Yonuk Chong

Practical high-resolution programmable Josephson Voltage standards using double- and triple-stacked MoSi/sub 2/-barrier junctions

We have developed vertically stacked superconductor normal-metal-superconductor Josephson junction technology for the next-generation quantum voltage standards. Stacked junctions provide a practical way of increasing the output voltage and operating margins. In this paper, we present fully functioning programmable voltage standard chips with double- and triple- stacked MoSi/sub 2/ barrier Josephson junctions with over 100 000 junctions operating simultaneously on a 1 cm /spl times/ 1 cm chip. The maximum output voltages of the double- and triple-stacked chips were 2.6 V and 3.9 V, with respective operating current margins of 2 mA and 1 mA. A new trinary-logic design is used to achieve higher voltage resolution. Thermal transport in these high-density chips will be briefly discussed.

Development of a 60 Hz Power Standard Using SNS Programmable Josephson Voltage Standards

We are implementing a new standard for 60 Hz power measurements based on precision sinusoidal reference voltages from two independent programmable Josephson voltage standards (PJVS): one for voltage and one for current. The National Institute of Standards and Technology PJVS systems use series arrays of Josephson junctions to produce accurate quantum-based DC voltages. Using stepwise-approximation synthesis, the PJVS systems produce sinewaves with precisely calculable RMS voltage and spectral content. We present measurements and calculations that elucidate the sources of error in the RMS voltage that are intrinsic to the digital-synthesis technique and that are due to the finite rise times and transients that occur when switching between the discrete voltages. Our goal is to reduce all error sources and uncertainty contributions from the PJVS synthesized waveforms to a few parts in 10 7 so that the overall uncertainty in the AC-power standard is a few parts in 106

Stacked SNS Josephson junction arrays for quantum voltage standards

NIST is using and developing superconductor-normal metal-superconductor (SNS) Josephson arrays for both programmable DC and AC voltage standards. Increasing the output voltage is difficult because the output voltage per junction is small; hence series arrays with large numbers of junctions are needed. The best way to generate higher voltages and achieve the best operating margins for the broadband drive signals is by densely packing the junctions into shorter arrays. NIST has been working on stacked SNS junctions to achieve this goal. By stacking junctions in the array, more junctions may be placed per length, while preserving a lumped microwave element. In this paper we introduce our results on stacked SNS junctions using MoSi/sub 2/ and Ti as barrier materials. These barriers were chosen because they can be reactive-ion etched (RIE) in contrast to our standard PdAu barriers, which must be wet etched. Using RIE, alternating layers of barrier material and Nb may be etched in a single step. We indirectly quantify the junction uniformity in the arrays by measuring the current range of the constant-voltage steps when the arrays are biased with a microwave drive.

Analog-to-digital conversion for low-frequency waveforms based on the Josephson voltage standard

A waveform synthesizer adopting a superconductor–normal metal–superconductor junction array has been developed, which can generate arbitrary stepwise waveforms with a number of quantum-voltage steps up to 1 V level amplitude. As an application of the synthesizer, we have built a sampling voltmeter that measures the differential voltages between a sinusoidal waveform produced by a semiconductor-based ac source and the Josephson waveforms. We carried out extensive sampling measurements for a 50 Hz sine wave with 1 V amplitude, applying sampling apertures in the range of 55 µs ≤ta ≤ 130 µs and using Josephson waveforms with 32, 60, 80 and 100 quantum steps. From the measurements, the amplitude of the ac waveform was determined with a type A uncertainty (k = 2) of 0.15 µV. Also, we elucidated how the phase jitter in the ac waveform is reflected in the overall uncertainty for the measurements. The type B uncertainty due to the jitter is at least one order of magnitude smaller than the type A uncertainty.

Thermal transport in stacked superconductor–normal metal–superconductor Josephson junctions

Nb/MoSi2/Nb stacked superconductor–normal metal–superconductor (SNS) Josephson junctions has proven to be a good candidate for high-density series arrays for Josephson voltage-standard applications. As the junction density increases, self-heating becomes an issue because the high power density per junction (1 W/cm2) generates significant power dissipation under typical operating conditions. In this letter, we analyze the heating effect of these sandwich-type SNS junctions using a model to quantitatively estimate and predict thermal-transport properties of the stacked structures. We describe several strategies that reduce heating and demonstrate improved properties of stacked-junction arrays with enhanced cooling capacity.

Design of SNS Josephson Arrays for High Voltage Applications

The voltage from a single, microwave-biased Josephson junction is a small quantity; thus useful voltages are generated only through series arrays of many thousands of junctions. Arrays of superconductor-normal metal-superconductor junctions have been fabricated and tested with as many as 16,500 junctions per array. The arrays are optimized for the highest voltage operation with the largest operating margins for the current bias. Measurements show that these arrays, driven with 20 GHz microwaves, generate a dc voltage greater than 680 mV per array with a dc bias margin over 1 mA. To increase the microwave uniformity across the array, the transmission line impedance has been tapered. By use of this technique, ac Josephson voltages over 110 mVrms per array have been generated, also with over 1 mA dc bias margin.

Stacked Nb-MoSi/sub 2/-Nb Josephson junctions for AC voltage standards

Superconductor-normal metal-superconductor (SNS) Josephson junctions have proven to be a critical technology for voltage standards. NIST has used SNS junctions for both dc and ac programmable voltage standards. Previous devices have used primarily PdAu as a normal-metal barrier material. In this paper we present measurements of circuits having MoSi/sub 2/ barriers. Stacking enables the junctions to be packed more densely, thus increasing design flexibility and margins for the microwave circuits. In this work, measurements are presented from two- and three-junction stacks for application to ac Josephson voltage standards, which show output voltage and distortion near our best previously published results.

Electrical properties of Nb–MoSi2–Nb Josephson junctions

We present a detailed study of the electrical properties of planar Nb–MoSi2–Nb Josephson junctions. The Nb–MoSi2–Nb junction is an excellent system to study proximity coupling in junctions with rigid superconductor/normal metal boundaries by precisely and independently controlling the barrier thickness and the temperature. With regard to applications, the Josephson properties are very reproducible, and the characteristic voltage can be tuned easily over more than two orders of magnitude while still maintaining a practical critical current density. The characteristic voltage can be controlled within ±5% at 4K, with an exponential dependence on the barrier thickness. The proximity-coupled junction theory fits the temperature dependence of the critical current density, allowing us to quantitatively extract material parameters.

Simulations of collective synchronization in Josephson junction arrays

We developed a virtual model of a Josephson junction series array based on a rigorous dynamic simulation and investigated coupling effect of Josephson oscillations. Our virtual model for a superconductor-insulator-normal metal-insulator-superconductor junction array reveals that a time-lag junction instability should be avoided for global synchronization in a long array with a large number of junctions. We found that the coupling between the self-generated Josephson oscillations through a microwave transmission line plays an important role in the collective synchronization of the Josephson array.

Superconducting junction of a single-crystalline Au nanowire for an ideal Josephson device

We report on the fabrication and measurements of a superconducting junction of a single-crystalline Au nanowire, connected to Al electrodes. The current-voltage characteristic curve shows a clear supercurrent branch below the superconducting transition temperature of Al and quantized voltage plateaus on application of microwave radiation, as expected from Josephson relations. Highly transparent (0.95) contacts very close to an ideal limit of 1 are formed at the interface between the normal metal (Au) and the superconductor (Al). The very high transparency is ascribed to the single crystallinity of a Au nanowire and the formation of an oxide-free contact between Au and Al. The subgap structures of the differential conductance are well explained by coherent multiple Andreev reflections (MAR), the hallmark of mesoscopic Josephson junctions. These observations demonstrate that single crystalline Au nanowires can be employed to develop novel quantum devices utilizing coherent electrical transport.