Chiral density waves in the NJL$_2$ model with quark number and isospin chemical potentials

Abstract: We investigate the phase portrait of the (1+1)-dimensional massless two-flavored NJL$_2$ model containing a quark number chemical potential $\mu$ and an isospin chemical potential $\mu_I$ in the limit of a large number of colors $N_c\to\infty$. Particular attention is paid to the question to what extent the inclusion of an isospin asymmetry affects chiral condensates to have a spatial inhomogeneity in the form of the so-called chiral density waves (CDW) (chiral spirals). It is shown that at zero temperature and comparatively small values of $\mu$, i.e. at $\mu<\mu_c\approx 0.68M_0$ ($M_0$ is the dynamical quark mass in the vacuum) only the homogeneous charged pion condensation phase is realized for arbitrary nonzero values of $\mu_I$. Contrary to this, for large values of $\mu>\mu_c$, two CDW phases appear in the $(\mu_I,\mu)$-phase diagram of the model. In the first phase, CDWs are clockwise twisted chiral spirals and in the second phase they are counterclockwise. The influence of nonzero temperature on the formation of the CDW phases is also investigated.