Measurements of the delay time between a critical current pulse and the first resistive response in superconducting niobium strips

We have measured in superconducting niobium filaments the delay time t_d that separates the initiation of a pulse of overcritical current (I > I_c) from its first resistive response. The experiments were performed at various temperatures, typically 4, 5, and 6 K, well below the critical temperature T_c, for delays in the range 0.3 ns < t_d < 80 ns, divided into two parts for technical reasons. The data t_d(T/T_c, I/I_c) were analyzed through a time-dependent Ginzburg-Landau theory leaving the gap relaxation time τ _d as an unknown parameter. In a restricted range of current amplitudes (I/I_c < 1.15), a fit is obtained by choosing τ _d between 1.65 and 1.75 ns, which we interpret as a film cooling time of 23 ps per nm thickness, almost independently of the temperature.