A novel technique employing $B$ splines is introduced that permits the precise evaluation of multiple sums over excited states occurring in higher-order many-body perturbation theory. As a test of the technique, the accurately known nonrelativistic expression for the second-order energy of helium in the $\frac{1}{Z}$ approximation is first calculated. The corresponding relativistic expression is then evaluated over the entire helium isoelectronic sequence with attention to the proper treatment of negative-energy states.

Doubly differential cross sections for electron emission coincident with projectile ionization are measured for fast ${\mathrm{He}}^{+}$-He collisions. The results show the importance, and details, of simultaneous target-projectile ionization events. The details allow us to assess the theoretical treatment of these collisions which is found to be inadequate in explaining the simultaneous ionization process.

A one-component plasma has been studied by molecular-dynamics calculations to simulate the behavior of charged particles in heavy-ion storage rings. The Hamiltonian used confines the plasma in the directions lateral to the direction of travel in the ring in the frame of reference which is moving with the beam. The results show an unexpected stratification of density in the lateral direction, and a tendency towards a first-neighbor coordination of 14 (8+6) seems incipient. On each shell we observe a triangular pattern of particle arrangement.

The sawtooth oscillations in tokamak discharges with Ohmic and neutral-beam heating could be suppressed when a large part of the plasma current was driven by lower hybrid waves ($\frac{I_{\mathrm{HF}}}{I_p}\approx 0.5$). The stabilization is due to a flattening of the current profile $j\left(r\right)\mathrm{}$ and an increase of $q\left(0\right)\mathrm{}$ above 1. Higher central electron temperatures are obtained with neutral-beam heating if the sawteeth are stabilized. The increase in total energy content in this case was 30% higher than in the presence of sawteeth.

We report the experimental observation of quantum beats in the coherently scattered radiation from nuclei excited by synchrotron-radiation pulses. The measurements were made with ${}_{}{}^{57}{}_{}{}^{}\mathrm{Fe}$-enriched yttrium-iron-garnet crystals, by reflection from two parallel crystals using the pure nuclear (002) Bragg reflection. The resulting beat pattern gives a sensitive direct measure both of the hyperfine splittings of individual iron nuclei, and of the energy shifts between different nuclear sites.

The energy surfaces of polymeric sulfur have been calculated with use of a parameter-free density-functional method. We study a wide range of geometries with interesting results. The calculated ground-state geometry is in good agreement with experiment, and the flat energy surfaces near this energy minimum are consistent with the extreme flexibility of the molecule. There are other geometries with energies only slightly above the ground-state energy, and stretching of the molecule results in metallic behavior.

A tight-binding theory is derived for Coulomb electron-electron interactions in tetrahedrally coordinated pseudobinary alloys of the form $A_{0.5}{B}_{0.5}C$ Coulomb contributions to the mixing enthalpy are driven by the bond polarity mismatch, as distinct from the bond length mismatch customarily considered. The Coulomb energy shows a strong dependence on the alloy configuration, which arises from the configuration dependence of charge fluctuations. The mixing enthalpy is of order 0.10 eV, commensurate with that of the strain contribution customarily considered.

The reconstruction of the Ir(100) surface as well as the transition of the regular bulklike structure into the reconstructed phase by thermal activation at $T>\mathrm{}800\mathrm{}$ K has been observed in a field-ion microscope. The field-ion microscope image of the thermally stable reconstructed surface supports the rippled quasihexagonal structure model for Ir(100) (1×5) already developed from LEED structure analyses.

Quasiperiodic ordering is studies in a GaAs-AlAs Fibonacci superlattice by high-resolution x-ray scattering. The data are consistent with the predicted dense set of diffraction vectors. Moderately large growth fluctuations in the sequential deposition of GaAs and AlAs layers do not appear to disturb seriously the quasiperiodic order. The effects of randomness are analyzed in a computer simulation.

The magnetization of ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ adsorbed on Grafoil has been measured as a function of coverage (two to three layers) at millikelvin temperatures. The observation of a well-defined peak substantially above the free-spin (Curie) value proves unambiguously the existence of a surface ferromagnetic effect.

The time dependence of the thermoremanent magnetization has been investigated for times ${10}^{-2\mathrm{}}$ $s<~t<~{10}^2$ s in ${\mathrm{Cd}}_{0.6}$${\mathrm{Mn}}_{0.4}$Te. The sudden appearance of a slow relaxation regime at temperature $T_{f0\mathrm{}}=13.10\mathrm{}\pm 0.03$ K, superimposed on a much faster regime varying continuously through $T_{f0\mathrm{}}$, supports the phase-transition hypothesis.

The scaling properties of a random-anisotropy system (amorphous DyNi) are given for the first time. The phase transition is shown to be of the spin-glass type, with a divergence of the nonlinear magnetization. However, critical exponents are different from those of canonical Heisenberg spin-glasses (e.g., CuMn or $a-\mathrm{GdAl}$). This result strongly suggests that random-anisotropy systems belong to a different universality class. This allows us to understand the scatter observed in the exponents of different Heisenberg spin-glasses as caused by a hyperscaling crossover near the freezing temperature.

The character of the magnetic transition is studied for a magnetic glass with strong random-anisotropy and ferromagnetic interactions. Field-dependent ac susceptibility measurements are presented near the ordering temperature of ${\mathrm{Tb}}_{64}$${\mathrm{Fe}}_{20}$${\mathrm{Ga}}_{16}$. The data are analyzed with a scaling theory for the singular susceptibility. An excellent collapse of the $H$, $T$ data is obtained for reduced temperatures in the range 0.002-0.13. These results, which are compared with recent theoretical work, provide strong evidence for a true phase transition in a random-anisotropy system.

Copper phthalocyanine iodide is a molecular metal whose conducting stacks incorporate a one-dimensional array of local moments strongly coupled to conduction electrons. Below 20 K the EPR $g$ value of the coupled system increases anomalously and at 8 K the EPR signal broadens abruptly and becomes unobservable. Anomalies in the proton NMR spin-lattice relaxation are observed at the transition temperature. Magnetic susceptibility measurements and NMR linewidth data both indicate that there is little if any static magnetic order below 8 K.

Results of numerical calculations of the electrical resistivity of the following primary solidsolution alloys are presented: Cu(Zn), Cu(Ga), Cu(Ge), Ag(Pd), and Ni(Mo). Our theoretical model is one reported earlier by Butler and uses a charge-self-consistent Korringa-Kohn-Rostoker coherent-potential approximation. The calculations are valid for strong as well as weak scattering, and for the first time, vertex corrections are included. Excellent agreement is obtained with experiment for the resistivity.

We report on the low-temperature reaction of ammonia with Si(100)-(2×1). The dangling bonds in the clean Si surface promote N${\mathrm{H}}_3$ dissociation even at temperatures as low as 90 K. The N atoms thus produced occupy subsurface sites, while the H atoms bind to surface Si atoms, tie up the dangling bonds, and inactivate the surface. Thermal or electronic-excitation-induced hydrogen desorption restores the dangling bonds and the reactivity of the surface. Silicon nitride film growth is achieved at 90 K by simultaneous exposure of the Si surface to N${\mathrm{H}}_3$ and an electron beam.

We present a study of nonstationary multiple-scattering noise resulting from pulse reflection from a one-dimensional randomly layered half-space. It is shown that the noise power spectrum exhibits model-independent universal behavior when it is analyzed in terms of a single dimension-less variable $x$, defined as the ratio between the mean distance traveled by the pulse and the frequency-dependent localization length. Results of numerical simulation indicate that the spectrum is a peaked function of $x$, where the height of the peak defines the expected value of a noise upper bound.

The divergence of the C VI 182-Å lasing line generated in a rapidly recombining, magnetically confined plasma column was measured with soft—x-ray spectrometers equipped with multichannel detectors. In addition to measurements of the relative divergence, an absolute divergence of ∼9 mrad at a magnetic field of 20 kG and ∼5 mrad at a magnetic field of 35 or 50 kG was obtained by a direct scan of the 182-Å axial radiation. Based on these data a peak 182-Å power of ∼100 kW is obtained. Calculations of the spatial distribution of gain in the plasma were in very good agreement with the experimental data.

Observations of Thomson scattering at uhf from Langmuir waves by a scheme involving the linear frequency modulation (chirping) of a uhf transmitter and the demodulation (dechirping) of the received signals show that a high-power hf wave used for ionospheric modification creates small-scale plasma depletions instantly. For a plasma frequency of 5.1 MHz, plasma frequency gradient of about 50 kHz/km, and power density input of 8×${10}^{-5\mathrm{}}$ W/${\mathrm{m}}^2$, the depletion ranged from 3% to 55%. This provides direct evidence that the hf-induced modifications involve Langmuir waves trapped in density cavities.

A theory is presented for the successive sixfold Fourier components, $C_{6n}$, in the bondorientational order in the neighborhood of a $\mathrm{s}\mathrm{m}\mathrm{e}\mathrm{c}\mathrm{t}\mathrm{i}\mathrm{c}-A-\mathrm{h}\mathrm{e}\mathrm{x}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{c}-B$ phase transition. Near the transition we predict that $C_{6n}\sim {C_6^{\sigma }}^n$ with ${\sigma }_n=n+x_{n}n(n-1\mathrm{})\mathrm{}$ where $x_n$ depends weakly on $n$. General arguments are presented for the topology of the phase diagram in the vicinity of the smectic "liquid-hexatic-crystal" triple point which lead to the existence of a tricritical point on the $\mathrm{s}\mathrm{m}\mathrm{e}\mathrm{c}\mathrm{t}\mathrm{i}\mathrm{c}-A-\mathrm{h}\mathrm{e}\mathrm{x}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{c}-B$ line.