Suchergebnisse

We present a comprehensive quasiclassical approach for studying transport properties of superconducting diffusive hybrid structures in the presence of extrinsic spin-orbit coupling. We derive a generalized Usadel equation and boundary conditions that in the normal state reduce to the drift-diffusion theory governing the spin-Hall effect in inversion symmetric materials. These equations predict the nondissipative spin-galvanic effect, that is the generation of supercurrents by a spin-splitting field, and its inverse—the creation of magnetic moment by a supercurrent. These effects can be seen as counterparts of the spin-Hall, anomalous Hall, and their inverse effects in the superconducting state. Our theory opens numerous possibilities for using superconducting structures in magnetoelectronics. ; The work of F.S.B. was supported by Spanish Ministerio de Economia y Competitividad (MINECO) through the Project No. FIS2014-55987-P and the Basque Government under UPV/EHU Project No. IT-756-13. I.V.T. acknowledges support from the Spanish Grant FIS2013-46159-C3-1-P, and from the "Grupos Consolidados UPV/EHU del Gobierno Vasco" (Grant No. IT578-13) ; Peer reviewed

arXiv:1409.4563v1 ; We present a full microscopic theory to describe the Josephson current through an extended superconductor-normal metal-superconductor (SNS) diffusive junction with an intrinsic spin-orbit coupling (SOC) in the presence of a spin-splitting field h. We demonstrate that the ground state of the junction corresponds to a finite intrinsic phase difference 0 < φ0 < 2π between the superconductor electrodes provided that both h and the SOC-induced SU(2) Lorentz force are finite. The nontrivial φ0 is closely related to the appearance of an equilibrium spin current in the normal metal with the spin projection parallel to the exchange field direction. In the particular case of a Rashba SOC we present analytic and numerical results for φ0 as a function of the strengths of the spin fields, the length of the junction, the temperature and the properties of SN interfaces. ; This work was supported by the Spanish Ministry of Economy and Competitiveness under Projects No. FIS2011-28851-C02-02, FIS2013-46159-C3-1-P, FIS2014-55987-P, and the Basque Government under UPV/EHU Project No. IT-756-13. IVT acknowledges funding by the Grupos Consolidados UPV/EHU del Gobierno Vasco (Grant No. IT578-13). ; Peer Reviewed

We investigate the proximity effect in diffusive superconducting hybrid structures with a spin-orbit (SO) coupling. Our study is focused on the singlet-triplet conversion and the generation of long-range superconducting correlations in ferromagnetic elements. We derive the quasiclassical equations for the Green's functions including the SO coupling terms in form of a background SU(2) field. With the help of these equations, we first present an interesting complete analogy between the spin diffusion process in normal metals and the generation of the triplet components of the condensate in a diffusive superconducting structure in the presence of SO coupling. From this analogy it turns out naturally that the SO coupling is an additional source of the long-range triplet component (LRTC) besides the magnetic inhomogeneities studied in the past. This analogy opens a range of possibilities for the generation and manipulation of the triplet condensate in hybrid structures. In particular we demonstrate that a normal metal with SO coupling can be used as source of LRTC if attached to a superconductor-ferromagnet bilayer. We also demonstrate an explicit connection between an inhomogeneous exchange field and SO coupling mechanisms for the generation of the LRTC and establish the conditions for the appearance of the LRTC in different geometries. Our work gives a global description of the singlet-triplet conversion in hybrid structures in terms of generic spin fields and our results are particularly important for the understanding of the physics underlying spintronic devices with superconductors. © 2014 American Physical Society. ; The work of F.S.B was supported by the Spanish Ministry of Economy and Competitiveness under Project No. FIS2011-28851-C02-02 the Basque Government under UPV/EHU Project No. IT-756-13. I.V.T. acknowledges funding by the "Grupos Consolidados UPV/EHU del Gobierno Vasco" (IT-578-13) and Spanish MICINN (FIS2010-21282-C02-01). ; Peer Reviewed

The long-range proximity effect in superconductor-ferromagnet (S/F) hybrid nanostructures is observed if singlet Cooper pairs from the superconductor are converted into triplet pairs which can diffuse into the ferromagnet over large distances. It is commonly believed that this happens only in the presence of magnetic inhomogeneities. We show that there are other sources of the long-range triplet component (LRTC) of the condensate and establish general conditions for their occurrence. As a prototypical example, we consider first a system where the exchange field and spin-orbit coupling can be treated as time and space components of an effective SU(2) potential. We derive a SU(2) covariant diffusive equation for the condensate and demonstrate that an effective SU(2) electric field is responsible for the long-range proximity effect. Finally, we extend our analysis to a generic ferromagnet and establish a universal condition for the LRTC. Our results open a new avenue in the search for such correlations in S/F structures and make a hitherto unknown connection between the LRTC and Yang-Mills electrostatics. © 2013 American Physical Society. ; The work of F. S.B was supported by the Spanish Ministry of Economy and Competitiveness under Project No. FIS2011-28851-C02-02 and the Basque Government under UPV/EHU Project No. IT-366-07. I.V. T. acknowledges funding by the ''Grupos Consolidados UPV/EHU del Gobierno Vasco'' (IT-319-07) and the Spanish MICINN (FIS2010-21282-C02-01). ; Peer Reviewed

We have studied the interaction between magnetism and superconductivity in a pseudo-spin-valve structure consisting of a Co/Cu/Py/Nb-layer sequence. We are able to control the magnetization reversal process and monitor it by means of the giant magnetoresistance effect during transport measurements. By placing the superconducting Nb-film on the top of the permalloy (Py) electrode instead of putting it in between the two ferromagnets, we minimize the influence of spin scattering or spin accumulation onto the transport properties of Nb. Magnetotransport data reveal clear evidence that the stray fields of domain walls (DWs) in the pseudo-spin valve influence the emerging superconductivity close to the transition temperature by the occurrence of peaklike features in the magneto-resistance characteristic. Direct comparison with magnetometry data shows that the resistance peaks occur exactly at the magnetization reversal fields of the Co and Py layers where DWs are generated. For temperatures near the superconducting transition the amplitude of the DW-induced magnetoresistance increases with decreasing temperature, reaching values far beyond the size of the giant magnetoresistive response of our structure in the normal state. 2012 American Physical Society. ; We acknowledge the Diputación Foral de Giupuzkoa Ref. 99/11, program Red Guipuzkoana de Ciencia Tecnologia e Innovacion, funding from the Basque Government under Program No. PI2009-17 and UPV/EHU Project IT-366-07, and the Spanish Ministry of Science and Innovation under Projects No. MAT2009-07980 and FIS2011-28851-C02-02. ; Peer Reviewed

arXiv:1412.0998v1 ; We propose and analyze a mesoscopic Josephson junction consisting of two ferromagnetic insulator-superconductors (FI-Ss) coupled through a normal metal (N) layer. The Josephson current of the junction is non-trivially affected by the spin-splitting field induced by the FIs in the two superconductors. In particular, it shows sizeable enhancement by increasing the amplitude of the exchange field (hex) and displays a switchable current-phase relation which depends on the relative orientation of hex in the FIs. In a realistic EuS/Al-based setup this junction can be exploited as a high-resolution threshold sensor for the magnetic field as well as an on-demand tunable kinetic inductor. ; The work of ES and FG was partially funded by the European Research Council under the European Union's Seventh Framework Programme (FP7/2007-2013)/ERC grant agreement No. 615187-COMANCHE, by the Marie Curie Initial Training Action (ITN) Q-NET 264034 and by the Marie Curie Individual Fellowship MSCA-IFEF-ST No. 660532-SupeMag. The work of FSB was supported by Spanish Ministerio de Economía y Competitividad (MINECO) through the Project FIS2014-55987-P and the Basque Government under UPV/EHU Project No. IT-756-13. ; Peer Reviewed

arXiv:1411.0990v3.-- et al. ; We have studied mesoscopic Josephson junctions formed by highly n-doped InAs nanowires and superconducting Ti/Pb source and drain leads. The current-voltage properties of the system are investigated by varying temperature and external out-of-plane magnetic field. Superconductivity in the Pb electrodes persists up to ∼7 K and with magnetic field values up to 0.4 T. Josephson coupling at zero backgate voltage is observed up to 4.5 K and the critical current is measured to be as high as 615 nA. The supercurrent suppression as a function of the magnetic field reveals a diffraction pattern that is explained by a strong magnetic flux focusing provided by the superconducting electrodes forming the junction. ; Partial financial support from the Marie Curie Initial Training Action (ITN) Q-NET 264034 and the Tuscany region through the project :TERASQUID" is acknowledged. The work of F.G. has been partially funded by the European Research Council under the European Union's Seventh Framework Programme (FP7/2007-2013)/ERC Grant 615187-COMANCHE. The work of F.S.B. was supported by the Spanish Ministry of Economy and Competitiveness under Projects No. FIS2011-28851-C02-02. ; Peer Reviewed

We propose a phase-controlled heat-flux quantum valve based on the proximity effect driven by a superconducting quantum interference proximity transistor (SQUIPT). Its operation relies on the phase-dependent quasiparticle density of states in the Josephson weak-link of the SQUIPT which controls thermal transport across the device. In a realistic Al/Cu-based setup the structure can provide efficient control of thermal current inducing temperature swings exceeding ∼ 100 mK, and flux-to-temperature transfer coefficients up to ∼ 500 mK/Φ 0 below 100 mK. The nanovalve performances improve by lowering the bath temperature, making the proposed structure a promising building-block for the implementation of coherent caloritronic devices operating below 1 K. ; The work of E.S. and F.G. was partially funded by the European Research Council under the European Union's Seventh Framework Programme (FP7/2007-2013)/ERC grant Agreement No. 615187-COMANCHE, and by the Marie Curie Initial Training Action (ITN) Q-NET 264034. The work of F.S.B. was supported by the Spanish Ministry of Economy and Competitiveness under Project No. FIS2011-28851-C02-02 the Basque Government under UPV/EHU Project No. IT-756-13. ; Peer Reviewed

A Josephson junction made of a generic magnetic material sandwiched between two conventional superconductors is studied in the ballistic semiclassic limit. The spectrum of Andreev bound states is obtained from the single valuedness of a particle-hole spinor over closed orbits generated by electron-hole reflections at the interfaces between superconducting and normal materials. The semiclassical quantization condition is shown to depend only on the angle mismatch between initial and final spin directions along such closed trajectories. For the demonstration, an Andreev-Wilson loop in the composite position–particle-hole–spin space is constructed and shown to depend on only two parameters, namely, a magnetic phase shift and a local precession axis for the spin. The details of the Andreev-Wilson loop can be extracted via measuring the spin-resolved density of states. A Josephson junction can thus be viewed as an analog computer of closed-path-ordered exponentials. ; The work of F. S. B. and F. K. was supported by Spanish Ministerio de Economía y Competitividad (MINECO) through Project No. FIS2014-55987-P and the Basque Government under UPV/EHU Project No. IT-756-13. I. V. T. acknowledges support from the Spanish Grant No. FIS2013-46159-C3-1-P and from the "Grupos Consolidados UPV/EHU del Gobierno Vasco" (Grant No. IT578-13). ; Peer reviewed

Under the terms of the Creative Commons Attribution License 3.0 (CC-BY). ; The concept of thermophase refers to the appearance of a phase gradient inside a superconductor originating from the presence of an applied temperature bias across it. The resulting supercurrent flow may, in suitable conditions, fully counterbalance the temperature-bias-induced quasiparticle current therefore preventing the formation of any voltage drop, i.e., a thermovoltage, across the superconductor. Yet, the appearance of a thermophase is expected to occur in Josephson-coupled superconductors as well. Here, we theoretically investigate the thermoelectric response of a thermally biased Josephson junction based on a ferromagnetic insulator. In particular, we predict the occurrence of a very large thermophase that can reach π/2 across the contact for suitable temperatures and structure parameters; i.e., the quasiparticle thermal current can reach the critical current. Such a thermophase can be several orders of magnitude larger than that predicted to occur in conventional Josephson tunnel junctions. In order to assess experimentally the predicted very large thermophase, we propose a realistic setup realizable with state-of-the-art nanofabrication techniques and well-established materials, based on a superconducting quantum interference device. This effect could be of strong relevance in several low-temperature applications, for example, for revealing tiny temperature differences generated by coupling the electromagnetic radiation to one of the superconductors forming the junction. ; F. G. acknowledges the Marie Curie Initial Training Action (ITN) Q-NET 264034 and the European Research Council under the European Union's Seventh Framework Programme (FP7/2007-2013)/ERC Grant agreement No. 615187-COMANCHE for partial financial support. The work of F.S.B has been supported by the Spanish Ministry of Economy and Competitiveness under Project No. FIS2011-28851-C02-02, and the work of T. T. H. has been supported by the ERC (Grant No. 240362-Heattronics) and the Academy of Finland through its Center of Excellence program. ; Peer Reviewed

Under the terms of the Creative Commons Attribution License 3.0 (CC-BY). ; We present an exhaustive theoretical analysis of charge and thermoelectric transport in a normal-metal-ferromagnetic-insulator-superconductor junction and explore the possibility of its use as a sensitive thermometer. We investigate the transfer functions and the intrinsic noise performance for different measurement configurations. A common feature of all configurations is that the best temperature-noise performance is obtained in the nonlinear temperature regime for a structure based on an Europium chalcogenide ferromagnetic insulator in contact with a superconducting Al film structure. For an open-circuit configuration, although the maximal intrinsic temperature sensitivity can achieve 10 nK Hz-1/2, a realistic amplifying chain will reduce the sensitivity up to 10 μK Hz-1/2. To overcome this limitation, we propose a measurement scheme in a closed-circuit configuration based on state-of-the-art superconducting-quantum-interference-device detection technology in an inductive setup. In such a case, we show that temperature-noise can be as low as 35 nK Hz-1/2. We also discuss a temperature-to-frequency converter where the obtained thermovoltage developed over a Josephson junction operated in the dissipative regime is converted into a high-frequency signal. We predict that the structure can generate frequencies up to approximately 120 GHz and transfer functions up to 200 GHz/K at around 1 K. If operated as an electron thermometer, the device may provide temperature-noise lower than 35 nK Hz-1/2 thereby being potentially attractive for radiation-sensing applications. ; F. G. acknowledges the European Research Council (ERC) under the European Union's Seventh Framework Program No. FP7/2007-2013 ERC Grant No. 615187-COMANCHE for funding. F. G. and P. S. acknowledge the MIUR-FIRB2013 Project Coca (Grant No. RBFR1379UX) for partial financial support. P. S. has received funding from the European Union Program No. FP7/2007-2013 under REA Grant No. 630925 COHEAT. A. B. thanks for the support of the MIUR-FIRB2012 Project HybridNanoDev (Grant No. RBFR1236VV). The work of F. S. B is supported by the Spanish Ministerio de Economía y Competitividad through the Project No. FIS2014-55987-P and Grupos Consolidados UPV/EHU del Gobierno Vasco (Grant No. IT-756-13). ; Peer Reviewed

We present a theoretical study of the effect of spin filtering on the Josephson and dissipative quasiparticle currents in a superconducting tunnel junction. By combining the quasiclassical Green's functions and the tunneling Hamiltonian method, we describe the transport properties of a generic junction consisting of two superconducting leads with an effective exchange field h separated by a spin-filter insulating barrier. We show that in addition to the tunneling of Cooper pairs with total spin projection S z=0 there is another contribution to the Josephson current due to triplet Cooper pairs with total spin projection S z 0. The latter is finite and not affected by the spin-filter effect provided that the fields h and the magnetization of the barrier are noncollinear. We also determine the quasiparticle current for a symmetric junction and show that the differential conductance may exhibit peaks at different values of the voltage depending on the polarization of the spin filter, and the relative angle between the exchange fields and the magnetization of the barrier. Our findings provide a plausible explanation for existing experiments on Josephson junctions with magnetic barriers, predict further effects, and show how spin-polarized supercurrents in hybrid structures can be created. © 2012 American Physical Society. ; The work of F.S.B and A.V. was supported by the Spanish Ministry of Economy and Competitiveness under Project No. FIS2011-28851-C02-02 and the Basque Government under UPV/EHU Project No. IT-366- 07. A.F.V. is grateful to the DIPC for hospitality and financial support. ; Peer Reviewed

We present a theoretical study of the quasiparticle and subgap conductance of generic X/Isf/SM junctions with a spin-filter barrier Isf, where X is either a normal N or a ferromagnetic metal F and SM is a superconductor with a built-in exchange field. Our study is based on the tunneling Hamiltonian and the Green's-function technique. First, we focus on the quasiparticle transport, both above and below the superconducting critical temperature. We obtain a general expression for the tunneling conductance which is valid for arbitrary values of the exchange field and arbitrary magnetization directions in the electrodes and in the spin-filter barrier. In the second part, we consider the subgap conductance of a N/I sf/S junction, where S is a conventional superconductor. In order to account for the spin-filter effect at interfaces, we heuristically derive boundary conditions for the quasiclassical Green's functions. With the help of these boundary conditions, we show that the proximity effect and the subgap conductance are suppressed by spin filtering in a N/Isf/S junction. Our work provides useful tools for the study of spin-polarized transport in hybrid structures both in the normal and in the superconducting state. © 2012 American Physical Society. ; The work of F. S. B. and A. V. was supported by the Spanish Ministry of Economy and Competitiveness under Project No. FIS2011-28851-C02-02 and the Basque Government under Universidad del Pais Vasco (UPV)/EHU Project No. IT-366-07. A. F. V. is grateful to the Donostia International Physics Center for hospitality and financial support. ; Peer Reviewed

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 nonzero 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. © 2014 American Physical Society. ; The work of F. S. B and A. O. has been supported by the Spanish Ministry of Economy and Competitiveness under Project No. FIS2011-28851-C02-02 and the Basque Government under UPV/EHU Project IT-756-13. The work of T. T. H. and P. V. has been supported by the Academy of Finland, the European Research Council (Grant No. 240362-Heattronics), and the EU-FP 7 INFERNOS (Grant No. 308850) program. The work of A. O. has also been supported by the CSIC and the European Social Fund under JAE-Predoc program and the EU-FP 7 MICROKELVIN project (Grant No. 228464). ; Peer Reviewed

This Colloquium discusses the recent progress in understanding the properties of spin-split superconductors under nonequilibrium conditions. Recent experiments and theories demonstrate a rich variety of transport phenomena occurring in devices based on such materials that suggest direct applications in thermoelectricity, low-dissipative spintronics, radiation detection, and sensing. This text discusses different experimental situations and presents a theoretical framework based on quantum kinetic equations. This framework provides an accurate description of the nonequilibrium distribution of charge, spin, and energy, which are the relevant nonequilibrium modes, in different hybrid structures. This Colloquium also reviews experiments on spin-split superconductors and shows how transport measurements reveal the properties of the nonequilibrium modes and their mutual coupling. In particular, the emphasis of the text is on spin injection and diffusion and very large thermoelectric effects in spin-split superconductors. ; This work was supported by the Academy of Finland Center of Excellence (Project No. 284594), Research Fellow (Project No. 297439), and Key Funding (Project No. 305256) programs, the Spanish Ministerio de Economía y Competitividad (MINECO, Project No. FIS2014-55987-P), the Spanish Ministerio de Ciencia, Innovación y Universidades (MICINN, Project No. FIS2017-82804-P), and the European Research Council under the European Union's Seventh Framework Program (FP7/2007-2013)/ERC Grant agreements No. 240362-HEATTRONICS and No. 615187-COMANCHE. ; Peer Reviewed