Collective modes in an imbalanced nodal-line semimetal

In this work, we investigate collective modes in a nodal line semimetal with two nodal lines that have opposite spin polarization in the presence of spin population imbalance. We find the components of the polarization operator, taking into account the electron-electron exchange interaction, and obtain the dispersion relations of collective modes for the bulk and surface states. There exist four modes in the bulk, among which one is gapless and other three are gapped. The gapless surface mode is sensitive to the boundary conditions.