We have obtained a coherent understanding of spin relaxation processes of electrons, holes, and excitons in quantum walls by investigating subpicosecond dynamics of luminescence polarization. We show that the spin behavior for electrons and holes in quasi-two-dimensional systems is distinct from that in bulk semiconductors and that many-body effects and formation process play an important role in exciton spin relaxation.

The high-resolution hyperfine spectroscopy, modulated adiabatic passage of oriented nuclei (MAPON), has been applied for the first time to high-purity, elemental systems. Detailed comparisons between the electric-field gradients (efg’s) and, in particular, their distributions are obtained for ${}_{}{}^{60}{}_{}{}^{}\mathit{Co}$ where the hosts are a single crystal of hcp cobalt and a polycrystalline cobalt foil of predominantly fcc character. Further comparisons between these MAPON data and earlier ${}_{}{}^{59}{}_{}{}^{}\mathrm{Co}$ NMR data on hcp cobalt metal strongly suppport additivity of efg’s between lattice and local ion contributions.

We compare the magnetic and transport properties of the low-temperature metal in nonstoichiometric and compressed vanadium sesquioxide. Antiferromagnetic order is robust for the full range of vanadium vacancy concentrations, but it can be suppressed with modest hydrostatic pressures. The temperature-pressure phase diagram for ${\mathrm{V}}_2$${\mathrm{O}}_3$ thus includes a new T=0 boundary between antiferromagnetic and paramagnetic metals. Furthermore, analysis of the T→0 diffusive metallic conductivity at the approach to the insulator-metal transition indicates a different role for correlations depending upon the means employed to effect the transition.

The structure of the unpinned vortex fluid in layered oxide high-temperature superconductors is investigated numerically using appropriate classical liquid-state theory, for magnetic fields B perpendicular to the layer plane. The direct correlation function so obtained is used as input for a first-principles, parameter-free calculation of the discontinuous fluid-to-Abrikosov-lattice transition boundary ${\mathit{T}}_{\mathit{f}}$(B). This transition boundary is found to have a shape and location similar to that observed in strongly anisotropic materials, e.g., Bi-Sr-Ca-Cu-O.

We show that the retarded interaction between quasiparticiles on a 2D square lattice induced by the exchange of antiferromagnetic paramagnons leads uniquely to a transition to a superconducting state with ${\mathit{d}}_{\mathit{x}}^2$-${\mathit{y}}^2$ symmetry. With a spin-excitation spectrum and a quasiparticle-paramagnon coupling determined by fits to normal-state experiments, we obtain high transition temperatures and energy-gap behaviors comparable to those measured for ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$, ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{6.63}$, and ${\mathrm{La}}_{1.85}$${\mathrm{Sr}}_{0.15}$${\mathrm{CuO}}_4$.

The ${\mathrm{K}}^{+}$-ion optic phonons couple strongly to carriers on ${\mathrm{C}}_{60}$ molecules and can lead to an effective attraction at low energies. The fcc lattice is a frustrated lattice for a charge-density-wave state, opening the door to superconductivity. The competition between these instabilities is shown explicitly within mean-field theory.

Measurements of the small-angle neutron scattering by polymer clusters near the gelation threshold have been made possible, in the bulk state, using mixtures of deuterated and hydrogenated polyurethane samples. The exponent which links the scattered intensity to the transfer vector q is predicted to be (3-τ)${\mathit{D}}_{\mathit{p}}$, where τ is the exponent which characterizes the mass distribution and ${\mathit{D}}_{\mathit{p}}$ the fractal dimension of polymer clusters in the bulk state. This exponent is found to be equal to 2.00±0.05. Using this result and the τ value determined in a previous experiment on the same system, we verify the hyperscaling law and deduce ${\mathit{D}}_{\mathit{p}}$=2.50±0.06.

Simulations of a colloidal particle suspended in a two-dimensional fluid are reported. The dissipative and fluctuating hydrodynamic forces acting on the particle are modeled by a lattice gas. Our results indicate that large long-time tails are present in both the translational and the rotational velocity correlation functions; these are the first observations of a rotational long-time tail. The strong translational tail leads to an observable renormalization of the diffusion coefficient; our results suggest that experimental observation of the latter effect is possible.

The Korteweg–de Vries, modified Korteweg–de Vries, and Harry Dym hierarchies of integrable systems are shown to be equivalent to a hierarchy of chiral shape dynamics of closed curves in the plane. These purely local dynamics conserve an infinite number of global geometric properties of the curves, such as perimeter and enclosed area. Several techniques used to study these integrable systems are shown to have simple differential-geometric interpretations. Connections with incompressible, inviscid fluid flow in two dimensions are suggested.

The fluctuations of the advancing interface observed in experiments on two-phase fluid flow in porous media are investigated using several evaluation techniques. We find that the amplitudes η of these fluctuations are distributed according to a power law of the form P(η)∼${\mathrm{\eta }}^{\mathrm{-}(\mathrm{\mu }+1)}$, with μ≊2.7. Our results together with a recent model of anomalous roughening suggest a plausible explanation for the nonuniversal values of the exponents observed in the experiment on two-phase flow.

The half-life for the decay of ${}_{}{}^{238}{}_{}{}^{}\mathrm{U}$ to ${}_{}{}^{238}{}_{}{}^{}\mathrm{Pu}$ has been measured to be (2.0±0.6)×${10}^{21}$ yr by chemically isolating and measuring from the resultant alpha particles the amount of plutonium that had accumulated in 35 yr from 8.4 Kg of purified uranyl nitrate. Other sources of ${}_{}{}^{238}{}_{}{}^{}\mathrm{Pu}$ have been studied and found negligible.

We have observed suppression of ionization in Rydberg atoms irradiated by an intense short-pulse laser field. We find that single-photon ionization of the outer electron in a Ba 6snk Stark state can be severely suppressed relative to high-order (≥3 photon) nonresonant multiphoton ionization of the tightly bound 6s core electron. This population trapping occurs in intense short laser pulses, where the bandwidth of the ionizing laser is greater than the spacings between adjacent Stark states, and may be due to a substantial modification of the initial Stark state via stimulated Raman processes through the continuum. The ${\mathrm{Ba}}^{+\mathrm{*}}$ ion should be left in a higly excited superposition state, or wave packet.

We have formed polarized muonic ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ by stopping unpolarized negative muons in a gaseous ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ target which was polarized by spin exchange with laser optically pumped Rb vapor. We measured ${\mathit{P}}_{\mathrm{\mu }}^{\mathit{N}}$, the resulting muon polarization normalized to the nuclear polarization of the ${}_{}{}^3{}_{}{}^{}\mathrm{He}$. In addition to being a property that characterizes the muon cascade in the μ${\mathrm{-}}^3$He system, ${\mathit{P}}_{\mathrm{\mu }}^{\mathit{N}}$ is of practical value for future experiments on nuclear muon capture. Our result, ${\mathit{P}}_{\mathrm{\mu }}^{\mathit{N}}$=0.072±0.008, is a factor of 2 smaller than predicted by simple atomic cascade theory.

We have studied multiphoton ionization of Ba 6snl Rydberg states by intense ps pulses and have shown that the duration of the laser pulse compared to the orbit time of the Rydberg electron determines the laser-atom interaction. For high-n states with a long orbit time, the loosely bound Rydberg electron is not affected by the intense laser pulse. Instead the inner electron is ejected by absorption of five photons, leaving the ${\mathrm{Ba}}^{+}$ ion in a Rydberg state. For low-n states the outer electron is ejected and the residual ${\mathrm{Ba}}^{+}$ ion can subsequently be multiphoton ionized to ${\mathrm{Ba}}^{++}$.

We report on a successful implementation of the diabatic-by-sector method as applied to the computation of positive-energy diamagnetic Rydberg spectra of the alkali-metal atoms in a laboratory field (∼6 T). The method provides the quantum-defect parameters directly, and opens up the possibility of accounting for various spectral features quantum mechanically.

Focusing of and imaging with atoms by means of a spherical Fresnel zone plate has been observed for the first time. An intense beam of metastable helium atoms with atomic de Broglie wavelength ${\lambda }_{\mathrm{dB}}$=0.5–2.5 Å is passed through either a single or a double slip with dimensions in the 10-μm range. This transverse intensity distribution is imaged by a Fresnel zone plate, 210 μm in diameter and with an innermost zone diameter of 18.76 μm. For ${\lambda }_{\mathrm{dB}}$=1.96 Å the focal length is 0.45 m. The imaging properties of the zone plate are presented and compared with numerical calculations.

A driven-oscillator model is developed to examine the orbital-velocity harmonics of off-axis electrons in combined helical wiggler and axial guide magnetic fields. The gyroresonance effect is found to occur at harmonics of the wiggler frequency in addition to the well-known fundamental-harmonic resonance. Resonance at the first negative harmonic is potentially important for providing an explanation of the unexpected observations of a recent free-electron-laser experiment. Exploration of the gyroresonance effect as a harmonic selection mechanism for the free-electron laser is discussed.

We study interfacial pattern formation during directional growth from the isotropic phase of a cholesteric liquid crystal that, at equilibrium, has a length 2π/${\mathit{q}}_0$. We find an oscillatory first instability to the cellular pattern and a second bifurcation to an oscillatory mode (breathing mode) when the pattern’s wave number, q is 0.5${\mathit{q}}_0$<q<${\mathit{q}}_0$. The breathing-mode frequency is linear in q. This is the first observation in pattern-forming systems of an oscillatory mode due to competition between two incommensurate but comparable lengths.

We suggest a mechanism for the appearance of intermittency in fully developed turbulence consistent with the pictures recently presented by Kraichnan [Phys. Rev. Lett. 65, 575 (1990)] and She [Phys. Rev. Lett. 66, 600 (1991)]. The key features in our model are (i) an inverse cascade associated with the spectral density of an additional finite flux motion invariant, leading to (ii) an attempt to form large-scale structures which (iii) are intrinsically unstable to a broadband spectrum of perturbing modes resulting in a secondary transfer of energy to small dissipative scales in intermittent bursts.

The experimental study of a linear array of vortices shows the existence of spatially modulated states of oscillation above onset. A model of coupled oscillators is proposed, and the measurements of the corresponding first- and second-neighbor couplings show that these regimes are the result of frustration.