Mixtures of polyisoprene and poly(ethylene-propylene) have been studied using time-resolved elastic light scattering. For off-critical quenches just into the two-phase coexistence region, highly monodisperse spheres are observed, whose radii grow with a power law R(t)=${\mathit{R}}_0$${\mathit{t}}^{1/2}$. The monodispersity and growth law are explained as a heterogeneous nucleation process with a simple model equation for the growth of isolated spheres, and allow the study of the kinetics of domain growth for metastable thermodynamic processes in unprecedented detail.

We report the observation of two kinds of novel nonhydrodynamic spin-transport phenomena of quasiparticles in superfluid ${}_{}{}^3{}_{}{}^{}\mathit{B}$. We find a drastic low-temperature depression of the transverse quasiparticle spin-diffusion coefficient. In addition, we have done the first measurements of the Leggett-Takagi spin-relaxation time far outside the hydrodynamic regime. The observations of spin diffusion are shown to be in quantitative agreement with a kinetic-equation approach to quasiparticle spin dynamics.

The photodoping process in Ag/${\mathrm{As}}_{25}$${\mathrm{S}}_{75}$ layers subjected to hydrostatic compression up to 100 kbar has been studied. With an increase in pressure, the response becomes faster and then quenched above a threshold pressure, which is 70–80 kbar when light is incident upon the Ag surface. The characteristic suggests that migration of Ag ions between As-S molecular clulsters and/or accumulation of electronic carriers in the Ag-doped region is responsible for the photodoping.

Crystalline ${\mathrm{Ni}}_3$B ribbons have been irradiated at low temperature with GeV heavy ions (Kr,Xe,U) in order to study the disordering process induced in this material by ion electronic energy loss. The atomic rearrangements produced by irradiation were monitored in situ via electrical resistance measurements and characterized after annealing at room temperature by electron diffraction. The results show a huge electrical resistivity increase above an electronic-energy-loss threshold, which can be ascribed to partial amorphization of the irradiated alloy.

We examine the classical diffusion of independent adatoms on periodic substrates using a recently developed microscopic theory. We show how the universal properties of surface diffusion arise from this theory in the high-friction limit at low temperatures. At high temperatures, the theory crosses over correctly to that of a Brownian particle in a viscous medium.

Low-energy electron-diffraction observations indicate the existence of quasicrystalline order at surfaces parallel and perpendicular to the periodic tenfold axis of the decagonal quasicrystal ${\mathrm{Al}}_{65}$${\mathrm{Cu}}_{15}$${\mathrm{Co}}_{20}$, and of a disordering transition near 715 K at the surface perpendicular to the tenfold axis.

The forces on dopant impurities and other defects are crucial in determining defect motion and diffusion in semiconductor heterostructures. Impurity ions and other charged defects feel electrostatic forces, just as electrons do. However, at heterojunction interfaces, electrons feel additional forces associated with band-edge distontinuities. Here, the analogous forces for ions and charged defects are derived, and given a simple physical interpretation. The donor or acceptor level is found to play much the same role for the ion that the band edge plays for the electron or hole in defining the effective potential. These forces can in general be spatially discontinuous, because of their dependence on charge state.

Optical-absorption and luminescence measurements on ${}_{}{}^{13}{}_{}{}^{}\mathrm{C}$ and ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$ diamonds establish that the indirect energy gap is 13.6±0.2 meV higher for ${}_{}{}^{13}{}_{}{}^{}\mathrm{C}$ than for ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$ diamond. Zero-parameter calculations, allowing for the changes in electron-phonon coupling and lattice parameter, give an estimated shift of 16.5±2.5 meV.

The electronic structure and bonding in the ${\mathrm{Y}}_2$${\mathrm{O}}_3$ crystal is completely elucidated by first-principles self-consistent band-structure and optical calculations. Results agree well with experiments. The unique type of bonding in ${\mathrm{Y}}_2$${\mathrm{O}}_3$ is emphasized; it does not support the fully ionic picture and is best descibed as ${\mathrm{Y}}_2^{+2.16\mathrm{}}$${\mathrm{O}}_3^{\mathrm{-}1.44}$.

The full semiconductor Bloch equations are solved to analyze the ultrafast exciton dynamics in semiconductors. Transient adiabatic following is obtained for large detunings and arbitrary intensities, leading to exciton Stark shift, bleaching, and recovery on the time scale of the pump pulse. The many-body Coulomb effects strongly influence the semiconductor response for resonant excitation conditions, almost doubling the effective Rabi frequency of the applied field.

Treating the electrons in the Hartree-Fock approximation, we study the wave-vector dependence of the spin-density excitations and the charge-density excitations of a double-quantum-well (DQW) system in a strong magnetic field with total ν=1. We conclude that as the distance between the wells increases, the DQW system undergoes a phase transition, probably to a charge-density-wave state. We discuss the possible effects of this phase transition on the quantum Hall effect.

We have measured the positions of the longitudinal resistance peaks in the fractional quantum Hall regime and found that they agree well with those predicted by theory in which quantum effects are crucial to the existence of extended states.

We have measured the magnetoconductance in the hopping regime of two thin films of crystalline ${\mathrm{In}}_2$${\mathrm{O}}_{3\mathrm{-}\mathit{x}}$. The films are 18 nm thick and have large (macroscopic) areas 2×2 and 1×0.3 ${\mathrm{mm}}^2$. We observe a large positive magnetoconductance and reproducible conductance fluctuations. The conductance fluctuations are easy to observe below 100 mK and their amplitude is consistent with simple ensemble averaging of phase-coherent regions. When the field is rotated into the plane of the film, the field scale for the conductance fluctuations increases by a factor of 2 suggesting that they are partly due to orbital effects.

The anomalous broadening of the ${\mathit{B}}_{1\mathit{g}}$-like phonon of R${\mathrm{Ba}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ observed in Raman scattering is studied as a function of phonon frequency by substituting various rare earths for R. The onset of broadening starts abruptly at a phonon energy which allows us to unambiguously determine a single, sharp superconducting gap at 2Δ=316 ${\mathrm{cm}}^{\mathrm{-}1}$. 2Δ/${\mathit{kT}}_{\mathit{c}}$=4.95±0.10, i.e., R${\mathrm{Ba}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ is a strong-coupling superconductor. We find qualitative and quantitative agreement with theory.

Reversible flux penetration in uniaxial layered superconductors is studied, taking into account the possible trapping of vortex cores between layers. This is found to induce high intrinsic critical currents parallel to the layers. In low and intermediate fields, the flux lines experience a lock-in transition towards the layer plane, as a function of the field direction or intensity. Application to anisotropic high-${\mathit{T}}_{\mathit{c}}$ materials is discussed.

It is argued that with increasing applied magnetic field, a disordered, superconducting thin film will undergo a zero-temperature transition into an insulating state. At this superconductor-insulator transition the field-induced vortices Bose condense. A scaling theory for this field-tuned transition is described. Right at the transition, both the longitudinal and Hall resistivities are predicted to be finite, nonzero, and have universal values.

A magnetic field is used to tune through a new superconducting-insulating transition of amorphous-composite indium oxide films at various stages of disorder. The results are in accord with scaling theory which identifies a universal sheet resistance separating a superconducting phase of localized vortices and Bose-condensed electron pairs from an insulating phase of Bose-condensed vortices and localized electron pairs. A unity dynamical exponent is confirmed and scaling behavior of the resistance over a wide range of temperatures and magnetic fields is found.

We present the results of Monte Carlo simulations for a variant of the chiral clock model in an attempt to clarify the microscopic origins of the ripple (${\mathrm{P}}_{\mathrm{\beta }\prime }$) phase that occurs in cewrtain lipid bilayers. Using independently determined values of the molecular interactions the simulations exhibit a ripple phase whose properties are in good agreement with certain features of synchrotron x-ray data and scanning tunneling micrographs of the ${\mathrm{P}}_{\mathrm{\beta }\prime }$ phase. The simulation data are compared to mean-field results which suggest the possibility that the ripple phase of the model is metastable. © 1990 The American Physical Society

The coherent scattering rate from n particles bound in a periodic lattice of N potential wells is considered. The quantum theory predicts a total coherent scattering rate proportional to ${\mathit{n}}^2$/N, whereas the conventional semiclassical approach predicts a rate proportional to n. These effects allow an experimental test of certain nonlocal assumptions inherent in quantum statistical mechanics and may be useful in analyzing the properties of crystalline materials.

A path-integral quantization is presented for the chiral Schwinger model on a Riemann surface. Gauge invariance is maintained by integrating over all gauge potentials without the usual gauge fixing. All local degrees of freedom of the gauge field are suppressed after the integration of the anomalous effective action over a gauge orbit. The resulting theory is a topological one for the surviving global gauge excitations. The general implications for consistent quantization of chiral gauge theories are also discussed.