Kouhei Nakayama

Improvement in reproducibility of multilayer and junction process for HTS SFQ circuits

We have developed an HTS multilayer structure including a superconducting ground plane and interface-engineered ramp-edge Josephson junctions for single flux quantum (SFQ) circuits, and tried to improve reproducibility in the multilayer and junction process. We found that one of the crucial factors to improve the reproducibility is use of a ground plane with small average surface roughness (R/sub a/) of typically less than 2 nm and a high T/sub c/ above 87 K. By employing La/sub 0.2/-Y/sub 0.9/Ba/sub 1.9/Cu/sub 3/O/sub x/ (La-YBCO) thin films deposited by dc off-axis magnetron sputtering, such high quality was maintained for 3-month successive use of a ceramic target. Use of such a high-quality ground plane reproducibly led to SrSnO/sub 3//La-YBCO/SrSnO/sub 3//La-YBCO //MgO multilayer structures with R/sub a//spl les/2 nm and 100-junction arrays with 1-/spl sigma/I/sub c/ spread of 6-17% at 4.2 K depending on the average I/sub c/ level. The run-to-run reproducibility of average junction I/sub c/ and sheet inductance of both the base- and counter-electrodes for three samples was /spl plusmn/12%, /spl plusmn/3.0%, and /spl plusmn/3.4%, respectively, which are substantially smaller than those previously reported.

Fabrication and logic operation of oxide SFQ-circuit-components

Basic SFQ circuit components, such as SFQ-dc converter, set-reset flip-flop (RS-FF), confluence buffer and splitter, have been fabricated by the YBa/sub 2/Cu/sub 3/O/sub 7-x/-ramp-edge-junction technology and their logic operations were investigated. Circuit parameters were optimized in order to widen the margins of these oxide SFQ circuit components. The SFQ-dc converter that comprised double SQUIDs connected in series showed the output voltage as much as 0.3 mV at 22 K owing to high I/sub c/R/sub n/ value of the ramp-edge junction. Logic operations of the RS-FF, the confluence buffer and the splitter were correctly performed by using the SFQ-dc converter. Finite operating margins were obtainable for these circuits up to the temperature near 30 K.

Fabrication of high-temperature superconductor single-flux-quantum circuits using a multilayer structure with a smooth surface

We have developed a multilayer structure with a smooth surface and fabrication processes for HTS single-flux-quantum (SFQ) circuits. The multilayer structure with surface roughness Ra less than 2 nm, composed of a La0.2–Y0.9Ba1.9Cu3Ox ground plane and base electrode layer, and SrSnO3 insulating layers, was deposited by off-axis magnetron sputtering. We have fabricated interface-engineered junctions based on the multilayer structure using an La0.2–Yb0.9Ba1.9Cu3Ox counter-electrode layer prepared by pulsed laser deposition. The fabricated junctions exhibited excellent Josephson characteristics with a 1-σ spread in Ic as low as 8% for 1000 junctions. The sheet inductance values below 50 K were 0.8–1.0 pH per square. Operation of several elementary SFQ circuits including a toggle flip-flop, a confluence buffer, a set–reset flip-flop and other SFQ elements has been successfully demonstrated at 30–60 K.