Optical pumping strongly enhances nuclear spin polarizations in an n-doped GaAs/AlGaAs multiple quantum well, allowing NMR signals of the GaAs wells to be investigated by direct, radio-frequency detection in a 7.05 T field. The optically pumped NMR signals exhibit unanticipated dependences on the pumping wavelength, polarization, and power. The temperature dependence of the ${}_{}{}^{71}{}_{}{}^{}\mathrm{Ga}$ NMR Knight shift is consistent with a ${\mathit{g}}^{\mathrm{*}}$ value of 0.49±0.02 for electrons in the wells, with no exchange enhancement. We present evidence that the doped and photoexcited electrons do not reach a common spin temperature under optical pumping.

We investigate the O(n) model on the honeycomb lattice, using its loop representation in the limit of full packing. The universal properties, which we calculate by means of finite-size scaling and transfer-matrix techniques, are different from the branches of O(n) critical behavior known thus far. The conformal anomaly of the model varies between -1 and 2 in the interval 0≤n≤2. The universality class of the model is characterized as a superposition of a low-temperature O(n) phase, and a solid-on-solid model at a temperature independent of n.

Sonoluminescence (SL) is generated from single, stably oscillating bubbles in a stationary, time-periodic acoustic field. By measuring the time dealy between flashes, the dynamics of the phenomenon has been investigated. While other researchers have concentrated on the remarkable periodic stability of the system, present results indicate that, for small variations in the governing parameters, period doubling, chaos, and quasiperiodicity can occur. The implications of the complex temporal behavior for the problem of determining the mechanism(s) for production of SL are discussed.

Light-scattering measurements have enabled us to determine that the transition to sonoluminescence is characterized by a bifurcation in the dynamics of a trapped pulsating bubble. These experiments also reveal that in the sonoluminescence (SL) state, changes in bubble radius of only 20% are associated with factors of 200 in the intensity of emitted light. This sensitivity of SL suggests that it originates from the kind of singular behavior that arises from the implosion of a shock wave. Theoretical extrapolations of this model to energy scales for fusion are discussed.

Cellular automata models for the formation of Liesegang structures are proposed. This novel approach, which takes into account the fluctuations for the first time, describes the problem at a microscopic scale, in terms of reaction, diffusion, nucleation, and aggregation processes. We present large scale numerical simulations which provide clear verifications of the time and spacing laws and predict a novel behavior for the widths of the patterns. We show that two different microscopic reaction schemes are possible for producing Liesegang structures and we propose a phase diagram showing the different types of possible patterns.

A method is proposed for the experimental determination of the amplitude and phase structure of a quasimonochromatic wave field in a plane normal to its propagation direction. The wave field may represent either a scalar electromagnetic (EM) field or the quantum mechanical (QM) wave function of a matter wave. For coherent EM fields or pure QM states, the method uniquely reconstructs the complex wave fields. For partially coherent EM fields or mixed QM states, it reconstructs the two-point correlation function or density matrix. The experiment uses only intensity measurements and refractive optics (lenses), and the data analysis algorithm is noniterative and requires no deconvolution.

We propose a classical model for the non-Abelian Chern-Simons theory coupled to N pointlike sources and quantize the system using the Becchi-Rouet-Stora-Tyutin technique. The resulting quantum mechanics provides a unified framework for fractional spin, braid statistics, and Knizhnik-Zamolodchikov equation.

We outline a procedure whereby higher-order corrections to the semiclassical propagator can be systematically computed within classical mechanics. The application of this result to actual calculations is straightforward.

Page conjectured very recently [Phys. Rev. Lett. 71, 1291 (1993)] that if a quantum system of Hilbert space dimension mn is in a random pure state, the average entropy of a subsystem of dimension m≤n is given by ${\mathit{S}}_{\mathit{m},}$n=${\mathit{tsum}}_{\mathit{k}=\mathit{n}+1\mathrm{}}^{\mathit{m}\mathit{n}}$1/k-(m-1)/2n. We outline a proof of this elegant formula.

The Newtonian gravitational constant has been determined at an effective interaction distance of 112 m. A high-precision balance was used to compare the weights of two 1-kg stainless stell masses located above and below the variable water level of a pumped-storage lake. Water-level changes up to 43 m produced a maximum weight difference of 1290 μg, which could be measured with a resolution of <1 μg. The data yield a value for the gravitational constant G of (6.6700±0.0054)×${10}^{\mathrm{-}11}$ ${\mathrm{m}}^3$ ${\mathrm{kg}}^{\mathrm{-}1}$ ${\mathrm{s}}^{\mathrm{-}2}$ (1σ), in agreement with laboratory determinations. New limits for the strength of possible new intermediate-range forces are placed.

We have measured total cross sections for the reaction ${\mathrm{\pi }}^{+}$p→${\mathrm{\pi }}^{+}$${\mathrm{\pi }}^{\mathrm{-}}$p at incident pion kinetic energies of 190, 200, 220, 240, and 260 MeV. We use this result to deduce a new value of the chiral symmetry breaking parameter, ξ=-0.25±0.10, in a global constrained fit of the five ππN near-threshold amplitudes. Consequently, we report new soft pion model values for the s-wave ππ scattering lengths.

Three rotational bands have been discovered in the nucleus ${}_{}{}^{109}{}_{}{}^{}\mathrm{Sb}$ that extend in angular momentum to ≃81/2ħ. The rotational frequency at the highest spins reaches ħω≃1.4 MeV, the highest frequency so far observed in a heavy nucleus. With increasing frequency the collective moments of inertia of all three bands smoothly diminish to unexpectedly low values, ${\mathit{scrJ}}^{\mathrm{}\left(2\right)\mathrm{}}$∼13${\mathrm{ħ}}^2$ ${\mathrm{MeV}}^{\mathrm{-}1}$, approximately one-third the rigid-body value, and, contrary to expectations, the moments of inertia become nearly equal to one another and to those of rotational bands in the core nucleus ${}_{}{}^{108}{}_{}{}^{}\mathrm{Sn}$.

The nucleus ${}_{}{}^{176}{}_{}{}^{}\mathrm{W}$ has been studied using the reaction ${}_{}{}^{150}{}_{}{}^{}\mathrm{Nd}$${(}^{30}$Si,4n), with prompt and delayed γ-γ coincidence techniques. A K=14 isomer (${\mathit{t}}_{1/2}$≊70 ns) is found to exhibit a unique decay pattern, primarily decaying to K=0 states, in contrast to all previously studied high-K isomers. Calculations have been perfomed of both Coriolis mixing and tunneling through a potential barrier in the triaxial degree of freedom to understand these unusual decays. While Coriolis mixing models do not reproduce the variations in the decay patterns in neighboring nuclei, the tunneling calculations are remarkably successful.

The relativistic excitation of the double giant dipole resonance in ${}_{}{}^{136}{}_{}{}^{}\mathrm{Xe}$ has been calculated, describing the resonance as a harmonic vibration of the mean field, damped through its coupling to doorway states. While the predictions associated with the one-phonon states provide an overall account of the experimental findings, the calculated cross section for the two-phonon states is much smaller than that extracted from the involved analysis of the data. The experimental elucidation of this discrepancy seems essential for assessing the validity of the standard picture of giant resonances.

The 2${\mathit{S}}_{1/2\mathrm{-}}$2${\mathit{P}}_{3/2}$ interval in hydrogen has been measured using a fast atomic beam and a separated oscillatory field technique at a beam energy of 96.5 keV using standard X-band waveguides as interaction regions. The experimental value for the 2${\mathit{S}}_{1/2\mathrm{-}}$2${\mathit{P}}_{3/2}$ hydrogen fine structure interval is 9911.200(12) MHz. This implies a Lamb shift of 1057.839(12) MHz, in fair agreement with the theoretical value of 1057.866(5) MHz that was calculated using the smaller of the two discrepant values of the proton radius in the literature.

Relative yields of ${\mathrm{Ca}}^{+}$ and ${\mathrm{Ca}}^{\mathrm{-}}$ ions from thermal-energy collisions between highly excited ${\mathrm{Ca}}^{\mathrm{*}}$ 4snd ${}_{}{}^1{}_{}{}^{}$D Rydberg atoms and Ca ground-state atoms are measured as a function of n. The collisions take place within an effusive Ca atomic beam. The cross section for a collision resulting in a bound ${\mathrm{Ca}}^{\mathrm{-}}$ and a ground-state ${\mathrm{Ca}}^{+}$ is expected to be large when the binding energy of the excited Rydberg state nearly matches the electron affinity of Ca. We observe a large increase in positive and negative ions for n=25 only. This result strongly suggests that the binding energy of ${\mathrm{Ca}}^{\mathrm{-}}$ is 24±1.4 meV.

We have observed the first laser-induced resonant transitions in antiprotonic helium atoms. These occur between metastable states and Auger dominated short lived states, and show that the anomalous longevity of antiprotons previously observed in helium media results from the formation of high-n high-l atomic states of p¯${\mathrm{He}}^{+}$. The observed transition with vacuum wavelength 597.259±0.002 nm is tentatively assigned to (n,l)=(39,35)→(38,34).

A control scheme is devised for taming the dynamics of convectively unstable extended systems exhibiting chaotic behavior. We show how complex spatiotemporal phenomena can be eliminated in arrays of coupled chaotic elements, in favor of a coherent state in which all elements are synchronized to a prescribed periodic orbit of the uncoupled system. The propagation of the synchronizing front and the resulting steady state properties of the array are anlayzed in the presence of noise.

The level of variance maintained in a stochastically forced asymptotically stable linear dynamical system with a non-normal dynamical operator cannot be fully characterized by the decay rate of its normal modes, in contrast to the case of normal dynamical systems. The nonorthogonality of modes in a non-normal system may lead to transient growth which supports variance far in excess of that anticipated from the decay rate given by the eigenvalues of the operator. As an example, the variance maintained by stochastic forcing in a canonical shear flow is examined in this work and found to increase with a power of the Reynolds number between 1.5 and 3. This great amplification of variance suggests a fundamentally linear mechanism underlying shear flow turbulence.

The coherent synchrotron radiation process in a waveguide is theoretically investigated. A single, short bunch propagating through a wiggler is considered. In a waveguide, two very distinct regimes are possible. At grazing, where the beam velocity matches the wave group velocity, the bunch emits a single, ultrashort chirped pulse whose duration is determined by the interaction bandwidth and the waveguide dispersion. Away from grazing, where slippage dominates, two distinct pulses are radiated at the Doppler upshifted and downshifted frequencies. Both the time and frequency domain expressions for the radiation characteristics are derived.