Y.M. Boguslavskij

YBa2Cu3Ox/PrBa2Cu3Ox/YBa2Cu3Ox Josephson ramp junctions

A detailed study of the fabrication process, current voltage (I‐V) characteristics, and Josephson and normal‐state properties of the YBa2Cu3Ox(YBCO)/PrBa2Cu3Ox(PBCO)/YBCO ramp junctions is presented. The I‐V characteristics can be well described by the resistively shunted junction model. It was found that the critical current Ic and the normal‐state conductance 1/Rn scale linearly with the junction area, whereas Ic, the excess current Iex, and IcRn products decrease with increasing barrier thickness. These junctions with cross‐sectional area A have a good controllability, low capacitance, and high values of IcRn and RnA products. The coherence length ξn 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 the microwave power as well as the modulations of critical current Ic(H) with applied magnetic field are shown. A modulation depth of more than 95% has been observed. Small proximity effect parameters and jun...

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.

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.

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 and reduction of I/sub c/R/sub n/ product in YBa/sub 2/Cu/sub 3/O/sub x//PrBa/sub 2/Cu/sub 3/O/sub x//YBa/sub 2/Cu/sub 3/O/sub x/ ramp-type Josephson junctions

The mechanisms of current passage and the causes of I/sub c/R/sub n/ (critical-current normal-resistance) product reduction of YBCO/PBCO/YBCO ramp-type junctions are analyzed. At PBCO (PrBa/sub 2/Cu/sub 3/O/sub x/) barrier thicknesses L=8-20 nm the junction characteristics are determined by the thickness of the PBCO barrier and its nature. The boundary resistance and depression of the YBCO (YBa/sub 2/Cu/sub 3/O/sub x/) superconducting parameters near the interface do not strongly affect the junction parameters. The behaviour of the YBCO/PBCO/YBCO junctions cannot be described by simple SNS (superconductor-normal metal-superconductor) weak-link or SIS (superconductor-insulator-superconductor) tunnel models. A strong pair-breaking effect and a one-center inelastic tunneling process are taken into account to explain the Josephson and normal state characteristics of these junctions.<<ETX>>

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 analyzed

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.

Multilayers for high-Tc superconducting electric field effect devices☆

Epitaxial multilayers, consisting of a PrBa2Cu3O7-x buffer layer, ultrathin YBa2Cu3O7-x and SrTiO3, have been grown for application in electric field effect devices. Different analytical techniques indicate a sharp interface between the layers and good dielectric properties of the SrTiO3-layer. First measurements show clear modification of the superconductors current-voltage characteristics upon applying electric fields of 0.1-1 MV/cm.

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.

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.