Hong Zhang

A TEM-EELS study of hole concentrations near strongly and weakly coupled grain boundaries in electromagnetically characterized YBa2Cu3O7−δ bicrystals

Abstract High-spatial-resolution electron energy loss spectroscopy in a transmission electron microscope (TEM-EELS) was used to obtain hole-concentration profiles near grain boundaries in flux-grown YBa 2 Cu 3 O 7−δ bicrystals with misorientation relationships of 7°, 12°, 28°, and 31° about their common [001] rotation axes. A key aspect of the work is that the electromagnetic properties and coupling character of each of the bicrystals also were determined explicitly. The 28° and 31° bicrystals showed Josephson-junction-like weak coupling, and evidence for severe hole depletion at the boundary was present in the oxygen K near-edge fine structure (ELNES) of the EELS spectra obtained from them. The hole-deficient zone extended 20 to 40 nm into each crystal. In striking contrast, the 7° and 12° bicrystals were strongly coupled and dramatically less hole depleted at the grain boundary. These results show that the boundary region within which the electronic structure differs from that of the bulk is substantially wider than both the YBa 2 Cu 3 O 7−δ coherence length and the grain-boundary structural and compositional width suggested by high-resolution TEM images and cation non-stoichiometry studies. They also suggest a continuous transition from strongly to weakly coupled character.

Two-dimensional growth of continuous Cu2O thin films by magnetron sputtering

We present results on the in situ, two-dimensional growth (as opposed to the more commonly encountered island-coalescence mechanism) of continuous epitaxial Cu2O films on MgO(011) using dc facing-magnetron sputtering from metallic Cu targets in an oxygen/argon atmosphere. Film growth was studied as a function of deposition time and the dc power applied to the guns. Control of the latter leads to either continuous or island-/rodlike film morphologies.

Hole formation and charge transfer in Y1−xCaxSr2Cu2GaO7, a new oxide superconductor

Abstract The technique of transmission electron energy loss spectrometry (EELS) in parallel detection has been utilized to analyze the fine structures associated with the O K and Cu L 3 absorption edges of recently discovered Y 1− x Ca x Sr 2 Cu 2 GaO 7 (0.1 ≤ x ≤ 0.4) series of compounds, which exhibit superconductivity when annealed under high-pressure oxygen atmosphere. We find subtle but significant changes in EELS spectral features of nonsuperconducting and superconducting specimens of these compounds. A broad pre-edge feature is observed below the O K absorption edge, at about 528.2 eV which emerges with Ca-doping alone. Evidence is presented for another O K secondary pre-edge feature, at about 1.1 eV below the first O K pre-edge feature in only those specimens which have undergone high-pressure oxygen treatment (i.e. superconducting specimens). We interpret the O K pre-edge feature as due to formation of holes on oxygen sites and propose that the first broad pre-edge feature (at 528.2 eV) is associated with holes on oxygen sites other than the CuO 2 planes, which are responsible for normal conductivity. The second smaller pre-edge feature (at 527.1 eV) is most probably associated with holes in the CuO 2 planes, which are associated with superconductivity in this system. The presence of a secondary smaller O K pre-edge feature in the superconducting specimens appears to correlate well with a broad shoulder on the high energy side of the Cu L 3 edge. The results are interpreted in terms of oxidation of CuO 2 planes through charge transfer between copper and oxygen in the CuO 2 planes, i.e. covalent mixing of O and Cu orbitals at the Fermi level as a result of high oxygen pressure annealing.

Reduction of magnesium orthovanadate Mg3(VO4)2

Abstract Reduction of polycrystalline and crystalline magnesium orthovanadate Mg3V2O8 by hydrogen was investigated in the temperature range of 560–920°C. A single crystal of orthorhombic Mg3V2O8 with a cation-deficient-spinel structure (25% of the magnesium atoms on octahedral sites are vacant) was transformed into cubic Mg3V2O6 which has a cation-stuffed-spinel structure (8a and 16c sites were partially occupied by magnesium and 16d sites were fully occupied by magnesium and vanadium) at 560°C in 7% H2 in N2.

Transmission electron microscopy of C70 single crystals at room temperature

Transmission electron microscopy (TEM) techniques have been employed to study the room temperature solid state form of chromatographically purified C 70 . Tilting and electron diffraction experiments in three-dimensional reciprocal space, on samples prepared by crystallization from several different solvents, show that C 70 crystallites adopt hexagonal close packed (hcp) structure with a = 1.01 ± 0.05 nm and c = 1.70 ± 0.08 nm. The extinctions and observed reflections conform to the P 6 3 / mmc space group. High resolution TEM images reveal the molecular order and periodicity associated with C 70 crystallites in real space. The experimental results are in agreement with the preliminary computations of crystal structure within acceptable error limits.

Suitability of metalorganic chemical vapor deposition-derived PrGaO3 films as buffer layers for YBa2Cu3O7-x pulsed laser deposition

Phase‐pure thin films of the YBa2Cu3O7−x (YBCO) lattice matched and low loss tangent perovskite insulator PrGaO3 have been grown in situ on single‐crystal (110) LaAlO3 substrates by metalorganic chemical vapor deposition (MOCVD). Films were grown at temperatures of 750–800u2009°C using β‐diketonate precursors M(dpm)3 (M=Pr,Ga; dpm =dipivaloylmethanate). YBCO films were then grown on the MOCVD‐derived PrGaO3 by pulsed laser deposition (PLD). Scanning electron microscopy reveals that the PrGaO3 films have smooth, featureless surfaces. As assessed by x‐ray diffraction, the PrGaO3 films grow epitaxially on LaAlO3 with a high degree of (001) and/or (110) plane orientation parallel to the substrate surface, and the subsequent YBCO films grow with a (00l) orientation. Rocking curve and φ‐scan analyses reveal that the PrGaO3 and YBCO films grow epitaxially. Cross‐sectional high resolution electron microscopy and transmission electron microscopic selected area diffraction confirm that the PrGaO3 and YBCO layers grow e...

Combined HRTEM, X-ray microchemical and EELS fine structure analysis of planar defects in YBa2Cu3O7-δ

Abstract High temperature superconductors in the systems YBa2Cu3O7−δ (123) and decomposed YBa2Cu4O7−δ (124) often contain planar faults with (001) as the nominal fault plane. Such planar defects have been analyzed using a unique combination of high resolution TEM (HRTEM) imaging and high resolution analytical electron microscopy (HRAEM). HRTEM confirmed that these planar faults are stacking faults with double or multiple insertion of crystal planes parallel to the c-plane of the crystal, most probably Cuue5f8O planes. X-ray microanalysis clearly showed enrichment of copper associated with these defects. EELS fine structure of Cuue5f8L23 edge from the fault regions resembled that of pure CuO. The combined results indicate that the planar defects are in fact insertions of extra Cuue5f8O planes in an otherwise regular 123 crystal unit cell.

Transmission high energy electron energy loss spectrometrp (eels) of cuprate superconductors

Abstract The formation of charge carriers (holes) and charge transfer correlated with different tppes of dopants in cuprate superconductors has been studied by means of transmission high energy electron energy loss spectrometry (EELS). The O-K absorption edges of p - type doped systems exhibit a distinct pre-edge feature which is related to the hole formation on oxygen sites. The CuL 3 edge of the doped compounds also exhibit a pronounced shoulder on the higher energy side of the CuL 3 edge which represents the charge transfer between copper and oxwgen. Er123 EELS data are similar to the p123 where the hole density shops a strong dependency on the oxygen doping. Co-substitution in p123 on Cu(I) sites does not seem to effect the hole density, but perhaps their mobility. The newly discovered Ga-based cuprate superconductors exhibit additional significant features. EELS studies on O-K edge appear to reveal two tppes of holes in this system, one induced by hole doping alone (Ca-doping) and the other by high pressure oxygenation treatment. Only the latter tppe of holes appears to be associated with occurrence of superconductivity. All the EELS absorption data are consistent pith the doped charge-transfer insulator description of the electronic states.

A new series of layered pure perovskites (ACuO2.5)2(ATiO3)m

Abstract A new homologous series of layered “pure” perovskites (ACuO 2.5 ) 2 (ARiO 3 ) m ( m = ∼ 4) was found. The distance between CuO 2 planes can be changed by simply changing the number of TiO 6 blocking layers. Among them, LaYCaBa 2 Cu 2 Ti 3 O 14 is a potential new superconductor.