Enhancement of the superconducting transition temperature of La _2-x Sr_x CuO₄ and La_1.875 Ba _0.125 CuO₄ bilayers: Bilayer and reference film prepared on the same wafer

Recently, a paper was published by Yuli [Phys. Rev. Lett. 101, 057005 (2008)], in which a large T_c enhancement was reported in bilayers of the nonsuperconducting La_1.65 Sr_0.35 CuO₄ and superconducting La_2-x Sr_x CuO₄ in the underdoped regime, in comparison with the same bare La_2-x Sr _x CuO₄ film. This result however, was not reproduced in the present experiments and an effort is made trying to resolve this puzzle. The difference in the present study is that both the bilayer and the bare reference film were prepared in the same deposition run on different parts of the same wafer. This is in contrast to the previous experiments where the bilayers and bare reference films were prepared in different runs and on different wafers. Nevertheless, a small but clear T_c enhancement effect of 1.4-2 K is found in the present study in a similar system of La_1.65 Sr _0.35 CuO₄ -La_1.875 Ba_0.125 CuO ₄ bilayers, which is in line with the theoretical arguments presented previously.