A nonlinear transformation performs the Levy-constrained search formulation of the density functional for the electronic energy through a minimization of the energy with respect to a set of variational coefficients. The construction requires a complete set of arbitrary functions as the auxiliary basis. Truncation of the basis set provides an upper bound to the energy functional. Practical approaches to obtain accurate upper bounds to this functional are discussed, and a density-functional alternative to the standard Hartree-Fock method is described.

We have developed a new method for measuring the electromagnetic fields (wake fields) induced in high-frequency accelerating devices by intense relativistic electron bunches. These fields are probed by a second collinear electron bunch which follows the primary bunch at a variable delay. Initial results for several iris-loaded rf structure geometries are presented. This technique is expected to be important for the study of new acceleration methods such as the plasma wake-field accelerator. Another possible application is the measurement of parasitic wakes in conventional accelerator components.

Experiments in the JET tokamak with additional heating power (ion cyclotron resonance heating and/or neutral beam injection) above 5 MW show that the plasma can undergo a transition to a new regime. In this regime, the sawtooth instability is suppressed for periods up to 1.6 s and the level of long-wavelength, coherent MHD activity is very low. An improvement in the global energy confinement time of up to 20% is observed. Possible mechanisms for the stabilization of the $m=1\mathrm{}$ instability and the implications for the near-ignition regime are discussed.

Molecular-dynamics calculations have been carried out for a system of long-chain molecules supported on a physisorbing planar substrate and constrained to a surface density characteristics of that observed in lipid and fatty acid monolayers at the air-water interface. Results are presented for the density distribution normal to the surface, the structure factor parallel to the surface, and the number of chain conformational defects as the surface area per chain is increased at room temperature. The driving force for the observed structural and conformational changes appears to be related to the tendency for the alkyl-chain methylene groups to close-pack.

We report the observation, by transmission electron microscopy, that single-crystal silicon and germanium are converted to an amorphous state at room temperature directly under both Vickers and Knoop indentations. The effect is seen for crystal orientations of [001], [011], and [111], and with applied loads between 0.1 and 0.5 N. We also observe that the materials become electrically conducting under load and that the process is reversible on subsequent unloading and reloading. Furthermore, the transformed phase is found to make Ohmic contact to the surrounding, untransformed, semiconductor.

It is shown that the x-ray diffraction patterns from the Hendricks-Teller model for layered systems and the icosahedral glass models for the icosahedral phases show large fluctuations between nearby scattering wave vectors and from sample to sample, that are quite analogous to laser speckle. The statistics of these fluctuations are studied analytically for the first model and via computer simulations for the second. The observability of these effects is discussed briefly.

We report on the experimental study of a novel time-dependent localized state in the electroconvection of a nematic liquid crystal inside a container of very large aspect ratio. At threshold the convective rolls propagate inside isolated domains well localized in space. The phase propagation is attributed to a slow lateral drift of the ionic charges, while the localization, which is not due to finite-size effects, could result from the nonlinear focusing of a broad band of dispersive modes.

The $x$ dependence of the normalized basal spacing, $d_n\mathrm{}\left(x\right)\mathrm{}$ of pillared vermiculite (Vm) has been measured for the mixed-layer system ${\left[{\left(\mathrm{C}{\mathrm{H}}_3\right)}_4{\mathrm{N}}^{+}\right]}_x{\left[{\left(\mathrm{C}{\mathrm{H}}_3\right)}_3\mathrm{N}{\mathrm{H}}^{+}\right]}_{1-x}-\mathrm{Vm}$ and compared with that of ${\mathrm{Cs}}_x{\mathrm{Rb}}_{1-x}-\mathrm{Vm}$. Both systems exhibit a nonlinear $d_n\mathrm{}\left(x\right)\mathrm{}$ with approximate thresholds of $x\cong 0.2 \mathrm{and} 0.5$, respectively. A model which related $d_n\mathrm{}\left(x\right)\mathrm{}$ to layer rigidity and the binding energies of gallery and defect sites yields excellent fits to the basal spacing data and to monolayer simulations if collective effects are included. This model should be applicable to other types of lamellar solids.

High-resolution x-ray scattering measurements have revealed a second critical length scale at the cubic-to-tetragonal transition in SrTi${\mathrm{O}}_3$, corresponding to very large correlation lengths of the order of 2500 Å at $T_c+1\mathrm{}$ K. The correlation length becomes substantially larger still after the sample temperature has been cycled once through $T_c$, a novel memory effect in large-length-scale critical behavior of this kind. We discuss the interpretation of these observations in terms of the effect of extended defects close to a weakly first-order transition.

The scanning tunneling microscope (STM) has been used to study the $(2\mathrm{}\times 4)-c(2\mathrm{}\times 8)$ reconstruction on the arsenic-rich surface of GaAs(001). The STM images show that the 4× periodicity is due to a regular array of missing arsenic dimers. The (2×4) units are arranged so as to give small domains of either (2×4) or $c(2\mathrm{}\times 8)$ reconstructions. These images are the first high-resolution STM images obtained from a surface grown by molecular-beam epitaxy. Features are seen which may be important in the mechanism of growth.

A microdomain structure was found in the alloy ${\mathrm{V}}_{41}$${\mathrm{Ni}}_{36}$${\mathrm{Si}}_{23}$ after rapid solidification. It behaves like a structural constituent giving electron diffraction patterns similar to those from a decagonal quasicrystal though its high-resolution electron microscopic image shows a microdomain structure. This is regarded as an intermediate structure between the decagonal quasicrystal and stable phases (Frank-Kasper phases in this case).

We performed parameter-free, self-consistent calculations of the electronic structures, total energies, and forces of the As antisite, of an As-interstitial-Ga-vacancy defect pair, and of various configurations between these limits. The total-energy surface exhibits an interesting metastability. The theoretical results can explain all established properties of the so-called $\mathrm{EL}2\mathrm{}$ defect.

We propose that a neutral As-antisite defect in GaAs has a stable fourfold and a metastable, threefold interstitial configuration differing by 0.24 eV in their energies. The barrier height from the metastable to the normal state is calculated to be 0.34 eV. The metastable geometry is predicted to have a low optical excitation probability and to be electron-paramagnetic-resonance inactive. The two structural states of the antisite provide the simplest explanation for the unusual properties of the $\mathrm{EL}2\mathrm{}$ defect center in GaAs.

A new mechanism is proposed to explain high-$T_c$ superconductivity in copper-oxide-based, open perovskitelike systems. It is shown that, should the oxygen ions be moving in a double-well potential, an order-of-magnitude enhancement of the electron-lattice coupling follows automatically from a consistent treatment of this motion. Both theoretical and experimental evidence for the presence of such double wells is cited.

A strongly anisotropic resistivity tensor was measured from $T_c=81\mathrm{} \mathrm{to} 600$ K in crystals of the new high-$T_c$ superconductor ${\mathrm{Bi}}_2$${\mathrm{Sr}}_{2.2}$${\mathrm{Ca}}_{0.8}$ ${\mathrm{Cu}}_2$${\mathrm{O}}_8$. The $a-b$ plane anisotropy is $\frac{{\rho }_a}{{\rho }_b}\approx 2$, with ${\rho }_a$ and ${\rho }_b$ linear in $T$. The perpendicular component ${\rho }_c\sim 10$ Ω cm is ${10}^5$ greater than ${\rho }_a$ or ${\rho }_b$. Evidence for nonmetallic temperature dependence is found for the $\frac{{\rho }_c}{{\rho }_{\mathrm{ab}}}$ resistivity ratio. The data are consistent with the notion of quasiparticle tunneling between planes, but an alternative explanation in terms of weak interplanar coupling by topological defects is also presented.

Temperature-dependent positron-electron momentum densities have been studied by two-dimensional angular correlation of annihilation radiation from 10 to 320 K in $\mathrm{Y}{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-x}$ samples. The positron ground-state charge density, computed by the linearized augmented-plane-wave method, indicates that in Y${\mathrm{Ba}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ delocalized positrons sample preferentially the linear copper-oxygen chains. Positron localization due to disorder in these chains is invoked to explain the striking differences observed between superconducting ($x\simeq 0.02$) and nonsuperconducting ($x\simeq 0.70$) samples.

We report strong, anisotropic magnetic relaxation of the field-cooled and zero-field-cooled magnetization along the principal axes of an Y-Ba-Cu-O single crystal and interpret it with a thermally activated flux-creep model. A simple scaling argument shows that high thermal activation causes magnetic irreversibilities and critical currents to drop below the threshold of detectability at a reduced temperature difference $1-t$ proportional to $H^{\frac{2}{3}}$, a power frequently observed in experiment and in particular in our crystal.

We show that scattering processes in anisotropic superconductors have some unexpected asymmetries when the normal-state phase shift is neither small nor resonant. The scattering rate is not symmetric about the Fermi surface, which gives rise to large thermoelectric effects. For states with gaps that have nontrivial phase variations over the Fermi surface, certain components of transport coefficient tensors can be finite even though they vanish in the normal state.

A formalism for the optical absorption in a coupled electron-ion system is developed. The formalism assumes an initial trajectory with frequency much lower than electronic energy scales, and a short-memory condition, i.e., the excited-state trajectories diverge away from the initial trajectory within one period. The result shows nonadiabatic features such as level broadening, sidebands, and tails in nonclassical regimes. We apply the formalism to breather modes in polyacetylene and show that they can account for the intragap absorption tail and for the observed photoinduced absorption at 1.35 eV.

Dynamical correlation functions at low temperature for one-dimensional systems that involve the decay of a single elementary excitation into three others show logarithmic singularities at the frequency of the original excitation, as well as discontinuities that become sharper as the temperature is lowered. The discontinuities are the result of an elementary catastrophe that occurs as the phase space available for the decay varies with frequency. They should be observable, for instance, as dramatic changes in the out-of-plane spin-wave linewidth in CsNi${\mathrm{F}}_3$ as a function of temperature and wave vector.