Theory of Helical Fermi liquids

We extend the Landau Fermi Liquid (FL) theory to include spin-orbit coupling (SOC). In particular, the Rashba SOC is chosen as an example. It is shown that although ``charge-part'' quantities, such as the charge susceptibility and effective mass, are determined solely by the quasi-particles, ``spin-part'' quantities, such as the spin susceptibility, have contributions from the damped states in between the two Fermi surfaces induced by the SOC. However, contributions to the lowest order in the SOC can still be extracted from the theory. The nature of the instabilities of such spin-orbit coupled FL is discussed.