Characteristics of advanced YBa2Cu3Ox/PrBa2Cu3Ox/YBa2Cu3Ox edge type junctions

Abstract

This letter describes the scaling behavior and Josephson properties of improved YBCO/PBCO/ YBCO edge-type junctions. The critical current, normal-state resistance, and I& product scale with barrier thickness and junction area. The coherence length of the PBCO barrier is estimated to be between 5 and 8 nm. As unambiguous evidence of the Josephson behavior, the microwave response as a function of microwave power, as well as the current modulation with applied magnetic field, have been studied: well-developed Shapiro steps at 10 GHz have been observed, and the modulation of I,(H) shows Fraunhoferlike behavior with 95% Ic suppression.We have previously reported the fabrication of highquality epitaxial, all high-l', edge-type YBa&!usO, (YBCO)/PrBa&!u~O,(PBCO)/YBCO Josephson junctions,"' prepared with our modified off-axis sputtering technique.3s4 These junctions have shown Josephson behavior up to rather high temperatures, and dc SQUIDS with such junctions are promising for practical applications. However, the physical properties of these junctions are far from clear. For fabrication of really controllable and uniform junctions, these properties must be studied. The scaling behavior, intrinsic parameters, and magnetic field modulation, as well as the microwave response of our edge junctions, are presented here. the intrinsic nature of the PBCO might play a role.Our junctions show good uniformity in the scaling of the critical current and the inverse normal resistance with the junction area, defined as A = wXd/sin i) (w is the junction width, d is the thickness of the base electrode, and 6 is the angle of the junction edge). This is shown in Fig. 1 for junctions with the same barrier thickness. The values for the smallest junctions may be somewhat too low due to the larger influence of the damaged edge by ion etching. At any rate, Fig. 1 demonstrates uniform behavior with different junction areas.The dependence of J, on the-R;Rk product has -been investigated as well. The data shown in Fig. 3 are distributed along the line J,-(R&) -m, with m = 1.2. For SNS structures with L&, one would expect J,-(Rd) -2, whereas for superconductor/insulator/normal-metal/ superconductor (SINS) structures J,-(R&) -' (Refs. 5 and 6). The observed dependence differs from the predjction for SNS structures and more closely resembles that for SINS structures. On the other hand, the temperature dependence of the critical-current density,.-( 1 -T/T,)', is in good agreement with the SNS type model, as we reported earlier.' This discrepancy may 'be attributed to the interface resistance between the base elcctrode and PBCO barrier, and the influence of the speci:iic resistance of PBCO, which may be different for Lscn or Lci5rIn addition, the I,$, products have been found to scale nearly linearly with L, ranging from 0.8 to 5 mV. TG highest values obtained were 8 mV. The damage of the edge surface due to our ex situ process could also detri-A strong dependence of the critical current density J, on the PBCO barrier thickness L has been observed....

Transport processes in YBa2Cu3Ox/PrBa2Cu3Ox/YBa2Cu3Ox ramp type Josephson junctions

“…~ 5-8 nm, as we reported and discussed earlier [14,15]. The Josephson and normal state junction characteristics depend strongly on the interlayer boundary resistance, the thickness of the PBCO barrier layer and the physical nature of the PBCO barrier material.…”

“…This geometrical value is close to the value of the junction area determined from the modulation of the critical current in an applied magnetic field, as we reported earlier [ 14]. The normal state resistance R, was determined from the linear part of the current-voltage characteristics at high voltages V~ 10-30 mV.…”

“…Characterization of the YBCO/PBCO/YBCO junctions with different areas and barrier thicknesses [ 14,15 ] shows clear Josephson behavior in an applied magnetic field and under microwave irradiation as well as the uniformity of the junction parameters and their scaling with junction area. The fabrication process is controllable and reproducible for this type of all high-To junctions.…”

“…(PBCO) is a convenient semiconductor-like material which is isomorphic to the high-To superconductor (HTS) YBa2Cu3Ox (YBCO) and can be used as a barrier material. Our study of the YBCO/PBCO/YBCO multilayers and Josephson ramp junctions [10,[13][14][15] shows high quality crystal structures and interlayer boundaries as well as reproducible characteristics, including IcR, products up to 8 mV at 4.2 K, clear Josephson behavior and scaling of the junction parameters with the junction area. For better understanding of the Josephson effect in the YBCO/PBCO/YBCO ramp structures a more detailed study of the mechanisms of the current passage through this barrier layer is needed.…”

“…We have previously reported on the fabrication process of high quality epitaxial high-T~ ramp type YBCO/PBCO/YBCO Josephson junctions [ 10,15 ]. Briefly, all layers were deposited by a modified offaxis sputtering technique.…”