We present a model for the cluster-impact-fusion experiments of Buehler, Friedlander, and Friedman, Calculated fusion rates as a function of bombarding energy for constant cluster size agree well with experiment. The dependence of the fusion rate on cluster size at fixed bombarding energy is explained qualitatively. The role of correlated, coherent collisions in enhanced energy loss by clusters is emphasized.

Neon field-ion image spots of impurities deposited from a Ta wire source on the Ir(100) surface show a halo-ring structure. Using an atom-probe field-ion microscope, these impurities are tentatively identified as tantalum hydrides. This halo-ring image-spot structure is most probably produced by the perturbation of the electronic charge-density distribution of the substrate around the impurity by its adsorption.

A new ordering vector k=(2π/a)(0, 2/3, 2/3) for fcc antiferromagnets has been found by neutron-diffraction experiments at nanokelvin temperatures in the nuclear-spin system of a ${}_{}{}^{65}{}_{}{}^{}\mathrm{Cu}$ single crystal. The corresponding reflection together with the previously observed (100) Bragg peak show the presence of three antiferromagnetic phases, separated by two first-order phase transitions, as the external magnetic field is varied between zero and ${\mathit{B}}_{\mathit{c}}$=0.25 mT. The kinetics of the transitions was observed to be on the order of seconds.

Experimental studies are made of the response of bound pairs of magnetic holes (nonmagnetic microspheres in ferrofluid) subjected to a rotating magnetic field. For increasing driving frequency, the motion of the system goes through a transition from a state where the pair axis follows the magnetic field with a constant phase delay to a state where the phase delay increases in a series of kinks. The equation of motion for the rotating pair is found analytically and numerical solutions show good agreement with experiments.

We have measured the luminescence and optical-absorption change induced by optical excitation at the exciton band perturbed by a triplet self-trapped exciton (STE) in RbI. We have identified two channels of deexcitation of the exciton complexes thus generated, comprising an exciton and a STE: the transformation to an F-H pair with a yield of 0.04 and the transition to the singlet luminescent state of a STE with a yield of 0.38. These results are interpreted in terms of exciton fusion, either accompanied with specific lattice relaxation or due to Auger transitions.

We discuss the existence of electromagnetic Stark ladder in some optic systems by studying the propagation of light. The transmission coefficient is calculated by means of the transfer-matrix method. It is shown that although the transfer matrix is different from the quantum case it can give, in some particular configurations, a Stark-ladder resonance structure similar to the one observed in quantum systems. The relation between these resonances and a Fabry-Perot-like interference is discussed. Consideration is given to the observability of these ladders in the laboratory.

The localization properties of noninteracting electrons in the lowest Landau band of a disordered two-dimensional system are investigated using a new version of the numerical finite-size scaling method which includes a quantitative statistical procedure of data evaluation. In contrast to other suggestions, universal one-parameter scaling behavior is obtained. The localization length diverges at the center of the band with a critical exponent ν=2.34±0.04. The critical region is determined to be about half of the bandwidth. Comparison with experimental quantum-Hall-effect data yields the temperature exponent of the phase breaking time p=2.0±0.2.

We report on a new technique, spin-polarized electron emission spectroscopy. Using grazing-angle, surface scattering of ${\mathrm{H}}^{+}$ and ${\mathrm{He}}^{+}$ ions at Ni(110), we find that the angle-resolved energy distribution of emitted spin-polarized electrons is significantly different from that of electron-induced secondary (cascade) electrons and exhibits a series of characteristic ‘‘peaks’’ (including element-specific Auger electrons). We obtain characteristic, spin-dependent information on the surface electronic structure of Ni(110) and on atom-surface charge-transfer processes.

The magnetic states occurring in the two-dimensional Hubbard model in the vicinity of half-filling of the conduction band are studied in the case of weak correlations, using Hartree-Fock theory. It is shown that at zero temperature the commensurate antiferromagnetic state is unstable against domain-wall formation for arbitrarily small deviations from half-filling. The resulting incommensurate antiferromagnet remains initially insulating. The predictions of the model for the polarization and modulation wave vector of the incommensurate antiferromagnet are discussed.

Correlation functions of the spin-1/2 2D Heisenberg antiferromagnet at low temperatures are computed via a large-scale Monte Carlo simulation on 128×128 lattices. The correlation length is found to be accurately described by the exponentially divergent form typical of classical Heisenberg spins in two dimensions. The large correlation lengths measured are directly compared with the neutron-scattering experiments on ${\mathrm{La}}_2$${\mathrm{CuO}}_4$. The excellent fit provides a first-principles determination of the exchange coupling: J=1450 ±30 K.

The photodissociation of HI in clusters of the type ${\mathrm{Xe}}_{\mathit{N}}$HI is studied theoretically to explore the possible occurrence and dynamics of the cage effect in small clusters (from N=1 to 12). The method used describes the H atom quantum mechanically, the heavy atoms classically. It is found that a cage effect, corresponding to transient vibrations of the H between heavy atoms, exists for all clusters, including XeHI. For N<5 these resonances are short lived and contribute little. For N>5 the resonances are long lived and dominate the process. Steric and structural factors cause this behavior.

Rheological measurements have been performed on pectin biopolymers close to the sol-gel transition. From these measurements scaling exponents were determined independently for the viscosity, s=0.82(5), for the elastic modulus, t=1.93(8), for the frequency-dependent modulus, Δ=0.71(2), and for the relaxation times below and above the transition, νz=2.67(12) and νz’=2.65(9). The exponents satisfy the scaling relations predicted by the theory and their numerical values agree with those from scalar elasticity percolation. Universal scaling functions were constructed from the data for the complex modulus and the dynamic viscosity.

An extensive set of measurements of low-frequency conductivity σ of a three-component microemulsion system, sodium-di-2-ethyl-hexylsulfosuccinate (AOT)-water-decane, as a function of temperature (T from 10 to 50 °C) and volume fraction of the dispersed phase (φ from 0.098 to 0.65) has been made. The measurements were made at a fixed value of the water-to-surfactant molar ratio equal to 40.8, where the microemulsion consists of AOT-coated water droplets, 85 Å in radius, dispersed in oil. σ can be quantitatively calculated at low φ by means of a charge fluctuation model of Eicke et al., while it can be interpreted for higher vlaues of φ in terms of power-law behavior with indices related to the dynamic percolation picture below the threshold and the static one above it. The percolation locus in φ-T plane has been determined starting from the vicinity of the lower consolute point up to φ=0.65. This line can be successfully interpreted in terms of a modified version of the analytical theory of percolation given by Xu and Stell.

Dynamical properties of the neural networks with asymmetrical synaptic couplings respecting the Dale hypothesis are studied. The time evolution of the networks is assumed to obey stochastic dynamics of the Little type with time delay. Using a nonlinear master equation, exact equations are derived for the time evolution of the overlaps of instantaneous configuration with p embedded patterns and with the characteristic pattern representing the configuration of excitatory and inhibitory neurons. It is shown that the networks exhibit noise-driven chaotic motions in the retrieval process.

Femtosecond studies of the formation, decay, and absorption spectra of the wet or presolvated electron are presented. The discovery of an isosbestic wavelength shows for the first time that the wet-to-solvated electron transition involves only two states. The model of the wet electron as an excited state of the solvated electron and the wet-to-solvated electron transition, viewed as a radiationless transition, is discussed. An ultrafast component of the visible absorption signal is believed to be ${\mathrm{H}}_2$${\mathrm{O}}^{+}$.

For any Ising model on a planar lattice without bond crossings, we prove that the anisotropic interface tension is given by the lattice Green’s function of a free random-walk problem defined on the dual lattice. This fact, derived via Vdovichenko’s combinatorial method, reveals a general relation between the bulk free energy and the interface tension for two-dimensional Ising models. As an important consequence, we show that the equilibrium crystal shape corresponds to imaginary zeros of the partition function. We also discuss generalizations to non-Ising and non-solvable systems.

A new theorem of inverse formula is introduced for a kind of infinite series. Thus some new results for important inverse problems in physics are presented in this paper. These are the inverse problems for obtaining the phonon density of states, the inverse blackbody radiation problem for remote sensing, and the solution for inverse Ewald summation. Of more importance, it shows the possibility of the application of number theory to physical problems.

It is shown that one can convert a chaotic attractor to any one of a large number of possible attracting time-periodic motions by making only small time-dependent perturbations of an available system parameter. The method utilizes delay coordinate embedding, and so is applicable to experimental situations in which a priori analytical knowledge of the system dynamics is not available. Important issues include the length of the chaotic transient preceding the periodic motion, and the effect of noise. These are illustrated with a numerical example.

We examine propagation of gravitational wave packets in ${\mathit{T}}^3$×R spacetimes with one and two global spatial Killing vectors. The analysis migrates from a regime where the linearized solutions are valid to domains that can only be addressed by numerical solution, with the intention of understanding the first nonlinear effects of the Einstein equations.

A test of Newton’s inverse-square law of gravitation by Eckhardt, Jekeli, Lazarewicz, Romaides, and Sands using gravity measurements on a tall tower yielded evidence for non-Newtonian gravity. We have refined the analysis of that experiment by including detailed topographic information and conclude that, in fact, no such evidence exists. The accuracy of the experiment constrains the Yukawa-potential coupling constant α to be less than 0.001.