  We argue that both pocket-disappearing and neck-disrupting types of Lifshitz
transitions can be realized in two-dimensional spin-density wave models for
underdoped cuprates, and study both types of transitions with impurity
scattering treated in the self-consistent Born approximation. We first solve
for the electron self-energy from the self-consistent equation, and then study
the low-temperature electrical conductivity and thermopower. Close to the
Lifshitz transition, the thermopower is strongly enhanced. For the
pocket-disappearing type, it has a sharp peak, while for the neck-disrupting
type, it changes sign at the transition, with its absolute value peaked on both
sides of the transition. We discuss possible applications to underdoped
cuprates.
