Conventional proximity effect in bilayers of superconducting underdoped La1.88Sr0.12CuO4 islands coated with nonsuperconducting overdoped La1.65Sr0.35CuO4

Following a recent study by our group in which a large T-c enhancement was reported in bilayers of the nonsuperconducting La1.65Sr0.35CuO4 and superconducting La1.88Sr0.12CuO4 films [Yuli , Phys. Rev. Lett. 101, 057005 (2008)], we checked if a similar effect occurs when superconducting La1.88Sr0.12CuO4 islands are coated with a continuous layer of the nonsuperconducting La1.65Sr0.35CuO4. 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 similar to 15-18 K. When overcoated with a La1.65Sr0.35CuO4 film, it becomes superconducting with a T-c onset of 16.6 K, which is less than that of a thick La1.88Sr0.12CuO4 film (25.7 K). We therefore conclude that the lower T-c in the bilayer is due to a conventional proximity effect.