We study the effect of an external magnetic field H on an individual tunneling system by measuring the time-dependent bistable conductance of a submicron Bi sample near 1 K. The amplitude of the dynamic conductance fluctuations is a random function of H. The transition rates are significantly modified by the field, from which we infer an H-dependent energy splitting. This splitting is symmetric near H=0, but exhibits nonmonotonic features at higher fields, possibly arising from the coupling of local electron-density fluctuations to the defect.

We have created one-dimensional quantum wells (quantum wires) by laterally straining a GaAs quantum well with patterned carbon stressors 180 nm in width. We find four well-resolved one-dimensional subbands in the excitation spectra, whose constant spacing of 2.4 meV confirms quantitatively that there is quantum confinement in the parabolic well predicted by continuum elasticity theory. The lateral width of the electron ground state is 35 nm.

We observe light-activated switching between discrete resistance states in GaAs/AlGaAs single-barrier tunnel structures. This switching is caused by a change in charge state of a single hole trap which modulates the current through a small region of low barrier height generated by a crystalline dislocation propagating through the barrier. Each switching event corresponds to the capture or subsequent release of a single photogenerated hole. We are therefore able to probe this single charge trap by optically inducing changes in the occupancy of its two charge states.

The Barkhausen effect was studied in an amorphous alloy. The data exhibit all the attributes of self-organized critical behavior enumerated by Bak, Tang, and Weisenfeld: The distribution of lifetimes and areas of discrete Barkhausen pulses follow power-law distributions, which have been modified to account for finite-size effects as suggested by Kadanoff, Nagel, Wu, and Zhou. The directly measured power spectral density has the form of flicker noise, with exponent and form consistent with those to be expected from the measured distribution of pulse areas and lifetimes in the light of the work of Jensen, Christensen, and Fogedby.

The Gutzwiller wave function is shown to be the exact solution of a supersymmetric t-J-type model. The model realizes a Fermi-liquid state in one dimension with a discontinuity in the momentum distribution. Analytic results are obtained for spin and charge susceptibilities, and the specific-heat coefficient with the help of the Luttinger-liquid theory. In the high-density limit the model exhibits a Mott-Hubbard gap and reduces to an antiferromagnetic spin chain with long-range exchange solved by Haldane and Shastry.

Using the technique of spin-polarized tunneling, we studied tunnel junctions with EuS/Al bilayer electrodes. We found a large effective internal field in the Al film, which gives rise to extra Zeeman splitting in the superconducting quasiparticle density of states and which is attributed to the exchange interaction between the Eu ions and the Al conduction electrons. This exchange field, acting only on the electron spins, is inversely proportional to the thickness of the Al, causes no observable orbital depairing in the Al, and leads to a first-order transition to the normal state.

The magnetic-flux noise generated by a patterned film of ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathit{x}}$ is reversibly suppressed by the application of a supercurrent. The total noise power decreases by an order of magnitude, in quantitative agreement with a model involving thermally activated vortex hopping between pairs of pinning sites of nearly equal pinning energy. The model yields a force constant of 6×${10}^{\mathrm{-}5}$ N/m for a vortex at a typical pinning site, and a hopping distance which increases with temperature from 13 nm at 4.2 K to 64 nm at 60 K. The number density of vortices is at least 0.02 μ${\mathrm{m}}^{\mathrm{-}2}$, even in a magnetic field of less than 1 μT.

We report the first measurement of the positron-electron momentum density in superconducting single-crystal ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{CaCu}}_2$${\mathrm{O}}_{8+\mathrm{\delta }}$ (${\mathit{T}}_{\mathit{c}}$≊90 K). The observed anisotropy exhibits a twofold (rather than fourfold) symmetry, which is attributed to the superlattice modulation along the b axis of the ${\mathrm{BiO}}_2$ layers. Subtraction of the superlattice contribution also reveals a pair momentum distribution consistent with the ${\mathrm{CuO}}_2$ and ${\mathrm{BiO}}_2$ Fermi surfaces, and in reasonable agreement with the theoretical pair momentum density derived from band theory.

We have studied the thermally activated flux-flow resistance in ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$/${\mathrm{PrBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ (YBCO/BrBCO) artificial superlattices in magnetic fields perpendicular to the a-b plane. We find for multilayers with thick enough (>24 Å) insulating PrBCO layers and for YBCO thicknesses smaller than 300 Å that the activation energy has a logarithmic magnetic field dependence and that is is proportional to the YBCO thickness in the multilayer.

The superconducting properties of ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$-based superlattices are shown to depend strongly on the electronic properties of the barrier layers. The resistive transition width decreases significantly as the hole carrier density in the barrier layers is increased. However, ${\mathit{T}}_{\mathit{c}}$(onset) does not change, contrary to predictions of hole-filling models. ${\mathit{T}}_{\mathit{c}}$(onset) is apparently determined by the ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ layer thickness, while the transition width, determined by long-range phase coherence of the superconducting wave function, depends on the electronic properties of the isolating barrier layers.

We have found for the first time that superconductivity can occur in a one-unit-cell-thick layer of ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ (YBCO). The layer is grown, while monitoring with RHEED oscillations, on a superconducting ${\mathrm{PrBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ (PrBCO) buffer layer and covered by a PrBCO layer by reactive evaporation. A reduction of the onset temperature is found to be mainly due to a decrease of the hole carriers but not due to the absence of interlayer couplings. This covering PrBCO layer has been found to provide the YBCO layer with carrier holes sufficient to induce superconductivity.

The general form of the Green’s-function solution for the spherically symmetric Smoluchowski equation serves as a unified starting point for deriving identities. This approach enables one to obtain a nonlinear differential equation for the dependence of the Green’s function on the location of the boundary, relations between Green’s-function solutions for various boundary conditions, and their asymptotic expansions.

Recent simulations of the evolution of cosmic strings indicate the presence of small-scale structure on the strings. We show that wakes produced by such ‘‘wiggly’’ cosmic strings can result in the efficient formation of large-scale structure and large streaming velocities in the Universe without significantly affecting the microwave-background isotropy. We also argue that the motion of strings will lead to the generation of a primordial magnetic field. The most promising version of this scenario appears to be the one in which the Universe is dominated by light neutrinos.

A hadron mass calculation is reported using two flavors of dynamical, staggered fermions with masses of ma=0.01, 0.015, 0.02, and 0.025 on a ${16}^3$×32 lattice. The masses of hadrons containing valence quarks with ma=0.004, 0.07, 0.1, 0.14, and 0.25 were also computed to study the limit ma→0 and the strange particle spectrum. For ma=0.01, the mass ratio ${\mathit{m}}_{\mathit{N}}$/${\mathit{m}}_{\mathrm{\rho }}$=1.527(11) is 25% larger than experiment while the results ${\mathit{f}}_{\mathrm{\pi }}$/${\mathit{m}}_{\mathrm{\rho }}$=0.14(1) and (${\mathit{m}}_{\mathit{K}}^2$-${\mathit{m}}_{\mathrm{\pi }}^2$)/(${\mathit{m}}_{\mathit{K}}^{\mathrm{*}}$-${\mathit{m}}_{\mathrm{\rho }}$)${\mathit{m}}_{\mathrm{\rho }}$=2.25(3) agree somewhat better.

Arguments based on unitarity indicate that hadronic loop diagrams should produce large violations of the Okubo-Zweig-Iizuka rule. The mechanism by which these corrections are evaded has long been a mystery. We have found that there is an exact cancelation of all such loops in a particular limit and that, at least for the ρ-ω-φ system which we have studied in detail, the cancellation is maintained in a realistic calculation which takes into account departures from this limit.

We study photoproduction of η mesons off protons in the effective Lagrangian approach. Unlike the ${\mathrm{\pi }}^0$ case, η photoproduction at threshold is strongly dominated by resonance excitation. The quantity (χ${\mathrm{\Gamma }}_{\mathrm{\eta }}$${)}^{1/2}$${\mathit{A}}_{1/2}$/${\mathrm{\Gamma }}_{\mathit{T}}$, characteristic of the photoexcitation of the ${\mathit{S}}_{11}$(1535) resonance and its decay into the η-nucleon channel, of interest to hadron models, is determined to be (0.22±0.02)×${10}^{\mathrm{-}3}$ ${\mathrm{MeV}}^{\mathrm{-}1}$ from the existing data.

The coupling constant of the charged pions to nucleons is extracted from a partial-wave analysis of antinucleon-nucleon scattering data below ${\mathit{p}}_{\mathrm{lab}}$=950 MeV/c. For the value at the pion pole we find ${\mathit{f}}_{\mathit{c}}^2$=0.0751±0.0017 or equivalently ${\mathit{g}}_{\mathit{c}}^2$=13.6±0.3. This result is in agreement with the value found in the recent VPI&SU analysis of πN scattering data. Comparing with the neutral-pion coupling constant as determined in the Nijmegan phase-shift analysis of proton-proton scattering data, we see no evidence for a charge dependence of the pion-nucleon coupling constants.

Prescission charged-particle multiplicities, following fusion of ${}_{}{}^{164,167,170}{}_{}{}^{}\mathrm{Er}$${+}^{28}$Si, have been measured. The multiplicities at the lowest bombarding energies limit the statistical model level density parameters. More importantly, the α data restrict the time spent near equilibrium and suggest evaporation occurs predominantly from larger deformations.

The relative strength (ΔS=1)/[(ΔS=0)+(ΔS=1)] of isovector spin excitations, ${\mathit{P}}_{\mathrm{sf}}$, in the high-lying resonance region of ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$ was investigated with the ${(}^7$Li${,}^7$Be) reaction at ${\mathit{E}}_{\mathit{L}}$=26 MeV/nucleon and ${\mathrm{\theta }}_{\mathit{L}}$=0° by separately measuring the ${(}^7$Li${,}^7$${\mathrm{Be}}_{\mathrm{g}.\mathrm{s}.\mathrm{}}$) and ${(}^7$Li${,}^7$${\mathrm{Be}}_{0.43}$ MeV) reaction channels with the ${}_{}{}^7{}_{}{}^{}\gamma$ coincidence technique. The relative strength ${\mathit{P}}_{\mathrm{sf}}$ was derived up to an excitation energy of 18 MeV in ${}_{}{}^{12}{}_{}{}^{}\mathrm{B}$ and found to have a constant value of 0.4–0.5 for the continuum irrespective of excitation energy and 0.5 for the isovector dipole resonance centered at ${\mathit{E}}_{\mathit{x}}$=7.6 MeV.

The mixed-symmetry $2^{+}$ state of ${}_{}{}^{56}{}_{}{}^{}\mathrm{Fe}$ is investigated by a large-scale shell-model calculation. We can reproduce the experimental energy levels by the Kuo-Brown interaction, as well as the E2 and M1 transition probabilities. The (e,e’) form factors are also reproduced by including the core-polarization effect. By inspecting the shell-model wave functions thus tested, it is found that the $2_2^{+}$ and $2_4^{+}$ states share a large fraction of the mixed-symmetry component.