  The effects of planar hole content, p, on the temperature-dependent uniform
(q = 0) magnetic susceptibility, c(T), of polycrystalline Y1-xCaxBa2Cu3O7-d
compounds with 0.0 </= x </= 0.20 were investigated over a wide range of oxygen
deficiency (d). Systematic variations in c(T) were found with changing p. The
characteristic pseudogap temperature scale, Eg(p)/kB, was extracted from the
analysis of the c(T,p) data. We have found clear evidence that the pseudogap in
the quasi-particle spectral weight appears abruptly at a planar hole content of
p ~ 0.19, and that it grows rapidly with decreasing p.
