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(a) SEM of a sample showing the measurement configuration. The outermost two larger electrodes serve as the cooler junctions, while a pair of inner electrodes acts as a NIS thermometer to sense the electron temperature of the Cu wire. (b) High resolution SEM of a single junction showing the tapered Nb edge (blue) covered with Al (green), contacting the Cu (red). (c) Measured dI/dV − V characteristics of a NIS junction pair, indicated by the red dots, at a bath temperature of 150 mK, for samples with three different thicknesses of the Al layer (7, 10, 20 nm). Theoretical fits using a parallel junction model are represented by the lines, with parameters shown in the figure. The measured total tunneling resistances for each NIS pair were 60, 22, and , for the 7, 10, and 20 nm thick Al layers, respectively.
Bath temperature dependence of (a) I − V and (b) dI/dV − V characteristics of a NIS pair with 7 nm thick aluminium, between 0.15 and 1.2 K. The dots are the experimental data while the lines are theoretical fits for , 1.2 K, and 6 K (normal state), with electron temperature constant and equal to . The dotted line in (a) represents a constant current bias of 3.5 nA.
Thermometry characteristics for the sample with (a) 7 nm Al, bias current , and (b) 10 nm Al, bias current 20 nA. The solid lines indicate the corresponding theoretical fits. (c) Measured electron temperature of the copper wire as a function of cooler voltage at several bath temperatures, for the sample with 7 nm Al.
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