Complete local-bonding geometries in a disordered molecular overlayer have been determined for the first time, together with the relative occupation of different coexisting adsorption sites. Diffuse-LEED intensities were measured with a new, highly sensitive, low-noise, digital-LEED detector, and calculated with an extension of the efficient beam-set-neglect theory. At one-third of a monolayer of carbon monoxide on Pt(111) at 160 K, (88 ± 5)% of the molecules are found to occupy top sites and (12 ± 5)% bridge sites, with bond lengths consistent with earlier results for ordered layers.

We have investigated the structural and elastic properties of three distinct multilayer systems: Mo/Ni, Pt/Ni, and Ti/Ni. We demonstrate that the commonly observed lattice expansion perpendicular to the film plane is not a bulk effect, but is localized at the interface between the contacting metals. Incorporating the measured interfacial expansion into a universal binding relation, we show that the measured elastic softening versus compositional-repeat distance can result from the interfacial strain.

A channel of a two-dimensional electron gas in GaAs-AlGaAs heterostructures was irradiated with ballistic phonons at 1.2 K. No magnetic field was applied. A phonon-drag voltage was measured along the channel when phonons were absorbed in the channel and if the wave vector of these phonons had a component directed along the channel. The anisotropic nature of this phonon-drag effect was established with the phonon-imaging technique. From the comparison of the data with theory it follows that the electron-phonon coupling is of the piezoelectric type.

The resistivity of nearly solid-density Al was measured as a function of temperature over 4 orders of magnitudfe above ambient by observing the self-reflection of an intense, <0.5 psec, 308-nm light pulse incident on a planar Al target. As an increasing function of electron temperature, the resistivity is observed initially to increase, reach a maximum which is relatively constant over an extended temperature range, and then decrease at the highest temperatures. The broad maximum is interpreted as "resistivity saturation," a condition in which the mean free path of the conduction electrons reaches a minimum value as a function of temperature, regardless of the extent of any further disorder in the material.

A unique ballistic-electron spectroscopy technique has been employed to measure directly semiconductor-band-structure properties at a subsurface interface for the first time. Further, the method, based on scanning tunneling microscopy, enables spatially resolved carrier-transport spectroscopy of interfaces. A theoretical treatment has been developed which accurately accounts for the observed spectroscopic features.

We show that the subgap dissipation in sufficiently high-quality Josephson junctions, as measured directly from the current at which the junctions switch from the voltage state into the zero-voltage state, is controlled by thermally excited quasiparticle tunneling. The dissipation is in excellent agreement with a recent theory of Chen, Fisher, and Leggett.

We discuss the short-range resonating-valence-bond system as realized by a quantum hard-core dimer gas of arbitrary density on a two-dimensional square lattice. When the dimers completely cover the lattice, we argue that there is a first-order transition from a dimer crystal to an insulating quantum liquid state which possesses low-energy, neutral, spinless excitations which we call "resonons." For less than close-packed densities, the ground state is a superfluid. In addition to the usual Goldstone mode, there are low-energy, spinless zone-boundary excitations.

We present a mean-field analysis of a many-body dynamical system which models charge-density-wave transport in the presence of random pinning impurities. Phase slip between charge-density-wave domains is modeled by a coupling term that is periodic in the phase differences. When driven by an external field, the system exhibits a first-order depinning transition, hysteresis, and switching between pinned and sliding states, and a delayed onset of sliding near threshold.

The Bohr-Sommerfeld and Maslov phases, which occur in the Einstein-Brillouin-Keller quantization rule of semiclassical mechanics, are shown to be examples of Berry's phase. The generalization of Berry's phase due to Aharonov and Anandan, which does not require adiabaticity, is employed.

The first experimental observation of a bifurcation sequence of patterns in a reaction-diffusion system is reported. The experiment on a one-dimensional system, maintained away from equilibrium, reveals steady, periodic, quasiperiodic, frequency-locked, period-doubled, and chaotic spatiotemporal states. Results from a model reaction-diffusion system agree qualitatively with the experiment and provide insight into the physical mechanism that drives the observed behavior.

In this Letter, we describe the connection between the classical concepts of nonintegrability and chaos and the quantum concept of dynamical symmetry breaking. The existence of unbroken dynamical symmetry implies integrability of the mean-field motion in a quantum phase space, defined as a symplectic coherent-state parametrization of the coset space of the overall dynamical group. Broken dynamical symmetry leads to nonintegrability, and thus chaotic solutions to Hamilton's equations in the quantum phase space. We illustrate the general ideas with results obtained for a model of two coupled spins.

We calculate the effects of interactions on the density fluctuations of an $n$, $p$, $e$ plasma. The scattering of neutrinos arising from the Fermi part of the neutral current interactions with protons, under supernova conditions, is practically extinguished by Coulomb effects in the plasma. Charged-current reactions are discussed in terms of off-diagonal density fluctuations. Inclusion of the symmetry energy term in the nuclear interaction gives rise to a large reduction in the Fermi part of the cross section for $\nu +n\to e^{-}+p$.

We shall develop an approximation scheme to construct the metric representing a realistic clumpy universe in general relativity. Spatial averaging is used to express the fact that the expansion of the universe is generated collectively by the clumps of matter. The dynamics of the clumps is treated by the post-Newtonian-type approximation. The scheme allows one to calculate the back reaction due to the growth of clumps on the expansion and vice versa. An expression for the deviation from homogeneous and isotropic expansion is derived in terms of inhomogeneities of the gravitational field generated by clumps. It should also be used as the correct interpretation of the observation of gravitational lenses.

A laboratory experiment searching for intermediate-range forces coupling to $N-Z$ of nuclei has been performed. This yields the bound on the strength of any such force to be less than 3.0 × ${10}^{-3\mathrm{}}$ of gravity per baryon, for all ranges $\lambda >3\mathrm{}$ m.

A quantum field theory for electroweak interaction and gravitational interaction with local scale invariance and local SU(2) ⊗ U(1) gauge invariance is proposed. The requirement of local scale invariance leads to the existence of Weyl's vector meson which absorbs the Higgs particle remaining in the Weinberg-Salam model.

We report on a study of the inclusive production properties of $\frac{D}{\overline{D}}$ mesons in $\mathrm{pp}$ collisions at 800 GeV/c and compare our results to measurements made at lower energies and to the expectations of the QCD fusion model.

A search is described for muonium (${\mu }^{+}{e}^{-}$) conversion to antimuonium (${\mu }^{-}{e}^{+}$) with a coupling strength approximately equal to that of the weak interaction. A silica-powder target produced thermal muonium in a vacuum. A unique sequence of radioactivity following the capture of a ${\mu }^{-}$ in tungsten was the signal for conversion. No events were seen, implying an upper limit on the effective four-fermion weak coupling constant for conversion of $0.88G_{\mathrm{F}}$ (90% confidence level), corresponding to a branching ratio of less than 2×${10}^{-5\mathrm{}}$ with respect to normal muon decay.

We have studied the fragmentation of 6.4-TeV ${}_{}{}^{32}{}_{}{}^{}\mathrm{S}$ nuclei in Pb, Cu, and Al targets. For Pb, the electromagnetic spallation cross sections are larger than the nuclear cross sections. For Pb and Cu the electromagnetic cross sections decrease with projectile charge loss $\Delta Z$ as $\sim \exp \left(0.6\mathrm{}\Delta Z\right)$ and show a dependence on target charge consistent with $\sim Z_T^{1.8}$ as predicted by theory.

An exact solution of the time-dependent problem has been obtained for a degenerate two-level system interacting with a harmonic boson field. Unlike the Weisskopf-Wigner solution this solution takes into account various many-boson processes and is non-Markovian. There is no exponential decay and the asymptotic behavior explicitly depends on the initial conditions. Both spontaneous and stimulated processes have been considered.

We report the operation of a saturated 96.9-nm laser in Cs vapor that has an extrapolated small-signal gain of exp(83) in a total length of 17 cm. We believe that lasing occurs from a core-excited level embedded in the continuum of the valence electron. The laser is pumped by soft x rays from a synchronous, traveling-wave, laser-produced (2.5 J, 20 ps, 1064 nm) plasma.