Timothy R. Dinger

Crystallography and microstructure of Y1Ba2Cu3O9−x, a perovskite‐based superconducting oxide

We have investigated the crystallography and microstructure of Y1Ba2Cu3O9−x with transmission electron microscopy and x‐ray diffraction. Y1Ba2Cu3O9−x is a distorted, oxygen‐defect perovskite with ordering of the yttrium and barium ions. Its unit cell is orthorhombic with space group Pmm2 and lattice parameters a=3.893 A, b=11.688 A, and c=3.820 A. The structure is heavily twinned on {101} type planes, possibly due to a tetragonal‐to‐orthorhombic transition above room temperature.

Reliable single‐target sputtering process for high‐temperature superconducting films and devices

We report a simple, single‐target magnetron sputtering process for films of high‐temperature superconductors involving an off‐axis sputtering geometry. The process lends itself both to film growth with high‐temperature post‐anneals and to low‐temperature in situ film growth. The post‐anneal process routinely yields YBa2Cu3O7−x films on SrTiO3 substrates that are fully superconducting at 86–89 K. Current densities at 77 K range from 104 to 8×105 A/cm2. A single‐level superconducting quantum interference device (dc SQUID), made by photolithographically patterning a low current density film, has a flux noise level at 77 K of 3×10−4 Φ0/(Hz)1/2 at 20 Hz, dominated by low‐frequency noise associated with flux motion in the film.

Fracture toughness measurements of YBa2Cu3Ox single crystals

We report fracture toughness measurements on single crystals of YBa2Cu3Ox, the phase responsible for superconductivity above liquid‐nitrogen temperatures. Indentation crack length measurements on the (010) orthorhombic crystal growth faces revealed the (100) and (001) planes as preferred fracture planes. The toughness of these planes is Kc=1.1±0.3 MPa m1/2, and the hardness H=8.7±2.4 GPa. The observed growth of both radial and lateral cracks in ambient air suggests that these crystals are susceptible to moisture‐enhanced nonequilibrium crack propagation.

The anisotropic nature of the superconducting properties of single crystal Y1Ba2Cu3O7 − x

Abstract A series of noncontact magnetic measurements on high-quality single crystals of Y1Ba2Cu3O7 − x have enabled us to demonstrate that the superconducting-state properties of Y1Ba2Cu3O7 − x are those of a conventional, anisotropic superconductor with the anisotropy being associated with the highly conducting Cu O sheets in the Y1Ba2Cu3O7 − x crystal structure. The anisotropy in the superconducting state is reflected most strongly in the critical current anisotropy, which is as large as 20 to 1 at low temperatures and low fields and gets arbitrarily large at higher temperatures and higher fields. The upper critical field HC2 shows an anisotropy that varies from 6:1 to 10:1 in different crystals of high quality. Along the copper-oxygen sheets the upper critical field is enormous, extrapolating to ∼60 Tesla by 77 K and implying a low-temperature Ginzburg-Landau coherence length normal to the layers of 2A− 4A. Strong upward curvature is evident in the HC2 data in both orientations of applied field. This curvature can be interpreted in terms of thermodynamic fluctuations, and, if this explanation is correct, the critical region is enormous, extending more than 25 K below TC.

Observation by electron spin resonance of a pseudo-cubic site in YBa2Cu3O7−x.

Abstract We report electron paramagnetic resonance experiments in single crystals of the high Tc superconductor YBa2Cu3O7−x. No EPR signal is observed in the spectra of the properly annealed crystals with Tc≅90K. However, we observe a strong, almost isotropic Cu2+ signal in the spectra of the as-grown (unannealed) single crystals with Tc≅40K. The observed signal has a slightly anisotropic g-factor which becomes totally isotropic at T≅90K. The linewidth, dominated by the hyperfine interaction, remains anisotropic at all temperatures, showing that the signal originates from a site which does not have real cubic symmetry.

Identification and preparation of single phase 90 K oxide superconductor and structural determination by lattice imaging

Abstract We have investigated various compositions of Ba-Y-Cu-O, resulting in the preparation of single phase 90 K superconducting material of composition (Ba 0.67 Y 0.33 )CuO 3− δ and the identification of the structure of this superconducting phase by lattice imaging as a layered perovskite. The single phase material had a resistive transition with a midpoint at 92 K and a 1.3 K width (10%–90%) as well as a sharp low-field (H = 0.01 Oe) susceptibility transition.