  We use holography to compute spectral functions of certain fermionic
operators in three different finite-density, zero-temperature states of ABJM
theory with a broken U(1) symmetry. In each of the three states, dual to
previously studied domain wall solutions of four-dimensional gauged
supergravity, we find that the fermionic operators have gapped spectra. In one
case the gap can be traced to the small charge of the fermions, while in the
other cases it is due to a particular interaction that mixes particles and
holes.
