Discontinuity of the ultrafast electronic response of underdoped superconductingBi2Sr2CaCu2O8+δstrongly excited by ultrashort light pulses

Abstract

We report the experimental evidence of an abrupt transition of the ultrafast electronic response of underdoped superconducting ${\text{Bi}}_{2}{\text{Sr}}_{2}{\text{CaCu}}_{2}{\text{O}}_{8+\ensuremath{\delta}}$, 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 ${\ensuremath{\Phi}}_{\text{th}}\ensuremath{\simeq}70\text{ }\ensuremath{\mu}\text{J}/{\text{cm}}^{2}$. 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 ${T}_{\text{eff}}$ and ${\ensuremath{\mu}}_{\text{eff}}$ 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.