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IEEE Transactions on Magnetics

Gap suppression devices

B. Hunt, R. P. Robertazzi, Robert A. Buhrman

Published Mar 1, 1985• 12 citations
Publisher

Abstract

In this paper we discuss some general features of three terminal superconducting devices which operate on the basis of superconducting energy gap suppression by quasiparticle injection. These features include latching behavior at greater than unity power gain and, in the conventional three layer sandwich geometry, a rather long device switching time. We then describe three terminal gap suppression experiments carried out with the use of a double edge junction geometry. The problems of obtaining large gain in this higher speed geometry are discussed. Finally, we point out that these problems are alleviated by using the gap suppression effect to switch a supercurrent biased junction rather than to modify the quasiparticle I-V curve.

DOI
10.1109/tmag.1985.1063735

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In this paper we discuss some general features of three terminal superconducting devices which operate on the basis of superconducting energy gap suppression by quasiparticle injection. These features include latching behavior at greater than unity power gain and, in the conventional three layer sandwich geometry, a rather long device switching time. We then describe three terminal gap suppression experiments carried out with the use of a double edge junction geometry. The problems of obtaining large gain in this higher speed geometry are discussed. Finally, we point out that these problems are alleviated by using the gap suppression effect to switch a supercurrent biased junction rather than to modify the quasiparticle I-V curve.
This paper concerns a new three terminal superconducting weak link device, having a third electrode attached to the weak link part for injecting quasiparticles. The structure, the fabrication procedure and the result of the measurement of its static I-V characteristics are presented. As a result, a current gain larger than that of any other Josephson switching devices is obtained. It is found that the larger current gain depends strongly upon the input resistance between the third electrode and the weak link part. The switching speed of the device is also theoretically discussed.
The development of three-terminal superconducting devices with transistor-like characteristics may be crucial for a number of applications of superconducting electronics. A three-terminal Josephson device which in principle could have gain, and which depends on the magnetoelectric effect is described. The magnetoelectric effect is a property of the antiferromagnetic phase of certain compounds. When an electric field is applied across a magnetoelectric material, a magnetization develops. If one Josephson junction of a two-junction, three-electrode sandwich contained a magnetoelectric barrier, a magnetic field would result if that junction were switched to the finite voltage state. This field, when coupled into the second junction, would cause it to switch from the zero-voltage to the finite voltage state. If the second junction had a larger critical current than the drive junction, then the three-terminal device would have gain.
The QUITERON is the first three terminal superconducting switch possessing true transistor-like characteristics as well as ultra-low switching energy. Its action is based on non-equilibrium superconductivity in the regime of heavy external and self injection of quasiparticles leading to gap suppression. Experimental confirmation of the operating principle is presented, including the demonstration of large signal power gain, high switching speed, as well as other digital and analog transistor-like features.
New effects of quasiparticle injection on microwave irradiated Josephson triodes are demonstrated. The device comprises Pb-insulator (Nb2O5)-Nb SIS junction with direct contact of the Nb film to n+-GaAs gate substrate. In this device, the very thin Nb film (300 - 600 Å) as a base electrode of the SIS detector also serves as an acceptor of the quasiparticles injected from GaAs - Nb Sehottky barrier. The emphasis was placed on injections of hot quasiparticles with the energy in excess of 0.5 eV. The zeroth step height of microwave (10 GHz) irradiated Josephson current Iowas efficiently modulated by a sufficiently low density quasiparticle injection current Iinj(1 - 2 A/cm2), though no sign of change was found in the superconductive gap parameter and the Josephson critical current itself. These characteristic features became less pronounced as the energy of injected quasiparticles was lowered below 50 meV. Though the present experiment is in a preliminary stage, a current amplification (dIo/dIinj) of higher than two has been attained on a test specimen.
A superconducting transistor with large current gain and bandwidth at low temperatures would have many applications. We have fabricated and tested a new three-terminal superconducting device with transistor-like properties at an operating temperature of 4.2 K. It is based on a stacked double tunnel junction structure where the intermediate film is a bilayer of superconducting Nb and an Al quasiparticle trap which can work either in the superconducting or in the normal metal state. Current amplification factors of up to 2.0 are observed at a temperature of 4.2 K when the Al is superconducting, while large current gains of more than 50 are observed when the Al is in the normal state. The device shows a high degree of unidirectionality. The results can be explained on the basis of the recently proposed QUAsiparticle TRApping TRANsistor, which should have wide applications in detection systems operating at low temperatures.