The discrepancy shown by the methods of Monte Carlo and molecular dynamics in hard-disk systems concerning the solid-liquid coexistence region is analyzed. Using a system containing 224 disks we were able to obtain the van der Waals-type loop curve which has so far only been obtained by the molecular-dynamics procedure for 870 hard disks. This result is due to an improvement in the choice of the most representative configurations in the Monte Carlo procedure.

The energy liberated in the recombination of a vacancy-interstitial pair in electron-irradiated $n$-type GaAs has been measured by differential thermal analysis. The stored energy is found to be on the order of 8 eV, a value similar to the theoretical one.

A moving nematic-isotropic interface is subject to several instabilities. Some are traceable to phenomena typical of liquid crystals; others are tentatively identified with a Mullins-Sekerka instability.

Total-energy calculations based on self-consistent effective-medium theory are used to investigate icosahedral phase stability in aluminum-transition-metal binary alloys. We identify the mechanism of cohesion and explicate trends observed in diffraction experiments.

By mapping the two-dimensional percolation problem on a Coulomb gas, we obtain the exact fractal dimension of the external perimeter (or "hull") of the infinite percolation cluster: $D_H=\frac{7}{4}$, in agreement with numerical estimates and a recent conjecture. We also determine an infinite set of exact exponents associated with various topologies of this hull. We argue finally that the different fractal dimensions observed recently by Grossman and Aharony, who modified the definition of the hull, are all equal to $D_e=\frac{4}{3}$.

X-ray scattering carried out in situ during electrochemical doping indicates an unusual sequence of structural transitions. On doping to ≈6-7 mole%, lightly doped regions coexist with a discommensurate-domain channel structure. As the Na concentration increases, the density of domain walls decreases. When the discommensuration density falls to zero, the Na channels form a commensurate $\sqrt{3}\times \sqrt{3}$ superlattice with respect to the (average) triangular lattice of ${\left(\mathrm{CH}\right)}_x$ chains. At still higher concentrations, the Na lattice becomes unstable and transforms into a disordered "fluid" phase within the ordered polyacetylene structure.

We have studied the effect of isotopic substitution on the superconducting $T_c$ in the 90-K superconductors ${\mathrm{Ba}}_2$Y${\mathrm{Cu}}_3$${\mathrm{O}}_7$ and ${\mathrm{Ba}}_2$Eu${\mathrm{Cu}}_3$${\mathrm{O}}_7$ by replacing ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$ with the heavier isotope ${}_{}{}^{18}{}_{}{}^{}\mathrm{O}$. Samples with approximately 75% of the ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$ replaced by ${}_{}{}^{18}{}_{}{}^{}\mathrm{O}$ were prepared by gas-phase ion exchange. In these samples the phonon frequencies, measured by Raman spectroscopy, are reduced by the expected ∼4%. The transition temperatures, however, are found to change by less than 0.2%. This change in $T_c$ is much less than that expected for strongly coupled phonon-mediated superconductivity.

An isotope effect has been searched for in the high-$T_c$ superconductor $\mathrm{Y}{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$ through substitution of ${}_{}{}^{18}{}_{}{}^{}\mathrm{O}$ for ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$. No shift in the superconducting transition temperature $T_c$ is observed by electrical resistivity or magnetic susceptibility measurements. We discuss the implications of this result for mechanisms of superconductivity in the high-$T_c$ oxides.

Single-crystal micro-Raman spectroscopy is used to characterize the structures and phonon properties of the $\mathrm{Y}{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{6+x}$ and ${\mathrm{Y}}_2$BaCu${\mathrm{O}}_5$ phases present in the original 93-K Y-Ba-Cu-O superconducting compound. The vibrational Raman spectrum of the opaque $\mathrm{Y}{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{6+x}$ phase consists of prominent bands at 142, 338, 483, and 595 ${\mathrm{cm}}^{-1\mathrm{}}$. Polarized spectra of 10-30-μm single crystals of this phase are consistent with the tetragonal average structure determined by single-crystal x-ray diffraction measurements on the same samples.

Conductance fluctuations in one-dimensional lines of length $L$ shorter than the phase-coherence length $L_{\phi }$ are not universal but diverge as $L^{-2\mathrm{}}$. Using the Onsager relations and voltage additivity, we show that the voltage fluctuations are independent of the distance between voltage probes. The antisymmetric (Hall-type) contribution to the voltage fluctuations is constant for all values of $L$. Measurements of the voltage fluctuations and correlation function between different regions in Au and Sb lines confirm these results.

Multiterminal measurements of magnetoresistance fluctuations in silicon inversion-layer nanostructures are extended to probe spacings $L\ll L_{\phi }$, the phase-preserving diffusion length. Unlike for $L>\mathrm{}{L}_{\phi }$, the sizes of the voltage fluctuations are independent of $L$, and have novel correlations consistent with independent potential fluctuations of each probe. The corresponding "conductance" fluctuations $\delta G\left(L\right)\mathrm{}$ are $\gg \frac{e^2}{h}$; however, this can be understood if each pair of probes effectively measures voltage fluctuations at scale $L_{\phi }$, determined by the condition $\delta G\left(L_{\phi }\right)=\frac{e^2}{h}$.

We present ESR data obtained in the high-field phase of the spin-Peierls system tetrathiafulvalene-Au${\mathrm{S}}_4$${\mathrm{C}}_4$${\left(\mathrm{C}{\mathrm{F}}_3\right)}_4$ (TTF-AuBDT). The measurements were performed on single crystals, and are the first ESR measurements performed in the high-field phase of this system. Our results demonstrate for the first time a marked effect of the magnetic structure of the high-field phase of a spin-Peierls system on the ESR spectrum. The data are analyzed in terms of the available theories for spin-Peierls systems. The results are in good agreement with the predictions of the theory.

The mobility of hot electrons in gallium arsenide has been measured with subpicosecond time resolution, following injection by 2-eV femtosecond pulses. The mobility immediately after injection was measured to be less than 500 ${\mathrm{cm}}^2$/V.s, indicating efficient electron transfer into the satellite $L$ valleys. The electron mobility then rose to a quasiequilibrium value of 4200 ${\mathrm{cm}}^2$/V.s, limited by electron-hole scattering. The rise time was observed to be between 1.8 and 3.2 ps depending on injected carrier density.

We have observed the delayed time spectrum of coherently scattered synchrotron-radiation pulses following a single nuclear Bragg reflection in ${}_{}{}^{57}{}_{}{}^{}\mathrm{Fe}$-yttrium iron garnet. A new quantum-beat spectrum with more hyperfine information was obtained by our taking $B_{\mathrm{int}}$ perpendicular to the scattering plane ($k_0$, $k_f$). The speedup of coherent decay at the Bragg angle due to "enhancement," and the quantumbeat modulation of the coherent decay rate, are shown.

Two-dimensional angular correlation of annihilation radiation (2D ACAR) experiments have been performed to study the electron-positron momentum densities in potassium-intercalated graphite and in highly oriented pyrolytic graphite. Magnetic-field-dependent experiments indicate that the high-intensity, anisotropic 2D ACAR component observed in ${\mathrm{C}}_{24}$K is at least partially due to quasi-2D positronium formation. In the highly oriented pyrolytic graphite 2D ACAR spectra, a low-intensity, nearly isotropic, thermally activated positronium component is found.

The dependence of cation-substitutional transition-metal impurity levels upon the host semiconductor is calculated self-consistently in a defect-molecule approach. Heterojunction band lineups and Schottky-barrier heights can be calculated within the same tight-binding model. In all three systems, the characteristic behavior is determined by an approximate local charge-neutrality condition imposed by electrostatic self-consistency ("pinning"), with a dangling-bond level playing the role of the "neutrality level." In this way we explain the observed correlation between transition-metal impurity levels, heterojunction band lineups, and Schottky barriers.

The Ising-type ferromagnetic Dy${\mathrm{}\left(\mathrm{OH}\right)\mathrm{}}_3$ shows a remarkable dynamic instability at temperatures below its Curie point. This instability is observed, over a wide range of frequencies in ac susceptibility measurements, as a large discontinuous jump in the response as a function of the ac-field amplitude. The jumps generally occur at two different field amplitudes corresponding to dynamic hysteresis. We have developed a model, confirmed by further measurements, to account for these phenomena.

When a charge-density-wave system is prepared in an unpolarized state the transient response to a step current shows a sharp divergence near the threshold for depinning. We interpret this singularity as a phase transition from the ordered pinned phase to the moving state.

The intrinsic oxide decomposition reaction Si+Si${\mathrm{O}}_2$→2SiO↑ at the Si${\mathrm{O}}_2$/Si interface is shown to be nucleated at existing defect sites prior to the growth of physical oxide voids. At lower temperatures than needed for void formation, these defects become electrically active, leading to low-field dielectric break-down unless sufficient ${\mathrm{O}}_2$ is available (low concentrations). The systematics of the required ${\mathrm{O}}_2$ suggests strongly that it reverses the initial decomposition by reoxidizing the SiO product at the interface.

It is argued in the context of coupled quadratic maps that macroscopically chaotic states do not occur in many-body systems with local interactions and random initial conditions. Such systems can exhibit chaos, but only locally; their collective behavior is periodic or stationary. The phase diagram for the coupled-map system as a function of control parameter and noise is presented, and the universality classes of the phase transitions identified.