Classical periodic orbits are stationary-phase points in path integral representations of quantum propagators. We show that complex solutions of the stationary-phase equation, not corresponding to real classical periodic orbits, give additional contributions to the propagator which can be important, especially near bifurcations. We reveal the existence and relevance of such periodic ghost orbits for a kicked top.

We have mesured the polarization of 375-GeV/c ${\mathrm{\Sigma }}^{+}$ and Σ¯ ${}_{}{}^{\mathrm{-}}{}_{}{}^{}$ hyperons produced by 800-GeV/c protons incident on a Cu target. We find that the ${\mathrm{\Sigma }}^{+}$ polarization rises with increasing ${\mathit{p}}_{\mathit{t}}$ to a maximum of 16% at ${\mathit{p}}_{\mathit{t}}$=1.0 GeV/c and then decreases to 10% at ${\mathit{p}}_{\mathit{t}}$=1.8 GeV/c. We compare this ${\mathrm{\Sigma }}^{+}$ polarization with data at lower energies. The Σ¯ ${}_{}{}^{\mathrm{-}}{}_{}{}^{}$ polarization has been measured for the first time. It has the same sign as the ${\mathrm{\Sigma }}^{+}$ but smaller magnitude in a similar kinematical region.

Dipole transitions linking signature partner superdeformed bands in ${}_{}{}^{193}{}_{}{}^{}\mathrm{Hg}$ have been observed. Measurements of the photon decay branching ratios, taken together with the average superdeformed quadrupole moment measured in neighboring nuclei, enable the absolute M1 strengths to be determined. From these data, using the strong coupling model, we find (${\mathit{g}}_{\mathit{K}}$-${\mathit{g}}_{\mathit{R}}$)K/${\mathit{Q}}_0$=-0.14±0.01 (e b${)}^{\mathrm{-}1}$, ${\mathit{g}}_{\mathit{K}}$=-0.65±0.14 with Ω=2.8±0.8. These data are consistent with the superdeformed neutron orbital being [512]5/$2^{\mathrm{-}}$ as predicted by cranked Woods-Saxon calculations.

Systematic nuclear resonance fluorescence experiments, including model independent parity determinations, provided evidence for enhanced E1 excitations near 2.5 MeV excitation energy in the deformed nuclei ${}_{}{}^{150}{}_{}{}^{}\mathrm{Nd}$, ${}_{}{}^{160}{}_{}{}^{}\mathrm{Gd}$, and ${}_{}{}^{162,164}{}_{}{}^{}\mathrm{Dy}$. The corresponding ${\mathit{J}}^{\mathrm{\pi }}$=$1^{\mathrm{-}}$ states are interpreted as two-phonon excitations due to the coupling of octupole (J=$3^{\mathrm{-}}$,K=1) and quadrupole γ vibrations (J=$2^{+}$,K=2). The results can be explained in the framework of the sdf interacting-boson-approximation model and the dynamic collective model describing octupole vibrations and their coupling to quadrupole vibrations in deformed nuclei.

The Lyman α transitions of hydrogenlike uranium associated with electron capture were measured in collisions of stored bare ${\mathrm{U}}^{92+}$ ions with gaseous targets at the storage ring ESR. By applying x-ray–particle coincidence techniques the ground-state transition energies in the rest frame could be determined with a precision of about 600 ppm. The experimental result is in excellent agreement with the theroretical prediction for the ground-state Lamb shift in hydrogenlike uranium and is already sensitive to the 2s Lamb shift.

We have used fast-ion-beam laser spectroscopy to measure the 1s2${\mathit{s}}^3$s-1s2${\mathit{p}}^3$${\mathit{P}}^{\mathit{o}}$ intervals in Be iii to an absolute accuracy of 1 part in ${10}^8$. Our results are (in ${\mathrm{cm}}^{\mathrm{-}1}$): σ(1s2${\mathit{s}}^3$${\mathit{S}}_1$-1s2${\mathit{p}}^3$${\mathit{P}}_0^{\mathit{o}}$=28 864.6120(4), σ(1s2${\mathit{s}}^3$${\mathit{S}}_{1\mathrm{-}}$1s2${\mathit{p}}^3$${\mathit{P}}_1^{\mathit{o}}$=26 853.0534(3), σ(1s2${\mathit{s}}^3$${\mathit{S}}_{1\mathrm{-}}$1s2${\mathit{p}}^3$${\mathit{P}}_2^{\mathit{o}}$) =26 867.9484(3). These results are an improvement over previous measurements by nearly 3 orders of magnitude and show evidence of uncalculated O(${\mathrm{\alpha }}^4$${\mathit{Z}}^4$) relativistic corrections. Once those terms are computed, our measurement will allow QED calculations in two-electron atoms to be tested at the 100 ppm level.

Electron-energy-resolved measurements of the threshold shift of negative chlorine ions have been performed under the influence of a strong (4.5×${10}^{13}$ W/${\mathrm{m}}^2$) infrared laser field. The threshold shift is found to be in accordance with a full ponderomotive shift. Explanations are proposed for previously measured smaller-than-expected threshold shifts.

We report a technique for measuring the lifetimes of long-lived excited levels in highly charged ions that fall into the range from ${10}^{\mathrm{-}2}$ to ${10}^{\mathrm{-}7}$ sec, inaccessible to existing methods. Employing a fast-switching electron beam to produce and excite electrostatically trapped ions, lifetimes are determined by observing the fluorescent decay of metastable levels. A value of 90.5±1.5 μsec is obtained for the 1s2s ${}_{}{}^3{}_{}{}^{}$${\mathit{S}}_1$→1${\mathit{s}}^2$ ${}_{}{}^1{}_{}{}^{}$${\mathit{S}}_0$ transition in heliumlike neon, in good agreement with theoretical predictions of 91.1 and 92.0 μsec.

We have observed the spectrum of long range molecular states of singly excited ${\mathrm{Na}}_2$ formed by photoassociation during ultracold associative ionization collisions of laser-cooled Na atoms. The low kinetic energy of the collision allows us to obtain very high resolution spectra and to observe states close to the dissociation limit. The spectrum shows regular vibrational series characteristic of the 1/${\mathit{R}}^3$ long range portion of the $1_{\mathit{g}}$ potential (asymptotically ${\mathit{S}}_{1/2}$+${\mathit{P}}_{3/2}$) as well as molecular hyperfine structure within these peaks. We extract potential parameters and compare with recent theory.

We study the photoassociative collisional loss of laser-cooled Rb atoms from a far-off resonance optical dipole force atom trap. We obtain a well-resolved photoassociation spectrum from 50 ${\mathrm{cm}}^{\mathrm{-}1}$ to 980 ${\mathrm{cm}}^{\mathrm{-}1}$ below the first excited dissociation limit of the ${\mathrm{Rb}}_2$ molecule. Two vibrational series associated with excited ${\mathrm{Rb}}_2$ ${}_{}{}^3{}_{}{}^{}\mathrm{\Sigma }_{\mathit{g}}^{+}{}_{}{}^{}$ states are clearly visible. Oscillations in the associated Franck-Condon factors reflect the structure of the triplet ground state wave function. Our results clearly demonstrate the potential of photoassociation spectroscopy as a new probe of molecular structure.

Partial dynamical symmetry is a situation in which the Hamiltonian does not have a certain symmetry yet a subset of its eigenstates does. It is shown that partial dynamical symmetry may cause suppression of chaos even in cases where the fraction of states which has the symmetry vanishes in the classical limit. The average entropy associated with the symmetry is a sensitive quantum measure of the partial symmetry and its effect on the chaotic dynamics.

We show that in a chaotic scattering system the stable and unstable foliations of isolated chaotic invariant sets can become heteroclinically tangent to each other at an uncountably infinite number of parameter values. The first tangency, which is a crisis in chaotic scattering, provides the link between the chaotic sets. A striking consequence is that the fractal dimension of the set of singularities in the scattering function increases in the parameter range determined by the first and the last tangencies. This leads to a proliferation of singularities in the scattering function and, consequently, to an enhancement of chaotic scattering.

We show that spatiotemporally complex patterns in the presence of a symmetry imposed by the container geometry can have ordered spatial and temporal averages. The system studied was the Küppers-Lortz unstable state in rotating thermal convection. As the control parameter ΔT was increased above onset, fluctuations about the ordered averaged state increased and the correlation of individual patterns with the averaged state decreased. The pattern fluctuations about the averaged state were highly correlated with the instantaneous heat transport fluctuations.

The transition of a radial electric field from a negative to a positive value is observed in the compact helical system when the electron loss is sufficiently enhanced by the superposition of the off-axis second harmonic electron cyclotron heating on the neutral beam heated plasmas. Existence of the threshold for the enhanced particle flux required to cause the transition is experimentally certified. The observed threshold is compared with a theoretical prediction.

Schottky barrier height fluctuations of Au films on Si(100) are directly imaged with nm-scale resolution by ballistic electron emission. Fluctuations are made visible by using a highly doped (${\mathit{N}}_{\mathit{d}}$≊${10}^{17}$ ${\mathrm{cm}}^{\mathrm{-}3}$) substrate. Randomly distributed (approximately ${10}^{13}$ ${\mathrm{cm}}^{\mathrm{-}2}$) spots (about 2 nm in diameter) of reduced barrier height (typical ΔΦ=20–50 meV) are observed. The microscopic distribution of barrier heights effective in emission is consistent with mean barrier height values measured by standard techniques.

The present level of understanding of charged polymer chains is far less than that of neutral chains. Since they pose difficult theoretical and experimental tasks, they especially warrant simulation studies. We performed molecular dynamics simulations on multichain systems of flexible chains full Coulomb interactions of the monomers and counterions are treated explicitly. This model produces osmotic pressure data that agree excellently to experiments and extend them. The chain conformation changes from stretched to coiled as the density increases to semidilute. The persistence lengths show discrepancies with the Odijk et al. wormlike chain picture.

We have used light scattering to study the kinetics of polymerization-induced phase separation in a liquid-crystal–polymer mixture. The evolution of the structure factor is compared with scaling predictions for thermally quenched systems. We have also observed a cascading phenomenon where phase separated domains become unstable and undergo phase separation for a second time.

A time-dependent small-angle light scattering (SALS) study is presented which demonstrates the feasibility of electric-field-induced phase transitions in nonionic binary liquids near the critical point. Mixing of two-phase solutions induced by electric fields is shown to be a universal feature shared by a wide class of systems including upper and lower critical point polymer solutions and mixtures of low-molecular-weight molecules in a solvent. The shift of the critical temperature is computed from SALS measurements and is quadratic in the electric-field strength.

We present theoretical ideas which allow us to understand part of the scaling laws recently observed on branched cracks. We argue that some features are common to all critical branched structures, which barely survive when propagating. These ideas are illustrated by the directed percolation problem, which serves as an excellent toy model. Finally, we propose a Langevin equation for unbranched cracks in three dimensions, which naturally leads to self-affine structures.

The critical anomaly of the complex shear modulus has been studied for ideal polymer solutions in the frequency range from 1 Hz to 400 Hz. The logarithmic critical divergence is observed for both the real and imaginary parts of the complex shear modulus. It is found that the strength of the critical anomaly decreases with an increase in the degree of polymerization N, while the noncritical background increases with N, indicating the significance of the viscoelastic effect. The critical anomaly is likely suppressed by the viscoelastic effects through the spontaneous coupling between the stress field and the order parameter fluctuation.