Condensed-Matter Physics Seminar

A short overview of recent results on the ballistic transport in nanostructured ferromagnet/superconductor/ferromagnet (FISIF) and superconductor/ferromagnet/superconductor (SIFIS) double-barrier junctions with arbitrary interface (I) transparency will be presented.

In clean FISIF structures two characteristic features of finite size and coherency are the subgap electronic transport and oscillations of the differential conductance. Periodic vanishing of the Andreev reflection at the energies of geometrical resonances above the superconducting gap is a striking consequence of the quasiparticle interference, which could be applied for the ballistic spectroscopy of superconductors. In contrast with the case of incoherent transport, a non-trivial spin polarization without the excess spin accumulation is found for the antiparallel alignment of the electrode magnetizations.

For SIFIS double junctions with finite interface transparency spin split quasibound states amplify the supercurrent and trigger the transitions between 0 and p states. Instead of the critical current reaching a peak value (which happens when the Andreev states approach the Fermi surface in SINIS junctions), a narrow dip opens up exactly at the peak due to the competition of partial currents flowing in opposite directions. This is related to the spin polarization of the Andreev bound states and explains the coexistence of stable and metastable 0 and p states in the vicinity of the transition. Large coexistence region in ballistic junctions can be exploited in the design of a p SQUID which operates as a typical device with an effectively two times smaller flux quantum. Temperature-induced transitions both from 0 to p and from p to 0 states are studied by computing the phase diagram (temperature vs. junction width) for different interfacial transparencies varying from transparent to the tunnel limit.