Positrons implanted with varying energies (0-20 keV) have been used to study silicon epilayers grown by molecular-beam epitaxy on Si(100) substrates. Defects at the initial growth interface and throughout the overlayer have been observed and depth profiled. In addition, field-driven positron drift observed in some of the epilayers is shown to be consistent with estimated concentrations of (active) interfacial impurities. The study demonstrates that positrons can be used nondestructively to profile structural defects and electric fileds in thin films and at interfaces.

A microscopic theory for obtaining the polarized Raman spectrum of Fibonacci chains is developed and applied to GaAs-AlAs heterostructures. The results of the theory, without adjustable parameters, are compared with experimental data, and remarkable agreement is attained. The treatment is performed in real space and, although this system is nonperiodic in a strict sense, it retains many properties usually associated with the translational invariance of crystals. Because of its computational efficiency, this theory is suitable to other finite-size systems.

After recoil implantation of Coulomb-excited ${}_{}{}^{57}{}_{}{}^{}\mathrm{Fe}$ into $\alpha -\mathrm{Z}\mathrm{r}$ at 24 K, 30% of the implanted atoms end up on interstitial positions where they experience large electric field gradients. Between 40 and 100 K the quadrupole splitting shrinks by a factor of 2 accompanied by a sharp decrease of the line intensity. This effect is interpreted as a localized diffusion of the Fe atoms jumping in the octahedral cage of the hcp Zr lattice.

The empirical rule connecting transition-metal levels in semiconductors with band-edge offsets at semiconductor heterojunctions is theoretically justified by a complete Green's-function calculation. It is shown from a detailed analysis that the transition-metal levels are effectively pinned to the average dangling-bond energy, the latter being at the same time interpreted as the charge neutrality level which controls the band-edge offset of a heterojunction.

We report neutron-scattering experiments on single crystals of the layered perovskite ${\mathrm{La}}_2\mathrm{Ni}{\mathrm{O}}_{4+\delta }$. Three-dimensional magnetic order sets in at $T_N=70\mathrm{}$ K for $\delta \cong 0.05$. Strong two-dimensional magnetic correlations exist for $T$ between $T_N$ and the tetragonal-to-orthorhombic transition temperature, $T_s\approx 240$ K. As $T$ is increased through $T_s$, the 2D correlation length decreases from more than 50 to 10 Å. The excitation spectra for the 3D ordered state are characterized by large effective in-plane spin-wave velocities which, like $T_N$, depend strongly on $\delta$. For $\delta =0\mathrm{}\pm 0.001$, $c\gtrsim 0.2$ eV-Å, while for $\delta =0.05\mathrm{}$, $c\approx 0.1$ eV-Å.

We carried out a mean-field calculation of the antiferromagnetic instability of orthorhombic ${\mathrm{La}}_{2-x}{\mathrm{Sr}}_x\mathrm{Cu}{\mathrm{O}}_4$ as a function of the doping $x$ by looking at the eigenvalues of the vertex function. We found that there are two bands close to the Fermi energy that participate strongly in the instability. As a consequence of the tetragonal-to-orthorhombic transition, a saddle point with high local density of states is developed close to the Fermi energy. The magnetic instability is caused by a kind of Fermi-surface nesting with a wave vector connecting the saddle points. Good agreement of the transition temperature as a function of doping is found.

We present the first calculations of the electronic structure of the high-temperature superconductors in the disordered phase using the tight-binding coherent-potential approximation. These calculations are used to assess the influence of increasing oxygen vacancies when La is alloyed with $M$ (=Sr or Ba) in the system ${\mathrm{La}}_{2-x}{M}_x\mathrm{Cu}{\mathrm{O}}_{4-y}$. Contrary to simple doping arguments, it is found that deoxygenation does not raise the Fermi level ($E_F$). The increase in $T_c$ in the range $0.05<x<\mathrm{}0.15\mathrm{}$ and the decrease in $T_c$ beyond the peak at $x=0.15\mathrm{}$ follow the density of states at $E_F$.

We propose that a homologous series of transient ordered structures may form during the spinodal decomposition of the nonstoichiometric superconducting oxide $\mathrm{Y}{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-x}$. These transient phases provide the best kinetic accommodation of the local vacancy concentration. Their structures should be similar to those of the ${\mathrm{Ti}}_m{\mathrm{O}}_{2m-1}$ and ${\mathrm{Mo}}_m{\mathrm{O}}_{2m-1}$ series of Magneli phases. The transient phases in the oxide have the stoichiometry $\mathrm{Y}{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{\frac{7-n}{\mathrm{}(2n+1)\mathrm{}}}$, where $n$ is an integer. The structure of the $n$ phase has a diffraction maximum at the point ($\frac{n}{\mathrm{}(2n+1)\mathrm{} 00}$). Examples have been experimentally observed.

The critical current densities across grain boundaries have been measured as a function of misorientation angle in the basal plane of bicrystals of $\mathrm{Y}{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$. For small misorientation angles, the ratio of the grain-boundary critical current density to the bulk critical current density is roughly proportional to the inverse of the misorientation angle; for large angles, this ratio saturates to a value of about $\frac{1}{50}$. These results imply that achieving a high degree of texture both normal to and within the basal plane is important for the obtaining of very high critical currents in pure polycrystalline samples.

Anomalous relaxation observed in diluted antiferromagnets in a homogeneous external field at low temperature is explained for real, magnetic systems. For systems with weak crystal-field anisotropy the domain size is controlled by the Bruinsma-Aeppli mechanism and is field dependent, whereas bond randomness is irrelevant. If the anisotropy is strong the walls are pinned by random bonds. Random fields are merely responsible for readjustments of the interface on small scales explaining the relaxation of the magnetization after a decrease in the field.

Electron paramagnetic resonance studies on several donor- and acceptor-doped, oxygen-rich, silicon samples have been performed. The intensity of the spectrum Si-NL10 has been investigated as a function of heat-treatment time, temperature, and illumination. The results provide evidence for an acceptor character of the Si-NL10 center strongly supporting the recently proposed idea of relating it to the silicon thermal donor level T${\mathrm{D}}^{-}$.

It is argued by way of a renormalization-group analysis that the lower critical dimension of macroscopic mutual entrainment in a class of populations of oscillators satisfies a certain inequality which is sensitive to the tail of the distribution of native frequencies. This result is supported in part by numerical simulations as well as a proof of the absence of long-range order in one dimension.

We study three ways of processing of temporal sequences of patterns stored in a neural network; retrieval, counting, and recognition. These three processes are studied analytically in a strongly diluted neural network. The results compare qualitatively to simulations in fully connected networks.

High-power hf electromagnetic waves have been used to generate large electron-density cavities in the night-time $F$-region ionospheric plasma. Previous modification experiments have described induced density perturbations of only a few percent. Here we report the first observations of density depletions exceeding 50%, extending hundreds of kilometers along the geomagnetic field. These cavities are thermally driven, with electron temperatures in the depletions increased by factors of 3 to 4. The depletion dynamics exhibits many of the characteristics associated with thermal cavitons.

Sound velocity and damping measurements, taken at various angles $\theta$ between the normal to the smectic layers and the sound propagation direction, are reported for TBBA near its $\mathrm{Sm}A-\mathrm{S}\mathrm{m}C$ transition. Both velocity and damping present marked pretransitional anisotropic effects above $T_{\mathrm{Ac}}$. The anisotropy of the transition also manifests itself on the positions of the damping peaks of which that for $\theta =0^{\circ }$ moves towards low temperatures as the frequency increases, whereas no displacement is observed for 0=45° and 90°. The results are compared with the predictions of the current theories.

A simple model, describing a network of springs moving against friction, is used to study the evolution of surface patterns on a gel undergoing uniaxial expansion. The nonlinear growth equation obtained adequately describes the key experimental observations, such as the scaling of the typical size of patterns and the formation of cusps. The dynamics is carried on a computer, and the patterns obtained are in qualitative agreement with experiments.

We present novel calculations for the archetypal bistability dynamics driven by a correlated Gaussian random force (colored noise). Our focus is on the behavior at weak noise, which is difficult to solve theoretically as well as numerically. Precise numerical results for the smallest eigenvalue ${\lambda }_1\left(\tau \right)$ [or rate of escape $\Gamma \left(\tau \right)=\frac{1}{2}{\lambda }_1\left(\tau \right)$] at small to moderate to large noise correlation times $\tau$ are compared and interpreted versus a whole set of recent but conflicting theoretical predictions.

The existence of long-range order (LRO) at low temperature in the isotropic $\mathrm{XY}$ model with spin $S\geqq 1$ on simple cubic lattice in three or more dimensions is shown. This completes the proof of the existence of LRO at low temperature in the isotropic $\mathrm{XY}$ model with arbitrary spin size in three or more dimensions, as the proof for $S=\frac{1}{2}$ was given earlier by Dyson, Lieb, and Simon. The existence of LRO in the ground state is shown for $S\geqq \frac{3}{2}$ in two dimensions.

The laser-induced collective rotation of molecules in a liquid crystal is treated as a novel stimulated light-scattering process in which the Stokes shift is not constant but is induced by a pump field. The solution following the usual coupled-wave approach appears as a nonlinear eigenvalue problem and exhibits bistable behavior.

The optical Stark shift of the exciton is due to a coupling between the exciton and "semivirtual" biexcitonic states (bound and unbound). We show that at large detuning the excitonic shift is the same as the "dressed atom" blue shift. We predict that at small detuning the exciton-exciton interaction plays an important role and leads to an excitonic red shift at the two-photon absorption threshold when the molecular biexciton is stable.