Sophie Beck

We reveal a mechanism that may explain the nonmetallic out-of-plane resistivity in layered metals. By carefully examining how the Drude-Boltzmann expression for the c -axis conductivity emerges out of the Kubo formula, we find, besides the standard metallic term proportional to the carrier lifetime τ , a non-Drude contribution proportional to 1 / τ . The Drude behavior breaks down when 1 / τ > 2 η * , the crossover value η * being small (and hence observable) when the c -axis velocities vary rapidly with the distance from the Fermi surface. We consider the Hund metal Sr 2 RuO 4 as a test case, which we study within a realistic dynamical mean-field theory approach. The non-Drude behavior observed experimentally in c -axis transport is reproduced and explained by our considerations, showing that earlier invoked extrinsic mechanisms that involve either impurities or phonons are unnecessary. Although the small value of η * arises from a peculiar accidental cancellation due to destructive interference, characteristic of body-centered tetragonal lattices, we emphasize and discuss the generality of this mechanism.

As an alternative platform to unravel some of the perplexing characteristics of strontium ruthenate, we propose to study the isostructural and more correlated material strontium ferrite. Using density functional theory combined with dynamical mean-field theory, we attribute the experimentally observed insulating behavior at zero pressure to strong local electronic correlations generated by Mott and Hund's physics. At high pressure, our simulations reproduce the reported insulator-to-metal transition around 18 GPa. Along with distinctive features of a Hund metal, the resulting metallic state is found to display an electronic structure analogous to that of strontium ruthenate, suggesting that it could exhibit similar low-energy properties.