Demodulation by Superconductivity

Abstract

WE have examined further the anomalous fluctuations in the superconducting bolometers previously reported by Andrews, Milton and Desorbo¹, and find that, in part, such fluctuations are due to the absorption by the superconductor of modulated broadcast radio waves and the conversion of the modulation wave, by the superconductor, to simple audio-frequency. Our experiments were carried out with superconducting bolometers using small ribbons of columbium nitride mounted in cryostats, and employing circuits as described in the preceding reference. Following the discovery on December 2, 1946, of demodulation of broadcast waves at 1,090 kilocycles, we generated waves in the laboratory at frequencies ranging from 200 to 30,000 kilocycles, and found that demodulation occurred only in four bands centred approximately at 1, 3, 5 and 16 megacycles. We have found this demodulation to occur only within a narrow temperature zone corresponding to a part of the transition interval between the normal and the superconducting state. The temperature for maximum den odulation was not affected by changing the radio-frequency. Quality of reception was comparable with good standard radio reception. Although no tuned radio-frequency circuit or antenna was used, the signal generated was estimated to be of the order of ten to one hundred microvolts at the terminals of the ribbon. Demodulation could be reduced or eliminated by passing small direct currents through the superconductor.