We have searched for the two-photon production of the ${\eta }_c$, ${\chi }_0$, and ${\chi }_2$ charmonium states at the SLAC $e^{+}{e}^{-}$ collider PEP in the channels $\gamma \gamma \to K^{\pm }{K}_S^0{\pi }^{\mp }$, $\gamma \gamma \to K^{+}{K}^{-}{\pi }^{+}{\pi }^{-}$, $\gamma \gamma \to {\pi }^{+}{\pi }^{-}{\pi }^{+}{\pi }^{-}$, and $\gamma \gamma \to K^{+}{K}^{-}{K}^{+}{K}^{-}$. We identify four ${\eta }_c$ candidates in the $K^{+}{K}^{-}{K}^{+}{K}^{-}$ channel on a negligible background; the one $\phi \phi$ event among them implies a 95%-confidence-level (C.L.) lower limit for ${\Gamma }_{\gamma \gamma }\left({\eta }_c\right)$ of 1.7 keV. In the other channels we find no evidence for any of the three states. We establish 95%-C.L. upper limits ${\Gamma }_{\gamma \gamma }\left({\eta }_c\right)<\mathrm{}15.5\mathrm{}$ keV, ${\Gamma }_{\gamma \gamma }\left({\chi }_0\right)<\mathrm{}17.0\mathrm{}$ keV, and ${\Gamma }_{\gamma \gamma }\left({\chi }_2\right)<\mathrm{}4.2\mathrm{}$ keV. From all channels combined, we obtain the value ${\Gamma }_{\gamma \gamma }\left({\eta }_c\right)={6.4}_{-3.4\mathrm{}}^{+5.0\mathrm{}}$ keV.

Multihadronic $e^{+}{e}^{-}$ annihilation events containing final-state leptons have been investigated with a 4.7-${\mathrm{pb}}^{-1\mathrm{}}$ data sample at c.m. energies of 50 and 52 GeV. The number of low-thrust events ($T\le 0.8$) with isolated leptons is consistent with the expectation of the standard model with five quark flavors. Limits on the production of new quarks and leptons are reported.

We have measured relative cross sections in the angular range of 26° to 65° at 8.5 and 30 eV. Our 30-eV data show a monotonic increase with decreasing scattering angle, in disagreement with three theoretical calculations which predict a structure of the cross section in this range, but in qualitative agreement with a prediction of Joachain and Potvliege, derived from optical-potential calculations at and above 100 eV. At 8.5 eV, below all inelastic thresholds, our data agree very well with the theoretical results of McEachran and Stauffer as well as Nakanishi and Schrader but disagree with other calculations.

Numerically exact, fully three-dimensional quantum mechanical reactive scattering calculations are reported for the ${\mathrm{H}}_2$Br system. Both the exchange (H + H′Br→ H′ + HBr) and abstraction (H + HBR→ ${\mathrm{H}}_2$ + Br) reaction channels are included in the calculations. The present results are the first completely converged three-dimensional quantum calculations for a system involving a highly exoergic reaction channel (the abstraction process). It is found that the production of vibrationally hot ${\mathrm{H}}_2$ in the abstraction reaction, and hence the extent of population inversion in the products, is a sensitive function of initial HBr rotational state and collision energy.

A theory, supported by new experimental data, for recently observed anomalous large pseudospark and back-lighted thyratron cathode emission into a superdense glow discharge is reported. The current density at the cathode surface, ≅ 10 kA/${\mathrm{cm}}^2$, is produced by an ion "beam," extends over a surface area of ≅ 1 ${\mathrm{cm}}^2$, and is orders of magnitude larger than that of heated thermionic cathodes.

It is proposed that the ponderomotive force due to applied ion-cyclotron-resonance-frequency waves can stabilize the internal-kink mode in tokamaks. The sufficient stability criterion is derived and the necessary power estimated. It is concluded that at the rf power level present in the Joint European Torus experiment, the ponderomotive force effects are significant and may be responsible for the modification of the sawtooth activity observed in recent experiments.

Several recently proposed models for the interatomic potential in silicon are used to generate the interatomic force constants. These are then used to calculate the elastic constants and selected normal-mode frequencies. A comparison of the calculated and experimental values provides a test of the models. None of the models tested is completely satisfactory.

The ${\mathrm{Cu}}_3$Au(100) surface has been studied close to the bulk order-disorder transition temperature $T_0$ by x-ray diffraction under total external reflection. The order-parameter profile which has been probed in various depths is analyzed in terms of a surface-induced disorder transition. It is demonstrated that the temperature dependence of the superlattice intensity is consistent with the assumption of a disordered surface layer which exists between the ordered bulk and the vacuum at temperatures below $T_0$ and grows logarithmically as $T_0$ is approached.

A new model of a hierarchical space with arbitrary transfer rates is introduced. The topology is determined by a self-similar coupling scheme. It is closely related to the topology of a corresponding ultrametric space. The model allows exact solutions to the random-walk problem, the intermediate scattering function, and the autocorrelation function. The solutions are identical in structure to the exact solution of the problem of energy transfer in disordered media which corresponds to parallel rather than to sequential coupling schemes.

We present the first precise characterization of the profile of the precursor films which progressively develop ahead of the macroscopic edge of a spreading liquid drop. Experiments are performed on a model system (nonvolatile high-molecular-weight polydimethylsiloxane on smooth silicon wafers), using an ellipsometer with a high spatial resolution. Both the film profile and its evolution with time are in good qualitative agreement with recent models which include long-range force effects. Clear quantitative discrepancies may reveal specific polymeric effects.

The scaling behavior of interfaces is studied for ideal and random quasicrystals in two and three dimensions, and its consequences for the equilibrium crystal shape are discussed. For a 3D decagonal phase, a facet with a fivefold symmetry axis is found to undergo a roughening transition. For a 3D icosahedral phase, such a facet is likely to stay smooth at all temperatures, $T$, in the ideal case, but is predicted to be rough on sufficiently large scales for $T>\mathrm{}0\mathrm{}$ in the random case. The singular behavior of the equilibrium crystal shape near the edge of a facet is also determined.

Scanning tunneling microscopy (STM) images of the (3 × 3) superlattice of benzene and carbon monoxide coadsorbed on the Rh(111) surface reveal a well-ordered array of ringlike features associated with individual benzene molecules, while CO is not resolved. Images further show translational domain boundaries, step-edge structures, and evidence for surface diffusion. The origin of the STM image contrast of these molecules and implications for STM imaging of other molecular adsorbates are briefly discussed.

A simple idea of statistical independence of thermal and chemical disorders in perfectly disordered solid solutions suggests the universality of their thermodynamic and transport properties at high temperatures. Our experimental data on diagrams of state, heat capacities, and resistivities of mixtures of heavy alkali metals convincingly support this idea. A generalization of Lindemann's criterion of melting for disordered alloys has been suggested and applied to a variety of mixtures.

We argue that for ballistic transport through a narrow conductor (of width $W$) a threshold magnetic field exists below which the Hall resistance vanishes. The field is of order $\left(\frac{h}{e}\right)k_{\mathrm{F}}^{}{}_{}{}^{-1\mathrm{}}{W}^{-3\mathrm{}}$, and is reached when the transverse wavelength of quantum edge states becomes comparable to the width. This is offered as a mechanism for the quenching of the Hall effect discovered experimentally in a narrow two-dimensional electron-gas wire by Roukes et al.

The electronic properties of localized resonance states in III-V compound semiconductors are analyzed. In general, resonance states recombine rapidly with the band states. However, a new class of resonance states is found to be long-lived if their coupling to the lattice is sufficiently large. The recombination of these states with band states is thermally activated which leads to extremely long lifetimes at low temperatures. Such states can explain recent experiments suggesting long-lived resonance states associated with persistent photoconductivity and "$\mathrm{DX}$ centers" in $n$-doped ${\mathrm{Al}}_{1-x}{\mathrm{Ga}}_x\mathrm{As}$ for $x<\mathrm{}0.2\mathrm{}$.

We have fabricated small tunnel junctions with a variety of ratios of charging energy $E_c$ to Josephson coupling energy $E_{\mathrm{J}}$ and measured their $I-V$ characteristics. For $E_c\sim E_{\mathrm{J}}$, the $I-V$ curve is resistive at all currents and the critical current is greatly reduced, in agreement with our theoretical estimates. As $E_{\mathrm{J}}$ is further reduced by application of a magnetic field, we find a novel regime in which aspects of the Coulomb blockade of tunneling coexist with features typical of Josephson tunneling. At still higher magnetic fields, the junction becomes normal and shows the simple Coulomb blockade.

Long-range nuclear antiferromagnetic order in the nuclear-spin system of a ${}_{}{}^{65}{}_{}{}^{}\mathrm{Cu}$ single crystal has been observed by neutron diffraction. By use of a linear position-sensitive detector, the time evolution of the antiferromagnetic (100) Bragg peak was followed during the warmup of the nuclear-spin system. The peak intensity was found to depend strongly on the external magnetic field between $B=0\mathrm{}$ and the critical field $B_c=0.25\mathrm{}$ mT.

The predicted perpendicular surface anisotropy is observed for ferromagnetic fcc Fe/Cu(100) via in situ polar Kerr-effect measurements. Square hysteresis loops are obtained for films 1.5 to 5.7 monolayers thick for 100-K growth. The region of stability of the perpendicularly magnetized state as a function of growth temperature and film thickness is delineated. Combined longitudinal and polar Kerr-effect measurements show that the anisotropy reverts to being in plane for films thicker than ≃6 monolayers.

We have observed that a strong electric field $\mathcal{E}$ shifts to higher energies the photoluminescence and photocurrent peaks of a GaAs-${\mathrm{Ga}}_{0.65}$${\mathrm{Al}}_{0.35}$As superlattice of period $D$ (=65 Å), which we explain by a field-induced localization of carriers to isolated quantum wells. Good agreement is found between observed and calculated shifts when the large field-induced increase of the exciton binding energy is taken into account. At moderate fields [≅(2-3)×${10}^4$ V/cm], the coupling between adjacent wells is manifested by four additional peaks that shift at the rates $\pm e\mathcal{E}D$ and $\pm 2e\mathcal{E}D$ and correspond to transitions that involve different levels of the Stark ladder.

A density-functional theory for superconductors at arbitrary temperature is described. It leads to equations of the Kohn-Sham type, which incorporate exchange and correlation effects into the Bogoliubov-de Gennes equations for an inhomogeneous superconductor. Further, this formalism yields exchange-correlation corrections to Eilenberger's expression for the thermodynamic potential of a superconductor, and the Ginzburg-Landau equation. Practical aspects of the application of the formalism are discussed.