Charge transport and quantum phase transitions in singlet-superconductor/ferromagnet/singlet-superconductor junctions

We study the Josephson current, I-J, in a junction consisting of two s-wave superconductors that are separated by a ferromagnetic barrier possessing magnetic and nonmagnetic scattering potentials, g and Z, respectively. We discuss the general dependence of I-J on g, Z, and the phase difference phi between the two superconductors. Moreover, we compute the critical current, I-c, for given g and Z, and show that it possesses two lines of nonanalyticity in the (g,Z) plane. We identify those regions in the (g,Z) plane where the Josephson current changes sign with increasing temperature without a change in the relative phase between the two superconductors, i.e., without a transition between the 0 and pi states of the junction. Finally, we show that by changing the relative phase phi, it is possible to tune the junction through a first-order quantum phase transition in which the spin polarization of the two superconductors' combined ground state changes from < S-z >=0 to < S-z >=1/2.