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Characteristic voltage at 4 K as a function of the barrier thickness . The thicknesses are inferred from the deposition time and the nominal deposition rate of .
Temperature dependence of in junctions with different barrier thicknesses. Symbols are the measured data, and the lines are fit using Eqs. (1) and (2) with the fitting parameters given in Table I. The inset is a comparison of the steepness of the temperature dependence for junctions with three different barrier materials. These junctions have different barrier ’s but similar values at 4 K, and the characteristic voltage ’s at 4 K are 48, 31, and , respectively, for , and barriers. The barrier has been shown to have no superconducting transition down to 50 mK.
characteristics of a series array of 2000 double-junction stacks with (solid) and without (dashed) 15 GHz microwave bias at 4 K. The array produces a flat Shapiro step with a current range of . The inset shows that the Shapiro step is indeed flat and at the proper voltage.
Microwave response of the size of the critical current and first Shapiro step for (a) a -barrier junction array and (b) a -barrier junction array. Both arrays have 4400 junctions in series. The presented power dependences are measured at 13 GHz, but they show similar behavior for frequencies up to 18.5 GHz. The arrows indicate the power at which the current range of the first step is maximized.
Values of the fitting parameters and from the measured temperature dependence of the current density for barrier junctions. Here ’s are inferred from measured curves, and is calculated using Eq. (2) and . The superconducting transition temperature of the barrier was measured to be 1.72 K, and the of the Nb yielding the best overall fit was 9.0 K.
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