The elastic constants of Ni are calculated with use of molecular dynamics; at ∼ 5% [111] axial strain there is a large (∼ 35%) softening of the $C_{44}$ shear modulus and a stiffening of the in-plane Young's (∼9.5%) and biaxial (∼8%) moduli. These calculations are in remarkable agreement with experimental $C_{44}$ for Mo/Ni superlattices. The experimentally observed transition to a highly disordered state occurs at the same strain value at which in the calculation the crystal becomes unstable.

For three-dimensional ordered media of periodic and partially periodic structure, homotopy theory allows the existence of nonsingular stable distortion fields, which are finite in size and thus resemble extended particles. It is shown that because of integrability conditions homotopy arguments are not adequate for these systems. By methods of differential topology it is proved that in three dimensions topological particles of this type are unstable, but that there are analogous fields in higher dimensions that cannot decay to a uniform ground state.

It has been discovered that a single relation between binding energy and interatomic spacing can describe the energetics of diatomic molecules, bulk metals, bimetallic interfaces, and gas atom-metal surface interactions.

A critical wetting (or interface delocalization) transition occurs when the interface between two fluid phases becomes infinitesimally bound to an attracting wall. It is shown that the critical exponents at this transition depend continuously on the parameter $\omega \equiv \frac{k_{\mathrm{B}}{T}_w}{\left(4\mathrm{}\pi {{\xi }_b}^2\sigma \right)}$, where $\sigma$ is the surface tension of the free interface, ${\xi }_b$ is the bulk correlation length in the attracted fluid phase, and $T_w$ is the transition temperature.

Extended x-ray-absorption fine-structure measurements on ${\mathrm{Nb}}_3$Ge at temperatures from 8 to 300 K using the Nb $K$ edge show large anisotropies in nearest-neighbor vibrational correlations arising from one-dimensional transition-metal chains in the $A15\mathrm{}$ structure. A simple view of these vibrations involves combinations of optical phonons which cause chains of Nb atoms to vibrate, nearly undistorted within the bcc lattice of Ge atoms, uncorrelated with vibrations of adjacent, perpendicular Nb chains.

The critical dynamics of Heisenberg spin systems at the percolation threshold is obtained from the transformation of the equations of motion under a length scaling acheived by climination of sites. The method is applied exactly to $d$-dimensional nonrandom fractals representing the infinite-cluster backbone and approximatcly to a truly random twodimensional system to yield dynamic scaling forms for response function and characteristic frequency, and associated static and dynamic critical exponents.

Ultrahigh-resolution transmission-electron micrographs display $3a_0$ periodic distortions of the crystal lattice in $2H$-Ta${\mathrm{Se}}_2$ associated with charge-density waves below 120 K. Approximate structure-factor magnitudes and signs within single domains of the commensurate phase are determined. The data fit well only one of the models recently derived from ${}_{}{}^{77}{}_{}{}^{}\mathrm{Se}$ NMR. The charge-density-wave amplitude is 30% less in the orthorhombic direction for this model. The center of symmetry within the orthorhombic unit cell is determined. Images from incommensurate phases are discussed.

A simple model showing a $\frac{1}{f}$ behavior is proposed. It is argued, on the basis of a scaling argument, that is has ${\left(\ln f\right)}^k$ corrections. Numerical simulations confirm this picture.

It is shown that a nonlinear Schrödinger soliton behaves stochastically with random phases in both time and space in the presence of small external oscillating fields and emits small-amplitude plane waves with random phases. Statistical properties of random phases give the energy spectra of the soliton and plane waves.

The time evolution of expanded and contracted solitary waves of the Dirac field with scalar self-interaction is exhibited. It is shown that the positivity of the second variation of the energy functional is not a necessary condition for the stability of these waves as has been recently suggested.

A search for fractional-charge impurities in semiconductors, using photothermal ionization spectroscopy, is described. It is shown that, depending on the concentration of compensating impurities, the detection sensitivity for fractional-charge impurities in ultrapure germanium is ${10}^7$ ${\mathrm{cm}}^{-3\mathrm{}}$ in a conventional configuration and, after modification ${10}^5$ ${\mathrm{cm}}^{-3\mathrm{}}$—i.e., one fractional charge per ${10}^{19}$ nucleons. From measured spectra of an ultrapure Ge sample it was deduced that it contained less than 1.5× ${10}^{11}$ ${\mathrm{cm}}^{-3\mathrm{}}$ acceptorlike fractional-charge impurities.

A measurement of the cross section for production of collinear muon pairs based upon a sample of about 3000 events observed in the MAC detector at the storage ring PEP is presented. From the angular asymmetry $A_{\mu \mu }=0.076\mathrm{}\pm 0.018$ the axial-vector weak neutral coupling is found to be given by ${g_A}^e{g_A}^{\mu }=0.31\mathrm{}\pm 0.08$.

In addition to being bounded from below by the Prasad-Sommerfiled limit, monopole masses are also bounded from above by the limit in which the scalar field variables are frozen to their vacuum values. This upper bound is close to the lower one: The single-, double-, and triple-strength SU(5) monopoles are found to have their masses bound in $m<~M<~m\times 1.7867$, $2m<~M<~2m\times 2.0741$, and $3m<~M<~3m\times 2.3155$, respectively, where $m=\frac{3M_x}{8\alpha }$.

A new tool for the study of heavy-ion reactions is obtained by adapting a $4\pi$-neutron-multiplicity counter to accelerator experiments. The instrument registers almost all neutrons emitted in each single reaction event. For quasielastic reactions of 290-MeV ${}_{}{}^{20}{}_{}{}^{}\mathrm{Ne}$+${}_{}{}^{197}{}_{}{}^{}\mathrm{Au}$ the measured neutron multiplicity distributions allow a quantitative decompsotion of the cross section into two contributions: one with higher $Q$ value from transfer reactions, one with lower $Q$ value due to the decay of the decay of the projectilelike fragments.

One-particle, one-hole and two-particle, two-hole level densities are calculated in the local-density, i.e., Thomas-Fermi, approach. For the harmonic oscillator this leads to analytical expressions which are in excellent agreement with the exact quantum results.

The crossing of the ground and the $S$ band and the interaction between these two in ${}_{}{}^{198}{}_{}{}^{}\mathrm{Hg}$ are studied for states of sharp angular momenta by projection from deformed intrinsic states and subsequent diagonalization. The $S$ band crosses the ground band producing back-bend around $8\hslash$, in agreement with experiment. The spectra as well as the rotation alignment of $i\frac{13}{2}$ neutrons are presented.

The transition-probability amplitudes for absorption of two photons in positronium are computed from the $1{}_{}{}^3{}_{}{}^{}S_1^{}{}_{}{}^{}$ ground state to the excited states $2{}_{}{}^3{}_{}{}^{}S_1^{}{}_{}{}^{}$ and $3{}_{}{}^3{}_{}{}^{}S_1^{}{}_{}{}^{}$ as functions of the frequencies of the photons. Sharp resonances are found in correspondence to the intermediate states and transparencies occur between each two resonances and also before the first resonance in the $1{}_{}{}^3{}_{}{}^{}S_1^{}{}_{}{}^{}-3\mathrm{}{}_{}{}^3{}_{}{}^{}S_1^{}{}_{}{}^{}$ transition. The detailed structure of the transition probability in proximity of the intermediate ${}_{}{}^3{}_{}{}^{}P$ states reproduces the excited-state structure of positronium with inclusion of relativistic and radiative corrections.

The ionization of $\mathrm{He}\left[1s\right(n\simeq 30\left)s\right]{}_{}{}^3{}_{}{}^{}S_1^{}{}_{}{}^{}$ atoms by an oscillating electric field (9.92 GHz) is strongly dependent on the field amplitude. The ionization curves reveal structures which are absent in those for $\mathrm{H}\left(n\simeq 30\right)$ atoms. The ionization rate does not increase monotonically near threshold and is approximately constant over one or more regions of higher field. A quasistatic model is considered which relates the structure in the ionization curves of He to the occurrence of anticrossings in He Stark states.

Electron cross sections and asymmetries following photoexcitation of the CO $K$ shells were measured directly. Energy analysis of ejected electrons yielded cross sections exhibiting $\sigma$ shape resonances in the $\mathrm{C}\mathrm{}\left(1s\right)\mathrm{}$, $\mathrm{C}\mathrm{}\left(\mathrm{KVV}\right)\mathrm{}$, and $\mathrm{O}\mathrm{}\left(1s\right)\mathrm{}$ channels, and a $\pi$ resonance in the $\mathrm{C}\mathrm{}\left(\mathrm{KVV}\right)\mathrm{}$ channel, confirming earlier indirect measurements. Asymmetry measurements showed a $\sigma$ shape resonance in the $\mathrm{C}\mathrm{}\left(1s\right)\mathrm{}$ channel, in good agreement with the predictions of Dehmer and Dill. The $\mathrm{C}\mathrm{}\left(\mathrm{KVV}\right)\mathrm{}$ $\pi$ resonance showed nearly isotropic behavior.