Anisotropy in the Far Infrared Conductivity of Magnetically-Oriented YBa₂Cu₃O_7-δ Between 50–300K

Abstract

The far infrared reflectance of magnetically-oriented YBa₂Cu₃O_7-δ has been measured from 30–600 cm^-1 over the temperature range 300–50 K. A strongly temperature dependent optical anisotropy is observed in this region. The low energy Ē⊥c spectrum displays a high reflectivity associated with the dominant free carrier contribution. Peaks corresponding to phonons are present in the spectrum associated with the direction perpendicular to the high conductivity Cu - O planes. However the reflectivity for Ē∥c is not characteristic of an insulating material but rather shows a metallic-like rise with decreasing photon energy. The plasma frequencies for both polarizations, obtained from fits to the reflectance data, decrease with decreasing temperature. There is some evidence in the parallel conductivity data taken below T_c, for the onset of an energy gap that is substantially smaller than that deduced for the in-plane value; such a result suggests a large gap anisotropy. The parameters describing a number of low frequency phonon modes for Ē∥c have been determined from fits to the R∥ data. Most show a slight softening as T_c is traversed.