Experimental measurements of structure factors in GaAs have been compared with the results of ab initio calculations by the pseudopotential method. We measure the charge in the bond from charge-density maps to be 0.071 electron, in good agreement with theory. The measurements were obtained by an electron-diffraction technique which can be applied to submicron crystals, thus greatly extending the range of materials for which structure factors can be measured, and theoretical calculations tested.

We have determined experimental upper limits on any broadening which could be due to a charge-density wave in Na and K metal soft-x-ray absorption and emission spectra. The upper limit for absorption in Na is a factor of 4 below the expected theoretical value. These results contradict expectations based on the present theory of charge-density waves in alkali metals.

Optical and energy-loss data suggest that the soliton lattice, which accounts for the low Pauli susceptibility ${\chi }^{\mathrm{P}}$ at average doping concentrations $〈y〉<4\% \mathrm{to} 6\%$, persists to higher $〈y〉$. We have calculated the eigenvalues for a trans-polyacetylene chain with the electrons in soliton states for the geometry and $y$ values appropriate to metallic Na-doped polyacetylene and the Coulomb potential of the ions and solitons on other chains included. We find that, allowing for chains ≃ 100 C-H units in length, we can account for ${\chi }^{\mathrm{P}}$ in the metallic state.

We consider a class of singlet resonanting-valence-bond wave functions on a square lattice, with the bond-length distribution as a variational parameter. This class contains the two limiting cases of the dimer wave function and of the Néel state. We present numerical calculations of the energy and the spin-spin correlation functions up to very large lattice sizes (180×180) both for disordered states with exponentially decaying correlation functions and for ordered states. The energy of a disordered state can be within 0.1% of our best ordered state ($\frac{-0.3344J}{\mathrm{bond}}$).

We present results on the magnetic-field-induced metal-insulator transition in compensated GaAs. The electrical conductivity was found to obey $\sigma =a+b{T}^{\frac{1}{3}}$ down to 60 mK, on both sides of the transition, where $a$ and $b$ were constant for a given magnetic field. This indicates that the conductivity is determined by the shortest length scale, in this case the interaction length, on both the metallic and insulating sides of the transition. Thus, the behavior of the conductivity is essentially unchanged as the transition is scanned, as long as one length scale dominates.

The dynamics of the sol-gel transition is probed by use of quasielastic light scattering. A type of critical dynamics is observed that is associated with a divergent friction, rather than a singularity in a thermodynamic quantity. Several novel effects are reported, including power-law time decay of the intensity autocorrelation function, critical slowing down of the average relaxation time, and observation of a fractal time set in the scattered field.

Traditionally, radiation-scattering experiments are conceived in terms of cylindrical geometry where the symmetry axis is the line between the source and the target. When the target becomes extended, as is the case in many real-life measurements, the analysis or the prediction of spectra becomes difficult. By definition of the axis of symmetry as being between the source and the detector, the extended target can be described in toroidal geometry with the surface of each toroid being a contour of constant scattering angle. With the scattering cross section constant over each contour, the calculation of total scattering intensities is greatly simplified.

The critical $\mathrm{O}\mathrm{}\left(n\right)\mathrm{}$ model on a finite honeycomb lattice is solved by the Bethe-Ansatz method. Amplitudes of the dominant finite-size corrections to part of the eigenspectrum are obtained analytically. A comparison with Cardy's predictions from the theory of conformal invariance leads to the exact results for the conformal anomaly and scaling dimensions.

We study the static spherically symmetric Einstein-Yang-Mills equations with SU(2) gauge group and find numerical solutions which are nonsingular and asymptotically flat. These solutions have a high-density interior region with sharp boundary, a near-field region where the metric is approximately Reissner-Nørdstrom with Dirac monopole curvature source, and a far-field region where the metric is approximately Schwarzschild.

We describe the results of a balloon-borne apparatus searching for low-energy antiprotons in the Galactic cosmic rays. For energies less than 640 MeV at the top of the atmosphere, no cosmic-ray antiprotons were observed. This yields an upper limit to the $\frac{\overline{p}}{p}$ ratio of 4.6×${10}^{-5\mathrm{}}$ at the 85% confidence level.

With a residual interaction consisting of density-independent two- and three-body parts, we obtain within Hartee-Fock-Bogolyubov theory at the same time good results on the odd-even staggering of charge radii and on trends of the pairing energies. We present results for Sn and some isotopic chains in the Ba region.

Multiphoton ionization of Xe and Kr is investigated with intense 0.62-μm pulses of 900, 90, and 22 fsec duration. Deviation from the scaling of lowest-order perturbation theory is observed. For the shortest pulses a modified Keldysh theory closely fits the experimental data for xenon. The importance of dynamic resonances and the shifting ionization potential is discussed.

The two states ${}_{}{}^1{}_{}{}^{}\Sigma _u^{}{}_{}{}^{+}$ $B$ and $B^{\prime }$ of ${\mathrm{H}}_2$, dissociating adiabatically to $\mathrm{H}\mathrm{}\left(2p\right)+\mathrm{H}\mathrm{}\left(1s\right)\mathrm{}$ and $\mathrm{H}\mathrm{}\left(2s\right)+\mathrm{H}\mathrm{}\left(1s\right)\mathrm{}$, respectively, are simultaneously excited by vacuum-ultraviolet photon absorption, above threshold. The combination of the coherent excitation of the two continua and of a nonadiabatic transition occurring at large internuclear distance produces a quantum interference effect in the partial photodissociation cross section which strongly affects the branching ratio $\frac{{\sigma }_p({\mathrm{H}}_2\to \mathrm{H}\mathrm{}(2p)+\mathrm{H}\mathrm{}(1s\left)\right)}{{\sigma }_s({\mathrm{H}}_2\to \mathrm{H}\mathrm{}(2s)+\mathrm{H}\mathrm{}(1s\left)\right)}$. Comparisons with theoretical predictions are presented and discussed.

By the employment of "constant-scaled-energy spectroscopy" as a novel spectroscopic technique, the quasi-Landau resonance system of the diamagnetic H atom in even-partity $m=0\mathrm{}$ magnetic final states is observed for the first time in its entirety from the regular $\frac{1}{n}$ into the chaotic quasi-Landau regime. It evolves, fully unexpectedly, into a systematically structured hierarchy of generations of resonances, correlated to three physically different types of closed classical orbits.

We study the influence of a laser field on the dynamics of fast ($e,2e$) collisions on atomic hydrogen, in the asymmetric coplanar geometry. We find that the triply differential cross sections are strongly dependent on the "dressing" of the atomic target by the laser.

The evolution of morphological instability in solidifying succinonitrile has been investigated with high-resolution videomicroscopy. Quantitative analysis of the early development of the instability has demonstrated the critical role of noise, and indicates significant disagreement with predictions of the Mullins-Sekerka theory. The ratio of the tip radius ${\rho }^{*}$ of the steady-state dendrites to the length scale ${\lambda }_{max}$ of the initial instability was found to be approximately 0.5, in reasonable agreement with a recent theoretical prediction of Barbieri, Hong, and Langer.

We propose a simple mechanism that generates nonparabolicity in the dispersion of the positive charge carrier (snowball) in superfluid ${}_{}{}^4{}_{}{}^{}\mathrm{He}$. Specifically, we pursue the consequences of a model where (1) the snowball behaves as a rigid rotator and (2) the hydrodynamic contribution to its mass (with rotator axes fixed) is anisotropic. The nonparabolicity that we calculate leads to a linear temperature variation of the snowball effective mass. The model is consistent with recent measurements if we assume a mass anisotropy of (9-13)%. An additional $T^3$ mass enhancement by long-wavelength phonons is negligible for the snowball but may be observable for the negative charge carrier.

A silicon metal-oxide semiconductor field-effect transistor has been used to detect heat pulses generated by electrical heating of a thin metal film. In the presence of a quantizing magnetic field deep oscillations in the detected intensity are observed, the signal being a maximum when the Fermi level coincides with a Landau level. The appearance of an intermediate signal at higher heater temperatures is attributed to cyclotron phonon absorption. We show that this system can be used as a tunable phonon spectrometer.

A study is reported of finite-size, $L$, corrections for the surface tension of rough interfaces, i.e., for temperatures $T_R<T<\mathrm{}{T}_c$ in 3D, $0<T<\mathrm{}{T}_c$ in 2D. A universal leading contribution proportional to $\frac{\left(\ln L\right)}{L}$ is predicted in 2D. This term is a result of the characteristic rough interface fluctuations. In 3D, the leading corrections are of order $\frac{1}{L}$.