Temperature and electric-field dependencies of PBCO c-axis resistivity in YBCO/PBCO/Au structures

The current-voltage characteristics of YBCO/PBCO/Au planar structures reflect the resistance behavior of PBCO in the c-axis direction. With increasing applied voltages the PBCO barrier shows a transition from thermally-activated hopping conductivity to an activationless-hopping regime. Variable-range hopping and weak-localization models are discussed to explain the experimental data. An account of the Lifshitz correlation in the hopping conductivity gives an satisfactory agreement with the junction resistivity for c-axis PBCO barrier thicknesses of 10 to 40 nm

Electric-field effect devices made of YBa2Cu3O7-x/SrTiO3 epitaxial multilayers

Three terminal superconducting electric-field effect devices, consisting of a bufferlayer of PrBa2Cu3O7-x, an ultrathin layer of YBa2Cu3O7-x and a SrTiO3 gate isolation layer were fabricated and successfully operated. Transmission electron microscopy and laser scanning microscopy showed that all layers are highly epitaxial and uniform over the device area. This is essentially important in the analysis of the mechanism of the electric field effect and for the reproducible fabrication of devices. With a 5 nm thick YBa2Cu3O7-x layer and an applied electric field of 0.85 MV/cm the critical current was decreased by 5% at low temperatures and up to 36% close to Tc. Also enhancement was obtained

Influence of PrBa2Cu3-xGaX07 barrier material on electrical behaviour of ramp-type Josephson junctions

The use of PrBa2Cu3-xGaxO7 barrier material with increased resistivity by Ga doping (x=0; 0.05 and 0.1) in ramp-type DyBa2Cu3O7/PrBa2Cu3-xGaxO7/DyBa2Cu3O7 Josephson junctions has been investigated. All junctions have been fabricated with very smooth sputtered films and show good RSJ-like I-V characteristics with clear Josephson behaviour. Both critical current Ic and normal state resistance Rn are influenced by the doping level as well as the barrier thickness. The temperature dependence of the normal state resistance at different Ga doping levels and barrier thicknesses will be discussed

Quasiparticle-injection effect in YBa2Cu3Ox-based planar structures

The supercurrent IS of a YBCO bridge can be modulated by the quasiparticle-injection current IG from YBCO/Au or YBCO/PBCO/Au junctions. The behavior of these structures is determined by two effects: 1) summation of the currents IS and IG in the YBCO bridge; 2) nonequilibrium suppression of the supercurrent IS by the quasiparticle-injection. The current gain coefficient ¿IS/¿IG increases linearly with decreasing temperature, reaching a value of 1.5 for YBCO/Au structures at 65 K. The nature of the nonequilibrium state and the effectiveness of the PBCO barrier layer for the formation of the quasiparticles are analyze

Electric field effect on epitaxial YBa₂Cu₃O_7−x thin films

By applying a strong electric field perpendicular to the surface of an ultrathin, highly uniform epitaxial YBa2Cu3O7¿x film, the critical current was depressed and enhanced over 20% at temperatures close to Tc, and 5% at lower temperatures. Careful analysis of the electric field dependent I-V characteristics and Arrhenius plots indicate that the electric field effect can be interpreted as a change in the pinning potential and/or the vortex-antivortex interaction potential in the Kosterlitz-Thouless regime, as a result of a change in the carrier density in the superconductor

Electrical resistivity of PrBa2Cu3-xGaxO7-y (001) and (105) oriented thin films

In the past almost all studies on the anisotropy of the transport properties in 1-2-3 materials were performed on single crystals. This study is focused particularly on the anisotropy of the specific resistivity p as measured on almost single domain thin films of PrBa2Cu3-xGaxO7-y. Gallium doped PrBa2Cu3O7-y was deposited on (305) SrTiO3 to obtain (105) oriented, almost single domain thin films [1]. The films are deposited by rf magnetron sputtering in a one-step process, at low deposition rate. A relatively simple route for the preparation of single-phase gallium doped PrBa2Cu3O7-y target material by a citrate synthesis and pyrolysis [2] is presented

Quasiparticle injection effects in YBa2Cu3Ox-based planar structures at high operating temperatures

The modulation of the supercurrent Is of a YBCO bridge by the quasiparticle-injection current IG from the YBCO/Au or YBCO/PBCO/Au junctions at temperatures of 60¿85 K is determined by two effects: (1) summation of the currents IS and IG in the YBCO bridge, and (2) nonequilibrium suppression of IS by the quasiparticle injection. At a thickness of the PBCO barrier of 40 nm the modulation of IS can be described by the current-summation effect only. For YBCO/Au structures the current gain ¿IS/¿IG increases linearly with decreasing temperature, reaching the value of 2 at 60 K. Numerical simulations of the current- voltage characteristics show an increase of the effective temperature T* of the YBCO bridge under injection only at small thicknesses of the PBCO barrier. Visualization of the voltage drop over the junction area by laser scanning microscopy shows a qualitative agreement with the electrical measurements with respect to the current summation and nonequilibrium effects

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

A study of the YBCO/PBCO/YBCO ramp junctions with and without PBCO barrier shows that the Josephson and normal state behavior of these structures are determined by the thickness of the PBCO barrier and its nature. The boundary resistance and depression of the YBCO superconducting parameters near the interface do not strongly affect the junction characteristic. For thicknesses of 8 to 20 nm of the PBCO barrier the Josephson coupling is established through the high resistive barrier and the behavior of the junctions is better described by a SNINS model than by a SNS weak link model. Proximity effect, resonant tunneling and strong pair breaking mechanisms are discussed to explain the experimental characteristics. Good agreement with the experimental dependence of the IcRn product on the temperature and on the PBCO barrier thickness was obtained if a strong pair breaking mechanisms in the barrier is taken into account