  We report the effects of epitaxial strain and deoxygenation on high quality
[YBa2Cu3O7-x (YBCO)_N / PrBa2Cu3O7 (PBCO)_5] (1000 A thick) superlattices, with
1 <= N <= 12 unit cells. High spatial resolution electron energy loss
spectroscopy (EELS) shows that strained, fully oxygenated YBCO layers are
underdoped. Irrespective of whether underdoping is induced by strain or
deoxygenation, X-ray diffraction analysis shows that Tc correlates directly
with the separation of the CuO2 bilayers.
