Manifestation of Instabilities in Nambu-Jona-Lasinio type models

We study a Nambu-Jona-Lasinio (NJL) type model with two-flavor quark matter in $\beta$-equilibrium. It turns out that the system develops instabilities in the dispersion relations of the diquark fields, i.e., the velocity squared $v^2$ becomes negative in a certain region of the electron chemical potential. The critical point is the same as that of the chromomagnetic instability. The results imply the existence of spatially inhomogeneous diquark condensates in the genuine vacuum. We also discuss gauge equivalent classes between the inhomogeneous diquark condensates and gluon condensates.