The temperature dependences of the radio-frequency penetration depth ${\lambda }_{\mathrm{ab}}$, microwave surface resistance $R_s$, and the lower critical field $H_{c1}$ were measured for ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{\mathrm{y}}$ crystals. Pair-breaking effects of magnetic fields on ${\lambda }_{\mathrm{ab}}$, represented by a parameter k==d${\lambda }_{\mathrm{ab}}$/d($H^2$), were also measured. The temperature dependences of all these parameters ${\lambda }_{\mathrm{ab}}$, $H_{c1}$, k, and $R_s$ are in good agreement with the Bardeen-Cooper-Schrieffer temperature dependences for an s-wave superconductor, and suggest a mean-field behavior of the gap parameter.

An unusual loss mechanism has been commonly observed in the high-temperature superconductors in a magnetic field near the zero-field transition temperature ${\mathrm{T}}_{\mathrm{c}}$. The resistive transitions associated with this loss in highly c-axis-oriented ${\mathrm{Tl}}_2$${\mathrm{Ba}}_2$${\mathrm{CaCu}}_2$${\mathrm{O}}_{\mathrm{x}}$ thin films are presented as a function of field perpendicular to the c axis, for the current both perpendicular and parallel to the field. The data for these orientations are essentially the same over a span of 5 orders of magnitude below the normal-state resistance. Since the macroscopic Lorentz force is zero for the current parallel to the magnetic field, it can play, at most, a significantly reduced role in this loss mechanism, and our results seriously question any explanation of these losses based on magnetic flux motion.

Microscopic studies of the optical spectra of ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ have been done using the linear-muffin-tin-orbital (LMTO) method for solving the band-structure problem. Special attention has been paid to the midinfrared and visible spectral range where a number of peculiarities including the large anisotropy of the optical reflectance was observed. It is shown that the overall features of the observed optical spectra can be well explained in the framework of the usual band-structure approach. Moreover, the anisotropy of the reflectance in the a-b planes and the positions of the main interband transitions are also in good agreement with available experimental data.

Unoccupied bands of the Ni(110)/CO(2×1)p2mg chemisorption system were investigated by k-resolved inverse photoemission and by tight-binding extended-Hückel calculations. A detailed symmetry analysis reveals the effects of metal-CO backbonding in the CO 2π derived manifold. The CO 2π-metal sp interaction, although comparable to the d interaction of some CO 2π bands, effects the band topology only weakly and in a rather uniform manner. This study provides the first quantitative assessment of the CO 2π-metal d and sp interaction based on measurements of unoccupied bands.

We define a variant of the model of Bak, Tang, and Wiesenfeld of self-organized critial behavior by introducing a preferred direction. We characterize the critical state and, by establishing equivalence to a voter model, determine the critical exponents exactly in arbitrary dimension d. The upper critical dimension for this model is three. In two dimensions the model is equivalent to an earlier solved special case of directed percolation.

Gravitational collapse with rotation leaves a slowly decaying radiative tail which becomes infinitely blueshifted at the inner horizon of the resulting black hole. We study the gravitational effects of this on the inner structure of the hole, using a simple spherical model. In the presence of outflow from the collapsing star, the gravitational-mass parameter and the curvature are inflated at and within the inner horizon to values which, classically, are unlimited. Implications of this result are briefly discussed.

Using a sample of 484 000 B mesons collected with the CLEO detector at the Cornell Electron Storage Ring, we have measured B(B¯ ${}_{}{}^0{}_{}{}^{}\mathrm{D}_{}^{\mathrm{*}+}{}_{}{}^{}$${\mathrm{l}}^{\mathrm{-}}$ν) and found that the polarization of the $D^{\mathrm{*}+}$ is small. These results are compared with models of semileptonic B decay.

We report the results of an experimental search for neutrinoless double-β decay using isotopically enriched foils of ${}_{}{}^{100}{}_{}{}^{}\mathrm{Mo}$ interleaved in a stack of 40 silicon detectors. With an exposure of 69.9 mole days, we place lower half-life limits on the $J^P$${=0\mathrm{}}^{+}$→$0^{+}$ transition of 4×${10}^{21}$ yr (68% C.L.) and on the $0^{+}$→$2^{+}$ transition of 4×${10}^{20}$ yr (68% C.L.).

We have trapped single molecular ions in a precision Penning trap and, using cyclotron resonance techniques, have measured the mass ratio M(${\mathrm{CO}}^{+}$)/M(${\mathrm{N}}_2^{+}$) to be 0.999 598 8876(4). Accuracy is limited at 4×${10}^{\mathrm{-}10}$ predominantly by temporal instability in the magnetic field. All other systematic sources of error are ΔM/M≲${10}^{\mathrm{-}10}$. .AE

We report measurements of microwave intensity statistics in random configurations of polystyrene spheres contained in a cylinder with reflecting walls. As the sample thickness increases measurements of the intensity distribution and the intensity correlation function with frequency shift reveal a crossover from uncorrelated to correlated photon diffusion.

The spiking behavior of free-electron lasers, or sideband instability, has been observed for the first time in the time domain using an autocorrelator on the MKIII free-electron laser at Stanford University. The measurement is compared with simulations and qualitative theory.

Wetting immiscible displacement of one fluid by another in a porous medium yields self-affine (anisotropic) fractal interfaces between the two fluids. Water-air interfaces are characterized experimentally by a scale-dependent roughness w(L)=${\mathrm{AL}}^{\beta }$, with β=0.73±0.03, independent of the capillary number Ca, and A∝${\mathrm{Ca}}^{\mathrm{-}0.47\pm 0.06}$. The exponent β is related to the ‘‘box’’ and ‘‘divider’’ dimensions by $D_b$=2-β and $D_d$=1/β, respectively.

We use a concave spherical mirror to focus at 18-mm-diam beam of H atoms down to 0.5 mm. The mirror consists of a fused-quartz substrate polished to optical precision and coated with a liquid${\mathrm{-}}^4$He film to obtain high reflectivity. The temperature dependence of the focused beam intensity enables us to study the influence of the dynamic surface roughness on the reflection of the H atoms. Both zero-point fluctuations and thermal excitations turn out to be of importance. A monolayer of ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ does not significantly affect the results.

We study the dynamics of the roughening of a surface between two coexisting phases using a novel Monte Carlo algorithm. Our model Hamiltonian explicitly contains a latent heat whose value directly influences the growth law. We observe a crossover between short-wavelength short-time behavior controlled by the surface tension and long-wavelength long-time behavior controlled by the diffusion of heat. The results are in excellent agreement with recent theoretical predictions.

The boundary between the A and B phases of superfluid ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ possesses many interesting features because of the drastically different bulk order parameters. We study the transmission and Andreev reflection of ballistic wave packets of excitations incident onto the A-B phase boundary. We find that the A-B phase boundary is magnetically active and is capable of converting an unpolarized incident wave packet into transmitted and Andreev reflected wave pakets which are partially polarized. We also consider equilibrium properties due to bound states and resonances of excitations localized near the phase boundary. These states contribute to magnetic correlations and to mass and spin currents flowing in the plane of the interface.

We report NMR shift ΔK and $T_1$ data of ${}_{}{}^{89}{}_{}{}^{}\mathrm{Y}$ taken from 77 to 300 K in ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{6+\mathrm{x}}$ for 0.35<x<1, from the insulating to the metallic state. A Korringa law and therefore a Fermi-liqud picture is found to apply for the spin part $K_s$ of ΔK. The spin contribution ${\chi }_s$(x,T) to ${\chi }_m$ is singled out, as the T variation of ΔK scales linearly with the macroscopic susceptibility ${\chi }_m$. This implies that Cu(3d) and O(2p) holes do not have independent degrees of freedom. Their hybridization, which has a σ character, hardly varies with doping. These results put severe constraints on theoretical models of high-$T_c$ cuprates.

Total energy calculations for prototype metals like Al and Ga on Si(001) 2×2, 2×1, and 1×2 surfaces predict important new structures and provide their bond strengths. It is estimated that at about 0.5-monolayer coverage the surface reconstruction is lifted and metallic dimers become stable. New low-energy Peierls-distorted structures provide a more natural explanation for the 5×2 and 3×2 LEED patterns. The reported trend in desorption energy as a function of coverage is reproduced, but more importantly specific conditions for observing this trend are obtained.

We inject electrons at arbitrary energy into the band structure of a GaAs/AlGaAs semiconductor superlattice. The electrons traverse a finite number of periods and are then analyzed for the electron momentum distribution using a hot-electron spectrometer. By this technique, we are able to give the first demonstration of miniband conduction through high-order minibands. We relate the transmission and energy-loss characteristic to tight-binding models of the superlattice.

Measurements have been made of the transition rates between fluxoid states of a system with two Josephson junctions having two macroscopic degrees of freedom. The data, taken over a temperature range of 0.85 to 4.41 K, are in excellent agreement with the predictions for thermally activated transitions. No evidence is seen for recently reported apparent rate suppression below that predicted for thermal activation.

A collective, many-electron response of a host crystal to the electric field of a photon can enhance the oscillator strength for a quadrupole transition of an ion (imbedded in the crystal) by 8 orders of magnitude, e.g., from ${10}^{\mathrm{-}5}$ to ${10}^3$.