Strain-induced inhomogeneity of the superconducting state in strongly underdoped La2−<i>x</i>Sr<i>x</i>CuO4 thin films

The correlation between film morphology and homogeneity of the superconducting (SC) state is studied in thin, underdoped films of La2−xSrxCuO4 with substrate-induced compressive strain, which enhances SC transition temperature, Tc. Imaging techniques are used to evaluate the film surface, while the resistance and magnetoresistance are measured and subsequently analyzed by effective medium theory (EMT). It is found that the thinnest films, with the thickness of about 10 nm, exhibit the smoothest surface, the largest build-in strain, and the highest onset of Tc. However, the EMT indicates that the SC state in these films is strongly inhomogeneous, and the magnetoresistance behavior resembles that observed for Josephson junction arrays. When the film thickness is increased up to 70–110 nm, the films become less smooth and the Tc enhancement is reduced; however, homogeneity of the SC state improves dramatically. Thus, homogeneity cannot be inferred solely from surface morphology. We suggest that strong inhomogeneity of the thinnest films results from spatial variability of unrelaxed build-in strain, likely accompanied by inhomogeneous distribution of carriers. In thicker films, partial strain relaxation occurs leading to the improved homogeneity of the SC state.