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Anomalous dispersion of the collective modes of an ultracold $^6Li-$ $^{40}K$ mixture in a square optical lattice

We report numerical calculations of the collective excitation spectrum and the speed of sound of the superfluid phase of an atomic Fermi-Fermi mixture of population-imbalanced Lithium-6 and Potassium-40 atoms in a square lattice. It is predicted that in the exotic states of matter, known as the Fulde-Ferrell phase, an anomalous dispersion of the collective modes may be realized at some values of polarization, interacting strength and temperature, i.e. the collective-mode dispersion initially bends upward before bending over as the quasimomentum increases.