Laser beams are crossed in a two-dimensional colloidal liquid of strongly interacting spherical particles to produce an external periodic potential and stimulate a density modulation. If the modulation wave vector is at the peak in the static structure factor, a sufficiently large potential induces a phase transition to a structure which exhibits solidlike order having density modes other than those directly excited. A Landau theory is presented and discussed in terms of our results.

A new approach is developed to calculate electron-phonon interactions in metals. The method is fully self-consistent and does not assume the rigid-ion or the rigid-muffin-tin approximation. Application is made to aluminum to compute the wave-vector—dependent electron-phonon coupling. The results are in good agreement with experimental estimates of the average coupling $\lambda$ and some previous calculations of this quantity.

Tip-stable, tip-oscillating, and tip-splitting types of dendrites were observed experimentally in a two-dimensional supersaturated N${\mathrm{H}}_4$Cl solution. The relation between the tip velocity $V$ and the tip curvature $K$ were measured for the tip-stable type. It was found that $V$ is proportional to $K$ instead of $K^2$ which is expected from the marginal-stability hypothesis in a two-dimensional melt-growth system.

The first observation of the overtone infrared absorption of the wagging mode of H on W(100) at saturation coverage is reported. This measurement provides the first unambiguous evidence for electron-hole-pair coupling of an adsorbate vibration on a metal substrate.

An analysis of electron diffraction patterns is used to describe the structure of rapidly cooled Al-Mn alloys displaying icosahedral point-group symmetry and long-range orientational order. The underlying structure is provided by a lattice, the unit cell of which involves 32 atomic sites, which satisfy icosahedral symmetry. The appearance of the electron diffraction patterns can be understood by the coexistence of two independent three-dimensional lattices which can be found from this unit-cell structure.

By means of the ion-channeling technique, we have shown the existence of a very low-energy (∼3 meV) excitation for deuterium impurities trapped to vacancies in nickel. The excitation is associated with a change of the D location from near octahedral to near tetrahedral. On the basis of calculations of the D potentials within the effective-medium theory and the quantum treatment of the D motion, we suggest that the observed state can be regarded as an excited standing wave of a particle (D) on a sphere (internal surface around the vacancy).

Analysis of the high-temperature susceptibility series for the random-field Ising model with a Gaussian distribution of random fields demonstrates the existence of a fluctuation-driven first-order transition below four dimensions, $d<\mathrm{}4\mathrm{}$. This, and the behavior of the susceptibility exponent in three dimensions, suggest that $d=4\mathrm{}$ is a "critical" dimension for the model.

We have used the resistance fluctuations of very small-area ($0.03 \mu {\mathrm{m}}^2<A<\mathrm{}2\mathrm{}\mu {\mathrm{m}}^2$) tunnel junctions to study the population kinetics of individual localized electron traps in the tunnel-barrier material. We present evidence that the rate-limiting kinetic step for charge fluctuations above ∼ 15 K is thermal activation between two low-energy ionic configurations. Below 15 K the transitions between these potential wells appear to be dominated by direct configurational tunneling of the ions forming the trap.

Experimental data on the transverse magnetoresistance in Nb${\mathrm{Se}}_3$, in the temperature range from 1 to 59 K, in magnetic fields of up to 227 kG, and for a variety of electrical currents are presented. They constitute clear evidence that (1) the observed behavior is caused neither exclusively by the normal galvanomagnetic properties of the conduction electrons nor by the conduction properties of the sliding charge-density wave and (2) the low-temperature phase of Nb${\mathrm{Se}}_3$ is not, as it is ordinarily thought, a simple charge-density wave but exhibits a strong magnetic field dependence. The most probable structure of that phase is a mixed density wave, involving charge- and spin-density characteristics as well as a net magnetization.

A model based on the elastic scattering of exciton polaritons from residual neutral impurities is proposed to account for the line shape of the exciton-polariton photoluminescence emission in GaAs at low temperatures. The good agreement obtained with experimental spectra emphasizes the importance of impurity scattering on exciton-polariton transport for modes in the vicinity of the exciton energy. This model is applicable to any solid possessing dipole-active excitons.

A magnetic-exchange—induced valence transition has been observed for the first time for different compositions $x$ of ${\mathrm{Tm}}_{1-x}{\mathrm{Eu}}_x\mathrm{Se}$. This new effect is explained in terms of an exchange splitting $2\Delta E$ of the $5d$ conduction band which leads to a crossing of the lower $5d$ band edge with the ${\mathrm{Tm}}^2$ $4f^{13}$ state at a certain temperature. The polar magneto-optical Kerr effect of the ${\mathrm{Eu}}^{2+}$ $4f^7\to 4f^{6}5d$ transition is used to study $\Delta E$ as a function of temperature via the magnetic red shift. Volume changes of more than 1% have been observed which correspond to an exchange-induced valence change of the Tm ions by 0.15.

The density of states of two-dimensional electron systems in GaAs/AlGaAs single-layer and multilayer heterostructures has been determined through measurements of the high-field magnetization. Our results reveal a substantial density of states between Landau levels, even in high-mobility single quantum wells. There is no existing theoretical explanation for this anomaly.

On the lateral surfaces of a semiconductor superlattice, the structure along the direction of the superlattice axis displays the superlattice periodicity while along the perpendicular direction it is translationally invariant. We investigate the surface-plasma and magnetoplasma modes of such a surface as a function of the components of the wave vector along these two dissimilar directions.

Frequency-dependent cyclotron-resonance measurements are reported on ${\mathrm{Ga}}_{0.47}$${\mathrm{In}}_{0.53}$As-InP and ${\mathrm{Ga}}_{0.47}$${\mathrm{In}}_{0.53}$${\mathrm{A}\mathrm{s}-\mathrm{A}\mathrm{l}}_{0.48}$${\mathrm{In}}_{0.52}$As heterojunctions. Discontinuities in the effective mass occur at two frequencies as a result of resonant polaron coupling with both optic-phonon modes present in the ${\mathrm{Ga}}_{0.47}$${\mathrm{In}}_{0.53}$As alloy. The coupling occurs at the frequencies at the TO phonons, in contrast to measurements on bulk materials. Possible changes in the screening and polarization of the optic-phonon modes are considered.

Helical instabilities in an electron-hole plasma in Ge in parallel dc electric and magnetic fields are known to exhibit chaotic behavior. By fabricating probe contacts along the length of a Ge crystal we study the spatial structure of these instabilities, finding two types: (i) spatially coherent and temporally chaotic helical density waves characterized by strange attractors of fractal dimension $d\sim 3$, and (ii) beyond the onset of spatial incoherence, instabilities of indeterminately large fractal dimension $d>\mathrm{}8\mathrm{}$. A transition between these two types of behavior is observed as the applied fields are increased.

We present results for the gravitational radiation from the collapse to a black hole of rotating relativistic polytropes. The wave form closely resembles that emitted by a test particle infalling into a black hole, but the amplitude is reduced and opposite in sign. Less than 0.07% of the star's mass is converted to gravitational-wave energy. With sufficient rotation, the star bounces and no black hole forms. These results were obtained with a fully general-relativistic computer code that evolves rotating axisymmetric configurations and directly computes their gravitational-radiation emission.

We have studied the shape of large clusters in the lattice-animal, percolation, and growing-percolation models. By calculating the radius of gyration tensor we find that in these models the clusters have an anisotropic shape. The results suggest that the critical droplets in related isotropic equilibrium models, such as the Ising model, may also be anisotropic. We have also determined the leading nonanalytic correction-to-scaling exponent by analyzing the anisotropy data and find that for percolation in two dimensions $\theta \simeq 0.47$.

We study the correspondence between quantum and classical dynamics with Wigner and Husimi distribution functions, respectively. The nature of quantum-mechanical regular and irregular behaviors is discussed with the use of Wigner and Husimi distribution functions. By numerical studies of the chaotic state, the Husimi distribution function is found to be a better representation than the Wigner distribution function, because coarse graining is usually involved implicitly in an observational process using the Wigner distribution function.

A low-temperature approach for the critical Ising spin dynamics on percolation clusters is proposed. The average energy barrier to overturning a cluster of $s$ spins is shown to scale as $\ln s$. This basic result is supported by exact numerical calculations of the energy-barrier distribution for two-and three-dimensional percolation clusters, and is argued to be a consequence of scale invariance. The resulting temperature as well as size dependence of the relaxation-time scales agrees very well with the results of our Monte Carlo simulations.

A new and quite tractable model for spreading phenomena is proposed, which contains as a special case the Eden model and a model for epidemics. Two exponents are defined, one static and one kinetic ("growth"). The surprising feature is that the kinetic exponent can be continuously tuned while the static one does not change. Thus the dynamic universality classes are quite independent of the static one. This is the first one-cluster growth model showing dynamic universality classes unrelated to static ones and thereby yields insight into a generic feature for growth models.