Epitaxial strain and superconductivity in La2-xSrxCuO4 thin films.

The report that T(c) was doubled in underdoped La2-xSrxCuO4 films under compressive epitaxial strain has stirred great interest. We show that such films are extremely sensitive to oxygen intake, even at very low temperature, with startling consequences including colossal lattice expansion and a crossover from semiconductor to metallic behavior. We can bring T(c) up to 40 K in La2CuO4 films on SrTiO3 substrates-without any Sr doping and under tensile strain. On LaSrAlO4 substrates, we reached T(c)=51.5 K, the highest so far in La2-xSrxCuO4.