Infrared properties of epitaxial La2−xSrxCuO4 thin films in the normal and superconducting states

Abstract

The CuO2-plane optical reflectance of superconducting La2−xSrxCuO4 thin films (Tc≃31 K) has been measured over a wide frequency and temperature range. The optical conductivity in the normal state is well described by a temperature-dependent weak-coupling (λ≊0.25) free-carrier term plus an overdamped, weakly temperature-dependent, midinfrared component. The free-carrier plasma frequency is nearly constant, ωpD=6300 cm−1, whereas the relaxation rate varies linearly with temperature above Tc. In the superconducting state, according to our two-component approach, most of the Drude oscillator strength condenses to a δ(ω) function. A two-fluid analysis gives a rapid drop in the quasiparticle damping rate below Tc. A reasonable estimate (∼2750 Å) for the ab-plane London penetration depth is obtained from the superfluid density. We observe that the midinfrared strength increases below Tc, suggesting that some (∼15%) of the free carriers do not condense into superconducting pairs and may have a strong interaction with pair-breaking excitations. Two absorption edges around 80 cm−1 (3.7 kBTc) and 400 cm−1 (18 kBTc) are seen but neither is assigned to the superconducting gap. Comparisons with a one-component picture described by a frequency-dependent scattering rate and effective mass are made and discussed. The far-infrared ab-plane phonons show systematic changes with temperature, which are associated with the structural transition near 250 K.

Source-reported materials — not catalogue approval

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