We have studied the transition rate from metastable wells in the two-dimensional potential of a dc SQUID as a function of applied flux and temperature. We observe a crossover from thermally activated escape to macroscopic quantum tunneling at a flux-dependent temperature. The thermal rates are significantly suppressed, suggesting that the potential barrier for activation is effectively enhanced by the interaction of the macroscopic degrees of freedom in the device.

The negative Knight shift measured at 293 K in $\mathrm{Y}{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{6+x}$ is found to decrease progressively for decreasing oxygen content and to vanish below $x\approx 0.4$, in the insulating phases. The corresponding decrease of the macroscopic susceptibility is found to be dominated by the variation of the Pauli susceptibility of the Cu${\mathrm{O}}_2$ planes. These results imply that the chain contribution to the density of states at the Fermi level is not significant enough to explain the high $T_c$. It is also concluded that strong coupling between charge and spin degrees of freedom occurs in this material.

We report the discovery of three new phases in the Tl-Ca-Ba-Cu-O system, ${\mathrm{Tl}}_1{\mathrm{Ca}}_{n-1\mathrm{}}{\mathrm{Ba}}_2{\mathrm{Cu}}_n{\mathrm{O}}_{2n+3}$ $n=1,2,3\mathrm{}$, one of which $n=3\mathrm{}$ exhibits bulk superconductivity at ≃100 K. The crystal structures of these new materials are closely related to that of ${\mathrm{Tl}}_2{\mathrm{Ca}}_2{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_x$, the recently discovered 125-K superconductor. All of these compounds contain Cu perovskitelike units, but whereas these are separated by bilayer Tl-O sheets in the 125-K material, they are separated by monolayer Tl-O sheets in the new structures, enabling an examination of the effect of the coupling between successive Cu perovksite units on superconducting properties.

The dynamics of the short-range Ising spin-glass ${\mathrm{Fe}}_{0.5}$${\mathrm{Mn}}_{0.5}$Ti${\mathrm{O}}_3$ has been investigated with a SQUID magnetometer. The dynamic spin-correlation function, $q\mathrm{}\left(t\right)\mathrm{}$, as reflected in low-field ac susceptibility and time-dependent magnetization measurements, was studied in the time interval ${10}^{-6\mathrm{}}$-${10}^4$ sec. The functional form of $q\mathrm{}\left(t\right)\mathrm{}$ shows a remarkable agreement with the results obtained from Monte Carlo simulations on a three-dimensional Ising spin-glass. The temperature dependence of the relaxation times above the spin-glass temperature is well described by conventional power-law scaling.

The temperature dependence of the surface magnetization $M_S\mathrm{}\left(T\right)\mathrm{}$ in the spin-wave regime is obtained from measurement of the spin polarization of the low-energy cascade electrons. With the $T^{\frac{3}{2}}$ law, the ratio $k$ of the mean spin deviation in surface and bulk is determined, where $k$ reflects the perpendicular exchange interaction $J\perp$. In the ferromagnetic glass FeNi${\mathrm{B}}_{0.5}$, $J\perp$ depends critically on the chemistry of the first layer and is weakened even with a bulklike surface composition.

Resonant two-mode polarons due to phonon-induced electron transitions between the conduction band and the heavy-hole valence band are considered and shown to describe well the observed effect in the zero-gap semimagnetic semiconductor ${\mathrm{Hg}}_{0.95}$${\mathrm{Mn}}_{0.05}$Te. It is demonstrated that the interband electron-phonon interaction provides unique possibilities to investigate an involved structure of the Landau-Luttinger magnetic subbands in zero-gap materials.

We present a theory of quantum chaos of the Hadamard-Gutzwiller model, a quantum mechanical system which describes the motion of a particle on a surface of constant negative curvature. The theory is based on periodic-orbit sum rules that can be rigorously derived from the Selberg trace formula and which provide an exact substitute, appropriate for our strongly chaotic system, for the Bohr-Sommerfeld-Einstein quantization rules. Our recent enumeration of the classical periodic orbits enables us to evaluate the sum rules numerically and to demonstrate thereby that the theory provides also a practical method to study the quantum chaos of spectra.

We have constructed a positron reemission microscope and taken images of a number of targets using it. The unique image contrast of this device is determined by the probability that positrons are remitted from a specimen surface. The surface and near-surface defect sensitivity of the positron reemission microscope is demonstrated, as well as the feasibility of imaging biological specimens and semiconductor devices. Applications are discussed.

A 1500-Å Ni(100) film is back illuminated with 5-keV positrons from a brightness-enhanced slow positron beam. Some of the positrons diffuse through the film and are emitted from the front surface by virtue of the Ni having a negative positron affinity. An immersion objective and projector lens form a 1150× image of the reemitted positrons at a 2D position-sensitive detector. With a resolution of 3000 ± 1000 Å we observe patches attributable to positron trapping at boundary layers inside the film. The advantages and ultimate capabilities of the positron reemission microscope are also discussed.

This is a study of the nonlinear interactions of solitary waves or impulses in homogeneous, extended media. We obtain the set of ordinary differential equations for the positions of the impulses in the nearest-neighbor approximation. Solutions with constant velocity lead to pattern maps that give the successive spacings of the impulses. From among the infinitely many metastable patterns given by the maps, the system chooses one asymptotically through a hierarchical evolution.

Random walk in a one-dimensional random medium of length $N$ is analyzed. It is rigorously shown that in most realizations of the medium, the mean first-passage time, $\overline{t}$, bears the following relation to $N$, for large $N:log\overline{t}\propto \sqrt{N}$. The average of $\sqrt{t}$ over the realizations of the medium, $〈t〉$, satisfies $log〈t〉\propto N$. Our formalism, though being exact, employs only elementary means and makes transparent the physics of the delay experienced by the random walker: It is due to the existence of subsegments in which the bias against motion towards the desired end is largest. Some implications of these results concerning the replica method are briefly discussed.

A new formalism of the phase-space path integral based on our previously proposed phase-space distribution function is presented. A semiclassical theory for this path integral is also developed. It is shown that the amplitude factor associated with the semiclassical "kernel" is always finite and closely related to Greene's residue, which was devised to predict the occurrence of global chaos in classical systems.

A class of nondiagonal cylindrically symmetric algebraically general solutions of the Einstein-Maxwell equations is given. These solutions are interpretable as gravitational colliding waves supporting an electromagnetic field; they are generalizations of the Nutku-Halil solution. The new metric, derived by a Harrison transformation, is endowed with four free parameters.

Interesting limits are set on candidates for cold-dark-matter particles in the halo of our Galaxy from their interaction with a very-low-background Ge detector used to search for double-$\beta$ decay. Dirac neutrinos constituting all of dark matter are excluded for masses between 12 GeV/${\mathit{c}}^2$ and 1.4 TeV/${\mathit{c}}^2$. There are slightly better limits on magninos and cosmions, proposed massive particles which also explain the solar-neutrino problem but which interact more strongly with Ge. In addition, millicharged shadow matter is ruled out as the main form of dark matter.

It is shown that noncommutative electric fields ($E$) are essential to preserve algebraic consistency in anomalous gauge theories. A consistency condition is used to reveal different structures for the $E-E$ commutator.

An explicit example of a three-dimensional quantum field theory with exotic statistics and fractional spin is presented.

The QCD Lagrangean expressed in terms of dual potentials exhibits, at zero temperature, spontaneous symmetry breaking giving rise to dual superconductivity and therefore to color confinement. At a finite temperature, the spontaneous symmetry breaking disappears and so does confinement. We estimate the deconfining transition temperature using a high-temperature expansion.

We have studied proton-antiproton elastic scattering at $\sqrt{s}=1800\mathrm{}$ GeV at the Fermilab Collider, in the range $0.02<\mathrm{}\left|t\right|<0.13\mathrm{}$ ${(\mathrm{G}\mathrm{e}\mathrm{V}/\mathit{c})}^2$. Fitting the distribution by $\exp \mathrm{}(-B|t\left|\right)\mathrm{}$, we obtain a value of $B$ of 17.2±1.3 ${(\mathrm{G}\mathrm{e}\mathrm{V}/\mathit{c})}^{-2\mathrm{}}$.

${\chi }^2$ mappings in the necessary, but generally unsearched, sixth parameter of the ${}_{}{}^3{}_{}{}^{}P_2^{}{}_{}{}^{}-{}_{}{}^3{}_{}{}^{}F_2^{}{}_{}{}^{} N-N$ partial wave around 1 GeV reveal a strong preference for single-channel dominance of the inelasticity. This is consistent with the constraint imposed by fixing the sixth parameter in the VPI analyses but it is contrary to the hidden constraint imposed upon all other analyses of nucleon-nucleon elastic scattering.

We report the first observation of a double isovector giant dipole state in nuclei (i.e., an isovector giant dipole resonance built on another giant dipole). Two new resonances were observed in (${\pi }^{+},{\pi }^{-}$) double charge exchange (at $T_{\pi }=292\mathrm{}$ MeV) on ${}_{}{}^{\mathrm{nat}}{}_{}{}^{}\mathrm{S}$ at excitation energies of 24.7 and 28.7 MeV. The energy centroid is very close to the energy at which the double dipole ($J^{\pi }=0^{+},2^{+};T=2\mathrm{}$) state is expected to appear. The angular distributions for the resonances have a clear quadrupole shape. The measured cross sections and the angular distribution agree well with a simple sequential two-step calculation in which single charge exchange through the giant dipole resonance to the double dipole is evaluated.