Cavity-QED-enhanced stimulated visible emission was observed in 14-μm-diam rhodamine-6G–ethanol droplets pumped by cw 514.5-nm radiation. Use of droplets provides an excellent test of cavity QED theory for spherical geometries. The mode number and order of the spherical-cavity resonances responsible for the observed emission peaks were identified and their Q values calculated from Lorenz-Mie theory. By equating stimulated gain to calculated/measured cavity losses, it was determined that spherical-cavity Q’s of only ${10}^{3–}$${10}^4$ lead to cavity QED enhancements in excess of 120 in the emission cross section of rhodamine 6G, consistent with theory.

Experiment and theory for surface waves in square containers suggest that the effective symmetry may be larger than the geometric symmetry of the container cross section. The extra symmetries complicate the construction of normal forms and appear to stabilize effects in the experiments that would otherwise be nongeneric. These symmetries may be directly observed if the sidewalls are deformed to a nonsquare cross section that retains square symmetry. The effects of such a deformation are analyzed.

The eddy size and temperature distribution in the inertial subrange of turbulent Rayleigh-Bénard flow at a Rayleigh number Ra≊${10}^{11}$ is calculated from the Boussinesq equations. Velocity and temperature fields are decomposed into Fourier modes with geometrically scaling wave numbers. 1/3 scaling for both velocity u and temperature FTHETA is obtained in agreement with Shraiman and Siggia’s finding, but there is no indication of a new scaling range as claimed recently. We resolve this discrepancy. The isotropization by eddy decay is demonstrated in terms of the ${\mathit{u}}_3$FTHETA scaling, which appreciably differs from 2/3; testing this new quantity experimentally would be of great interest.

A new approach to enhance the production of negative ions in low-pressure hydrogen and deuterium discharges is described. The procedure, which requires on-off modulation of the discharge current, is applied to a conventional magnetic multicusp or ‘‘bucket’’ volume negative-ion source. The extracted ${\mathrm{D}}^{\mathrm{-}}$ current from a 15-A discharge at a filling pressure of 2.4 mTorr is increased from 0.5 to 2.0 mA/${\mathrm{cm}}^2$ by discharge-current modulation at 10 kHz and 23% duty cycle. The ${\mathrm{D}}^{\mathrm{-}}$ beam current is not significantly modulated (<5%). The extracted electron current density and average discharge current are reduced by over a factor of 4. The work is relevant to production of ${\mathrm{D}}^{\mathrm{-}}$ beams for neutral-beam injectors.

We study numerically and analytically the selection of Saffman-Taylor fingers in cells having the shape of a disk sector. For divergent fingers we find results which are qualitatively new compared to the standard linear geometry. Instead of a discrete set of solutions, all converging to a relative width of 1/2 for zero surface tension, we find that the fingers disappear below some surface tension. This happens by the merging of neighboring branches of the discrete set of solutions. This behavior is in (quantitative) agreement with experimental results.

Cracks in brittle materials have terminal velocities far below theoretical predictions. To address this problem we have investigated the propagation of cracks in a brittle plastic (polymethylmethacrylate). Velocity measurements with resolution an order of magnitude better than past experiments reveal the existence of a critical velocity at which the velocity begins to oscillate, the mean acceleration drops sharply, and a pattern is formed on the fracture surface. Thus the dynamics of cracks may be governed by a dynamical instability.

First-principles calculational results, that conflict with conventional wisdom, are reported for the orientation dependence of the energy of ${\mathrm{H}}_2$, a few bohrs above a Rh(001) surface. In geometric configurations near the saddle point where H-Rh bonds begin to form, the favored molecular axis orientation is along a 〈100〉 direction, corresponding to the attraction of each H atom to a twofold bridge site. This is true even though the final destinations of the H atoms are fourfold hollows, and even though the 〈100〉 orientation corresponds to relatively low symmetry, i.e., ${\mathit{C}}_{\mathit{s}}$ rather than ${\mathit{C}}_{2\mathit{v}}$.

The Si(001)2×1 surface is expected to undergo a phase transition from single to double-atonic-height steps with increasing angle of miscut. Here we show that this transition is quite different than previously believed, involving something like a ‘‘devil’s staircase’’ of transitions in a mixed phase consisting of a complex sequence of single and double steps. Even at low angles, where only single steps occur, the areas of 2×1 and 1×2 regions are unequal, in agreement with recent experimental results.

We consider the accuracy of holographic inversions of electron emission data at ∼${10}^3$ eV to yield atomic positions. Theoretical calculations for small clusters of 2–5 atoms in both single and multiple scattering show that self-interference effects may be present, and that forward-peaked scattering tends both to shift peaks by as much as 0.5–1.0 Å from the true positions and to elongate images along forward-scattering directions. However, eliminating forward-peaked intensities before inversion significantly improves image quality.

Time-of-flight cluster-impact fusion studies were carried out by pulsing low-velocity beams. Time-of-flight mass spectra and experiments in which the production of fusion protons was correlated with the arrival times of the cluster ions at the target show that low-molecular-weight ions containing oxygen (i.e., ${\mathrm{D}}_2$${\mathrm{O}}^{+}$,${\mathrm{D}}_3$${\mathrm{O}}^{+}$) cannot be responsible for observed fusion events.

The electron energy distribution from a single 1-nm-diam. Au cluster supported on a W(110) field-emission tip has been studied using energy-resolved field-emission microscopy. The emitted electrons provide information on the quantum electron energy states of the supported cluster and demonstrate that these states survive after deposition onto a substrate. A model that accounts for the salient features in the energy distribution is described.

We present results of doubly differntial cross-section measurements for production of convoy electrons from 36-MeV/u Ar ions in thin amorphous carbon targets. Comparison of convoy emission distributions as a function of target thickness reveals rapid evolution of component multipole strengths. Thin-target results are compared with binary ion-atom electron-loss-to-the-continuum predictions. A quantitative model of the multipole evolution bridging the transition from the single- to the plural-collision regime is presented which demonstrates significant competition between one-step and multistep production mechanisms.

The conductance of a mesoscopic conductor is shown to be a sensitive function of the dc voltage on the system. In the low-temperature limit, changing the potential drop across the system by an amount on the order of the level spacing of the system is sufficient to change the conductance by ${\mathit{e}}^2$/h. Arguments based on scattering theory, analogy with the universal conductance fluctuation, and numerical evidence are presented to support our conclusion. Our theory is in good agreement with the results of harmonic-generation experiments.

In the Perk-Schultz vertex model links can have n+1 different colors and the weights are ferroelectric or antiferroelectric depending on a discrete parameter ${\mathrm{\epsilon }}_{\mathit{a}}$=±1 (1≤a≤n). We compute the exact ground-state spins and find ferrielectric behavior for general signs ${\mathrm{\epsilon }}_{\mathit{a}}$. The exact free energy and excitation spectrum are found as well as the finite-size corrections yielding the central charge and conformal dimensions. The scaling limit yields a quantum field theory whose mass spectrum and S matrix are explicitly obtained through the light-cone approach. The mass spectrum surprisingly depends on the anisotropy parameter and the signs ${\mathrm{\epsilon }}_{\mathit{a}}$.

A theory of oscillations in the exchange coupling between two transition-metal ferromagnets separated by a nonmagnetic transition-metal spacer is developed for a one-band model. A close analogy between oscillations in the exchange and de Haas–van Alphen oscillations is established and exploited to show that the period, asymptotic decay, and temperature dependence of the oscillations are determined by properties of the Fermi surface in the spacer layer. The theory describes many features of the oscillations in the exchange coupling observed recently in Co/Ru, Co/Cr, and Fe/Cr superlattices.

The spin correlations of the S=1 Heisenberg antiferromagnetic chains in ${\mathrm{AgVP}}_2$${\mathrm{S}}_6$ have been studied by neutron scattering at temperatures of the order of T∼${10}^{\mathrm{-}2}$ J/${\mathit{k}}_{\mathit{B}}$. On the polycrystalline samples, with a careful choice of experimental conditions, we have observed a Haldane gap of ${\mathit{E}}_{\mathit{g}}$=26 meV at the antiferromagnetic point at q=π. The excitation spectra are consistent with a mode that has a spin-wave velocity C=150 meV and a correlation length ξ≊5.5, close to C/${\mathit{E}}_{\mathit{g}}$. These results are in quantitative agreement with the predictions and numerical results on the Haldane state.

We report the observation of a giant resonance in the Cu 2p resonant-photoemission spectra of CuO. The study allows the unambiguous identification of the local Cu 3${\mathit{d}}^8$ configuration in the valence-band photoemission spectrum, providing conclusive evidence for the charge-transfer nature of the insulating gap in the electronic structure of CuO. A comparison with 3p resonant-photoemission measurements is also presented to emphasize the strength of 2p resonant photoemission as a tool for studying the electronic structure of highly correlated 3d transition-metal systems.

We investigate a mechanism of generation of dislocations for the melting of flux lattices in high-${\mathit{T}}_{\mathit{c}}$ oxide superconductors. The free energy is the difference of the elastic energy and that from the interaction of the dislocation with the flux-lattice vibrations. For Bi-Sr-Ca-Cu-O and Y-Ba-Cu-O with the flux interaction potential derived by Sudbo/ and Brandt, the free energy of an edge dislocation becomes zero at temperatures that are within 20% of the experimental melting temperatures. Screw dislocations are more difficult to generate than edge dislocations.

We have performed an extensive set of tunneling measurements on the high-${\mathit{T}}_{\mathit{c}}$ perovskites ${\mathrm{BaPbO}}_3$,${\mathrm{BaPb}}_{0.75}$${\mathrm{Sb}}_{0.25}$${\mathrm{O}}_3$, ${\mathrm{BaPb}}_{0.75}$${\mathrm{Bi}}_{0.25}$${\mathrm{O}}_3$, and ${\mathrm{Ba}}_{0.7}$${\mathrm{K}}_{0.3}$${\mathrm{BiO}}_3$. In these materials, the superconducting transition temperature varies from less than 1 to 30 K. Tunneling data show a linear background conductance similar to that observed in the cuprates and we have strong experimental evidence that this linear conductance results from intrinsic properties of the density of states in these materials. We also observe a remarkable correlation between ${\mathit{T}}_{\mathit{c}}$ and the slope of this linear background conductance.

The orthorhombic-tetragonal structural phase transition of ${\mathrm{La}}_{2\mathrm{-}\mathit{x}}$${\mathrm{Sr}}_{\mathit{x}}$${\mathrm{CuO}}_4$ is quantitatively analyzed as a function of composition x within an anharmonic electron-phonon interaction model. The correct temperature dependence of the soft mode and the elastic constant ${\mathit{c}}_{66}$ is obtained. The double-well potential in the electron-phonon interaction is derived self-consistently and found to vary strongly with x. In the vicinity of the superconducting transition temperature ${\mathit{T}}_{\mathit{c}}$ electron–two-phonon interactions dominate the harmonic ones which may explain the high ${\mathit{T}}_{\mathit{c}}$’s observed.