Temperature dependence of the $cos\phi$ conductance in Josephson tunnel junctions determined from plasma resonance experiments

The microwave response at 9 GHz of Sn-O-Sn tunnel-junction current biased at zero dc voltage has been measured just below the critical temperature $T_c$ of the Sn films. The temperature dependence of the $cos\phi$ conductance is determined from the resonant response at the junction plasma frequency $f_p$ as the temperature is decreased from $T_c$. We used three different schemes for observation of the plasma oscillations: (a) second-harmonic generation (excitation at ∼ 4.5 GHz, $f_p\sim 4.5$ GHz); (b) mixing (excitations at ∼ 9 and ∼ 18 GHz, $f_p\sim 9$ GHz); (c) parametric half-harmonic oscillation (excitation at ∼ 18 GHz, $f_p\sim 9$ GHz). Measurements were possible in two temperature intervals, $0.994\le \frac{T}{T_c}\le 0.982$ and $0.965\ge \frac{T}{T_c}\ge 0.930$, with the result that as the temperature was decreased, the $cos\phi$ amplitude first increased from about zero to positive values and then at lower temperatures decreased approaching - 1 at the lowest temperatures of the experiment.