Mayumi Ishizaki

10V programmable Josephson voltage standard circuits using NbN∕TiNx∕NbN∕TiNx∕NbN double-junction stacks

Using NbN∕TiNx∕NbN∕TiNx∕NbN double-junction stack technology we have demonstrated a programmable Josephson voltage standard chip that operates up to 10.16V output voltage cooled with a two-stage Gifford–McMahon cryocooler. The circuit uses double-junction stacks, where two junctions are fabricated in each stack, in order to integrate 327 680 junctions into a 15.3mm×15.3mm chip. A 1-to-32 microwave distribution circuit is also integrated on the chip. The chip is divided into 22 cells, which perform as an 11-bit digital-to-analog converter. The 21 working cells include 307 200 junctions biased with 16GHz microwaves at 10.2K that generated flat voltage steps with current margins greater than 1mA, which indicates good uniformity of the stacked junctions.

Operation of a NbN-based programmable Josephson voltage standard chip with a compact refrigeration system

A refrigeration system was designed and constructed for realizing a liquid-He-free programmable Josephson voltage standard. The system is equipped with a two-stage Gifford-McMahon cooler, a thermal-radiation shield, a magnetic-field shield and semi-rigid coaxial cables to supply microwave power to a chip. The performance of the system was examined by use of a NbN-based 8-bit digital-to-analog converter (DAC) chip designed as a 1 V programmable voltage standard. When operated at 8.5 K on the cryocooler, constant-voltage steps with amplitudes greater than 1 mA were observed for every segment of junction arrays on the chip.

NbN/TiNx/NbN/TiNx/NbN double-barrier junction arrays for programmable voltage standards

A series array of NbN/TiNx/NbN/TiNx/NbN double-barrier junctions was fabricated on Si wafers, and their current–voltage (I–V) characteristics were measured with and without microwave power in order to investigate their performance for programmable voltage standards. By adjusting the thickness of the TiNx barrier, nearly identical critical currents were obtained for 128 lower and 128 upper junctions in an array. When applying 8 GHz microwave power to an array, a large constant-voltage step (∼3 mA) appeared on the I–V curve. The zero-voltage critical current and the height of the first (n=1) constant-voltage step showed microwave-power dependences coincident with theoretical prediction.

Operating Margins of a 10 V Programmable Josephson Voltage Standard Circuit Using

The operating margins of a 10 V programmable Josephson voltage standard circuit using Operating margins of a 10 V programmable josephson voltage standard circuit using NbN/TiNx/NbN/TiNx/NbN double-junction stacks was investigated as a function of microwave frequency and operating temperature. The circuit contained 32 arrays of 5 120 Operating margins of a 10 V programmable josephson voltage standard circuit using NbN/TiNx/NbN/TiNx/NbN double-junction stacks. In other words, the circuit contained 327 680 Operating margins of a 10 V programmable Josephson voltage standard circuit using NbN/TiNx/NbN/TiNx/NbN junctions. It was found that the operating margins (the heights of the constant-voltage step) of the arrays varied largely with microwave frequency and operating temperature. The microwave-frequency dependence was due to the resonance of the microwaves in the circuit. The temperature dependence of the circuit was due to the temperature dependence of the IcRn product of the junctions, where Ic was the critical current Rn and was the normal resistance. Fortunately, however, the operating margin was maximized by changing the temperature of the chip, mounted on the cold head of a cryocooler.

{\rm NbN}/{\rm TiN}_{x}/{\rm NbN}/{\rm TiN}_{x}/{\rm NbN}

Stacked double and triple Josephson junctions with NbN electrodes and TiN/sub x/ barriers were fabricated for the next-generation 10 V programmable Josephson voltage standard. Because of difficulties in the growth of uniform junctions in a stack with a constant barrier thickness, a stack with carefully engineered thicknesses was grown that exhibited uniform junction properties. The junction arrays on these chips were biased with microwave power at 16 GHz resulting in constant-voltage steps consistent with the total number of junctions in the array, including the multiple junctions in the stacks. The steps had a current range greater than 1 mA at 4.2 K.

Double-Junction Stacks

The results of direct comparisons between a 1-V NbN-based programmable superconductor–normal metal–superconductor (SNS) Josephson array and a conventional superconductor–insulator–superconductor (SIS) Josephson array showed an agreement with deviations of less than 0.4 nV at voltages between 0.1 and 1 V. Null detector fluctuations caused by external noises and environmental variations that couple to the measurement system limit the measurement accuracy.

Critical current control and microwave-induced characteristics of (NbN/TiN/sub x/)/sub n//NbN stacked junction arrays

The relationships between the height of the constant voltage step and the difference in the critical current Ic or normal resistance Rn of the upper and lower junctions were numerically calculated. The dependence of the step height on the distribution of the microwave power in a Josephson junction array was also numerically calculated. As a result, it was shown that the difference in Rn significantly reduced the height of the steps, while the uniformity of Rn was actually excellent in our previous experiments. On the other hand, neither the difference in Ic nor the distribution of the microwave power due to attenuation at junctions significantly reduced the step height. The numerical analysis implied that the standing waves in the array due to an impedance mismatch will also significantly reduce the step height.

Comparison between a 1-V NbN-Based Programmable and a Conventional Josephson Array

A programmable Josephson voltage standard chip using arrays of NbN/TiN/NbN/TiN/NbN double-junction stacks was fabricated and operated at 10 K. The circuit with over 260,000 junctions showed constant voltage steps at +/- 6 V when driven at 11 GHz microwave bias. Further investigation is required to achieve 10 V operation at the desired design frequency of 19 GHz

Reduction in Shapiro Step Height in Double-Barrier Josephson Junction Arrays

A NbN-based programmable Josephson voltage standard arrays (PJVAs) have been investigated. We show that these arrays can be utilized for programmable operation up to /spl plusmn/1 V. The results of direct comparisons between the PJVA and a conventional superconductor-insulator-superconductor Josephson array agree at voltages between 0.1 V and 1 V within an uncertainty of better than 1 nV. An indirect comparison via a Zener voltage reference also shows agreement within a 1/spl sigma/ type A uncertainty of 13 nV.

Programmable Josephson voltage standard circuits using arrays of NbN/TiN/NbN/TiN/NbN double-junction stacks operated at 10 K

We tried a direct comparison between a 1-V NbN-based programmable superconductor-normal metal-superconductor (SNS) Josephson array and a conventional superconductor-insulator-superconductor (SIS) Josephson array. In preliminary experiment the voltage difference of the two arrays was -4.3 times 10-9 V in average of 10 measurement sets. The deviation is resulted from the fluctuations of the background in the measurement set