Coherent pair creation during the collision of ${\mathrm{e}}^{+}$${\mathrm{e}}^{\mathrm{-}}$ beams in linear colliders is examined. This includes contributions from both real and virtual photons, where the real photons are predominantly from beamstrahlung. The pair creation probability is shown to be orders of magnitude larger than the corresponding incoherent processes. The energy spectrum is also calculated where the effective threshold energy is shown to be inversely proportional to the beamstrahlung parameter Υ. Implications of this effect on future ${\mathrm{e}}^{+}$${\mathrm{e}}^{\mathrm{-}}$, eγ, and γγ linear colliders are discussed.

Sum rules and causality are used to obtain a general relation connecting the dynamic and static properties of any dielectric system which supports polarization waves. The Lyddane-Sachs-Teller relation appears as a special limiting case. Inhomogeneous media provide illustrative examples.

Strong damping (depletion) of the driver wave (electron plasma wave) after electron acceleration has been observed in the ${\mathrm{v}}_{\mathrm{p}}$×B acceleration scheme in microwave-plasma interaction experiments. This depletion can be controlled by changing the rise time of the rf pump wave. Time- and space-resolved observations of high-energy electrons reveal the possible existence of different acceleration mechanisms near the resonance absorption layer.

The large-scale inhomogeneity of the solar wind is taken into account to estimate the turbulent flux due to nonlinear interactions among purely outward-traveling waves. The nonlinear interactions are mediated by secondary, incoming waves generated by the linear coupling of the dominant species to the large-scale gradients. A quasistationary self-similar turbulent cascade is possible, with a spectrum scaling as ${\mathrm{k}}^{\mathrm{-}1}$, close to what is found in the low-frequency range of solar-wind fluctuations near to the sun.

We have made the first systematic study of the pair-breaking edge in superfluid ${}_{}{}^3{}_{}{}^{}\mathit{B}$ over a wide range of pressures and frequencies. This direct measurement of the gap enables us to experimentally verify the predictions made for strong-coupling corrections to the gap. In addition, it indirectly lends support to the temperature scale of Greywall.

The driving force for atomic migration due to steady bombardment by a beam of lighter particles is described as a quasistatic force in the adiabatic approximation. It is shown that atomic recoil can substantially enhance the driving force for classical diffusion processes, and substantially reduce the driving force for quantum-mechanical diffusion processes. This recoil effect, which has been previously ignored in electromigration theories, affords an explanation of the Häffner effect in liquid-metal electromigration and is especially large for the migration of light impurities below room temperature.

The interface structure of strained-layer Si-Ge superlattices on Si(100) is investigated by selected-area electron-diffraction techniques and high-resolution transmission electron microscopy. Three types of strained-layer Si-Ge superlattices characterized by different strain distributions are compared. Diffraction evidence is presented for the existence of boundary layers at the interfaces consisting of chemically ordered domains with a bilayer stacking of Si and Ge along the 〈111〉 directions. This phenomenon is independent of the strain distribution between Si and Ge, i.e., of the choice of the substrate lattice parameter.

The initial dissociative sticking probability of ${\mathrm{H}}_2$ on Cu(110) has been measured using vibrationally hot, translationally cold-seeded beams. The results indicate a dissociative barrier which is accessed through energy partition in both translational and vibrational degrees of freedom in the im- pinging hydrogen molecule. The experiment has enabled a determination of the translational requirement in dissociating hydrogen in the first vibrational excited state, and the results place the barrier at ∼1 eV, in good agreement with theoretical predictions.

We have studied the morphology of thin epitaxial Ge films on Si(001). By terminating the surface with a monolayer of As during growth, Ge is forced to grow layer by layer, instead of the preferred mode, which is layer by layer for three monolayers, followed by islanding. In layer-by-layer growth, there are no nucleation sites for misfit dislocations to accommodate the 4% lattice mismatch. Instead, we observe by high-resolution transmission electron microscopy a novel strain relief defect combining two Σ9 boundaries and a twin. An estimate of the defect energy compares favorably with the energy of equivalent misfit dislocations.

Initial stages of Si epitaxial growth on vicinal Si(001) substrates were investigated using scanning tunneling microscopy. For a growth temperature of about 750 K it was found that initial growth occurs almost exclusively at one of the two nonequivalent types of step edge. This leads to the formation of a single-domain surface with an array of evenly spaced straight steps with biatomic height. This structure can be preserved by quenching the sample to room temperature.

The observation of statistical absorption fluctuations of an inhomogeneously broadened optical transition in a glass is reported. Frequency modulation spectroscopy in a 10-ppm ${\mathrm{Nd}}^{3+}$-doped-silica optical fiber has also revealed spectral holes that persist for days at 4.2 K. Homogeneous linewidths ${\Gamma }_h$ measured using hole burning and fluctuations are in good mutual agreement but are significantly larger than those inferred from photon echo decays in the same system. This difference confirms recent predictions of a time-dependent ${\Gamma }_h$(t) arising from the tunneling dynamics of the glass.

Theoretical lattice dynamics analysis and new optical experiments on KI:${\mathrm{Ag}}^{+}$ reveal a nearly unstable low-temperature defect-host configuration, whose population decreases dramatically with temperature, but whose dynamics are consistent with a temperature-independent harmonic model involving collective motion. This behavior is in marked contrast to the temperature-dependent evolution of the dynamics of ‘‘soft-mode’’ systems.

Ultraviolet photoelectron spectroscopy (UPS) has been used to observe changes in the π-band structure in poly(3-hexylthiphene) induced by doping using ${\mathrm{NOPF}}_6$. The charge-induced movement of the Fermi energy and a finite density of electronic states at the Fermi energy in the most highly doped (saturated) material are seen unambiguously with UPS for the first time in a conjugated polymer. The results imply the existence of a new, recently predicted theoretically, but hitherto unobserved, polaron-lattice state of this conjugated polymer at the saturation-doping level.

We studied the Schottky barrier for several metal-semiconductor interfaces using both deposition sequences: metal on semiconductor and semiconductor on metal. The interface morphology was found to depend on the deposition sequence, whereas both sequences produced the same interface chemical species and the same Schottky-barrier dependence on the metal electronegativity. These results clarify the nature of the Schottky term in the barrier height, supporting the effective-work-function theory of Freeouf and Woodall.

A new diluted magnetic III-V semiconductor of ${\mathrm{In}}_{1\mathrm{-}\mathrm{x}}$${\mathrm{Mn}}_{\mathrm{x}}$As (x≤0.18) has been produced by molecular-beam epitaxy. Films grown at 300 °C are predominantly ferromagnetic and their properties suggest the presence of MnAs clusters. Films grown 200 °C, however, are predominantly paramagnetic, and the lattice constant decreases with increasing Mn composition; both are indicative of the formation of a homogeneous alloy. These films have n-type conductivity and reduced band gaps.

We have measured the frequency dependence of the Young’s-modulus (Y) and internal-friction anomalies in the charge-density-wave conductors ${\mathrm{NbSe}}_3$ and ${\mathrm{TaS}}_3$. Measurements were made on overtones of flexural resonances using a helical resonator detector between 1 and 100 kHz. Temperature-dependent anomalies were independent of frequency, whereas electric-field-dependent anomalies decreased rapidly with frequency; e.g., ΔY/Y∝${\omega }^{\mathrm{-}p}$, where p∼(3/4. These results are inconsistent with all models proposed of the elastic properties of the charge-density-wave state.

A model for ballistic transport based on classical mechanics of electrons at the Fermi level is shown to exhibit a variety of magnetoresistance anomalies found experimentally in narrow-channel two-dimensional electron gases. Among the phenomena considered are quenched and negative Hall resistances, the last Hall plateau, bend resistances, and geometrical resonances.

We have obtained convincing experimental evidence of the time correlation of single-electron tunneling (SET) events, i.e., of SET oscillations with the average frequency $f_s$ fundamentally related as $f_s$=I/e to the dc current I. The experiments have been carried out with one-dimensional N-junction arrays of Al/oxide/Al tunnel junctions of area S≊0.006 μ${\mathrm{m}}^2$ at T≊60 mK. The correlation showed up as resonance peaks of the dynamic resistance dV/dI as a function of I, arising at I=$I_n$=nef (n=±1 and ±2) at irradiation of the N=15 and 19 samples by microwaves of frequency f=0.7–5 GHz.

${}_{}{}^{17}{}_{}{}^{}\mathrm{O}$ nuclear-magnetic-resonance spectra are obtained, using uniaxially aligned powder of ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$, and resonance lines from the four inequivalent oxygen sites are clearly distinguished. Principal components of the electric field gradient (EFG) and the shift tensors are obtained in the normal state. Assigning the direction of the largest EFG components of Cu-O bond axes, the anisotropy of the shift suggests that the spin density at the plane O(2,3) and the bridging O(4) sites resides on the $p_{\sigma }$ orbitals. The domonant contribution to the spin susceptibility comes from Cu d states.

With the vibrating-reed technique we have studied the coupling of the flux-line-lattice (FLL) to the atomic lattice between 4.2 and 100 K at constant applied fields 0.014T≤${\mathrm{B}}_{\mathrm{a}}$≤4 T in three different Bi-based polycrystalline samples. Depinning of the FLL is observed at temperatures above ${\mathrm{T}}_{\mathrm{D}}$ where a peak occurs in the damping of the vibrating sample. With increasing temperature, the FLL dissipation changes at ${\mathrm{T}}_{\mathrm{D}}$ from a hysteretic to a viscous regime. The depinning temperature ${\mathrm{T}}_{\mathrm{D}}$<${\mathrm{T}}_{\mathrm{c}}$(${\mathrm{B}}_{\mathrm{a}}$) follows a logarithmic magnetic field dependence which is similar to the dependence of the depinning temperature ${\mathrm{T}}^{\mathrm{*}}$ obtained from measurements of the electrical resistivity due to thermally activated flux motion.