The alloying behavior of zinc atoms into gold clusters has been studied using a 200 kV transmission electron microscope equipped with a double-source evaporator in the specimen chamber. Isolated gold clusters were first prepared on a supporting film of amorphous carbon, and zinc atoms were then deposited onto the same film. Upon the deposition of zinc, the gold clusters quickly changed into Au-Zn solid solution clusters, disoroderd ${\mathrm{\alpha }}_1$-${\mathrm{Au}}_3$Zn clusters, or ordered β’-AuZn clusters depending on the amount of deposited zinc. The spontaneous alloying which results in the formation of compound clusters is observed here for the first time.

α-quartz becomes x-ray amorphous when compressed, at ambient temperature, by pressures of 25 to 30 GPa. [Hemley et al. Nature 334, 5 (1988)]. The relationship between pressure-amorphized and melt-quenched silica is of great interest. The most fundamental characteristic of melt-quenched silica is its lack of periodicity at the atomic level. We report here that high-resolution electron microscopy shows that α-quartz, pressure amorphized at 30.5 GPa is, as is conventional melt-quenched silica, amorphous at the atomic level.

We characterize the orientational dynamics of ${\mathrm{C}}_{60}^{3\mathrm{-}}$ anions in ${\mathrm{K}}_3$${\mathrm{C}}_{60}$, using two-dimensional NMR exchange spectroscopy. We find an activated temperature dependence for the reorientation rate between 195 and 220 K, with ${\mathit{E}}_{\mathit{a}}$=460±60 meV. Molecular reorientation proceeds via small (<44°) rotational jumps, implying that ${\mathrm{K}}_3$${\mathrm{C}}_{60}$ is more highly orientationally disordered than previously proposed.

The dynamics of a simple model for step bunching during crystal growth is studied by propagating a disturbance into an unstable system of equidistant steps. Depending on the initial step spacing, we find a wide range of different bunching modes, leading to distinctive spatial patterns: periodic (with subharmonic bifuractions), chaotic, and intermittent. Marginal stability theory gives extremely accurate predictions of the velocity of the propagating front and the existence and location of one of the bifurcations.

On the basis of a detailed stochastic simulation we find that nonequilibrium growth in 2+1 dimensions, within the simple solid-on-solid approximation, is surprisingly rich with its dynamical universality depending sensitively on the local atomistic relaxation rules of the growth model. We establish connections between our computed dynamical growth exponents for various physically plausible local growth models with those given by a recently proposed fourth-order nonlinear continuum differential equation.

We present a general theory of compounds with partial long-range order. We derive a simple formula that determines the properties of a partially ordered compound from those of the perfectly random alloy and the fully ordered compound. The formula makes accurate predictions of both formulation energies and electronic band structures. We also use the formula to predict the band gaps of ${\mathrm{Al}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Ga}}_{\mathit{x}}$As/GaAs superlattices.

Dynamic scaling behavior has been observed during the growth of Fe films on Fe(001) using high-resolution low-energy electron diffraction technique. The interface width grows with time according to the power law w∼${\mathit{t}}^{\mathrm{\beta }}$, with β=0.22±0.02. Time-invariant self-affine behavior on the short-range scale has also been observed with the roughness exponent α=0.79±0.05. The time-invariant characteristic agrees with the recent prediction based on the dynamic scaling theory. From the measured growth exponents, we suggest that the growth of Fe film is more consistent with a conservative growth process.

The thermal expansion of single-crystalline ${\mathrm{C}}_6$ is studied using high-resolution capacitance dilatometry. In addition to the orientational-ordering transition at 261 K, we observe a transition at ${\mathit{T}}_{\mathit{g}}$≊90 K due to the freezing in of orientational disorder, which has the character of a glass transition. The relaxation near ${\mathit{T}}_{\mathit{g}}$ is studied by varying the heating (cooling) rate over more than two decades. It is found to obey an Arrhenius behavior with ${\mathit{E}}_{\mathit{a}}$=288±5 meV within a simple glass-transition model.

Dilute aqueous polystyrene suspensions are found to exhibit a novel vapor-liquid condensation. Upon deionization, weakly interacting homogeneous suspensions below a critical particle concentration condense into a concentrated phase with liquidlike order and a dilute vapor phase. This phenomenon strongly suggests net attraction between particles at interparticle separation several times the particle diameter. The present results are understood on the basis of an effective interparticle model potential.

Using a molecular dynamics computer simulation technique, evaporation and condensation processes of liquid methanol are studied at room temperature. We observe evaporation of a hydrogen-bonded dimer, which was experimentally suggested. The condensation coefficient (the number of ratio of condensed molecules to incident ones) seems to be less than unity as several groups predicted. Since various modes of dynamic processes at the surface are found, more careful analysis and theoretical consideration are necessary.

Results of an ab initio pseudopotential total energy calculation of the atomic structure and surface energy of the 3×2 reconstruction of the (113) surface of Si are presented. Despite many reports in the literature that (113) is a particularly low energy surface of Si, the calculated surface energy of 0.138 eV A${\mathrm{̊}}^{\mathrm{-}2}$ is not small in comparison with similar calculations of the surface energy of Si(111) and Si(100). This discrepancy is discussed with reference to the role of carbon contamination in stabilizing the (113) surface.

The microscopic mechanisms for Zn diffusion in GaAs and Zn-induced interdiffusion in GaAs/AlAs superlattices are investigated by ab initio molecular dynamics. Among the various proposed mechanisms for Zn diffusion, kick-out by Ga interstitials has the lowest activation energy. Zn in-diffusion generates nonequilibrium group-III interstitials, which are bound to Zn by Coulomb forces. The interstitials follow the Zn diffusion front and disorder the superlattice. The calculated activation energies for these processes are in good agreement with the experimental data.

An effective medium Hamiltonian is used to describe the electronic excitations in a continuous random network model of amorphous graphite. The effective medium is obtained from an operator which describes the structural relationships of the model. This operator commutes with a Hamiltionian which differs from the network Hamiltonian by a perturbing potential which is shown to be exponentially localized on odd-membered rings. The localization of this potential transforms the extended odd-membered ring defects into pointlike impurity defects, which may be treated in an effective Hamiltonian theory.

The thermoelectric power of K- and Rb-doped ${\mathrm{C}}_{60}$ superconductors prepared from single crystals grown from a ${\mathrm{CS}}_2$ solution is reported. Nearly linear temperature dependences of the thermoelectric power with negative sign are observed above the superconducting transition temperatures for both materials. From the slope of the linear correlation, the ratio of the density of states at the Fermi energy, ${\mathit{N}}^{\mathrm{Rb}}$(${\mathit{E}}_{\mathit{F}}$)/${\mathit{N}}^{\mathrm{K}}$(${\mathit{E}}_{\mathit{F}}$), is estimated to be 1.5–1.8.

We report on the first observation of coherent terahertz radiation, tunable from 1.4 to 2.6 THz, emerging from GaAs/${\mathrm{Al}}_{0.3}$${\mathrm{Ga}}_{0.7}$As single quantum wells after the coherent optical excitation of both light hole and heavy hole excitons. We attribute the radiation to charge oscillations following the coherent excitation of the excitons in an electric field. Terahertz radiation is also emitted when no electric field is present in the sample suggesting that valence band mixing leads to a significant far-infrared transition dipole moment between the light hole and heavy hole subbands.

Tunneling between parallel two-dimensional electron gases in double quantum wells is examined at both zero and high perpendicular magnetic field. The measured I-V characteristics show the magnetic field qualitatively alters the tunneling density of states, creating both a wide gap at the Fermi level and a much broader density of final states than is present at zero field. We suggest the origin of these effects lies in the strong Coulomb correlations characteristic of Landau quantized two-dimensional systems.

We have used magnetic decoration to study the pattern of vortices formed near an isolated sawtooth twin boundary in single-crystal ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$. When the pitch of the sawtooth is comparable to both the intervortex spacing and the penetration depth, we find an unusual vortex structure which has a reduced symmetry relative to the sawtooth itself. This vortex pattern arises from the magnetic interaction between the vortices and the twin boundary which occurs for an asymmetric twin. This is the first observation of a vortex structure caused by the magnetic interaction with a crystalline defect.

Measurements of the irreversibility line H(${\mathit{T}}^{\mathrm{*}}$) of ${\mathrm{Y}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Pr}}_{\mathit{x}}$${\mathrm{Ba}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{6.97}$, a system in which the superconducting transition ${\mathit{T}}_{\mathit{c}}$ can be varied from 0 to 92 K, show that H(${\mathit{T}}^{\mathrm{*}}$) obeys a universal scaling relation characterized by an m=3/2 power law near ${\mathit{T}}_{\mathit{c}}$, with a crossover to a more rapid temperature dependence below T/${\mathit{T}}_{\mathit{c}}$≊0.6. The applicability of the scaling formula to other high ${\mathit{T}}_{\mathit{c}}$ superconductors is tested, with the results suggesting that both the crossover phenomenon and the power law temperature dependence are ubiquitous, scale-invariant properties of high temperature superconductors.

A large shift of the onset of flux-flow resistivity and the irreversibility line ${\mathit{H}}_{\mathrm{irr}}$(T) to higher temperatures is observed in ${\mathrm{Tl}}_2$${\mathrm{Ba}}_2$${\mathrm{Ca}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{10}$ films containing linear defects created by ${\mathrm{Ag}}^{+21\mathrm{}}$ ion irradiation. The ${\mathit{H}}_{\mathrm{irr}}$(T), which has a characteristic ssL shape in highly anisotropic Tl and Bi based cuprates, becomes more like that of ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ in the presence of these defects. The ${\mathit{J}}_{\mathit{c}}$ at 77 K also shows a large increase as a result of flux localization at the defects. The transport data indicate that in the H-T plane above ${\mathit{H}}_{\mathrm{irr}}$(T) of the unirradiated material, an ensemble of unoccupied defects is required for effective pinning of each flux line in the system.

The occurrence of superconductivity due to nonretarded attraction is demonstrated in a class of repulsively interacting electron systems that consist of a carrier band interacting with an insulating band. The superconductivity, which can be mapped to that in the attractive Hubbard model, has been confirmed from both the canonical transformation of the Hamiltonian and quantum Monte Carlo results. We indicate essential differences of the present models from existing models, including the d-${\mathit{p}}_{\mathrm{\sigma }}$ model, and also discuss the relevance of our models to high-${\mathit{T}}_{\mathit{c}}$ materials.