Josephson tunnel junctions with ferromagnetic \Fe_{0.75}\Co_{0.25} barriers

Josephson tunnel junctions with the strong ferromagnetic alloy \Fe_{0.75}\Co_{0.25} as the barrier material were studied. The junctions were prepared with high quality down to a thickness range of a few monolayers of Fe-Co. An oscillation length of $\xi_{F2}\approx 0.79\:{\rm {nm}}$ between 0 and $\pi$-Josephson phase coupling and a very short decay length $\xi_{F1}\approx 0.22\:{\rm {nm}}$ for the amplitude of the superconducting pair wave function in the Fe-Co layer were determined. The rapid damping of the pair wave function inside the Fe-Co layer is caused by the strong ferromagnetic exchange field and additional magnetic pair breaking scattering. Josephson junctions with Fe-Co barriers show a significantly increased tendency towards magnetic remanence and flux trapping for larger thicknesses $d_{F}$.Comment: contains 5 figure

On the Puzzle of Odd-Frequency Superconductivity

Since the first theoretical proposal by Berezinskii, an odd-frequency superconductivity has encountered the fundamental problems on its thermodynamic stability and rigidity of a homogenous state accompanied by unphysical Meissner effect. Recently, Solenov {\it et al}. [Phys. Rev. B {\bf 79} (2009) 132502.] have asserted that the path-integral formulation gets rid of the difficulties leading to a stable homogenous phase with an ordinary Meissner effect. Here, we show that it is crucial to choose the appropriate saddle-point solution that minimizes the effective free energy, which was assumed {\it implicitly} in the work by Solenov and co-workers. We exhibit the path-integral framework for the odd-frequency superconductivity with general type of pairings, including an argument on the retarded functions via the analytic continuation to the real axis.Comment: 6 pages, in JPSJ forma

Study of the possibility of realization of a spin valve on the basis of superconductor/ferromagnet multilayers

The ways of realization of two different schemes of a spin valve for the superconducting current on the basis of the superconductor/ferromagnet proximity effect are studied. First, we have studied the superconducting proximity effect in the thin film system Fe/Cr/V/Cr/Fe where the Cr layers play the role of screening layers between the superconducting V-layer and the strongly pair breaking Fe-layers. Besides the new results concerning the magnetic phase transitions in the Cr layers we found the upper limit of the thickness of the screening Cr layers for operation a spin valve based on the Fe/V/Fe trilayer. Second, we studied the superconducting spin valve effect of a V-layer coupled to an antiferromagnetic [Fe/V]-superlattice. The parallel upper critical magnetic field exhibits an anomalous temperature dependence up to the ferromagnetic saturation field of the superlattice, indicating that the superconducting transition temperature Tc decreases when rotating the relative magnetization directions of the sublattice from antiparallel to parallel. © 2006 WILEY-VCH Verlag GmbH & Co. KGaA

Possible Odd-Frequency Superconductivity in Strong-Coupling Electron-Phonon Systems

A possibility of the odd-frequency pairing in the strong-coupling electron-phonon systems is discussed. Using the Holstein-Hubbard model, we demonstrate that the anomalously soft Einstein mode with the frequency $\omega_{\rm E}\ll\omega_{c}$ ($\omega_{c}$ is the order of the renormalized bandwidth) mediates the s-wave odd-frequency triplet pairing against the ordinary even-frequency singlet pairing. It is necessary for the emergence of the odd-frequency pairing that the pairing interaction is strongly retarded as well as the strong coupling, since the pairing interaction for the odd-frequency pairing is effective only in the diagonal scattering channel, $(\omega_{n},-\omega_{n})\to(\omega_{n'},-\omega_{n'})$ with $\omega_{n'}=\omega_{n}\gtrsim \omega_{\rm E}$. Namely, the odd-frequency superconductivity is realized in the opposite limit of the original BCS theory. The Ginzburg-Landau analysis in the strong-coupling region shows that the specific-heat discontinuity and the slope of the temperature dependence of the superfluid density can be quite small as compared with the BCS values, depending on the ratio of the transition temperature $T_{c}$ and $\omega_{c}$.Comment: 6 pages, 7 figures, submitted to J. Phys. Soc. Jp

Spin-polarized supercurrents for spintronics: a review of current progress

During the past 15 years a new field has emerged, which combines superconductivity and spintronics, with the goal to pave a way for new types of devices for applications combining the virtues of both by offering the possibility of long-range spin-polarized supercurrents. Such supercurrents constitute a fruitful basis for the study of fundamental physics as they combine macroscopic quantum coherence with microscopic exchange interactions, spin selectivity, and spin transport. This report follows recent developments in the controlled creation of long-range equal-spin triplet supercurrents in ferromagnets and its contribution to spintronics. The mutual proximity-induced modification of order in superconductor-ferromagnet hybrid structures introduces in a natural way such evasive phenomena as triplet superconductivity, odd-frequency pairing, Fulde-Ferrell-Larkin-Ovchinnikov pairing, long-range equal-spin supercurrents, $\pi$-Josephson junctions, as well as long-range magnetic proximity effects. All these effects were rather exotic before 2000, when improvements in nanofabrication and materials control allowed for a new quality of hybrid structures. Guided by pioneering theoretical studies, experimental progress evolved rapidly, and since 2010 triplet supercurrents are routinely produced and observed. We have entered a new stage of studying new phases of matter previously out of our reach, and of merging the hitherto disparate fields of superconductivity and spintronics to a new research direction: super-spintronics.Comment: 95 pages, 23 Figures; published version with minor typos corrected and few references adde

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

Our friends in the Commonwealth: The paradox of the post-EU immigration question

We have studied Josephson junctions with barriers prepared from the Heusler compound Cu 2 MnAl . In the as-prepared state the Cu 2 MnAl layers are nonferromagnetic and the critical Josephson current density j c decreases exponentially with the thickness of the Heusler layers d F . On annealing the junctions at 240   ° C the Heusler layers develop ferromagnetic order and we observe a dependence j c ( d F ) with j c strongly enhanced and weakly thickness dependent in the thickness range 7.0 < d F < 10.6   nm . We interpret this feature as an indication of a triplet component in the superconducting pairing function generated by the specific magnetization profile inside thin Cu 2 MnAl layers

Study of the possibility of realization of a spin valve on the basis of superconductor/ferromagnet multilayers

The ways of realization of two different schemes of a spin valve for the superconducting current on the basis of the superconductor/ferromagnet proximity effect are studied. First, we have studied the superconducting proximity effect in the thin film system Fe/Cr/V/Cr/Fe where the Cr layers play the role of screening layers between the superconducting V-layer and the strongly pair breaking Fe-layers. Besides the new results concerning the magnetic phase transitions in the Cr layers we found the upper limit of the thickness of the screening Cr layers for operation a spin valve based on the Fe/V/Fe trilayer. Second, we studied the superconducting spin valve effect of a V-layer coupled to an antiferromagnetic [Fe/V]-superlattice. The parallel upper critical magnetic field exhibits an anomalous temperature dependence up to the ferromagnetic saturation field of the superlattice, indicating that the superconducting transition temperature Tc decreases when rotating the relative magnetization directions of the sublattice from antiparallel to parallel. © 2006 WILEY-VCH Verlag GmbH & Co. KGaA