Spin filtering in a quantum ring with Rashba coupling

We study the effect of Rashba spin-orbit coupling on the spin interference in a non-interacting one-dimensional ring connected to two lead theoretically within the non-equilibrium Greens´ function formalism. We compute the charge and spin currents and analyze their Aharonov-Bohm oscillations. The geometry of the system is conveniently described by the angle ¿ between the two leads. We show that for ¿=180° (i.e for symmetrically coupled leads) a good filtering of up or down spin orientation is obtained around half-integer multiples of ¿/¿₀. These particular flux values correspond to degeneracy points for clockwise and counter-clockwise propagating state related to the same spin orientation in the local spin frame of the ring. In contrast, for the asymmetric coupling, i.e., ¿=135° the filter efficiency is maximum around integer multiples of ¿/¿₀. The numerical results suggest that the spin filtering is obtained when the clockwise or counter-clockwise states interfere destructively. The spin filtering regime is stable against variations of the bias applied on the system.