EMISSION OF HIGH FREQUENCY RELAXATION PHONONS BY SUPERCONDUCTING ALUMINIUM TUNNELLING JUNCTIONS

Abstract

Up to now the emission of relaxation phonons of energy E > 2 A by tin tunnelling junctions could not be observed by reason of the strong reabsorption of such phonons in the generator junction. We now report experiments in which the emission of relaxation phonons of energy E r 2 A by aluminium tunnelling junctions is observed. The conditions necessary for the emission of high frequency relaxation phonons are discussed. It was first pointed out by Eisenmenger and Dayem (I) that superconducting tunnel junctions could be used as phonon generators. The measure- ments showed (I), 121, that the phonon spectrum radiated by such a diode consists mainly of two parts : a 4 A most of the phonons with E > 2 A become reabsorbed within the generator film by pair-breaking and subsequent decay of the secondary quasiparticles under emission of low ener- getic phonons. The last result was confirmed by expe- riments by Eisenmenger (3) and Narayanamurti (4), in which no phonons of energy E > 2 A leaving a Sn-generator junction could be observed. We now report on the observation of phonons of energy E > 2 A radiated by A1-I-A1 tunnelling junctions. We used a Sn-I-Sn tunnelling junction as phonon detector. Both the AI-I-A1 generator and the Sn-I-Sn detector junction were evaporated onto opposite faces of a sapphire plate (3 mm thich, c-cut). The experiments were performed at T = 1.0 K ; at this temperature 2 A,, = 0.28 meV, 2 As, = 1.16 meV. The tin detector responds only to phonons of energy E 2 2 As,, therefore, only phonons of energy E 2 8.4 A,, can be observed. Under pulsed generator current conditions the first derivative of the detector signal with respect to the generator current, i. e. the differentiated transmission-reception characte- ristic (DTRC) was directly measured by means of a sampling and modulation technique.