Traditional studies that combine spintronics and superconductivity have mainly focused on the injection of spin-polarized quasiparticles into superconducting materials. However, a complete synergy between superconducting and magnetic orders turns out to be possible through the creation of spin-triplet Cooper pairs, which are generated at carefully engineered superconductor interfaces with ferromagnetic materials. Currently, there is intense activity focused on identifying materials combinations that merge superconductivity and spintronics to enhance device functionality and performance. The results look promising: it has been shown, for example, that superconducting order can greatly enhance central effects in spintronics such as spin injection and magnetoresistance. Here, we review the experimental and theoretical advances in this field and provide an outlook for upcoming challenges in superconducting spintronics.At the interface between materials with radically different properties, new physical phenomena can emerge. A classical example of such an interface is that between a superconductor and a ferromagnet where the opposing electron orders destructively interfere; however, it turns out that under the right conditions at a superconductorferromagnet interface both superconductivity and spin-polarization can unite to create a new superconducting state that offers tantalizing possibilities for spin transport in which Joule heating and dissipation are minimized.Spintronics offers the potential for creating circuits in which logic operations controlled by spin currents can be performed faster and more energy efficient [1] than the charge-based equivalent in semiconductor transistor technologies. Spintronics is one of the most active areas of research and while it offers control of spin and charge at the nanometer scale, it has also found sensory applications in hard disk drive read heads via the giant magnetoresistance effect [2,3]. The idea of combining superconductivity with spintronics has historically focused on the net spin-polarization of quasiparticles in superconductors. It is interesting to note that the first spin transport experiments [4-6] involved ferromagnet-superconductor bilayers and predated non-superconducting spin transport experiments [8]. As will be discussed in this review, it is possible to create pseudo-chargeless spin-1/2 excitations in superconductors [7] which have extremely long spin lifetimes.Recently, a more complete synergy between superconductivity and spintronics has been made possible through the discovery of spin-triplet Cooper pairs at superconductor-ferromagnet interfaces. Non-superconducting spin currents are generated by passing charge currents through ferromagnetic materials. As will be explained in this review, spin currents can also be generated by passing supercurrents through ferromagnetic materials. Charge flow within superconductors is carried by Cooper pairs which consist of interacting pairs of electrons [9]. The idea of combining superconducting and magnetic order wa...
We investigate the conductance spectra of a normal/superconductor graphene junction using the extended Blonder-Tinkham-Klapwijk formalism, considering pairing potentials that are both conventional (isotropic swave) and unconventional (anisotropic d-wave). In particular, we study the full crossover from normal to specular Andreev reflection without restricting ourselves to special limits and approximations, thus expanding results obtained in previous work. In addition, we investigate in detail how the conductance spectra are affected if it is possible to induce an unconventional pairing symmetry in graphene, for instance a d-wave order parameter. We also discuss the recently reported conductance-oscillations that take place in normal/superconductor graphene junctions, providing both analytical and numerical results.
We study proximity-induced superconductivity on the surface of a topological insulator (TI), focusing on unconventional pairing. We find that the excitation spectrum becomes gapless for any spin-triplet pairing, such that both subgap bound states and Andreev reflection is strongly suppressed. For spin-singlet pairing, the zeroenergy surface state in the d xy -wave case becomes a Majorana fermion, in contrast to the situation realized in the topologically trivial high-T c cuprates. We also study the influence of a Zeeman field on the surface states. Both the magnitude and direction of this field is shown to strongly influence the transport properties, in contrast to the case without TI. We predict an experimental signature of the Majorana states via conductance spectroscopy. PACS numbers:Topological insulators represent a new state of matter which presently is generating much interest [1][2][3][4]. While being insulating in the bulk due to a charge excitation gap, spindependent conducting channels are formed at the edges or surfaces of such materials. These states form as Kramer pairs which are topologically protected, persisting in the presence of disorder as long as time-reversal symmetry is preserved. The allure of topological insulators stems not only from their obvious interest from a fundamental physics point of view, but also because they may find use in spintronics [5]. Recent experiments have observed the surface Dirac states characteristic for 3D topological insulators [6].Another motivation for studying topological insulators is that they provide an arena for excitations that satisfy nonAbelian statistics: so-called Majorana fermions [7]. Elementary excitations with non-Abelian statistics form a centerpiece in recent proposals for topological quantum computation [8]. Majorana fermions have been shown to exist as surface states at the junction between a superconductor (S) and ferromagnetic insulator (FI) deposited on a topological insulator due to the proximity effect [9][10][11]. The formation of Majorana states at the interface between a superconductor and a topological insulator [12,13] and at the boundary of superfluid 3 He-B [14] has also been proposed. Very recently, an experimental study reported the observation of doping-induced superconductivity in the topological insulator (TI) Bi 2 Se 3 [15].By depositing superconducting materials with an unconventional pairing symmetry on top of a TI, an exciting prospect of an interplay between the internal phase of the superconducting order parameter ∆ and Majorana states opens up. In this Letter, we investigate how spin-triplet and spinsinglet d-wave pairing interact with the environment of a TI. We find that spin-triplet pairing universally gives rise to gapless excitations and that both bound-states and Andreev reflection are strongly suppressed. For spin-singlet pairing, we find that the zero-energy surface states in the d xy -wave case are now Majorana fermions in contrast to the case of the topologically trivial case of the high-T c cuprates. Several...
We study how the surface states in the strong topological insulator Bi 2 Se 3 are influenced by finite size effects and compare our results with those recently obtained for two-dimensional topological insulator HgTe. We demonstrate two important distinctions: ͑i͒ contrary to HgTe, the surface states in Bi 2 Se 3 display a remarkable robustness towards decreasing the width L down to a few nm thus ensuring that the topological surface states remain intact and ͑ii͒ the gapping due to the hybridization of the surface states features an oscillating exponential decay as a function of L in Bi 2 Se 3 in sharp contrast to HgTe. Our findings suggest that Bi 2 Se 3 is suitable for nanoscale applications in quantum computing or spintronics. Also, we propose a way to experimentally detect both of the predicted effects.
We study theoretically proximity-induced superconductivity and ferromagnetism on the surface of a topological insulator. In particular, we investigate how the Andreev-bound states are influenced by the interplay between these phenomena, taking also into account the possibility of unconventional pairing. We find a qualitative difference in the excitation spectrum when comparing spin-singlet and spin-triplet pairing, leading to non-gapped excitations in the latter case. The formation of surface-states and their dependence on the magnetization orientation is investigated, and it is found that these states are Majorana fermions in the d xy -wave case in stark contrast to the topologically trivial high-T c cuprates. The signature of such states in the conductance spectra is studied, and we also compute the supercurrent which flows on the surface of the topological insulator when a Josephson junction is deposited on top of it. It is found that the current exhibits an anomalous current-phase relation when the region separating the superconducting banks is ferromagnetic, and we also show that in contrast to the metallic case the exchange field in such a scenario does not induce 0-π oscillations in the critical current. Similarly to the high-T c cuprates, the presence of zero-energy surface states on the topological surface leads to a strong low-temperature enhancement of the critical current.PACS numbers: 74.45.+c arXiv:1003.4754v1 [cond-mat.supr-con]
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