It was observed that bi-directional phase changes can be triggered by genuine photoexcitation between two phases with different bond structures in polydiacetylenes. The observed photoinduced phase transition was found to be mediated by photogeneration of charge carriers (and not of excitons). The extremely high efficiency is perhaps due to cooperative interaction between the locally photoconverted domains.

A microscopic theory is presented of stress-assisted fluctuational breakaway of dislocation kinks from pinning centers. It is shown that a polaron lattice distortion near a pinned kink essentially affects the magnitude of energy fluctuation required to activate a jump over the pinning barrier. The effect of quantum lattice fluctuations on the low-temperature depinning rate is analyzed. The role played by the kink geometric width in depinning phenomenon is disclosed.

Electron-heating measurements were carried out to measure the electron-phonon scattering rates in CuCr thin films. We examined free-standing and supported films whose thickness and width span lengths comparable to the mean thermal-phonon wavelength. The measured scattering rate was proportional to ${\mathit{T}}^2$ from 0.5 to 10 K, independent of sample configuration. We observe that the quantization of the phonon spectrum required by sample dimensions has no effect on the temperature dependence of the electron-phonon scattering rate.

It has for the first time become possible to make quantitative determinations of differences in energy and jump rate for adatoms at the two types of binding sites on a close-packed fcc (111) plane. For iridium on Ir(111), variations in the potential energy of an atom over the surface are found to be small. Binding energies are, however, strongly dependent upon the chemical identity of the adatom. For rhenium and tungsten, differences in binding at fcc and hcp sites are sizable. In contrast, the dynamics of jumps out of the two types of sites are similar for the same atom.

We report the relative efficacy of vibrational and translational energy in overcoming the barrier to dissociative adsorption of ${\mathrm{D}}_2$ on a Cu(111) surface. This is determined by measuring variation of adsorption probability with kinetic energies, ${\mathit{E}}_{\mathit{i}}$, and vibrational temperature. At the highest energy (${\mathit{E}}_{\mathit{i}}$=0.83 eV) adsorption is predominantly due to ground-state ${\mathrm{D}}_2$; molecules in excited vibrational states play the dominant role for ${\mathit{E}}_{\mathit{i}}$ below ∼0.5 eV. A detailed analysis provides estimated adsorption probability versus energy functions for ${\mathrm{D}}_2$ in vibrational states v=0 to v=3.

We report experimental results describing the mechanical breakdown of random porous Au which is a new material specifically developed for this study. Digital image analysis was used to characterize the microstructures of the samples which varied by more than 2 orders of magnitude in length scale. The porous Au undergoes a microstructurally controlled ductile-brittle transition. Similar transitions have been found in numerical simulations by Kahng et al. [Phys. Rev. B 37, 7625 (1988)].

Crystallography is given a new formulation that abandons its traditional reliance on microscopic periodicity. Both periodic and quasiperiodic materials are treated on an equal footing by redefining the point group of a material in terms of indistinguishable (rather than identical) densities. The new paradigm is simpler and broader than the widely used ‘‘superspace’’ extension of the crystallographic categories to quasiperiodic materials. This is illustrated by a comparison of how the two schemes treat certain Bravais classes.

Raman spectroscopic data are presented on the rotational ordering fcc→sc transition in single-crystal ${\mathrm{C}}_{60}$. Splittings and activation of new modes reveal the influence of the cubic structure, and are in agreement with the crystal symmetry in both phases. The temperature dependence of the spectra is consistent with a sharp first-order phase transition near 252 K. A simple model is proposed for the explanation of the temperature dependence of the spectra, involving a large vibrational-rotational coupling in the fcc phase, and essentially no rotational disorder in the sc phase.

A simple dipole chain is the main building block of many physical models in solid state physics. Here, just such a chain of essentially identical dipoles is investigated by considering nearest-neighbor interactions of the individual charges in the dipoles. A three-dimensional model is found. In a restricted, two-dimensional picture, a nonlinear, second-order partial differential equation, more general than sine-Gordon, results. Soliton-kink solutions are found.

We report the near-steady-state and long-time photoinduced spectroscopies of pernigraniline base, a novel Peierls ground-state polymer with multiple order parameters. Two kinds of defects are distinguished by differing time domains, ir-active vibrations, and light-induced electron spin resonance. The short-lived defect has a lifetime of msec scale, defect mass of ∼5${\mathit{m}}_{\mathit{e}}$, and is spinless; in contrast, the long-lived defect (mass ∼300${\mathit{m}}_{\mathit{e}}$) has a lifetime of greater than several hours at low temperatures, and spin 1/2. We propose that the short-lived excitation is a soliton defect in the bond-length order parameter, while the long-lived excitation is a polaronlike defect in the ring-torsion-angle order parameter.

In a dilute, low-disorder, two-dimensional IholeR system at the GaAs/AlGaAs heterointerface, we observe a reentrant insulating behavior around the ν=1/3 fractional quantum Hall liquid at B≃5 T, strikingly similar to recent observations in low-disorder 2D IelectronR systems near ν=1/5. We interpret this behavior as manifesting a weakly pinned hole Wigner crystal around ν=1/3, and suggest that its observation at such large ν is a result of Landau level mixing which, in the case of much heavier holes, significantly modifies the ground-state energies of the fractional quantum Hall and Wigner crystal states of the system.

In situ measurements of surface conductance, combined with simultaneous observations of reflection high-energy electron diffraction, clearly demonstrated strong dependence of the conductance on substrate-surface structures and epitaxial growth styles at early stages of Ag and Au depositions on Si(111) surface at room temperature. The conductance showed a large change with a small amount of deposition (<0.1 monolayer) on the substrate of a metal-induced superstructure (√3 × √3 -Ag or 5×2-Au), while it scarcely changed for a clean 7×7 substrate. The results are discussed in terms of the Fermi-level pinning and space-charge layers.

We report on transport measurements in a novel system composed of two parallel 2D electron and hole gases separated by a barrier which is high enough to prevent tunneling and recombination while thin enough to allow for strong interlayer Coulomb interaction. Separate electrical contacts to each layer and independent control of both carrier densities facilitate a detailed study of the electron-hole interaction. Current driven in one layer is found to induce opposite current in the other layer. The measured electron-hole momentum-transfer rate is a factor of 5 to an order of magnitude larger than in previous experiments on electron-electron scattering and calculations based on Coulomb scattering theory.

We find evidence that quantum confinement of electrons in small semiconductor particles causes the nonlinear optical properties in the transparency region to differ markedly from those of bulk semiconductors. The optical Stark effect makes the dominant contribution to the third-order refractive nonlinearity.

We have measured the temperature dependence of valence and core x-ray photoelectron intensities for W(110). At 295 K, valence spectra are very sensitive to emission direction due to direct-transition effects, and Ienergy-integratedR valence and core photoelectron diffraction curves are similar in form, but with significant differences in fine structure. At 803 K, valence spectra are by contrast very insensitive to direction due to increased zone averaging, but valence and core diffraction curves retain differences due to the atomic-orbital character of the initial and final states involved.

IIn situR magnetic resonance measurements of ultrathin Ni(111) on W(110) prepared in ultrahigh vacuum have been carried out to determine the thickness dependence of the critical exponent β and the Curie temperature ${\mathit{T}}_{\mathit{c}}$. We find that β changes drastically from 0.30 to 0.13 between 7 and 5 monolayers. We attribute this behavior to a crossover from a three- to a two-dimensional magnetic behavior. The observed ${\mathit{T}}_{\mathit{c}}$’s and linewidths also exhibit the effect of the dimensional crossover. Ni films ≤4 layers thick show a truly two-dimensional magnetic phase transition with β=0.13.

Ultrathin epitaxial Fe films grown on W(001) surfaces prepared to yield uniform 25-Å-wide terraces are shown to exhibit layer-dependent in-plane uniaxial magnetic anisotropy having an easy axis perpendicular to the steps. Hysteresis loops measured by magneto-optical techniques when steps are aligned parallel to the applied field H manifest the flip of magnetization that occurs near H=0. The absence of magneto-optic effects for films less than or equal to one monolayer thick is consistent with recent Iab initioR calculations that predict significant moment reduction resulting from strong film-substrate hybridization.

We reconsider the problem of the inertial mass of vortex lines in both neutral and charged $s$-wave Fermi superfluids at $T=0\mathrm{}$, taking into account the difference between amplitude and phase motion, Fermi-liquid effects, and electromagnetic screening. Our results are consistent with earlier estimates as regards order of magnitude, but differ from them in detail.

We study theoretically the transport of a one-channel Luttinger liquid through a weak link. For repulsive electron interactions, the electrons are completely reflected by even the smallest scatterer, leading to a truly insulating weak link, in striking contrast to that for noninteracting electrons. At finite temperature (T) the conductance is nonzero, and is predicted to vanish as a power of T. At T=0 power-law current-voltage characteristics are predicted. For attractive interactions, a Luttinger liquid is argued to be perfectly transmitted through even the largest of barriers. The role of Fermi-liquid leads is also explored.

We present a detailed analysis of the critical properties of the two-dimensional XY model at full frustration. We calculate U(1) and ${\mathrm{Z}}_2$ gauge-invariant correlation functions, as well as the chiral magnetization and susceptibility, by using extensive Monte Carlo simulations in lattices up to 240×240. The best fits to our results give ${\nu }_{\mathrm{U}\left(1\right)\mathrm{}}$=0.3069, ${\mathrm{\eta }}_{\mathrm{U}\left(1\right)\mathrm{}}$(${\mathrm{T}}_{\mathrm{U}\left(1\right)\mathrm{}}^{\mathrm{c}}$=0.446)=0.1915, while ${\nu }_{{\mathrm{Z}}_2}$=0.875, 2${\mathrm{\beta }}_{{\mathrm{Z}}_2}$=0.1936, 2${\gamma }_{{\mathrm{Z}}_2}$=1.82, 2${\gamma }_{{\mathrm{Z}}_2}^{\prime }$=1.025. These novel results are quantitatively different from those expected for separate 2D XY and 2D Ising models.