The $I_t$=$J_t$ rule and the proportionality rule are two recently derived selection rules for meson-baryon scattering, valid to leading order in 1/$N_c$. Here, a firm connection to the Feynman diagrams of effective Lagrangians is established: If both rules are implemented at the level of one-boson exchange (which is easily done), then they will both continue to hold to all orders in the loop expansion.

Collisions of Si(14.5A GeV+Au are investigated in the relativistic-quantum-molecular-dynamics approach. The calculated pseudorapidity distributions for central collisions compare well with recent experimental data, indicating a large degree of nuclear stopping and thermalization. Nevertheless, nonequilibrium effects play an important role in such complex multihadron reactions: They lead to a strong enhancement of the total kaon production cross sections, in good agreement with the experimental data, without requiring the formation of a deconfined quark-gluon plasma.

A gas-filled, liquid-nitrogen-cooled cell on-line with an accelerator permits fluorescence spectroscopy of atoms of heavy-ion reaction products generated at rates less than ${10}^3$ ${\mathrm{s}}^{\mathrm{-}1}$. This low required rate of production allows the optical spectroscopy of radioactive atoms to be extended further from stability. With the cell we have measured isotope shifts of ${}_{}{}^{152,154,156,158,166}{}_{}{}^{}\mathrm{Yb}$, reaching N=82, four nuclides further than previously reached with other techniques. The change in rms charge radius near the shell closure is surprisingly large.

We study the irregular wave functions of a highly excited hydrogen atom in a uniform magnetic field. The scarring of wave functions by periodic orbits is quantitatively investigated. The shape of unperturbed scars is in good agreement with recent semiclassical predictions.

We demonstrate radiative resonant pumping using two spatially distinct laser-produced aluminum plasmas. An embedded microdot was irradiated to prepare a plasma that was predominantly in the Alxii $1^1$${\mathit{S}}_0$ ground state. This plasma was then pumped by a photon flux at 7.757 Å, the wavelength of the $1^1$${\mathrm{S}}_0$–2 ${}_{}{}^1{}_{}{}^{}P_1^{}{}_{}{}^{}$ transition. The absolute measurement of the resulting fluorescent signal was ≊1.5×${10}^{11}$ photons/sr.

Time-resolved, submicron density scale lengths in picosecond laser-produced plasmas are measured using resonance absorption profile spectroscopy. The density scale length at critical density is inferred from the angle and polarization dependence of the absorption of a picosecond laser pulse in a performed plasma. The early expansion of picosecond laser-produced plasmas produced from an Au target is studied, with measured scale lengths in the range of ∼1000–5000 Å.

The microcanonical ensemble of identical guiding centers in a two-dimensional, circular domain is characterized by means of a Monte Carlo simulation. For sufficiently large energies, the rotational symmetry is spontaneously broken so that a net displacement of the system accounts for a significant fraction of the angular momentum. The transition between axisymmetric and displaced statistical equilibria resembles a second-order phase transition. The description also applies to point vortices bounded by a circular equipotential surface.

The x-ray-photoelectron core-level linewidths show pronounced differences between atomic C, N, and O adsorbed on Ni(100). Furthermore, the widths are temperature dependent. The observed effects are reproduced within a model based on the creation of vibrational excitations in the core-ionization process. This explains the large linewidths observed for many adsorbate systems. The results have important consequences for the general understanding of line shapes in core-level spectroscopies.

A pool of potential energies at which defect states can be created is incorporated into a simple chemical-equilibrium description of defect formation in a-Si:H. For a wide enough defect-pool distribution, $D^{\mathrm{-}}$ lies deeper in the gap than $D^0$, in agreement with recent photoemission results, even though the correlation energy is positive. Bonded hydrogen is proposed as the physical origin of the defect pool.

The electronic structure of amorphous silicon models has been calculated with the tight-binding parameters of Chadi. An energy gap of ∼1.0 eV is obtained for a fully four-coordinated model, whereas gap states occur for models with coordination defects. Dangling-bond gap states are well localized on the three-coordinated site. Floating bonds lead to considerably less localized gap states with wave-function amplitudes on the neighbors of the five-coordinated atom. The hyperfine splittings of these coordination defects are calculated and compared to experiment.

The quasiparticle band energies for 1×1 and 2×2 GaAs/AlAs(001) superlattices have been calculated from first principles using a self-energy approach. Both superlattices have indirect band gaps. The pseudodirect gaps at the zone center derived from folding of the Brillouin zone are larger than the direct gaps. The calculated results explain the various experimental data quantitatively. Matrix elements for optical transitions near the zone center show significant anisotropy. The effects of disorder on the observed spectra are discussed.

A theoretical analysis is presented of the current from a field-emission tip terminated by a single atom. The calculations yield a relatively focused, high-current beam with a narrow energy distribution, in agreement with recent experimental studies. The results are physically interpreted in terms of channel filtering by an adiabatic constriction.

The generation of harmonics of a small ac modulation field is used, in conjunction with the standard current-voltage measurement, to study the depinning behavior in well-characterized and high-purity samples of the charge-density-wave conductor ${\mathrm{NbSe}}_3$. The results show that the current carried by the charge density wave obeys a scaling law: $I_{\mathrm{CDW}=\left[\right(\mathrm{V}\mathrm{-}{V}_T)/V_T{]}^{\zeta }}$ with ζ=1.23±0.07. Implications of this result on the various theoretical models of the dynamics of the sliding state are discussed.

The existence of classical phase diffusion in hysteretic junctions is demonstrated by quantitative agreement between experimental and simulated I-V curves. The simulations are based on a circuit that accurately models both the junction and its external shunting impedance at microwave frequencies. We show that the bias current at which the junction switches from the phase diffusion state to the voltage state is sensitive to dissipation at microwave frequencies.

We demonstrate experimentally the existence of a continuous phase transition between a normal and a true superconducting phase (with zero linear resistivity) in epitaxial films of Y-Ba-Cu-O in strong magnetic fields fields, H≫${\mathrm{H}}_{\mathrm{c}1}$. The nonlinear I-V curves show scaling behavior near the transition and the relevant critical exponents are extracted. These exponents are consistent with values expected for freezing into a superconducting vortex-glass phase.

The difference in Madelung site potential, Δ$V_M$, between a hole on a copper site and one on an oxygen site can control the hole’s site preference and ability to delocalize. Among a variety of copper oxide structures, eΔ$V_M$ is shown to vary by ∼3 eV and to clearly separate two classes of compounds: Those with eΔ$V_M$>47 eV are metallic with high $T_c$ and delocalized holes, whereas those with lower Δ$V_M$ are semiconducting because their holes are self-trapped on oxygen.

We use a renormalized mean-field theory of the t-J model to show that flux states closely related to those recently proposed by Anderson, Shastry, and Hristopoulos are stabilized, at values of J/t≳1, when the flux per plaquette is commensurate with the electron density in a way similar to that found recently for noninteracting electrons. These states (in the interacting t-J model) are characterized by a collective gauge variable which leads to superconductivity. At not too small values of J/t the state with half a flux quantum per plaquette is stable at low doping.

A new method is introduced for analytically continuing imaginary-time data from quantum Monte Carlo calculations to the real-frequency axis. The method is based on a least-squares fitting procedure with constraints of positivity and smoothness on the real-frequency quantities. Results are shown for the single-particle spectral-weight function and density of states for the half-filled two-dimensional Hubbard model.

We find a noise-induced transition to bistability in a Monte Carlo simulation of a model heterogeneous catalytic chemical reaction which is deterministically monostable. Analysis of the probability density function and the correlation integral of time series of this model indicates the existence and central role of noise in this transition. We find that the behavior of this system is a consequence of the interaction of noise with the spatial degrees of freedom on the model catalytic surface.

We solve an initial-boundary value problem for the Davey-Stewartson equation, a multidimensional analog of the nonlinear Schrödinger equation. It is shown that for large time, an arbitrary initial disturbance will, in general, decompose into a number of two-dimensional coherent structures. These structures exhibit interesting novel features not found in one-dimensional solitons.