Magnetotransport measurements in several GaAs/GaAlAs heterojunctions show a linear magnetoresistance at $30 \mathrm{K}<T<\mathrm{}120\mathrm{} \mathrm{K}$. At lower temperatures, where the quantum Hall effect is observed, the magnetoresistance is found to be proportional to the product of the magnetic field and the derivative of the Hall voltage with respect to the field. This can be seen as the extension of the classical linear magnetoresistance to the quantum regime. A relation between Hall voltage and magnetoresistance valid for all samples and temperatures is given.

Temperature-dependent inelastic-neutron-scattering measurements were performed on a single crystal of Ce${\mathrm{Pd}}_3$ over the energy range $-10<\hslash \omega <80\mathrm{}$ meV. The spectral response cannot be described by a single Lorentzian line as has been used for previous measurements on polycrystalline samples. The two energy scales which emerge from the data analysis correspond to those found in bulk measurements.

We deduce the evolution of electronic excitations in Kr clusters ($N=2-3000\mathrm{}$) from fluorescence excitation spectroscopy with synchrotron radiation. In small clusters ($N\le 30$), a broad absorption band appears, slightly red shifted compared with the atomic $5s{\left(\frac{3}{2}\right)}_1$ resonance line. Intermediate-size clusters ($50<N\le 200$) show absorption profiles which are mainly correlated with the surface excitons of the solid. The bulk excitons are pronounced only in large clusters ($N\ge 200$). Absorption processes at the surface and inside the cluster correlate well to the surface-to-volume ratio of the clusters.

Combining the methods of scattering theory and supersymmetric quantum mechanics we obtain relations between the $S$ matrix and its supersymmetric partner. These relations involve only asymptotic quantities and do not require knowledge of the dynamical details. For example, for coupled channels with no threshold differences the relations involve the asymptotic normalization constant of the bound state removed by supersymmetry.

The possibility is investigated that impurity spins in SQUID tunnel junctions might have a strong dissipative effect on macroscopic flux tunneling. A microscopic theory is given which shows that for temperatures lower than the Kondo temperature $T_K$ of the spins, there is a strong effect if $T_K\simeq {\omega }_J$, the Josephson plasma frequency. The connection with quantum measurement theory is discussed.

Limits on the galactic flux of quark nuggets are improved by many orders of magnitude on the assumption that pulsars exhibiting glitches must be neutron stars, not strange stars. Therefore no quark nuggets can have settled in the core of the neutron-star progenitor or have penetrated to the neutron-drip region after the pulsar was formed. This excludes nuggets with baryon number $A<\mathrm{}{10}^{35}$ as possible dark-matter candidates. The existence of strange stars in binary systems is questioned, and it is argued that quark nuggets are unlikely explanations of the solar neutrino problem.

Orbital angular momentum ($l$) distributions of ions in intermediate $n$ and $l$ states have been compared for 1.5-5-MeV carbon ions traversing C foils and He gases by projectile Auger spectroscopy. The degree of high-$l$ enhancement in C targets is quantitatively established. A picture involving multiple collisions between target atoms and the entrained electrons which accompany the ions explains the results.

Projectile-centered Rydberg states of highly charged, fast ions traversing thin solid targets show an unexpected abundance of high-$l$ states. We present a theory for the production of high-$l$ states based on classical stochastic dynamics. Diffusion into high-$l$ states is shown to be universal for single-particle orbits in three dimensions under the influence of a stochastic perturbation, i.e., largely independent of the details of the interaction potentials. Monte Carlo simulations using a Langevin equation for stochastically perturbed electrons in a dynamically screened Coulomb field yield quantitative agreement with experimental data.

A pair of correlated light quanta of 532-nm wavelength with the same linear polarization but divergent directions of propagation was produced by nonlinear optical parametric down conversion. Each light quantum was converted to a definite polarization eigenstate and was reflected by a turning mirror to superpose with the other at a beam splitter. For coincident detection at separated detectors, polarization correlations of the Einstein-Podolsky-Rosen-Bohm type were observed. We also observed a violation of Bell's inequality by 3 standard deviations.

Concentration contours in the displacement of a clear fluid by a colored fluid of the same viscosity and density in a two-dimensional porous medium are shown to be self-affine fractal curves with a fractal dimension $D\simeq 1.42\pm 0.05$. The dispersion front may on the average be described by the hydrodynamic dispersion with a longitudinal dispersion coefficient $D_{\parallel }=U{d}_{\parallel }$, where $U$ is the average flow velocity and $d_{\parallel }$ is a characteristic length of the order of a pore diameter. This result is valid for dispersion at high Péclet numbers $\mathrm{Pe}=\frac{U{d}_{\parallel }}{D_m}$, where $D_m$ is the molecular diffusion coefficient of the dye.

The collinear propagation of an arbitrary number of finite-amplitude waves is modeled by a system of coupled nonlinear Schrödinger equations. This model incorporates the effects of a coordinate-dependent external potential and coordinate-independent nonlinearities. A virial theorem is derived, which governs the nonlinear focusing of these coupled waves. For two light waves in a beat-wave accelerator, the coupling to each other and to the resonantly generated Langmuir wave has a significant effect on the focusing properties of the waves.

An experiment was designed to check theoretical predictions of charge homogenization and emittance growth in nonuniform space-charge-dominated beams due to conversion of field energy into transverse kinetic energy. Five beamlets were masked out of a solid 5-keV electron beam and injected into a periodic solenoidal focusing channel. Phosphor screen images showed that the beamlets merged into an almost uniform density single beam. Images of the five-beamlet configuration were observed at a distance of 7-8 lens periods. Experimental results on merging distance and emittance growth were compared with theory and simulation and good agreement was found.

Deuterium NMR of nematic droplets in a polymer matrix reveals the effect of confinement on molecular orientational ordering near the nematic-isotropic ($N-I$) phase transition. For large droplets of 0.35 μm in diameter, the $N-I$ transition is first order. For droplets of less than 0.035 μm in diameter, the isotropic phase shows a paranematic order and the $N-I$ transition is replaced by a continual evolvement from paranematic to nematic order. This result provides the first demonstration of Sheng's prediction that the $N-I$ coexistence curve terminates at a critical enclosure size.

The near-total-external-fluorescence technique was used to measure in situ the adsorption of a metal ion from a subphase solution to the liquid-air interface. For a monolayer formed by the spreading of stearic acid on a ${10}^{-3\mathrm{}}$-mole/l solution of Mn${\mathrm{Cl}}_2$, the ratio of Mn ions segregated to the interface to the number of stearate molecules on the surface was determined as 0.6 ± 0.2. Surface extended x-ray-absorption fine-structure experiments revealed local order of Mn ions at the surface at the condensed phase but no order showed in the expanded phase. We explain these findings using a self-consistent Poisson-Boltzmann calculation.

Based on a novel "exact enumeration" approach, we find evidence suggesting the existence of a phase transition in the multifractal spectrum of diffusion-limited aggregation. Above a critical point ${\beta }_c$, the moment expansion shows an infinite hierarchy of phases, while below ${\beta }_c$ we find a single phase. At ${\beta }_c$ we find fluctuations of all energy scales and singular behavior of the energy and specific heat. We also find that the maximum energy scales with system size $L$ as $E_{max}\mathrm{}\left(L\right)\mathrm{}\mathbf{\propto }\frac{L^2}{\mathrm{In}L}$. Consequently, for $\beta <{\beta }_c$ the partition function does not scale with $L$, which implies that the conventional moment expansion must break down.

We consider polymerized non-self-avoiding membranes fluctuating under constrained boundary conditions. Such constraints prevent the membranes from being crumpled at high temperature and introduce a tension. Upon lowering the temperature this tension reduces, and vanishes at a transition temperature below which the membrane assumes a buckled state. Buckling transition points, depending on boundary conditions, arrange on a line which terminates at the crumpling transition. We study the critical properties of these transitions by various field-theoretical techniques.

Direct methods have been studied for many years in the context of x-ray diffraction and it is the heavy atom technique of x-ray crystallography that inspires our approach to the surface problem. The idea is that we know the bulk structure lying behind the surface, and we can define a "reference surface" as near to the true surface structure as possible. It is subsequently treated as the "heavy atom" of the surface problem and all phases referred to the reference surface phases. The method is shown to work well for some simple examples. Future applications of greater sophistication are discussed.

The atomic structure occurring at the interface between Si(111) and thin GaAs epitaxial overlayers has a pronounced effect on the surface electronic structure, thus making it possible to infer structural information by comparing measured surface-state energies with those calculated for possible interface bonding arrangements. The atomically abrupt interface...SiAsGaAs, in addition to being energetically very unfavorable, is inconsistent with measured valence-band photoemission spectroscopy. Our results indicate that the interface is not atomically abrupt.

We have observed surface-molecule electron transfer from the semiconductor diamond (111) surface to the iodine molecule, following hyperthermal molecular-beam scattering. The negative-ion mass spectrum was dominated by undissociated molecular ions. No positive ions were observed. The threshold kinetic energy was ≅ 3 eV and an absolute yield of ≅ 1% was measured at 10-eV incident kinetic energy. The surface temperature effect on the yield was small. The ions' energy distribution was broad and the angular distribution was shifted towards the surface.

We extend the reptation model of DNA gel electrophoresis by replacing the tube by a chain of narrow "straits" connecting open "lakes." The DNA chain can accumulate easily in the lakes, thus allowing large fluctuations in the length of the chain. Unlike the standard model, this model, simulated on a computer, shows a sharp minimum in mobility as the period is varied with periodically reversing fields, similar to experiments.