase transitions in ${\mathrm{K}}_2$${\mathrm{SeO}}_4$ and that a double-well type of structure in the potential-energy surface is the driving mechanism of these phase transitions.

The spin dynamics of muonium diffusing in NaCl have been investigated using the technique of muon-spin relaxation. An effective coupling constant (${\delta }_{\mathrm{ex}}$) for the nuclear hyperfine interaction and the correlation time for motion were determined from measurements of the muon-spin-relaxation rate. At temperatures below 200 K ${\delta }_{\mathrm{ex}}$ is sharply reduced below that expected from the known hyperfine structure of interstitial hydrogen, providing clear evidence for delocalization of muonium.

First-principles calculations are used to evaluate the hyperfine and superhyperfine parameters for hydrogen and muonium at various sites in the Si lattice. Good agreement is found with muon-spin-rotation experiments, leading to an unambiguous identification of ‘‘anomalous muonium’’ with the bond-center site. The results establish the power and reliability of pseudopotential-spin-density-functional theory for calculating signatures of defects.

We have grown a new phase of Cu, bcc Cu, on bcc Fe(001). We have shown that bcc Cu(001) grows epitaxially on bcc Fe(001) and that bcc Fe(001) grows epitaxially on bcc Cu(001). bcc Cu(001) on Fe(001) maintains its structure for 10–11 monolayers before undergoing a structural transition. Ultrathin well-defined trilayers of Fe(001)/Cu(001)/Fe(001) with Cu thicknesses between 6–12 monolayers were grown. Ferromagnetic resonance, Brillouin light scattering, and surface magneto-optical Kerr effect were used to measure the coupling between the Fe layers. The coupling between the Fe layers changed from ferromagnetic to antiferromagnetic between 9–10 monolayers of Cu.