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The discovery 1 of high-temperature superconductivity in copper oxides raised the possibility that superconductivity could be achieved at room temperature. But since 1993, when a critical temperature (T c) of 133 K was observed in the HgBa 2 Ca 2 Cu 3 O 8+ δ (ref. 2), no further progress has been made in raising the critical temperature through material design. It has been shown, however, that the application of hydrostatic pressure can raise T c—up to∼ 164 K in the case of HgBa 2 Ca 2 Cu 3 O 8+ δ (ref. 3). Here we show, by analysing the uniaxial strain and pressure derivatives of T c, that compressive epitaxial strain in thin films of copper oxide superconductors could in principle generate much larger increases in the critical temperature than obtained by comparable hydrostatic pressures. We demonstrate the experimental feasibility of this approach for the compound La 1.9 Sr 0.1 CuO 4, where we obtain a …