Epitaxial-strain-induced insulator-superconductor transition in undoped and lightly doped La2CuO4

We have studied the epitaxial-strain effects on transport properties of pulsed-laser-deposited undoped and lightly doped ${\mathrm{La}}_{\mathrm{2}}{\mathrm{CuO}}_{\mathrm{4}}$ thin films. The epitaxial strain is controlled by using ${\mathrm{SrLaAlO}}_{\mathrm{4}}$ buffer layers of different thicknesses on ${\mathrm{SrTiO}}_{\mathrm{3}}$ substrates. The ${\mathrm{La}}_{\mathrm{2}}{\mathrm{CuO}}_{\mathrm{4}}$ films are insulating under tensile epitaxial strain, but when the strain is sufficiently compressive they become superconducting. Since the undoped and lightly doped ${\mathrm{La}}_{\mathrm{2}}{\mathrm{CuO}}_{\mathrm{4}}$ are known to be superconducting when interstitial oxygen is inserted, the epitaxial-strain-induced insulator–superconductor transition suggests a significant interplay between strain and oxygenation in the films.