F. Habbal

Superconducting properties of in situ formed Cu‐V3Ga composites

Cu‐V3Ga composites were prepared from in situ formed multifilamentary Cu‐V wires containing 20 vol.% of vanadium filaments. The highest value of the upper critical field at 4.2 K of the reacted composites was found to be 22.4 T, with a corresponding midpoint superconducting transition of 15.5 K. The overall critical‐current density compares favorably with commercial V3Ga composites over the entire field range (2×105 A/cm2 at 4 T, 104 A/cm2 at 18 T).

ac Josephson effect in small‐area superconducting tunnel junctions at 604 GHz

We have measured the ac Josephson effect in small‐area superconducting tunnel junctions at 604 GHz (λ=496 μm) using a resonant planar dipole antenna to couple the radiation into the junction. In a 176‐Ω junction, we have observed up to the 7th Josephson step (at 8.75 mV), a performance comparable to that seen in point contacts. We also compare our data with the results of the RSJ model and of the Werthamer theory.

Stress effects in superconducting Cu‐V3Ga in situ composites

The effect of the uniaxial tensile stress on the critical‐current density Jc, and upper critical field Hc2, of in situ formed Cu‐V3Ga composites has been investigated. Jc was found to gradually decrease at stresses greater than ∼200 MPa. The effect is small, amounting to a decrease of only 20% at 16 T and 1 GPa. However, no permanent degradation was observed even at stress levels in excess of 1 GPa. Similarly, Hc2 was found to be rather insensitive to applied stress. The observed decrease of about 0.5 T at 1 GPa was fully recovered after releasing the stress.

High magnetic field transport properties of liquid quenched Nb3Al and Nb3Al(Si,Ge) superconducting compounds

Critical current behavior and intrinsic superconducting properties of liquid quenched Nb3Al and its pseudobinaries Nb3 (Al,Si) and Nb3(Al,Ge) have been studied in fields up to 29 T. Our results show that flux pinning in these materials is dominated by strong precipitate pinning at low temperatures and by grain boundary pinning at high (>10 K) temperatures. The best results were obtained in Nb75Al19Ge6 samples with Tc of 20.0 K, estimated Hc2(0) of 43 T and Jc (4.2 K) in excess of 104 A/cm2 at 30 T.

Photon-assisted tunneling at 246 and 604 GHz in small-area superconducting tunnel junctions

We have observed photon‐assisted tunneling effects at 246 and 604 GHz in small‐area superconductor‐insulator‐superconductor (SIS) and superconductor‐insulator‐normal metal (SIN) superconducting junctions. We find excellent agreement with the Tien–Gordon theory, and infer that the responsivity of SIN junctions approaches the quantum limit at low temperatures for photon energies less than 2Δ.

Superconducting critical properties and AC losses in a large sample of in situ formed Cu‐Nb3Sn composite

Critical‐current density, upper critical field, and hysteresis ac losses have been evaluated for a large (100‐m long) sample of in situ formed Cu‐Nb3Sn composite with superconducting volume fraction λ≃0.23. The magnetic field dependence of the critical current, measured in wire and tape samples of different reductions, cannot be explained by a model based on a single pinning mechanism. The enhancement of the pinning force in highly reduced tape composites is attributed to surface flux pinning at the filament‐matrix interfaces. Hysteretic ac losses obtained by calorimetric and electronic measuring methods confirmed the results reported previously for small in situ samples.

In situ formed permanent magnet Cu‐Fe multifilamentary composites

Cu‐30 Vol % Fe multifilamentary composites were produced in situ. Magnetic hysteresis loops of wire and tape composites were obtained at room temperature as a function of both cross‐sectional area reduction, up to 99.9996%, and annealing conditions. Hci and (BH)max increase with cross‐sectional area reduction, reaching 250 Oe and 0.46 MG Oe, respectively, in the most heavily deformed unannealed samples. On proper annealing, Hci and (BH)max increase dramatically, up to 590 Oe and 1.35 MG Oe, respectively. Considering the simple preparation technique and inexpensive constituent elements, the in situ formed Cu‐Fe composites appear to have the potential for permanent magnet applications.

Superconducting and Mechanical Properties of in Situ Formed Cu-V3Ga Composites

V3Ga-based superconducting composites exhibit higher critical current densities at high fields (12 to 20 T) than any other practical superconductor, including Nb3Sn. Their low-field performance is limited by the difficulty of producing the material with sufficiently small grain size and not by its intrinsic properties [1]. Two recent reports on model V3Ga-based conductors have demonstrated that’these problems can be substantially alleviated. One successful approach consisted of reacting composites at lower temperatures (≲500°C) using a so-called modified bronze technique [12]; the other relied on manipulating the size and shape of V3Ga grains by the addition of ternary elements [3].

Superconducting properties of V3Ga prepared by rapid liquid quenching and solid‐state precipitation

A modified solid‐state precipitation process for the formation of stoichiometric A‐15 V3Ga is presented which results in high values of Tc=15.0 K and Hc2(4.2)=22 T, excellent phase homogeneity, and a high critical current Jc(4.2 K)=3×105–6×104 A/cm2 over the field range 0–18 T. We find that grain boundary pinning is dominant, producing a very high specific pinning force Qmax=6.4×104 dyn/cm2. The nonparamagnetically limited Hc2 is needed to explain our high flux‐pinning results.