Yoshinobu Tarutani

Superconducting characteristics of a planar‐type HoBa2Cu3O7−x −La1.5Ba1.5Cu3O7−y−HoBa2Cu3O7−x junction

A planar‐type junction of HoBa2Cu3O7−x−La1.5Ba1.5Cu3O7−y −HoBa2Cu3O7−x having an electrode spacing as small as 0.1 μm is fabricated and its electrical characteristics are measured. Supercurrent through the La1.5Ba1.5Cu3O7−y layer is detected up to the temperature of 69 K. A superconducting region extends from the YBa2Cu3O7−x electrodes into a normal La1.5Ba1.5Cu3O7−y layer with a submicrometer scale. The superconducting decay length is 65 nm, which is two orders of magnitude larger than the value calculated from the conventional proximity theory.

DC-SQUID with High-Critical-Temperature Oxide-Superconductor Film

Voltage vs magnetic-flux characteristics of a DC-SQUID using YBa2Cu3O7-δ film have been studied. YBa2Cu3O7-δ film SQUID was fabricated by lithographical patterning. The DC-SQUID operated up to 72 K.

Influence of Counter-Layer Deposition Condition on Critical Current Spread in Interface-Modified Ramp-Edge Junction Arrays

We have fabricated interface-modified ramp-edge junctions and junction arrays with sputtered La-doped YBa2Cu3Oy (La-YBCO) and La-doped YbBa2Cu3Oy (La-YbBCO) by pulsed laser deposition (PLD) as base and counter layers, respectively. The influences of deposition conditions for the La-YbBCO counter layers, in particular the target–substrate distance (Ht–s), on the junction properties and the Ic spread in the junction arrays is investigated. For a relatively short Ht–s of 60 mm, junctions with different edge directions exhibit substantially different Ic values, suggesting that the barrier recrystallization is significantly influenced by the incident angle of ablated atoms on the ramp surface. By employing a longer Ht–s of 80 mm, this influence can be lessened, and 1 σ spreads in Ic at 4.2 K of 7.9 and 9.5% can be obtained for 100- and 1000-junction series arrays, respectively.

Growth of SrTiO3 thin films by pulsed-laser deposition

Abstract SrTiO 3 thin films were grown on MgO(001) substrates at a substrate temperature of 600 °C by pulsed-laser deposition. The preferential a axis orientation is enhanced and the lattice parameter is strongly expanded as the oxygen pressure during growth is lowered. The thin film grown at 10 −6 Torr has a lattice parameter as large as 0.395 nm. This is thought to result from oxygen vacancies introduced during growth in a non-thermal equilibrium state. The lattice of the film has a cube-on-cube orientation. Such an epitaxial-like thin film was thought to arise because thin film growth at a low oxygen pressure brings about a molecular-beam-epitaxy-like condition and thus a large migration energy is retained on the substrate surface.

Investigation of ramp-type Josephson junctions with surface-modified barriers

We have investigated the properties of YBa/sub 2/Cu/sub 3/O/sub 7-x/ ramp-edge Josephson junctions with surface-modified barriers produced by Ar-ion irradiation followed by oxygen annealing. The fabricated junctions displayed RSJ-like I-V characteristics and excellent uniformity. The stray capacitance of the junctions was estimated from the ramp-edge structure. The junction capacitance was obtained by subtracting the stray capacitance from the shunting capacitance. We estimated the barrier thickness from the junction capacitance, and found that the critical current density of the junction increased exponentially with decreasing barrier thickness. The relative dielectric constant of the barriers ranged from 13 to 18.

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.

Ramp-edge junctions with interface-modified barriers fabricated on YBCO thick films

We fabricated ramp-edge junctions with an interface-modified barrier on YBa/sub 2/Cu/sub 3/O/sub y/ (YBCO) liquid phase epitaxy (LPE) thick films. The LPE thick films were used as ground-planes. For the insulating layer between the ground plane and base electrode, a SrTiO/sub 3/ (STO)/(LaAlO/sub 3/)/sub 0.3/-(SrAl/sub 0.5/Ta/sub 0.5/O/sub 3/)/sub 0.7/ (LSAT)/STO multilayer with the dielectric constant of approximately 32 was employed. We fabricated ramp-edge junctions with La-doped YBa/sub 2/Cu/sub 3/O/sub y/ (La-YBCO) and La-doped YbBa/sub 2/Cu/sub 3/O/sub y/ (La-YbBCO) as base and counter electrodes, respectively. The fabricated junctions exhibited resistively and capacitively shunted junction (RCSJ)-like characteristics and typical I/sub c/R/sub n/ products of 2.6 and 1.1 mV at 4.2 and 40 K, respectively. The 1 /spl sigma/-spread in I/sub c/ as small as 8.1% was obtained for 100-junction series-arrays. The use of ground plane reduced the sheet inductance of electrodes to a value of 0.76-0.94 pH/square at 4.2 K.

Electrical Characteristics of HoBa2Cu3O7-x-La1.5Ba1.5Cu3O7-y-HoBa2Cu3O7-x Junctions with Planar-Type Structures

A HoBa2Cu3O7-x-La1.5Ba1.5Cu3O7-y-HoBa2Cu3O7-x junction having a planar structure is fabricated and its electrical characteristics are investigated. The planar junction is formed by sectioning a HoBa2Cu3O7-x layer into two superconducting electrodes keeping a submicrometer between them. Supercurrent is detected for the junction with an electrode spacing smaller than 0.4 µm. Critical current is exponentially dependent on electrode spacing. Superconducting decay length for the La1.5Ba1.5Cu3O7-y layer is 65 nm and is almost independent of temperature up to 70 K. This extraordinary long decay length and anomalous superconducting characteristics are not shown to be explained by the conventional proximity theory, and phenomenologically outlined.

Niobium-based integrated circuit technologies

Metallurgical and electrical properties of Nb and NbN films for use as Josephson junction electrodes and wiring layers are investigated. The crystallographic and superconducting properties necessary for Nb-based integrated circuit processes are clarified. Tunnel barrier structures of NbN-Nb oxide-NbN (Pb alloy) and Nb-Al oxide-Nb Josephson junctions have been analyzed and correlated with junction characteristics and critical current uniformity. It was found that the surface structure of a base electrode should be smooth to ensure that Josephson junctions have low leakage current and uniform critical current distribution. New types of Josephson junctions with artificial tunnel barriers such as amorphous Si or Mg oxide are reviewed. A variety of Josephson junction structures or processes have been developed for Nb-based Josephson integrated circuits in order to improve circuit performance. These include junction miniaturization, planarization, and stacked junction structures. These structures are mainly intended for Nb-Al oxide-Nb Josephson circuits. The Nb-Al oxide-Nb Josephson junction technology is by far the most advanced and has been used in logic and memory circuits, for example a 4-bit*4-bit parallel multiplier, a Josephson logic gate array, a 16-bit arithmetic logic unit, a 4-bit microprocessor, and 1-kb and 4-kb memory circuits. >

Development of Integrated HTS SQUIDs With a Multilayer Structure and Ramp-Edge Josephson Junctions

We have fabricated high-temperature superconductor (HTS) dc superconducting quantum interference devices (SQUIDs) with a multilayer structure and ramp-edge Josephson junctions. For stable operation at 77 K, SmBa₂Cu₃O_y (SmBCO) and La_0.1-Er_0.95Ba_1.95Cu₃O_y (La-ErBCO) films with a <i>T</i> _c above 90 K were employed as materials for the base-electrode and counter-electrode layers, respectively. Junctions were fabricated by employing a Cu-poor La-ErBCO layer as a precursor of the barrier. The fabricated parallel gradiometers with a baseline of 1-3 mm exhibited the field-induced modulation voltage of 20-50 muV at 77 K, and a clear modulation up to 86.8 K. Noise measurements at 77 K revealed a white flux noise of 4.5-10 muPhi₀/Hz^1/2 at 1 kHz. The magnetometer with an integrated 20-turn flux transformer and the effective area of about 1.8 mm² exhibited a field noise of 25 fT/Hz^1/2 or less at 1 kHz.