J. R. Gavaler

Superconductivity in Nb–Ge films above 22 K

Niobium‐germanium films which remain superconducting up to 22.3 K have been prepared by a high‐pressure dc sputtering process. The high Tcs of these films are attributed to the formation of a more nearly perfect stoichiometric Nb3Ge compound than has previously been obtainable.

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.

Critical parameters in the single‐target sputtering of YBa2Cu3O7

The critical parameters in the single‐target magnetron sputtering of YBa2Cu3O7 have been identified and sufficiently optimized to allow the reproducible deposition of films with Tc’s of ≳90 K and Jc’s of ≫ 106 A/cm2 at 77 K. It was found that during film growth the bombardment of the YBa2Cu3O7 by energetic particles must be minimized and also a stronger oxidizing agent than molecular oxygen must be present to obtain films with these properties. Otherwise, films are deposited that, by x‐ray diffraction and energy dispersive x‐ray spectroscopy analyses, are indistinguishable from the highest‐Tc 1:2:3 stoichiometric material but which have critical temperatures of ≪90 K. Films need not have 1:2:3 overall stoichiometry to have optimum superconducting properties. In such cases the excess elements are present as second‐phase particles.The critical parameters in the single‐target magnetron sputtering of YBa2Cu3O7 have been identified and sufficiently optimized to allow the reproducible deposition of films with Tc’s of ≳90 K and Jc’s of ≫ 106 A/cm2 at 77 K. It was found that during film growth the bombardment of the YBa2Cu3O7 by energetic particles must be minimized and also a stronger oxidizing agent than molecular oxygen must be present to obtain films with these properties. Otherwise, films are deposited that, by x‐ray diffraction and energy dispersive x‐ray spectroscopy analyses, are indistinguishable from the highest‐Tc 1:2:3 stoichiometric material but which have critical temperatures of ≪90 K. Films need not have 1:2:3 overall stoichiometry to have optimum superconducting properties. In such cases the excess elements are present as second‐phase particles.

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.

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.

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.

Laser patterning of Y‐Ba‐Cu‐O thin‐film devices and circuits

We report our studies on electrical properties of Y‐Ba‐Cu‐O test devices and circuits fabricated using a laser‐writing patterning technique. The patterning procedure is noninvasive, does not require a patterning mask, and does not contaminate nor damage the surface of patterned films. Our laser‐written, oxygen‐rich lines (typically 4–100 μm wide) possess excellent superconducting properties with zero resistivity at 89.5 K and critical current densities of above 2 MA/cm2 at 77 K. On the other hand, oxygen‐poor regions are semiconducting and exhibit thermally activated transport, well described by a three‐dimensional, variable‐length hopping process. Their resistance below 100 K is above 10 MΩ/square. A number of test structures patterned by laser writing, such as a microbridge, coplanar transmission line, open‐ended microwave resonator, photoconductive switch, and Y‐Ba‐Cu‐O field‐effect transistor, have been presented.

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.

The nucleation of high‐Tc Nb3Ge 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.

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.