In-plane pressure effect in strainedYBa2Cu3O7/La1.85Sr0.15CuO4superlattices

Superconducting strained superlattices have been formed by epi- taxial stacking along the c axes of two kinds of unit layers, ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$ (YBCO) and ${\mathrm{La}}_{1.85}$ ${\mathrm{Sr}}_{0.15}$ ${\mathrm{CuO}}_4$ (LSCO). In these superlattices, the ab plane of YBCO is compressed and the c axis is elongated due to the LSCO layer, which has smaller lattice constant a than that of the YBCO layer. When the thickness of the YBCO layer is reduced, the superconducting transition temperature (${\mathit{T}}_{\mathit{c}\mathrm{end}}$) of YBCO gradually decreases. The decrease of ${\mathit{T}}_{\mathit{c}}$ is, however, much smaller in this system than that in ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$/${\mathrm{PrBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$ and ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$/(Nd,Ce${)}_2$ ${\mathrm{CuO}}_4$ superlattices. The ${\mathit{T}}_{\mathit{c}\mathrm{end}}$ for only two unit cells (2.4 nm) of the YBCO layers is as high as 72 K. The correlation between the ${\mathit{T}}_{\mathit{c}}$ and the in-plane strain along the ab axes suggests that the effect of pressure in changing the Cu-O bond length along the ${\mathrm{CuO}}_2$ plane of the superconducting YBCO layer is one of the important factors in raising the ${\mathit{T}}_{\mathit{c}}$ value.