The 0–π transition in Superconductor-Insulator-superconductor-Ferromagnet-Superconductor (SIsFS) Josephson junctions (JJs) was investigated experimentally. As predicted by theory, an s-layer inserted into a ferromagnetic SIFS junction can enhance the critical current density up to the value of an SIS tunnel junction. We fabricated Nb′ | AlOx | Nb | Ni60Cu40 | Nb JJs with wedge-like s (Nb) and F (Ni60Cu40) layers and studied the Josephson effect as a function of the s- and F-layer thickness, ds and dF, respectively. For ds = 11 nm, π-JJs with SIFS-type jc(dF) and critical current densities up to jcπ=60 A/cm2 were obtained at 4.2 K. Thicker ds led to a drastic increase of the critical current decay length, accompanied by the unexpected disappearance of the 0–π transition dip in the jc(dF) dependence. Our results are relevant for superconducting memories, rapid single flux quantum logic circuits, and solid state qubits.

Measurements performed on superconductive networks shaped in the form of planar graphs display anomalously large currents when specific branches are biased. The temperature dependences of these currents evidence that their origin is due to Cooper pair hopping through the Josephson junctions connecting the superconductive islands of the array. The experimental data are discussed in terms of theoretical models which predict, for the system under consideration, an inhomogeneous Cooper pair distribution on the superconductive islands of the network as a consequence of a Bose-Einstein condensation phenomenon.

We investigate experimentally the retrapping of the phase in a ϕ Josephson junction upon return of the junction to the zero-voltage state. Since the Josephson energy profile U0(ψ) in ϕ JJ is a 2π periodic double well potential with minima at ψ = ±ϕ mod 2π, the question is at which of the two minima −ϕ or +ϕ the phase will be trapped upon return from a finite voltage state during quasistatic decrease of the bias current (tilt of the potential). By measuring the relative population of two peaks in escape histograms, we determine the probability of phase trapping in the ±ϕ-wells for different temperatures. Our experimental results qualitatively agree with theoretical predictions. In particular, we observe an onset of the butterfly effect with an oscillating probability of trapping. Unexpectedly, these probability saturates at a value different than 50% at low temparatures.

We demonstrate experimentally the operation of a deterministic Josephson ratchet with tunable asymmetry. The ratchet is based on a ϕ Josephson junction with a ferromagnetic barrier operating in the underdamped regime. The system is probed also under the action of an additional dc current, which acts as a counter force trying to stop the ratchet. Under these conditions the ratchet works against the counter force, thus producing a non-zero output power. Finally, we estimate the efficiency of the ϕ Josephson junction ratchet.

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