TEMPERATURE-DEPENDENCE OF THE MIDINFRARED DIELECTRIC FUNCTION OF YBCOIN THE AB PLANE

We have excited mid-infrared surface plasmons in two YBCO thin films o f contrasting properties using attenuated total reflection of light an d found that the imaginary part of the dielectric function decreases l inearly with reduction in temperature. This result is in contrast with the commonly reported conclusion of infrared normal reflectance studi es. If sustained it may clarify the problem of understanding the norma l state properties of YBCO and the other cuprates. The dielectric func tion of the films, epsilon = epsilon(1) + i epsilon(2), was determined between room temperature and 80K: epsilon(1) was found to be only sli ghtly temperature dependent but somewhat sample dependent, probably as a result of surface and grain boundary contamination. The imaginary p art, epsilon(2), (and the real part of the conductivity, sigma(1),) de creased linearly with reduction in temperature in both films. Results obtained were: for film 1: epsilon(1) = - 14.05 - 0.0024T and epsilon( 2) - 4.11 + 0.086T and for film 2: epsilon(1) = - 24.09 + 0.0013T and epsilon(2) = 7.66 + 0.067T where T is the temperature in Kelvin. An un derstanding of the results is offered in terms of temperature-dependen t intrinsic intragrain inelastic scattering and temperature-independen t contributions: elastic and inelastic grain boundary scattering and o ptical interband (or localised charge) absorption. The relative contri bution of each is estimated. A key conclusion is that the interband (o r localised charge) absorption is only similar to 10%. Most importantl y, the intrinsic scattering rate, 1/tau, decreases linearly with fall in temperature, T, in a regime where current theory predicts dependenc e on frequency, omega, to dominate. The coupling constant, lambda, bet ween the charge carriers and the thermal excitations has a value of 1. 7, some fivefold greater than the far infrared value. These results im ply a need to restate the phenomenology of the normal state of high te mperature superconductors and seek a corresponding theoretical underst anding.