J.-M. Triscone

A-Axis--Oriented YBa2Cu3O7/PrBa2Cu3O7 Superlattices.

A modulated structure has been fabricated from high transition temperature superconductors where the individual CuO2 planes are composed of alternating superconducting and insulating strips. This structure is made by growing a-axis—oriented YBa2Cu3O7/PrBa2Cu3O7 superlattices by 90� off-axis sputtering on (100)SrTiO3 and (100)LaAlO3 substrates. Superlattice modulation is observed to a modulation wavelength of 24 angstroms (12 angstroms—YBa2Cu3O7/12 angstroms—PrBa2Cu3O7), both by x-ray diffraction and by cross-sectional transmission electron microscopy. Rutherford backscattering spectroscopy indicates a high degree of crystalline perfection with a channeling minimum yield of 3 percent. Quasi—one-dimensional conductivity should be obtainable in these structures.

Epitaxial Piezoelectric Pb(Zr0.2Ti0.8)O3 Thin Films on Silicon for Energy Harvesting Devices

We report on the properties of ferroelectric Pb(Zr0.2Ti0.8)O3 (PZT) thin films grown epitaxially on (001) silicon and on the performance of such heterostructures for microfabricated piezoelectric energy harvesters. In the first part of the paper, we investigate the epitaxial stacks through transmission electron microscopy and piezoelectric force microscopy studies to characterize in detail their crystalline structure. In the second part of the paper, we present the electrical characteristics of piezoelectric cantilevers based on these epitaxial PZT films. The performance of such cantilevers as vibration energy transducers is compared with other piezoelectric harvesters and indicates the potential of the epitaxial approach in the field of energy harvesting devices.

A-axis-oriented YBa2Cu3O7/PrBa2Cu3O7 superlattices : growth and transport properties

Recent results on the growth of a-axis-oriented YBa2Cu3O7/PrBa2Cu3O7 superlattices and their transport properties are reported. The films have extremely smooth surfaces and a high crystalline quality as revealed by X-ray, transmission electron microscopy and Rutherford-backscattering spectroscopy analysis. When a magnetic field is applied in the plane of the substrate, it is found that the resistive transitions of superlattices containing individual YBa2Cu3O7 layers 24 or 48 A thick separated by PrBa2Cu3O7 layers 24 or 48 A thick show a cross-over at a temperature T∗ which depends on the PrBa2Cu3O7 thickness; above T∗ the transition is insensitive to magnetic fields up to 8 T; below T∗ some broadening occurs. These results are related to an abrupt disappearance of the coupling between the YBa2Cu3O7 channels in the structure above T∗.

Vertical transport properties of A-axis oriented YBa2Cu3O7/PrBa2Cu3O7/YBa2Cu3O7 sandwich junctions

Abstract We report on the conduction mechanism in PrBa 2 Cu 3 O 7 thin films and on the transport properties of a-axis oriented YBa 2 Cu 3 O 7 /PrBa 2 Cu 3 O 7 /YBa 2 Cu 3 O 7 (YBCO/PBCO/YBCO) junctions. We find that the resistivity of a- and c-axis thin PBCO films is proportional to exp(T −1 4 ) which suggests that these materials exhibit Mott variable range hopping. For the junctions one finds two different types of behavior: either supercurrent-low resistivity junctions or no-supercurrent-high resistivity junctions. The supercurrent samples typically had barrier resistivities two to three orders of magnitude lower than the resistivity measured on single PBCO films suggesting that the supercurrents observed come from shorts in the barrier.

Verification of the Takahashi-Tachiki effect using multilayers consisting of Nb and Nb0.6Ti0.4

Abstract We observe, in Nb/Nb0.6Ti0.4 multilayers, the predicted anomalous upturn in the parallel upper critical field which reflects the shift of the nucleation field from the clean Nb layers to the dirty NbTi layers. The temperature at which the upturn occurs increases with modulation wavelength in accord with theory. We also present critical fields as a function of angle.

Investigations of coupled DyBa2Cu307-(Y1-xPrx)Ba2Cu307 multilayer structures

We report on the behavior of high Tc superconductor coupled multilayer structures in magnetic field perpendicular to the ab plane. In these structures the number N of superconducting layers in the sample has been systematically varied. We extract the activation energy for flux motion from resistive measurements of the superconducting transitions. For samples containing N 24 angstroms thick DyBa2Cu3O7 layers, each separated from the next by 96 angstroms of an (formula available in paper) alloy, we find that the activation energy U increases linearly with the number N of layers for N < 3 - 4, and then saturates. The linear increase of U is taken as an evidence that coupling through (formula available in paper) increases the correlated volume involved in the flux motion and thus U. For N equals 3 stacked pancake vortices move rigidly and the vortex lattice is 2D. For N > 3 - 4 shear of the vortex lattice becomes important, the vortex lattice dimensionally crosses-over to 3D, and the activation energy saturates.

Transport properties of a-axis YBa2Cu3O7/PrBa2Cu3O7 superlattices

Abstract We have measured the resistive transition of a-axis oriented YBa 2 Cu 3 O 7 /PrBa 2 Cu 3 O 7 superlattices in magnetic fields. We find for fields applied parallel to the substrate (i.e. perpendicular to the CuO 2 planes) that 80% of the transition of 48A/48A multilayer was unaffected by magnetic fields up to 8T. For a 24A/24A multilayer we observe that only the upper 30% part of the transition is insensitive to the magnetic field. Those results show that no substantial screening currents develop in the CuO 2 planes above the crossover implying that each plane is divided into independent superconducting channels separated by insulating channels where the respective channels are determined by whether the rare earth neighbor is Pr or Y.

Fabrication and Superconducting Propertiesof YBa 2 Cu 3 O 7 /PrBa 2 Cu 3 O 7 Artificial Superlattices

YBa2Cu3O7/PrBa2Cu3O7 (YBCO/PrBCO) superlattices grown by single target dc planar magnetron sputtering are used as a model system to investigate flux motion in high Tc superconductors. In parallel field (parallel to the a-b plane) we find for multilayers containing thin individual YBCO layers ( 24A). These results allow the estimation of the correlation length for YBCO along the vortices to be about 450 A.