We report the observation of L-valley-conduction intersubband absorption concurrent with Γ-valence intersubband absorption in optically excited undoped binary-binary short-period GaSb/AlSb superlattices. Under above-band-gap optical excitation, electrons in these structures occupy the lowest-energy L-valley subband, while holes occupy the highest-energy Γ subband. Due to the separation in k space, these populations have long lifetimes, which enables the observation of photoinduced intersubband absorption. Due to the symmetry of the L valleys, their contribution to the intersubband absorption is observed for both s- and p-polarized infrared light, in agreement with theoretical predictions.

Photoelectron energy spectra reveal discrete valence-electron states in the range 0–220 meV above ${\mathit{E}}_{\mathit{F}}$ for 1–3 atomic layers of Na on Cu(111). Apart from a stepwise dependence on the number of atomic Na layers the energy of the quantum-well states depends in a gradual manner on how full the layer is. The states are as well defined in energy as comparable surface states producing peak widths (50–90 meV), which depend on temperature and coverage.

The excitonic population lifetime and its temperature dependence is measured in nanometer-scale quantum wires. The measurements were performed on two cleaved-edge overgrown GaAs quantum-wire systems, in which the lateral dimensions are comparable to the dimensions of a reference quantum well and an easy comparison between them is straightforward. The excitonic population decay time in the wires is less sensitive to temperature changes than that in the wells. Whereas the latter is linear with temperature, the former is roughly proportional to the square root of the temperature. Consequently, the decay times of the photoluminescence from the wires are longer than those from the wells at low temperature, but shorter at higher temperatures. From the temperature dependence of the lifetimes, which is an unambiguous signature of a one-dimensional system, we find the intrinsic radiative lifetime of excitons in quantum wires. They are an order of magnitude longer than in quantum wells.

A dynamic theory of interaction between tunneling electrons and charge fluctuations on the electrodes of a mesoscopic tunnel junction is developed and accounts for the renormalization of the tunneling potential due to the interaction. It is shown that the magnitude of the potential renormalization depends on the relation between the characteristic frequency of the charge fluctuations and the traversal time of tunneling ${\mathrm{\tau }}_0$. The modes with frequencies larger than ${\mathrm{\tau }}_0^{\mathrm{-}1}$ do not affect the tunneling potential.

We investigate the low-field magnetoresistance ${\mathrm{\rho }}_{\mathit{xx}}$ perpendicular to a one-dimensional periodic potential which is imposed upon a two-dimensional electron gas in GaAs-${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As heterojunctions. Distinct low-field anomalies and a crossover from positive to large negative magnetoresistance with increasing potential strength unveil the important role of anisotropic relaxation processes in strongly modulated electron systems.

We report first-principles total-energy calculations that provide detailed reaction pathways for Sb-dimer rotation in evolving processes of ${\mathrm{Sb}}_4$ precursors on the Si(001) surface. We find that the total energy for the Sb dimer perpendicular to the subsurface Si dimers is lower than the parallel dimer by 0.45 eV. We also find a plausible reaction pathway for the Sb-dimer rotation during which only a single dangling bond is created. The calculated activation energy for the rotation is 1.0 eV, indicative that the dimer rotation is rate-determining in the evolving process of ${\mathrm{Sb}}_4$ precursors.

Two-stage melting of submonolayer xenon adsorbed on single-crystal graphite has been observed using transmission high-energy electron diffraction. Diffraction patterns were obtained from the solid, hexatic, and liquid phases for 11<p<41×${10}^{\mathrm{-}3}$ torr and 110<T<120 K. The aligned solid melts via a hexatic phase to an essentially isotropic liquid within a 1-K interval at constant pressure. The lattice parameter and spot profiles of the hexatic phase are reported, and the order of the transitions discussed in relation to previous thermodynamic and diffraction measurements.

From an interplay between x-ray diffraction and scanning tunneling microscopy the atomic structure of the (5√3×2)S reconstruction on Ni(111) has been revealed. It consists of four squarelike tetramers of Ni atoms, rotated in pairs ∼14° clockwise and counterclockwise, onto which eight fourfold-coordinated S atoms are chemisorbed. The results have a significant bearing on the understanding of the detailed structure of postulated pseudo-(100) reconstructions of fcc(111) surfaces and support a general principle of increased adsorbate coordination as being decisive for the reconstruction.

Angle-resolved-photoemission studies of the dispersion of the quantum-well states in copper thin films deposited on a Co(001) substrate reveal that hybridization in the interface leads to a large increase in the effective mass of the electrons. These observations have implications for theories of the oscillatory exchange coupling in the related magnetic multilayers, particularly where Fermi-surface spanning vectors away from the center of the zone are invoked as in the case of the short-period oscillation in the Co/Cu(001) multilayers.

A time-domain measurement of the relaxation time of photoexcited electrons as a function of excitation energy at a single-crystal Cu(100) surface has been made with time-resolved two-photon photoemission. The relaxation lifetime of the excited electrons is found to decrease rapidly with the amount of excitation energy above the Fermi level, and we compare the measured lifetimes with a calculation based on Fermi-liquid theory. The results reveal a qualitative agreement with the theory, and indicate that effects due to transport of the electrons away from the surface and into the bulk are important.

Cryogenic scanning tunneling microscopy is used to study local electrical-transport properties of thin granular Au/${\mathrm{Al}}_2$${\mathrm{O}}_3$ films in the vicinity of the percolation threshold. The current-voltage characteristics are found to vary dramatically from one tip position to another over distances of the order of a few nanometers. These characteristics often exhibit interesting Coulomb-staircase structures having unusual variations in step widths and heights due to complex tunneling paths. A triple-barrier tunnel-junction model accounts quantitatively for the experimental results.

We analyze the core structure of string defects in transparent condensed-matter systems and argue that in certain cases the variation of the refractive index near the core may be such that it can lead to total internal reflection of light traveling along the string core. Such strings will thus behave as optical fibers. We consider several cases, such as superfluid helium and liquid crystals, where this possibility may be realized.

The initial domain formation process in a ferroelastic crystal ${\mathrm{KD}}_3$(${\mathrm{SeO}}_3$${)}_2$ is observed by a method that utilizes laser light refraction at the ferroelastic domain structures. The high sensitivity of this method enables us to detect the refracted light even above the transition point (${\mathit{T}}_{\mathit{c}}$). The intensity level extends down three orders of magnitude smaller than that caused by the domains below ${\mathit{T}}_{\mathit{c}}$. This implies the embryos of the ferroelastic domains already exist in the high-temperature phase as the inhomogeneous strain field.

On the basis of a Ginzburg-Landau model for ferroelectric perovskites, we calculate an effective free energy for a polarization field by eliminating elastic fields which are coherently induced by inhomogeneous configurations of polarizations. We obtain elastic long-range interaction between polarization fields on an adiabatic approximation. Computer simulations of a two-dimensional time-dependent Ginzburg-Landau (TDGL) model are performed by taking into account those long-range interactions. Charge-neutral 90° twins and its time evolution in the tetragonal phase are obtained with appropriate parameters of a phenomenological free energy. Resultant domain patterns are in agreement with experimental observations of tetragonal ${\mathrm{BaTiO}}_3$ ceramics. It is shown that anisotropic long-range interaction between polarization fields caused by elastic effects plays a major role in determining both the domain morphology and domain growth law in ferroelectric perovskites.

The triple-coincidence technique, coupled with magnetic fields, can be used to feature the behavior of positron bound systems in condensed matter. In particular, data concerning the anomalous magnetic quenching of positronium in some nonmetallic solids are presented; comparisons with results from lifetime spectroscopy are made. Possible applications of the technique are discussed.

Avrami’s treatment of nucleation and growth kinetics considers that active nucleation sites are randomly distributed throughout the volume and that grains may impinge upon grains grown from other sites, causing growth to cease on the common interface. This treatment resulted in the well-known Avrami equation. As a result of the reassessment of these basic assumptions, an integral equation is proposed for the time-dependence evaluation of the transformed phase volume fraction in crystallization processes, instead of the Avrami equation. The proposed model fits very well the whole range of experimental data for ${\mathrm{NiZr}}_2$ and ${\mathrm{Te}}_{92}$${\mathrm{Pb}}_8$ amorphous to crystalline transformation.

The instability of the shape of two-dimensional new-phase nuclei is studied using an order-parameter field equation. The criteria of instability of angular perturbations are obtained for systems in which the order-parameter nonconservation processes are negligible. These coincide with criteria obtained within the framework of the phenomenological Zeldovich-Volmer theory and the continual diffusion-limited aggregation model in the diffusive limit. The influence of small angular perturbations on the fractal dimensionality of the nuclei is discussed.

Solid ${\mathrm{H}}_2$S was studied by x-ray diffraction under pressure up to 20 GPa at room temperature. An F→P symmetry change in the cubic lattice was observed at 8 GPa. At 11 GPa where a previous Raman study indicated a pressure-induced transition, an obvious crystal structure change from the cubic symmetry to a lower symmetry was observed.

The structural changes within the copper(I) halides induced by hydrostatic pressure have been investigated using the powder neutron-diffraction technique. The expected transition from the ambient zinc-blende structure with tetrahedral coordination to the octahedrally coordinated rocksalt structure is observed in both CuCl and CuBr. No rocksalt phase is observed in CuI, presumably due to the limited maximum pressure of our apparatus (∼10 GPa). The lower symmetry phases which occur as intermediate structures between the zinc-blende and rocksalt phases have been studied in detail. CuCl and CuI undergo abrupt transitions whilst CuBr has three sluggish transitions, involving extensive coexistence of neighboring phases. There is one intermediate phase in CuCl (CuCl-IV), two in CuBr (CuBr-IV and CuBr-V) and at least two in CuI (CuI-IV and CuI-V). Three different structure types have been identified, in space groups Pa3¯ (CuCl-IV and CuBr-V), P4/nmm (CuBr-IV and CuI-V) and R3¯m (CuI-IV). The intermediate structures all retain the tetrahedral cation coordination of the ${\mathrm{Cu}}^{+}$, though the environment is rather more distorted in CuCl and CuBr than in CuI. The evolution of the structures of these I-VII compounds is very different from that observed in the more covalent II-VI and III-V systems.

Defect states in cold-worked iron specimens electrolytically hydrogen charged were characterized by a combination of optical microscopy, scanning electron microscopy, and high-resolution electron microscopy. For deformations of up to 20% and 13% (for cold-rolled and elongated specimens, respectively), the density of hydrogen-induced cracks and surface blisters decreased with the increases in the amount of deformation. Vacancy clusters with sizes of the order of 1 nm were observed in cold-rolled and hydrogen-charged iron specimens. The size and the density of the observed vacancy clusters were of the same order as those predicted previously based on positron-annihilation results. The results are explained by hydrogen trapping.