Strongly correlated fermions on superlattices: Magnetism and superconductivity

We consider fermions in one-dimensional superlattices (SL's), modelled by site-dependent Hubbard-U on-site couplings, and use Lanczos diagonalization to investigate their properties. The "magnetic" SL is made up of a repeated pattern of repulsive (i.e., U > 0) and free (U = 0) sites. Depending on the band-filling: Spin-Density-Wave correlations are wiped out for some patterns, while they are enhanced for others. Also, below half-filling, the local-moment maxima are displaced from the repulsive to the free sites; the peak in the magnetic structure factor is also shifted from q = pi, for the alternating configuration when rho = 1/3 and 5/3. The "superconducting" SL, on the other hand, consists of a repeated pattern of attractive (i.e., U < 0) and either free or repulsive sites. We find that some patterns give rise to ground states with dominant superconductivity, others with spin-density-wave (SDW), and others with coexistence of both; some ground states have no dominant correlations. A systematic study of the behavior with band filling and layer thickness reveals a sensitive dependence of these properties with the superlattice configuration. while coexistence is more frequent above half-filling, superconductivity is more frequent below. Further, since in the case of coexistence pairing occurs predominantly on the free sites, the substitution of repulsive for free sites wipes out superconductivity in these cases.