Gaetano Annunziata

Spin-sensitive long-range proximity effect in ferromagnet/spin-triplet-superconductor bilayers

, calls for experimental methods to identify the presence of spin-triplet pairing. We here demon-strate a method which accomplishes this in an appealingly simple manner: a spin-sensitive proximity effect ina ferromagnetjtriplet superconductor bilayer. It is shown how the orientation of the field can be used to unam-biguously distinguish between different spin-triplet states. Moreover, the proximity effect becomes long-rangedin spite of the presence of an exchange field and even without any magnetic inhomogeneities, in contrast toconventional SjF junctions. Our results can be verified by STM-spectroscopy and could be useful as a tool tocharacterize the pairing state in unconventional superconducting materials.

Proximity effect with noncentrosymmetric superconductors

We describe the superconducting proximity effect taking place in a contact between a noncentrosymmetric superconductor and a diffusive normal/ferromagnetic metal within the quasiclassical theory of superconductivity. By solving numerically the Usadel equation with boundary conditions valid for arbitrary interface transparency, we show that the analysis of the proximity-modified local density of states in the normal side can be used to obtain information about the exotic superconductivity of noncentrosymmetric materials. We point out the signatures of triplet pairing, the coexistence of triplet and singlet pairing, and particular orbital symmetries of the pair potential. Exploiting the directional dependence of the spin polarization pair breaking effect on the triplet correlations, we show how the order relation between triplet and singlet gaps can be discriminated and that an estimation of the specific gap ratio is possible in some cases.

Charge and spin transport through a ferromagnet/insulator/unconventional superconductor junction

We analyze the charge and spin transport through a ballistic ferromagnet/insulator/superconductor junction by means of the Bogoliubov-de Gennes equations. For the ferromagnetic side we assume that ferromagnetism may be driven by an unequal mass renormalization of oppositely polarized carriers, i.e. a spin bandwidth asymmetry, and/or by a rigid splitting of up-and down-spin electron bands, as in a standard Stoner ferromagnet, whereas the superconducting side is assumed to exhibit a d-wave symmetry of the order parameter, which can be pure or accompanied by a minority component breaking time-reversal symmetry. Several remarkable features in the charge conductance arise in this kind of junction, providing useful information about the mechanism of ferromagnetism in the ferromagnetic electrode, as well as of the order parameter symmetry in the superconducting one. In particular, we show that when a time-reversal symmetry breaking superconductor is considered, the use of the two kinds of ferromagnet mentioned above represents a valuable tool to discriminate between the different superconducting mixed states. We also explain how this junction may mimic a switch able to turn on and off a spin current, leaving the charge conductance unchanged, and we show that for a wide range of insulating barrier strengths, a spin bandwidth asymmetry ferromagnet may support a spin current larger than a standard Stoner one.

Josephson effect in S/F/S junctions: Spin bandwidth asymmetry versus Stoner exchange

We analyze the dc Josephson effect in a ballistic superconductor/ferromagnet/superconductor junction by means of the Bogoliubov-de Gennes equations in the quasiclassical Andreev approximation. We consider the possibility of ferromagnetism originating from a mass renormalization of carriers of opposite spin, i.e. a spin bandwidth asymmetry. We provide a general formula for Andreev levels which is valid for arbitrary interface transparency, exchange interaction, and bandwidth asymmetry. We analyze the current-phase relation, the critical current, and the free energy in the short junction regime. We demonstrate that a larger number of

Field-induced transition from chiral spin-triplet to mixed-parity Fulde-Ferrell-Larkin-Ovchinnikov superconductivity

0-\pi

Magnetic Josephson junctions with noncentrosymmetric superconductors

transitions caused by a change in junction width or polarization magnitude is expected when ferromagnetism is driven by spin bandwidth asymmetry compared to Stoner magnetism, and that the same mechanism is likely responsible for the flowing of a larger critical current. We compare the phase diagrams of two identical junctions differing only in the mechanism by which the mid layer becomes magnetic, i.e. Stoner magnetism or bandwidth asymmetry. In particular, we point out that for these two cases, the phase difference across the junction in the ground state need not be the same, even if the polarizations are equal.