Absence of gross inhomogeneity in HTS cuprates

The possible importance of inhomogeneity in thephysics of high temperature superconducting (HTS)cuprates has been stressed since the earliest investiga-tions of these materials. Of necessity they involve chemi-cal disorder as substitutional dopants in the parent com-pounds are usually required to introduce carriers. Fre-quently observed forms of inhomogeneity include substi-tutional, interstitial, reconstructive, twinning and incom-mensuration [1–4]. Despite this potentially high degreeof structural and compositional disorder (which is quitevariable from one compound to another) the cupratesdisplay universal phase behaviour [5–7]. Their physi-cal properties vary systematically with doping in verymuch the same way from one compound to another pro-vided simply that the substitutional disorder lies out-side of the active CuO