Fig. 1.
A simplified tetragonal representation of the crystal structure of Bi-2212. The actual material is orthorhombic, with a long-period modulation wave along the b axis that is incommensurate with respect to this simplified representation. The superconducting energy gap associated with the CuO2 planes has a modulation that matches this superlattice, and the size of the gap is conjectured to be anticorrelated with the distance between the copper atoms and the apical oxygen atoms (1). The microscopic origin of this anticorrelation effect could be an important key in deciphering the mystery of the high superconducting transition temperature seen in this material.