We have discovered that the apical oxygen with a 2.35-Å La-O bond length is removed when La is substituted by Sr in polycrystalline ${\mathrm{La}}_{2\mathrm{-}\mathit{x}}$${\mathrm{Sr}}_{\mathit{x}}$${\mathrm{CuO}}_4$ under normal preparation conditions. This apical oxygen can be partially filled by oxygen annealing. It is reasoned that a defect oxygen is trapped at an interstitial site near the Sr atom. We present evidence that this defect oxygen is intrinsic to Sr doping, independent of processing conditions. We propose that this defect oxygen serves as a mechanism for hole doping similar to that in the superconducting oxygen-rich ${\mathrm{La}}_2$${\mathrm{CuO}}_{4+\mathit{y}}$.

We investigate the single-band Hubbard model with random site energies in the strong-repulsion limit, as represented by the slave-boson formulation of the t-J model. It is essential to include the correlations between the slave-boson mean field and the disorder. This leads to the widening of the non-Fermi-liquid (‘‘generalized Mott insulating’’) region around half filling. In the Fermi-liquid regime we apply the renormalization-group method. The resulting phase diagram has superconducting, magnetic, and insulating phases. The disorder generally suppresses superconductivity and gives rise to the formation of localized magnetic moments.

Using a random-phase-approximation expression to approximate the dynamic susceptibility χ(q,ω) of a 2D Hubbard model, we have calculated the spin-relaxation rates and the neutron-scattering intensity. Here, using recent static magnetic measurements to set parameters which enter the hyperfine form factors, we show that this simple form for χ(q,ω) exhibits the features of the spin fluctuations that are required to fit the NMR data, and explore its consequences for the temperature and energy dependence of the magnetic neutron scattering.

The random-loose-packing fraction of uniform spheres at the limit of zero gravitational force is determined to be 0.555±0.005. This structure corresponds to a sphere packing at its rigidity-percolation threshold. The onset of dilatancy was also measured to be at approximately the same packing fraction. Preliminary evidence also indicates that shear thickening of suspensions with macroscopic spheres and a negative pore liquid pressure develops at the rigidity threshold. These provide further evidence that a hard-sphere glass transition could exist in the packing fraction range of 0.555 to 0.645.

The replica-symmetry-breaking (RSB) solution of the infinite-range Ising spin glass in the presence of a transverse field is obtained. The quenched free energy and the phase boundary of the glass transition temperature versus the transverse field are calculated at first-step RSB without using the static approximation. We demonstrate that replica symmetry (RS) has to be broken in the spin-glass phase by comparing the free energies of the RSB and RS solutions. No evidence is found to support an intermediate spin-glass phase with replica symmetry.

In the upper and lower quadrants of flat spacetime, (±T>‖X‖) it is possible to introduce a system of coordinates in which the metric is explicitly time dependent. The conventional interpretation of quantum field theory, based on a Fock basis, cannot be implemented in a natural fashion in such coordinates. This difficulty suggests that quantum theory is covariant only for a subset of the coordinate transformations which are allowed in the classical theory.

We study the strong phase of QED with more than one flavor ${\mathit{N}}_{\mathit{f}}$>1, and spontaneously broken chiral symmetry, using an effective Lagrangian. The effective Lagrangian contains a massless ${\mathit{e}}^{+}$${\mathit{e}}^{\mathrm{-}}$ bound state (the ‘‘electropion’’) and the photon. This theory is shown to predict the existence of a fermionic soliton. We argue that this soliton must be the electron. The mass of the electron is calculated to be in the range 2.23F<m<3.33F, where e is the electron charge and F is the electropion decay constant. Since the mass is finite, we conclude that strongly coupled QED does not show confinement.

A precise measurement of the atomic-mass dependence of dimuon production induced by 800-GeV protons is reported. Over 450 000 muon pairs with dimuon mass M≥4 GeV were recorded from targets of ${}_{}{}^2{}_{}{}^{}\mathrm{H}$, C, Ca, Fe, and W. The ratio of dimuon yield per nucleon for nuclei versus ${}_{}{}^2{}_{}{}^{}\mathrm{H}$, R=${\mathit{Y}}_{\mathit{A}}$/${\mathit{Y}}_{2\mathit{H}}$, is sensitive to modifications of the antiquark sea in nuclei. No nuclear dependence of this ratio is observed over the range of target-quark momentum fraction 0.1<${\mathit{x}}_{\mathit{t}}$<0.3. For ${\mathit{x}}_{\mathit{t}}$<0.1 the ratio is slightly less than unity for the heavy nuclei. These results are compared with predictions of models of the European Muon Collaboration effect.

Peripheral ${}_{}{}^{40}{}_{}{}^{}\mathrm{Ar}$${+}^{232}$Th collisions at a bombarding energy E/A=30 MeV were studied by measuring coincidences of fission fragments with projectile fragments detected near the grazing angle. It is found that the sequential fission of the highly excited target residue nuclei is severely hindered if their excitation energies, determined from the measured recoil momenta, exceed a value of abour 50–75 MeV. A transient time of fission of the order of ${10}^{\mathrm{-}20}$ s is deduced.

The novel phenomenon of induced focusing occurring in a self-defocusing nonlinear medium is discussed theoretically. Induced focusing of a weak optical beam occurs when it copropagates with an intense pump beam whose intensity peaks at a place different from that of the weak beam. The physical mechanism behind induced focusing is cross-phase modulation that couples the two beams. The conditions under which induced focusing can occur are discussed by solving numerically the coupled amplitude equations which incorporate the effects of diffraction, self-phase modulation, and cross-phase modulation.

The shape and the dynamics of the electron charge cloud for capture to the 2p states in ${\mathrm{B}}^{3+}$-He collisions have been studied by coupled-channel methods. We predict strong propensity for electron capture into oriented 2p states, characterized by preferred angular momentum perpendicular to the collision plane. This effect, present for velocities above 0.1 a.u. and intermediate impact parameters, is probably a general aspect of capture to multiply charged ions dominated by p-state capture. A new feature of reaction dynamics is a slippage phenomenon, i.e., the major axis of the 2p charge density lags behind the internuclear axis as the projectile traverses the turning point.

Two-color photon pairs are selected by two double apertures placed to satisfy the phase-matching conditions at a down-conversion crystal. The different wavelengths are superposed at spatially separated points on a beamsplitter and coincident two-photon detections are measured. On adjusting phase plates in the beams before the beamsplitter an apparent nonlocal fourth-order interference effect is seen which violates Bell’s inequality by several standard deviations.

The spectral and temporal response of an optical cavity resonantly coupled to an ensemble of barium atoms has been investigated experimentally. The empty-cavity trnasmission resonances are found to split in the presence of the atoms and, under these conditions, the cavity’s temporal response is found to be oscillatory. These effects may be viewed as a manifestation of a vacuum-field Rabi splitting, or as a simple consequence of the linear absorption and dispersion of the intracavity atoms.

We discuss the superdiffusive motion of a random walk in a medium containing random velocity fields. For a two-dimensional layered medium with y-dependent random velocities in the x direction u(y), 〈${\mathit{x}}^2$(t)〉∼${\mathit{t}}^{2\nu }$, with 2ν=3/2, and with strong sample-to-sample fluctuations. The probability distribution of displacements, averaged over environments, takes a non-Gaussian scaling form at large time, 〈P(x,t)〉∼${\mathit{t}}^{\mathrm{-}3/4}$f(x/${\mathit{t}}^{3/4}$), where v(u)∼exp(-${\mathit{u}}^{\mathrm{\delta }}$) for u≫1, with δ=4/3. For an isotropic two-dimensional medium with ${\mathit{u}}_{\mathit{x}}$(y)=f(y) and ${\mathit{u}}_{\mathit{y}}$(x)=f(x), we find ν=2/3 and δ=1-ν${)}^{\mathrm{-}1}$=3.

We investigate a high-Q cavity partially filled by a medium with ${\mathrm{\chi }}^3$ nonlinear susceptibility with a coherent signal having the form of a series of short pulses injected into it. We show that the electromagnetic field which builds up in the cavity has strong nonclassical features. For appropriate parameters Anderson localization occurs leading to the generation of states with a narrow photon-number probability distribution. We demonstrate that by adiabatic changing of the size of the cavity one may generate from the vacuum a nearly perfect Fock state of a chosen mode.

A relativistic electron beam propagating through a plasma in the ion-focused regime exhibits an electromagnetic instability with peak growth rate near a resonant frequency ω∼2${\gamma }^2$${\mathrm{\omega }}_{\mathrm{\beta }}$. Growth is enhanced by optical guiding in the ion channel, which acts as a dielectric waveguide, with fiber parameter V∼1(I/${\mathit{I}}_{\mathit{A}}$${)}^{1/2}$. A 1D theory for a laser making use of this instability is formulated, scaling laws are derived, and numerical examples are given. Possible experimental evidence is noted.

A spontaneous acoustic oscillation of a gas column induced by temparature gradients is periodically perturbed by an external force. Nonlinear coupling between two oscillating modes can generate abundant nonlinear phenomena, quasiperiodicity, frequency locking, onset of chaos, and quenching phenomenon. Adjusting the frequency ratio to the golden mean, we study the scaling universality for the quasiperiodic transition to chaos. The first experiment in acoustics demonstrates that thermoacoustic systems follow universal scaling properties of the circle map in spite of the complexity of the system.

Quantum turbulence in an antisymmetric-tensor model of superfluids at zero temperature is an effect analogous to vacuum polarization in electrodynamics: it is due to a quantum-mechanical instability in the presence of a strong background field. In place of pair creation there is the spontaneous formation of expanding loops of vorticity. The phenomenon can be described by a linear theory of vortices interacting with a two-index antisymmetric-tensor potential, much like electrodynamics. The rate at which turbulence sets in is estimated for supersonic flow and flow around a wedge. It is argued that turbulence and renormalization of the string mass per unit length are closely related.

The nuclear magnetic susceptibility of submonolayer ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ films adsorbed on a graphite substrate has been measured for temperatures between 6 and 800 mK. Three distinct surface-density regimes are identified for fractional monolayer coverages below perfect registry. These appear to be a fluid phase with some atoms localized by substrate heterogeneity, a solid-fluid coexistence region, and a registered solid phase with defects. ${\mathit{T}}_2$ values inferred from NMR linewidth measurements are also reported.

We report measurements of near-surface critical x-ray scattering in ${\mathrm{Fe}}_3$Al. The radial distribution of the critical diffuse intensity around a superlattice reflection exhibits a cusplike shape at the critical temperature ${\mathit{T}}_{\mathit{c}}$. From this distribution we determined for the first time the critical surface exponent ${\mathrm{\eta }}_{\mathrm{?}}$=1.52±0.04, which agrees well with the prediction for the ordinary transition of a semi-infinte Ising system and shows that the lateral correlations near the surface decay much faster than in the bulk.