No abstract

No abstract

No abstract

No abstract

Measurements have been made on the superconducting properties of thin films of lead-bismuth alloys ranging in composition from pure lead to 70% bismuth. These were deposited in 300-Å-thick layers on glass and polypropylene substrates. At T=4.2°K, Hc rises with increasing bismuth concentration to a maximum of 23 kG at 50% bismuth and then falls abruptly to zero for greater bismuth concentrations. At 1.9°K this maximum increases to 33 kG at 50% bismuth. Current densities of 107 A/cm2 were reached, corresponding to 7 A in a 2-mm-wide sample. Measurements were made of the low-frequency ac superconducting losses by the L-C circuit Q method. Frequencies in the order of a few hundred cps were achieved by vacuum deposition of circuit elements several hundred feet long. The only dielectric material present was polypropylene whose dissipation factor was found to be 5.5×10−6 (Q=1.8×105). With this background loss the superconducting losses were barely detectable in lead-bismuth, with a lower limit on Q of about 106. Losses became large when currents were sufficiently high for the formation of normal regions. The onset of these losses correlated very well with dc resistance measurements. For the long circuit elements this resistive onset occurs at low current densities, about 104 A/cm2. Present work is aimed at raising the current-carrying capacity of the long circuits to that of the short samples, and measuring ac superconducting losses at high current densities.