Factors Controlling Transport Properties of Interfaces in High- T c Superconductors
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ABSTRACT A comprehensive understanding of the transport properties of interfaces in high-Tb cuprates has been obtained by considering their microstructure, the possibility of bending of the electronic band structure in these materials, and the predominant d×2.y2 -symmetry of the order parameter in most high-Ta cuprates. These factors are of central importance for the critical current density and the normal state resistivity of grain boundaries and their dependencies on boundary misorientation and on applied magnetic and electrical fields. In addition, some of these factors play an important role for the transport properties of other interfaces involving high-Ta superconductors, such as superconductors-normal metal contacts. Based on the improved understanding of the mechanisms controlling interface transport properties, we have been able to meet a long-standing challenge in high-Ta superconductivity and have increased the critical current densities of grain boundaries by large factors, using appropriate doping. INTRODUCTION The electronic transport properties of interfaces in high-To superconductors are of central importance for numerous applications of these materials, varying from electronic devices to large-current carrying cables and tapes. Also in many fundamental studies on high-Ta cuprates interfaces play a crucial role. Understanding and control of the transport properties of such interfaces, e.g. grain boundaries and superconductor-normal metal contacts, is therefore of great relevance. For several reasons, the physics of interfaces in high-To superconductors is considerably richer and more complex than that of interfaces in the classical, metallic, low-Th superconductors. First, the high-Th cuprates are characterized by short coherence lengths •, due to which the boundary-conditions imposed on the pair-potential by interfaces, structural defects or impurities may cause a drastic reduction of the order parameter [1,2]. Second, important effects arise from the predominant dx2.y2 -symmetry of the order parameter for many high-Tc superconductors [3], as will be pointed out in more detail below. Third, the small carrier densities of the cuprates in combination with their large dielectric constants are expected to lead to significant bending of the electronic band structure and the formation of space-charge layers at interfaces, as we have discussed recently for the first time [4-6]. As the transport properties of the high-Ta cuprates depend sensitively on the carrier density, these space-charge layers have strong implications for the electronic properties of interfaces. Considering ways to diminish the detrimental effects of band-bending, we have been able by appropriately doping the high-Tc superconductors to enhance the critical current densities Jc of grain boundaries far beyond the hitherto established upper limits.
261 Mat. Res. Soc. Symp. Proc. Vol. 574 © 1999 Materials Research Society
In the following, first a brief overview will be given of structural and electronic properties of two types of inter
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