Cooper pair transport and macroscopic quantum dynamics in Josephson junctions through ferromagnetic insulators

We theoretically investigate the cooper pair transport and the macroscopic quantum dynamics of a Josephson junction with a superconductor (S) and a ferromagnetic insulator (FI). By use of the recursive Green’s function method, we numerically found the formation of the π junction in the case of the fully spin-polarized FI. On the other hand, the Josephson current through the spin filtering barrier does not show the π junction behaviors. By deriving the effective action from microscopic Hamiltonian, a π junction flux qubit is proposed. In this qubit, a quantum two-level system is spontaneously generated and the effect of the quasiparticle dissipation is found to be very weak. These features make it possible to realize a quiet qubit with high coherency.