We formulate a microscopic theory for the quasiparticle interaction between ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ atoms adsorbed to the surface of liquid ${}_{}{}^4{}_{}{}^{}\mathrm{He}$. The calculated magnetic susceptibility of the ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ system is in good agreement with recent experiments by Valles et al. The coverage dependence of the magnetic susceptibility probes the momentum dependence of the quasiparticle interaction.

Pulse-time-of-flight sound propagation (group velocity) studies of the squashing mode of superfluid ${}_{}{}^3{}_{}{}^{}\mathrm{He}-B$ in a magnetic field have revealed four of the five expected Zeeman sublevels for this $J=2^{-}$ mode. Variation of the pressure at very low temperatures $\frac{T}{T_c}\le 0.35$ was used to sweep through the squashing mode at constant frequency (137.6 MHz). Observation of very low group velocities (down to ∼6 m/s) and pulse-shape analysis enabled the identification of the $J_z=+2, +1, 0, \mathrm{and} -1\mathrm{}$ sublevels. The Landé $g$ factor was found to be 0.042 near $T=0\mathrm{}$ and $P=19\mathrm{}$ bars as determined from the linear dependence of the $J_z=+2\mathrm{}$ Zeeman component.

Uniformly sized microspheres interacting via long-range magnetic dipolar forces are used to study diffusion-limited cluster aggregation in a plane. The results show that it is possible to scale the temporal evolution of the cluster size distribution and that there is a crossover in fractal dimension from $D=1.52\mathrm{}\pm 0.05$ to $D=1.16\mathrm{}\pm 0.05$ in the limit of weak and strong dipolar coupling. External magnetic fields are shown to produce pronounced chaining with $D$ approaching 1. The results compare favorably with computer simulations of aggregation of the same type of particles.

We present a growth model of the dodecagonal quasicrystal consisting of hexagonal antiprisms on a square and equilateral-triangle tesselation derived from the close structural relationship with the well-known $\sigma$ phase.

The interface of an Ising lattice-gas model of ions, driven by an external electric field parallel to the interface, is studied in two bulk dimensions with extensive Monte Carlo simulations. Even for weak field, numerical results indicate strong suppression of interfacial capillary-wave excitations. This is qualitatively similar to the effects of gravity on fluid interfaces, but some differences are observed in the structure factors. We argue that the physical picture may apply in general to driven systems in steady state, and in higher dimensions.

High-resolution x-ray scattering measurements on heavily doped Si:As (5×${10}^{21}$ As ${\mathrm{cm}}^{-3\mathrm{}}$) show lattice compression relative to pure silicon, $\frac{\Delta a}{a}=-0.0019\mathrm{}\pm 0.0003$, although extended x-ray-absorption fine-structure measurements show that the As-Si bond length is 0.06±0.02 Å greater than the usual Si-Si bond length. The overall lattice compression is attributed to increased population of conduction-band states which reduces Si-Si bond lengths. These measurements provide the first direct measurement of the hydrostatic deformation potential for the conduction-band edge in silicon, +3.3±0.7 eV.

We have studied both the energy and angular dependence of vibrational losses for the formate species chemisorbed on Ni(110), using high-resolution electron-energy-loss spectroscopy. Temporary negativeion formation, at an incident electron energy of ∼ 14 eV, leads to a dramatic enhancement in the intensity of the symmetric O-C-O stretch. Electron emission shows a strong angular dependence which peaks at high emission angles away from the surface normal. This effect is compared to previous photoemission studies of adsorbed CO, where the scattering in the $\sigma$-shape resonance is directed along the C-O bond.

The structure of the Si(111)-Ca${\mathrm{F}}_2$ interface has been determined with medium-energy ion scattering. Ca bonds to the Si substrate in a geometric arrangement virtually identical to Ca${\mathrm{Si}}_2$. Half of the F atoms at the interface are missing; there is no F-Si bonding and the interface is charge neutral. The structure we find has not been considered previously and provides a natural explanation for the results of published photoemission studies.

We report the observation of skipping motion of 200-2000-eV ${\mathrm{Si}}^{+}$ beams on a smooth Cu(111) surface under grazing (1°-14°) angles of incidence. The kinetic-energy distributions of specularly scatered ${\mathrm{Si}}^{+}$ and ${\mathrm{Si}}^{-}$ ions exhibit discrete energy-loss features corresponding to an integral number of reflections. Energy losses up to 20% of the incident beam energy, associated with five reflections in a binding potential, are identified. Classical trajectory calculations imply that the beam is trapped following an energy loss, by substrate electron-hole pair or collective excitation, which exceeds the incident-beam-energy component normal to the surface.

Supercell band-structure calculations of semiconductor heterojunctions were carried out with a significantly simplified description of self-consistency. The results show that the interface dipole, which is an important part of the band offset, is essentially dielectrically screened. A new simple model, emphasizing the charge transfer in the interfacial bonds, is proposed. Satisfactory results for a large number of lattice-matched (110) heterojunctions are presented.

We study the effect of on-site Coulomb repulsion on the process of resonant tunneling. We find that the tunneling peak results from a crossover from the high-temperature Kondo phase to the low-temperature mixed-valence phase of the system when the chemical potential is varied across the on-site localized-state energy. Consequently, the line shape is non-Lorentzian, with rather unusual temperature dependence. Moreover, a magnetic field does not split the tunneling peak, but the line shape is modified. The effect of coupling between localized states is also discussed.

By use of ab initio density-functional total-energy calculations a point defect with a negative Hubbard $U$ is identified, for the first time, in a chalcogenide system. It is found that in $c-{\mathrm{As}}_2{\mathrm{Se}}_3$ two noninteracting neutral Se antisite defects are unstable toward the formation of a pair of noninteracting oppositely charged Se antisites. The correlation energy gained is 0.3 eV per electron pair.

We report measurements of the resistance and narrow-band noise in Nb${\mathrm{Se}}_3$ for magnetic fields from 0 to 10 T and temperatures from 30 to 55 K. We find that a magnetic field effects a change of less than 5% on the charge-density-wave carrier concentration obtained from the slope of the current versus frequency curves. This disagrees with some previously reported measurements and resolves an existing controversy. Our work also implies that the increase in the resistance anomaly with magnetic field is not related to an increase in the carrier concentration.

Activated dynamics are established in ${\mathrm{Rb}}_2$${\mathrm{Cu}}_{0.782}$${\mathrm{Co}}_{0.218}$ ${\mathrm{F}}_4$, which is a model compound for the two-dimensional Ising spin-glass with random nearest-neighbor bonds. Cole-Cole analysis of the ac complex susceptibility reveals an extremely wide distribution of relaxation times, and a divergence, extracted to cover sixteen decades, of the median relaxation time ${\tau }_c$ towards $T_c=0\mathrm{}$ K according to $\ln \left(\frac{{\tau }_c}{{\tau }_0}\right)\propto T^{-1-\psi \nu }$, with $\psi \nu =2.2\mathrm{}\pm 0.2$. Activated dynamic scaling of the out-of-phase susceptibility is found to hold over a wide range of temperatures and frequencies. Estimates for the critical exponents $\gamma$, $\nu$, and $\psi$ are given.

We study a generalized Hubbard model in the $U=\infty$ limit with use of the slave-boson technique. The $N=\infty$ theory describes a Fermi liquid with a strongly renormalized effective mass. The $\frac{1}{N}$ corrections give rise to different superconducting instabilities depending on the band structure and the filling factor. For low densities there is a $p$-wave instability while close to half filling in the cubic lattice we find a $d$-wave instability. We also calculate Fermi-liquid parameters.

As a consequence of the charged nature of a dissolved DNA polymer and its sheath of counterions we predict that there should exist normal modes of the system characterized by substantial longitudinal electric fields and relatively small motion of the atoms and ions. This longitudinal phonon is the one-dimensional counterpart of the plasmon excitation of three-dimensional conductors, and of the surface plasmon of two-dimensional charged systems. The frequency of the DNA plasmon, as a function of excitation wavelength, is linear in inverse wavelength at long wavelengths.

The eigenenergies for the spin $S$ generalization of the critical $\mathrm{XXZ}$ model are calculated by solving numerically, for finite chains, their associated Bethe-Ansatz equations. The conformal anomaly and scaling dimensions are obtained from the conformal invariance of the infinite chain. Our results reveal that these models are governed by a conformal field theory with central charge $c=\frac{3S}{\mathrm{}(S+1\mathrm{})\mathrm{}}$ and having continuously varying critical exponents, like the eight-vertex model. We also show that they are related to a large class of unitary conformal field theories.

The problem of instanton interactions is considered for the case of dissipative tunneling out of a metastable state. The complex ground-state energy is expressed in terms of the pressure of the interacting instanton gas evaluated at a purely imaginary fugacity. With the Yang-Lee theory it is shown that for sufficiently small barrier height a gas-liquid condensation occurs. As a consequence the decay rate becomes much smaller than the corresponding ideal gas result. This is compared with experiments on tunneling in strongly damped superconducting quantum-interference devices.

We reanalyze the constraints on atmospheric ${\nu }_e$ and ${\nu }_{\mu }$ fluxes due to three-neutrino oscillations and investigate the anomalous neutrino fluxes recently observed at Kamiokande. We find that it is difficult to explain the data by ${\nu }_e\leftrightarrow {\nu }_{\mu }$ oscillations alone. We also show that models of three neutrino flavors with neutrino-mass-squared differences of order ${10}^{-2\mathrm{}}$ ${\mathrm{eV}}^2$ can solve both the solar and the atmospheric neutrino problems simultaneously.

We show that in three dimensions the effective electromagnetic actions of the $C{P}^1$ model and of a charged fermion coincide in the vacuum and one-soliton sectors, to lowest order in inverse mass. This implies equivalence between the fermion and $C{P}^1$ current operators. In the vacuum sector, the first-order fermion-current-current self-interaction correction, required for equivalence at next-to-leading order, is also obtained. In the soliton phase, the two next to leading order terms appear with coefficients differing by simple numerical factors.