The occurrence and nature of low-energy excitations associated with two-level tunneling systems in the metallic glass ${\mathrm{Pd}}_{0.775}$${\mathrm{Si}}_{0.165}$${\mathrm{Cu}}_{0.060}$ annealed to various levels of relaxation in the amorphous state and into crystalline phases have been studied by ultrasonics. In one of the crystalline phases the same density of two-level systems was found as in the amorphous phase.

The effect of 30-ns, 6943-Å, high-power laser irradiation on the surface of pristine and ion-implanted graphite has been studied by Rutherford-backscattering channeling spectrometry (RBS). The experimental results are explained by a model in which at a threshold energy density of 0.6 J/${\mathrm{cm}}^2$ the graphite surface melts, and the melt-depth measurements provide a simple estimate of the melting temperature (∼ 4300 K) and the latent heat of fusion of graphite (∼ 16 ± 5 kcal/mol).

NMR measurements were performed with nuclear-spin—polarized alkali-metal atoms (${}_{}{}^6{}_{}{}^{}\mathrm{Li}$, ${}_{}{}^7{}_{}{}^{}\mathrm{Li}$, ${}_{}{}^{23}{}_{}{}^{}\mathrm{Na}$) adsorbed on a clean and an oxygen-covered W(110) surface. For the desorbed alkali-metal ions the change of polarization due to the radio-frequency field was detected by beam-foil spectroscopy. Energy splittings of the nuclear states were found arising from the interaction of nuclear quadrupole moments with electric field gradients. They reflect the spatial charge distribution in the vicinity of the adsorbed alkali-metal atoms.

We have studied the localization of normal modes in a model glass as a function of several parameters and found that the fraction of modes which are localized remains small even when the disorder is very large. We have also studied how anharmonic effects, such as the Grüneisen parameter, are influenced by whether or not a mode is localized or extended.

The overlayer-plasmon dispersion in a K adlayer on Si(001)2×1 has been measured by angle-resolved electron-energy-loss spectroscopy. The dispersion of the interband-plasmon mode shows an azimuth-dependent anisotropy, which is in good agreement with recent theories that assume a strongly anisotropic electronic structure. Besides, the intraband-plasmon mode was observed, which identifies a metallic character of the K chains. It is proposed that the K chains adsorbed on Si(001) are in the nature of a one-dimensional metal.

With the use of the random-phase approximation, we obtained the dispersion relation of the inter- and intra-surface-band plasmon of the parallel-rod model, which simulates the alkali-metal adatom chains on Si(100)2×1 surfaces. There are two interband plasmon modes: One corresponds to the polarization vertical to the surface and the other to the polarization vertical to both the rod axis and the surface normal. The results explain fairly well the plasmon modes of the Si(100)(2×1) K surface observed recently.

We report on measurements of both the ac conductivity ($\sigma$) and the ac dielectric constant ($\epsilon$) of thin Au films near their percolation threshold for frequencies between 100 Hz and 10 MHz. In the critical regime, $\sigma \simeq {\omega }^x$ and $\epsilon \simeq {\omega }^{-y}$. We obtain $x=0.95\mathrm{}\pm 0.05$ and $y=0.13\mathrm{}\pm 0.05$, in agreement with a general relation $x+y=1\mathrm{}$. These results are significantly different from predictions based on finite size scaling of the dc conductivity or diffusion on independent percolation clusters, providing evidence for the importance of electron-electron interaction effects.

The anisotropy of optical absorption saturation by intense linearly polarized light pulses is measured with subpicosecond resolution in GaAs at 77 K, at a photon energy slightly above the band gap. For an electron-hole pair density of about 6×${10}^{17}$ ${\mathrm{cm}}^{-3\mathrm{}}$, the momentum orientational relaxation time is determined to be 190 fs, most likely due to carrier-carrier scattering.

The consequences of an incommensurate lattice modulation on the electronic energy levels have been studied by optical transmission experiments on ${\mathrm{Rb}}_2$Zn${\mathrm{Br}}_4$. The results show the appearance of new energy levels and are analyzed with a simple tight-binding model in which the superspace symmetry of the crystal is taken into account.

Classical and quantum models of dynamically reversible computers are considered. Instabilities in the evolution of the classical "billard ball computer" are analyzed and shown to result in a one-bit increase of entropy per step of computation. "Quantum spin computers," on the other hand, are not only microscopically, but also operationally reversible. Readoff of the output of quantum computation is shown not to interfere with this reversibility. Dissipation, while avoidable in principle, can be used in practice along with redundancy to prevent errors.

It is shown that by a mere bending of the drift tube of a two-cavity klystron into a circular arc, the current bunching near the output cavity is significantly enhanced. This increase in the bunching results from the "negative mass" behavior of a rotating electron beam. Operations with both low- and high-voltage beams are suggested.

We compute numerically the eigenvalues of a family of two-dimensional Hamiltonians which give rise to regular or chaotic classical motion depending on the particular choice of parameters. A close relationship is found between the spectral statistics and the fraction of classical phase space covered by chaotic trajectories. In the extreme regular and chaotic cases the system displays Poisson and Gaussian-orthogonal-ensemble statistics, respectively. A one-parameter random-matrix model is proposed to describe the spectral statistics in intermediate situations.

A simple and direct method is described for constructing exact quasiperiodic solutions of the focusing nonlinear Schrödinger equation. An algebraic constraint is imposed on the initial values of a set of auxiliary variables. The solution, expressed in terms of these auxiliary variables, is quasiperiodic in general, and represents a wider class of solutions than those obtained from the inverse-scattering transform. As an example of our method, we present solutions corresponding to a modulated plane wave.

We have developed a distorted-wave strong-potential Born approximation for charge transfer at large scattering angles in asymmetric ion-atom collisions, and applied it to the calculation of electron capture in resonant nuclear collisions. A strong variation of the capture probability across the resonance is predicted.

The reaction $\mathrm{F}+p-{\mathrm{H}}_2\to \mathrm{HF}+\mathrm{H}$ was studied in a high-resolution crossed-molecular-beams experiment at a collision energy of 1.84 kcal/mole. Center-of-mass translational energy and angular distributions were determined for each product vibrational state. The $v=3\mathrm{}$ product showed intense forward scattering while the $v=2\mathrm{}$ product was backward peaked. These results suggest that dynamical resonances play an important role in the reaction dynamics of this system.

Using high-precision laser spectroscopic techniques we have measured the predicted shift of Rydberg energy levels induced by blackbody radiation. Fractional shifts of ∼ 2×${10}^{-12\mathrm{}}$ are consistent with theoretical predictions.

X-ray studies of relativistic heavy-ion-atom collisions allow the direct observation of the radiative-electron-capture photons. The angular distribution of these photons is approximately ${sin}^2{\theta }_{\mathrm{lab}}$, because of the cancellation of electron retardation effects when the cross sections are subjected to Lorentz transformation into the laboratory frame. Comparison of measured and calculated cross sections reveals the number of equilibrium projectile $K$ vacancies present in the solid targets, which can be compared with charge-state measurements behind the target.

A constant of the motion exact for a valence-shell Hamiltonian is explicitly constructed for second- and third-row atoms. It is analyzed into sums of one-electron and two-electron operators, and the relative contribution of each is determined.

The onset of time dependence of Rayleigh-Bénard convection in water is studied in a rectangular container with aspect ratio 9.5×4.6. Quasiperiodic states are observed exhibiting four and five incommensurate frequencies. All but one of the modes are predominantly localized in different parts of the cell; this localization may explain the coexistence of more than three incommensurate frequencies without chaos.

We report narrow-band spectra ($\frac{\Delta \omega }{\omega }\approx 0.02$) from a tunable ($7<~\frac{\omega }{2\pi }<~18$ GHz), Raman, free-electron laser operating in a single T${\mathrm{E}}_{11}$ waveguide mode. rf power levels of 100 kW at an efficiency of ∼ 12% have been achieved. Measured dispersion characteristics are in good agreement with theory.