INFRARED STUDIES OF HIGH-TEMPERATURE SUPERCONDUCTORS

Infrared measurements have been used both to study the superconducting energy gap and the normal state electronic character of the high TC oxide superconductors. While the question of the existence of a gap is not yet completely resolved, we find strong evidence for a very large energy gap, 2Δ8kTC, in the a-b plane response of crystal Y1Ba2Cu3O7. Simple calculations also support the assertion that the much smaller gap values, 2Δ4kTC, inferred from polycrystalline data are associated with the c-axis response. The relationship between infrared, tunnelling and NQR measurements of the energy gap is not yet clear. We find that the normal state conductivity above about 1000cm-1 is primarily Drude-like with modest interband contributions beginning at about 6000cm-1. The area of our Drude curve corresponds to a density of 5×1021 cm-3 assuming a mass of 1 at high frequency. Below 1000cm-1 we suggest that a frequency dependent scattering rate will be required to obtain an appropriate correspondence between the infrared conductivity and the strongly temperature dependent d.c. conductivity.