Characteristic features of the ac Josephson effect above the gap voltage in niobium point contacts

Abstract We have investigated the strength of the ac Josephson effect, I MAX 1 /I C , above the gap voltage V g in Nb point contacts. We found that its dependence on voltage is empirically formulated as I MAX 1 /I C = exp{- α ( V - β )}, where the damping constant α has a strong correlation with the magnitude of V g . From the existence of the threshold at ≈V g in the damping of I MAX 1 /I C , we conclude that the exponential damping of the strength of the ac Josephson effect above the gap voltage is caused by the injection of quasiparticles. By finding of the correlation between the gap voltage and the cooling efficiency of the point contact, we confirm that the damping constant α is determined by the cooling efficiency of the point contact.