Larkin-Ovchinnikov-Fulde-Ferrell state in two-color quark matter

Abstract

We explore the phase structure of two-color and two-flavor QCD in the space of the quark chemical potential ${\ensuremath{\mu}}_{\mathrm{q}}$ and the isospin chemical potential ${\ensuremath{\mu}}_{\mathrm{I}}$. Using a mean-field model we calculate the chiral and diquark condensates, $\ensuremath{\sigma}$ and $\ensuremath{\Delta}$, self-consistently. In weak coupling and in the chiral limit, we confirm the interval of the isospin chemical potential, $0.71{\ensuremath{\Delta}}_{0}<{\ensuremath{\mu}}_{\mathrm{I}}<0.75{\ensuremath{\Delta}}_{0}$, in which a single plane-wave Larkin-Ovchinnikov-Fulde-Ferrell (LOFF) phase is favored over isotropic superfluidity and normal quark matter. The LOFF window becomes slightly wider at high density. For stronger coupling with nonzero quark mass, which is relevant to currently available numerical simulations in lattice two-color QCD, the single plane-wave LOFF phase appears only at sufficiently high density. The prediction obtained for the LOFF region could be tested with lattice since we can prove that the present system is free from the fermion sign problem. We draw the energy landscape on which local minima corresponding to the isotropic superfluid phase and the LOFF phase and a local maximum corresponding to the gapless phase are manifest. Our results clearly illustrate the path from the unstable gapless phase down to the LOFF phase.