  We study the spectral function of fermions in a holographic set up with bulk
Dirac mass in the regime beyond the conformal unitarity bound, and find that
spectral function has the dispersion relation with tachyonic behavior,
indicating an instability. Based on linearity between the density and the
position of the tip of the k-gap, we suggest that this instability is toward
the charge density wave(CDW) and the position of the tip can be identified as
the wave vector of CDW. For the physical origin, we point out the similarity of
unitarity violation in our non-Fermi Liquid theory and nesting phenomena in the
Fermi liquid theory as the mechanism of CDW instability.
