M.J. Fluss

Atomic-layer engineering of cuprate superconductors

A technique for atomic layer-by-layer synthesis of cuprate superconductors and other complex oxides has been developed. Thin films with excellent transport properties and atomically flat surfaces and interfaces are obtained. The samples are engineered by stacking molecular layers of different compounds to assemble multilayers and superlattices, by adding or omitting atomic monolayers to create novel compounds, and by doping within specified atomic monolayers to fabricate, for the first time, intra-cell barriers. Apart from manufacturing trilayer Josephson junctions withIcRn>5 mV, this technique enables one to customize both the materials and the devices according to the needs of a specific experiment. A number of fundamental issues, such as the dimensionality of the HTSC state, existence of long-range proximity effects, occurrence of resonant tunneling with a specified number of hops, etc., have been addressed in this way. Synthesis of the first “artificial” metastable HTSC compounds is also reported.

Improving 351-nm damage performance of large-aperture fused silica and DKDP optics

A program to identify and eliminate the causes of UV laser- induced damage and growth in fused silica and DKDP has developed methods to extend optics lifetimes for large- aperture, high-peak-power, UV lasers such as the National Ignition Facility (NIF). Issues included polish-related surface damage initiation and growth on fused silica and DKDP, bulk inclusions in fused silica, pinpoint bulk damage in DKDP, and UV-induced surface degradation in fused silica and DKDP in a vacuum. Approaches included an understanding of the mechanism of the damage, incremental improvements to existing fabrication technology, and feasibility studies of non-traditional fabrication technologies. Status and success of these various approaches are reviewed. Improvements were made in reducing surface damage initiation and eliminating growth for fused silica by improved polishing and post- processing steps, and improved analytical techniques are providing insights into mechanisms of DKDP damage. The NIF final optics hardware has been designed to enable easy retrieval, surface-damage mitigation, and recycling of optics.

Engineered defects for investigation of laser-induced damage of fused silica at 355 nm

Embedded gold and mechanical deformation in silica were used to investigate initiation of laser-induced damage at 355 nm (7.6 ns). The nanoparticle-covered surfaces were coated with between 0 and 500 nm of SiO2 by e-beam deposition. The threshold for observable damage and initiation site morphology for these engineered surfaces was determined. The gold nanoparticle coated surfaces with 500 nm SiO2 coating exhibited pinpoint damage threshold of <0.7 J/cm2 determined by light scattering and Nomarski microscopy. The gold nanoparticle coated surfaces with the 100 nm SiO2 coatings exhibited what nominally appeared to be film exfoliation damage threshold of 19 J/cm2 via light scattering and Nomarski microscopy. With atomic force microscopy pinholes could be detected at fluences greater than 7 J/cm2 and blisters at fluences greater than 3 J/cm2 on the 100-nm-coated surfaces. A series of mechanical indents and scratches were made in the fused silica substrates using a non-indentor. Plastic deformation without cracking led to damage thresholds of approximately 25 J/cm2, whereas indents and scratches with cracking led to damage thresholds of only approximately 5 J/cm2. Particularly illuminating was the deterministic damage of scratches at the deepest end of the scratch, as if the scratch acted as a waveguide.

A positron study of the electronic structure of YBa2Cu3O7- delta

The authors have measured the two-dimensional electron-positron momentum density (resolved momentum components along (100) and (010)) for single crystals of YBa2Cu3O7- delta at a significantly higher level of statistical precision for this orientation than that embraced in earlier published works. They have found in their data structures suggesting that the positron annihilates predominantly on oxygen sites in the copper-oxygen chains. These structures are also present in the results of a companion ligand field calculation and arise from spatial variations in the electron and positron wavefunction overlaps. Even at the present level of statistical precision, the authors are unable to argue for or against the existence of a Fermi surface.

Multiple ordered phases in the filled skutterudite compound PrOs4As12

Magnetization, specific heat, and electrical resistivity measurements were made on single crystals of the filled skutterudite compound PrOs{sub 4}As{sub 12}. Specific heat measurements indicate an electronic specific heat coefficient {gamma} {approx} 50-200 mJ/mol K{sup 2} at temperatures 10 K {le} T {le} 18 K, and {approx} 1 J/mol K{sup 2} for t {le} 1.6 K. Magnetization, specific heat, and electrical resistivity measurements reveal the presence of two, or possibly three, ordered phases at temperatures below {approx} 2.3 K and in fields below {approx} 3 T. The low temperature phase displays antiferromagnetic characteristics, while the nature of the ordering in the other phase(s) has yet to be determined.

Electron momentum density studies in high-Tc materials by positron annihilation spectroscopy: theory and experiment

The authors discuss positron behaviour and in particular the ground state positron probability in high-Tc materials. Special attention is paid to La2CuO4, for which they report detailed high-resolution two-dimensional angular correlation of positron annihilation radiation spectroscopy measurements in three sample orientations and as a function of temperature. Besides a large isotropic core-like electron contribution ( approximately=85%), the remaining anisotropic contribution is attributed to valence electrons which are modelled by a linear combination of atomic orbitals-molecular orbital method in conjunction with a localised ion scheme, within the independent particle model approximation. The covalency structure so derived is used to discuss the case of the parent doped material and YBa2Cu3O7. The authors also comment on the application of the Lock-Crisp-West theorem to La2CuO4, before adding some concluding remarks.

Electron-positron P- and K-space densities in YBa2Cu3O7-δ

Abstract We have performed an ultra-high precision measurement (5 × 10 8 coincidence counts) of the basal-plane electron-positron momentum density (MD) in well oxygenated, twin-free, single crystals of YBa 2 Cu 3 O 7δ . The anisotropies of the raw and processed MD spectra and LCW-transformed spectra (k-space densities) not only show the D 2 symmetry appropriate to the untwinned crystals but, more importantly, unambiguously show a dear image of a major Fermi surface sheet. The form of the FS image is in substantial agreement with theoretical predictions of a Γ-X electron ridge section associated with states in the Cu-O chains.

Self-decay-induced damage production and micro-structure evolution in fcc metals: An atomic-scale computer simulation approach

In this paper, we discuss the development and application of a hybrid molecular dynamics/kinetic Monte Carlo simulation to model defect production and microstructure evolution in irradiated fcc metals. The molecular dynamics results show that the primary damage state produced by high-energy 30xa0keV recoils in low melting point, high-Z fcc metals such as Pb and Au is dominated by populations of large vacancy and interstitial clusters, and only a small percentage of the damage is produced as isolated point defects. Kinetic Monte Carlo simulations were used to calculate the fraction of the defects produced by the 30 keV recoils that are able to avoid recombination within their nascent cascade and freely migrate through the lattice. We show that because of in-cascade clustering this fraction is different for vacancies and interstitials and depends strongly on temperature. The results are used to provide a qualitative explanation for the experimentally observed differences in void swelling between fcc and bcc metals. The kinetic Monte Carlo simulations were also used to model damage accumulation in Pb under 30xa0keV self-recoil irradiation as a function of dose rate and temperature.

Non-uniform solute segregation at semi-coherent metal/oxide interfaces

The properties and performance of metal/oxide nanocomposites are governed by the structure and chemistry of the metal/oxide interfaces. Here we report an integrated theoretical and experimental study examining the role of interfacial structure, particularly misfit dislocations, on solute segregation at a metal/oxide interface. We find that the local oxygen environment, which varies significantly between the misfit dislocations and the coherent terraces, dictates the segregation tendency of solutes to the interface. Depending on the nature of the solute and local oxygen content, segregation to misfit dislocations can change from attraction to repulsion, revealing the complex interplay between chemistry and structure at metal/oxide interfaces. These findings indicate that the solute chemistry at misfit dislocations is controlled by the dislocation density and oxygen content. Fundamental thermodynamic concepts – the Hume-Rothery rules and the Ellingham diagram – qualitatively predict the segregation behavior of solutes to such interfaces, providing design rules for novel interfacial chemistries.

Positron annihilation in perovskite superconductors; Theory and experiment

Abstract Positron Annihilation Spectroscopy is shown to be of potential value for probing the electronic structure and the changes accompanying the superconducting transition of the new high Tc materials. The experimental results of electron-positron momentum distribution for La2CuO4 agree with a ligand field approach, suggesting a strong electron localization and the importance of the covalency.