THz conductivity of La_2-xSr_xCuO₄ in the pseudogap region and in the superconductivity state

We measure the complex conductivity, σ̃(ω,T), of LSCO thin films with various dopings at THz frequencies. Several features characteristic of a superconductor were observed, including the superconducting gap in the spectrum of the real part, σ₁(ω). The imaginary part of the conductivity, σ₂, shows a rapid increase with decreasing temperature below some temperature, T_c^THz, which corresponds to the onset of superconducting fluctuation. T_c ^THz agrees well with the upper-limit temperature for superconductiving fluctuation, T_c^MF (or T_c ^CF), previously estimated by our microwave investigation. This suggests that the Nernst signal observed at much higher temperatures in these materials has an origin different from superconductivity. It should be noted that another small increase of σ₂(T) was observed at a much higher temperature, T₀, in underdoped samples. It turned out that this temperature agreed well with the so-called pseudogap temperature, suggesting that the quasiparticle (QP) life time starts to increase because of the suppression of the QP scattering by the pseudogap opening.