Introduction

Although Y123 and Bi2212 both contain CuO2 planes and have similar values of Tc, there are notable differences which have affected the course of the study of each class of material. We have already mentioned two such differences, the reproducible cleavage of Bi2212 and the stability of the cleaved surface in ultrahigh vacuum. The cleavage plane(s) of Y123 is not really known and may vary from cleave to cleave. Moreover, a single plane is unlikely, and the atomic nature of the exposed surface may change at cleavage steps. Surface reconstruction, or at least relaxation, may be possible, even at low temperature. The surfaces of Y123 generally are not stable in ultrahigh vacuum except at temperatures below about 50 K. However, exceptions to this have been found by several groups, but the reasons for this stability are not yet known. Tc may be varied in Bi2212 by the addition or removal of oxygen and there is an optimum oxygen content at which Tc is a maximum. YBa2Cu3Ox also has variable oxygen stoichiometry, but between x = 6.8 and x = 7, Tc remains very close to its maximum value. In Bi2212 the oxygen content changes occur on or adjacent to the Bi–O planes. In Y123 the changes occur in the Cu–O chains, although the holes provided by the oxygen atoms are believed to reside on the CuO2 planes (Cava et al., 1990). Starting from x = 7, removing oxygen atoms from the chains should lower the hole concentration.