Narrow YBa 2 Cu 3 O 7 films, excited by pulses of supercritical current, had their nanosecond electric response monitored in zero applied magnetic field. Delayed voltage steps plus constant differential resistance, characteristic of phase-slip centers (PSC), are observed at all temperatures. The duration of the initial zero voltage state is well fit by Ginzburg-Landau based theories, with a gap relaxation time controlled by phonon escape. At higher levels of excitation, PSC's give birth to slowly spreading normal hot spots. [S0031-9007(98)06897-5] PACS numbers: 74.76. Bz, 74.25.Kc, 74.40. + k, 74.50. + r Superconductivity is perturbed, not suppressed, by a critical current. Thanks to vortex motion, the growing phase differences of the order parameter wave function relax by multiples of 2p [1], a process which allows the material to preserve long range order while sustaining a voltage. In filamentary structures, similar quantum phase jumps occur in short, fixed, zones named phaseslip centers [2] (PSC). So, dissipation arises in a way fundamentally different from a transition into the normal state. A second singular feature is the long persistence of the zero voltage state after the application of the supercritical current, much beyond the natural (picosecond) time h͞D (Planck constant divided by the energy gap). Pals and Wolter [3] discovered this delay, or PSC nucleation time, t d on aluminum film strips. Interpreting it as the time required to achieve complete collapse of the order parameter, and using a timedependent Ginzburg-Landau (TDGL) equation based on the nonequilibrium energy-mode gap relaxation time [4], they could account fairly for their experimental data.It would be difficult to make any predictions for cuprate superconductors, due to their utterly short coherence length. Nevertheless, the dc current-voltage ͑I-V ͒ curves of high-T c narrow bridges indeed display the steps and the expected response to microwave radiation [5] related to PSC's. This Letter reports on the first observation of the delay t d in epitaxial YBa 2 Cu 3 O 7 thin films, and its interpretation through TDGL. At the same time, we tackle directly the troublesome problem of heating in a dissipative center.Our experiments were performed in zero applied magnetic field on c-axis textured films with typical misorientation 0.5 degree of arc. Sample LL-109N is a 75 nm thick film evaporated [6] on ͓100͔ MgO with cationic composition YBa 1.88 Cu 3.04 . Our bridge has resistivity r͑300 K͒ 350 mV cm, a ratio RRR r͑300 K͒͞r͑100 K͒ 2.8, and a critical temperature T c 89 K. Samples TO-34 and TO-S35, obtained by dc hollow cathode sputtering [6], had similar electrical characteristics. Finally, sample LZCYB77 is a 35 nm thick film deposited by laser ablation [6] on ͓100͔ Si covered with a buffer layer (70 nm YSZ; 10 nm CeO 2 ). Our bridge had r͑300 K͒ 1.9 mV cm, and RRR 2.6.According to the standard PSC model [7], each phaseslip event produces a burst of quasiparticles, whose diffusion eventually determines the length of the dissipative zone. T...

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Granular YBaCuO films in their current-induced dissipative state were exposed to light pulses of small fluence and the decay of the photoresistance was observed on the nanosecond time scale in the range 1 K<T<35 K. The response appears bolometric down to 10 K. Below, we resolve a second decay rate having a temperature dependence characteristic of the electron-phonon interaction. Using the known lattice specific heat as a reference, we deduce the Sommerfeld coefficient (γ=2.5 mJ/K2 mol) of the electronic specific heat.