By Johan Chang, University of Zürich
Based on article published in Nature Communications
Exposed to pressure, the lattice parameters of a material generally shrink. In turn, the electronic nearest neighbour hopping integral t increases, due to larger orbital overlap. In a Mott insulator this enhancement can trigger a bandwidth-controlled insulator-to-metal transition. Indeed, the ratio, U/t, of the electron-electron (Coulomb) interaction U and the hopping t may be driven below its critical value. This premise has led to prediction of a pressure-induced insulator-to-metal transition in hypothetical solid hydrogen. Experimentally, pressure-induced metallisations have been realised, e.g., in NiS2 and organic salts. However, besides its impact on the bandwidth, pressure also influences (in a complex fashion) the electron-electron interaction U—an effect that has received little attention so far. The fate of Mott insulators exposed to external pressure therefore remains an interesting (and unresolved) problem to consider.
In the case of layered copper-oxide materials (cuprates), superconductivity emerges once the Mott insulating state is doped away from half-filling. In fact, it is commonly believed that the Mott state is a precondition for cuprate high-temperature superconductivity. While the optimal doping has been established for all known cuprate systems, the ideal configuration—for superconductivity—of the parent Mott state has not been identified. Typically, it is reported that hydrostatic pressure has a positive effect on T. However, the microscopic origin of this finding remains elusive. In particular, how pressure influences the local Coulomb interaction U and the inter-site magnetic-exchange interaction—to lowest order—J = 4t2/U, is an unresolved problem.
We have carried out a combined x-ray absorption spectroscopy (XAS) and resonant inelastic x-ray scattering (RIXS) study of the La2CuO4 Mott insulating phase. By straining thin films, the crystal field environment, as well as the energy scales t and U that define the degree of electronic correlations, was tuned. In stark contrast to predictions for elementary hydrogen and observations on standard Mott insulating compounds, we demonstrate that U/t remains approximately constant with in-plane strain. In La2CuO4, both U and t are increasing with compressive strain. In-plane strain is therefore not pushing La2CuO4 closer to the metallisation limit. Instead, strain enhances the stiffness, i.e., the exchange interaction J. For superconductivity, originating from the anti-ferromagnetic pairing channel, the exchange interaction is a key energy scale. Our study demonstrates how Jeff can be controlled and enhanced with direct implications for the optimisation of superconductivity.