Epitaxial strain effects on superconducting and transport properties of La_2-xSr_xCuO_4+δ

Epitaxial strain in La_2-xSr_xCuO_4+δ thin films (0 < x < 0.30) is controlled by using SrLaAlO₄ buffer layers of different thicknesses on SrTiO₃ substrates. We found that compressive epitaxial strain results in higher T_c for all the Sr concentrations. Better oxygenation by cooling the films in ozone/molecular oxygen mixture also leads to higher T_c. In undoped and lightly-doped ultrathin films, the samples are insulating under tensile strain, but superconducting when the strain is sufficiently compressive. We suggest that the epitaxial strain affects the insertion of interstitial oxygen, which is responsible for the observed effects. Hall measurements confirm the change in carrier density in films of different strain. The Hall angle also changes with epitaxial strain. The epitaxial strain dependence of the slope in the T² dependence of the cotangent of the Hall angle is different for underdoped and optimally-doped samples.