Direct Correlation between Tc and CuO2 Bilayer Spacing in YBa2Cu3O7-x
  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. 
M. Varela, D Arias, Z. Sefrioui, C. Leon, C. Ballesteros, S. J.   Pennycook, J. Santamaria