Date of Award

9-29-2003

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

First Advisor

Tar-Pin Chen

Abstract

This dissertation described herein includes the fabrication of and experimental studies on the family of insulators PrBa2(Cu1−xM x)3O7−δ (PBCMO), with M = Al, Fe, Ni, Zn, Co, Ga, and x = 0, 0.05, 0.10, 0.15, and 0.20. The samples were grown by the solid state reaction technique. The purposes was to find a better buffer layer material for fabricating superconducting devices from an YBa2Cu 3O7−δ (YBCO) superconductor. An ideal buffer layer must have the same crystal structure as YBCO, with well-matched lattice parameters, and must also provide adequate electrical resistivity to serve as an insulating layer for YBCO/PBCMO multilayers. If this buffer layer is being used for a Josephson junction, it should be made very thin to provide large Josephson current. X-ray diffraction data revealed that the structure of the members of the PBCMO family is the same as that of YBCO and their lattice parameters are also very close to those of YBCO, while the four-probes resistivity data showed that these materials have much higher electrical resistivity than PrBa2Cu3O7 (PBCO), a material which was previously considered as the ideal buffer layer but failed to insulate YBCO layers in YBCO/PBCO multilayers. Laser ablation was used to fabricate expitaxial multilayers or superlattices of YBCO/PrBa2(Cu0.8Al0.2)O7 and YBCO/PrBa2(Cu0.8Ga0.2)O 7. The superlattices of different YBCO and/or PBCMO buffer layer thickness were made in order to explore the effect of PBCMO layer thickness on YBCO superconducting layer. Transport studies on the superlattices indicated that no superconductivity exists above 4.2 K for YBCO thickness of 12.5 Å (the thickness of a unit cell). The experiments showed that increasing YBCO thickness increases the superconducting transition temperature, T c. These allowed us to extract the superconducting coupling length of YBCO. The coupling length is determined by the thickness of YBCO at which Tc reaches a plateau, i.e., a saturation value. The numerical value obtained for the coupling length is about 150 Å ± 50 Å. It was also found that when the thickness of the buffer layers, PBCMO, in the superlattices was varied while the thickness YBCO films were kept constant, the transition temperature of the superlattices remains the same even when the PBCMO layer was made as thin as 12 Å. This result differs from other measurements reported in the literature in which Tc decreases with increasing the thickness of PBCO. This difference indicates that PBCMO is better buffer layer and is adequate to insulate YBCO films in the superlattice and to cut off the superconducting coupling between them. This suggests that PBCMO compounds can be better buffer materials than any other materials. PBCMO films may be the ideal insulating layer materials for superconductor-insulator-superconductor Josephson junctions and is the solution for high-Tc superconducting electronic devices.

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