We present a fully-quantum-mechanical theory for the phonon modes of a two-dimensional Wigner crystal in the strong-magnetic-field limit. Our theory is based on a time-dependent Hartree-Fock approximation of the density-density response function χ, which is exact in the harmonic limit but allows for arbitrarily large anharmonicity. Our calulation is facilitated by identities, valid in the strong-magnetic-field limit, which allow χ to be expressed solely in terms of the ground-state electron density.

Because of phase coherence across the A-B interface, the hypercooled phase transition is not an onset phenomenon in this system, but rather the limiting behavior of very large interface velocities. At lower velocities, the chemical potential is continuous across the interface and the growing B phase is colder than the receding A phase.

Low-frequency rotational and lattice-phonon spectra of solid ${\mathrm{H}}_2$ have been measured to 162 GPa at 77 to 295 K. Rotational bands broaden continuously up to and through the 150-GPa phase transition, with no sign of orientational transitions. The optical phonon, identified at low pressures as the transverse-optical mode of the hexagonal-close-packed structure, persists into the high-pressure phase, which indicates the underlying lattice remains hexagonal to these pressures. The measurements are consistent with the 150-GPa transition being largely electronic in origin and associated with metallization.

The shear strain of a mixed cyanide crystal has been measured under field-cooling conditions. The time decay after field removal follows a Kohlrausch-Williams-Watts law. The results are analogous to spin glasses, except that residual permanent strains can be frozen-in.

Raman scattering in high-pressure solid orthodeuterium has been used to make the first observation of the J=4 roton band in o-${\mathrm{D}}_2$, not confined to k=0. The results are consistent with an almost dispersionless band. A determination of the J=2 roton band density of states is in agreement with our computer calculations.

A mean-field theory is developed for scaling in radial immiscible viscous fingering with constant areal flow rate. Excellent agreement is found between the predictions of this theory and existing experimental data. It also provides a clear physical insight into the origin of scaling.

Strong intensity oscillations have been found in reflection high-energy electron diffraction during epitaxial growth of ${\mathrm{BaTiO}}_3$, ${\mathrm{La}}_2$${\mathrm{CuO}}_4$, and ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathit{x}}$ with perovskite-type structures. The oscillation period corresponds to the height of the minimum unit of each oxide that satisfies the chemical composition and electrical neutrality. The data provide a basic understanding of the epitaxial growth process of ionic oxides.

The single-ended, cw impedance technique has been employed to study the squashing mode of superfluid ${}_{}{}^3{}_{}{}^{}\mathit{B}$ in a sound cell with a path length of 381 μm with ultrasound frequencies of 115.8 and 141.6 MHz. A doublet splitting of the squashing mode has been observed and the width of the splitting is dependent on the thermal gradient within the acoustic cell, which strongly suggests that the observed phenomena are induced by superflow.

Using glancing-angle x-ray-scattering techniques we investigated the order-disorder transition of ${\mathrm{Cu}}_3$Au near the (111) surface. Upon approach the bulk disordering temperature, the superstructure reflections exhibit a very unusual double-Gaussian line shape. The narrow component is due to the usual long-range order, while the broad Gaussian component indicates that small disordered clusters of a well-defined size of about 25 Å are present in the surface region. These clusters coexist with large ordered domains of about 5000 Å in size. We interpret the small clusters as surface heterophase fluctuations and discuss their implications.

We propose a new variational method, based on the ab initio Hartree-Fock methods, for the purpose of calculating efficiently both the equilibrium geometry and the stability of alkali-metal microclusters. Applying this method to lithium clusters, up to ${\mathrm{Li}}_{36}$, we are able to find a close correlation between the observed magic numbers and the calculated stable cluster size. Furthermore, we find that lithium clusters larger than ${\mathrm{Li}}_{26}$ have an ordered structure, while clusters smaller than this do not. This is suggestive of the evolution of the clusters to the bulk structure of solid lithium.

Stable icosahedral ${\mathrm{Al}}_{85\mathrm{-}\mathit{x}}$${\mathrm{Cu}}_{\mathit{x}}$${\mathrm{Ru}}_{15}$ (15<x<20) alloys of good metals are characterized by conductivities below the ‘‘minimum metallic value,’’ and resistivity ratios ρ(0.5 K)/ρ(295 K) as large as 4. Strong temperature dependences which lead to changes of sign in the Hall coefficient and thermopower are also observed. Electronic densities of states are found to vary rapidly with alloy composition. The unusual electronic properties of this new class of barely metallic systems are discussed in light of current theories on electronic structure and transport of icosahedral crystals.

A wide-terrace single-domain Si(001)c(4×2) surface has been obtained by cooling a wide-terrace single-domain Si(001)2×1 surface to 200–80 K. Angle-resolved ultraviolet photoelectron spectra have been measured for the Si(001)c(4×2) surface at 200–100 K and compared with those for the Si(001)2×1 surface. The electronic structure of the Si(001)c(4×2) surface appears to be explained as that expected for ‘‘antiferromagnetic’’ order of asymmetric dimers of surface Si. The electronic structure of the Si(001)2×1 surface appears to be reminiscent of the c(4×2) surface.

Femtosecond dynamics of photogenerated quasiparticles in ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ superconducting thin films shows, at T≤${\mathit{T}}_{\mathit{c}}$, two main electronic processes: (i) quasiparticle avalanche production during hot-carrier thermalization, which takes about 300 fsec; (ii) recombination of quasiparticles to form Cooper pairs, which is completed within 5 psec. In contrastr, nonsuperconducting epitaxial films such as ${\mathrm{PrBa}}_2$${\mathrm{Cu}}_2$${\mathrm{O}}_7$ and ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_6$ show regular picosecond electronic response.

Resonant neutron-absorption spectroscopy has been used to determine the temperature dependence of the average kinetic energy 〈E〉 of the Cu in ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{CaCu}}_2$${\mathrm{O}}_8$ for the directions along and perpendicular to the c axis. 〈E〉 is directly related to the Cu phonon density of states and shows essentially normal behavior as a function of temperature for the c-axis direction. However, a sudden drop in 〈E〉 is observed at the superconducting transition for the motions in the Cu-O planes.

Experiments clearly prove that narrow peaks in the fluorescence-excitation spectrum of a pentacene-doped p-terphenyl crystal stem from single molecules. This claim is supported by the distribution, width, and height of the peaks, as well as by the correlation of the emitted light and the sudden drops and surges of the emission of certain peaks. We attribute these to the hole burning of a single molecule. These results show the feasibility of the optical study of a single molecule and its local environment.

We report on novel interaction phenomena between different Stark ladders in a strongly coupled GaAs-AlAs superlattice. We observe an anticrossing behavior between certain exciton states if the electric field perpendicular to the layers of the superlattice is such that the energy spacing between the lowest two conduction subbands is an integer multiple of the Stark-ladder spacing. This anticrossing is accompanied by a spatial delocalization of the electron states and can therefore be resolved by photocurrent spectroscopy. Our experimental results are confirmed by a numerical calculation of the field dependence of the conduction-subband energies.

High-${\mathit{T}}_{\mathit{c}}$ Cu oxides such as ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ and the Bi-Sr-Ca-Cu-O systems are characterized by a near-zero oxygen-isotope effect. Recent results by Crawford et al. show, however, that the isotope effect in the ${\mathrm{La}}_{2\mathrm{-}\mathit{x}}$${\mathrm{Sr}}_{\mathit{x}}$${\mathrm{CuO}}_4$ system depends strongly on the doping level (x) and can significantly exceed the BCS limit. Within the framework of the BCS phonon-mediated pairing, a logarithmic (2D) Van Hove singularity in the density of states can provide a basis for understanding these anomalous isotope effects and possibly the origin of high-temperature superconductivity.

The electronic structure of seven ideal metal/GaAs interfaces is calculated self-consistently, within the local-density approximation. Calculated pinning positions ranged from 0.3 to 1 eV above the valence-band maximum. The metal d electorns are found to play a significant role in the electronic structure of the ideal interface, and in determining the Schottky-barrier height. These calculations contradict models that invoke intrinsic interface states to explain the experimentally observed Fermi-level pinning.

We have calculated the mgnetic coupling of two layers of iron through an intervening layer of chromium. We use the mixing interaction between iron atoms at the interfaces with the conduction electrons of chromium and the band structure of paramagnetic chromium. When we take into account the roughness of the interface, our calculated coupling reproduces the essential features of the coupling found in Fe/Cr superlattices.

We present a Landau-Ginzburg theory for oil-water-surfactant mixtures with a scalar order parameter ψ for the oil-water subsystem and a vector τ for the surfactant part. A physical interpretation of the parameters in terms of single-membrane energetics is given. A detailed density-functional calculation yields three-phase coexistence between oil-rich, water-rich, and microemulsion phases, and modulated phases including lamellar, columnar, and cubic phases. The computed structure factor for water in the microemulsion phase shows a variety of trends in qualitative accord with experiments.