  We study mass and population imbalance effect on density (in-phase) and spin
(out-of-phase) collective modes in a two-component normal Fermi gas. By
calculating eigenmodes of the linearized Boltzmann equation as well as the
density/spin dynamic structure factor, we show that mass and population
imbalance effects offer a variety of collective mode crossover behaviors from
collisionless to hydrodynamic regimes. The mass imbalance effect shifts the
crossover regime to the higher-temperature, and a significant peak of the spin
dynamic structure factor emerges only in the collisionless regime. This is in
contrast to the case of mass and population balanced normal Fermi gases, where
the spin dynamic response is always absent. Although the population imbalance
effect does not shift the crossover regime, the spin dynamic structure factor
survives both in the collisionless and hydrodynamic regimes.
