FEMTOSECOND OPTICAL-ABSORPTION STUDIES OF NONEQUILIBRIUM ELECTRONIC PROCESSES IN HIGH-TC SUPERCONDUCTORS

We report the results of femtosecond optical transient absorption experiments performed on <m1;&1p>the superconducting compounds YBa2Cu3O7-x(x ∼ 0) and Bi 2Sr2Ca2Cu3O 10+δ(δ ∼ 0) and nonsuperconducting YBa 2Cu3O6+y(y<0.4) for sample temperatures ranging from ∼7 K to room temperature. Nonequilibrium heating was found to occur on a subpicosecond time scale. A distinct, dramatic increase in the relaxation time was observed for the superconducting samples as the sample temperature was lowered below the critical temperatures of the respective films. Accompanying the increase in relaxation time was an increase in the peak fractional transmissivity change. No such changes were observed for the nonsuperconducting YBCO sample. We believe the above described behavior is electronic in origin and intimately related to the superconductivity of the compounds.