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Some properites of thin film aluminum superconductors for use as a megahertz second sound detector Lenz, John William

Abstract

Some properties of thin film aluminum superconductors (80-130 Å) were investigated for use as a high frequency second sound detector. The films' transition temperatures, Tc, were raised to near the lambda point of He by a surface enhancement of the BCS coupling constant. The films were useful as sensitive thermometers over the temperature range of their transition, Δ Tc= 0.06 K, with a sensitivity given by the transition slope, 2-3 x 10³ ohms/K. The films mechanically and electrically withstood the thermal shock of five thermal cycles from room temperature to 2 K. The instability of Tc after cycling was at most 0.05 K. By increasing the bias current, Tc (the He bath temperature at the transition) could be decreased by at most 0.025 K. The slope of the transition curve, dR/dT, increased by at most 30% upon thermal cycling and within experimental accuracy the bias current had no effect on dR/dT, for the range 2-60μA. From measurements of one film's time response to thermal excitation, the time constant was found to be less than 45.5 n sec, which means that the film is capable of fully responding to second sound of frequencies of at least 3.5 MHz.

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