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Notes: Using a serial bilayer model we show that heterogeneous ferroelectric systems may exhibit substantial Maxwell–Wagner piezoelectric relaxation. The relaxation has its origin in the coupling of the dielectric and piezoelectric properties. The model predicts both retardation (positive phase angle and counterclockwise charge–pressure hysteresis) and relaxation (negative phase angle and clockwise charge–pressure hysteresis) of the longitudinal direct piezoelectric coefficient. The theoretical results are confirmed experimentally in ferroelectric ceramic–ceramic bilayers and in a single-phase ferroelectric Aurivillius compound ceramic in which neighboring grains with strongly anisotropic properties may behave as basic Maxwell–Wagner units. © 2001 American Institute of Physics.

Notes: We describe a prototype linear vacuum-ultraviolet x-ray pinhole camera based on the luminescence of a phosphor screen working in reflection mode. The luminescence emitted by the phosphor, in response to radiation selected by a broadband filter, is detected using a 1024 pixel intensified linear array detector. By operating this detector with integration times from 20 to 50 ms, random plasma fluctuations can be smeared out, so that structures related to the magnetic topology can be searched for. © 2002 American Institute of Physics.

Notes: The interaction potential energy surface (PES) of H4 is of great importance for quantum chemistry as a test case for molecule–molecule interactions. It is also required for a detailed understanding of certain astrophysical processes, namely collisional excitation and dissociation of H2 in molecular clouds, at densities too low to be accessible experimentally. The 6101 ab initio H4 energies reported in 1991 by Boothroyd et al. demonstrated large inaccuracies in analytic H4 surfaces available at that time. Some undesirable features remained in the more accurate H4 surfaces fitted to these energies by Keogh and by Aguado et al., due in part to the relatively sparse coverage of the six-dimensional H4 conformation space afforded by the 6101 ab initio energies. To improve the coverage, 42 079 new ab initio H4 energies were calculated, using Buenker's multiple reference (single and) double excitation configuration interaction program. Here the lowest excited states were computed as well as the ground state, and energies for the original 6101 conformations were recomputed. The ab initio energies have an estimated rms "random" error of ∼0.5 millihartree and a systematic error of ∼1 millihartree (0.6 kcal/mol). A new analytical H4 PES was fitted to these 48 180 ab initio energies (and to an additional 13 367 points generated at large separations), yielding a significant improvement over previous H4 surfaces. This new PES has an rms error of 1.43 millihartree relative to these 48 180 ab initio energies (the fitting procedure used a reduced weight for high energies, yielding a weighted rms error of 1.15 millihartree for these 48 180 ab initio energies). For the 39 064 ab initio energies that lie below twice the H2 dissociation energy, the new PES has an rms error of 0.95 millihartree. These rms errors are comparable to the estimated error in the ab initio energies themselves. The new PES also fits the van der Waals well to an accuracy of about 5%. For relatively compact conformations (energies higher than the H2 dissociation energy), the conical intersection between the ground state and the first excited state is the largest source of error in the analytic surface. The position of this conical intersection forms a somewhat complicated three-dimensional hypersurface in the six-dimensional conformation space of H4. A large portion of the position of the conical intersection has been mapped out, but trying to include the conical intersection explicitly in an analytic surface is beyond the scope of the present paper. © 2002 American Institute of Physics.

Notes: This paper reports experimental results showing that coherent helical structures are formed in the reversed field pinch (RFP) self-organizing plasma core as a result of transitions to states where the n-spectrum of the m=1 modes is dominated by a single (1,nmax) geometrical helicity. These states are dubbed quasi-single helicity (QSH) states. Their magnetic and thermal properties measured in the reverse field experiment (RFX) [G. Rostagni, Fusion Eng. Des. 25, 301 (1995)] device are described. The present theoretical understanding of QSH states is discussed and some recent theoretical results are presented. The role of aspect ratio is discussed. These results represent a significant step to open a path beyond the standard paradigm that a bath of magnetohydrodynamic (MHD) modes is intrinsic to the RFP. © 2000 American Institute of Physics.

Notes: Transport in the collisional regime is analyzed here for tokamaks and compact toroid of any shape. Toroidal axisymmetry is assumed. The treatment is performed with an improved system of coordinates, here described. Pfirsch–Schlüter current is determined in a general case. The average velocity around a magnetic surface is found as a function of closed geometrical integrals and separated factors that include the electric and magnetic fields. The contribution of the induced electric field Ecursive-phi is analyzed and the similarities with the Ware pinch are shown. The general equations have been applied to families of nested curves with elliptic shapes. © 2000 American Institute of Physics.

Notes: Data are presented on high-power AlGaInN flip-chip light-emitting diodes (FCLEDs). The FCLED is "flipped-over" or inverted compared to conventional AlGaInN light-emitting diodes (LEDs), and light is extracted through the transparent sapphire substrate. This avoids light absorption from the semitransparent metal contact in conventional epitaxial-up designs. The power FCLED has a large emitting area (∼0.70 mm2) and an optimized contacting scheme allowing high current (200–1000 mA, J∼30–143 A/cm2) operation with low forward voltages (∼2.8 V at 200 mA), and therefore higher power conversion ("wall-plug") efficiencies. The improved extraction efficiency of the FCLED provides 1.6 times more light compared to top-emitting power LEDs and ten times more light than conventional small-area (∼0.07 mm2) LEDs. FCLEDs in the blue wavelength regime (∼435 nm peak) exhibit ∼21% external quantum efficiency and ∼20% wall-plug efficiency at 200 mA and with record light output powers of 400 mW at 1.0 A. © 2001 American Institute of Physics.

Notes: Capacitance measurements are used to investigate a series of single-barrier n-i-n GaAs/AlAs/GaAs heterostructures incorporating a layer of self-assembled InAs quantum dots into the AlAs barrier. They reveal a low-density, excess positive charge in the AlAs barrier which we attribute to defects associated with quantum dot formation. The quantity of positive charge is proportional to the amount of AlAs deposited on top of the dots. © 2000 American Institute of Physics.

Notes: YBa2Cu3O7−δ (YBCO) films were fabricated via a fluorine-free metal organic deposition (MOD) method followed by high-temperature, low oxygen partial pressure annealing. Trimethyl acetate salts of copper, yttrium, and barium hydroxide were used as the precursors, which were dissolved in proponic acid- and amine-based solvents. After spin-coating and burnout, samples were annealed at 740°C in 180 ppm oxygen partial pressure and exposed to humid atmosphere for different times. A critical transition temperature, Tc(R=0) of 90.2 K and a transport critical current density (Jc) of 0.55 MA/cm2 (77 K and self-field) were obtained for 0.2 μm YBCO films on CeO2-buffered yttria-stabilized zirconia (YSZ) substrates. X-ray studies shows that the YBCO films have sharp in-plane and out-of-plane texture for all samples; however, the porosity of the YBCO film varies with the time of exposure to the humid atmosphere. A reaction between YBCO and CeO2 during the high-temperature anneals and formation of the reaction product BaCeO3 was confirmed by X-ray diffraction (XRD) studies. The XRD and transmission electron microscopy analysis indicated that the epitaxial relations in the film were YBCO (00l)//CeO2 (00l)//YSZ (00l) and YBCO [100]//CeO2 [110]//YSZ [110].

Notes: The effect of a localized SiO2 codoping on the conversion of polycrystalline, MgO-doped Al2O3 tubes to single-crystal sapphire was investigated. Codoping with SiO2 before sintering intentionally triggered abnormal grain growth, which resulted in the full conversion of tube surfaces to single crystal without adversely affecting densification to a almost pore-free, translucent state. The degree of surface conversion was strongly dependent on experimental variables, which included furnace temperature and codoping amount. Surface-converted tubes had excellent physical properties, which included good thermal cycling resistance and optical properties superior to unconverted, polycrystalline Al2O3.

Notes: The presence of rigid inclusions in a powder compact leads to a reduction in the densification rate of the compact and may also lead to processing defects. In this paper, the densification rate and the constitutive parameters of both homogeneous YBa2Cu3O6+x and composite powder compacts (YBa2Cu3O6+x powder with 10 vol% dense inclusions of YBa2Cu3O6+x) are reported. A small amount of liquid phase, which formed during sintering, was present in the samples. However, even with the presence of a liquid phase, the addition of inclusions still reduces the densification rate of the composite and increases its viscosity. The results have been compared with a published analysis of the problem using measured values of the constitutive parameters. Both the viscosity and viscous Poisson's ratio of the porous body have been measured.