Ethan Y Cho, Yuchao W Zhou, Jennifer Y Cho, Shane A Cybart
Applied Physics Letters 113 (2), 022604

We report the fabrication of nanoscale wires and Josephson junctions in 25 nm thick YBa2Cu3O7–δ thin films with wire widths as narrow as 50 nm. Our approach utilizes a finely focused gas field ion source from a helium ion microscope to directly modify the material on the nanometer scale to convert irradiated regions of the film into insulators. In this manner, the film remains intact and no material is milled or removed. Transport data show that the electrical properties, critical current and conductance, scale linearly with the lithographically defined width ensuring that the actual and lithographically defined dimensions are commensurate with each other. Unlike in typical ion damage proximity effect Josephson junctions, we observe a low temperature saturation of the critical current and near temperature interdependent resistance which we attribute to a narrower and more resistive barrier.

SA Cybart, A Herr, V Kornev, CP Foley
Supercond. Sci. Technol 30 (090201), 5pp.

Richard Feynman’s Lectures in Physics, Volume 3 [1] final lecture predicted that multiple Josephson junction devices would improve the sensitivity and bandwidth of superconducting devices. Since then there has been the development of both low temperature and high temperature devices and systems for a very wide range of applications, many requiring multiple Josephson junctions and arrays of junctions to operate. Examples are the Josephson voltage standard, superconducting computers, ratchet devices, antennas, THz receivers and detectors, data storage and single flux quantum devices. This focus issue on superconducting devices with multiple Josephson junctions features seven original research papers presenting new concepts, modeling, fabrication and applications. Furthermore this growing research field has regular breakthroughs that then are rapidly superseded. The international development of superconducting electronics in a range of applications combined with the advent of improved miniature cryocooling makes broad application of superconducting electronics a nearer term reality. Figure 1 shows the increasing complexity of different Josephson junction circuits as time progresses.

E. Y. Cho, Meng K. Ma, Chuong Huynh, R. C. Dynes and Shane A. Cybart
Microscopy and Microanalysis 21.S3 (2015): 1997-1998.

Electrical circuits fabricated from high-transition temperature superconductors (HTS) are very difficult to pattern due to the lack of a reliable etching process. Chemical etching can be used for large features, but undercutting limits the feature size to tens of microns. Dry etching is required for smaller features, but there is no anisotropic reactive ion etch for these materials. Therefore dry etching must be done with isotropic argon ion milling. The ion milling process generates excess heat and unfortunately oxide superconductors are sensitive to high temperature. Overheating the material by ion milling causes it to deoxygenate which turns the superconductor into an insulator. Therefore to prevent overheating the critical dimension for argon ion milling is typically limited to a few microns.

E Y Cho, K Kouperine, Y Zhou, R Dynes, SA Cybart
Superconductor Science and Technology, Volume 29, Number 9

We have fabricated the two-dimensional arrays of superconducting quantum interference devices (SQUIDs) using YBa2Cu3O7-δ ion-irradiated Josephson junctions, and we have studied the effects of post-annealing the arrays at 100 ◦C in oxygen. The maximum voltage modulation, V B, in a magnetic field for DC biased arrays at 50 K is initially 1.2 mV, but increases to 3 mV after annealing. Furthermore, the temperature where the largest V B occurs increases from 45 K to 48.5 K after annealing. We present and simulate a model where annealing causes diffusion and recombination of the low-energy oxygen defects that narrows the barrier, resulting in an increase in the Josephson binding energy. We show that this process stabilizes after 40 minutes of annealing and leads to a significant improvement in the properties of the array.

S Cybart, P Roediger, K Chen, J Parker, E Cho, T Wong, R Dynes
IEEE

We investigate the temporal stability of YBa2Cu3O7 Josephson junctions created by ion irradiation through a nano-scale implant mask fabricated using electron beam lithography and reactive ion etching. A comparison of current-voltage characteristics measured for junctions after fabrication and eight years of storage at room temperature show a slight decrease in critical current and increase in normal state resistance consistent with broadening of the weaklink from diffusion of defects. Shapiro step measurements performed 8 years after fabrication reveal that device uniformity is maintained and is strong evidence that these devices have excellent temporal stability for applications.

SA Cybart, K Chen, RC Dynes
Applied Superconductivity, IEEE Transactions on 15 (2), 241-244

Reproducible low resistance lumped element Josephson junction arrays are desired for many microwave applications. Using our established process of electron beam lithography and ion damage, we have fabricated and demonstrated high quality YBa2Cu3O7-δ superconductor-normal superconductor-superconductor (SS'S) in-plane Josephson junctions. Single junctions and multiple junction arrays with as many as 50 junctions in series have been investigated. These junctions are in close proximity and offer several possible applications. Current-voltage characteristics for single junctions are consistent with the resistively shunted junction model. Junction in pairs have been fabricated which show nearly identical characteristics. Rounding near the critical current occurs for larger number arrays which we attribute to junction nonuniformity. Microwave measurements reveal sharp giant Shapiro steps for junction pairs and 10 junction arrays, rounded steps appear for larger arrays.

K Chen, SA Cybart, RC Dynes
Applied Superconductivity, IEEE Transactions on 15 (2), 149-152

Using electron beam lithography and ion damage, high quality YBa2Cu3O7-δ superconductor-normal metal-superconductor in-plane Josephson junction pairs have been fabricated. These junctions operate at temperatures between 60 and 85 K and have spacing ranging between 150 nm and 800 nm. Central electrodes connecting to the area between the two junctions were made, allowing for simultaneous measurements of the individual junctions as well as the series. Josephson junction pairs with 150 nm spacing were found to have extraordinary properties under microwave radiation with different power. At low power the structure functioned as two independent Josephson junctions in series, while at high power it behaved as one single Josephson junction. A possible origin of this behavior could be that the applied microwave radiation induces weak link behavior in the YBa2Cu3O7-δ between the two junctions.

K Chen, SA Cybart, RC Dynes
Applied physics letters 85 (14), 2863-2865

We have fabricated in-plane high-Tc Josephson junction pairs and series arrays using our established nanolithography and ion damage process. Junctions in a pair showed nearly identical electrical properties. The ten-junction array exhibited current–voltage characteristics that can be described by the resistively shunted junction model at 78K. Flat giant Shapiro steps were observed in both cases. We believe that the ion-damaged high-TcsuperconductingJosephson junction is a good candidate to form large numbers of junctions in series arrays that can function above 77K for quantum voltage standards and other applications.