Discontinuity of the ultrafast electronic response of underdoped superconducting Bi2Sr2CaCu2O8+δ strongly excited by ultrashort light pulses

We report the experimental evidence of an abrupt transition of the ultrafast electronic response of underdoped superconducting Bi2Sr2CaCu2O8+delta, under the impulsive photoinjection of a high density of excitations, using ultrashort laser pulses and avoiding significant laser heating. The direct proof of this process is the discontinuity of the transient optical electronic response, observed at a critical fluence of Phi(th)similar or equal to 70 mu J/cm(2). Below this threshold, the recovery dynamics is described by the Rothwarf-Taylor equations, whereas, above the critical intensity, a fast electronic response is superimposed to a slower dynamics related to the superconductivity recovery. We discuss our experimental findings within the frame of the available models for nonequilibrium superconductivity, i.e., the T-eff and mu(eff) models. The measured critical fluence is compatible with a first-order photoinduced phase transition triggered by the impulsive shift of the chemical potential. The measured value, significantly in excess of the condensation energy, indicates that, close to the threshold, the largest amount of energy is delivered to phonons or to other gap-energy excitations strongly coupled to Cooper pairs.