Conservation of angular momentum and the isotropy of space prohibit orthopositronium (o-Ps) from annihilating into two photons. Nevertheless, an observed o-Ps decay rate in excess of theory tempts one to speculate on the existence of a ubiquitous field that would couple to Ps, allowing a small partial rate for the long-lived o-Ps state to annihilate into two photons. We apply a 3.4-kG magnetic field to a sample of Ps to quench the m=0 states in the laboratory frame of reference, and search for a 12-h periodicity in the 3γ yield. The observed effect is consistent with the assumption that space is isotropic and, at the 80% confidence level, is too small to explain the observed o-Ps decay-rate discrepancy with theory.

We study shock waves propagating in a rigid heat conductor at low temperature using a generalized Maxwell-Cattaneo equation. The existence of a critical temperature θ̃, characteristic of the material, for which the structure of the shock changes is proved. When the unperturbed temperature ${\mathrm{\theta }}_0$ is less than θ̃ the temperature ${\mathrm{\theta }}_1$ behind the shock wave front is such that ${\mathrm{\theta }}_1$>${\mathrm{\theta }}_0$ (hot shock), and, vice versa, if ${\mathrm{\theta }}_0$>θ̃ then ${\mathrm{\theta }}_1$<${\mathrm{\theta }}_0$ (cold shock. We find θ̃=15.36 K for NaF and θ̃=3.38 K for Bi. These temperatures are very close to the values at which the second sound was identified experimentally in pure NaF and Bi crystals.

For anisotropic porous media of arbitrary topology, it is shown that there exists a rigorous relation between the fluid permeability tensor k and the diffusion-controlled trapping constant γ, namely, k≤${\gamma }^{\mathrm{-}1}$I. It is demonstrated that the equality is achieved for a certain class of microstructures and that the bound can be relatively sharp for other media. The important fundamental as well as practical implications of this relation are discussed.

The transport of photons through a slab of random medium is shown to deviate from the diffusion approximation when z/${\mathit{l}}_{\mathit{t}}$ is small, where z is the thickness of the slab and ${\mathit{l}}_{\mathit{t}}$ is the transport mean free path. When z/${\mathit{l}}_{\mathit{t}}$=10 and z=10 mm, the average time of arrival is about 0.9 times that predicted by diffusion theory. Photons are found to arrive earlier than that predicted by the diffusion theory as z/${\mathit{l}}_{\mathit{t}}$ becomes smaller or the anisotropic scattering increases.

Electron plasma waves generated by both the absolute-stimulated-Raman scattering and the two-plasmon-decay instabilities in a ${\mathrm{CO}}_2$-laser–produced plasma are studied by picosecond-resolution Thomson scattering. The spatial and wave-vector distribution covering the regions of both stimulated Raman scattering and two-plasmon decay are observed. The experiments show no temporal nor wave-number separation between plasma waves generated by both instabilities in contrast to predictions from linear theory. The fluctuation spectra in the plane of polarization show a sharp peak at k=2${\mathit{k}}_0$.

Allowing for the presence of an expected moderate concentration (∼10%) of low-energy ions in excess of the Maxwellian distribution causes a dramatic change in the neoclassical prediction for momentum transport, leading to an agreement in magnitude with experiment.

A new program of characterization of multifractal moments is proposed. For the circle map, the usual multifractal moments are described by both scaling exponents and amplitudes. These amplitudes depend sensitively on the starting point of the time series used to define these moments. This leads naturally to a statistical description whcih is universal and analogous to that used in critical phenomena and in other fields where multifractals occur. This description can be used not only at the critical point, but also in the crossover region where the analog of the correlation length is large but not infinite.

Reduced sedimentation velocities are reported for suspensions of nearly hard PMMA (polymethylmethacrylate) spheres as a function of volume fraction. The absolute sedimentation velocities are sufficiently slow compared to crystal-growth rates so that phase separation is achieved during the sedimentation process. As a result the analytic behavior of the measured sedimentation velocity changes as a function of volume fraction at the freezing and melting points. This transition serves as a definitive marker for comparison with theoretical predictions of sedimentation velocities for hard-sphere liquids and crystals.

We propose a structure for the self-trapped exciton in α-quartz on the basis of ab initio Hartree-Fock cluster calculations. It involves the displacement of a single oxygen atom in a direction nearly parallel to the hexagonal axis so that the shorter of its two silicon-oxygen bonds is lengthened. The energy and polarization of the recombination luminescence, the fine-structure parameters of the triplet state, and the magnitude and sign of the transient volume change induced by self-trapping are all correctly predicted. The calculations show no evidence for hole self-trapping in a similar geometry.

The solid-state reaction in an Al/Pd lateral diffusion couple starts with the growth of the crystalline ${\mathrm{Al}}_3$${\mathrm{Pd}}_2$(δ) phase. When the growth of ${\mathrm{Al}}_3$${\mathrm{Pd}}_2$ reaches its critical length, the quasicrystalline decagonal ${\mathrm{Al}}_3$Pd(γ’) phase starts to grow and both the ${\mathrm{Al}}_3$${\mathrm{Pd}}_2$ and ${\mathrm{Al}}_3$Pd phases continue to grow afterwards. We have fitted this simultaneous growth with a recently proposed model of kinetic suppression of the second phase. The kinetic analysis directly links the appearance of the decagonal phase as the second phase to its interfacial reaction barrier. We also suggest that the growth of the metastable decagonal ${\mathrm{Al}}_3$Pd phase over the equilibrium phase is a result of its relatively lower interfacial reaction barrier.

The diffusion of ${}_{}{}^{63}{}_{}{}^{}\mathrm{Ni}$ has been measured in ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ epitaxial thin films on (100) ${\mathrm{SrTiO}}_3$ substrates in the 550–650 °C range (orthorhombic phase) in ${\mathrm{O}}_2$ pressure of 1 bar, keeping δ in the 0.3–0.4 range. The diffusion behavior is described by D=1.3 exp[(-2.7 eV)/kT] ${\mathrm{cm}}^2$/sec and is many orders of magnitude slower than anion (oxygen) diffusion reported in the literature. A diffusion mechanism involving thermal vacancies and atomic jumps in diagonal directions of the type 〈110〉, 〈301〉, and 〈031〉 is proposed for long-range diffusion of cations occupying the Cu sites.

A new spectroscopy has been developed for the first direct probe of carrier-carrier scattering in materials. This spectroscopy provides spatial and energy resolution of the scattering process and has been used to investigate transport, scattering phenomena, and hot-carrier creation in two important metal-semiconductor systems. A theoretical treatment of this scattering spectroscopy yields excellent agreement with experimental spectra and provides direct evidence that carrier-carrier scattering is a dominant energy-loss mechanism in hot-carrier transport.

X-ray-photoelectron diffraction has been measured from an Al(001) single crystal for the free-electron-like valence band and the well-localized 2s core level, using Mg Kα radiation. Both show the same high anisotropy of 65% in the azimuthal distribution of the photoemitted electrons at 45° away from the surface normal. Moreover, the measured intensity patterns are essentially identical, indicating a strong similarity in the photoemission final states. This similarity provides evidence for a strong degree of hole localization in high-energy valence-band photoemission.

We report the first observation of a crossover with decreasing temperature of the resistivity of a doped semiconductor from the ${\mathrm{expT}}_0$/T${)}^{1/4}$ behavior characteristic of Mott variable-range hopping to the exp(${\mathrm{T}}_0^{\prime }$/T${)}^{1/2}$ form expected in the presence of a Coulomb gap. Simultaneous determination of ${\mathrm{T}}_0$ and ${\mathrm{T}}_0^{\prime }$ for several dopant concentrations yields the empirical relation ${\mathrm{T}}_0^{\prime }$∝${\mathrm{T}}_0^{\mathrm{p}}$, with p≈23. If current theory is valid and applicable, this has unexpected implications regarding the critical behavior of the localization length and the dielectric constant at the metal-insulator transition. © 1990 The American Physical Society

We have fabricated an array of ultrasmall tunnel junctions which acts like a turnstile for single electrons. When alternating voltage of frequency f is applied to a gate, one electron is transferred per cycle through the device. This results in a current plateau in the current-voltage characteristic at I=ef. The overall behavior of the device is well explained by the theory of Coulomb blockade of electron tunneling. We discuss the accuracy limitations of this device.

A pronounced difference in magnetic behavior at distinct lattice sites has been found for recoil-implanted ${}_{}{}^{54}{}_{}{}^{}\mathrm{Fe}$ ions in fcc cerium metal using the perturbed-angular-distribution method. Of the implanted ions 25(5)% show predominantly spin magnetism while 75(5)% appear to be nonmagnetic. Based on the Anderson model, the magnetic fraction is assigned to regular lattice sites and the nonmagnetic fraction is inferred to occupy interstitial positions. This interpretation is supported by Monte Carlo simulations describing the transport of ions in matter.

The magnetic phases which have earlier been observed when a magnetic field is applied in the plane of the helical magnetic structure of Ho are identified with structures, intermediate between the helix and the fan, which we call helifans. A number of helifan structures have been calculated by a self-consistent mean-field method, and one of them accounts very well for the observed neutron-diffraction pattern. Different sequences of helifans may, in principle, be produced by varying the magnetic field and temperature, and modifying the exchange by alloying.

Synthetic Gd/Dy superlattices show novel magnetic properties which we investigate both experimentally and theoretically. In this system competing exchange and Zeeman interactions lead to many different possible magnetic structures. Of particular interest is an alternating-helicity phase where the magnetic unit cell is twice the length of the chemical unit cell. A simple theoretical model gives results in good agreement with the experimental measurements.

We have investigated the susceptibility and entropy in the thermally isolated system of silver nuclei down to 0.8 nK and, at negative temperatures, up to -4.3 nK. Low-frequency SQUID-NMR techniques were employed to measure the dynamic sysceptibility. Curie-Weiss behavior was observed for the static susceptibility both at T>0 and T<0; for FTHETA we deduce -4.4±1.0 nK. Our results show directly that antiferromagnetic nuclear alignment at positive temperatures transforms into ferromagnetic orientation at T<0 in the nuclear-spin system of silver, dominated by exchange interaction.

The superconducting-vortex-core spectra show a zero-bias peak which splits within a coherence length of the core. Further away from the core these split peaks merge gradually with the gap edge and give a direct local measure of the superfluid velocity. The vortex-core states are imaged both at the Fermi energy and just below the gap to reveal two different sixfold star-shaped structures. The anisotropy and size of these images may be a consequence of the crystalline band structure with its charge-density-wave gap as well as the interaction of the neighboring vortices of the Abrikosov flux lattice.