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When a soliton propagates in a discrete lattice it excites small-amplitude linear waves in its wake. In a dc current-biased Josephson-junction (JJ) array these manifest as electromagnetic (EM) waves excited by a (magnetic field induced) chain of propagating magnetic vortices.When the vortex velocity and the phase velocity of one of the excited EM waves match, phase-locking occurs. This produces resonant steps in the current-voltage characteristics where amplification of EM radiation occurs. We report the first observation of phase-lockinginduced amplification of EM radiation at 77K and above in JJ arrays made of high temperature superconductors.

We have developed a Josephson vortex-flow transistor based on a parallel array of 440 YBa2Cu3O7−δ bicrystal grain boundary Josephson junctions. The array's critical current Ic was measured as a function of the control current Ictrl through a control line that is inductively coupled to the array. The device has a highly asymmetric Ic(Ictrl) curve with several regions where a switching behaviour is observed characterized by a maximum current gain gmax = ∂Ic/∂Ictrl of 19 and a significant dynamic range of 20 μA at 77 K. In the range 4.7–92 K gmax versus temperature is non-monotonic with a maximum recorded at 77 K.

Several Josephson ratchets designed as asymmetrically structured parallel-series arrays of Josephson junctions made of YBa2Cu3O7−δ have been fabricated. From the current-voltage characteristics measured for various values of applied magnetic field, B, in the temperature range of 10–89 K, we demonstrate that the devices work as magnetic field-tunable highly reversible vortex diodes. Thus, at 89 K, the ratchet efficiency η could be reversed from +60% to −60% with a change in B as small as 3 μT. By decreasing the operation temperature, η improves up to −95% at 10 K while the dynamics in the B-tunability degrades. The ratchet designs we propose here can be used to control unidirectional vortex flow vortices in superconducting devices as well as building integrated nano-magnetic sensors. Numerical simulations qualitatively confirm our experimental findings and also provide insight into the related and more general problem of the control of the transport of nano/quantum objects in thin films.

Fig. 4b and from 0.5 mA to 1mA (-1 mA to -0.5 mA) in Fig. 5a. Also, the amplitude of voltage modulation is considerable larger for negative currents relative to the positive currents. It is important to note that the families of V(I ctrl ) recorded for different current biases I plotted in Figs. 3b, 4b and 5b, they all lack inversion symmetry with respect to 0. This is because the PAJJ is not magnetically screened and therefore the presence of the earth magnetic field breaks such symmetry. Since the perpendicular component to the PAJJ planar structure of the earth magnetic field B normal is about 25 T it requires a significant shifting along the I ctrl current axis in order to compensate for the earth magnetic field and therefore recover the inversion symmetry. Indeed, the magnetic field corresponding to I ctrl =1.8mA (the current period of V(I ctrl ) curves) is about 2.4 T which is much smaller than B normal . The differences in the IVCs and V(I ctrl ) between the positive and the negative current biases are far less pronounced at temperatures below 60K. An example is shown in Fig. 6 for T=57.5 K. Similar characteristics have been measured at several other temperatures below 60K, namely, at 4.7K, 10K, and 30K.Indeed, the IVCs have a good degree of symmetry at these temperatures (see Fig.6a). Also, in this temperature range the V(I ctrl ) response becomes aperiodic (see Fig.6b) due to a significant suppression of the flux-flow modes due to a decrease in the dissipation coefficient 1/R N (or damping) with temperature (here R N is the normal resistance of the Josephson junction). A similar suppression of the flux-flow with decreasing T has been previously observed (both theoretically and experimentally) in PAJJs made of niobium low temperature superconductors [4,5].8 ConclusionIn conclusion, we have shown that an inductive asymmetric loop configuration within the array implemented in our PAJJ leads to a significant difference in the flux-flow for positive current biases relative to negative current biases and consequently to a corresponding different qualitative flux-to-voltage response. To achieve both an enhanced current amplification [11] and an enhanced microwave power generation [12] a PAJJ has to be operated in a regime of enhanced flux-flow, i.e., it should be biased with a positive current.The degree of asymmetry of our design can be further extended to enhance the contrast in the directional flux-flow and to improve PAJJs performances in these applications.