The structure of the Si(111)√3 × √3 R30°-Ag surface has been analyzed with a novel form of low-energy ion-scattering spectroscopy and energy-minimization calculations. The topmost layer of the surface is formed by Ag atoms with a honeycomb-chained-trimer arrangement in which the intratrimer Ag-Ag distance is 5.1±0.2 Å. At 0.75±0.07 Å below the Ag layer there exists a Si layer with three Si atoms per surface unit cell. The lower layer of the subsequent bulklike Si double layers is split into a honeycomb and a √3 × √3 layer with a large interlayer distance of about 0.6 Å.

We present molecular-dynamics computer-simulation studies of 25-keV displacement cascades in Cu at low temperature. We observe the splitting of a cascade into subcascades and for the first time show by molecular dynamics that cascades in metals may lead to the formation of both vacancy and interstitial dislocation loops. We propose a new mechanism of defect production based on interstitial prismatic dislocation-loop formation and discuss its consequences regarding the primary state of damage in irradiated metals.

Surface anharmonicity has been measured on Cu(110) by examining the temperature dependence of surface phonons with atomic motion normal to the surface using high-resolution electron-energy-loss spectroscopy. At the Brillouin zone center Γ¯ the energy of the ${\mathit{MS}}_7$ resonance decreases by 3.2 meV and the intrinsic width increased by 8.6 meV as the temperature increases from 21 to 766 K. Energy decrease and width increase are also observed at the Brillouin zone edge Y¯ in the ${\mathit{S}}_3$ phonon. The percentage energy decrease is the same in both phonons. Direct comparison with bulk-phonon measurements shows that the anharmonicity at the surface is 4.1–4.8 times greater than in the bulk.

We report detailed observations of a continuous hexatic-to-liquid melting transition in two-dimensional magnetic-bubble lattices in magnetic garnet films using digital-imaging techniques. Topological defects, diffraction patterns, and translational and orientational correlation functions show that a second-order melting transition occurs via the formation of progressively larger transient defect clusters.

The anisotropic thermal conductivity of ${\mathrm{Al}}_{65}$${\mathrm{Ni}}_{20}$${\mathrm{Co}}_{15}$ and ${\mathrm{Al}}_{62}$${\mathrm{Si}}_3$${\mathrm{Cu}}_{20}$${\mathrm{Co}}_{15}$ single quasicrystals has been measured. While a nearly isotropic phonon contribution was observed, the electronic thermal conductivity is almost completely suppressed in the quasicrystalline plane in contrast to the periodic direction where the materials behave like the usual 3D alloy except for a markedly lower electronic contribution.

The optical activity of rare-earth ions in semiconductors is discussed taking into account the large 4f-level correlation energy. In addition to crystal-field effects, three many-body excitation mechanisms of 4f-4f transitions are identified; coherent or incoherent energy transfer via electron-hole pairs and nonequilibrium electron excitation. The first two give rise to novel nonlinear optical effects, while the last promises a simple means of achieving optical gain.

We report the first application of valence-band photoemission to a quantum-dot system. Photoemission spectra of cadmium sulfide quantum dots, ranging in size from 12 to 35 Å radius, were obtained using photon energies of 20 to 70 eV. The spectra are qualitatively similar to those obtained for bulk cadmium sulfide, but show a shift in the valence-band maximum with size.

We have introduced an artificial array of scatterers into a macroscopic two-dimensional conductor nearly devoid of intrinsic defects. This generates pronounced structure in the magnetoresistance, anomalous low-field Hall plateaus, and a quenching of the Hall effect about B=0. Our calculations show that the predominant features in the data arise from commensurate classical orbits impaled upon small groups of the imposed scatterers.

A dynamic current ${\mathit{I}}_{\mathrm{\omega }}$ in a mesoscopic ring subjected to a time-dependent external magnetic field H̃(t)=${\mathit{H}}_0$+${\mathit{H}}_{\mathrm{\omega }}$ cosωt is calculated in the limit of small ${\mathit{H}}_{\mathrm{\omega }}$ using linear-response theory and the supersymmetry method. It is shown that the current ${\mathit{I}}_{\mathrm{\omega }}$ remains finite in the limit ω→0 and exceeds currents obtained from thermodynamic considerations. It can be of the order of ${\mathit{eD}}_0$/${\mathit{L}}^2$ for magnetic fluxes φ∼${\mathrm{\phi }}_0$, where ${\mathit{D}}_0$ is the classical diffusion coefficient, L is the circumference, and ${\mathrm{\phi }}_0$=hc/e is the flux quantum, and has a component which oscillates in flux with the period ${\mathrm{\phi }}_0$/2. The derived current can explain the experiment by Levy et al.

The surface magnetoelastic coupling coefficient ${\mathit{B}}^{\mathit{s}}$ is determined for cobalt-rich and iron-rich amorphous alloys by measuring the field and strain dependence of the spin polarization of secondary electrons emitted from their surfaces. ${\mathit{B}}^{\mathit{s}}$ is found to be approximately 3 times the bulk value for the cobalt-rich alloy and half the bulk value for the iron-rich alloy.

It is shown how large domain walls, containing ≥${10}^{10}$ spins, can behave as quantum objects at low temperatures. They move quantum diffusively, and exhibit macroscopic tunneling from defect pinning centers. The dissipation is calculated and shown to be very small; it does not involve the usual Caldeira-Leggett environmental couplings. The theory can also be used to treat smaller single-domain-wall behavior.

The low-frequency dielectric response has been measured in the spin-density-wave state of (tetramethyltetraselenafulvalene${)}_2$${\mathrm{PF}}_6$. The dielectric constant is significantly larger than that expected for single-particle excitations and is dominated by the internal modes of the spin-density-wave condensate. We also established the relation between the frequency-dependent and nonlinear response.

Ferromagnetism of ${}_{}{}^{19}{}_{}{}^{}\mathrm{F}$ nuclear spins in a monocrystalline ${\mathrm{KMgF}}_3$ sample has been observed by NMR in the presence of a high magnetic field and in the microkelvin temperature range. The three ${}_{}{}^{19}{}_{}{}^{}\mathrm{F}$ spins of one crystalline cell are equivalent when the field is along the [111] axis of the crystal and inequivalent for the field along the [100] axis. At negative temperature, the structures are ferromagnetic with domains for both orientations. In the [100] orientation, it is a two-sublattice ferromagnetism: Spins belonging to different sublattices of ${}_{}{}^{19}{}_{}{}^{}\mathrm{F}$ have different polarizations within the same domain.

We show that the low-temperature fixed-point behavior of the negative-U Anderson model involves a ‘‘charge Kondo effect,’’ where the local pair behaves as a Heisenberg rather than an xy degree of freedom. Interactions of the local pair with the conduction sea generate a highly polarizable Fermi liquid with enhanced linear specific heat and charge and pair susceptibilities.

We investigate charge transport in two capacitively coupled 1D arrays of small tunnel junctions, attached to two different circuits. The electron and hole placed in adjacent electrodes of different arrays form a sort of ‘‘exciton’’ that moves as a whole. It was shown that under certain conditions only these excitons contribute to the charge transport along the junction arrays, so that the currents in the circuits are exactly equal in magnitude and opposite in direction. The existence of these excitons provides an interesting possibility to simulate some features of microscopic excitons in a mesoscopic system.

The differential resistance of the voltage-current characteristic generated by an isolated phase-slip center in a superconducting tin whisker has been measured down to 50 mK below its critical temperature. As a function of the temperature the differential resistance shows a resonancelike behavior with a maximum at the temperature where charge-imbalance waves excited by the phase-slip center have their maximum decay length, suggesting that the observed maximum of the differential resistance gives experimental evidence for charge-imbalance waves arising from a phase-slip center.

We report linear electrical transport measurements in ${\mathrm{Mo}}_{77}$${\mathrm{Ge}}_{23}$/Ge multilayres and a single layer of ${\mathrm{Mo}}_{77}$${\mathrm{Ge}}_{23}$ which is nominally the same as the constituent layers of each multilayer. In the multilayers, we observe a crossover from a regime where the vortex motion displays interlayer coupling to a regime where the vortex segments in each layer move effectively independent. Our observations bear some strong resemblances to those in high-temperature superconductors, and we discuss the possible relevance to our system of some concepts developed to interpret the high-temperature superconductors.

Meissner-effect and microwave-absorption measurements on bulk samples show that ${\mathrm{Rb}}_{\mathit{x}}$${\mathrm{C}}_{60}$ is superconducting with a maximum transition temperature of 28 K. This is a 10-K (60%) increase over the K-doped material. Only ${\mathrm{Ba}}_{0.6}$${\mathrm{K}}_{0.4}$${\mathrm{BiO}}_3$ and the cuprate superconductors have higher transition temperatures.

The possibility of eliminating chaos in a dynamical system or of decreasing the leading Liapunov exponent by applying a weak periodic external forcing to the system is demonstrated through the example of a periodically driven pendulum. The applications of the external forcing also results in other striking changes in the dynamics such as a stabilization of narrow subharmonic steps and the achievement of very low winding numbers.

One of the few experimental handles on the nonlinear properties of the gravitational interaction is to test the ‘‘strong equivalence principle,’’ i.e., to test whether the ratio ${\mathit{m}}_{\mathrm{gravitational}}$/${\mathit{m}}_{\mathrm{inertial}}$ is 1 for self-gravitating bodies. We point out that existing observational data on the class of small-eccentricity long-orbital-period binary pulsars already provide a limit (namely ‖${\mathit{m}}_{\mathit{g}}$/${\mathit{m}}_{\mathit{i}}$-1‖<1.1×${10}^{\mathrm{-}2}$; 90% C.L.) which goes beyond corresponding solar-system limits in probing strong-gravitational-field effects. Possible observational ways of improving this limit are suggested.