The far-infrared response of the ceramic high temperature superconductor YBa 2Cu 4O 8

Abstract

Far-infrared reflectivity measurements on ceramic samples of YBa 2Cu 4O 8, a high- T c superconductor with T c = 80 K, were made over the temperature range of 20–300 K and frequency range of 20–1000 cm −1. Six phonons were observed at 130 cm −1, 192 cm −1, 300 cm −1 (doublet), 500 cm −1, and 603 cm −1 and their temperature dependences were studied. Through comparisons with 1–2–3 compounds, these phonons were identified and assigned to the respective lattice vibrations of B 1u symmetry (i.e. along the c-axis). The observed temperature dependences of phonon frequencies, lineshapes and reflectivity ratios all point to the existence of two superconducting energy gaps, one in the c direction at about 150 cm −1 and another (somewhat more speculatively) in the ab plane at about 450 cm −1. While the c-axis gap occurs at a nearly BCS-like value of ∼ 2.7 kT c, the a−b plane gap corresponds to ∼ 8 kT c, in agreement with the work of others on single crystals of YBa 2Cu 3O 7.