SNIS Junctions Boost Voltage and Temperature Range
in AC Quantum Standards
The development of accurate DACs for a huge variety of applications, spanning from
precision space sensors to power efficiency systems and environmental safety, requires
standards traceable over a frequency range extending from zero up to and in excess of 1 MHz
with quantum accuracy.
This has been pursued for the low frequency range by programmable Josephson voltage
standards (PJVS).
Our aim was to test the performance of SNIS junctions as alternative candidate for the realization of binary-divided array DACs with:
- a minimum of junctions per Volt
- working temperature of the device above 4.2 K, for easier cryo-cooler operation
Overdamped Nb/Al-AlOx/Nb superconductor-normal metal-insulator-superconductor (SNIS) Josephson
junctions have shown the possibility of simultaneous optimization of both Jc and Vc,
and in general, the possibility to trim these parameters, according to a specific application.
Binary-divided 1 V Josephson quantum DACs with 8192 overdamped Nb/Al-AlOx/Nb junctions, fabricated
within a joint cooperation between INRIM and PTB, have been tested at 4.2 K and from 6 to
7 K.
The characteristic voltage Vc of the fabricated junctions varies from 0.25 to 0.6 mV. For the
lower limit of Vc we demonstrated at 4.2 K proper operation of the 1 V circuits
with current margins larger than 1 mA for all array segments. In the case of larger values for
Vc the arrays can be operated at the second and third Shapiro step, when radiated with
microwaves from 69 to 74 GHz. Unsloped 3rd harmonic steps whose width ranged from 0.3 mA
up to and above 1 mA have been measured on all sub-segments providing voltages up to
3.73 V at 73.7 GHz across the whole array.
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Good unsloped 1 V step in PJVS with 8192 SNIS junctions under microwave irradiation at
70.29 GHz. |
Devices with high values of Vc (at 4.2 K) have been also successfully operated at 6-7 K, by optimizing the first Shapiro step. Quantized voltages up to 1.24 V with current margins between 0.5 and 0.8 mA have been observed at these temperatures. Working at these temperatures, the device seems well suited for operation in a cryocooler.
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Curve of current vs. voltage (IV) of a 1 V PJVS at T= 6 K under microwave irradiation at 73.6 GHz, showing the step at 1.247 V. Junction parameters: Ic=2.2 mA, Vc=0.21 mA. |
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In some cases circuits with a high Vc displayed unsloped steps corresponding to the 2nd and 3rd harmonic of the irradiated microwave signal. The figure presents the IV curve observed with an array designed to operate on the first step (1 V), showing an unsloped step at 3.60296 V. |
From these results we conclude that SNIS junctions can be a possible candidate for 10 V PJVSs.
References
- V. Lacquaniti, N. De Leo, M. Fretto, S. Maggi, and A. Sosso: Nb/Al-AlOx overdamped josephson junctions above 4.2 K for voltage metrology, Applied Physics Letters, Vol. 91, no. 25, pp. 252--254, 2007.
- V. Lacquaniti,C. Cagliero, S. Maggi, and R. Steni: Overdamped Nb/Al-AlO_x/Nb Josephson Junctions, Applied Physics Letters, Vol. 86, no. 4, p. 042501, 2005.
- V. Lacquaniti, D. Andreone, N. De Leo, M. Fretto, S. Maggi, A. Sosso, and M. Belogolovskii: Analysis of the Temperature Stability of Overdamped Nb/Al-AlOx/Nb Josephson Junctions, IEEE Trans. Appl. Supercond., vol. 17, no. 2, pp. 609-612, June 2007.
The research within this EURAMET joint research project leading to these results has received in part funding from the European Community's Seventh Framework Programme, ERA-NET Plus, under Grant Agreement No. 217257 (JoSy project).
