Dynamic study of conduction carriers in ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ thin films using a pulsed-laser-induced transient-thermoelectric-effect method

The pulsed-laser-induced transient thermoelectric effect (TTE) has been measured for c-axis-oriented ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ thin films over a wide time range (50 ns to 2 ms) and temperature range (10–300 K). The analysis of the decay curves of TTE voltages has revealed that the ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ system has multiple conduction carriers, the semiconducting holes in the one-dimensional (1D) CuO chains and two types of holes (light-mass and heavy-mass holes) arising from the metallic 2D ${\mathrm{CuO}}_2$-derived band. From the observed relaxation times for thermal diffusions of light and heavy holes, we have estimated their mobilities, which show a ‘‘critical slowing-down’’-like anomaly near the superconducting transition temperature ${\mathit{T}}_{\mathit{c}}$. The temperature dependence of the hole mobilities can be reasonably explained by considering a critical divergent nature of the diffusion coefficients for conduction holes and a ‘‘quasiparticle lifetime’’ ${\mathrm{\tau }}^{\mathrm{*}}$ in the superconducting state. In the superconducting state we have observed the stepwise-, shunt-, and plateau-type TTE signals above and/or below a characteristic temperature ${\mathit{T}}_{\mathit{c}}^{\mathrm{*}}$ (=35 K). The presence of ${\mathit{T}}_{\mathit{c}}^{\mathrm{*}}$ is indicative of an additional superconducting transition from phase I to II of the quasiparticle system.