Strain-engineering Mott-insulating La(2)CuO(4)

The transition temperature T(c) of unconventional superconductivity is often tunable. For a monolayer of FeSe, for example, the sweet spot is uniquely bound to titanium-oxide substrates. By contrast for La(2−x)Sr(x)CuO(4) thin films, such substrates are sub-optimal and the highest T(c) is instead obtained using LaSrAlO(4). An outstanding challenge is thus to understand the optimal conditions for superconductivity in thin films: which microscopic parameters drive the change in T(c) and how can we tune them? Here we demonstrate, by a combination of x-ray absorption and resonant inelastic x-ray scattering spectroscopy, how the Coulomb and magnetic-exchange interaction of La(2)CuO(4) thin films can be enhanced by compressive strain. Our experiments and theoretical calculations establish that the substrate producing the largest T(c) under doping also generates the largest nearest neighbour hopping integral, Coulomb and magnetic-exchange interaction. We hence suggest optimising the parent Mott state as a strategy for enhancing the superconducting transition temperature in cuprates.