Josephson weak link based on proximity effect in long bi-metallic thin-film bridge

Abstract

We prepared a bi-metal Sn/Al thin film bridge of 1 × 5 µm2 in size and exposed it to microwave irradiation in a frequency range of 7 to 40 GHz to explore the Shapiro steps in the current-voltage characteristics, which served as a reliable indicator for assessing current-phase relation (CPR). The measurements were made in the temperature range (0.89 … 0.99)Tc with Tc = 3.66 K. No fractional steps are observed at 10 GHz, while all integer steps are present, and their widths oscillate with microwave field amplitude, which suggests a non-skewed quasi-sine CPR. Therefore, the normal-metal covering alters the resistive state of the long thin-film strip containing phase-slip centers so that the bi-metallic long bridge exhibits characteristics similar to a Josephson weak link. Considering a simple fabrication procedure, it may be utilized in making Josephson-effect-based devices such as DC and RF SQUIDs, especially in low-budget projects. Additional small-scale oscillations of the step widths found between the main peaks and the missing first step at a higher frequency of 20 GHz near Tc may be associated with Landau–Zener transitions between Andreev states and require further detailed study.