Properties of tunnel Josephson junctions with a ferromagnetic interlayer

We investigate superconductor/insulator/ferromagnet/superconductor (SIFS) tunnel Josephson junctions in the dirty limit, using the quasiclassical theory. We formulate a quantitative model describing the oscillations of critical current as a function of thickness of the ferromagnetic layer and use this model to fit recent experimental data. We also calculate quantitatively the density of states (DOS) in this type of junctions and compare DOS oscillations with those of the critical current.Comment: 9 pages, 8 figures, to be published in Phys. Rev.

Theory of proximity effect in ferromagnet/superconductor heterostructures in the presence of spin dependent interfacial phase shift

We study the proximity effect and charge transport in ferromagnet (F)/superconductor (S) and S/F/I/F/S junctions (where I is insulator) by taking into account simultaneously exchange field in F and spin-dependent interfacial phase shifts (SDIPS) at the F/S interface. We solve the Usadel equations using extended Kupriyanov–Lukichev boundary conditions which include SDIPS, where spin-independent part of tunneling conductance GT and spin-dependent one Gφ coexist. The resulting local density of states (LDOS) in a ferromagnet depends both on the exchange energy Eex and Gφ/GT. We show that the magnitude of zero-temperature gap and the height of zero-energy LDOS have a non-monotonic dependence on Gφ/GT. We also calculate Josephson current in S/F/I/F/S junctions and show that crossover from 0-state to

Density of states in SF bilayers with arbitrary strength of magnetic scattering

We developed the self-consistent method for the calculation of the density of states $N(\epsilon)$ in the SF bilayers. It based on the quasi-classical Usadel equations and takes into account the suppression of superconductivity in the S layer due to the proximity effect with the F metal, as well as existing mechanisms of the spin dependent electron scattering. We demonstrate that the increase of the spin orbit or spin flip electron scattering rates results in completely different transformations of $N(\epsilon)$ at the free F layer interface. The developed formalism has been applied for the interpretation of the available experimental data.Comment: 5 pages, 8 figure

The current-phase relation in Josephson tunnel junctions

The $J(\phi)$ relation in SFIFS, SNINS and SIS tunnel junctions is studied. The method for analytical solution of linearized Usadel equations has been developed and applied to these structures. It is shown that the Josephson current across the structure has the sum of $\sin \phi$ and $\sin 2\phi$ components. Two different physical mechanisms are responsible for the sign of $\sin 2\phi$. The first one is the depairing by current which contributes positively to the $\sin 2\phi$ term, while the second one is the finite transparency of SF or SN interfaces which provides the negative contribution. In SFIFS junctions, where the first harmonic vanishes at 0 - $\pi$ transition, the calculated second harmonic fully determines the $J(\phi)$ curve.Comment: 6 pages, 2 figure

Quasiparticle interference in multiband superconductors with strong coupling

We develop a theory of the quasiparticle interference (QPI) in multiband superconductors based on strong-coupling Eliashberg approach within the Born approximation. In the framework of this theory, we study dependencies of the QPI response function in the multiband superconductors with nodeless s-wave superconductive order parameter. We pay a special attention to the difference of the quasiparticle scattering between the bands having the same and opposite signs of the order parameter. We show that, at the momentum values close to the momentum transfer between two bands, the energy dependence of the quasiparticle interference response function has three singularities. Two of these correspond to the values of the gap functions and the third one depends on both the gaps and the transfer momentum. We argue that only the singularity near the smallest band gap may be used as an universal tool to distinguish between $s_{++}$ and $s_{\pm}$ order parameters. The robustness of the sign of the response function peak near the smaller gap value, irrespective of the change in parameters, in both the symmetry cases is a promising feature that can be harnessed experimentally.Comment: 16 pages, 16 figure

Disorder induced transition between s_+- and s_++ states in two-band superconductors

We have reexamined the problem of disorder in two-band superconductors, and shown within the framework of the T-matrix approximation, that the suppression of T_c can be described by a single parameter depending on the intraband and interband impurity scattering rates. T_c is shown to be more robust against nonmagnetic impurities than would be predicted in the trivial extension of Abrikosov-Gor'kov theory. We find a disorder-induced transition from the s_{\pm} state to a gapless and then to a fully gapped s_{++} state, controlled by a single parameter -- the sign of the average coupling constant . We argue that this transition has strong implications for experiments.Comment: 5 pages, 4 figures; suppl. material: 3 pages, 2 figures; published versio

Inverse ac Josephson Effect for a Fluxon in a Long Modulated Junction

We analyze motion of a fluxon in a weakly damped ac-driven long Josephson junction with a periodically modulated maximum Josephson current density. We demonstrate both analytically and numerically that a pure {\it ac} bias current can drive the fluxon at a {\it resonant} mean velocity determined by the driving frequency and the spatial period of the modulation, provided that the drive amplitude exceeds a certain threshold value. In the range of strongly ``relativistic'' mean velocities, the agreement between results of a numerical solution of the effective (ODE) fluxon equation of motion and analytical results obtained by means of the harmonic-balance analysis is fairly good; morever, a preliminary PDE result tends to confirm the validity of the collective-coordinate (PDE-ODE) reduction. At nonrelativistic mean velocities, the basin of attraction, in position-velocity space, for phase-locked solutions becomes progressively smaller as the mean velocity is decreased.Comment: 15 pages, 26 kbytes, of text in plain LaTeX. A uuencoded, Z-compressed tar archive, 21 kbytes, containing 3 PostScript, [email protected], [email protected], [email protected]

Method for reliable realization of a varphi Josephson junction

We propose a method to realize a $\phi$ Josephson junction by combining alternating 0 and $\pi$ parts (sub junctions) with an intrinsically non-sinusoidal current-phase relation (CPR). Conditions for the realization of the $\phi$ ground state are analyzed. It is shown that taking into account the non-sinusoidal CPR for a "clean junction with a ferromagnetic (F) barrier, one can significantly enlarge the domain (regime of suitable F-layer thicknesses) of the $\phi$ ground state and make the practical realization of $\phi$ Josephson junctions feasible. Such junctions may also have two different stable solutions, such as 0 and $\pi$, 0 and $\phi$, or $\phi$ and $\pi$