K. Joosse

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.

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

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.

Electric field effect on epitaxial YBa2Cu3O7-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.