A microscopic but schematic model is discussed for defects (such as oxygen vacancies ${\mathrm{O}}^{\mathrm{\square }}$) in compounds such as ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7+\mathrm{\delta }}$${\mathrm{O}}_{2\mathrm{-}\mathrm{\delta }}^{\mathrm{\square }}$ and ${\mathrm{La}}_{3\mathrm{-}\mathit{x}}$${\mathrm{Ba}}_{3+\mathit{x}}$${\mathrm{Cu}}_6$${\mathrm{O}}_{14\mathrm{-}\mathrm{\delta }}$${\mathrm{O}}_{4\mathrm{-}\mathrm{\delta }}^{\mathrm{\square }}$ in relation to the superconductive properties of these materials. The discussion shows in detail how $T_c$ can reach its maximum value at or near the metal-semiconductor transition $T_{\mathrm{ms}}$. It introduces a new mechanism for enhancing of electron-phonon interactions that is separate from, and empirically superior to, Fermi-surface nesting.

We report ${}_{}{}^{63}{}_{}{}^{}\mathrm{Cu}$ nuclear spin-lattice relaxation data for ${\mathrm{Ba}}_2$${\mathrm{YC}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ in the superconducting state obtained by nuclear quadrupole resonance in zero applied magnetic field. The temperature dependences of the relaxation rates reveal striking differences in the excitation spectra for quasiparticles on the chain-forming Cu1 and planar Cu2 lattice sites, corresponding to substantially different pairing energies for electrons on the planes and chains. Values of the gap ratio 2Δ/${\mathrm{kT}}_{\mathrm{c}}$ 8.3 and 2.4 are obtained for the Cu1 and Cu2 sites, respectively.

The prompt ${\nu }_e$ signal from the supernova explosion in the Large Magellanic Cloud presumably detected by Kamiokande II does not necessarily mean that the Mikheyev-Smirnov-Wolfenstein effect on the solar-neutrino flux is not operative. The electron neutrino, once rotated to a different-flavor neutrino in the progenitor star, can come back via the matter-oscillation effect in the Earth, or a residual ${\nu }_e$ flux from the progenitor can directly hit the detector, saving the Mikheyev-Smirnov-Wolfenstein explanation of the solar-neutrino problem for a range of mixing parameters.

Replica-symmetric solutions of the graph-bipartitioning problem with finite connectivity are presented. With the constraint ${sum}_{\mathrm{i}=1\mathrm{}}^{\mathrm{N}}$${\mathrm{S}}_{\mathrm{i}}$=0 strictly enforced, another solution can be obtained, which gives a lower cost function than that given by the spin-glass solution. It is believed that this new solution is currently the best candidate for the true solution of the graph-bipartitioning problem.

We show in this Letter exact results for the Ising model on the two-dimensional Kagomé lattice with nearest- and next-nearest-neighbor interactions ${\mathrm{J}}_1$ and ${\mathrm{J}}_2$. In some regions of phase space, we find a nonzero critical temperature despite a finite zero-point entropy. For a narrow range of ${\mathrm{J}}_2$/${\mathrm{J}}_1$ we find successive transitions with a reentrance at low temperature. We studied the nature of ordering by Monte Carlo method and found that in these regions one sublattice remains disordered below the transition and down to zero temperature except in the reentrant region. Thus disorder can coexist with order at equilibrium.

We present experimental evidence for deviations from universality in the transition to chaos from quasiperiodicity in a nonlinear dynamical system. The dimension of the quasiperiodic set at the transition is 0.795±0.005, not 0.87 as predicted by the standard model based on the circle map. We also deviate from predictions of the model at the golden mean: The power spectrum, tongue convergence rate, and spectrum of critical exponents, f(α), all differ from the theory.

The relationship between the phenomenon of species doubling on the lattice and the Abelian anomaly is generalized to the case of the non-Abelian anomaly. It is shown that each species individually gives the correct consistent anomaly. In total, however, the anomaly is canceled by opposite-handed species. It is thus demonstrated that a wide class of lattice fermions can reproduce the non-Abelian anomaly by decoupling the doublers.

Within the formalism and theoretical framework of asymptotic ${\mathrm{SU}}_{\mathrm{F}}$(4) symmetry plus the constraint algebras involving the generators of the underlying symmetries of QCD, we derive an important relation between meson isoscalar-isovector matrix elements of the electromagnetic current and corresponding matrix elements of the axial-vector charge $A_{\pi }$. A lower bound is calculated for the rate Γ(ι→ργ) which indicates that Γ(ι→ργ) is of the order of 1 MeV, with its exact value depending upon the rates Γ(ι→δπ) and Γ(δ→ηπ). The estimate of the lower bound is independent of the specific quark and glue content of the ι.

The spin-spin correlation parameters $C_{\mathrm{LL}}$=(L,L;0,0)=$A_{\mathrm{LL}}$ and $C_{\mathrm{SL}}$=(S,L;0,0)=$A_{\mathrm{SL}}$ for np elastic scattering were measured for incident polarized-neutron–beam kinetic energies of 484 and 634 MeV over the center-of-mass angles from ≃80° to 180°. The data are important for determining the I=0 nucleon-nucleon amplitudes. These results are compared with phase-shift calculations.

The University of California, Irvine ${}_{}{}^{82}{}_{}{}^{}\mathrm{Se}$ double-beta-decay experiment has now accumulated 7960 h of live time. After imposition of kinematic cuts, thirteen candidate events remain in the sum-energy window between 2.0 and 3.0 MeV. If these are interpreted as neutrinoless double-beta decay with the emission of a Goldstone boson (known as the Majoron), we obtain a half-life limit of 4.4×${10}^{20}$ yr (90% confidence level). This gives, dependent upon the various calculations of the nuclear matrix elements, a limit of the majoron coupling constant ${\mathrm{g}}_{\mathrm{ee}}$≤(2–30)×${10}^{\mathrm{-}4}$. .AE

Relativistic Dirac-Brueckner calculations, based on a one-boson-exchange interaction, are presented for the equation of state of asymmetric nuclear matter. For decreasing values of the ratio Z/A it is shown that the saturation shifts to lower densities and the compression modulus at saturation drops. however, at high densities for all values of Z/A a stiff equation of state is observed, in contrast to the outcome of supernova calculations. The possibility of the occurrence of a ${\mathrm{\pi }}^{\mathrm{-}}$ condensate is discussed.

A tensorial method for the derivation of new sum rules for oscillator strengths and their moments and for expressions of similar mathematical structure is presented. For illustration, the method is applied to dipole transitions. Here are found new rules generalizing the Wigner-Kirkwood sum rules from one-electron to many-electron atoms. As a second example a new total sum rule for coherently excited states is reported and its relation to partial sum rules for fixed angular momentum transfer but incoherently excited states is shown.

We report the experimental observation of laser confinement of neutral atoms in optical-wavelength–size regions. A well collimated atomic beam was crossed at right angles by a one-dimensional standing wave and atoms were observed to be channeled into paths between the peaks of the standing wave.

We have observed subpicosecond electrical pulses to propagate on 5-μm coplanar transmission lines at velocities faster than the phase velocity in the underlying dielectric. This situation produces an electromagnetic shock wave in a manner similar to Cherenkov radiation and electro-optic Cherenkov radiation. Using time-domain spectroscopy we have measured the strong frequency-dependent loss of energy in the propagating electrical pulse due to this radiation.

A new approximation to the short-wavelength scattering amplitude from an impenetrable sphere is presented. It is uniform in the scattering angle and it is more accurate than previously known approximations (including Fock’s theory of diffraction) by up to several orders of magnitude. It remains valid in the transition to long-wavelength scattering. It leads to a new physical picture of diffraction, as tunneling through an inertial barrier.

Experiments with refractive-index–matched materials show that evaporation of a liquid from a porous medium produces a modified form of immiscible displacement. In this process the displaced fluid is removed from the pore structure by counterflow in the same pore channels as the displacing fluid. Pressure gradients that arise from fluid flow result in displacement fronts that, unlike conventional displacement fronts, are inherently stable irrespective of the relative viscosities of the fluids.

We show experimentally that the presence of static field asymmetries can affect the stability of a confined pure electron plasma. A new induced-scattering instability is observed whose threshold and growth rate depend on the amplitude of static field asymmetries. The presence of the instability is associated with a twelvefold enhancement of radial plasma transport. The qualitative features of this instability are found to be in agreement with recent theoretical work.

The equilibrium fluctuations of interfaces in two-dimensional quasiperiodic lattices, such as Penrose tilings, are considered. By a transfer-matrix formulation, this problem is mapped to the one-dimensional Schrödinger equation with a quasiperiodic potential; from the scaling properties of the eigenstates near the band edge, one can extract the exponents ζ characterizing the interface roughness. For a cosine potential, we find a true roughening transition in d=2. For a Fibonacci tiling, which approximates the Penrose tiling, we find that ζ is nonuniversal, ζ<(1/2, and ζ→0 continuously as T→0. .AE

The question whether quasicrystal shapes should be faceted is studied in a simple model of quasicrystalline order. At T=0, the model is proved to yield a completely faceted equilibrium shape in both two and three dimensions. At T>0, an interface model is derived for a two-dimensional Penrose tiling. By mapping it onto a one-dimensional quasiperiodic Schrödinger equation, we show that the roughness exponent varies continuously with T at low T.

Optical reflectance and reflectance-difference changes resulting from abrupt changes in As and Ga fluences during molecular-beam-epitaxy growth on GaAs(001) are shown to provide surface chemical information, thereby complementing the structural data available from reflection high-energy electron diffraction. Polarization and spectral dependences suggest that the observed optical anisotropies arise from absorption associated with Ga—Ga surface dimer bonds.