M. A. Janocko

Preparation and properties of high‐Tc Nb–Ge films

Using a previously reported low‐energy sputtering technique, Nb‐Ge films have been prepared with superconducting Tcs up to ∼23 K. High‐field, x‐ray, and annealing results are reported for these highest‐Tc films. A discussion of the critical parameters required to achieve the highest transition temperatures in the Nb–Ge system is given. Also further arguments in support of the theories that these highest Tcs are due to the formation of the stoichiometric Nb3Ge phase in the sputtered films are presented.

Low- and high-temperature superconducting microwave filters

Stripline and microstrip filters at X-band were designed and fabricated using low- and high-temperature superconductors in quarter-wave, parallel-coupled section configurations. Low-temperature superconducting niobium thin films, deposited on single-crystal sapphire, were used to build to six-pole stripline filters with adjacent passbands and approximately 3 dB crossovers and 1.2% bandwidth. Four- and six-pole microstrip filters were made with in situ epitaxial YBa/sub 2/Cu/sub 3/O/sub 7/ (YBCO) films on LaAlO/sub 3/ substrates. All the YBCO filters showed 77 K passbands with clean skirts and high out-of-band rejection. The six-pole filters had adjacent passbands with -28 dB crossovers and 1.5% bandwidth. >

Very High Critical Current and Field Characteristics of Niobium Nitride Thin Films

Previous results indicated that NbN thin films possess critical current and field characteristics significantly superior to that of bulk NbN having a similar Tc. Further measurements made on NbN films, with thicknesses between 50 A and 8 μ, now show that, at 4.2°K, certain of these films exhibit the highest current densities of any presently known superconductor in all fields from zero up to the limit of our measurement capability (210 kOe). In addition, anomalously high current and field values have been measured in very thin (<300 A) films. These thinner films show no depressions in Jc (measured at 4.2°K) or in Hc2 (O) values despite decreases in Tc from almost 16°K (in the thicker films) down to 11°K.

Upper critical fields of Nb3Ge thin film superconductors

The upper critical field Hc2(T) of the highest Tc(∼23K) Nb3Ge superconducting films has been found to be ≈370kG at 4.2K. Measurements on lower Tc films show very broad transitions reflecting nonuniformity. The Hc2(T) characteristics are consistent with other Nb3X type II superconductors.

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.

High temperature superconducting space-qualified multiplexers and delay lines

A high temperature superconducting (HTS) four-channel multiplexer and a delay line were fabricated, space qualified and tested as part of the U.S. Navys High Temperature Superconductivity Space Experiment II (HTSSE-II). The multiplexer had an architecture that included two branch-line hybrids and two identical parallel-coupled line filters per channel. Its operation was centered at 4 GHz, with 50-MHz-wide channels. It was fully integrated, with microstrip interconnections between channels and thin-film load terminations in the out-of-phase port of the output hybrid. The delay line was made up of two cascaded modules for a total delay of 45 ns between 2 and 6 GHz. Both devices were made using 5-cm-diameter LaAlO/sub 3/ wafers coated with epitaxial thin film YBa/sub 2/ Cu/sub 3/O/sub 7/, on both sides in the case of the delay line. Both devices operated at 77 K.

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.

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.

A‐15 structure Mo–Re superconductor

An investigation of the properties of Mo–Re thin films has been undertaken. Superconducting transition temperatures of up to ∼15°K have been observed in films which have an approximate composition of Mo0.7Re0.3. This increase in Tc of 2–3°K over the corresponding bulk values is attributed to the formation of a metastable A‐15 phase in the sputtered films of this composition.

High-temperature superconducting wide band delay lines

Stripline non-dispersive delay lines with 20 GHz of bandwidth were designed and fabricated using YBCO thin films deposited on both sides of 2-inch-diameter, 10-mil thick LaAlO/sub 3/ wafers. A double-wound spiral configuration was used yielding a line length of approximately 150 cm. Novel approaches were used to minimize the effect of air gaps between the two substrates needed to form the stripline structure, and to realize a wide band transition from the coaxial external connections to stripline. Experimental results showed 5 dB of insertion loss at 20 GHz, with 1 dB amplitude ripple over most of the 0-20 GHz, range for a 22.5 ns delay.<<ETX>>