  We investigate the BCS-BEC crossover in three dimensional degenerate Fermi
gases in the presence of spin-orbit coupling (SOC) and Zeeman field. We show
that the superfluid order parameter destroyed by a large Zeeman field can be
restored by the SOC. With increasing strengths of the Zeeman field, there is a
series of topological quantum phase transitions from a non-topological
superfluid state with fully gapped fermionic spectrum to a topological
superfluid state with four topologically protected Fermi points (i.e., nodes in
the quasiparticle excitation gap) and then to a second topological superfluid
state with only two topologically protected Fermi points. The quasiparticle
excitations near the Fermi points realize the long-sought low-temperature
analog of Weyl fermions of particle physics. We show that the topological phase
transitions can be probed using the experimentally realized momentum resolved
photoemission spectroscopy.
