The first measurements of the switching current distribution of an extremely underdamped Josephson junction are presented at various temperatures. Careful fitting of the data provides an experimental verification of the thermal activation theory in the very low damping limit. Moreover, the fitting allows us to obtain the ‘‘effective’’ resistance of a Josephson tunnel junction, thus providing an important indication as to the proper junction resistance to be used in the resistively shunted junction model. These values of junction resistance show the temperature dependence of a subgap resistance, i.e., exp(Δ/$k_B$T), due to activation of quasiparticles over the superconductor energy gap Δ.

• Received 1 June 1987

DOI:https://doi.org/10.1103/PhysRevB.37.1525