Observation of the Bloch oscillations in an ultrasmall Josephson junction

We have studied the low-temperature behavior of lead-alloy Josephson tunnel junctions with area S≊0.01 μ${\mathrm{m}}^2$, isolated from their electromagnetic environment by high-resistance metallic resistors inserted into the current and voltage leads. Under irradiation with microwaves frequencies, f=3.5–10 GHz, the dc differential resistance dV/dI, as a function of the dc current I, showed peaks at I=±2ef. This effect, and other observations, arises due to the periodic electrical recharging of the junction by discrete Cooper pairs, and can be explained by the ‘‘orthodox’’ theory of Bloch oscillations.