Abstract

The near infrared and optical absorption of several high-Tc superconductors is investigated using the KBr powder method. The spectra show broad NIR absorption peaks. Examples of absorption spectra obtained using transmission data of thin film work for Ba1−xKxBiO3 (BKB) and single-crystal measurements for Bi2Sr2Ca1−xYxCu2O8+δ (BSCC) are given for comparison. They confirm the results of the KBr spectra. An example of similar investigations on NiO also shows good agreement over the whole spectral range under investigation. These facts provide some evidence that the line profiles deduced by the KBr technique may indicate a correct absorption line shape. A possible explanation of the NIR line profile is discussed.

Introduction

The absorption of various high- Tc systems measured using Kubelka-Munk [1, 2] or standard KBr powder techniques [3–7] is known to possess broad peaks in the near infrared spectral range. Similar features have earlier been discussed in connection with a small-polaron absorption mechanism in the non-stoichiometric compounds of systems like TiO2−x [8], WO3−x [9], NbO2.5−x [10,11] and the ternary compounds NbxW1−xOy [12]. For the superconductors of the system YBa2Cu3O7−δ, La2CuO4+δ and Bi2Sr2CaCu2O8+δ [3–5] it has been shown that the NIR absorption cross-sections increase approximately linearly with decreasing temperature down to the superconducting transition temperatures (Tc). Below Tc the slope decreases to smaller values. Dewing et al. [3, 4] have explained this effect as evidence for Bose–Einstein condensation of small bipolarons for YBa2Cu3O7.