A new optically detected magnetic resonance spectrum in ${\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}\mathrm{As}$ is reported and assigned to native Ga interstitials. Luminescence-quenching signals were observed over the energy region from 0.75 to 1.1 eV. The optically detected magnetic resonance is nearly isotropic with spin-Hamiltonian parameters $g=2.025\mathrm{}\pm 0.006$, central hyperfine splitting $A\left({}_{}{}^{69}{}_{}{}^{}\mathrm{Ga}\right)=0.050\mathrm{}\pm 0.001$ ${\mathrm{cm}}^{-1\mathrm{}}$, and $A\left({}_{}{}^{71}{}_{}{}^{}\mathrm{Ga}\right)=0.064\mathrm{}\pm 0.001$ ${\mathrm{cm}}^{-1\mathrm{}}$ for H near [001]. The strong hyperfine coupling denotes an electronic state of $A_1$ symmetry, which current theories predict for the Ga interstitial but not the Ga antisite. The slight anisotropy probably indicates that the ${\mathrm{Ga}}_i$ is paired with a second, unknown defect.

The phase diagram of particles interacting through a repulsive screened Coulomb (Yukawa) potential has been calculated. Such interactions describe charge-stabilized colloidal suspensions and provide a potential of variable shape which can be used to test general ideas about phase transitions, including the phenomenological Hansen-Verlet and Lindemann rules. Competition between bcc and fcc solid phases was explored through calculations of the free-energy difference. For a range of screening lengths we found a transition from a low-temperature fcc phase to a high-temperature bcc phase.

Constant-pressure molecular dynamics has been used to study the effect of random strains on the low-temperature structure of ${\left(\mathrm{KBr}\right)}_{1-x}{\left(\mathrm{KCN}\right)}_x$ mixed crystals with $x\approx 0.75$. In accord with experiment, systems with a random arrangement of ${\mathrm{Br}}^{-}$ ions transform on cooling from a cubic phase to either a rhombohedral or a monoclinic structure. For an ordered $A_{3}B$ anion sublattice, however, the ground-state structure is similar to that of pure KCN, namely orthorhombic. It is concluded that strain-rotation coupling plays a critical role in the destabilization of the orthorhombic phase.

Dislocation generation is studied in a generalized two-dimensional Frenkel-Kontorova model which allows consideration of dislocations of arbitrary Burgers vector and line shape. Two new mechanisms for dislocation generation at high stresses have been found: (1) heterogeneous nucleation of dislocations on preexisting dislocations and (2) generation of dislocation loops in the wake of a moving dislocation. These new mechanisms provide an understanding of how materials generate the density of dislocations required to accommodate arbitrary strains at high strain rates.

We studied a class of mistakes or faults in quasilattices. The effect of a random distribution of mistakes on the diffraction of 1D, and a special class of 3D, quasilattices is calculated exactly. Mistakes change the diffraction pattern qualitatively: Some Bragg peaks decrease in intensity as expected, but some are enhanced. As a result some spots disappear and some new ones appear. The diffuse scattering is also calculated. Results are given comparing calculated diffraction patterns in fivefold, threefold, and twofold symmetry directions for a 3D quasicrystal with and without mistakes.

The flow of a polarized Fermi gas through channels from one cell to another can be used to study its transport properties. If the mean free path $\lambda$ in the gas is smaller than the radius $r$ of a channel, the time constant for the filling of an empty chamber can yield the viscosity of the gas in both the Boltzmann and degenerate cases. If $\lambda >r$ and the gas is degenerate, a novel effect is predicted: Because the Fermi velocities of the two spin species are different, the polarization in the second cell will initially have a value greater than the equilibrium value in the first cell.

X-ray diffraction has been used to measure the intensity profile of the two-dimensional rods of scattering from a single interface buried inside a bulk material. In both $\frac{\mathrm{Si}\mathrm{}\left(111\right)\mathrm{}}{a-\mathrm{Si}}$ and Si(111)/Si${\mathrm{O}}_2$ examples there are features in the perpendicular-momentum-transfer dependence which are not expected from an ideal sharp interface. The diffraction profiles are explained by models with partially ordered layers extending into the amorphous region. In the $\frac{\mathrm{Si}\mathrm{}\left(111\right)\mathrm{}}{a-\mathrm{Si}}$ case there is clear evidence of stacking faults which are attributed to residual 7×7 reconstruction.

We report a high-resolution x-ray study of a model multilayer fluid membrane system in the lyotropic $L_{\alpha }$ phase of a quaternary microemulsion system. The structure factor exhibits power-law behavior characteristic of a Landau-Peierls system. As a function of intermembrane distance $3.8 \mathrm{nm}\le d\le 16.3 \mathrm{nm}$, the exponent $\eta \mathrm{}\left(d\right)\mathrm{}$ which describes the algebraic decay of layer correlations is predicted by the model of Helfrich where entropic steric repulsions dominate intermembrane interactions.

The energy of an electron-hole pair in laser-excited semiconductor microcrystallites is computed with plasma screening and dielectric polarization taken into account. A strong excitation-induced blue shift of the absorption is predicted which causes a large optical nonlinearity for crystallite sizes exceeding the bulk-exciton Bohr radius.

Using diagrammatic Green's-function methods we investigate the criterion for the localization of an electromagnetic wave in a dielectric medium containing randomly distributed metallic spheres. We show by calculating the scattering length using the $t$-matrix approach and Mie theory that this criterion can be satisfied for reasonable values of the concentration of metallic spheres and other experimental parameters. It is proposed that effects of this strong localization can be observed in optical-absorption or -transmission experiments on a suspension of silver particles.

Spatially resolved electron-energy-loss scattering has been used to study changes in the inelastic scattering near the bulk band-gap energy for locations near the GaAs-${\mathrm{Ga}}_{0.85}$${\mathrm{In}}_{0.15}$As interface. We observe the expected bulk band gap on either side of the interface. At a single interface-misfit dislocation we observe scattering which is consistent with an excitation of transitions between a localized state near the dislocation and the crystal conduction band. Within this interpretation, the energy of the state is estimated to be 0.7 ± 0.05 eV above the GaAs valence-band maximum.

In YbPd, ${}_{}{}^{170}{}_{}{}^{}\mathrm{Yb}$ Mössbauer measurements show that two different Yb charge states, one magnetic and one nonmagnetic, are present in essentially equal proportions. For the magnetic fraction the saturated spontaneous magnetic moment is $1.15{\mu }_{\mathrm{B}}$, which is lower than that in any of the ${\mathrm{Yb}}^{3+}$ cubic crystal-field levels, and there is no evidence of any anisotropy in the $4f$-shell charge distribution. These results suggest the presence of $4f$-conduction-band hybridization. Both above and below the ordering temperature of the magnetic fraction, the remaining Yb possess a negligibly small magnetic susceptibility and an isomer-shift value characteristic of a charge state close to ${\mathrm{Yb}}^{2+}$.

A model in which photoexcited electrons in small metal particles are scattered elastically from the boundaries and travel inelastically within is included in a free-electron theory which successfully explains the enhanced photoelectric quantum yield of these systems relative to macroscopic metal surfaces. Application of the theory to the Ag-O-Cs photoemitter where elastic scattering dominates, and to Ag aerosols, where optical-thickness and energy-threshold differences between particles of different diameters dominate, gives good results with experiment.

A diamond-anvil cell is used to measure the pressure dependence of the superconducting transition temperature $T_c$ in both primitive hexagonal and hexagonal-close-packed Si up to 45 GPa. The experimental results verify the prediction of superconductivity in hcp Si and the measured curve of $T_c$ versus pressure is found to be in good qualitative agreement with theory, in contrast to a recent experiment by Lin et al. who claim complete disagreement with theory.

The effects of uniaxial pressure parallel to [001] on the magnetic ordering and on the critical fluctuations of CeSb have been studied by neutron diffraction and diffuse critical neutron scattering. We find that the system exhibits a critical end point at $p=2.0\mathrm{}\pm 0.3$ kbar and $T=18.1\mathrm{}\pm 0.2$ K, which is understood in terms of competing bilinear and higher-degree magnetic interactions favoring antiferromagnetic type-I ordering with a second-order transition and a modulated ordering with a first-order transition, respectively.

We propose a variational formalism, based on supersymmetry, for the calculation of the tunneling exchange integral. Good numerical results are obtained with minimal effort.

We consider a semiconductor of arbitrary dimension subject to a strong monochromatic laser beam in the transparency region below the exciton resonance. We calculate the spectrum of collective excitations and explain the recently observed dynamical Stark effect of the exciton. For the first time, we derive from first principles the nonresonant nonlinear susceptibility ${\chi }^{\mathrm{}\left(3\right)\mathrm{}}$ including exciton correlations.

Precessing nuclear quadrupole moments are observed to induce oscillating electric dipole moments in neighboring atoms. The ${}_{}{}^{35}{}_{}{}^{}\mathrm{Cl}$ nuclei of a single crystal of NaCl${\mathrm{O}}_3$ placed between the plates of a capacitor are excited into precession by a rf pulse. After the pulse, the electric polarization induced by the chlorine nuclear quadrupole moments produces a voltage across the capacitor, which is amplified with a dc SQUID. This technique provides information on the location and polarizability of atoms near nuclear quadrupole moments in noncentrosymmetric crystals.

The effects of surface scattering on quantum transport in thin films and wires are considered. A nontrivial channel-mixing problem arising from random boundary conditions is solved exactly by a new method that we present here. We find that the full quantum-mechanical treatment of very pure systems leads to results qualitatively different from those of currently used quasiclassical theory. Our theory explains some recent experimental observations.

An $\mathrm{NP}$-complete problem which is not a spin-glass is exhibited. The $\mathrm{NP}$-complete problem 3-satisfiability is also embedded into a continuous analog system with no hills in the energy landscape obstructing solution of the problem. There is, however, a large flat plateau. This shows how sculpting of the energy surfaces of continuous analog systems to remove hills may fail to aid solution of embedded combinatorial optimization problems.