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Superconductor Science and Technology

Large constant voltage generated with a single array including 65 536 Nb/TiN/Nb Josephson junctions

Hirotake Yamamori, Chiharu Urano, Michitaka Maruyama, Hitoshi Sasaki, Satoshi Kohjiro

Published Jun 16, 2010• 5 citations
Publisher

Abstract

A 2 V Shapiro step with a current height of about 0.3 mA was successfully generated at 4.2 K with a single array including 65 536 Nb/TiN/Nb Josephson junctions under microwave irradiation at 16 GHz. The critical current density was 89 kA cm − 2 , and the junction size was 3 µm. The n = 1 step height decreased exponentially as the number of junctions N increased when the number of junctions was less than 30 000, a result that agreed well with the dissipated junction model. On the other hand, the step height was nearly constant when the number of junctions was more than about 30 000, a figure much larger than that theoretically calculated from the model. Using an array including such a large number of Josephson junctions represents a promising way for ac waveform synthesis to increase output voltage to about 1 V rms .

DOI
10.1088/0953-2048/23/7/075011

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A 2 V Shapiro step with a current height of about 0.3 mA was successfully generated at 4.2 K with a single array including 65 536 Nb/TiN/Nb Josephson junctions under microwave irradiation at 16 GHz. The critical current density was 89 kA cm − 2 , and the junction size was 3 µm. The n = 1 step height decreased exponentially as the number of junctions N increased when the number of junctions was less than 30 000, a result that agreed well with the dissipated junction model. On the other hand, the step height was nearly constant when the number of junctions was more than about 30 000, a figure much larger than that theoretically calculated from the model. Using an array including such a large number of Josephson junctions represents a promising way for ac waveform synthesis to increase output voltage to about 1 V rms .
Binary-divided 1 V arrays were fabricated for programmable Josephson voltage standards in close cooperation between INRIM and PTB. The arrays, consisting of 8192 overdamped Nb/Al-AlO(x)Nb SNIS Josephson junctions, were successfully operated at microwave frequencies around 70 GHz. While the characteristic voltage of the junctions ranged from 0.25 to 0.6 mV, the current margins were larger than 1 mA for the Shapiro step n = 1. In addition, flat Shapiro steps of higher order (n = 3 and 4) were generated at increased microwave power levels due to the large characteristic voltage, demonstrating the small parameter spread of the SNIS junctions. Operation at temperatures up to 6-7 K has also been verified, measuring quantized voltages up to 1.25 V on the n = 1 step.
Voltage pulses with a determined amplitude and width might be the basis for high-precision ac voltage measurements in the future. The pulse width modulation technique makes possible the generation of arbitrary waveforms. In this paper, arrays of SINIS Josephson junctions were investigated which produce a 1 V voltage step under irradiation at a frequency of the order of 70 GHz. This step has an amplitude of 1 mA whereof a region of 0.3 mA overlaps with the critical current. This allows the generation of a 1 V voltage pulse by switching the microwave power at constant dc bias current. The results of this investigation will be presented and also the array parameters which are required for obtaining a relative uncertainty of 0.1 ppm in ac voltage measurements.
Design and fabrication of Nb-PdAu-Nb trilayer Josephson junctions are described. The microwave response of an array of 1000 of these junctions was measured, and constant-voltage step heights mere characterized as a function of the microwave amplitude and frequency. Experimental results fit well to point-junction simulations at the 3 GHz design frequency of the microwave distribution network. The observed step height of 3.8 mA shows that the array and microwave distribution are sufficiently uniform for application in programmable Josephson voltage standards.
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Series arrays of Nb–PdAu–Nb Josephson junctions were fabricated with characteristics ideally suited for application in programmable voltage standards and D/A converters with fundamental accuracy. Large arrays of junctions with applied microwave power showed constant voltage steps with current amplitudes as large as 7 mA. A novel coplanar waveguide design enabled uniform microwave power coupling to a five-segment array of 8192 junctions, so each segment had constant voltage steps over the same bias range. The 8192-junction device generated 1.1 mA steps at 186 mV with 11 GHz power and a maximum constant voltage step of 260 mV at 15.34 GHz.
A 10-V programmable Josephson voltage standard (PJVS) circuit including 524 288 vertically stacked NbN/TiN/NbN Josephson junctions was successfully fabricated without any defects. The chip was cooled with a compact cryocooler at a temperature of 9.8 K and generated an output voltage of 17.3 V with the first Shapiro step height of 1.0 mA under microwave irradiation at 16 GHz. Furthermore, the circuit design having the high output voltage increased the number of available chips for the 10-V PJVS. The fabrication yield for the chip, which generates greater than 10 V, was 36%, which is ten times greater than that for a perfect chip without any defects. This method was not effective for the chip that had serious defects, such as a disconnection across the array or short between the array and ground; thus, efforts to reduce such defects, e.g., frequent maintenance inside vacuum chambers for film preparation and etching, were found to be essential.