Femtosecond thermomodulation studies of low and high-Tc superconductors

The authors 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 electron-phonon coupling constant λ. The agreement between the λ values measured and those obtained by other techniques is excellent, confirming the theoretical predictions of P.B. Allen (1987). 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 a 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 ε₂. The observed changes in ε₂ are related to the dynamics of the Cu d to O p band charge transfer excitation occurring in the CuO₂ planes. By depleting a YBa₂Cu₃O_7-δ sample of oxygen, one can simultaneously vary the Fermi level and the T_c and make dramatic changes in the pump-probe signal. An estimate of λ, in several high-T_c materials, is also made using Allen´s theory to fit the relaxation behavior of ε₂