Anisotropy of the Lower Critical Field in YBa₂Cu₃O_7-δ

Abstract

The crystal structure¹ of YBa₂Cu₃O_7-δ is composed of chains and planes of Cu-O bonds which are arranged in layers perpendicular to the c direction. The one-dimensional chains are oriented along the b direction and are located at the top and bottom of the unit cell shown in Fig. 1. The planes are composed of a corrugated two-dimensional network of Cu-O bonds oriented along the a and b directions and located near the center of the unit cell just above and below the Y site. The metallic character of YBa₂Cu₃O_7-δ is provided by strong overlap between neighboring Cu and O ions in the chains and planes. There is almost no metallic character along the c direction because there are no oxygen atoms along the cell edges between the Cu2 sites. Thus the Cu-O plane just above the Y site does not overlap the plane just below the Y site. In the layer of chains there is virtually no metallic character in the a direction because there are no oxygen atoms between the Cul sites along a. The layer of chains and the layer of planes are connected by the bridging O4 site which overlaps both the neighboring Cul and Cu2 sites. However, this O4 is displaced considerably toward the Cul site so that it is bound much more strongly to the chains than to the planes. Thus, the coupling between the chains and planes is relatively weak, and the system may be thought of as composed of two nearly independent subsystems showing one- and two-dimensional character.²