We compare the classical and quantum mechanics of a system whose classical dynamics are dominated by chaos. We observe a quantum "scars" of short periodic orbits. By examining the states on a quantum surface of section, we find that the stable and unstable manifolds of the periodic orbits have a clear effect on the eigenstates, in addition to the orbits themselves. A local density operator, constructed from a band of energy eigenstates, is strongly scarred by short periodic orbits. We also demonstrate a relation between the quantum eigenvalue spectrum and the actions of the periodic orbits.

Ab initio molecular $R$-matrix calculations are presented for collisions of low-energy positrons with the hydrogen fluoride molecule as a function of HF internuclear separation. Calculations presented are for $\Sigma$ total symmetry with and without polarization effects. These calculations mirror recent electron-HF scattering calculations by Morgan and Burke. Strong evidence is obtained for the presence of a bound state of the $e^{+}\mathrm{HF}$ system. The experimental consequences of this result are discussed.

The angular distribution of departing ${\mathrm{N}}^{+}$ ions from ${\mathrm{N}}_2$ stimulated by $K$-shell photoexcitation has been measured by use of a time-of-flight mass spectrometer. A detailed analysis of the flight-time spectra shows that at 401 eV, i.e., the resonance transition from $1s{\sigma }_{\mathrm{u}}$ to $2p{\pi }_{\mathrm{g}}$, the asymmetry parameter $\beta$ is close to -1. Between 405 and 410 eV, $\beta$ shows an oscillatory variation due to the core-electron excitations to the Rydberg states. In the ${\sigma }_{\mathrm{u}}$-type shape resonance, $\beta$ is larger than 0. The results obtained are discussed in relation to the characteristics of the $K$-shell excited states.

Resonancelike structures have been observed by means of photon-induced fluorescence spectroscopy in the $\mathrm{Ar}-3s$ photoionization cross section between the threshold and the Cooper minimum. They coincide with resonances classified in published absorption spectra of Ar by ${3s}^2{3p}^4\mathrm{nl}{n}^{\prime }{l}^{\prime }$ excitations. Moreover, structures were found to correlate with resonances in the satellite production also studied by photoninduced fluorescence spectroscopy.

A nondestructive type of measurement, thermal-wave spectroscopy, is proposed to determine the porosity, the distribution of length scales, and the dynamically connected pore-size parameter, $\Lambda$, of an electrically insulating porous medium. The idea is to heat selectively with a modulated source the water-saturated porous medium and to study the resulting temperature distribution, e.g., by monitoring the electrical conductivity. Exact expressions for the low- and high-frequency behavior are derived.

The nonlinear dynamics of transverse particle oscillations in the Fermilab Tevatron is studied experimentally and compared with prediction. Accurate measurements of various phase-space features are obtained. A theoretically expected metastable state of the accelerator, with particles captured on nonlinear resonance islands, is demonstrated experimentally, and stability of the state is investigated.

We have measured the wake fields induced by short, intense relativistic electron bunches in a slow-wave structure consisting of a dielectric-lined tube, as a test of the dielectric wake-field acceleration mechanism. These fields were used to accelerate a second electron bunch which followed the driving bunch at a variable distance. Results are presented for different dielectrics and beam intensities, and are compared with theoretical predictions.

It is demonstrated that, independent of its polarization, an electromagnetic wave packet propagating along a plasma-vacuum boundary excites surface waves associated with the boundary. The corresponding rate of loss is evaluated and it is shown that the rate is a maximum when the wavelength of surface wave is comparable with the width of the packet in the direction of propagation of the surface wave.

An entirely analytic treatment for second-harmonic mode conversion is presented, which uses the wave-phase-space $\mathbf{x}-\mathbf{k}$ space) method. Explicit expressions are derived for the coefficients of transmission, reflection, conversion (magnetosonic wave to ion-Bernstein wave), and absorption. A conservation law for wave energy flux in phase space is also presented.

Measurements of three-halves harmonic emission from laser-produced plasmas are presented. The emission from molybdenum targets irradiated with 1.06-μm laser light is simultaneously spectrally and spatially resolved. The data show a clear spatial separation between the blue-shifted and red-shifted components of the three-halves emission. Spectral structure suggests that ion mode coupling is involved in the scattering process.

Solidification controlled by vacancy diffusion is studied by Monte Carlo simulations of a two-dimensional Ising model defined by a Hamiltonian which models a thermally driven fluid-solid phase transition. The nonequilibrium morphology of the growing solid is studied as a function of time as the system relaxes into equilibrium described by a temperature. At low temperatures the model exhibits fractal growth at early times and crossover to compact solidification as equilibrium is approached.

We study the relaxation of 2D quasicrystalline elastic networks when their constituent bonds are perturbed homogeneously. Whereas ideal, quasiperiodic networks are stable against such perturbations, we find significant accumulations of strain in a class of disordered networks generated by a growth process. The grown networks are characterized by root mean square phason fluctuations which grow linearly with system size. The strain accumulation we observe in these networks also grows linearly with system size. Finally, we find a dependence of strain accumulation on cooling rate.

A new technique, combining a pulsed supersonic molecular-beam source and time-resolved surface infrared spectroscopy, has been devised to measure the microscopic diffusion of molecules at surfaces. Following a rapid dose of a periodically stepped Pt(111) crystal face to a low total coverage of CO (∼0.006 monolayer), the time evolution of CO molecules migrating from terrace to step sites was monitored with a temporal resolution as fast as 5 msec. When compared with a simple kinetic model, the data reveal statistical microscopic hopping rates for CO diffusion on the (111) plane over the $T=95-195\mathrm{}$ K range investigated.

Time, pressure, and temperature dependences of the optically determined surface coverage by Ga-containing species reveal that the kinetic limits to crystal growth in atmospheric-pressure organometallic chemical-vapor deposition on (001) GaAs are determined by reversible excluded-volume chemisorption and subsequent decomposition of trimethylgallium at surface lattice sites.

The motion of hydrogen in the B-H complex in silicon has been studied. An applied stress is used to produce a preferential alignment of the B-H complex at temperatures sufficiently high for the H to move within the complex (above ∼60 K). This alignment of the complexes is detected by comparing the optical-absorption strengths of the H stretching vibration for light polarized parallel and perpendicular to the stress axis. From the kinetics of the decay of alignment after the stress is removed we show that the motion of H from bond-centered site to bond-centered site around the B atom is thermally activated with an activation energy of 0.19 eV.

A new theoretical method of the multi-configuration self-consistent field type is proposed to treat the Anderson-localized states in strongly correlated disordered systems. Performing a cluster simulation for a Si:P system below the critical concentration, we find that most of the electrons form spin-singlet pairs and the remaining bonds are related to spin-triplet pairs. The nature of the metal-insulator transition is reflected by a single Slater determinant constructed from extended one-electron orbitals.

The full time evolution of the coherent deexcitation of the 14.4-keV nuclear resonance, excited by synchrotron radiation in a single crystal of the simple antiferromagnet ${}_{}{}^{57}{}_{}{}^{}\mathrm{Fe}_2^{}{}_{}{}^{}$${\mathrm{O}}_3$, has been measured. The buildup from zero decay probability at time zero following excitation, increasing to a maximum 2.4 nseclater, has been observed experimentally for the first time. The results are in good agreement with calculations based on the dynamical theory of nuclear Bragg scattering.

Magnetoresistance measurements in a $\frac{{\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}\mathrm{As}}{\mathrm{GaAs}}$ heterojunction with a gate covering a small region of the sample reveal quantized values for certain ranges of gate voltage; these are explainable in terms of a Landauer resistance formula. The quantization occurs when the voltage probes are located in a region of dissipationless current flow, where the quantum Hall effect provides a physical realization of ideal leads. The measurements distinguish dramatically between different multichannel generalizations of the Landauer formula.

The quantized Hall effect comprises edge states flowing in opposite directions on opposite edges of a two-dimensional electron gas. In a narrow wire, the presence of a potential barrier in the wire can cause the state to be reflected back into the oppositely directed state at the outer edge of the sample. Experiments on narrow (∼1 μm) $\mathrm{GaAs}-{\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}\mathrm{As}$ wires with short cross gates confirm that backscattering is caused by the barrier drawn up by the gate. The barrier, by reflecting some fraction of the states, causes quantized plateaus in any four-probe measurement of resistance.

The torque on a superconductor in a magnetic field $H$ has been thought to be dominated by trapped flux or sample shape effects, but it has recently been suggested that an anisotropic type-II material should experience an intrinsic torque for $H_{c1\mathrm{}}\ll H<\mathrm{}{H}_{c2\mathrm{}}$. The predicted phenomenon results from a transverse magnetization of the Abrikosov lattice. Measurements are presented on copper-oxide superconductors which delineate the experimental regime in which extrinsic effects are negligible and confirm the existence of the predicted intrinsic torque.