The perpendicular peak velocity and critical field have been measured in a series of GaAs/AlAs superlattices as a function of the miniband width and miniband position relative to the AlAs X confined levels. The data cannot be explained by the electric-field localization mechanism [R. Tsu and G. Döhler, Phys. Rev. B 12, 680 (1975)] in the range of superlattice parameters investigated, but is qualitatively understood in terms of electron acceleration within the miniband [L. Esaki and R. Tsu, IBM J. Res. Develop. 14, 61 (1970)].

The diffusion coefficient of the dominant (g=2.0055) paramagnetic defect (D center) in amorphous silicon is determined to be less than half the diffusion coefficient for hydrogen. High-D-center densities are found to trap hydrogen. These results eliminate the possibility that the D center is responsible for hydrogen motion in hydrogenated amorphous silicon.

The coupling of a photon probe to electron-hole magnetoplasmas in quantum wells is calculated for the first time including electron-hole pairing and excitonic effects. Optical experiments on modulation-doped quantum wells support the validity of the mean-field approximation involved. The condensation of quasi-two-dimensional, neutral, electron-hole plasmas into an excitonic-insulator state is accompanied by strong modifications of magneto-optical spectra. Significant magnetic field and well-width dependences on dynamical Stark effect and exciton saturation are predicted.

The nonlinear dynamics of the main magnetoexchange branch of a circular thin (0.45 μm) film of yttrium iron garnet are investigated at perpendicular resonance. Distinct ‘‘fingers of auto-oscillation’’ arise from each magnetoexchange mode in the low-power spectrum. A set of equations of motion derived from a microscopic Hamiltonian are integrated to compare with experiment. In the first finger, the model predicts the power for onset of auto-oscillation to within 1 dB of the experiment and the frequency to within 15%.

The inelastic-neutron-scattering technique was used to measure magnetic excitations of ${\mathrm{Ho}}^{3+}$ dimers in ${\mathrm{Cs}}_3$${\mathrm{Ho}}_2$${\mathrm{Br}}_9$. The usual phenomenological models failed to reproduce the observed energy spectra. An exchange-tensor formalism, on the other hand, which treats the ${\mathrm{Ho}}^{3+}$ ground state as an effective S=2 state, provides an excellent description of the observed energy splittings and intensities.

The spectral density of the magnetic flux noise measured in high-temperature superconductors in low magnetic fields scales approximately as the inverse of the frequency and increases with temperature. We use the temperature and frequency dependence of the noise to determine the pinning energies of individual flux vortices in thermal equilibrium. The distribution of pinning energies peaks below 0.1 eV in ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ and near 0.2 eV in ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{CaCu}}_2$${\mathrm{O}}_{8+\mathrm{\delta }}$. The noise power is proportional to the ambient magnetic field, indicating that the vortex motion is uncorrelated.

It is shown that if a periodical faulting of the double-period orthorhombic (1/200) phase in ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ produces a Magneli series, ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7(\mathrm{n}/(2\mathrm{}\mathrm{n}+1)}$, with a regular superlattice spots at [1/2+1/2(2n+1),0,0], the random faulting results in either broadening the (1/200) diffraction maximum or its splitting into two maxima, (1/2±ε,0,0), depending on the stoichiometry. The observation of these effects is a strong argument in favor of an assumption that this compound is inherently inhomogeneous due to the accommodation of nonstoichiometry by means of the formation of interstitial plane defects.

Measurements of the superconducting transition temperature ${\mathrm{T}}_{\mathrm{c}}$ in the electron-doped copper-oxide compounds ${\mathrm{L}}_{2\mathrm{-}\mathrm{x}}$${\mathrm{M}}_{\mathrm{x}}$${\mathrm{CuO}}_{4\mathrm{-}\mathrm{y}}$ under pressure P to ∼20 kbar reveal that, in general, ${\mathrm{T}}_{\mathrm{c}}$ decreases with P, in contrast to the behavior of hole-doped copper-oxide compounds. The quantity d ln${\mathrm{T}}_{\mathrm{c}}$/d lnV exhibits a dependence on ${\mathrm{T}}_{\mathrm{c}}$ similar to that of hole-doped compounds, but opposite in sign, and can be as large as ∼24. Correlation of structural and transport properties indicates competition between interlayer and intralayer effects in these materials.

The frequency-dependent conductivity of laser-deposited ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ thin films shows an onset of midinfrared absorption at ∼140 ${\mathrm{cm}}^{\mathrm{-}1}$ and structure in the 400–500-${\mathrm{cm}}^{\mathrm{-}1}$ region. These low-energy absorptions occur both above and below $T_c$, making them unlikely to be the superconducting gap in the usual BCS sense. The absorption across the gap is weak because the high-$T_c$ materials are in the clean limit; this weak absorption is masked by the midinfrared absorption.

We derive scaling equations for a 2D square Heisenberg model where frustration spontaneously breaks the $Z_4$ lattice symmetry. At short distances, the model behaves as two interpenetrating Néel sublattices. Short-wavelength fluctuations couple these sublattices, driving a crossover to single-lattice behavior at long distances and generating an Ising order parameter. When the spin-correlation and crossover lengths become comparable, there exists a finite-temperature Ising phase transition independent of the subsequent development of a sublattice magnetization.

We point out that the radiative-neutrino-decay mechanism proposed by Fukugita to explain the observed excess microwave background radiation can be naturally realized in a class of gauge models, where the primary decay mode of the heavy neutrino is ${\nu }_H$→3${\nu }_e$ with a radiative branching ratio B(${\nu }_{\mathit{H}}$→${\nu }_{\mathit{e}}$+γ)≊${10}^{\mathrm{-}4}$–${10}^{\mathrm{-}5}$. These models also provide a natural understanding of the smallness of the neutrino masses via the seesaw mechanism. Constraints from muon-decay experiments require the existence of a light Higgs particle with $M_H$ below 100 GeV which couples dominantly to the neutrinos.

A geometric mapping of dimers on a square lattice onto surfaces of a 3D crystal is constructed. The mapping transforms the dimer-RVB (resonating valence bond) problem to the quantum roughening problem: Low-energy excitations correspond to the crystal surface fluctuations; holes are represented by screw dislocations. A phase diagram is discussed and the existence of a quantum-liquid state with gapless excitations is demonstrated using the results obtained previously for the roughening problem.

We show that—over a range of length scales r—the shapes of quasi-two-dimensional retinal neurons are fractal objects, and hence may be quantitatively characterized in part by their fractal dimension ${\mathrm{d}}_{\mathrm{f}}$. We analyze the shapes of numerous retinal neurons, both in vivo and in vitro. The neurons in vivo are found to have a fractal dimension ${\mathrm{d}}_{\mathrm{f}}$ of 1.68±0.15. We also propose an explanation of certain stages of neuronal shape development in terms of a diffusion-limited-aggregation model, which predicts ${\mathrm{d}}_{\mathrm{f}}$=1.70±0.1.

We report the first study of the effect of high-energy mechanical deformation on amorphous iron-based metallic alloys. The structural changes happening in amorphous iron-based materials containing Co or Ni during mechanical deformation show that the structural stability of an amorphous alloy against a thermal and a mechanical process are not related. Therefore, the concept of a high local effective temperature during the milling process cannot be singled out as the only reason for the observed structural transformations.

A new quantum mechanism for the suppression of chaotic ionization of highly excited hydrogen atoms explains the appearance of anomalously stable states in the microwave ionization experiments of Koch et al. A novel phase-space representation of the perturbed wave functions reveals that the inhibition of quantum transport is due to the selective excitation of wave functions that are highly localized near unstable periodic orbits in the chaotic classical phase space. These ‘‘scarred’’ wave functions provide a new basis for the quantum description of variety of classically chaotic systems.

We summarize the results of high-resolution simulations of cosmic-string evolution in an expanding universe. The string network is found to relax to a scaling solution in accordance with Kibble’s ‘‘one-scale model.’’ However, the long strings are found to have significant structure on scales much smaller than the scale length of the long-string network, and the size of the stable loops produced by the network is determined by this small-scale structure. This implies that the long strings rather than the loops will probably play a dominant role in the observational effects of cosmic strings.

We present measurements by the Mark II experiment of the ratios of the leptonic partial widths of the Z boson to the hadronic partial width. The results are ${\mathrm{\Gamma }}_{\mathit{ee}}$/${\mathrm{\Gamma }}_{\mathrm{had}}$=0.${037}_{\mathrm{-}0.012}^{+0.016\mathrm{}}$, &=0.${053}_{\mathrm{-}0.015}^{0.020}$, and ${\mathrm{\Gamma }}_{\mathrm{\tau }\mathrm{\tau }}$/${\mathrm{\Gamma }}_{\mathrm{had}}$=0.${066}_{\mathrm{-}0.017}^{+0.021\mathrm{}}$, in good agreement with the standard-model prediction of 0.048. From the average leptonic width result, ${\mathrm{\Gamma }}_{\mathit{ll}}$/${\mathrm{\Gamma }}_{\mathrm{had}}$=0.${053}_{\mathrm{-}0.009}^{+0.010\mathrm{}}$, we derive ${\mathrm{\Gamma }}_{\mathrm{had}}$=1.${56}_{\mathrm{-}0.24}^{+0.28\mathrm{}}$ GeV. We find for the vector coupling constants of the tau and muon ${\mathit{v}}_{\mathrm{\tau }}^2$=0.31±0.${31}_{\mathrm{-}0.30}^{+0.43\mathrm{}}$ and ${\mathit{v}}_{\mathrm{\mu }}^2$=0.05±0.${30}_{\mathrm{-}0.23}^{+0.34\mathrm{}}$.

We have measured the antiproton-proton total cross section at √s =1.8 TeV at the Fermilab Tevatron Collider; the value obtained is 78.3±5.9 mb. B, the nuclear slope parameter for elastic scattering, was measured to be 16.3±0.5 (GeV/c${)}^{\mathrm{-}2}$. From these data, we derive a value for the total elastic cross section.

An analysis of CP-violating effects in $e^{+}$$e^{\mathrm{-}}$→${\tau }^{+}$${\tau }^{\mathrm{-}}$ is outlined. Some tensor observables sensitive to CP-odd couplings arising from possible electric or weak dipole moments $d_{\tau }$,d${̃}_{\tau }$ of the τ are presented. For the case where both τ’s decay via τ→π${\nu }_{\tau }$ we estimate the accuracies with which d${̃}_{\tau }$ and $d_{\tau }$ can be measured at the Z resonance and at a τ-charm or B factory, respectively.

A self-dual field is described by the Lagrangian for an ordinary scalar field with a term added to it to take care of the self-duality constraint. A self-consistent Hamiltonian formulation is obtained using Dirac’s method. The constraints are second class, the auxiliary field drops out of the Hamiltonian, and the quantized theory does not show any violation of causality.