The possibility of a semiconducting quasicrystal in boron-rich solids made of ${\mathrm{B}}_{12}$ icosahedra is investigated. On the basis of crystallographic data and a molecular-orbital calculation, we predict the existence of a semiconducting quasicrystal, based on α-rhombohedral units of boron icosahedra, and its boron-carbon composition. We have experimentally discovered a new 1/0-1/0-0/1 orthorhombic approximant phase of an icosahedral quasicrystal by annealing amorphous phase ${\mathrm{B}}_{100\mathrm{-}\mathit{x}}$${\mathrm{C}}_{\mathit{x}}$ (x=3, 5, 7) above 1200 °C for 10 min. It must be a semiconductor like other boron-rich compounds.

Starting from a nondegenerate-ground-state Su-Schrieffer-Heeger-like model Hamiltonian for poly(phenylene vinylene) we study the effect of electron-lattice coupling on the radiative recombination of polaron-excitons, using a lattice relaxation theory which explicitly takes into account multimode effects. We provide a detailed microscopic description of the decay process with emphasis on the fact that the phonon modes of the excited state—the polaron-exciton—are different from the ground-state phonon modes. We find that at zero temperature all accepting modes are within an almost dispersionless ground-state phonon branch. Therefore, as long as the polaron-exciton is in its vibrational ground state a simple, empirical, single-mode description of the recombination (photoluminescence) is feasible. However, the complicated phonon structure of the polaron-exciton—including localized as well as extended modes—renders an Einstein desciption of promoting modes meaningless. We thus expect that in processes where promoting modes are involved a single-mode model breaks down.

Small polarons, being composite particles in which the charge degrees of freedom and the lattice vibrational modes are locked together, show unique features in their single-particle spectral properties. The occupation number ${\mathit{n}}_{\mathbf{k}}$ of the polaronic charge carriers is finite and roughly constant throughout the entire Brillouin zone; with only a small jump at ${\mathit{k}}_{\mathit{F}}$. Strongly temperature-dependent incoherent contributions to the photoemission spectrum and the occurrence of a polaron-induced vibrational mode in the neutron scattering cross section are indications for small polarons in band states.

The classical antiferromagnetic Heisenberg model on the triangular lattice is studed using Monte Carlo techniques. The behavior of the antiferromagnetic structure factor, antiferromagnetic correlation length, and spin stiffness at low temperatures is consistent with recent low-temperature renormalization-group predictions based on a nonlinear sigma model (NLσ) description of the system. Our results support the validity of the NLσ model approach to the study of frustrated systems.

The one-dimensional XXZ model is studied in the presence of disorder in the Heisenberg exchange integral. Recent predictions obtained from renormalization-group calculations are investigated numerically using a Lanczos algorithm on chains of up to 18 sites. It is found that in the presence of strong X-Y-symmetric random-exchange couplings, a ‘‘random singlet’’ phase with quasi-long-range order in the spin-spin correlations persists. As the planar anisotropy is varied, the full zero-temperature phase diagram is obtained and compared with predictions of Doty and Fisher [Phys. Rev. B 45, 2167 (1992)]. In addition, we observed a novel reentrant transition of the ordered phases when exchange disorder is included.

For thermally activated flux creep, which can be characterized by a temperature-magnetic-field-, and current-density-dependent activation energy U(j,T;${\mathit{B}}_{\mathit{e}}$), we show that this function can be determined from a combination of magnetization and magnetization dynamical relaxation rate data. As an illustration the method is applied to creep data on epitaxial ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ thin films. In contrast to previously proposed procedures, the present inversion scheme is much more general as it does not require a priori assumptions about the explicit temperature or field dependence of U.

The revival of superconductivity in the electron-excess T’-type compound ${\mathrm{Nd}}_{1.82}$${\mathrm{Ce}}_{0.18}$${\mathrm{CuO}}_{4\mathrm{-}\mathrm{\delta }}$ by hole doping has been observed. The normal-state resistivity (ρ) changes from semiconducting (dρ/dT<0) to metallic (dρ/dT>0) as the hole doping increases up to a certain level.

The low-temperature dependence of the microwave complex surface impedance (17 GHz) of pure and Zn-doped high-quality YBCO single crystals is reported. The surface resistance of the pure crystal shows a peaked structure around 40 K which disappears with Zn doping. The in-plane penetration depth varies linearly with temperature below 25 K; a ${\mathit{T}}^2$ contribution appears at the lowest temperature when Zn impurities are added. This suggests that the former temperature dependence is intrinsic to pure crystals whereas the ${\mathit{T}}^2$ behavior, also observed in thin films, would be due to defects. The linear temperature dependence is expected for superconducting gaps with lines of nodes.

The motion of the flux front in ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ with the field applied parallel to the layers is studied using a scanning Hall probe. By tracking the peak positions versus field in the remanence, we determine directly the critical current density perpendicular to the layers (${\mathit{J}}_{\mathit{c}}^{\mathrm{\perp }}$=8.3×${10}^4$ A/${\mathrm{cm}}^2$ at 4.2 K). The observed critical-current anisotropy (∼500) is much larger than indicated by high-temperature measurements. By scanning the flux profiles in the direction perpendicular to the layers, we find direct evidence that flux motion occurs parallel to the layers. A model that accounts for the observed mountain-pass topography in the remanence is presented.

After preparation of the remanent critical state, the normal component of the local time-dependent magnet field ${\mathit{B}}_{\mathit{z}}$ near the surface of a ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ thin film was measured before, during, and after the application of a transport current. The results are shown to be consistent with new calculations of ${\mathit{B}}_{\mathit{z}}$ for the thin film geometry. The calculated behavior in a thin film is shown to be substantially different from that of the Bean critical state in a long slab in a parallel field. In slab geometry, starting from the remanent critical state, ‖J(x)‖ is always equal to ${\mathit{J}}_{\mathit{c}}$, whereas in thin films a continuous distribution of current densities J(x) is found. This feature results in a unique distribution of the magnetic field ${\mathit{B}}_{\mathit{z}}$ and unusual magnetic relaxation behavior in the presence of a transport current.

Results of neutron-scattering experiments on a single crystal of ${\mathrm{Ni}}_{62.5}$${\mathrm{Al}}_{37.5}$ as a function of uniaxial stress and temperature are reported. Emphasis is placed on the behavior of the low-energy part of the [ζζ0]-${\mathrm{TA}}_2$ phonon branch and its associated central peak. The dip in the phonon-dispersion curve is stress dependent and shifts from ζ=0.14 for zero stress to ζ=0.18 for an applied stress of 85 MPa at room temperature. As the temperature decreases the satellite shifts further, the phonon energy decreases considerably, and the linewidths become broad. These results are interpreted in terms of Clapp’s localized soft-mode theory of nucleation in martensite.

We measure the effect of dc-electric-field–temperature sequences on the finite-frequency dielectric constant ɛ(ω), and the thermally stimulated depolarization current of single crystals of Pb (${\mathrm{Mg}}_{1/3}$${\mathrm{Nb}}_{2/3}$)${\mathrm{O}}_3$ (PMN). The results are interpreted in terms of a field-induced change in the low-temperature phase of PMN from glassy to rhombohedral ferroelectric. The latter transition has first-order characteristics, and occurs only if a dc field along 〈111〉 of 1.7 kV/cm or more is applied. The criterion for glassiness is the apparition of a long-lived metastable polarization, proportional to the applied field, without accompanying anomalies on the small-signal dielectric constant. It is believed that the well-known broad dispersive maximum in ɛ(ω) near 270 K simply reflects thermal slowing down of strongly non-Debye dynamics, and is not related to a ferroelectric phase transition; the existence of a divergence in the static dielectric constant is unlikely. A phase diagram in the electric-field–temperature plane is proposed.

Semiempirical cluster calculations are performed on positive (${\mathrm{E}}_1^{\prime }$ center) and neutral oxygen-vacancy defects in α-quartz. A comparison is made of the various available Hamiltonians and the use of unrestricted wave functions. The results predict that these two defects form a bistable system, exhibiting the same two metastable configurations but differing in which configuration is lower in energy. The neutral charge state favors a configuration in which the silicon atoms adjacent to the vacancy relax only slightly away from their perfect crystal positions. The positive charge state yields a minimum energy configuration with large relaxation of the silicon on the long-bond side of the vacancy. This silicon atom, in fact, moves through its basal oxygen plane and bonds with an oxygen atom, making the oxygen atom threefold coordinated. Unpaired spin localization is consistent with experimental data from electron-paramagnetic-resonance studies on irradiated quartz.

Cu-Co has been chosen as a model system to study phase formation during mechanical alloying in systems with components exhibiting a positive heat of mixing. Phase formation is monitored by scanning electron microscopy, x-ray diffraction, and calorimetric measurements. Although the occurrence of a single fcc Bragg pattern cannot definitely prove the alloy formation on an atomic scale, the quantitative agreement of the heat release upon decomposition of the alloy with the heat of mixing, calculated from the available thermodynamic data of the system, is regarded as clear evidence for the formation of an fcc solid solution. As a mechanism of alloy formation the dissolution of Cu and Co particles during milling is discussed. It is proposed that the formation of a homogeneous solid solution is energetically favored if the particle size is reduced below a critical value, which amounts to about 1–2 nm, depending on the composition of the alloy and the temperature. The driving force is provided by the chemical contribution of the Cu/Co interface enthalpy and by the high configurational entropy of the randomly distributed atoms in the solid solution.

Using a family of volume-dependent interatomic pair potentials derived from first principles, we calculate phonon properties and thermodynamic functions in the quasiharmonic approximation for the hcp and bcc phases of Mg over a wide range of volume and temperature. At atmospheric pressure, the calculated hcp phonon-dispersion curves and thermodynamic properties agree well with experiment. The pressure dependence of the Raman-active transverse-optical phonon mode also agrees well with very recent measurements. At high pressure, the temperature dependence of the hcp-bcc phase line is predicted, with values of the transition pressure ranging from 52 GPa at zero temperature to about 28 GPa at 1000 K.

The anisotropic growth of twinned platelets having a cubic crystal structure is examined by a ball-model simulation of the growth process. The simulation has suggested that the long-accepted growth mechanism, which relies on the self-regenerating {111} faceted-side-face structure, may need revision. Such a side-face structure is likely to transform into a different structure where the twin bands are bound by {100} planes. Experimental evidence is presented to support this structure for the silver halide system.

We propose an ion-beam-mixing model that accounts for compound formation at a boundary between two materials during ion irradiation. It is based on Fick’s law together with a chemical driving force in order to simulate the chemical reaction at the boundary. The behavior of the squared thickness of the mixed layer, ${\mathit{X}}^2$, with the irradiation fluence, Φ, has been found in several mixing experiments to be either quadratic (${\mathit{X}}^2$α${\mathrm{\Phi }}^2$) or linear (${\mathit{X}}^2$αΦ), a result which is qualitatively reproduced. Depending on the fluence range, compound formation or diffusion is the limiting process of mixing kinetics. A criterion is established in terms of the ratio of the diffusion coefficient D due to irradiation to the chemical reaction rate squared which allows us to predict quadratic or linear behavior. When diffusion is the limiting process, D is enhanced by a factor which accounts for the formation of a compound in the mixed layer. Good agreement is found between the calculated mixing rates and the data taken from mixing experiments in metal/Si bilayers.

Point-defect energetics and diffusion mechanisms in graphite are investigated using a semi-empirical tight-binding-force model. Possible diffusion processes associated with point-defect (i.e., vacancies and interstitials) and nondefect (i.e., atomic exchange) mechanisms are analyzed. It is found that self-diffusion in graphite in the direction parallel to the basal plane can be mediated by vacancies. However, since the calculated vacancy- and interstital-formation energies are nearly equal, it is argued that Frenkel pairs could exist as equilibrium defects. In this case, at high enough temperatures, self-diffusion parallel to the basal plane should occur by an interstitial mechanism because the migration energy of the interstitial is much lower.

The photoionization of ${\mathrm{Ce}}^{3+}$ in ${\mathrm{CaF}}_2$ has been reinvestigated with ${\mathrm{CaF}}_2$:${\mathrm{Ce}}^{3+}$ and ${\mathrm{CaF}}_2$:${\mathrm{Ce}}^{3+}$, ${\mathrm{Na}}^{+}$. Irradiation of the singly doped crystals with 308-nm light at room temperature produces ${\mathrm{Ce}}^{2+}$. However, at 80 K, we found no ${\mathrm{Ce}}^{2+}$ and observed only that the ${\mathit{F}}^{\mathrm{-}}$ interstitial charge compensator moved from the (001) site to the (111) site. (Units are a/2, the unit cell dimension.) In the doubly doped crystals, we observed the production of ${\mathrm{Ce}}^{2+}$ at 80 K. Diffusion processes at higher temperatures lead to formation of other color centers. A mechanism that accounts for these observations is proposed.