L. G. Turner

Y-Ba-Cu-O superconducting thin films by simultaneous or sequential evaporation

Superconducting thin films of Y‐Ba‐Cu‐O near the 1:2:3 stoichiometry were produced by simultaneous (coevaporation) and sequential (multilayer) evaporation in the same evaporator. The best film obtained on yttria‐stabilized zirconia (YSZ) had a superconducting onset temperature of 104 K, a midpoint Tc of 92 K, and zero resistance at T≤74 K. Stoichiometry was deduced by inductively coupled plasma emission spectroscopy, and elemental depth profiles were obtained by x‐ray photoelectron spectroscopy. Film stoichiometry changes only near the film/substrate boundary for films on YSZ. Films on Si/SiO2 were not superconducting; depth profiling shows severe changes of film composition with depth. A major theme of this work is process reproducibility, which was found to be poor for coevaporation but improved considerably for sequential evaporation.

Characterization of thin films of Y‐Ba‐Cu‐O on oxidized silicon with a zirconia buffer layer

Thin films of the high‐temperature superconductor Y‐Ba‐Cu‐O with zero‐resistance transition temperatures up to 83 K have been recently reported by using a zirconia buffer layer on the primary materials of interest for electronics, Si and SiO2. In this letter, various characteristics of these films are discussed. Microstructural analysis using transmission electron microscopy shows the complex morphology of the unoriented polycrystalline films. Elemental depth profiling by x‐ray photoelectron spectroscopy shows the effectiveness of the zirconia buffer layer in preventing interdiffusion; fluorine is found throughout the film at an abundance of 4 at. % The critical current density was measured as a function of temperature; its value is 5 kA cm−2 at 4.2 K.

Epitaxial growth and critical current density of thin films of YBa2Cu3O7−x on LaAlO3 substrates

Thin films of the high‐temperature superconductor YBa2 Cu3 O7−x have been produced on (100) LaAlO3 substrates by coevaporation and furnace annealing. A 14‐μm‐wide and 400‐μm‐long constriction patterned on a 0.8‐μm‐thick film had a zero resistance transition temperature of 90 K, a transition width of 1.5 K, and a critical current density of 8×104 A cm−2 at 77 K. Although x‐ray diffraction shows a definite c‐axis alignment normal to the substrate plane, further analysis reveals that c‐axis alignment in the substrate plane is also present. The detailed microstructural picture is revealed by transmission electron microscopy: a continuous layer, about 0.2 μm thick adjacent to the substrate, with c axis normal to the substrate plane, and the remaining top portion of the film, with the c axis in the film plane. In spite of the bilayer structure, the film remains epitaxial (the axes of the superconductor are parallel to the 〈100〉 directions of the substrate).

Thickness and annealing dependence of the superconducting transition temperature of YBa2Cu3O7−x thin films on oxidized silicon and polycrystalline alumina substrates

Superconducting thin films of YBa2Cu3O7−x in the thickness range of 0.2–0.9 μm were tested in this study. A zirconia buffer layer was used to minimize interdiffusion on oxidized silicon and polycrystalline alumina substrates. The highest zero resistance transition temperatures (85 K for oxidized silicon and 86 K for polycrystalline alumina) were obtained for the thicker films; these are the highest values reported for thin films of this superconductor on these substrates. The thickness and annealing dependence of the transition temperature suggests that interdiffusion limits the performance of the thinner samples.

LOWER TEMPERATURE POST-ANNEALING OF THIN FILMS OF YBA2CU3O7 AT LOWER OXYGEN PARTIAL PRESSURE

Thin films of YBa2Cu3O7 formed by ambient temperature deposition and furnace post‐annealing have been obtained at annealing temperatures around 750 °C and an oxygen partial pressure of 29 Pa. The zero resistance transition temperature of these smooth films on LaAlO3 was 89 K, and a critical current density in excess of 1 MA cm−2 at 77 K was found by transport measurements.

High temperature superconducting resonator for use in nuclear magnetic resonance microscopy

We describe surface coil resonators made with thin‐film Y1Ba2Cu3O7 (YBCO) for use in high‐resolution nuclear magnetic resonance imaging. At the operating temperature of the coil, 10 K, we have measured Q values (quality factor) in excess of 50 000 at 300 MHz in magnetic fields as high as 9 T. Noise temperature measurements of this superconducting probe indicate that the dominant noise source is Johnson noise. We describe a coupling circuit comprised of the YBCO coil that is inductively coupled to a room‐temperature copper resonator. This circuit preserves the high signal‐to‐noise ratio that accrues to the YBCO resonator.

Large differences of critical current density in thin films of superconducting YBa2Cu3O7−x

Thin films of YBa2 Cu3 O7−x with three types of c‐axis alignment have been prepared by evaporation: unaligned films on oxidized silicon with zero‐resistance transition temperatures as high as 88 K (the highest value reported for thin films of this superconductor on this substrate), films with regions aligned along each of the three 〈100〉 directions of the (100) SrTiO3 substrate, and films with the c‐axis perpendicular to the (100) SrTiO3 plane. Typical values of critical current density (A cm −2 ) at 77 K are 102 , 3×104 , and 106, respectively. The temperature dependence of the critical current density is similar for the three types of films; it increases linearly with decreasing temperature, which is suggestive of a flux creep‐limited model.

Thermal fluctuation and 1/f noise in oriented and unoriented Y1Ba2Cu3O7−x films

We report on electrical noise measurements made on YBCO (Y{sub 1}Ba{sub 2}Cu{sub 3}O{sub 7{minus}{ital x}}) films on SrTiO{sub 3}, on bulk silicon with a ZrO{sub 2} buffer layer, and on thin dielectric membranes. We have found that 1/{ital f} noise predominates in the unoriented films and that thermal fluctuation noise is the chief source of noise in good films on SrTiO{sub 3}.

Correlations of radon and carbon isotopic measurements with petroleum and natural gas at Cement, Oklahoma

Radon in soil gas and the carbon isotopic composition of carbon compounds in soil have been measured at 98 sites over the Cement, Oklahoma oil and gas field. Strong correlations were observed between the density of oil and gas wells and storage tanks and soil 222Rn, field 222Rn, vertical gas flow, and δ13C gradients. No meaningful correlation is seen with δ13C. If further study shows that these results are general, the types of measurement done here could facilitate exploratory drilling.

Film thickness dependence of critical current density and microstructure for epitaxial YBa2Cu3O7-x films

Films of nominal composition YBa2Cu3O7-x(YBCO) were made on (100) SrTiO3 substrates by coevaporation and furnace annealing. Film thickness was in the range 0.2-2.4 mu m. Chemical analysis by inductively coupled plasma emission spectroscopy reveals that the thinner films had an excess of Cu and Ba relative to Y, which led to Cu- and Ba-rich particles on the film surface. Transmission electron microscopy was used to define the epitaxial nature of the films: a continuous layer of about 0.4 mu m thickness adjacent to the substrate and with the c axis normal to the substrate plane, followed by material with the c-axis in the film plane. A simple model based on microstructural observations and the strongly anisotropic transport properties of YBCO is compared with the measurements of room-temperature resistivity and critical current density at 77 K as a function of film thickness. The critical current density is found to decrease as a function of increasing film thickness by a much larger factory than the simple model predicts.