Origins of bad-metal conductivity and the insulator–metal transition in the rare-earth nickelates

Abstract

For most metals, increasing temperature (T) or disorder hastens electron scattering. The electronic conductivity (σ) decreases as T rises because electrons are more rapidly scattered by lattice vibrations. The value of σ decreases as disorder increases because electrons are more rapidly scattered by imperfections in the material. This is the scattering rate hypothesis, which has guided our understanding of metal conductivity for over a century. However, for so-called bad metals with very low σ this hypothesis predicts scattering rates so high as to conflict with Heisenberga's uncertainty principle. Bad-metal conductivity has remained a puzzle since its initial discovery in the 1980s in high-temperature superconductors. Here we introduce the rare-earth nickelates (RNiO₃, R = rare-earth) as a class of bad metals. We study SmNiO₃ thin films using infrared spectroscopy while varying T and disorder. We show that the interaction between lattice distortions and Ni-O covalence explains bad-metal conductivity and the insulator-metal transition. This interaction shifts spectral weight over the large energy scale established by the Ni-O orbital interaction, thus enabling very low σ without violating the uncertainty principle.