Direct correlation between T_(c) and CuO_(2) bilayer spacing in YBa_(2)Cu_(3)O_(7-x)

We report the effects of epitaxial strain and deoxygenation on high quality [YBa_(2)Cu_(3)O_(7-x)(YBCO)_(N) /PrBa_(2)Cu_(3)O_(7)(PBCO)_(5)]_(1000 Å) superlattices, with 1 < N < 12 unit cells. High-spatialresolution electron energy loss spectroscopy 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 T_(c) correlates directly with separation of the CuO_(2) bilayers