O. Nakamura

High Tc thin films with roughness smaller than one unit cell

We have developed a method for the growth of epitaxial high Tc superconducting thin films with roughness smaller than one unit cell using conventional magnetron sputtering. In this method the substrate is positioned above one edge of the target (off axis) to avoid resputtering, and oscillated back and forth between the two symmetrical edges of the target to improve film thickness homogeneity. Finite size peaks in the x‐ray diffraction spectra of thin GdBa2Cu3O7−δ films and satellite peaks on a GdBa2Cu3O7−δ/YBa2Cu3O7−δ superlattice show the excellent thickness control and smoothness obtained with this technique.

Synthesis and properties of a‐axis and b‐axis oriented GdBa2Cu3O7−δ high Tc thin films

We report the growth and properties of a‐axis oriented GdBa2Cu3O7−δ high Tc thin films on (100) SrTiO3 substrates by dc magnetron sputtering. It is found that GdBa2Cu3O7−δ films on (100) SrTiO3 exhibit a‐oriented growth at higher substrate temperatures compared with YBa2Cu3O7−δ films. By utilizing low‐temperature‐grown a‐axis GdBa2Cu3O7−δ films (200 A in thickness) as a self‐template, pure a‐axis films can be grown at elevated temperatures. The growth of b‐axis film on vicinal (100) SrTiO3 under similar growth conditions is also reported.

Thickness dependence of the superconducting transition temperature of YBCO

Abstract We have performed detailed structural and superconducting studies of YBCO single layers, YBCO/PrBCO/YBCO tri-layers, and YBCO/PrBCO superlattices. The results for single layers indicate that a nominally one unit cell thick YBCO film sandwiched between PrBCO layers is not superconducting. However, a superlattice with nominally one unit cell thick YBCO layers has a measurable superconducting temperature onset ( T c o ). These apparently contradictory results are reconciled by experiments on YBCO/PrBCO/YBCO tri-layers which, due to coupling across the PrBCO insulating layer, exhibit superconductivity for PrBCO thicknesses below 200 A. Structural studies show that the length scale of the superconductive coupling is much larger than the length scale of the step disorder and interdiffusion present in the film.

Scaling of critical currents in high-temperature superconducting superlattices and thin films

We report the measurement of transport critical current densities in a wide variety of high‐temperature superconducting (high Tc) superlattices and thin films. We find that the temperature dependence of the critical current density Jc for all samples may be collapsed into a single curve through the scaling relation Jc(t)=Jc(0)(1−t)1.6, where t=T/Tc(H,Θ). This scaling relation remains valid in fields of up to 5 T and in all field directions Θ with respect to the c axis of the sample with the field kept perpendicular to the current.

New buffer layer for high‐temperature superconducting ceramics on sapphire: LaBa2Cu3Oy/Ag bilayers

We have grown highly oriented LaBa2Cu3Oy (110)/Ag (111) bilayers on sapphire (1120). This structure constitutes a potentially excellent buffer layer for the growth of 1‐2‐3 ceramic oxides on sapphire substrates. This bilayer does not show superconductivity in resistive measurements. A DyBa2Cu3Oy film grown on this bilayer also exhibited (110) orientation.

Interface structure in high-Tc superlattices

The authors have studied the structure of high-Tc superlattices and films through refinement of the X-ray spectra. The results indicate that interdiffusion, layer thickness fluctuations and strains are present and cannot be neglected.

Disorder and superconductivity in YBa2Cu3O7−δ/GdBa2Cu3O7-δ superlattices☆

Abstract The structure of c oriented YBa 2 Cu 3 O 7−δ /GdBa 2 Cu 3 O 7−δ superlattices is analyzed throug refinement of the X-ray diffraction spectrum. This novel technique, similar in spirit to the well known Rietveld refinement of powder X-ray diffraction spectra, allows quantitative determination of the interfacial roughness. The results show that for these superlattices the roughness consists in step disorder of one unit cell around the average thickness of the layer and interdiffusion of 15 to 25% between the Y and Gd sites within one unit cell from the interface.

Anisotropic pressure dependence of the critical temperature of REBa2Cu3O7−δ

Abstract The hydrostatic pressure dependence of the superconducting critical temperature, T c , was measured in highly crystalline oriented REBa 2 Cu 3 O 7−δ (RE=Yb, Dy, Y, Gd) thin films grown on different substrates. The anisotropic strain field induced by the substrate and the different crystalline orientations of the films allows the three strain derivatives of the critical temperature to be estimated. A large anisotropy is found in the ab plane with T c increasing on compression parallel to the CuO chains and decreasing on compression perpendicular to them. The results on c textured films indicate a non linear relation between T c and the microscopic parameter which determines its pressure dependence.