Conventional proximity effect in bilayers of superconducting underdoped $La_{1.88}Sr_{0.12}CuO_4$ islands coated with non superconducting overdoped $La_{1.65}Sr_{0.35}CuO_4$

Abstract

Following a recent study by our group in which a large $T_c$ enhancement was reported in bilayers of the non-superconducting $La_{1.65}Sr_{0.35}CuO_4$ and superconducting $La_{1.88}Sr_{0.12}CuO_4$ films [Phys. Rev. Lett. \textbf{101}, 057005 (2008)], we checked if a similar effect occurs when superconducting $La_{1.88}Sr_{0.12}CuO_4$ islands are coated with a continuous layer of the non superconducting $La_{1.65}Sr_{0.35}CuO_4$. We found that no such phenomenon is observed. The bare superconducting islands film behaves as an insulator where transport occurs via hopping or tunneling between islands, but exhibits a weak signature of localized superconductivity at $\sim$18 K. When over coated with a $La_{1.65}Sr_{0.35}CuO_4$ film, it becomes superconducting with a $T_c$ onset of 16.6 K, which is less than that of a thick $La_{1.88}Sr_{0.12}CuO_4$ film (25.7 K). We therefore conclude that the lower $T_c$ in the bilayer is due to a conventional proximity effect.