Properties of the Normal and Superconducting States of High-Tc Superconductors Determined by the Infrared Conductivity

L. D. Rotter ； Z. Schlesinger ； R. T. Collins ； F. Holtzberg ； C. Field ； U. W. Welp ； G. W. Crabtree ； Y. Fang ； K. G. Vandervoort ； S. Fleshler ； J. Z. Liu

《Chinese Journal of Physics》 30卷2期 (1992/04) Pp. 271-283

並列摘要

The infrared conductivity provides valuable information regarding the dynamics of the carriers in the normal state, as well as the size and nature of the superconducting gap, or characteristic energy scale, in the high-Tc superconductors. The two are interrelated in that the interpretation of conductivity features found in the normal state influences the interpretation of features found in the superconducting state. The development of various interpretations of the data will be traced, while emphasizing how the the highly anisotropic nature of the materials affects the results. We will concentrate on the highly studied YBa2Cu3O7-y system, including new results on reduced-Tc materials, and show how polarized reflectivity measurements on single-domain samples lead to an understanding of the normal state, and, hence, the superconducting state as well. We find that the CuO2 plane conductivity can be well described in the normal state by an unconventional Drude-like term with a frequency dependent scattering rate and mass. Features which have been assigned to a mid-infrared mode are shown to be related to the CUO chains and are not present in the CuO2 plane conductivity. The CuO2 plane conductivity shows a ~500 cm^(-1) threshold at low temperatures. This threshold shows strong temperature dependence just below Tc which demonstrates its essential connection with the development of the superconducting state. Moreover, its persistence above Tc suggests an increasing tendency towards pair formation, or spin gap formation, in the normal state as Tc is reduced.