We study the Josephson effect in short ballistic SINIS and SIFIS double-barrier junctions, consisting of clean superconductors (S), a normal metal (N) or ferromagnet (F), and insulating interfaces (I). For SINIS double-tunnel junctions, the critical Josephson current as a function of the junction width shows sharp peaks because of resonant amplification of the Andreev process when the quasi-bound states of the normal interlayer enter the superconducting gap and morph into phase-sensitive bound states. For SIFIS double-tunnel junctions the corresponding quasi-bound states are spin-split, they amplify the supercurrent less efficiently, and trigger transitions between 0 and π states of the junction. In contrast to SINIS junctions where the critical current reaches a peak value when the Andreev bound states cross the Fermi surface, here a narrow dip opens up exactly at the peak due to compensation of partial currents flowing in opposite directions. With increased barrier transparency, the described mechanism is modified by the broadening and overlap of quasi-bound states. Temperature-induced transitions both from 0 to π and from π to 0 states are studied by computing the phase diagram (with temperature and junction width as the variables) for different interfacial transparencies varying from transparent metallic to the tunnel limit.
We present a microscopic theory of coherent quantum transport through a superconducting film between two ferromagnetic electrodes. The scattering problem is solved for the general case of ferromagnet/superconductor/ferromagnet (FSF) double-barrier junction, including the interface transparency from metallic to tunnel limit, and the Fermi velocity mismatch. Charge and spin conductance spectra of FSF junctions are calculated for parallel (P) and antiparallel (AP) alignment of the electrode magnetization. Limiting cases of nonmagnetic normal-metal electrodes (NSN) and of incoherent transport are also presented. We focus on two characteristic features of finite size and coherency: subgap tunneling of electrons, 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. Also, the non-trivial spin-polarization of the current is found for FSF junctions in AP alignment. This is in contrast with the incoherent transport, where the unpolarized current is accompanied by excess spin accumulation and destruction of superconductivity. Application to spectroscopic measurements of the superconducting gap and the Fermi velocity is also discussed.
Assuming tunneling perpendicular to the layers, Andreev bound states are studied theoretically in atomic-scale S/F superlattices, where S is a conventional superconductor, and F is a ferromagnet. The nonmonotonic dependence of the critical current on the exchange field h in F layers is a consequence of (pi) -phase formation. The shape of the spectra of spin-splitted densities of states (DOS) depends strongly on h and on the transfer integral t between the layers; for an appropriate choice of these parameters zero-energy bound states (ZES) may appear. Considering in-plane tunneling in high-Tc S/F/S Josephson weak links with thin, ferromagnetic metal barrier, DOS is calculated assuming that the a-axes of the crystals in two S electrodes are misoriented for an angle (theta) . In the anisotropic d-wave case the number of peaks in DOS, corresponding to the Andreev bound states, can be greater than in the isotropic s-wave case. DOS strongly depends on h, (theta) , and on the macroscopic phase difference (phi) across the link.
We study theoretically the Josephson coupling in a ferromagnetic weak link between both isotropic s-wave, and layered d-wave superconductors. For strong ferromagnetic barrier influence, the conventional coupling is obtained, with ground state phase difference across the link 0 < (phi) gs <EQ (pi) . When the weak link is a part of a superconducting ring, this is accompanied by the flow of spontaneous supercurrent, of intensity which depends on the reduced inductance l equals 2(pi) LIc(T)/(Phi) 0, and is non-zero only for l greater than a critical value. For l >> 1, another consequence of the unconventional coupling is the anomalous quantization of the magnetic flux.
The quasiparticle excitation spectrum of an isolated vortex in a clean layered d-wave superconductor is calculated, using both the Bogoliubov-de Gennes and the quasiclassical Eilenberger equations. Analytical solutions are obtained within the model of step-variation of the gap function, adjusting the Caroli, de Gennes and Matricon approach for low- lying excitations in cuprates. A large peak in the density of states in the 'pancake' vortex core is found, caused by the two-dimensional and strong coupling nature of high-temperature superconductivity.
Exchange field effects and d-wave pairing in superconductors may have similar manifestations, as for example (pi) -phase states and spontaneous currents in Josephson devices. Influence of exchange field on the Josephson tunneling between d-wave superconducting electrodes through a ferromagnetic barrier is studied theoretically. Implications of combined exchange field and d-wave pairing effects are discussed in the context of the experimental verification of d-wave superconductivity.
Recent progress in the proximity effect theory of superconductor-ferromagnet superlattices is reviewed. The phase diagram calculations, transition temperature Tc and upper critical fields (formula available in paper), are presented. Characteristic features in Tc and Hc2(T) dependence on layers thicknesses, including the predicted unusual oscillatory variations and new inhomogeneous superconducting state with nontrivial phase difference between neighboring superconducting layers, are discussed and compared with experimental data for V/Fe and Nb/Gd superlattices.
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