Physics 1973 =l6(V)+l"(V)cos(4)+l1(r)sin(4) [6] At finite voltages the linear increase with time of 4d implies that the only contribution to the dc current comes from the first term, which is the same as Giaever's prediction, thus extending the results of Cohen et al. to the two- superconductor case. The second term had a form consistent with my expecta tions of a Ag dependence of the current due to tunnelling of quasi-particles The third term, however, was completely unexpected, as the coefficient 11(v), unlike L(V, was an even function of V and would not be expected to vanish when V was put equal to zero. The 4g dependent current at zero voltage had the obvious interpretation of a supercurrent, but in view of the qualitative argument mentioned earlier I had not expected a contribution to appear to the same order of magnitude as the quasiparticle current, and it w days before I was able to convince myself that I had not made an error in the calculation Since sin(4)can take any value from-I to 1, the theory predicted a value of the critical supercurrent of I,(0). At a finite voltage V an ac supercurrent of amplitude {[4()2+[P frequency 2ev/h was expected. As mentioned earlier, the only contribu n to the dc current in this situation (V* 0)comes from the Io(v) term, so that a two-section current-voltage relation of the form indicated in Fig. 2 is expec I next considered the effect of superimposing an oscillatory voltage at fre- quency v on to a steady voltage V. By assuming the eff voltage to be to modulate the frequency of the ac supercurrent 1 concluded that constant-voltage steps would appear at voltages V for which the frequency of the unmodulated ac supercurrent was an integral multiple of v, i.e. when V=nhu/ge for some integer n The embarrassing feature of the theory at this point was that the effects order of magnitude as the jump in current occurring as the voltage through that at which production of pairs of quasi-particles becomes possible. xamination of the literature showed that possibly dc supercurrents of this magnitude had been observed, for example in the first published observation of tunnelling between two evaporated-film superconductors by Nicol, Shapiro and Smith(12)(fig. 3). Giaever(13)had made a similar observation, but ascribed the supercurrents seen to conduction through metallic shorts through the barrier layer. As supercurrents were not always seen, it seeme lanation in terms of shorts might be the correct one, and the whole theory might have been of mathematical interest only (as was indeed suggested in the literature soon after)160 Physics 1973 (6) At finite voltages the linear increase with time of ,LI@ implies that the only contribution to the dc current comes from the first term, which is the same as Giaever’s prediction, thus extending the results of Cohen et al. to the two￾superconductor case. The second term had a form consistent with my expecta￾tions of a A@ dependence of the current due to tunnelling of quasi-particles. The third term, however, was completely unexpected, as the coefficient 11 (V), unlike IO (V) , was an even function of V and would not be expected to vanish when V was put equal to zero. The A@ dependent current at zero voltage had the obvious interpretation of a supercurrent, but in view of the qualitative argument mentioned earlier I had not expected a contribution to appear to the same order of magnitude as the quasiparticle current, and it was some days before I was able to convince myself that I had not made an error in the calculation. Since sin (A(@) can take any value from e-1 to + 1, the theory predicted a value of the critical supercurrent of I 1 (0). At a finite voltage V an `ac supercurrent’ of amplitude and frequency 2eV/h was expected. As mentioned earlier, the only contribu￾tion to the dc current in this situation (V ¹ 0) comes from the IO (V) term, so that a two-section current-voltage relation of the form indicated in Fig. 2 is expected. I next considered the effect of superimposing an oscillatory voltage at fre￾quency v on to a steady voltage V. By assuming the effect of the oscillatory voltage to be to modulate the frequency of the ac supercurrent 1 concluded that constant-voltage steps would appear at voltages V for which the frequency of the unmodulated ac supercurrent was an integral multiple of V, i.e. when V = nhv/2e for some integer n. The embarrassing feature of the theory at this point was that the effects predicted were too large! The magnitude of the predicted supercurrent was proportional to the normal state conductivity of the barrier, and of the same order of magnitude as the jump in current occurring as the voltage passes through that at which production of pairs of quasi-particles becomes possible. Examination of the literature showed that possibly dc supercurrents of this magnitude had been observed, for example in the first published observation of tunnelling between two evaporated-film superconductors by Nicol, Shapiro and Smith (12) (fig. 3). Giaever (13) had made a similar observation, but ascribed the supercurrents seen to conduction through metallic shorts through the barrier layer. As supercurrents were not always seen, it seemed that the explanation in terms of shorts might be the correct one, and the whole theory might have been of mathematical interest only (as was indeed suggested in the literature soon after)