Ratchet Effect in Triangle-Shaped Vertical Nb/TiO/Nb Josephson Junctions

We fabricate vertical Nb/TiOx/Nb Josephson junctions with the shape of right-angled isosceles triangles of different sizes. The TiOx layer is primarily metallic, indicating that our junctions belong to the superconductor-normal metal-superconductor (S-N-S) junction. When applying bipolar dc currents across the junctions in parallel magnetic fields, there are many obvious differences between the positive and negative parts of the current-voltage characteristics. We attributed this asymmetry behavior (or ratchet behavior) mainly to the asymmetric pinnings for Josephson vortex penetration at the asymmetric edges (the right-angled corner and the hypotenuse) of the triangled junction, in addition to the effect of current-induced magnetic self-fields. For comparison with superconducting thin-film ratchets, the rectified dc voltages of our junctions with applied ac currents are investigated at different temperatures, magnetic fields, and driving frequencies. The waveforms of the time-resolved voltage with sinusoid currents are also measured. The ratchet effect in the junctions decreases monotonically with increasing temperature, increases to the peak first, and then decreases with the increasing magnetic field. It remains almost unchanged in the driving frequency range of 0.1 to 100 kHz. Compared with Abrikosov vortex ratchets, the ratchet effect in the junctions is much more sensitive to temperatures far from Tc, demonstrating the larger temperature-regulating window. This article shows the distinct influence of the asymmetric junction boundaries on the characteristics of the junctions under certain conditions, which can be useful for further understanding and inspiring new applications of Josephson junctions.