UC Riverside

We study density wave instabilities in a doubly degenerate Fermi-Fermi mixture with SU(2)×SU(2) symmetry on a square lattice. For sufficiently large on-site inter-species repulsion, when the two species of fermions are both at half-filling, two conventional (s wave) number density waves are formed with a π-phase difference between them to minimize the interspecies repulsion. Upon moving one species away from half-filling, an unconventional density wave with dxy-wave symmetry emerges. When both species are away from the vicinity of half-filling, superconducting instabilities dominate. We present results of a functional renormalization-group calculation that maps out the phase diagram at weak couplings. Also, we provide a simple explanation for the emergence of the dxy-density wave phase based on a four-patch model. We find a robust and general mechanism for dxy-density-wave formation that is related to the shape and size of the Fermi surfaces. The density imbalance between the two species of fermions in the vicinity of half-filling leads to a phase-space discrepancy for different interspecies umklapp couplings. Using a phase-space argument for leading corrections in the one-loop renormalization-group approach to fermions, we show that the phase-space discrepancy in our system causes opposite flows for the two leading intraspecies umklapp couplings and that this triggers the dxy-density-wave instability. © 2014 American Physical Society.