We report the first experimental observation of the two-node thickness dependence of the critical current in Josephson junctions with a ferromagnetic interlayer. Nodes of the critical current correspond to the transitions into the pi state and back into the conventional 0 state. From the experimental data the superconducting order parameter oscillation period and the pair decay length in the ferromagnet are extracted reliably. We develop a theoretical approach based on the Usadel equations taking into account the spin-flip scattering. Results of numerical calculations are in good agreement with experiments.

The difference between the phases of superconducting order parameter plays in superconducting circuits the role similar to that played by the electrostatic potential difference required to drive a current in conventional circuits. This fundamental property can be altered by inserting in a superconducting circuit a particular type of weak link, the so-called Josephson π-junction having inverted current-phase relation and enabling a shift of the phase by π. We demonstrate the operation of three superconducting circuits -two of them are classical and one quantum -which all utilize such π-phase shifters realized using superconductor-ferromagnet-superconductor sandwich technology. The classical circuits are based on single-flux-quantum cells, which are shown to be scalable and compatible with conventional niobium-based superconducting electronics. The quantum circuit is a π-phase biased qubit, for which we observe coherent Rabi oscillations and compare the measured coherence time with that of conventional superconducting phase qubits. 1 arXiv:1005.1581v1 [cond-mat.supr-con]

We have studied vortex configurations in mesoscopic superconducting disks using the Bitter decoration technique. For a broad range of vorticities L the circular geometry is found to lead to the formation of concentric shells of vortices. From images obtained on disks of different sizes in a range of magnetic fields we traced the evolution of vortex states and identified stable and metastable configurations of interacting vortices. Furthermore, the analysis of shell filling with increasing L allowed us to identify magic numbers corresponding to the appearance of consecutive new shells.

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We report the observation of the 0-junction state in superconductor-ferromagnet-superconductor Josephson junctions arising from variations in the effective barrier thickness d / F . By varying the temperature, the junctions can be tuned from the 0-to the -state through an intermediate regime in which parts of the junction are in each state. A signature of the 0-regime is that the usual peak in the critical current diffraction pattern at zero magnetic field becomes a dip, evidence for sign changes in the critical current density. In some junctions, the zero-field critical current does not vanish at any temperature and half-integer Shapiro steps are observed, indicating the presence of spontaneous supercurrents circulating within the junctions around the 0-interfaces.In the past few years considerable attention has been directed toward the understanding and realization of Josephson junctions. 1 Transitions between 0 and junction states were demonstrated in transport measurements on superconductor-ferromagnet-superconductor ͑SFS͒ junctions as a function of temperature 2 and barrier thickness, 3 and in mesoscopic superconductor-normal metal-superconductor junctions as a function of the barrier thermalization voltage. 4,5 Further experiments provided phase-sensitive evidence of the junction state, including dc superconducting quantum interference device ͑SQUID͒ interferometry, 6,7 direct measurements of the current-phase relation, 8 and probes of spontaneous currents in dc ͑Refs. 6 and 9͒ and rf SQUIDs. 10 These findings demonstrate the feasibility of basic junction circuits that have been proposed for both quantum and ultrafast classical computing implementations. [11][12][13] In this Rapid Communication, we report a study of SFS junctions in which the critical current density varies spatially within the junction due to small inhomogeneities in the thickness and/or strength of the exchange field in the ferromagnetic barriers. As a result, regions of the junctions transition into the junction state at different temperatures, resulting in a 0-junction with regions in each state. 14 We are able to manipulate the 0 and critical current densities by changing the temperature, and monitor the spatial variation of the critical current via magnetic field modulation measurements. We find that when the net critical currents of the 0 and regions become comparable in magnitude, the energy of the junction is minimized by generation of spontaneous circulating currents flowing around the interfaces between them. These currents prevent the overall junction critical current from vanishing at any temperature, the usual signature of the 0-transition. It also couples to applied microwave fields, producing the half-integer Shapiro steps that we observe in a narrow temperature range around the minimum critical current. A similar effect has been observed in nearly symmetric dc SQUIDs with an applied magnetic flux of 1 2 ⌽ 0 ͑⌽ 0 = h /2e͒. 15 A Josephson junction is characterized by a negative critical current. The mechanism of the state in ...