Enhancement of transition broadening by elastic strain in superconducting YBa2Cu3O7 and YBa2Cu3O7/PrBa2Cu3-xGaxO7 superlattices

The authors report measurements of surface impedance in YBa2Cu3O7 (YBCO) ultra-thin films and YBCO/PrBa2Cu3-xGaxO7 superlattices deposited by laser ablation on SrTiO3 with different thickness d of the YBCO sublayer. The authors find that the Tc is unchanged in relaxed or strained structures while the width follows two distinct regimes: it is narrow and slightly decreases with d in the former and is broad and follows the law Δ Tc ∼ √d in the latter. They propose a simple argument which explains these two regimes in terms of statistical fluctuations of the number of carriers respectively in single relaxed crystalline domains and in the whole strained sublayer