Effects of selective lattice deformation on YbBa2Cu4O8 and YBa2Cu3O7 epitaxial films.

Alternating current magnetic measurements of YbBa₂Cu₄O₈ (Yb-124) and YBa₂Cu₃O₇ (Y-123) epitaxial films were performed under hydrostatic pressure (HP). Here, the strain under HP results in uniaxial strain along the c-axis, in addition to the biaxial strain due to mismatching with the substrate. This uniaxial effect on Yb-124 film brings about a prominent increase in the superconducting transition temperature (T_c) against the initial strain. However, a nearly optimal Y-123 film hardly exhibits an initial change in T_c, and even an underdoped one only exhibits an increase in T_c, that is, one-third as much as that in the Yb-124 film. Effective carrier doping by the use of effective out-of-plane contraction prominently appears in the 124-structure rather than in the 123-structure. [ABSTRACT FROM AUTHOR]