  We propose to detect phonon and roton excitations in a two-dimensional
Bose-Einstein condensate with Raman-induced spin-orbit coupling by perturbing
the atomic cloud with a weak barrier. The two excitation modes can be observed
by moving the barrier along different directions in appropriate parameter
regimes. Phonon excitations are identified by the appearance of solitary waves,
while roton excitations lead to distinctive spatial density modulations. We
show that this method can also be used to determine the anisotropic critical
velocities of superfluid.
