A. I. Braginski

Evidence for two superconducting components in oxygen-annealed single-phase YBaCuO☆

Abstract The complex susceptibility of a sintered YBaCuO superconductor is strongly dependent on a.c. field amplitude, h . Very small values of h must be used for the real part of susceptibility, χ′, to reach a value corresponding to bulk diamagnetism just below the critical temperature, T c . The imaginary part, χ″, represents hysteresis loss in the sample. Thus, χ″ versus temperature becomes positive when h exceeds the lower critical field, H cl of the superconductor. Annealing the material in oxygen gives rise to two distinct components, a relatively high- T c , high- H cl superconductor (denoted as ‘G’ or ‘good’) and a relatively low-T c , low- H cl superconductor (denoted as ‘B’ or ‘bad’). Curves of susceptibility versus increasing temperature reflect the dual nature of the annealed sample: χ′ has an inflection point at T c of the B component and approaches zero at T c of the G component, while χ″ has a peak at each T c . Both critical temperatures decrease linearly with increasing h , though at very different rates. H cl of the G component is considerably greater than H cl of the B component. The lower critical fields are linearly decreasing functions of temperature. Two models might explain the susceptibility data. In the grain model, the G component consists of superconducting grains and the B component is either intergranular material, unfavourably orientated anisotropic grains, or oxygen-depleted grain boundaries. In the surface model, the G component is in the interior of the sample and the B component is at the samples surface. This condition could arise if there was oxygen depletion at the surface subsequent to total enrichment during annealing.

Surface stability of NbN single-crystal films

The surfaces of NbN(100) and NbN(111) single‐crystal films were analyzed as a function of annealing temperature in ultrahigh vacuum by low‐energy electron diffraction and x‐ray photoelectron spectroscopy. The NbN(100) surface changed from a (1×1) to a ((2)1/2×(2)1/2)45° structure at 900 °C accompanied by a loss of nitrogen. The NbN(111) surface, initially richer in nitrogen, transformed from a (1×1) to a (2×2) structure at 800 °C with an even greater loss of nitrogen from the surface. Increasing the carbon content of the films increased the temperatures at which reconstructions were observed. Structural models of the surfaces are presented that are consistent with these observations. At higher temperatures (up to 1100 °C), additional nitrogen was lost from films of both orientations. Measurements of superconducting transition temperatures and energy gaps indicated that the entire film was nitrogen deficient after annealing, with the greatest nitrogen loss at the surface.

Chemical vapor deposition of YBa2Cu3O7 using metalorganic chelate precursors

Superconducting films of YBa2 Cu3 O7 were prepared using metalorganic chelate precursors at ambient pressure. Ba and Y were transported using fluoride‐substituted β‐diketonates, while a β‐keto‐imide chelate was used for Cu. Deposition at 500 °C on Al2 O3 and SrTiO3 substrates by reaction with O2 saturated with room temperature water vapor formed crystalline mixtures of BaF2 , YF3 , and CuO. Superconducting YBa2 Cu3 O7 formed after annealing in Ar/H2O at 835 °C followed by O2 at 900 and 400 °C with a slow cooling in O2 to below 200 °C. A 2.3 μm film on a (100) SrTiO3 substrate had an onset Tc of 90 K with zero resistance at 70 K. The film showed a mixture of c‐ and a‐axis orientation. A similar film on (1102) Al2O3 showed zero resistance at 65 K.

Optical response of epitaxial films of YBa2Cu3O7−δ

We present the results of measurements of optical detection in epitaxial films of YBa2Cu3O7−δ, at wavelengths of 0.63 and 10.6 μm. In contrast to the behavior observed in granular materials, these films appear to show no evidence of nonequilibrium response (breaking of Cooper pairs by photons), but only a bolometric effect (heating of the sample by radiation) in the resistive transition regime. This suggests that epitaxial films do not contain intrinsic links weak enough to be modulated by the incident radiation. For 0.63 μm radiation, mechanically chopped at 725 Hz, measurements of a 10 μm ×90 μm bridge yield a bolometric responsivity of approximately 4×103 V/W, and a detectivity D* of more than 108 cm Hz1/2/W.

Chemical vapor deposition of YBa/sub 2/Cu/sub 3/O/sub 7/ using metalorganic chelate precursors

Superconducting films of YBa2 Cu3 O7 were prepared using metalorganic chelate precursors at ambient pressure. Ba and Y were transported using fluoride‐substituted β‐diketonates, while a β‐keto‐imide chelate was used for Cu. Deposition at 500 °C on Al2 O3 and SrTiO3 substrates by reaction with O2 saturated with room temperature water vapor formed crystalline mixtures of BaF2 , YF3 , and CuO. Superconducting YBa2 Cu3 O7 formed after annealing in Ar/H2O at 835 °C followed by O2 at 900 and 400 °C with a slow cooling in O2 to below 200 °C. A 2.3 μm film on a (100) SrTiO3 substrate had an onset Tc of 90 K with zero resistance at 70 K. The film showed a mixture of c‐ and a‐axis orientation. A similar film on (1102) Al2O3 showed zero resistance at 65 K.

Dimensional effects on current and field properties in NbN films

We have investigated dimensional effects, i.e., variations in thickness, width, grain size, and separation between grains, on the current and field properties of NbN films. The films, all of which had T c s of ∼ 16K were prepared by reactive sputtering, Self-field current densities measured at 4.2K ranged from \sim 5 \times 10^{5} to > 107Amps/cm2. Measured upper critical fields at 4.2K varied from 220 kG. Extrapolated H c2 s of over 500 kG were calculated from data taken near T c . All of these results are correlated with transmission electron microscopy studies. The very highest upper critical fields are attributed to an H c3 arising from a column-void microstructure. In general, we conclude that dimensional effects are a dominant factor in achieving the very high current and field values observed in these films.

Properties of La1.8Sr0.2CuO4 superconductors

Critical current, critical field, and carrier density measurements have been made on bulk samples of La1.8Sr0.2CuO4 to assess the potential of such oxide superconductors for practical applications. The importance of preparing samples in a high oxygen pressure was documented. The upper critical field at T=0 was estimated to be 530 kOe. From magnetization hysteresis loops, critical current densities were determined between 0 and 60 kOe. At 60 kOe, the values were 2×103 A/cm2 at 4.2 K and 4×102 A/cm2 at 18 K in samples that exhibited characteristics of weak flux pinning. The effective carrier density at 48 K was 1×1021 cm−3, approximately half of the expected upper limit. A set of microscopic superconducting parameters has been derived from transition temperature, resistivity, and upper and lower critical field measurements made on a single specimen.

Nucleation of high-T/sub c/ Nb/sub 3/Ge in the presence of impurities

Analyses of high‐Tc Nb3Ge films show that they all have a peak in oxygen concentration near the substrate‐film interface and that their lattice parameters in that region are abnormally large. It is proposed that high‐Tc Nb3Ge is a metastable phase which is formed via a homoepitaxial process from a large lattice parameter A15 Nb‐Ge phase. This phase near the interface is believed stable due to an expanded lattice resulting from the presence of impurities.

Progress toward a practical Nb-Ge conductor

Properties of high-T c Nb-Ge films deposited by sputtering and by chemical vapor deposition (CVD) have been investigated. Results of sputtering in the presence of controlled levels of O 2 , N 2 , Si, and of reactive sputtering in Ar-GeH 4 , suggest that the high-T c A15 phase is impurity- or defect-stabilized. In CVD deposits two tetragonal modifications were found: σ and T2, the latter probably stabilized by C1 2 . High critical current densities, J c (H, T) of fine-grained sputtered films are attributed to flux pinning on A15 grain boundaries. In coarse-grained CVD films high self-field J c s, 106to 107A cm-2at T = 4.2 K, are attributed to pinning on dispersed σ-phase. Comparably high J c s were also obtained in CVD A15 films doped with impurities. Low field ac losses p (H,T) were correlated with J c and coating geometries. The feasibility of fabricating multifilamentary composite conductors by CVD was demonstrated experimentally and a fabrication process for long Nb 3 Ge CVD tapes is being developed.

Optical response of epitaxial and granular films of YBa/sub 2/Cu/sub 3/O/sub 7- delta / at temperatures from 25 K to 100 K

The authors present the results of optical detection in epitaxial and granular films of YBa/sub 2/Cu/sub 3/O/sub 7- delta / at wavelengths of 0.63, 3.39, and 10.6 mu m and at temperatures from 25 K to 100 K. Both types of film exhibit only bolometric detection, with no evidence for nonequilibrium effects in this temperature range. For 0.63- mu m radiation, mechanically chopped at 725 Hz, measurements of a 10*90 mu m/sup 2/ epitaxial bridge yield a bolometric responsivity of approximately 4*10/sup 3/ V/W, and detectivity D* of more than 10/sup 8/ cm square root Hz/W. Granular films biased above their critical current are found to exhibit two-level switching noise, resulting in Lorentzian deviations from an otherwise 1/f noise spectrum. >