We consider novel unusual effects in superconductor-ferromagnet (S/F) structures. In particular we analyze the triplet component (TC) of the condensate generated in those systems.This component is odd in frequency and even in the momentum, which makes it insensitive to non-magnetic impurities. If the exchange field is not homogeneous in the system the triplet component is not destroyed even by a strong exchange field and can penetrate the ferromagnet over long distances. Some other effects considered here and caused by the proximity effect are: enhancement of the Josephson current due to the presence of the ferromagnet, induction of a magnetic moment in superconductors resulting in a screening of the magnetic moment, formation of periodic magnetic structures due to the influence of the superconductor, etc. We compare the theoretical predictions with existing experiments.

We analyze the proximity effect in a superconductor/ferromagnet (S/F) structure with a local inhomogeneity of the magnetization in the ferromagnet near the S/F interface. We demonstrate that not only the singlet but also the triplet component of the superconducting condensate is induced in the ferromagnet due to the proximity effect. Although the singlet component of the condensate penetrates into the ferromagnet over a short length ξ h = D/h (h is the exchange field in the ferromagnet and D the diffusion coefficient), the triplet component, being of the order of the singlet one at the S/F interface, penetrates over a long length D/ǫ (ǫ is the energy). This long-range penetration leads to a significant increase of the ferromagnet conductance below the superconducting critical temperature Tc.In recent experiments on S/F structures a considerable increase of the conductance below the superconducting critical temperature T c was observed [1][2][3]. Although in a recent work [4] it was suggested that such an increase may be due to scattering at the S/F interface, a careful measurement of the conductance demonstrated that the entire change of the conductance was due to an increase of the conductivity of the ferromagnet [1,2].Such an increase would not be a great surprise if instead of the ferromagnet one had a normal metal N. It is well known (see for review [5,6]) that in S/N structures proximity effects can lead to a considerable increase of the conductance of the N wire provided its length does not exceed the phase breaking length L ϕ . However in a S/F structure, if the superconducting pairing is singlet, the proximity effect is negligible at distances exceeding a much shorter length ∼ ξ h . This reduction of the proximity effect due to the exchange field h of the ferromagnet is clear from the picture of Cooper pairs consisting of electrons with opposite spins. The proximity effect is not considerably affected by the exchange energy only if the latter is small h < T c . As concerns such strong ferromagnets as F e or Co used in the experiments [1,2], whose exchange energy h is by several orders of magnitude larger than T c , a singlet pairing is impossible due to the strong difference in the energy dispersions for the two spin bands. At the same time, an arbitrary exchange field cannot destroy a triplet superconducting pairing because the spins of the electrons forming Cooper pairs are already parallel. A possible role of the triplet component in transport properties of S/F structures has been noticed in Refs. [7,8], where the triplet component arose only as a result of mesoscopic fluctuations. However, in both cases the corrections to the conductance are much smaller than the observed ones.In this paper, we suggest a much more robust mechanism of formation of the triplet pairing in S/F structures, which is due to a local inhomogeneity of the magnetization M in the vicinity of the S/F interface. We show that the inhomogeneity generates a triplet component of the superconducting order parameter with an amplitude co...

We calculate the dc Josephson current IJ for two types of superconductor-ferromagnet (S/F) Josephson junctions. The junction of the first type is a S/F/S junction. On the basis of the Eilenberger equation, the Josephson current is calculated for an arbitrary impurity concentration. If hτ ≪ 1 the expression for the Josephson critical current Ic is reduced to that which can be obtained from the Usadel equation (h is the exchange energy, τ is the momentum relaxation time). In the opposite limit hτ ≫ 1 the superconducting condensate oscillates with period vF /h and penetrates into the F region over distances of the order of the mean free path l. For this kind of junctions we also calculate IJ in the case when the F layer presents a nonhomogeneous (spiral) magnetic structure with the period 2π/Q. It is shown that for not too low temperatures, the π-state which occurs in the case of a homogeneous magnetization (Q = 0) may disappear even at small values of Q. In this nonhomogeneous case, the superconducting condensate has a nonzero triplet component and can penetrate into the F layer over a long distance of the order of ξT = D/2πT . The junction of the second type consists of two S/F bilayers separated by a thin insulating film. It is shown that the critical Josephson current Ic depends on the relative orientation of the effective exchange field h of the bilayers. In the case of an antiparallel orientation, Ic increases with increasing h. We establish also that in the F film deposited on a superconductor, the Meissner current created by the internal magnetic field may be both diamagnetic or paramagnetic.

We calculate the dc Josephson current for two superconductor/ferromagnet (S/F) bilayers separated by a thin insulating film. It is demonstrated that the critical Josephson current I(c) in the junction strongly depends on the relative orientation of the effective exchange field h of the bilayers. We found that in the case of an antiparallel orientation I(c) increases at low temperatures with increasing h and at zero temperature has a singularity when h equals the superconducting gap Delta. This striking behavior contrasts with the suppression of the critical current by the magnetic moments aligned in parallel and is an interesting new effect of the interplay between superconductors and ferromagnets.

We show that a huge thermoelectric effect can be observed by contacting a superconductor whose density of states is spin-split by a Zeeman field with a ferromagnet with a non-zero polarization. The resulting thermopower exceeds kB/e by a large factor, and the thermoelectric figure of merit ZT can far exceed unity, leading to heat engine efficiencies close to the Carnot limit. We also show that spin-polarized currents can be generated in the superconductor by applying a temperature bias.PACS numbers: 74.25.fg, 74.25.F-, 72.25.-b Thermoelectric effects, electric potentials generated by temperature gradients and vice versa, are intensely studied because of their possible use in converting the waste heat from various processes to useful energy. The conversion efficiency η =Ẇ /Q, the ratio of output powerẆ to the rate of thermal energy consumedQ, in thermoelectric devices however typically falls short of the theoretical Carnot limit and is low compared to other heat engines, which has motivated an extensive search for better materials. [1] In electronic conductors a major contributor to thermoelectricity is breaking of the symmetry between positive and negative-energy charge carriers (electrons and holes, respectively) [2]. Within Sommerfeld expansion, this is described by the Mott relation [3], which predicts thermoelectric effects of the order ∼ k B T /E 0 , where T is the temperature and E 0 a microscopic energy scale describing the energy dependence in the transport. This is usually a large atomic energy scale (in metals, the Fermi energy), so that E 0 k B T even at room temperature and these effects are often weak. Larger electron-hole asymmetries are however attainable in semiconductors, as the chemical potential can be tuned close to the band edges, where the density of states varies rapidly. [1,4] The situation in superconductors is superficially similar to semiconductors. The quasiparticle transport is naturally strongly energy dependent due to the presence of the energy gap ∆, which can be significantly smaller than atomic energy scales. However, the chemical potential is not tunable in the same sense as in semiconductors, as charge neutrality dictates that electron-hole symmetry around the chemical potential is preserved. This implies that the thermoelectric effects in superconductors are often even weaker than in the corresponding normal state, in addition to being masked by supercurrents [5,6].We show in this Letter that this problem can be overcome in a conventional superconductor by applying a spin-splitting field h. It shifts the energies of electrons with parallel and antiparallel spin orientations to opposite directions. [7] This breaks the electron-hole symmetry for each spin separately, but conserves charge neutrality, as the total density of states remains electron-hole symmetric. In this situation, thermoelectric effects can be obtained by coupling the superconductor to a spinpolarized system.