Wednesday, July 10, 2013

1307.2242 (Chen-Yen Lai et al.)

d_{xy}-density wave in fermion-fermion cold atom mixtures    [PDF]

Chen-Yen Lai, Wen-Min Huang, David K. Campbell, Shan-Wen Tsai
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 {\pi}-phase difference between them to minimize the inter-species repulsion. Upon moving one species away from half-filling, an unconventional density wave with d_{xy}-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 d_{xy}-density wave phase based on a four-patch model. We find a robust and general mechanism for d_{xy}-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 phase- space discrepancy for different inter-species 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 intra-species Umklapp couplings, and this triggers the d_{xy}-density-wave instability.
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