A. Mogro‐Campero

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.

Thin films of Y-Ba-Cu-O on silicon and silicon dioxide

Thin films of the high‐temperature superconductor Y‐Ba‐Cu‐O near the 1:2:3 stoichiometry were deposited on single‐crystal silicon and oxidized silicon with a zirconia buffer layer. Zero‐resistance transition temperatures up to 83 K have been measured on the films formed by a process of sequential evaporation and furnace annealing.

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.

Association of subsurface radon changes in Alaska and the northeastern United States with earthquakes

Abstract Changes of radon concentration in the ground have been reported to correlate with seismic activity in many places over widely varying distances from the earthquake epicenters, sometimes over 100s and occasionally over 1000s of km depending on the magnitude M of the earthquakes. For proper use of such correlations in forecasting the location and size of impending earthquakes, it would be useful to establish a relation between M and the maximum distance x M at which radon can be significantly altered by preseismic effects. We have monitored radon in the ground continuously at Blue Mountain Lake, NY starting Dec. 3. 1975 and at three locations in Alaska—Icy Bay, Yakataga, and Sand Point (starting Nov. 12, 1979; March 5, 1980; and June 29, 1980 respectively). Sample correlations from visual examination of the recent radon record and tentative use of a proposed scaling relation show a number of possible earthquake-related signals at these locations. The data are compatible with scaling relations that were derived from two separate models of premonitory elastic strains. In this work x M = 10 0.48 M , where x M is in units of km and M ≥ 3. Since tilt and strain signals correlate similarly with magnitude and distance, it is likely that most earthquake-related radon signals are mechanically induced.

A review of high-temperature superconducting films on silicon

Work on high-temperature superconducting films on silicon is reviewed. High-temperature superconducting thin films of YBa2Cu3O7-x (YBCO) have been made on single crystal SrTiO3 with zero-resistance transition temperatures (Tc) of 90 K and critical current densities (Jc) up to 5*106 A cm-2 at 77 K. Both thick and thin films of YBCO have been made on silicon or on silicon with a buffer layer with Tc up to 85-90 K. Polycrystalline superconducting films with unoriented grains exhibit weak link behaviour and have low values of Jc (hundreds of A cm-2 at 77 K), whereas the considerably higher value of 6*104 A cm-2 77 K has been achieved for YBCO deposited on a double epitaxial buffer layer on silicon.

Radon emanation over an ore body: Search for long-distance transport of radon

Abstract A major hope for discovering subsurface uranium ore is that measurable concentrations of the radioactive gas 222 Rn can be recognized near the surface of the earth. Integrated measurements, made over several weeks time, show promise of giving greater reproducibility than short-term measurements, which are more subject to meteorological variability. The use of improved methods of integrated radon measurements — free of 220 Rn, of thermal track fading, and of moisture-condensation effects — allow readings to be made that generally are highly stable over time. At a site 16 km north of Thoreau, NM, readings at 60 cm depth taken over a 13-month interval for a set of 55 positions give different, but nearly constant monthly readings at each position, the typical standard deviation being 22%. Superimposed on that stable pattern have been three periods during which spatially grouped radon readings increased by a factor of two or more over their normal values. The simplest tenable description of the source of the increases are sporadic puffs of upflowing gas, originating at as yet unknown depths. The measurements are consistent with an upward velocity of flow of ∼ 10 −3 cm/s. If this velocity is maintained to depth it is still insufficient to transport detectable amounts of radon from the ore body which is at 90 m depth, but it would be sufficient to reveal ore at 50 m or less. Down-hole measurements of permeability yield values that generally are too low to allow signals to be delivered from the ore body by any of the mechanisms that have been modeled. Occasional localized regions of adequately high permeability have been found, but their orientation and extent have not been measured. Although it is improbable that this ore body was discovered by distant transport of radon, the possible existence of occasional gas flow in the earth gives encouragement that at some sites discovery will be aided by such flow.

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.