Pairing interaction from three-dimensional acoustic plasmon demon modes in Sr_2⁢RuO₄

We investigate the properties of the recently observed "demon" mode, a 3D acoustic plasmon, in Sr2⁢RuO4 with an emphasis on evaluating its role for the pairing interactions in this superconductor. The demon mode is a low-energy electronic excitation, and it has been suggested that it could contribute to a reduced Coulomb repulsion and even a possible attractive interaction between electrons. In this study we explicitly calculate the dynamically screened Coulomb interaction for Sr2⁢RuO4 by using a renormalized tight-binding band structure and the random phase approximation for the dielectric function. Although the focus here is on Sr2⁢RuO4, this material is considered mainly as a prototype system, having an observed demon mode, and our results should be considered as a guide for applications to other systems. Our calculations show that there are regions in (𝐪,𝜔) space where the Coulomb interaction becomes attractive. We find that although the demon mode is not capable of producing a total attractive electron-pairing interaction in Sr2⁢RuO4, it does contribute to a significant reduction in the Coulomb repulsion at the relevant pairing energy scale.