Impact of Strain on the Superconducting Properties of Strongly Underdoped $La_{2-x}Sr_xCuO_4$ Thin Films

Using pulsed laser deposition we have grown films of $La_{2-x}Sr_xCuO_4$ with x in close vicinity of the superconductor-insulator transition, x=0.051 and x=0.048, on $SrLaAlO_4$ substrates, and of different thickness d (from 25 nm to 250 nm). The X-ray diffraction shows that for each d the films grow with variable degree of compressive in-plane strain, with the largest strain achieved in thinnest films. The resistivity measurements show strong enhancement of superconductivity with increasing strain, so that the onset of superconductivity at temperature as high as 27 K is observed. With increasing strain the character of resistivity changes from the insulating to metallic.