In the theoretical analysis of dispersive transport, it is essential to take account of the actual initial conditions. In transient photoconductivity experiments, all the particles are initially in a transport state, but in those on atomic diffusion the atoms are often initially in thermal equilibrium at a temperature less than that at which the diffusion is measured. We show that the multiple-trapping model, which explains well the photoconductivity results, cannot account for those on the diffusion of H atoms in a-Si:H, for which it has often been proposed.

A structure model for the decagonal quasicrystals AlCuCo and AlNiCo is proposed. The model agrees with available experimental data, i.e., with three-dimensional and five-dimensional Patterson analysis of x-ray-diffraction data and with direct-space atomic patterns found by high-resolution electron microscopy. The model allows for a dualistic description. It can be viewed both as a set of specifically decorated overlapping decagonal clusters and as a cutting and projecting of only two simple ‘‘atomic surfaces.’’

Detailed atomic-resolution images of ordered Au(100) surfaces in aqueous 0.1M ${\mathrm{HClO}}_4$ as obtained by in situ scanning tunneling microscopy (STM) are reported as a function of electrode potential. Below about -0.25 V (versus saturated calomel electrode), the (1×1) surface reconstructs to form corrugated quasihexagonal domains. Multiple distinct, yet related, structures are formed that resemble those postulated from diffraction measurements. The reconstruction can be lifted by returning to 0.2 V. The results demonstrate the promise of atomic-resolution STM for examining reconstruction at ordered electrochemical surfaces.

Field-ion-microscope observations and molecular statics calculations using embedded-atom-method potentials have identified the surface diffusion modes for Pt dimers and trimers on Pt(001). Dimers migrate by a series of displacements involving exchange between one of the dimer atoms and a substrate atom and have a lower activation barrier for diffusion than monomers. Trimer diffusion involves both exchange and hopping displacements and has an activation barrier comparable to monomers.

It is found for the first time that a molecule can induce a displacive reconstruction in the substrate on which it is adsorbed. This is observed in a refined structure determination of Rh(111)(2×2)-${\mathrm{C}}_2$${\mathrm{H}}_3$, in which ${\mathrm{C}}_2$${\mathrm{H}}_3$ (ethylidyne) is adsorbed on the Rh(111) single-crystal surface. This result strengthens an earlier hypothesis that catalytic reactions are usually accompanied by distortions in the catalyst, which can profoundly affect reaction rates and selectivities.

The band gaps and exciton energies of CdS and GaP clusters are calculated for the first time using pseudopotentials. The calculated exciton energies of CdS over a wide range of cluster sizes are in excellent agreement with experiment. Furthermore, the exciton states of clusters with zinc-blende and hexagonal lattices are similar in large clusters, but differ dramatically in small clusters. Finally, the spectra of small GaP clusters shift to red, instead of to the blue, with decreasing cluster size. These effects provide novel ways of tuning the optical properties of clusters.

The use of femtosecond laser pulses to excite electron-hole plasmas in semiconductors has become a major method of studying fast processes. The transition times from the central Γ valley in GaAs to the satellite X and L valleys are comparable to the reciprocal of the frequency of the phonon involved, bringing into question the use of standard perturbation-theory approaches. We investigate the role of retardation arising from the collision duration required for such a transition to occur, through the use of a path-integral form of the quantum kinetic equation. A secondary self-scattering process and a probability of completed collisions are used to cast this equation in a semiclassical form.

The behavior of carrier-carrier screening is investigated in a GaAs-GaAlAs quantum-well structure by measuring the band-to-band polarization dephasing with femtosecond photon echoes. The variation of the electron-hole-polarization dephasing time with the carrier concentration reveals the two-dimensional character of the short-range screening between the interacting carriers.

Size effects in the orbital magnetic susceptibility of small metallic systems are investigated by considering some simple model systems. The atomlike properties at low temperatures gradually evolve towards a diamagnetism close to the bulk Landau value at high temperatures, with only small size-dependent deviations. For ${\mathit{k}}_{\mathit{B}}$T smaller than the energy-level spacing, the susceptibility of an ensemble average of different particles is expected to show orbital paramagnetism. This mechanism is proposed as an explanation for the observations of Kimura and Bandow.

Structural and magnetic properties of the rare-earth compounds ${\mathit{R}}_2$${\mathrm{Fe}}_{17}$${\mathrm{N}}_{\xi }$ have been studied with neutron-diffraction measurements and self-consistent spin-polarized electronic-structure calculations. The diffraction results indicate for the Nd compound that N goes into two sites in two or more phases of varying fractional N occupations. For the Y compound N occupies only one site. Electronic-structure calculations for ${\mathrm{Y}}_2$${\mathrm{Fe}}_{17}$ and ${\mathrm{Y}}_2$${\mathrm{Fe}}_{17}$${\mathrm{N}}_3$ give excellent results for site-dependent Fe moments, and, with spin-fluctuation theory, explain the large change in the Curie temperature on nitrogenation.

We report the realization of a microstructure which leads to very strong high-temperature flux pinning in ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ crystals. Aligned discontinuous columns of damaged material, about 50 Å in diameter and more than 15 μm long, are produced by 580-MeV Sn-ion irradiation. The enhancement of flux pinning is largest when the applied magnetic field is aligned with these tracks. At high temperatures and fields the pinning is much greater than that produced by random point defects, and causes a considerable enlargement of the irreversibility region in the H-T plane.

The mechanism for the dissociation of ${\mathrm{CH}}_4$ at metal surfaces has been and is still controversial. In large part, this is because many of the experimental data suggest a direct tunneling mechanism, which is not obviously compatible with an observed strong dependence on the surface temperature. We propose here that a new phenomenon, thermally assisted tunneling, caused by a coupling of the tunnel barrier to the lattice, resolves this controversy. Explicit calculations using a simplified model for this process agree qualitatively with a wide range of experimental data for ${\mathrm{CH}}_4$ dissociation on Pt(111).

We discuss the two time-interval sequences which play a crucial role in studies of escape times in bistable systems driven by periodic functions embedded in noise. We demonstrate that the probability density of escape times for one of the sequences exhibits all the substantive features of experimental interspike interval histograms recorded from real, periodically forced sensory neurons. Our analysis relies on linking this interval sequence to the firing-reset mechanism of real neurons, and illustrates the importance of the noise, without which the substantive features cannot exist, for the transmission of sensory information.

We present exact solutions for breather solitons in the (2+1)-dimensional Chern-Simons gauge theory with external magnetic field B. The breather soliton is a time-dependent solution whose size is oscillating and whose center of mass moves ina cyclotron motion. These solitions describe how the Jackiw-Pi solitons behave in the external magnetic field. We then apply the Bohr-Sommerfeld semiclassical quantization to the breathing mode and the cyclotron motion. It is found that the orbital angular momentum takes on an integer value and that the quantum soliton is in the Landau level.

Vortex solitons of the planar, gauged nonlinear Schrödinger equation are quantized semiclassically in the presence of an external magnetic field.

We relate the complex solutions of the thermodynamic Bethe-ansatz equations to the excited states in factorizable massive models. This approach permits one to recover exactly the ultraviolet behavior of these excitations. In order to illustrate our method we consider several unitary and nonunitary integrable models. We also comment on the calculation of critical exponents in massless integrable models.

In this Letter we give the gauge-independent divergent part of the one-loop correction to the effective action for the six-dimensional Einstein-Hilbert action, including a cosmological constant. We extend the methods of Barvinsky-Vilkovisky from four to higher dimensions, including the Vilkovisky-DeWitt correction, to obtain the unique counterterms for an arbitrary background field. All one-loop operators are given for N dimensions; however, due to the complexity of the Schwinger-DeWitt expansion, going beyond six dimensions to eight or ten will probably require restricting the background.

We use a configuration-mixing quasiparticle Tamm-Dancoff approximation to calculate the expected event rate for an ${}_{}{}^{127}{}_{}{}^{}\mathrm{I}$ solar neutrino detector. Our cross section for ${}_{}{}^8{}_{}{}^{}\mathrm{B}$ solar neutrinos is 2.2×${10}^{\mathrm{-}42}$ ${\mathrm{cm}}^2$ [corresponding to about 13 solar neutrino units (SNU) in the standard solar model], a factor of more than 3 below an estimate by Haxton. This value may increase by about 30% when quadrupole correlations are included. The cross section for ‘‘Be neutrinos is 2.0×${10}^{\mathrm{-}45}$ ${\mathrm{cm}}^2$ (9.4 SNU), in agreement with a calculation by Dellagiacoma and Iachello. The total event rate per nucleus from all solar neutrinos is about 3 times that in ${}_{}{}^{37}{}_{}{}^{}\mathrm{Cl}$.

The strength function ${\mathit{S}}_{\mathrm{\beta }}$(E) and the cross section of the reaction ${\nu }_{\mathit{e}}$${+}^{127}$I→${\mathit{e}}^{\mathrm{-}}$${+}^{127}$Xe are calculated in the framework of the theory of finite Fermi systems. The results obtained are used to estimate the possibility of using an iodine-xenon detector in solar-neutrino experiments. The advantages of the iodine-xenon detector over the current chlorine-argon one are shown.

The ${\mathit{K}}^{\mathrm{\pi }}$=35/$2^{\mathrm{-}}$, five-quasiparticle isomer in ${}_{}{}^{179}{}_{}{}^{}\mathrm{W}$ is shown to decay into the region of a backbend in the 7/$2^{\mathrm{-}}$[514] band, allowing for the first time the identification of a full set of aligned-band states. Destructive interference results from level mixing in the band-crossing region. The deduced γ-ray branching ratios are used to establish the mixing matrix elements and to show that the aligned band has a high value of the K quantum number. The properties of well-defined alignment and yet also high K provide the first clear example of a Fermi-aligned s band. The anomalous decay of the isomer itself is now explained.