Doping in bismuth sites of Bi-2212 ceramic superconductors

Abstract

The phenomenon of high-Tc superconductivity has been extensively studied but, until now, not very well understood. In the Bi₂Sr₂CaCu₂O₈ high-Tc ceramic superconductor, with superconducting critical temperature Tc of approximately 80 K, superconductivity is mainly attributed to the presence of holes which act as charge carriers within the two dimensional CuO₂ planes. Studies of band structure and density of carriers near the Fermi energy, E_F, however show the important role the Bi₂O₂ layer plays in the superconductivity of Bi₂Sr₂CaCu₂O₈ (Bi-2212). The main band features near the Fermi level include a pair of nearly half-filled two-dimensional Cu-O 3d-2p bands and a pair of slightly filled Bi 6p bands. The Bi₂O₂ layers provide additional hole charge carriers to the CuO₂ planes by draining its electrons and bringing it to the Bi₂O₂ layers. Electronic structure calculations show that BiO bands are coupled to the CuO₂ planes allowing the holes to pass from the Bi₂O₂ layers to the CuO₂ planes.
Doping of Sn and Zn in the bismuth sites of Bi-2212 affect the Bi₂O₂ layers by providing donor and acceptor states, respectively. To investigate the effects of the dopant states in the Bi-2212 system, ac magnetic susceptibility measurements were performed.