We present evidence for the emission of atomic Ge and ${\mathrm{Ge}}_2$ molecules accompanying dynamic fracture at room temperature of single-crystal Ge under flexure loading. The emission follows the completion of crack propagation by several milliseconds and occurs in bursts, typically 1–2 ms in duration. A dislocation-driven mechanism for this emission is presented suggesting that considerable dislocation activity is accompanying failure of these samples.

Small dislocation loops with characteristic diffraction contrast of black-white lobes have been observed experimentally in annealed ${\mathrm{Al}}_{76}$${\mathrm{Si}}_4$${\mathrm{Mn}}_{20}$ quasicrystalline icosahedral phase by using transmission electron microscopy. The variation of the features of the diffraction-contrast images of the small dislocation loops with experimental conditions has been observed and computer simulated systematically. By taking the Burgers vector b and the normal n of the loops to be along the [100100] direction, i.e., parallel to one of the twofold symmetry axes, we have arrived at good agreement between the observed and computer-simulated features of the diffraction-contrast images.

Absorption spectra of GaAs/GaAlAs Fibonacci superlattices (FSL) with magnetic fields parallel to the layers show self-similarity at field values scaled by ${\mathrm{\tau }}^2$ [τ=(1+ √5 )/2]. The one-dimensional nonperiodicity of the FSL affects the carrier motion perpendicular to the layers very differently from that parallel to the layers: In ${\mathit{B}}_{\mathrm{\parallel }}$, where carriers have to cross several barriers to complete a cyclotron orbit, clear Landau levels are observed, whereas in ${\mathit{B}}_{\mathrm{\perp }}$ the merging of the magnetic levels superimposed on the irregular eigenvalue spectrum of the FSL does not yield equally spaced spectra.

${\mathrm{Ga}}_{0.5}$${\mathrm{In}}_{0.5}$P alloys order spontaneously during growth into a (111) monolayer superlattice despite the fact that this is not the lowest-energy structure of the three-dimensional bulk compound. Using first-principles total-energy calculations, we show that a novel electronically driven surface reconstruction provides a driving force for such ordering.

Excitation of a KI crystal by a high-power laser in the visible (around 630 nm) leads to ‘‘reflected’’ light at the third harmonic (around 210 nm). There is a pronounced structure in the spectrum between the longitudinal and the transverse exciton, which is interpreted as a nonlinear reststrahl band. Study of third-harmonic generation in transmission also reveals pronounced structure. An analysis according to Maxwell boundary conditions leads to a good fit to the experimental results.

Picosecond-resolution spectroscopy of the three intrinsic self-trapped-exciton luminescence bands in Rb, i.e., σ, ${\mathit{E}}_{\mathit{x}}$, and π, is reported. For T≥17 K, the ${\mathit{E}}_{\mathit{x}}$ and π bands each display a rapid rise coincident with the 15-ps rise time of the σ band, followed by a second-stage growth matching the 3.4-ns decay of the σ luminescence. The results provide confirmation of recent theoretical models attributing all three luminescence bands to progressive degrees of ‘‘off-center’’ relaxation of the self-trapped exciton, in contrast to the well-known ${\mathit{V}}_{\mathit{k}}$+e model.

We have calculated the nonlocal dielectric function of the alkali metals in the region of the p-core excitation threshold. The theory is at the level of the random-phase approximation and includes the self-consistent response of both valence and core electrons. We find that the peaking of threshold absorption is due to valence-electron–hole excitations which are induced by the dynamic dipole field of the core electrons. Our results are in good agreement with experiment and offer an alternative explanation of threshold behavior to that provided by the Mahan–Nozières–de Dominicis theory.

The kinetics of charge transport in quantum point contacts has been studied by low-frequency noise spectroscopy. Temperature and frequency (Lorentzian and 1/f) dependences of the noise spectral density are found to vary strongly from device to device, but the low-T conductance dependence universally exhibits a strong quantum size effect. Based on the direct observation in the time domain of spontaneous resistance switching, the noise is identified to be due to trapping processes which affect the local electrostatic potential. A model is presented which explains the main experimental observations.

Confirming theoretical predictions more than 25 years old, we show that Co slabs are indirectly exchange coupled via thin Cu layers with a coupling that alternates back and forth between antiferromagnetic and ferromagnetic. Four oscillations are observed with a period of ≃10 Å. Moreover, the antiferromagnetically coupled Co/Cu superlattices exhibit extraordinarily large saturation magnetoresistances at 300 K of more than 65%.

We study doping effects in the antiferromagnetic state. It is shown that hole-driven spiral fluctuations will dominate over conventional spin waves in the long-wavelength, low-energy limit, leading to a disordered state at finite doping. In such a background, the single-hole propagator will lose its coherence beyond the characteristic scale of the spin correlation, whereas a pair of holes could still keep their coherent amplitude in the singlet channel. Superconducting condensation is discussed after the interlayer coupling is included.

Anomalous spectral weight transfer at the superconducting transition of single-crystalline ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{CaCu}}_2$${\mathrm{O}}_{8+\mathrm{\delta }}$ was observed by high-resolution angle-resolved photoemission spectroscopy. As the sample goes superconducting, not only is there spectral weight transfer from the gap region to the pileup peak as in BCS theory, but along the Γ-M¯ direction there is also some spectral weight transfer from higher binding energies in the form of a dip. In addition, we note that at the superconducting transition there is a decrease (increase) in the occupied spectral weight for the spectra taken along Γ-M¯ (Γ-X).

We have measured the anisotropic magnetoresistivity tensor of single-crystal ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{CaCu}}_2$${\mathrm{O}}_8$ in the normal and in the mixed state. For magnetic fields applied perpendicular to the Cu-O (a-b) planes, the out-of-plane (c-axis) resistivity displays a huge magnetic enhancement, and a large apparent depression in the resistively measured transition temperature ${\mathit{T}}_{\mathit{c}}$ (c axis). No such depression is observed in ${\mathit{T}}_{\mathit{c}}$(a-b plane). Although these results suggest a field-induced anisotropy in ${\mathit{T}}_{\mathit{c}}$, we propose an alternate model of novel Lorentz-force-independent fluctuation-induced dissipation in the mixed state. The model may also account for the anomalous anisotropic magnetoresistance observed in layered low-${\mathit{T}}_{\mathit{c}}$ superconductors.

New features of the domain structure in the two-species annihilation reaction, A+B→0, are reported. A simple scaling argument predicts that the gaps between domains grows as ${\mathit{t}}^{\mathrm{\zeta }}$ with ζ=3/8 and 1/3, respectively, in spatial dimension d=1 and 2. The average density profile in a single domain exhibits a power-law tail near the domain edge. This feature has surprising implications for the spatial correlations of same-species reactants.

We investigate experimentally the scaling of the average time τ between intermittent, noise-induced bursts for a chaotic mechanical system near a crisis. The system studied is a periodically driven (frequency f) magnetoelastic ribbon. Theory predicts that for deterministic crises where τ scales as τ∼‖f-${\mathit{f}}_{\mathit{c}}$${\mathrm{\Vert }}^{\mathrm{-}\gamma }$ (f<${\mathit{f}}_{\mathit{c}}$, f=${\mathit{f}}_{\mathit{c}}$ at crisis), the characteristic time between noise-induced bursts (f≥${\mathit{f}}_{\mathit{c}}$) should scale as τ∼${\mathrm{\sigma }}^{\mathrm{-}\gamma }$g(‖f-${\mathit{f}}_{\mathit{c}}$‖/σ), where σ is the noise strength and γ is the same in both cases. We determine γ for the low-noise (‘‘deterministic’’) system, then add noise and observe that the scaling for τ is as predicted.

A direct search has been made for supermassive relics (heavier than about ${10}^{12}$ GeV/${\mathit{c}}^2$) with a 2000-${\mathrm{m}}^2$ array of CR-39 track-etch detectors deployed underground for 2.1 yr. The nonobservation of penetrating tracks places a new upper limit for the velocity-dependent flux at 3.2×${10}^{\mathrm{-}16}$ ${\mathrm{cm}}^{\mathrm{-}2}$ ${\mathrm{s}}^{\mathrm{-}1}$ ${\mathrm{sr}}^{\mathrm{-}1}$, for magnetic monopoles carrying various magnetic charges, for electrically charged relics, and strange matter. This flux limit corresponds to a limit on the relic abundance of order ${\mathit{n}}_{\mathit{x}}$/${\mathit{n}}_{\gamma }$=${10}^{\mathrm{-}29}$ relative to the 3-K photons.

We formulate two-dimensional topological gravity starting from local N=2 superconformal geometry. The theory is free from the very beginning. The usual ‘‘twisting’’ of the N=2 algebra emerges from symmetry breaking when we expand about a nonzero value for one of the ghost fields. The mysterious linear term in the supercurrent emerges automatically, as does a full superfield formalism for the whole system including ghosts. We analyze the moduli space of the ‘‘semirigid’’ super Riemann surfaces associated with this theory, including their allowed degenerations.

Analyzing powers ${\mathit{A}}_{\mathit{y}}$ were measured for ${\mathrm{\pi }}^{+}$ and ${\mathrm{\pi }}^{\mathrm{-}}$ elastic scattering from polarized ${}_{}{}^{13}{}_{}{}^{}\mathrm{C}$ at energies near the ${\mathit{P}}_{33}$ resonance. At ${\mathit{T}}_{\mathrm{\pi }}$=132 MeV the values of ${\mathit{A}}_{\mathit{y}}$ are significantly different from zero for ${\mathrm{\pi }}^{\mathrm{-}}$. For ${\mathrm{\pi }}^{+}$ at 132 MeV and for both ${\mathrm{\pi }}^{\mathrm{-}}$ and ${\mathrm{\pi }}^{+}$ at all other energies, the ${\mathit{A}}_{\mathit{y}}$ are mostly consistent with zero. These data differ from the predictions of present pion-nucleus reaction theories, especially at large momentum transfers.

The photon-number fluctuation of the external field from a semiconductor laser—which was reduced to below the standard quantum limit—is shown to be correlated with the measured junction-voltage noise. The spectral density of the sum of the photon-number fluctuation and junction-voltage fluctuation falls below the squeezed photon-number fluctuation. This confirms the theoretical predictions that this correlation, which originates in the dipole interaction between the internal field and electron-hole pairs, extends into the quantum regime.

High-intensity Cherenkov radiation (CR), emitted in air from a high-current 30-MeV electron bunch (2×${10}^{11}$ electrons/bunch) of the linear accelerator at the Institute of Scientific and Industrial Research was observed at wavelengths from 0.4 to 3 mm. The radiation was emitted at an angle of 38 mrad, with some spread, from the beam axis. The radiation measured was about ${10}^{11}$ times as intense as that calculated for ordinary CR. Such enhancement is attributed to the coherence which appears in CR emitted from electron bunches of relatively short length.

We report results of two-dimensional numerical solutions of Zakharov’s model for strong Langmuir turbulence excited by powerful electromagnetic pump waves at subcritical densities, as in hf modification of the ionisphere and laser-plasma interactions. The results show that truncated parametric decay cascades can coexist with caviton collapse for moderate pump power. A high level of ion density fluctuations, driven primarly by caviton collapse, produces energy and power spectra which are not consistent with conventional weak turbulence theory.