INFRARED PROPERTIES OF EPITAXIAL LA2-XSRXCUO4 THIN-FILMS IN THE NORMAL AND SUPERCONDUCTING STATES

The CuO2-plane optical reflectance of superconducting La2-xSrxCuO4 thin films (T(c) congruent-to 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 (lambda almost-equal-to 0.25) free-carrier term plus an over-damped, weakly temperature-dependent, midinfrared component. The free-carrier plasma frequency is nearly constant, omega(pD) = 6300 cm-1, whereas the relaxation rate varies linearly with temperature above T(c). In the superconducting state, according to our two-component approach, most of the Drude oscillator strength condenses to a delta(omega) function. A two-fluid analysis gives a rapid drop in the quasiparticle damping rate below T(c). A reasonable estimate (approximately 2750 angstrom) for the ab-plane London penetration depth is obtained from the superfluid density. We observe that the midinfrared strength increases below T(c), suggesting that some (approximately 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 k(B)T(c)) and 400 cm-1 (18 k(B)T(c)) 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.