Strain-Engineering Mott-Insulating La$_2$CuO$_4$

TL;DR

This study shows that applying compressive strain to La$_2$CuO$_4$ thin films enhances key microscopic interactions, suggesting strain engineering of the parent Mott state as a route to higher superconducting transition temperatures.

Contribution

The paper demonstrates how strain can be used to tune Coulomb and magnetic interactions in La$_2$CuO$_4$ thin films, linking these parameters to optimal superconductivity conditions.

Findings

Compressive strain enhances Coulomb and magnetic-exchange interactions.

Largest $T_c$ correlates with increased nearest neighbor hopping, Coulomb, and exchange interactions.

Strain engineering of the parent Mott state can improve superconducting properties.

Abstract

The transition temperature $T_\textrm{c}$ 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$_{2-\mathrm{x}}$Sr$_\mathrm{x}$CuO$_4$ thin films, such substrates are sub-optimal and the highest $T_\textrm{c}$ 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 $T_\mathrm{c}$ 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 $T_\textrm{c}$ 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.