Temporal relaxation of nonequilibrium in Y-Ba-Cu-O measured from transient photoimpedance response

Nonequilibrium in Y-Ba-Cu-O thin films biased with a dc current is photoinduced by exposure to 300-fsec 2-eV laser pulses. The photoinduced nonequilibrium transients were measured in the superconducting, transition, and normal states occurring between 7 and 200 K. The photoabsorption produced temporal changes in the sample’s impedance, which manifest themselves as transient voltage signals occurring across the samples during the nonequilibrium’s relaxation process. At and above ${\mathit{T}}_{\mathit{c}}$, the observed photoresponse is thermal. Below ${\mathit{T}}_{\mathit{c}}$, a quantum response is obtained corresponding to changes in the Cooper-pair populations. In the zero-resistance superconducting state, a positive signal corresponds to quasiparticle generation and a negative signal corresponds to quasiparticle recombination.