Strain-controlled band engineering and self-doping in ultrathin LaNiO3 films

We report on a systematic study of the temperature-dependent Hall coefficient and thermoelectric power in ultrathin metallic LaNiO3 films that reveal a strain-induced, self-doping carrier transition that is inaccessible in the bulk. As the film strain varies from compressive to tensile at fixed composition and stoichiometry, the evolution of the transport coefficients is strikingly similar to those of bulk hole-doped superconducting cuprates with varying doping level. Density functional calculations reveal that the strain-induced changes in transport properties arise from changes in the low-energy electronic band structure that induce self-doping, a transfer of charge between O p and Ni d states. The results suggest that thin-film epitaxy can serve as a means to vary the charge-carrier concentration in other (negative) charge-transfer gap transition-metal oxides without resorting to chemical substitution.