36 research outputs found
Huge thermoelectric effects in ferromagnet-superconductor junctions in the presence of a spin-splitting field
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
by a large factor, and the thermoelectric figure of merit 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.Comment: 5 pages, 4 figure
Supercurrent and Andreev bound state dynamics in superconducting quantum point contacts under microwave irradiation
We present here an extensive theoretical analysis of the supercurrent of a
superconducting point contact of arbitrary transparency in the presence of a
microwave field. Our study is mainly based on two different approaches: a
two-level model that describes the dynamics of the Andreev bound states in
these systems and a fully microscopic method based on the Keldysh-Green
function technique. This combination provides both a deep insight into the
physics of irradiated Josephson junctions and quantitative predictions for
arbitrary range of parameters. The main predictions of our analysis are: (i)
for weak fields and low temperatures, the microwaves can induce transitions
between the Andreev states leading to a large suppression of the supercurrent
at certain values of the phase, (ii) at strong fields, the current-phase
relation is strongly distorted and the corresponding critical current does not
follow a simple Bessel-function-like behavior, and (iii) at finite temperature,
the microwave field can enhance the critical current by means of transitions
connecting the continuum of states outside the gap region and the Andreev
states inside the gap. Our study is of relevance for a large variety of
superconducting weak links as well as for the proposals of using the Andreev
bound states of a point contact for quantum computing applications.Comment: 16 pages, 11 figures, submitted to Phys. Rev.
Andreev current enhancement and subgap conductance of superconducting hybrid structures in the presence of a small spin-splitting field
We investigate the subgap transport properties of a S-F-Ne structure. Here S
(Ne) is a superconducting (normal) electrode, and F is either a ferromagnet or
a normal wire in the presence of an exchange or a spin- splitting Zeeman field
respectively. By solving the quasiclassical equations we first analyze the
behavior of the subgap current, known as the Andreev current, as a function of
the field strength for different values of the voltage, temperature and length
of the junction. We show that there is a critical value of the bias voltage V *
above which the Andreev current is enhanced by the spin-splitting field. This
unexpected behavior can be explained as the competition between two-particle
tunneling processes and decoherence mechanisms originated from the temperature,
voltage and exchange field respectively. We also show that at finite
temperature the Andreev current has a peak for values of the exchange field
close to the superconducting gap. Finally, we compute the differential
conductance and show that its measurement can be used as an accurate way of
determining the strength of spin-splitting fields smaller than the
superconducting gap.Comment: 5 pages, 4 figure
Electron cooling in diffusive normal metal - superconductor tunnel junctions with a spin-valve ferromagnetic interlayer
We investigate heat and charge transport through a diffusive SIF1F2N tunnel
junction, where N (S) is a normal (superconducting) electrode, I is an
insulator layer and F1,2 are two ferromagnets with arbitrary direction of
magnetization. The flow of an electric current in such structures at subgap
bias is accompanied by a heat transfer from the normal metal into the
superconductor, which enables refrigeration of electrons in the normal metal.
We demonstrate that the refrigeration efficiency depends on the strength of the
ferromagnetic exchange field h and the angle {\alpha} between the
magnetizations of the two F layers. As expected, for values of h much larger
than the superconducting order parameter \Delta, the proximity effect is
suppressed and the efficiency of refrigeration increases with respect to a NIS
junction. However, for h \sim \Delta the cooling power (i.e. the heat flow out
of the normal metal reservoir) has a non-monotonic behavior as a function of h
showing a minimum at h \approx \Delta. We also determine the dependence of the
cooling power on the lengths of the ferromagnetic layers, the bias voltage, the
temperature, the transmission of the tunneling barrier and the magnetization
misalignment angle {\alpha}.Comment: 8 pages, 7 figure
Heat transport and electron cooling in ballistic normal-metal/spin-filter/superconductor junctions
Selected papers from the sixth Moscow International Symposium on Magnetism (MISM-2014).-- arXiv:1407.1977v2We investigate electron cooling based on a clean normal-metal/spin-filter/superconductor junction. Due to the suppression of the Andreev reflection by the spin-filter effect, the cooling power of the system is found to be extremely higher than that for conventional normal-metal/nonmagnetic-insulator/superconductor coolers. Therefore we can extract large amount of heat from normal metals. Our results strongly indicate the practical usefulness of the spin-filter effect for cooling detectors, sensors, and quantum bits.This work was supported by the Topological Quantum Phenomena (No. 23103520) KAKENHI on Innovative Areas, a Grant-in-Aid for Scientific Research (No. 25286046) from MEXT of Japan, the JSPS Institutional Program for Young Researcher Overseas Visits, the European Union Seventh Framework Programme (FP7/2007-2013) under Grant agreement “INFERNOS” no. 308850, the Spanish Ministry of Economy and Competitiveness under Project FIS2011-28851-C02-02, and the CSIC and the European Social Fund under JAE-Predoc programPeer Reviewe
Evidence for spin selectivity of triplet pairs in superconducting spin valves.
Spin selectivity in a ferromagnet results from a difference in the density of up- and down-spin electrons at the Fermi energy as a consequence of which the scattering rates depend on the spin orientation of the electrons. This property is utilized in spintronics to control the flow of electrons by ferromagnets in a ferromagnet (F1)/normal metal (N)/ferromagnet (F2) spin valve, where F1 acts as the polarizer and F2 the analyser. The feasibility of superconducting spintronics depends on the spin sensitivity of ferromagnets to the spin of the equal spin-triplet Cooper pairs, which arise in superconductor (S)-ferromagnet (F) heterostructures with magnetic inhomogeneity at the S-F interface. Here we report a critical temperature dependence on magnetic configuration in current-in-plane F-S-F spin valves with a holmium spin mixer at the S-F interface providing evidence of a spin selectivity of the ferromagnets to the spin of the triplet Cooper pairs.This work was funded by the Royal Society through a University Research Fellowship “Superconducting Spintronics” held by J.W.A.R. M.G.B acknowledges funding from the UK EPSRC and the European Commission through an ERC Advanced Investigator Grant "Superspin". C.B.S. and R.G.J.S were supported by the Erasmus exchange programme and the Leiden Outbound Grant. C.B.S. acknowledges Prof. Jan Aarts’ for scientific input. The work of F.S.B and A. O. have been supported by the Spanish Ministry of Economy and Competitiveness under Project FIS2011-28851-C02-02. The work of A. O. have 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).This is the accepted version of an article originally published in Nature Communications. The final version is available at http://www.nature.com/ncomms/2014/140109/ncomms4048/full/ncomms4048.html. © Nature Publishing Group. Reuse rights are available at http://www.nature.com/authors/policies/license.html
Detection of small exchange fields in S/F structures
Selected papers from the sixth Moscow International Symposium on Magnetism (MISM-2014).-- arXiv:1401.0646v2Ferromagnetic materials with exchange fields Eex smaller or of the order of the superconducting gap Δ are important for applications of corresponding (s-wave) superconductor/ferromagnet/superconductor (SFS) junctions. Presently such materials are not known but there are several proposals how to create them. Small exchange fields are in principle difficult to detect. Based on our results we propose reliable detection methods of such small Eex. For exchange fields smaller than the superconducting gap the subgap differential conductance of the normal metal–ferromagnet–insulator–superconductor (NFIS) junction shows a peak at the voltage bias equal to the exchange field of the ferromagnetic layer, eV=Eex. Thus measuring the subgap conductance one can reliably determine small EexΔ one can determine the exchange field in scanning tunneling microscopy (STM) experiment. The density of states of the FS bilayer measured at the outer border of the ferromagnet shows a peak at the energy equal to the exchange field, E=Eex. This peak can be only visible for small enough exchange fields of the order of few Δ.This work was supported by European Union Seventh Framework Programme (FP7/2007–2013) under Grant agreement “INFERNOS” No. 308850, by Ministry of Education and Science of the Russian Federation in the framework of the Federal Target Program >Research and development in priority areas of science and technology complex of Russia for 2014-2020>, Grant nos. 14Y.26.31.0007, 2014-14-588-0010-061, RFBR no. mol_a 14-02-31798, and by French National Agency for Research ANR-GUI-AAP-05 (electroVORTEX).Peer Reviewe
Superconducting spintronics
The interaction between superconducting and spin-polarized orders has recently emerged as a major research field following a series
of fundamental breakthroughs in charge transport in superconductor-ferromagnet heterodevices which promise new device
functionality. Traditional studies which 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 which merge superconductivity and spintronics in order 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 related to the new concept of superconducting spintronics.J.L. was supported by the Research Council of Norway, Grants No. 205591 and 216700.
J.W.A.R. was supported by the UK Royal Society and the Leverhulme Trust through an
International Network Grant (IN-2013-033).This is the accepted manuscript. The final version is available at http://www.nature.com/nphys/journal/v11/n4/full/nphys3242.html