N.G. Pugach

Controlled suppression of superconductivity by the generation of polarized Cooper pairs in spin valve structures

(Received 10 April 2014; revised manuscript received 13 January 2015; published 2 February 2015) Transport measurements are presented on thin-film superconducting spin-valve systems, where the controlled noncollinear arrangement of two ferromagnetic Co layers can be used to influence the superconducting state of Nb. We observe a very clear oscillation of the superconducting transition temperature with the relative orientation of the two ferromagnetic layers. Our measurements allow us to distinguish between the competing influences of domain averaging, stray dipolar fields, and the formation of superconducting spin triplets. Domain averaging is shown to lead to a weak enhancement of transition temperature for the antiparallel configuration of exchange fields, while much larger changes are observed for other configurations, which can be attributed to drainage currents due to spin triplet formation.

Remotely induced magnetism in a normal metal using a superconducting spin-valve

A switchable induced magnetic moment in a non-magnetic metal that is separated from a ferromagnet by a thick superconducting layer contradicts existing models.

The effect of normal and insulating layers on 0-π transitions in Josephson junctions with a ferromagnetic barrier

Using the Usadel approach, we provide a formalism that allows us to calculate the critical current density of 21 different types of Josephson junctions (JJs) with a ferromagnetic (F) barrier and additional insulating (I) or/and normal (N) layers inserted between the F layer and superconducting (S) electrodes. In particular, we obtain that in SFS JJs, even a thin additional N layer between the S layer and F layer may noticeably change the thickness of the F layer at which the 0-π transitions occur. For certain values of a 0-π transition can even be achieved by changing only the N layer thickness. We use our model to fit experimental data of SIFS and SINFS tunnel junctions.D. M. Heim, ∗ N. G. Pugach, 3 M. Yu. Kupriyanov, 4 E. Goldobin, D. Koelle, R. Kleiner, N. Ruppelt, M. Weides, and H. Kohlstedt Institut für Quantenphysik and Center for Integrated Quantum Science and Technology (IQ ), Universität Ulm, D-89069 Ulm, Germany Skobeltsyn Institute of Nuclear Physics, M. V. Lomonosov Moscow State University, 119991 Leninskie Gory, Moscow, Russia Royal Holloway University of London, Egham, Surrey, TW20 0EX, United Kingdom Moscow Institute of Physics and Technology, Dolgoprudny, Moscow Region, Russia Physikalisches Institut and Center for Collective Quantum Phenomena in LISA, Universität Tübingen, D-72076 Tübingen, Germany Nanoelektronik, Technical Faculty, University of Kiel, D-24143 Kiel, Germany Physikalisches Institut, Karlsruhe Institute of Technology, D-76131 Karlsruhe, Germany

Superconductor-insulator-ferromagnet-superconductor Josephson junction: From the dirty to the clean limit

The proximity effect and the Josephson current in a superconductor-insulator-ferromagnet-superconductor (SIFS) junction are investigated within the framework of the quasiclassical Eilenberger equations. This investigation allows us to compare the dirty and the clean limits, to investigate an arbitrary impurity scattering, and to determine the applicability limits of the Usadel equations for such structures. The role of different types of the FS interface is analyzed. It is shown that the decay length and the spatial oscillation period of the Eilenberger function may exhibit a nonmonotonic dependence on the properties of the ferromagnetic layer such as exchange field or electron mean free path. The results of our calculations are applied to the interpretation of experimentally observed dependencies of the critical current density on the ferromagnet thickness in Josephson junctions containing a Ni layer with an arbitrary scattering.

Examination of logic operations with silent phase qubit

Reduced control Hamiltonian of the silent phase qubit has been found and dependence of the Hamiltonian matrix elements on applied magnetic field has been analyzed. The obtained results yield in concept of qubit logic operations accomplished by means of magnetic field application. One can propose a set of the qubits, each designed for specific logic operation. The qubit logic specialization may be fixed by both the junction current-phase relation and the characteristic Josephson-to-Coulomb energy ratio s. As current-phase relation is assigned, the qubit specialization can be tuned by the ratio s which varies with Josephson junction area. Examples of some logic operations are considered. Estimation of both the decoherence time and the quality factor Q has been obtained for such a qubit based on high-Tc superconductors, which could provide the required current-phase relation.

Irreversible magnetization switching at the onset of superconductivity in a superconductor ferromagnet hybrid

We demonstrate that the magnetic state of a superconducting spin valve, that is normally controlled with an external magnetic field, can also be manipulated by varying the temperature which increases the functionality and flexibility of such structures as switching elements. In this case, switching is driven by changes in the magnetostatic energy due to spontaneous Meissner screening currents forming in the superconductor below the critical temperature. Our scanning Hall probe measurements also reveal vortex-mediated pinning of the ferromagnetic domain structure due to the pinning of quantized stray fields in the adjacent superconductor. The ability to use temperature as well as magnetic field to control the local magnetisation structure raises the prospect of potential applications in magnetic memory devices.

Ferromagnetic planar Josephson junction with transparent interfaces: a φ junction proposal

We calculate the current-phase relation of a planar Josephson junction with a ferromagnetic weak link located on top of a thin normal metal film. Following experimental observations we assume transparent superconductor-ferromagnet interfaces. This provides the best interlayer coupling and a low suppression of the superconducting correlations penetrating from the superconducting electrodes into the ferromagnetic layer. We show that this Josephson junction is a promising candidate for experimental φ junction realization.

Protected 0-π states in SIsFS junctions for Josephson memory and logic

We study the peculiarities in current-phase relations (CPR) of the SIsFS junction in the region of 0 to π transition. These CPR consist of two independent branches corresponding to 0- and π-states of the contact. We have found that depending on the transparency of the SIs tunnel barrier, the decrease in the s-layer thickness leads to transformation of the CPR shape going in the two possible ways: either one of the branches exists only in discrete intervals of the phase difference φ or both branches are sinusoidal but differ in the magnitude of their critical currents. We demonstrate that the difference can be as large as 10% under maintaining superconductivity in the s layer. An applicability of these phenomena for memory and logic application is discussed.

Extraordinary magnetic field behavior of SIFS Josephson junctions with an inhomogeneous transparency of the FS interface

Within the framework of the Usadel equations, the Josephson effect in a superconductor-insulator-ferromagnet-superconductor (SIFS) structure with a spatially heterogeneous transparency of the SF interface has been studied. It is shown that, at a certain thickness of the F layer, a stepwise variation of the transparency leads to the formation of a region of size ∼ξF (coherence length in a ferromagnet), where the Josephson supercurrents of different signs may flow. This may lead to the dependence of the junction critical current on the external magnetic field qualitatively different from the Fraunhofer pattern typically observed in usual Josephson junctions.

Critical-current oscillations in superconductor-ferromagnet-superconductor structure taking s-d scattering into account

The critical current in a Josephson junction with a weakly ferromagnetic layer of a transition-metal alloy is calculated. The Gor’kov equations are solved taking the s-d scattering in a ferromagnet into account. It is shown that consideration of this scattering, which results in the destruction of Cooper pairs, makes it possible to achieve good agreement with experiments which could not be explained by other models.