Paramagnetism in few-fermion clusters with repulsive interactions

We describe the possible states of four-fermion clusters loaded in one-dimensional optical lattices with an additional harmonic confinement when the interactions between particles of different spin are contact-like. We have found that, depending on the depth of the potential well, the wavelength of the laser, and the strength of the repulsive interactions, we can have ground states that have unpaired spins (paramagnetic), all the spins paired (diamagnetic) or a degenerate mixture of different configurations. In this last case, the cluster could be either a metal or a Mott insulator. The characteristics of the ground state can be inferred to some extent from the solutions of non-interacting Schrödinger equations for the same arrangements. (paper)