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(Color) Schematics and operating principles of the circuit. (a) The device is made of two super-conducting materials with different transition temperatures (e.g., niobium and tantalum). The circuit’s operation regime can be altered both by stretching or compressing the break junction and by varying the temperature. (b) Resistively shunted junction (RSJ) configuration with the MCBJ in the contact mode and the shunting segment in the normal state. (c) Stretched MCBJ in the tunnel mode allows measuring the superconducting gap of the junction material. (d) Further stretching yields the “broken” junction .
(Color) (a) Colorized scanning electron micrograph of the circuit with a diagram of the measurement setup. Metallic structures were deposited on top of an insulating thin polyimide film (dark blue) covering a flexible bronze substrate. The circuit mostly consists of superimposed tantalum and niobium films (green), each thick. Top tantalum layer was removed by reactive ion etching in the rectangular area around the break junction, leaving a purely niobium contact (brown). We used the energy dispersive x-ray spectroscopy (EDS) microanalysis to verify the absence of tantalum. Subsequently, niobium was wet etched from the shunt (red). The circuit was operated through two pairs of gold leads (yellow). (b) Magnified view of the tantalum shunt. Both the shunt and the break junction area were patterned by electron beam lithography. (c) The break junction area; the length and the width of the narrowest part are both . (d) Superconducting transitions in the device. Normally, we observe the “bulk” transition of niobium film before stretching the junction for the first time. Later in the experiment, the shunt resistance and transition temperature are measured in the “broken” junction configuration [Fig. 1(d)]. We routinely achieve thin-film for Nb and for Ta, which are slightly below corresponding bulk values.
Raw (a) and differential conductance (b) tunnel curves obtained from two different samples with identical . The normal-state QPC resistance was and that of the RSJ was (notice the difference in corresponding vertical scales). The QPC circuit did not have an embedded shunt. curves were produced by numerical differentiation with smoothing applied to the RSJ data. Broadening of MAR features and substantially higher voltage fluctuations were typical for all three RSJ samples.
Normalized characteristics taken in the same sample before (a) and after (b) destroying the shunt by excessive current at cryogenic temperatures. Both junctions shown have comparable conductances of . The presence of the shunt caused shift in the SGS structure. Solid lines represent best fits, which are of comparable quality.
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