FEMTOSECOND THERMOMODULATION STUDIES OF LOW AND HIGH-TC SUPERCONDUCTORS

We report femtosecond pump-probe measurements of electronic energy relaxation in conventional metallic and high-T(c) oxide superconductors. In conventional metallic superconductors, the energy relaxation rate of electrons is used to determine the electronphonon coupling constant lambda. The agreement between our measured lambda values and those obtained by other techniques is excellent, thereby confirming the theoretical predictions of P.B. Allen. A novel Cu overlayer technique was developed in order to measure certain metals which do not have a strong optical transition to states near the Fermi level at our laser energy of 1.98 eV. The effect of different Cu overlayer thicknesses has been studied. In the new copper-oxide high-T(c) superconducting materials, electronic energy relaxation is monitored by measuring changes in epsilon-2. The observed changes in epsilon-2 are related to the dynamics of the Cu d to O p band charge transfer excitation occurring in the CuO2 planes. By depleting a YBa2Cu3O7-delta-sample of oxygen, we can simultaneously vary the Fermi level and the T(c), and make dramatic changes in the pump probe signal. An estimate of lambda, in several high-T(c) materials, is also made using Allen's theory to fit the relaxation behavior of epsilon-2.