Far-Infrared Absorption by Small Particles of YBa₂Cu₃O_6.74

Abstract

The presence of the superconducting energy gap, 2Δ, in YBa₂Cu₃O_6.74 has been observed directly at 45±5 cm⁻¹ and at 1.6 K by far-infrared absorption measurements in small particles embedded randomly in a polyethylene film. This result indicates that the gap has the minimum of 2Δ_min\simeq0.7 kT_c, and thus the gap is finite along any direction in the crystal. Transverse effective charges associated with polar optical phonons at 144, 280, and 315 cm⁻¹ increase by a factor of 4∼5 upon a decrease in the temperature from 295 K to 1.6 K, suggesting the presence of an anomalous electron-phonon coupling in this material.